Aspects of Reductive Methods in Organophosphorus Chemistry

A Thesis presented to the Faculty of Science of the University of New South Wales in fulfilment for the Degree of

Doctor of Philosophy

by Neil Donoghue B.Sc. (Hons.), University of Adelaide

Department of Organic Chemistry School of Chemistry University of New South Wales May 1998 ii Abstract.

This study is concerned with the reductive cleavage of tetracoordinated organophos- + – phorus compounds (either quaternary phosphonium salts R4P X or tertiary phosphine oxides

R3P O) with either the naphthalene radical (naphthalenide) anion or lithium aluminium hydride in THF solution at room temperature (RT). Part 1 examines the reaction of lithium naphthalenide with both phosphonium salts and phosphine oxides. The reaction was demonstrated to cleave phenyl groups from both bis-salts and bis-oxides in the presence of 1,2- ethylene bridges; based upon this, parallel syntheses of either 1,4-diphosphabicyclo[2.2.2]octane or its P,P'-dioxide were attempted by using the commercially available ethane-1,2-bis- (diphenylphosphine) as the starting material in each case. Examination of the products of + – reductive cleavage of the series of benzylphenylphosphonium bromide [PhnP(CH2Ph)4-n] Br (where n = 0 to 3) with lithium naphthalenide leads to the proposal of a mechanism. Part 2 describes hydridic reductions of both quaternary phosphonium salts and tertiary phosphine oxides. Examination of the lithium aluminium hydride reduction of qua- 31 ternary phosphonium salts using P-NMR has confirmed tetraorganophosphoranes (R4PH; R = Ph, alkyl) as intermediates in the reaction; in addition, two previously unknown classes – of compounds, the triorganophosphoranes R3PH2 and the tetraorganophosphoranates R4PH2 , were also found to be intermediates. The behaviour of bis-phosphonium salts where the phosphonium centres are separated by either 1,2-ethylene or 1,3-propylene bridges are also examined. Formation of a monocation exhibiting a bridging hydride occurs when the cyclic bis-phosphonium salt 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide is reacted with lithium aluminium hydride. Mechanisms are proposed which are consistent with the observed experimental results. iii Acknowledgments.

This work is dedicated to my family Kathleen, Robert and Graham, and to the memory of my sister Judith.

Special thanks go to my Supervisor A/Prof. Mike Gallagher for all his suggestions and help over the years, and for being the number-one fan of the “dihydrophosphoranes.”

Many thanks also to Prof. Bruce Wild (of the RSC at the ANU, Canberra) for his interest in the “fundamental chemistry” of the hydrophosphoranes. Thanks must go to Dr. Graham Ball & Hildegard Stender (who ran all of the Bruker DMX 500 spectra), Dr. Tahany Ghazy & Dr. Jim Hook (for training and assistance on the Bruker ACP 300 and ACF 300 machines), Dr. Joe Brophy (Mass Spectroscopy); also to A/Prof. Roger Bishop, Prof. David Black, Prof. Ian Dance, Dr. Gavin Edwards, Dr. Jim Hook, Dr. Naresh Kumar, Dr. Grainne Moran, Prof. Mike Paddon-Row, and Dr. Roger Read, as well as Juan Araya, John Narayanan, Rane, Paul Sykes, and Thanh Vo Ngoc… …and also to Dr. Darryl McConnell, for putting up with me as a flatmate down at Maroubra Beach in 1993.

Sincere thanks go to each of the following: Prof. David Black and Niall O’Shea for thesis grammar checking; [Organic Chemistry, UNSW] Paul Ahn, Paul Harvey, Drs. Kate Jolliffe & Chris Marjo, Ashley Jones, Craig Muldoon, Tracey O’Leary and Sharadha Sakthi- Kumar; [International House, UNSW] Rebecca Findlow, Michael Harries, Dr. Tim Heseltine, Margaret Lloyd, Gwen McLay, Helen Pearce, Cheryl Richardson, Margaret Sharland and Bernhard Vogl; [Adelaide] Michelle Brennen & Simon Clinch, Dr. Maureen Bussitil, Margaret Curry, Ramesh Dhillon, Greg Newbold, Sara Rankin, Sonya Whitbread & her sister Reina; [Canberra] Suzie Callaghan, Chuck, Kerrie Langford, Linda Langford & Richard Pratt, and Joanne Pratt; [Bavarian Chess Club] Konrad Schöll, Waltraud Schweiger and Marion Gürtler, as well as all those people who deserve at lot more, but just refused to be catagorised properly: Kitty Ahn, Julie Harvey, Sophie McConnell, as well as Björn Marksteiner & Jane Rees. Thanks due for accommodation supplied (both recreational and essential) by: Suzie Callaghan & her sister Shauna, Linda Langford & Richard Pratt, Tracey O’Leary & her parents, Helen Pearce, and Cheryl Richardson…

…and to any other deserving soul who has been missed… sorry, and thanks! iv Abbreviations.

H* anti-bonding orbital (of H-symmetry) [F] concentration of F (in moles per litre) ˚C degrees Celsius = degrees Centigrade Å angstrom (1 Å = 10 nm = 10–8 cm) 13 GC C-NMR: chemical shift (in ppm from TMS) 2 GD H-NMR: chemical shift (in ppm from TMS) 1 GH H-NMR: chemical shift (in ppm from TMS) 31 GP P-NMR: chemical shift (in ppm from 85% H3PO4) S pi orbital V sigma orbital AO atomic orbital ax axial, parallel to the principle axis biph 2,2'-biphenylene bipy 2,2'-bipyridyl BPR Berry Pseudo-Rotation

Bn benzyl, C6H5CH2- (phenylmethyl) ca approximately

CDCl3 deuterated chloroform, d1-chloroform

CD3CN perdeuterated acetonitrile, d3-acetonitrile

C6D6 perdeuterated benzene, d6-benzene

C10H8 naphthalene

Cn E-cinnamyl, E-PhCH=CHCH2- (trans-3-phenylprop-2-enyl) C.N. coordination number d doublet (NMR) dd doublet of doublets (NMR) D deuterium, 2H

D2O deuterium oxide d8-THF perdeuterated tetrahydrofuran, d8-tetrahydrofuran DLE dynamic ligand exchange e doubly degenerate set of orbitals eq equatorial, approximately perpendicular to the principle axis

Et ethyl, CH3CH2-

Et2O diethyl ether fac facial isomer of octahedral symmetry gm gram Hz Hertz v

I nuclear spin n JA-B the n-bond coupling constant between nuclei A and B (in Hz) K Kelvin L litre = dm3 M moles per litre = mol.L–1

Me methyl, CH3- mer meridional isomer of octahedral symmetry mL millilitre = cm3 MO molecular orbital mol mole mp melting point MS mass spectrum or mass spectroscopy n Bu n-butyl, CH3CH2CH2CH2- NMR nuclear magnetic resonance

Ph phenyl, C6H5- ppm parts per million q quartet (NMR) qn quintet (NMR) RBF round-bottomed flask RT room temperature (about 25˚C) s singlet (NMR) sec second sx sextet t triplet (NMR) t Bu t-butyl, (CH3)3C- TFA trifluoroacetic acid THF tetrahydrofuran TMS tetramethylsilane TSR Turnstile Rotation wrt with respect to vi Table of Contents.

Abstract. ii

Acknowledgements. iii

Abbreviations. iv

Table of Contents. vi

Overview of the Thesis. ix

Part 1. Towards the Synthesis of some Phosphorus Heterocycles.

Chapter 1: Introduction to Part 1. 2

1.1 Trivalent phosphorus as a ligand to transition metals. 2 1.2 Barriers to inversion at trivalent phosphorus. 3 1.3 Structural degrees of freedom at phosphorus. 5 1.4 Some examples of phosphorus heterocycles. 6 1.5 Methodologies for the synthesis of phosphorus heterocycles. 13

Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene 16 Radical Anion.

2.1 Cleavage of phenyl versus loss of the ethylene bridge. 17 2.2 Cleavage of phenyl versus loss of the 1,3-propylene bridge. 20 2.3 Cleavage of phenyl versus cleavage of benzyl. 22 2.4 Approaches toward the 1,4,7-triphenyl-1,4,7-triphosphonane system. 25

Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene 30 Radical Anion.

3.1 Reductive cleavage of triphenylphosphine chalcogenides. 32 3.2 Attempts at phenyl cleavage from bis(phosphine oxides). 33 3.3 Attempts at phenyl cleavage with cyclic bis(phosphine oxides). 36 vii Part 2 The Hypervalent Chemistry of Phosphorus.

Chapter 4: Introduction to Part 2. 40

4.1 The search for main-group penta-coordinated molecules. 40 4.2 The stereochemical properties of penta-coordinated phosphorus. 46 4.3 The relationship between penta- and hexa-coordinated phosphorus. 52 4.4 Organic derivatives of penta- and hexa-coordinated phosphorus. 54 4.5 The discovery of hydrophosphoranes and hydrophosphoranates. 60

Chapter 5: Incorporation of Deuterium into Benzene and Toluene 61

cleaved using LiAlD4.

+ – 5.1 Toluene cleaved from Ph3PBn Br with LiAlD4.63 + – 5.2 Toluene cleaved from PhnPBn4-n Br (n = 0, 1, 2, 3) with LiAlD4.70 + – 5.3 Benzene cleaved from Ph4P Br with LiAlD4.75 5.4 Comparison of the percentage deuteration in the benzene and toluenes. 77

Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium 79 Aluminium Hydride.

+ – 6.1 The reaction of Ph4P Br with LiAlD4 in THF at RT. 80 + – 6.2 The reaction of PhnPBn4-n Br (n = 0, 1, 2, 3) with LiAlD4 in THF at RT. 91 – 6.3 Attempts to isolate Ph4PH, Ph3PH2 and Ph4PH2 .95 – 6.4 The multi-nuclear NMR spectroscopy of Ph4PH, Ph3PH2 and Ph4PH2 . 103 – 6.5 The bonding in Ph4PH, Ph3PH2 and Ph4PH2 . 107 6.6 Reaction of other mono-phosphonium salts. 117

Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium 127 Aluminium Hydride.

7.1 Reactions with bis-phosphonium salts containing a 2-carbon bridge. 127 7.2 Reactions with bis-phosphonium salts containing no 2-carbon bridges. 133 viii

Chapter 8: Further Reactions using Metal Hydride Reagents. 137

8.1 Additional reactions using lithium aluminium hydride in THF. 137

8.2 Reactions using LiBH4 in THF. 140 8.3 Reactions using 1.6 M Red-Al in toluene. 141 8.4 Reactions using KH in THF. 141

Chapter 9: The Reactions of Phosphonium Salts with Other 142 Nucleophiles.

9.1 Reaction of deuteroxide ion in D2O/THF. 142 9.2 Reaction of cyanide, azide and fluoride ions. 144

Part 3 The Experimental Details.

Chapter 10: Experimental. 147

10.1 Introduction. 147 10.2 Analysis of the commercial samples. 148 10.3 Analysis of the research samples. 151 10.4 Preparation and reactions of lithium phosphides. 153 10.5 Synthesis of the ammonium and phosphonium salts. 155 10.6 Synthesis of the phosphine chalcogenides. 162 10.7 The reaction of phosphine oxides with metallic lithium in THF. 165 10.8 The reaction of phosphine chalcogenides with lithium naphthalenide. 166 10.9 The reaction of phosphonium salts with lithium naphthalenide. 168 10.10 Towards the 1,4,7-triphenyl-1,4,7-triphosphonane system. 170

10.11 The reaction of mono-phosphonium salts with LiAlH4 in THF. 172

10.12 The reaction of bis-phosphonium salts with LiAlH4 in THF. 182

10.13 The reaction of other phosphorus compounds with LiAlH4 in THF. 185 10.14 The reaction of phosphorus compounds with other hydride donors. 187 10.15 The reaction of phosphonium salts with miscellaneous nucleophiles. 188

References. 192 ix

Overview of the Thesis.

The work described in this thesis has been split into two parts: Part 1 (Chapters 1 to 3) deals with some approaches towards the synthesis of phosphorus heterocycles containing at least three bonds to carbon. The focus is on the conversion of quaternary phenylphosphonium salts into tertiary phosphines through the reductive cleavage of phenyl groups, using naphthalene radical anion as a reducing agent. Using this approach, an attempt was made to synthesise the cyclic tris-phosphine sym-1,4,7-triphenyl-1,4,7-triphosphonane 29 from the corresponding tris-salt 1,1,4,4,7,7-hexaphenyl-1,4,7-triphosphonanium tribromide 36. Also discussed here are parallel reactions of tertiary phenylphosphine oxides with naphthalene radical anion.

In Part 2 (Chapters 4 to 9), a series of hydrophosphoranes R4PH (R = Ph, Bn, Me) are presented, representing a class of compounds previously unknown except for the spirocyclic hydrophosphoranes HP(L-L)2 (L-L = 2,2'-biphenyl) characterised by Hellwinkel. In addi- – tion, dihydrophosphoranes R3PH2 (R = Ph, Bn, and Me) and dihydrophosphoranates R4PH2 (R = Ph, Me) were also discovered, representing two entirely new classes of compounds. All of these species were produced through the reaction of quaternary phosphonium salts with

LiAlH4 in THF at RT, and represent intermediates in the hydride reductive cleavage of phosphonium salts to give phosphines. Characterisation for all of these compounds was achieved using 31P{1H}-NMR. The work presented in both Parts 1 and 2 rely heavily on characterisation of products and intermediates by 31P{1H}-NMR spectroscopy. Phosphorus lends itself well to this form 31 1 1 of analysis due to the high natural abundance (100% P, I = /2) and sensitivity of the NMR- active isotope. Additionally, because the nucleus has no electric quadrupole moment, the observed line-widths are reasonably narrow (ca 1 Hz) allowing resolution of fine structure, such as spin-spin coupling. It is doubtful that the hydrophosphoranes discussed in Part 2 would have been discovered if the spectrometer on which the 31P{1H}-NMR spectra were obtained was not readily available. Part 1: Towards the Synthesis of some Phosphorus Heterocycles.

It won’t happen overnight, but it will happen!

Rachel Hunter. Chapter 1: Introduction to Part 1.

1.1 Trivalent phosphorus as a ligand to transition metals.

Trivalent phosphorus compounds, particularly the tertiary organophosphines R3P, are well known for their behaviour as donors towards the softer (i.e. more polarisable) metal cations, such as those from the d-block transition series.2 In addition to functioning as a V-donor via donation of lone-pair electron density (a property in common with its nitrogen analogues

R3N), the R3P ligand also possesses low-lying vacant orbitals of S-symmetry (not available in the case of R3N) into which electron density from a d-block metal can flow. This synergistic effect (also known as “back-bonding”) results in a stronger M-P bond with some multiple bond character, as well as preventing the build-up of excessive electron density on the metal. In effect, the movement of V-electron density from the lone pair on phosphorus into the vacant V-orbital on the metal is balanced somewhat by movement of S-electron density of non-bond- 2 ing d-electrons on the metal into the low-lying LUMOs of the R3P ligand (Figure 1.1).

R R

MMPM P M P

R R R R

(a) (b)

Figure 1.1: The synergistic model of transition metal-phosphine bonding: (a) simple donation of V-electron density from the lone pair on phosphorus into a vacant V-orbital (coordination site) on the metal atom (ion), making the metal electron-rich while the phosphorus becomes electron-poor; (b) movement of S-electron density from a filled d orbital on the metal into a vacant S-orbital on phosphorus, ultimately reducing the electron build up on the metal, and decreasing the polarity of the M-P bond. (The filled orbitals are shaded, with the arrows indicating the flow of electrons.)

The explanation often proposed for “back-bonding” is the participation of vacant 3d orbitals of the appropriate symmetry localised on phosphorus. These orbitals lie much higher in energy than the valence 3p orbitals, and are believed to be fairly diffuse. However, the increasing positive charge building up on phosphorus as the lone pair is donated to the Lewis acid was thought to result in the contraction of these virtual 3d orbitals in space with a simultaneous lowering of their energy.3,4 This effect would be expected to increase with the elec- Chapter 1: Introduction to Part 1. 3

tronegativity of the attached groups on phosphorus (e.g. PF3); consistent with this are that both 5,6 7,8 the observed and calculated Ni-P bond length in [Ni(PF3)4] is significantly shorter (i.e., 9 there is more back-bonding) than for complexes with R3P (R = alkyl, aryl). The result would be the significant overlap of these contracted 3d orbitals with the filled d-orbitals on the transition metal. The inability of nitrogen to accept S-electron density was easily explained as due to the nature of its virtual 3d orbitals, which are much higher in energy than those of phosphorus, and so are much less likely to participate in this type of bonding. Computational studies of systems such as these have cast doubt on this view. Instead, the vacant S-orbitals on phosphorus in PX3 are considered to be the P-X V* (antibonding) or- 7,10,11 bitals. Because of the reasonably low electronegativity of phosphorus (FP = 2.2), the lobes of the P-X V* orbitals tend to localise on phosphorus, increasingly so as the electronegativity of X increases (in contrast, the lobes of the P-X V (bonding) orbitals build up on X). Hence, this model also predicts the S interaction to increase with the electronegativity of the X groups, as is observed. The inability of nitrogen to participate in S-bonding is now explained as follows: the lobes of the N-X V* orbitals are localised on the X groups, while the lobes of 4 the N-X V orbitals build up on nitrogen because of its high electronegativity (FN = 3.0). As the N-X V orbitals are necessarily filled, they cannot accept S-electron density from a transition metal. Only NF3, or perhaps N(CF3)3, would possess vacant N-F V* orbitals localised on nitrogen, giving rise to the possibility of back-bonding in transition metal complexes. As both models give qualitatively similar results, it is difficult to decide between them. Despite this, the second model (which involve P-X V* orbitals12 instead of virtual phosphorus 3d orbitals) is supported by the calculated orbital electron populations which indicate that there is very little (i.e. less than 15%) d-character in the P-M bond.7 Consistent with this is that the

LUMO of PX3 is found to be mainly made up of P 3p orbitals of e symmetry (i.e., the orbitals make up a doubly degenerate pair), matching the P-X V* orbitals.13 A similar result is also found for the P-O bond in phosphine oxide14 (with an even lesser extent in the corresponding phosphine sulphides, selenides and tellurides),15 whereas bonding in both penta- and hexa-coordinated phosphorus can be explained by invoking 3-centre-4-electron (3c-4e) bonds and does not require the use of phosphorus 3d orbitals.14,16-18

1.2 Barriers to inversion at trivalent phosphorus.

Compounds of trivalent phosphorus, such as the tertiary phosphines R3P, are known to –1 –1 possess relatively high barriers to geometric inversion (327 kJ.mol for PH3 to 712 kJ.mol 19 for PF3), unlike their nitrogen analogues which, because of their low inversion barrier, exhibit a high rate of inversion even at room temperature (RT).15,20 The lower rate of inversion in R3P makes it possible to separate enantiomers, with the possibility of using such asymmetric ligands as chiral auxiliaries in asymmetric synthesis. Loss of optical activity in chiral phosphines may be effected either by heating,21 or by exposure to a catalytic amount of free Chapter 1: Introduction to Part 1. 4

22 halogen X2 (X = Cl, Br, I) or organic peroxide ROOR (R = alkyl, aryl). In both cases, racemisation is believed to occur via an intermediate phosphorane (Figure 1.2). Oxidative addition of the halogen or peroxide to PXYZ (X Y Z) gives a phosphorane in which both halogen or oxygen atoms occupy the two axial positions of the trigonal bipyramidal molecule because of the higher electronegativity of these groups relative to alkyl or aryl groups.3,23 Hence, there is now a plane of symmetry lying in the trigonal plane of the phosphorane, and therefore all geometric asymmetry has been lost. This meso-phosphorane is able to oxidise another R3P molecule to the same phosphorane through an exchange reaction, itself being reduced back to R3P. Because all information concerning the original state of chirality has been lost, both enantiomers of the chiral phosphine R3P are formed with equal probability, and so the optical activity of the bulk sample will rapidly decrease to zero. The intermediate phosphorane may undergo dynamic ligand exchange (DLE) processes (two possible mechanisms will be described later in Section 4.2), which can also result in the racemisation of the bulk sample, but this is not necessary to explain the loss of optical activity here.

X R + X R + R P 2 R3 P 2 1 3 R1 P R3 P R1 R + R3P R + X2 R2 3 2 X meso phosphorane

Figure 1.2: Oxidative addition of a phosphine by X2 (X = Cl, Br, I, OR) gives a symmetrical phos-

phorane in which any chiral information has been lost. Subsequent loss of X2 by transfer to another

R3P molecule gives either enantiomer with equal probability.

Chiral phosphines are found to be stable to mutarotation in protic acids. For example, the optical rotation of a solution of the chiral bis-phosphine (RR)-o-phenylenebis(methylphenylphosphine) 1 in 10 M HCl was found to remain unchanged after 3 weeks at RT.21 Because the phosphino groups are ortho to each other, in acid solution they are believed to chelate a single proton (Figure 1.3), and can be crystallised out of solution as the mono-protonated salts.

Ph Me Ph Me Ph Me Ph Me + P P P P + H + H H H + P P P P 1 Ph Me Ph Me Ph Me Ph Me

Figure 1.3: The mono-protonated cation of (RR)-o-phenylenebis(methylphenylphosphine) 1 undergoes rapid proton exchange. Chapter 1: Introduction to Part 1. 5

In sharp contrast to chiral phosphines, chiral arsines are found to rapidly lose their optical activity from exposure to traces of HX.24 Arsines, like phosphines, are not protic bases, but the penta-coordinated arsorane, once formed, may be less likely to revert to either an arso- nium cation or the original arsine because of the reduced inter-ligand repulsions on the larger arsenic atom (the respective covalent radii1 are: P 1.10 Å, As 1.21 Å). Such an increase in the lifetime of the arsorane intermediate compared to the corresponding phosphorane would then increase the chance of DLE occurring. Also possible is a faster rate of DLE in the arsorane compared with the phosphorane: because of the reduced steric interactions between the ligands in the arsorane due to the larger arsenic atom, the transition state geometry in the DLE process (which is believed to be a square pyramid; see Section 4.2) would also be less crowded, and so perhaps lower in energy relative to the phosphorane case. This would decrease the energy barrier, thereby increasing the rate of DLE, and therefore also increasing the rate of racemisation, as observed. Indirect evidence consistent with this hypothesis is the case of pentaphenylantimony (Ph5Sb), in which two geometries occur: Ph5Sb has been shown to exist in both a trigonal bipyramidal geometry25 as well as a distorted square pyramid.26 The implication is that, in the case of the much larger antimony atom (covalent radius: Sb 1.41 Å1), the energy difference between the two geometries has become so small that crystal packing forces can decide between them.

1.3 Structural degrees of freedom at phosphorus.

In a phosphine molecule R3P, the phosphorus atom is directly involved in three bonds, about each of which free rotation can occur when all three R groups are independent from one another. Hence, the phosphorus atom may be considered to have a maximum of 3 degrees of freedom (this ignores the translational and rotational degrees of freedom for the molecule as a whole). This situation corresponds to cyclopentylphosphine 2, where free rotation can occur around both P-H bonds as well as the P-C bond. For this, and any, phosphine with three independent substituents, the phosphorus atom may be considered to have 3 degrees of freedom.

H H

P P H P P H 23 45

What this means is that the cyclopentyl ring can rotate by any angle with respect to either of the P-H bonds (there are no angular restrictions apart from those imposed by the bond angles at the atoms themselves). Of course, rotation about either of the P-H bonds will not be observable because of the cylindrical symmetry of the P-H bonds. (Amines, because of their rapid inversion, may be assigned an extra degree of freedom, giving a maximum of 4 degrees Chapter 1: Introduction to Part 1. 6 of freedom associated with the nitrogen atom; at RT, phosphines are not observed to undergo inversion, and so this extra degree of freedom may be ignored.) If two of the substituents on phosphorus are now connected, as in phospholane 3, the situation changes. Now, even though all three bonds are still inherently able to support rotation (they are all single V-bonds), the fact that both P-C bonds are part of the same 5-membered ring means that they are no longer independent from each other, and must rotate together. In addition, rotation may also occur about the single P-H bond. Hence, the number of degrees of freedom possessed by the phosphorus atom has decreased to 2 because of its incorporation into the ring (the analogous amine pyrrolidine would have 3 degrees of freedom associated with the N atom because of inversion). This results in a restriction of the phosphorus atom relative to the case of three independent groups. In the case of 1-phosphabicyclo[3.3.0]octane 4, the phosphorus atom is located at the junction of two cis-fused rings (the trans-fused isomer would experience excessive ring-strain). Clearly, each of the three P-C bonds is now linked to the other two, either directly or indirectly, resulting in no degrees of freedom for the phosphorus atom (i.e. no independent rotation about any of the three bonds can occur; only rotation of the whole molecule, a motion that has already been accounted for). For similar reasons, the phosphorus atom in 1-phosphabicyclo[3.3.3]- undecane 5 also has no degrees of freedom. It is likely that, for the corresponding amines 1- azabicyclo[3.3.0]octane and 1-azabicyclo[3.3.3]undecane, inversion at nitrogen would be restricted due to the bicyclic nature of the compounds. If this is so, then both compounds can be assigned zero degrees of freedom to nitrogen. For the remainder of this chapter, discussion will be presented of two areas of heterocyclic chemistry: the first deals with monocyclic phosphorus compounds, particularly bis- and tris-phosphorus heterocycles, while the second con- siders both bi- and tricyclic compounds containing the restricted bridgehead phosphorus.

1.4 Some examples of phosphorus heterocycles.

There have been several examples of cyclic bis-phosphines reported in the literature. The X-ray crystal structures of both the cis27 and trans28 isomers of 1,4-diphenyl-1,4-diphosphorinane-1,4-dioxide 6 show that, in each case, the ring adopts a chair conformation. In the cis isomer, one of the P=O bonds is axial while the other is equatorial,27 whereas both P=O bonds are axial in the trans isomer.28 In contrast, in trans-1,4-diphenyl-1,4-diphosphorinane (trans-7) itself, it is the phenyl groups that are found to be axial.29 Other examples29 of the preparation of cyclic bis-phenylphosphines are cis- and trans-1,3-diphenyl-1,3-diphospholane 8, cis- and trans-1,3-diphenyl-1,3-diphosphorinane 9, cis-1,4-diphenyl-1,4-diphosphorinane (cis-7), 1,3-diphenyl-1,3-diphosphepane 10, cis- and trans-1,4-diphenyl-1,4-diphosphepane 11 and cis- and trans-1,5-diphenyl-1,5-diphosphocane 12. The X-ray structure of both trans-12 and its 1,5-dioxide 13 have been determined,30 in which both oxygen atoms in 13 are found to be pseudo-axial, as is the oxygen atom in 1-phenyl-1-phospha-5-thiacyclooctane-1-oxide 14.30 Chapter 1: Introduction to Part 1. 7

Additionally, a number of cyclic tetrabenzyl bis-phosphonium salts have been prepared31 by quaternisation of D,Z-bis(dibenzylphosphino)alkanes with D,Z-dibromoalkanes in MeCN.

O O Ph

P P P Ph Ph O Ph P P P

Ph O Ph cis-6 trans-6 trans-7

Ph Ph Ph Ph Ph PP P P P P Ph cis- & trans-8 cis- & trans-9 cis-7

Ph Ph Ph Ph P P P P

P Ph P Ph

10 cis- & trans-11 cis- & trans-12 O Ph Ph P Ph P P S

O O 13 14

Another class of heterocycles worthy of the considerable interest it has provoked is that based on 1,4,7-trioxacyclononane 15 (also known as 9-crown-3). Calculations suggest that 15 32 + – itself should be capable of complexing to a lithium atom to produce Li(15)2] e 16, an ionic salt-like substance known as an electride, in which electrons are present in the crystal lattice in place of anions (Figure 1.4). The positioning of the three donor atoms is almost perfect for capping a trigonal face of an octahedral metal atom.

O O O O Li Li+ e– metal O O O O O

15 16 Figure 1.4: The formation of the electride 16 from the reaction between Li metal and 15 in an inert solvent. Chapter 1: Introduction to Part 1. 8

Analogues of the crown where some or all of the oxygen atoms have been replaced with nitrogen or sulphur have been synthesised, and are also found to exhibit peculiar coordinating n+ ability. For instance, many metal complexes of the type [ML2] have been characterised, where L is either 1,4,7-trithiacyclononane33-38 17, its nitrogen analogue 1,4,7-triazacyclonon- ane39-42 18, or the mixed donor ligand 1,4-diaza-7-thiacyclononane43 19. In contrast, the tri- substituted 1,4,7-trimethyl-1,4,7-triazacyclononane 20 is found to only give 1:1 complexes42 n+ of the type [LMXm] where m = 1, 2, or 3, presumably because of the increased steric strain imposed by the three Me groups. Again, 17 can also participate in many tetrahedral com- 44-46 n+ 47-50 n+ plexes of the type [LMX] as well as in octahedral complexes such as [LMX3] (where X is a unidentate ligand).

H Me

S N S N

S S N N N N N N H H H H Me Me

17 18 19 20

Besides the apparent geometric stability, metal complexes of the cyclononane-based n+ ligands [ML2] also exhibit unusual redox behaviour. For instance, in the case of gold, an II 2+ intermediate Au species, [Au(17)2] 21 (an exceedingly rare oxidation state for gold which is presumed to prefer a pseudo-octahedral geometry), was detected by ESR spectroscopy45 du- + 3+ ring redox experiments. The analogous [Au(17)2] 22 and [Au(17)2] 23 complexes were characterised by X-ray crystallography (Scheme 1.5), with the gold exhibiting its usual coordination geometries (tetrahedral and square planar, respectively).

3+ 2+ S S S S S S S + – – S S Au – e Au – e Au S S + e– + e– S S S S S S

22 21 23

Scheme 1.5: The redox chemistry of gold in the presence of 17 provides evidence for the formation of an intermediate gold(II) complex.

A possible explanation for the remarkable ability of 17 to complex to metal ions is suggested by the conformation adopted in the solid state. The X-ray crystal structure of 1751 Chapter 1: Introduction to Part 1. 9

shows that the ligand exists in a symmetrical C3 conformation where all three sulphur atoms are directed into the molecular cavity. The S…S distance was found to be 3.451Å, which is less than the sum of the van der Waals radii, suggesting the possibility of weak transannular sulphur-sulphur bonding. Because the C3 conformation is necessarily adopted upon coordination to a trigonal face of a metal ion, the lack of conformational change required in the lig- and will result in a relatively small activation energy of complexation.38 In addition to bind- ing to a trigonal face of both octahedral and tetrahedral metal ions, 17 has also been shown to bind to a square pyramidal metal ion. For the NiII complex cation [Ni(17)P-P]2+ 24 (where P-P is bis(diphenylphosphino)methane), the face-capping ligand 17 is found to coordinate to the nickel ion through 2 basal and the single apical positions (Figure 1.6).52

2+ Ph S Ph S P Ni CH2 S P Ph Ph

Figure 1.6: The square pyramidal complex cation 24.

More recently, a derivative in which phosphorus replaces one of the sulphur atoms in 17 was synthesised. 1-Phenyl-1-phospha-4,7-dithiacyclononane 25 has been shown to form a + + – 2:1 complex with tetrahedral Cu . The X-ray crystal structure of [Cu(25)2] PF6 26 shows one ligand capping a trigonal face of the copper ion, with the coordination of the metal com- pleted by the phosphorus atom of the second ligand,53 the two remaining sulphur atoms remaining non-coordinating in the crystal lattice (Figure 1.7).

Ph Ph S P P S + Cu Ph P S S S S

25 26

Figure 1.7: The ligand 1-phenyl-1-phospha-4,7-dithiacyclononane 25 and its copper(I) complex 26.

This strongly suggests that the phosphorus atom behaves as a much stronger donor than the sulphur atoms, as would be expected from comparison with the behaviour of the monoden- 54 tate R2S and R3P molecules. The structure of the complex is similar to that found for the Chapter 1: Introduction to Part 1. 10

+ – + complex [Cu(17)2] PF6 27, where one ligand caps a trigonal face of the tetrahedral Cu ion while the second ligand only coordinates to the copper ion through one of its sulphur atoms.46 No analogues of 17 containing more than one sulphur substituted by phosphorus have yet been reported in the literature. However, the analogous 12-membered ring tris-phosphino macrocycle 1,5,9-tris(2-propyl)-1,5,9-triphosphacyclododecane 28 has been prepared, isolated by cleavage from a molybdenum(II) complex with alcoholic NaOH.55 The X-ray structure shows all three phosphorus lone pairs on the same face of the ligand, as expected if the temperature is kept low enough to prevent inversion at phosphorus.15 The molecule is expected to be more flexible than its cyclononane analogues, and may therefore be able to coordinate to a wider range of different sized metal ions.

iPr iPr Ph Ph PP P P

P P

iPr Ph 28 29

Several attempts to synthesise cyclic tris-phosphine 29 have been made, including the attempted cyclisations of phenyldivinylphosphine 30 with ethane-1,2-bis(phenylphosphine) 31 and bis(2-(phenylphosphino)ethyl)phenylphosphine 32 with 1,2-dibromoethane; however, only polymeric products were obtained (Scheme 1.8).56

PH Ph Polymeric BrCH2CH2Br P P P Ph H Products Ph P PH H Ph 31 30 32 Ph

Scheme 1.8: The attempted cyclisations of both 31 with 30 and of 32 with 1,2-dibromoethane gave only polymeric material.

The attempt to prepare ethane-1,2-bis(phenylvinylphosphine) 33 from the reaction between dilithium ethane-1,2-bis(phenylphosphide) 34 and vinyl bromide gave an excellent yield of the cyclised product 1,4-diphenyl-1,4-diphosphorinane 7 as a mixture of cis and trans isomers, but none of the desired product 33 was detected (Scheme 1.9).56 Chapter 1: Introduction to Part 1. 11

Ph Ph Ph

P PLi P Br Br

P PLi P

Ph Ph Ph

33 34 7

Scheme 1.9: The reaction of the bis-phosphide 34 with vinyl bromide gave cis- and trans-7.

In the course of this work, a route to bis(phosphino)-phosphonium salt 35 was discovered (see Chapter 2). Cyclisation with 1,2-dibromoethane would then give 1,1,4,4,7,7- hexaphenyl-1,4,7-triphosphonanium tribromide 36, from which desired cyclic tris-phosphine 29 could then possibly be obtained by reductive phenyl cleavage using a solution of naphthalene radical anion in THF (Scheme 1.10).

Ph Ph Ph Ph Ph Ph P++P Ph P P + BrCH2CH2Br Li P PPh C H Ph 2 + 10 8 – PPh Br 2 P 3Br– P

Ph Ph Ph

35 36 29

Scheme 1.10: Proposed route to 1,4,7-triphenyl-1,4,7-triphosphacyclononane 29.

+ BnMgCl BrCH CH Br 2 2 + PBn3 PCl3 Bn3P Et2O Bn3P 2Br–

Bn LiAlH4 2Br– P Bn Bn LiAlH4 + + BrCH2CH2Br PBn P P 2 Bn2P cis & Bn Bn trans P Bn Bn

P BrCH2CH2Br ++ LiAlH4 Bn P P Bn P P cis 2Br– P Bn 37

Scheme 1.11: The synthetic route used by Mann to prepare the bridgehead bicyclic diphosphine 37. Chapter 1: Introduction to Part 1. 12

Progress has also been made in the area of bicyclic phosphorus heterocycles. In 1959, the synthesis of 1,4-diphosphabicyclo[2.2.2]octane (37), involving the repeated quaternisation and subsequent reductive benzyl cleavage using LiAlH4 from the bis-benzylphosphonium salts, was reported57 (shown above in Scheme 1.11). However, because the overall yield was extremely poor, this method does not constitute a practical synthesis of 37. Other bicyclic bridgehead phosphorus compounds that have been reported are 1-phosphabicyclo[3.3.1]nonane 38,58 1,5-diphosphabicyclo[3.3.1]nonane 3959 and its 1,5-disulphide 40,60 and 1-phospha-5-azabicyclo[3.2.1]octane 4161 along with its methylated derivative 42.62 The cis-fused 1,5-diphosphabicyclo[3.3.0]octane 43, 1,5-diphospha[4.3.0]nonane 44 and 1,6- diphosphabicyclo[4.4.0]decane 45,63 and the corresponding 1,6-diphosphatricyclo[4.4.4.0]te- tradecanediium dication 46,64 were prepared from a similar methodology used to synthesise bridgehead bicyclic diamines.65-68

S S P P P P P N P N P

38 39 4041 42

_ 2CF3SO3 + P P P P +P P P P

43 44 45 46

Compounds such as these, with the inherent freedom of the phosphorus atom severely restricted, can exhibit unusual properties well worthy of investigation. For instance, the phosphonium-oxyphosphorane 47, obtained from the alkaline hydrolysis (Scheme 1.12) of the tricyclic bis-salt 46, exhibits a substantial transannular interaction in the form of a P-P bond,64 1 detectable via through-bond coupling (i.e. JP-P = 115 Hz). The observed transannular interactions between the two phosphorus atoms observed in 1 1 both the fluoro (48; JP-P = 182 Hz) and methoxy (49; JP-P = 155 Hz) derivatives are even stronger than in the oxo species (47) shown above. All three compounds are obtained from – – – 64 reaction of 46 with Ph3SnF2 , MeO , and OH , respectively (Scheme 1.13). In addition, re- – action of 46 with BH4 gave the corresponding hydrido species 50. Chapter 1: Introduction to Part 1. 13

_ 2CF3SO3

+ + P _ P P OH +P P P _ O O

46 47a 47b

Scheme 1.12: The alkaline hydrolysis of 46 gives the monoxide 47.

_ 2CF3SO3 + _ _ P + CF SO P X 3 3 P +P X

48 (X = F) 46 49 (X = OMe) 50 (X = H)

Scheme 1.13: The reaction of 46 with some nucleophiles.

1.5 Methodologies for the synthesis of phosphorus heterocycles.

The work presented in Chapters 2 and 3 describes attempts to prepare the bicyclic bis- phosphine 1,4-diphosphabicyclo[2.2.2]octane 37 and its 1,4-dioxide 51, respectively. Al- though both compounds have previously been synthesised,57 the route followed gave meagre yields, mainly due to cleavage of ethylene bridges from the cyclic bis-phosphonium salts when treated with LiAlH4. It has been shown since by trapping effluent gases with a solution of Br2 69 in CCl4 that the ethylene bridge is lost as gaseous H2C=CH2. If a reaction could be devel- oped where reduction of the phosphonium salt occurred with the preservation of all ethylene bridges in the molecule, then an efficient synthesis of 37 and its derivatives might be develop- ed. In addition, syntheses of bicyclic bridgehead bis-phosphines in general may be possible through successive quaternisation of bis-phosphines and subsequent reduction of the phosphonium intermediates. The perceived advantages of this method are that the intermediate bis- phosphonium salts are both air-stable and – presumably – easily crystallised, unlike the alkali phosphides used by others.70,71

P P O P P O

37 51 Chapter 1: Introduction to Part 1. 14

Due to the commercial availability of the series of the phenyl-substituted bis-phosphines Ph2P(CH2)nPPh2 (n = 2 or 3), it was desirable to find a way to induce reductive phenyl cleavage from the corresponding bis-phosphonium salt (made from cyclisation of the bis-phosphines with D,Z-dibromoalkanes) while leaving any ethylene bridges intact. The conversion + – 72 by sodium naphthalenide of Ph4P Br to Ph3P in 65% yield and 2-aminophenyltriphenylphosphonium chloride 52 to the bidentate ligand 2-aminophenyldiphenylphosphine 53 in 77% overall yield73 suggested that the naphthalene radical anion was worthy of closer examination. Solutions of sodium naphthalenide in THF have already been used to reductively cleave phenyl groups from phenylphosphines to give phosphide anions,70 the reaction occurring over several days, but it was hoped that this would not occur to any significant extent at the low temperatures and relatively short reaction times used for reduction of the phosphonium salts.

NH2 NH2

+ _ PPh3 Cl PPh2

52 53

Chapter 2 describes the attempts to reach 1,4-diphosphabicyclo[2.2.2]octane 37 via the retrosynthetic pathway shown below (Scheme 1.14).

++ P P Ph P P Ph_ Ph PPPh 2Br cis-7 37 55 _ + 2Br PPh PPh Ph Ph + _3 2 ++ Ph3P 2Br Ph2P P P Ph Ph 54 Ph _ P + Br P Ph Ph Ph Ph Ph Ph

Scheme 1.14: The proposed retrosynthetic route to 37.

Because of the unanticipated trouble in the successful reduction of the cyclic bis-salt 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide 54 into the corresponding cyclic bis- phosphine 1,4-diphenyl-1,4-diphosphorinane 7 (of which only the cis isomer could be used to synthesise the bicyclic bis-salt 1,4-diphenyl-1,4-diphosphoniumbicyclo[2.2.2]octane dibrom- Chapter 1: Introduction to Part 1. 15 ide 55), the attempted synthesis had to be abandoned. Also discussed in Chapter 2 is the attempted synthesis of the cyclic tris-phosphine 29 from the corresponding cyclic tris-salt 36. Chapter 3 follows a similar approach with tertiary phenylphosphine oxides instead of the quaternary phenylphosphonium salts. Reduction of Ph3P=O in THF with either naphthalene radical anion or alkali metal (i.e. Li, Na, K),74 followed by work-up of the reaction mixture with EtBr, gives the phenyl-cleaved product Ph2P(O)Et. Extending this reaction provides a possible synthetic route to the bicyclic dioxide 51 (Scheme 1.15).

+ + O O __Li Li O P P O O P P O PP Ph Ph 51 cis-6

_ O + _ O Li + PPh2 Ph3P=O Ph2POLi PPh Ph2P PhP O _ + O Li Scheme 1.15: The proposed retrosynthetic route to 51. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion.

Recently, it has been shown that a solution of sodium naphthalenide in THF at –78˚C can cleave a phenyl group from 2-aminophenyltriphenylphosphonium chloride 52 giving 2- aminophenyldiphenylphosphine 53.73

NH2 NH2

+ _ PPh3 Cl PPh2

52 53

The reaction is presumed to proceed by transfer of the electron from the naphthalene radical anion to the LUMO of one of the phenyl groups bound to phosphorus (Scheme 2.1), giving an uncharged radical species stabilised by resonance between ylide-like structures. x Expulsion of a phenyl radical Ph would give the free phosphine R3P; the phenyl radical itself would accept another electron, giving phenyl anion Ph–. The electron is transferred to one of the unsubstituted phenyl groups instead of the single 2-aminophenyl group because donation of S-electron density into the ring from the amino group will tend to destabilise the LUMO.

_ e – R P _ 3

++ + R3P R3P R3P R3P

+ Scheme 2.1: The postulated mechanism for reductive phenyl cleavage of R3PPh : the expelled phenyl radical accepts another electron to form the phenyl anion.

Assuming the mechanism outlined above is correct, it seems that it may be possible to reductively cleave phenyl groups from phosphonium salts possessing other, more fragile, moi- eties. With regard to the synthesis of bridgehead phosphorus heterocycles, one obvious can- didate is a bis-salt in which the two phosphonium atoms are connected through an ethylene bridge. In such compounds, the ethylene bridge is cleaved by a variety of reagents (such as Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 17

75 69 methoxide or LiAlH4 ), appearing as H2C=CH2. Any method that is able to cleave phenyl groups from these salts, while at the same time leaving all of the ethylene bridges unaffected, would be extremely useful synthetically. The synthesis of 1,4-diphosphabicyclo[2.2.2]octane 37 (prepared in very low yield57 by Mann) via the bicyclic bis-salt 55 (Scheme 2.2) represents an appropriate target compound.

++ P P Ph P P Ph_ Ph PPPh 2Br cis-7 37 55 _ + 2Br PPh PPh Ph Ph + _3 2 ++ Ph3P 2Br Ph2P P P Ph Ph 54 Ph _ P + Br P Ph Ph Ph Ph Ph Ph

Scheme 2.2: The proposed retrosynthetic route to 37.

+ – The proposed route starts with conversion of Ph4P Br into Ph3P by reductive phenyl cleavage. As both these compounds, along with ethane-1,2-bis(diphenylphosphine) 56, are commercially available, a more practical starting point is the conversion of 56 into the cyclic bis-salt 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide 54.76 This is accomplished in virtually quantitative yield from the reaction between diphosphine 56 and excess 1,2-dibromoethane at 100˚C. The following section describes experiments concerning the attempted reductive dephenylation of 54 with the naphthalene radical anion.

2.1 Cleavage of phenyl versus loss of the ethylene bridge.

To examine whether phenyl groups can be cleaved from phosphorus in the presence of an ethylene bridge, the bis-salt 54 was chosen as a convenient starting point, and could be obtained in quantitative yield by heating a solution of 56 at 100˚C in an excess of 1,2-dibromoethane as the solvent. A solution of 0.5 M lithium naphthalenide in THF was prepared by the reaction of lithium metal with an excess of naphthalene in THF at RT, and an excess of the dried bis-salt 54 was added against a stream of argon to the solution at RT. The deep green colour of the naphthalene radical anion was quickly replaced by a brown colour, at which point the solution was examined using 31P{1H}-NMR, and was consistently found to contain Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 18 only the bis-phosphine 56, indicating the exclusive loss of the ethylene bridge with no phenyl cleavage. The reaction was repeated, this time adding the salt to an excess of the naphthalenide solution at RT, such that the solution remained green. When this solution was examined using 31P{1H}-NMR, again exclusive loss of the ethylene bridge was observed. At this point, it was decided to try the reaction again on a simpler system: the reaction + – was carried out with Ph4P Br , using either the salt as the limiting reagent or in excess, at both RT and –78˚C (with subsequent warming). In each case, Ph3P was the only product observed by 31P{1H}-NMR. When the reaction was carried out using an excess of the salt, the colour immediately changed from green to brown at RT, but remained green at –78˚C, becoming brown when the solution warmed up to about –60˚C. This was taken to indicate that the reaction does not commence until the solution has warmed up to around –60˚C. It is hard to account for the ready loss of the ethylene bridge from the bis-salt 54 if the mechanism described above in Scheme 2.1 is assumed to be operating. Results discussed in Section 2.3 indicate that there is an initial deprotonation of the salt by the naphthalene radical anion to give an ylide, which is then itself reduced by excess reducing agent. The bis-salt 54 is an example of an ethane-1,2-bis(phosphonium) salt, which are known to give vinyl salts upon treatment with strong base (Scheme 2.3).

_ _ _ 2Br Ph Br Ph Br Ph _ + Ph + H Ph Ph + P P ++P P P P Ph Ph Ph Ph Ph Ph 54 57 58

Scheme 2.3: Deprotonation of the bis-salt 54 leads to ring-opening.

Reaction of the bis-salt 54 with n-butyl lithium gave a quantitative yield of the ring- opened product 58, as determined by 31P{1H}-NMR, presumably formed via ylide 57. Could the formation of this intermediate explain the facile loss of the ethylene bridge from the bis- salt? Clearly, if this reaction is to be of any synthetic value, conditions must be found which disfavour deprotonation. For the sake of completeness, it was decided to attempt the reduction at a lower temperature. The bis-salt 54 was added to an excess of lithium naphthalenide solution, pre-chilled to –78˚C, and the mixture then allowed to warm to RT. This reaction gave the desired product 7 in highly variable yields, which ranged from exclusive loss of the ethylene bridge to a quantitative mixture of the cyclic bis-phosphines cis/trans-7 as determined by 31P{1H}-NMR. The reaction was repeated numerous times, but the usual result was either no product or only very poor yields of the desired heterocycles. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 19

P P P P Ph Ph Ph cis-7 trans-7

During one of the very few times the reaction worked, giving an almost quantitative yield of both isomers of 7 in about a 1:1 ratio, it was noticed that a solid layer of ice approximately ½ cm thick had formed over the flask as it was left to warm up. This had originated from water applied to the flask during the initial stages of the reaction in order to see into the flask more clearly. It was thought that the ice may have been acting as an insulator, thereby maintaining a low temperature inside for much longer, and allowing the contents to warm up at a slower rate. Frustratingly, attempting to deliberately create the icy layer by e.g. dipping the cold flask in water, or spraying water over the flask, were unsuccessful, usually just causing the flask to warm up too rapidly, and so reducing the observed yield. It seemed that the key to favouring reduction over deprotonation lay in the amount of time the reaction mixture remained at the critical temperature (the temperature at which reductive cleavage can begin). The acetone used in the cold temperature bath has a lower heat capacity than an alcohol like iso-propanol (126.4 J.mol–1.K–1 compared with 155.2 J.mol–1.K–1),77 and therefore a slurry of liquid nitrogen in iso-propanol was tried as a cold temperature bath. It was hoped that this solvent would warm up at a slower rate than does acetone, and so hopefully increase the proportion of phenyl cleavage. The reaction was repeated using the new bath, but results were no better than before. What was needed was to discover the critical temperature at which the reaction begins, and then try to maintain the flask at this temperature. Because the lithium naphthalenide solution has such a distinct colour, it could be used as an internal indicator as to the state of the reaction. It was fairly certain that no reaction between the bis-salt 54 and lithium naphthalenide occurred at –78˚C. Hence, if the reducing agent was cooled down to this temperature, and an excess of 54 added, then when the critical temperature was reached, the reaction should quickly exhaust all of the green naphthalenide anion present, and so change colour. It was hoped that, in this way, the critical temperature could be discovered by simply monitoring the temperature as the reaction warmed up. This procedure was carried out, but it was found that the exact point of transition from opaque dark green into opaque dark brown was very hard to see. What was noted was that the reaction seemed to become a definite brown by about –60˚C, indicating that the reaction had finished. Therefore, the critical temperature was probably between –70˚C and –60˚C. The reaction, now with an excess of lithium naphthalenide again, was repeated with the temperature allowed to drop to –61˚C in a slurry of CHCl3 and liquid nitrogen before the bis-salt 54 was added. The reaction was allowed to warm up to RT in the chloroform, and examination of the solution using 31P{1H}-NMR indicated the three bis-phosphines 56 and Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 20 cis/trans-7 were in approximately equal proportions. Although the yield was not as good as it had occasionally been, this result was found to be repeatable. By comparison, the analogous cyclic bis-salt 1,1,4,4-tetraphenyl-1,4-diphosphepanium dibromide 59 was reacted in the same way. This 7-membered ring bis-salt, previously unknown, was prepared by the same method used for 54, i.e. the bis-phosphine propane-1,3- bis(diphenylphosphine) 60 was dissolved in excess 1,2-dibromoethane, and heated for several hours at 100˚C (Scheme 2.4). Unlike the synthesis of 54, the bis-salt 59 did not precipitate from solution, and upon evaporation formed a glassy solid which resisted crystallisation from many solvents. Finally, the glassy salt was found to spontaneously crystallise after about 3 months, giving the bis-salt 59 in moderate yield.

Ph Ph _ PPh P 2 Ph + 2Br BrCH2CH2Br P +P P 100ºC Ph Ph PPh2 Ph

60 59 cis- & trans-11

Scheme 2.4: The preparation of the 7-membered bis-salt 59; its related cyclic bis-phosphine 11.

Reaction of the bis-salt 59 with an excess of 0.5 M lithium naphthalenide gave similar results to those obtained for 54. When the reaction was carried out at RT, exclusive ring- opening occurred, regenerating the acyclic bis-phosphine 60 (as determined by 31P{1H}-NMR spectroscopy). Repeating the reaction at –78˚C and then allowing the mixture to warm up to RT also gave mostly ring-opened product. As only low yields of the cyclic phosphines cis- and trans-11 were ever obtained, work with the bis-salt 59 was stopped.

2.2 Cleavage of phenyl versus loss of the 1,3-propylene bridge.

The previous section discussed the competition of reduction versus deprotonation with regard to the reaction of naphthalene radical anion with the cyclic bis-salts 54 and 59. As has already been discussed, bis(phosphonium) salts containing 2-carbon (or ethylene) bridges are well known to undergo cleavage of the bridge as ethylene under a variety of conditions. By means of a comparison, it was decided to carry out the reaction on a bis-salt that does not possess any ethylene bridges. The salt chosen was 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide 61, which has two 1,3-propylene bridges.29 _ _ 2Br Ph Ph _ 2Br Ph + Ph + 2Br P Ph Ph +P P +P ++P P Ph Ph Ph Ph Ph Ph 54 59 61 Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 21

Presumably, deprotonation to give the cyclic ylide-salt 62 would still occur, but subsequent ring-opening through expulsion of a tertiary phosphine, as in the case of the ethylene bridge, should not be possible (Scheme 2.5). This being the case, either the resulting ylide- salt 62 or bis-ylide 63, in addition to the free bis-salt 61, should be cleaved by excess naphthalene radical anion to give a quantitative yield of cis-,trans-1,5-diphenyl-1,5-diphosphocane 12.

2Br– Ph Ph Ph + Ph P – H P P +P + + Br– Ph Ph Ph Ph 61 62 – H+

H+/ [R]

[R] Ph Ph P P ] R + /[ Ph Ph 2H 63a

Ph Ph P P cis-12

Ph Ph 2H+/[R] Ph P P P P trans-12 Ph Ph Ph 63b

Scheme 2.5: In the case of 61, deprotonation should not lead to ring-opening via the mechanism described above in Scheme 2.3, but instead should give cis- and trans- isomers of 12.

The reaction was carried out by adding the dry bis-salt slowly to an excess of 0.5 M lithium naphthalenide at RT, as described in the previous section. The dark green mixture was examined using 31P{1H}-NMR, which indicated almost exclusive loss of the propylene bridge to give propane-1,3-bis(diphenylphosphine) 60. This result was unexpected, as pro- pano bridges are usually found to be quite resilient towards those reagents which are known to cleave ethylene bridged bis-salts.29 The reaction was repeated at –78˚C, and as before, a reasonable yield of the cyclic product cis-,trans-12 was obtained, along with some ring-opened product. How can these results be explained? It may be that the initially formed ylide- salt 62 is not stable, with the ylidic methine nucleophilically attacking one of the methylenes bound to the other phosphorus atom. Both of these possible reactions are shown below in Scheme 2.6. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 22

PPh2 Ph Ph Ph [R]? +P P Ph2P P+

Ph Ph Ph PPh2 62 64 60 [R]?

Ph Ph + P Ph Ph [R]? PPh2 +P P

Ph Ph PPh2 62 Ph2P 66 65

Scheme 2.6: Possible rearrangements of the ylide-salt 62. The proposed site of bond-breakage is indicated by .

Depending upon which methylene is attacked, either the cyclopropyl-salt 64 or the phospholanium salt 65 is formed. Cyclopropyl groups are susceptible to ring opening under radical conditions,78 and so the naphthalene radical anion may cause the ring-opening of 64, possibly giving either 60 or 66.

2.3 Cleavage of phenyl versus cleavage of benzyl.

Benzyl groups are well known to be good leaving groups under reducing conditions because of the ability for the benzyl anion to delocalise any acquired negative charge around the aromatic ring (see Figure 2.7). As an example, reductive cleavage of a benzyl group from quaternary benzylphosphonium salts with LiAlH4 gives the corresponding tertiary phosphine in good yields.79 This and other similar reactions will be discussed further in Part 2.

_ CH2 CH2 CH2 CH2

_ _

Figure 2.7: Delocalisation of negative charge around benzyl anion by resonance.

Because the naphthalene radical anion presumably transfers an electron directly onto the phenyl ring of a phenylphosphonium salt, it is conceivable that this method may be able to cleave phenyl groups ahead of benzyl groups. If this is found to occur, it would provide a complementary method of reductive cleavage to that using LiAlH4. A series of phosphonium Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 23

+ – + – salts, PhnPBn4-n Br (n = 0 to 3), was prepared and, together with Ph4P Br for comparison, were reacted with 0.5 M lithium naphthalenide in THF at –23˚C (in a slurry of CCl4 with liquid nitrogen). The results are shown below in Table 2A.

Table 2A: Phenyl Cleavage versus Benzyl Cleavage.*

Phosphonium Salt Loss of Phenyl Loss of Benzyl –––––––––––––– + – –––––––––––––– Ph P Br 100 % Ph P –––––––––––––– 4 3 –––––––––––––– + – Ph3PBn Br ca. 85 % Ph2PBn ca. 15 % Ph3P

+ – Ph2PBn2 Br not observed > 99 % Ph2PBn

+ – PhPBn3 Br not observed > 99 % PhPBn2 –––––––––––––– + – –––––––––––––– Bn P Br –––––––––––––– 100 % Bn P 4 –––––––––––––– 3 * Taken from the 31P-NMR spectra.

+ As can be seen, all of the salts reacted, all losing benzyl exclusively except for Ph4P – + – Br (with the trivial result of exclusive phenyl loss) and Ph3PBn Br (with approximately + – 85% phenyl cleavage). Clearly, the ready cleavage of Bn4P Br to Bn3P cannot depend upon the naphthalene radical anion transferring an electron to a phosphorus-bound phenyl group as there are none present. A more likely mechanism would involve electron transfer directly to the phosphonium centre of the cation, giving 67, as shown below in Scheme 2.8.

R R R _ _ + e R e R _ P P P R R R R R R R 67 68 _ _ – R – R

+ _ P e P R R R R R 69R 70

Scheme 2.8: Proposed mechanism for reductive cleavage of quaternary phosphonium salts, proceeding via either a phosphoranyl radical 67 or a phosphoranide anion 68. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 24

The phosphoranyl radical 67 can then either accept a second electron giving 68, or it can expel a carbanion giving 69. Both 68 and 69 species are transient under the conditions of the reaction, with the observed product R3P 70 resulting from either the expulsion of a carbanion from 68 or the 1-electron reduction of 69. For the work described here, R = Ph or Bn, and so the expelled carbanion in each case should be benzyl, excepting when R = Ph only. Hence, this mechanism would predict exclusive loss of benzyl for all of the salts where n < 4. This is indeed the case, except when n = 3, where there is not exclusive loss of benzyl, but in fact about 85% loss of phenyl. The discrepancy can be accounted for by assuming that, as the first step, the benzylphosphonium salts are deprotonated by the naphthalene radical anion, and then it is the resulting ylide (71) that is reduced (Scheme 2.9). Involvement of intermediate ylides have + – 80 been postulated in the reduction of Ph3PBn Cl to Ph2PBn in toluene with sodium metal. In this way, phenyl becomes a better leaving group than the benzylidene group, which already bears one negative charge. Of course, only one benzyl group would be deprotonated, so when there are two or more benzyl groups initially present in the salt, this ensures that one will always be available for expulsion before any phenyl groups present. Presumably, in the reduc- + – tion of Ph3PBn Br with naphthalene radical anion, approximately 15% of the product is derived from direct reduction of the salt, rather than via initial formation of the ylide.

_ CH2Ph CHPh CHPh + + _ H + P P P R R R R R R R 71 R R

_ _ CHPh CHPh _ R e R _ P P R R R R

_ _ _ R _ R

+ _ + P _ e P + H P R _ CHPh R R CHPh R R 72 R 70 R

Scheme 2.9: A possible reductive pathway going from quaternary phosphonium salts to tertiary phosphines, proceeding via ylide intermediates. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 25

Further support comes from 31P{1H}-NMR spectra obtained of the reaction mixtures, where initially the chemical shifts of the observed signals were close to, but did not corres- + – pond to, the expected chemical shifts. Upon addition of dry NH4 Cl to these solutions, the yellow colour was immediately dispersed, and re-examination using 31P{1H}-NMR showed that the signals had now shifted to the expected location for phosphines. The initial signals were presumed to be due to intermediate D-lithiophosphines (72), giving the free phosphines + – when quenched with NH4 Cl .

2.4 Approaches toward the 1,4,7-triphenyl-1,4,7-triphosphonane system.

+ – In 1968, Horner et al reported that the reaction between Ph4P and Ph2P in THF gave 81 two equivalents of Ph3P. The actual mechanism of the reaction is not known, but two path- – ways present themselves as possibilities (Scheme 2.10). In the first mechanism, the Ph2P + anion attacks one of the phenyl groups in Ph4P (presumably at the para position due to steric reasons), giving ylide 73, which upon deprotonation then eliminates Ph3P, giving carbanion 74. Subsequent protonation of the aryl lithium intermediate then gives the second equivalent of Ph3P. The second mechanism proceeds by the formation of an intermediate phosphino- – + phosphorane Ph4P-PPh2 from attack of Ph2P directly upon the phosphorus atom of Ph4P .

This phosphorane is then able to rearrange directly into two molecules of Ph3P.

B: H PPh2 PPh2 + LiX

_ + (i) LiPPh2 B + BH THF – PPh3 + + P P 74 Ph _ Ph Ph Ph Ph Ph X– 73

(ii) LiPPh2 – B THF

+ + PPh2 Ph Ph Ph DLE – PPh3 P Ph P PPh P 2 Ph Ph Ph Ph Ph Ph Ph + LiX where B = LiPPh2 or p-LiC6H4PPh2

+ – Scheme 2.10: Two possible mechanisms for the reaction between Ph4P and Ph2P : (i) the attack – of Ph2P at the para carbon, followed by rearomatisation and elimination of Ph3P; (ii) the attack of – + Ph2P on Ph4P giving a phosphino-phosphorane, which can then rearrange into two molecules of

Ph3P. (DLE = dynamic ligand exchange, to be discussed in Chapter 6.) Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 26

In order to determine whether this method could be used to dephenylate the cyclic bis- salt 54 to give the desired cyclic bis-phosphine 1,4-diphenyl-1,4-diphosphorinane 7, the reaction between 54 and LiPPh2 was attempted. A red-brown solution of LiPPh2 in THF was prepared from the reaction of Ph2PCl with lithium in THF under an atmosphere of dry argon. To this solution, against a flow of argon, was slowly added the oven-dried bis-salt 54 (0.5 eq. wrt LiPPh2). During the addition, the colour of the solution became yellow, and the solid suspension was observed to dissolve. After about half the bis-salt 54 had been added, the dissolution of further bis-salt became sluggish, and the colour changed little during the remainder of the addition. When all the salt had been added, undissolved white solid was observed, and was found later using 31P{1H}-NMR to be unreacted bis-salt 54.

46.8 46.4 46.0 45.6 32.4 32.0 31.6 (a) (ppm) (ppm)

46.8 46.4 46.0 45.6 32.4 32.0 31.6 (b) (ppm) (ppm) Figure 2.11: The 31P{1H}-NMR spectrum of 75 in MeOH solution: (a) with 2 Hz line broadening, and (b) with 0.1 Hz line broadening. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 27

31 1 P-NMR revealed the presence of Ph2PH (GP –39.9 ppm, JP-H = 222 Hz) in the solution, and another compound exhibiting a 1:2:1 triplet (GP +31.0 ppm, 1P) and a doublet (GP –12.1 ppm, 2P), with JP-P = 43 Hz, and was assigned as the bis(phosphino)phosphonium salt bis(2-diphenylphosphinoethyl)diphenylphosphonium bromide 35 (see Figure 2.13: page 28).

The relative proportion of Ph2PH to 35 was approximately 1:2. Characterisation of 35 was achieved through its disulphide 75, prepared by reaction with sulphur at RT, with the 31P- {1H}-NMR spectra obtained in MeOH solution, shown above in Figure 2.11. The observation that both the “doublet” and “triplet” shown above (Figure 2.11a) are in reality each a doublet of doublets (Figure 2.11b) suggests that the two terminal phosphorus atoms of 75 are in fact not equivalent. A possible explanation for this is shown below (Figure 2.12). Complexation of the phosphonium cation by one of the sulphur atoms would make the two phosphine sulphide groups inequivalent. If the rate of exchange between the environment of the two sulphur atoms was low, an asymmetry in the chemical shift of the phosphorus atoms may then be observed.

Ph Ph Ph Ph P Ph P Ph Ph Ph + Ph P P S S + P P S S Ph Ph Ph

Figure 2.12: Complexation of one of the sulphur atoms to the central phosphonium cation in 75 may explain the inequivalence of the two terminal phosphine sulphides observed in the 31P{1H}- NMR.

The mechanism proposed for the formation of the bis(phosphino)phosphonium salt 35 from the cyclic bis-salt 54 is shown below (Figure 2.13). Instead of removing phenyl groups – from 54, Ph2P causes the cyclic bis-salt to ring-open, presumably by deprotonation of one of the bridging methylene groups, explaining the formation of Ph2PH. The vinylphosphonium intermediate 58 is not observed directly because it behaves as a strong Michael-type acceptor – towards more of the strongly nucleophilic Ph2P anion. The product of this addition is ylide

76, which is quenched upon work-up, giving 35. The absence of any Ph3P produced in the – reaction with 54 suggests that Ph2P does not attack the para carbon of the phosphonium salt.

This strengthens the case for formation of the intermediate phosphino-phosphorane Ph2P-PPh4 + – in the reaction between Ph4P and Ph2P (Scheme 2.10), which cannot be ruled out complete- + ly due to the absence of any D-hydrogens in the case of Ph4P . Attempts were made to optimise the conditions for formation of the bis-phosphino- phosphonium salt 35. Addition of nBuLi (as a solution in hexane) after the complete addition of bis-salt to the LiPPh2 solution gave increased yields of the desired product at the expense of

Ph2PH. Good results were also obtained by preparing the LiPPh2 from the reaction of Ph3P Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 28 and lithium metal, with the PhLi by-product behaving as a convenient base in the subsequent reaction with the cyclic bis-salt. Using this method, the proportion of Ph2PH could be kept to below 10 mol% in the mixture.

_ Ph Ph ++ LiPPh2 2Br P P Ph PPPh THF Ph Ph +2Ph3P _ H 54 7 Ph2P LiPPh2, THF

Ph _ Ph Br + LiPPh P 2 P + LiBr PPh PPh Ph 2 THF Ph 2 PPh2 _ + + LiBr Ph2P 58H 76

Ph + P PPh Ph 2 _ PPh2 Br 35

54 Figure 2.13: The reaction of bis-salt with LiPPh2.

Cyclisation of the bis-phosphino-phosphonium salt 35 with 1,2-dibromoethane would give the cyclic tris-phosphonium salt 36 (Figure 2.14), closely structurally related to the as- yet unknown, but potentially very interesting, cyclic tris-phosphine 1,4,7-triphenyl-1,4,7-triphosphonane (29). The oily bis-phosphino-phosphonium salt 35 was refluxed with 1,2-dibromoethane in MeOH for 4 hours, by which time a white solid had precipitated. This solid was found to be insoluble in all solvents except trifluoroacetic acid (TFA), with examination 31 1 by P{ H}-NMR showing a single peak at GP +32.6 ppm, assigned to the desired cyclic tris- salt 36. Future work would involve attempting to purify the insoluble solid, possibly by crys- tallising from a solution in TFA by addition of MeOH. Once this had been achieved, further characterisation of the solid using both 1H- and 13C{1H}-NMR would be possible. Successful reduction with the naphthalene radical anion would give two stereoisomers, a symmetrical exo, exo, exo isomer, in which all three phenyl groups point away from the molecular cavity, and an exo, exo, endo isomer, where one of the phenyl groups points into the cavity (Figure 2.15). It is the symmetrical exo, exo, exo isomer which would be able to coordinate to a single metal atom in a tridentate manner. Chapter 2: The Reaction of Phosphonium Salts with the Naphthalene Radical Anion. 29

Ph Ph Ph Ph Ph ++ Ph P P Ph P P + BrCH2CH2Br Li P PPh Ph 2 + _ C10H8 _ PPh2 P P Br 3Br Ph Ph Ph

35 36 29

Figure 2.14: Cyclisation of 35 with 1,2-dibromoethane, followed by reductive phenyl cleavage, to give 29 as a mixture of symmetrical and unsymmetrical isomers.

Ph Ph Ph Ph P P P P Ph

P P

Ph exo, exo, exo exo, exo, endo

Figure 2.15: The two possible stereoisomers of 29. Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion.

Tertiary phosphine chalcogenides R3P=X (where X = O, S, Se, Te) may be considered a hybrid between the three possible resonance contributors.82,83 They range from 77b which has a dative P-X triple bond, to 77c with a P-X double bond, and 77a with P-X single bond. It has been suggested that, due to symmetry reasons, the real structure is a hybrid of 77a and 77b with no contribution from 77c (Figure 3.1).84 If the chalcogen atom in the 77a contributor is to partake in multiple bond formation with the phosphorus atom, it must be assumed to be sp-hybridised, with two of its three lone pairs in p-orbitals perpendicular to the P-X axis; as these two orbitals are degenerate, any contribution made by them to P-X multiple bonding must be identical. Therefore, the resulting structure is that of a partial triple bond.84 This view seems to be valid for X = O, but as the chalcogen atom becomes heavier, the structure is more accurately described by contributor 77a alone.84

_ + X X X + _ P P P R R R R R R R R R 77a 77b 77c

Figure 3.1: The resonance contributors for R3P=X (where X = O, S, Se, Te).

Because ofcontributor 77a, which has phosphonium character at phosphorus, it is conceivable that these compounds can be reductively cleaved using the same methods used with quaternary phosphonium salts.74,85,86 This cleavage gives trivalent phosphorus, and may conceivably proceed in two directions: (i) the chalcogen atom is cleaved off as X2– (also is equivalent to H2X), leaving the corresponding tertiary phosphine R3P; or (ii) one of the R – groups is lost, giving the secondary chalcophosphinite anion R2P-X (Scheme 3.2). _ _ X X __ 2e + 2e P P P R R R R (i)R (ii) R R R 77a

Scheme 3.2: Two possible pathways for reductive cleavage of the phosphine chalcogenide R3P=X: 2– – – (i) loss of chalcogen as X , giving R3P, or (ii) loss of R as R , giving R2P-X . Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 31

Despite the superficial similarity between reduction of species such as 77a and quaternary phosphonium salts, tertiary phosphine chalcogenides have one major advantage over the phosphonium salts, and that is solubility in ethereal solvents, especially THF. None of the salts discussed in Section 2.1 were appreciably soluble in THF, whereas simple phosphine chalcogenides such as Ph3P=O are freely soluble. Because of this, the reactions would now be homogeneous, and so hopefully they would proceed in a more consistent manner. – For the purposes of this work, reaction (ii) is desirable. The anion R2P-X can exist as two resonance contributors 78a and 78b, where the negative charge is either on the chalcogen atom or phosphorus atom, respectively (see Scheme 3.3). Structure 78a is presumably the main contributor because the highly electronegative oxygen atom would withdraw the charge onto itself. The addition of an electrophile E+ would then be expected to attack either of these two sites of negative charge, either O or P, depending on the nature of E+, yielding either a chalcophosphinous ester 79, or another phosphine chalcogenide 80; e.g., when E = H, the secondary phosphine oxide R2P(O)H is formed (the structure corresponding to 80), which 87 is the preferred tautomer of R2P-OH, the isomeric diorganophosphinous acid. _ X X _ P P R R R R 78a 78b

E+ E+

E X X

P P R R E R R 79 80

– 78a 78a Scheme 3.3: The two possible resonance contributors of R2P-X ( and ) corresponding to the two possible sites for electrophilic attack by E+, giving either 79 or 80.

The work described in the previous sections concerned the building up of phosphonium salts from phosphines, followed by reductive cleavage of one of the attached groups to yield another phosphine, which may then be realkylated, and so on. Following on from this, the possibility for a parallel synthetic procedure exists with the phosphine chalcogenides 77, via cleavage to 78, followed by realkylation to another phosphine chalcogen 80 (Scheme 3.4). Because of the ability of the naphthalene radical anion to cleave phenyl groups from quater- Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 32 nary phosphonium salts72,73 and tertiary phosphines,70 it was decided to see if it could also cleave phenyl groups from tertiary phosphine chalcogens.

+ + __Li Li O O O P P O O P P O PP Ph Ph 51 cis-6

_ O + _ O Li + PPh2 Ph3P=O Ph2POLi PPh Ph2P PhP _ O + O Li

Scheme 3.4: The proposed retrosynthetic route to 51.

3.1 Reductive cleavage of triphenylphosphine chalcogenides.

58,59 Of the three model compounds Ph3P=X (where X = O, S, Se), both the sulphide and the selenide88 were prepared by literature methods, and reduction of each was attempted with naphthalene radical anion in THF at RT. Both the heavier chalcogenides (Ph3P=S and 31 1 Ph3P=Se) gave a quantitative yield of Ph3P by P{ H}-NMR. (The conversion of Ph3P=S to 72 Ph3P by treatment with naphthalene radical anion has previously been reported. ) Hydro- lysis of the reaction mixture gave the characteristically repulsive odours of H2S and H2Se, respectively, and after a matter of hours, a deposit of red selenium was observed to form from 87 the latter, presumably from the aerial oxidation of H2Se. These results indicate that the chalcogen atom is cleaved off as the chalcogenide dianion X2– (X = S, Se), as described in pathway (i) of Scheme 3.2 above.

In the case of Ph3P=O however, pathway (ii) was followed with cleavage of a phenyl – group, giving the diphenylphosphinite anion Ph2P-O (GP +86.1 ppm). Confirmation of the assignment was made by quenching the mixture with ethanolic hydrochloric acid, yielding its 1 conjugate acid Ph2P(O)H (GP +23.3 ppm, JP-H = 497 Hz). There is precedence of this reaction when alkali metals are used as the reducing agent, the reaction requiring three equivalents of the metal74 instead of the expected two (see Scheme 3.5).

_ _ MM+ M Ph P=O M Ph P=O Ph P-OM Ph P-OM 3 3 – PhM 2 2 M = Na, K, Rb, Cs

Scheme 3.5: The reaction of Ph3P=O with alkali metals (Na, K, Rb, Cs) give radical anions of both – Ph3P=O and Ph2P-O . Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 33

89 Alkylation of this product with ethyl bromide at RT gave Ph2P(O)Et, confirming that the preferred site of alkylation is at phosphorus. A similar method, using Na in liquid NH3, has been used by others to substitute both phenyl and benzyl groups in Ph3P=O and Bn3P=O, respectively.90 Next, an attempt to extend this reaction to bis-phosphine dioxides was undertaken, to see if it could be used to prepare bridgehead phosphines.

3.2 Attempts at phenyl cleavage from bis(phosphine oxides).

If this reaction is to be useful for preparing the bridgehead heterocycles discussed in Chapter 1, it must be able to cleave phenyl groups from ethane-1,2-bis(phosphine oxides) while preserving the two-carbon bridge, as in the case of the ethane-1,2-bis(phosphonium salts). For a test reaction, ethane-1,2-bis(diphenylphosphine)-P,P'-dioxide 81 was chosen for the model compound, as it is easily prepared from the corresponding commercially available bis-phosphine 56 (Scheme 3.6).

O O H2O2 Ph2P PPh2 Ph2P PPh2 MeOH 56 81

Scheme 3.6: The oxidation of 56, giving the dioxide 81.

Unlike Ph3P=O, which is freely soluble in THF, the acyclic dioxide 81 is insoluble, and no reaction was observed with lithium metal either when stirring at RT or when sonica- ting the mixture. However, when the lithium was substituted with an excess of a solution of lithium naphthalenide in THF, the reduction proceeded smoothly at RT yielding a complex mixture by 31P-NMR, which, when acidified with a solution of glacial acetic acid in methanol, gave a clean solution containing only two signals due to phosphorus in approximately equal amounts at GP +25.4 and GP +25.1 ppm (both with a one-bond P-H coupling of J = 477 Hz). These compounds were assigned to the structures of the diastereomeric bis-phosphine oxides 82a (enantiomers SS and RR) and 82b (meso: SR = RS).91,92

O O O O O O

P P P P P P Ph Ph H H Ph H H H Ph Ph H Ph SS RR SR = RS 82a 82b

The complex spectrum observed by 31P-NMR prior to acidification was presumably largely due to paramagnetic broadening of radical anions present, consistent with the forma- Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 34

74 tion of radical anions such as those reported in the reduction of Ph3P=O with alkali metals

(see Scheme 3.5 above). Also present was a singlet at GP +98.3 ppm and a 1:1:1:1 quartet, centred at GP +90.8 ppm, with an observed peak separation of 55 Hz. This region of the spectrum is where phosphinite anions are found, and so these signals could be due to the bis- phosphinite dianion 83.

OLi OLi Ph P P Ph 83

7 3 6 Lithium is composed of two stable isotopes, 93% Li (I = /2) and 7% Li (I = 1) by natural abundance,1 and should couple to phosphorus to give a 1:1:1:1 quartet or a 1:1:1 triplet, respectively. Therefore, it was thought likely that the observed 1:1:1:1 quartet appearing 31 7 at GP +90.8 ppm could be due to a 1-bond P- Li coupling in the bis-phosphinite salt. This was confirmed by repeating the reaction using 95% isotopically enriched lithium-6, which 1 gave a 1:1:1 triplet at GP +90.8 ppm ( JP-6Li = 20 Hz), replacing the quartet previously observed at this position (Figure 3.7), with the singlet at GP +98.3 ppm remaining unaffected.

93 92 91 90 89 93 92 91 90 89 (ppm) (ppm) (a) (b)

Figure 3.7: The 31P-NMR spectra of (a) lithium-7 and (b) lithium-6 reaction mixtures, shown with identical scales. (The two low-intensity signals either side of the base of the triplet due to 6Li are probably the two outer peaks of the 1:1:1:1 quartet due to residual 7Li, while the signal occurring

at about GP +89.3 ppm in both spectra is an unidentified impurity.)

55 Hz = 2.75, The ratio of the coupling constants is: 20 Hz

J7 which compares well with: Li = 2.64 J6Li Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 35

1 Hence, JP-7Li = 55 Hz. Like the bis-phosphine oxide 82, the bis-phosphinite salts are expected to exist in meso and DL forms. The observed magnitude of the 1-bond 31P-7Li (55 Hz) compares well to literature93-97 values, although the majority of theses values were obtained at low temperatures from solutions of lithium phosphides in weakly coordinating solvents such as Et2O. Of the signals – + 98 observed at RT, [(Me3Si)2CH]2P Li 84 in Et2O exhibits a large phosphorus-lithium coup- 1 1 ling of JP-7Li = 80 Hz for the monomer and JP-7Li = 122 Hz for the cyclic dimer, while for

85 1 7 95 lithium (3,5-di-t-butylphenyl)-t-butylphosphide in Et3N, JP- Li = 58 Hz.

But

H H

tBu Me3Si P SiMe3 P Me3Si SiMe3 t Li Li Bu

84 85

The observation of 1-bond phosphorus-lithium coupling in the bis-phosphinite salts is remarkable because the phosphorus atoms are not thought to be anionic, as the oxygen atoms are expected to withdraw most of the negative charge. Secondly, the signal was obtained in THF (a reasonably strong donor solvent) at RT, whereas most of the examples published were acquired in Et2O, a comparatively weak donor solvent, and often at low temperature. An explanation is needed for the unusually strong phosphorus-lithium interaction.

OLi OLi O Li P P Ph P P Ph SS Ph Ph 83a Li O

OLi OLi Li O

RR P P Ph P P Ph 83b Ph Ph OLi

OLi OLi

SR = RS P P Ph 83c Ph

Figure 3.8: The postulated structural isomerism of the three dilithium bis-phosphinites. Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 36

The two enantiomers SS (83a) and RR (83b) can be rearranged into a bicyclo[2.2.2] configuration, where each lithium cation is chelated by one of the phosphorus atoms and the oxygen atom bound to the other phosphorus atom, whereas the meso isomer (83c) is unable to form this structure (shown above in Figure 3.8). Hence, the 1:1:1:1 quartet (or 1:1:1 triplet when lithium-6 is used) at GP +90.8 ppm was assigned to the SS/RR enantiomeric pair (83a and 83b), while the singlet observed at GP +98.3 ppm was assigned to the meso salt 83c. The reaction was repeated using sodium naphthalenide, but no corresponding 31P-23Na 23 1 3 coupling was detected ( Na: 100%, I = /2). Quenching of the mixture again gave a mixture of the isomeric meso and DL ethane-1,2-bis(phenylphosphine)-P,P'-dioxides 82, indicating that the phenyl cleavage still occurred, as expected (it is the naphthalene radical anion which reacts with ethane-1,2-bis(diphenylphosphine)-P,P'-dioxide 81, resulting in phenyl cleavage, whereas the alkali metal cation behaves only as a counter ion). Evidently, the bicyclic dilithium ethane-1,2-bis(phenylphosphinite) complexes 83a, 83b proposed above are only able to form because of the small size of the lithium cation, with Na+ presumably being too large to fit (the respective cationic radii1 being: Li+ 0.78Å, Na+ 0.98Å).

3.3 Attempts at phenyl cleavage with cyclic bis(phosphine oxides).

With the establishment of a method for reductively cleaving phenyl groups in the presence of two-carbon bridges, it was decided to examine the limits of the reaction by attempting to reduce 1,4-diphenyl-1,4-diphosphorinane-1,4-dioxide 6 with 0.5 M lithium naphthalenide in THF. This cyclic dioxide can be made by alkylating the bis-phosphinite 83 with 1,2-dibromoethane, but a more convenient synthesis is from the corresponding cyclic bis- phosphine 7, as is shown below (Scheme 3.9), using the vinyl bromide ring-closure reaction with dilithium ethane-1,2-bis(phenylphosphide) 34.56

Li Li Li Ph P PPh PhP PPh 2 2 THF, RT 56 34

Ph Ph Ph Ph cis-6 P P P P cis-7 O

O H2O2 O MeOH Ph Ph P P trans-6 P P trans-7 Ph O Ph Figure 3.9: Preparation of cis and trans 1,4-diphenyl-1,4-diphosphorinane-1,4-dioxide 6. Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 37

Contrary to the case of the acyclic dioxide 81, both isomers of the cyclic dioxides 6 were freely soluble in THF, but were shown by 31P-NMR not to react with either metallic Li (with or without sonication) or with lithium naphthalenide at RT (Scheme 3.10). The lack of reactivity cannot be a solubility problem, but instead it seems more likely that the dioxides 6 are less electrophilic than is 81, probably due to fewer phenyl groups attached to the phosphorus atoms. It is well known that phenyl groups stabilise phosphines towards aerial oxidation (e.g. Ph3P is quite air-stable, whereas the ease of oxidation increases with the extent of alkyl substitution). Because of the failure of the cyclic dioxides 6 to react with solutions of naphthalene radical anion, work involving tertiary phosphine oxides was stopped.

Ph O P Li Li/C H No Reaction10 8 No Reaction THF THF P Ph O 6

Scheme 3.10: The isomers of 6 were not able to be reductively cleaved.

The apparent inertness of aryldialkylphosphine oxides towards solutions of naphthalene radical anion in THF may be of value in an alternative synthesis of 1,4,7-triphenyl-1,4,7- triphosphonane 29; the retrosynthetic pathway is shown below (Scheme 3.11). The conversion of the cyclic bis-salt 54 into 35 was described in Section 2.4. Alkaline hydrolysis followed by oxidation with H2O2 would then give the tris(phosphine oxide) bis(2-diphenylphosphinoethyl)phenylphosphine-P,P',P"-trioxide 86. Similar compounds to 86 containing phosphorus in mixed oxidation states (i.e. both phosphine and phosphine oxide groups) have been previously used to prepare metal complexes of palladium and rhodium.99

+ Ph Ph Ph Ph Li _ _ P P P P O O O O O O Ph Ph Ph P P P P P

Ph 29 Ph 88 87

Ph O O Ph Ph ++ + O P P P P PPh PPh Ph Ph Ph _ 2 Ph 2 _ PPh PPh2 Br 2 542Br 35 86

Scheme 3.11: The proposed retrosynthetic pathway to 1,4,7-triphenyl-1,4,7-triphosphonane 29 via phosphine oxides. Chapter 3: The Reaction of Phosphine Oxides with the Naphthalene Radical Anion. 38

Reaction of the tris(phosphine oxide) 86 with naphthalene radical anion in THF solution should then give the bis(phosphinite)phosphine oxide 87, which may possibly be able to complex to a Li+ cation in solution. The nature of this complex automatically directs the 3 oxygen atoms to the same side of the ring, so that cyclisation with 1,2-dibromoethane should either give the desired cyclic trioxide 88 or form polymeric species. Once the cyclic trioxide

88 was obtained, deoxygenation using standard reagents (e.g. HSiCl3) should then give the cyclic tris-phosphine 1,4,7-triphenyl-1,4,7-triphosphonane 29. Part 2: The Hypervalent Chemistry of Phosphorus.

Lith’al’ and tetrahydrofurane Comprise a good mixture, it’s plain: I threw in some salt, And found (with a jolt), Phosphorane! phosphorane! phosphorane!

Neil Donoghue. Chapter 4: Introduction to Part 2.

Phosphorus can occur in molecular species (whether charged or uncharged) with coordination numbers up to and including six. For most species where the coordination number is less than five, Lewis structures can be written in which the Octet rule is obeyed, and where – + similar compounds involving nitrogen also exist (e.g. P2, H2P , PH3, PH4 ). However, when – the coordination number exceeds four (e.g. PF5, PF6 ), more than four electron pairs are situated around the phosphorus atom. Such compounds are termed “hypervalent” because they apparently violate the Octet rule, and the existence of such species has been previously explained by invoking the participation of the phosphorus 3d orbitals in the bonding. More recently, it has been suggested that the P 3d orbitals would be too high in energy to provide any significant orbital overlap with the valence orbitals of other atoms. Instead, the required vacant orbitals are provided by molecular LUMOs of the appropriate symmetry, which tend to be antibonding with respect to V overlap between phosphorus and the attached groups. The term “coordination number” (C.N.) refers to the number of atoms directly attached to the atom being considered. In contrast to this, terms such as “penta-coordinated” and “hexa-coordinated” will refer to the number of V-electron pairs (i.e. the sum of the bonding and non-bonding pairs) in the valence shell. Using these definitions, a molecule like phosphine (PH3) is classified as both having a coordination number of three (i.e. C.N. = 3) while also containing a tetra-coordinated phosphorus atom. In contrast, the phosphorus atom in

Me3P=O is also tetra-coordinated, but the C.N. = 4 (only the V-electron pairs are considered, with all S-bonding between phosphorus and oxygen ignored).

The hypervalent compounds discussed in Part 2 will be either phosphoranes (i.e. R5P) – or anionic phosphoranates (i.e. R6P ). In neither case does the phosphorus atom possess any lone pairs or participate in any formal multiple bonds. This naming system differs slightly from that used for inorganic derivatives i.e. Ph3P (triphenylphosphine), PF3 (phosphorus tri- – fluoride); Ph5P (pentaphenylphosphorane), PF5 (phosphorus pentafluoride); and Ph6P (hexa- – phenylphosphoranate anion, as yet unknown), PF6 (the hexafluorophosphate anion).

4.1 The search for main-group penta-coordinated molecules.

Many attempts to prepare penta-coordinated compounds of main group elements have been reported in the early chemical literature. Not surprisingly, these efforts have been con- centrated on the Group VB elements, and have been reviewed by Hellwinkel.100 An overview of that review will now be presented. The first comprehensive investigation of organo- Chapter 4: Introduction to Part 2. 41 lithium compounds as possible reagents in the attempt to prepare penta-coordinated species of nitrogen was performed by Wittig in the 1930s. Previous attempts had always met with failure, but the bonding theory of the day could not provide any explanation for this. The + – reaction between PhLi and Me4N Cl , carried out by Wittig, failed to give the desired penta- coordinated PhNMe4 89:

Ph Me Me N Me Me

However, the intermediate formed in the reaction was trapped with Ph2C=O, giving a zwitterionic ammonium alkoxide 90, isolated as the ammonium salt 91. The intermediate was therefore shown to be trimethylammonium methylide 92 (Scheme 4.1), and was termed an ylide by Wittig. Because phenyl lithium was known to be a strong nucleophile as well as + – a strong base, the proven deprotonation of Me4N Cl indicated that pentaorganonitrogen compounds were unlikely to ever be produced using this method. Research on nitrogen was therefore abandoned in favour of phosphorus, because of both the larger atomic radius of phosphorus (i.e. N: 0.70Å, P: 1.10Å)1 and the existence of penta-coordinated halides such as

PF5 and PCl5.

Ph Ph Ph Ph _ CH3 _ CH2 _ I _ + PhLi + Ph2C=O O HX OH N N X Me Me + + Me Me N N Me Me Me Me Me Me ylide Me Me 92 90 91

+ – + – 92 Scheme 4.1: The reaction of Me4N Cl with PhLi gave the zwitterion Me3N -CH2 .

+ – When Me4P I was reacted with phenyl lithium, an analogous sequence of events oc- + – curred (Scheme 4.2), with the intermediate ylide Me3P -CH2 93 (R = Me) being successfully + – trapped using Ph2C=O. However, when the same reaction was carried out with Ph3PMe I + – and the ylide Ph3P -CH2 93 (R = Ph) was quenched with Ph2C=O as before, the intermediate phosphonium alkoxide 94 rapidly decomposed into Ph3P=O and Ph2C=CH2. This reaction was later shown to proceed via a cyclic intermediate 95. The difference in reactivity between 94 (R = Me) and 94 (R = Ph) is consistent with the relative electrophilicities expected for the respective phosphorus atoms, because methyl is a stronger V-donor than phenyl. Hence, the alkoxide oxygen is able to attack the relatively electron deficient triphenylphosphonium group to give a cyclic oxyphosphorane 95, whereas Chapter 4: Introduction to Part 2. 42 the trimethylphosphonium group remains unaffected by the alkoxide oxygen. Cyclic oxyphosphorane 95 was not isolated, but instead expels Ph3P=O to give an olefin, Ph2C=CH2.

Ph Ph Ph Ph _ CH3 _ CH2 _ I _ + PhLi + Ph2C=O O HX OH P P X R R + (R = Me) + R R P P R R R Me R Me 93 R Me 94 (R = Ph) Ph Ph Ph – Ph3P=O CH2 Ph P Ph Ph Ph O 95 + – + Scheme 4.2: The reaction of Me4P I with PhLi was entirely analogous to the reaction of Me4N – + – Cl with PhLi. In contrast, when Ph3PMe I (R = Ph) was substituted, the intermediate alkoxide

rearranged (via a the cyclic oxyphosphorane) with elimination of Ph3P=O to give Ph2C=CH2.

+ – + – In both cases described above (Me4P I and Ph3PMe I ), the initial step in the reaction with phenyl lithium is deprotonation D to the phosphorus. If a salt was chosen possessing no D-hydrogens, perhaps nucleophilic addition to phosphorus would occur instead of + – deprotonation. The reaction with phenyl lithium was repeated using Ph4P I , and was found to give the desired phosphorane Ph5P (Scheme 4.3). Hence, through observation and logic, Wittig was able to prepare the first pentaorgano derivative of any element, and in doing so had also discovered a method for converting aldehydes and ketones into olefins, later known as the “Wittig reaction.”

Ph _ Ph I Ph + PhLi P P Ph Ph Ph Ph Ph Ph + – Scheme 4.3: The reaction of PhLi with Ph4P I , which contains no D-hydrogens, was found to give

the desired phosphorane Ph5P.

In many ways, Ph5P represents the parent pentaorganophosphorane (until the prepara- 101 102 tion of the as yet unknown Me5P). Like PF5, Ph5P has been shown to exist as trigonal bipyramidal molecules in the solid state. This geometry has been calculated103 to have the lowest energy on the basis of electrostatics alone, with the square pyramidal geometry only slightly less stable. Both geometries, and a possible mechanism for interconversion between them, will be discussed in more detail later. Chapter 4: Introduction to Part 2. 43

Preparation of spirocyclic phosphoranes, possessing either one or two 2,2'-biphenyl groups, could be accomplished via reaction of 2,2'-dilithiobiphenyl with an N-(p-toluenesulphonyl)triarylphosphine imine (Scheme 4.4). Reaction of a suspension of PCl5 with 2,2'-dilithiobiphenyl gave the interesting complex salt bis(2,2'-biphenyl)phosphonium tris(2,2'-biphenyl)phosphoranate 96 (Scheme 4.5), which directly corresponds to the structure of PCl5 in + – the solid state (i.e. PCl4 PCl6 ). The separation of the cation and anion was then achieved by addition of NaI, giving both bis(2,2'-biphenyl)phosphonium iodide and sodium tris(2,2'-biphenyl)phosphoranate.

Ar Ar P Ar (i) Ar p-TolSO2 N Ar Ar P (ii) Ar Ar

Ar P p-TolSO2 N Ar Ar Ar P (i)

(ii)

Ar P

Scheme 4.4: The synthesis of the pentaarylphosphoranes Ar5P, Ar3P(L-L) and ArP(L-L)2 (where Ar is a unidentate aryl group and L-L is 2,2'-biphenyl) involves the reaction of the appropriate N- (p-toluenesulphonyl)triarylphosphine imine with either (i) two equivalents of ArLi, or (ii) one equivalent of 2,2'-dilithiobiphenyl.

Li + _ PCl4 PCl6 _ P+ P

+ – Figure 4.5: The reaction of solid phosphorus pentachloride (as PCl4 PCl6 ) with 2,2'-dilithiobiphenyl gives bis(2,2'-biphenyl)phosphonium tris(2,2'-biphenyl)phosphoranate. Chapter 4: Introduction to Part 2. 44

Analogous derivatives of the heavier Group VB elements As, Sb, and Bi were also able to be prepared. Both the arsenic and antimony derivatives are found to be quite similar to those of phosphorus, except that Ar5Sb can react further with aryl lithium to give an anti- – monate anion Ar6Sb (Scheme 4.6). Neither Ar5P or Ar5As will accept a sixth aryl group when reacted with ArLi, but substitution of one aryl group for another does occur, suggesting – the possible involvement of a short lived hexa-coordinated anionic intermediate Ar6E (E = P, 104 As). The larger Ph5Bi will react with PhLi at low temperatures to give what is presumably – 100 Ph6Bi , but decomposition occurs upon warming to RT, presumably due to the weakness of the Bi-C bond.

_ Ph Ph Ph PhLi + Ph Ph (a) Sb Ph Li Sb Ph Ph Ph Ph Ph _ Ph p-Tol p-Tol Ph Ph p-TolLi + Ph Ph – PhLi (b) P Ph Li P P Ph Ph Ph Ph Ph Ph Ph Ph

Scheme 4.6: Addition reactions of Ph5E (E = P, As, Sb, Bi) with aryl lithiums: (a) the reaction of – Ph5Sb with PhLi gives Ph6Sb ; (b) the substitution reaction between Ph5P and p-TolLi.

Because the discovery that the spirocyclic phosphoranes tended to be more stable than

Ph5P, with a wider variety of groups able to be attached to phosphorus, attention was turned once again to nitrogen. Apart from the increase in steric congestion expected for the smaller atom, there is no reason why ammonium salts containing no D-hydrogens should not also behave as electrophiles towards aryl lithium reagents. A synthetic route to the spirocyclic salt bis(2,2'-biphenyl)ammonium iodide 97, was worked out, analogous to the known method of preparation of (2,2'-biphenyl)diphenylammonium iodide (Scheme 4.7). However, when the spirocyclic ammonium cation was reacted with either MeLi or PhLi, no penta-coordinated species 98 were detected. Instead, deprotonation of the aromatic rings ortho to the nitrogen was found to occur, with expulsion of the ammonium group to give a benzyne derivative 99, which was quickly quenched by more RLi. Subsequent acidic work-up gave the meta-substituted triarylamine 100. Hence, it may be concluded that either nitrogen is truly unable to bond to five groups via normal electron pair bonds, or the nitrogen atom in the spirocyclic salt is too sterically hindered to be approached by an incoming nucleophile. More recently, a different approach has been taken in the quest for nitrogen penta-coordination, focusing mainly on fluorinated 105-109 110 1 species, such as NF5 and H3NF2. Both the small size of the fluorine atom (0.64Å) Chapter 4: Introduction to Part 2. 45

–1 109 and the exceedingly weak bond strength of F2 ('HF-F = 37.9 kJ.mol ) contribute favour- ably towards the formation of high oxidation states and coordination numbers87 in molecular fluorides.

R RLi N + ' N+ N N 2 – N2

97 98

RLi

(i) RLi N N + (ii) H /H2O

R 100 99

Scheme 4.7: The spirocyclic ammonium salt 97 does not give the penta-coordinated compound 98 when reacted with RLi (R = Me, Ph).

Calculations have indicated that there is a significant increase in N-F bond length + when going from the known NF3 (1.365Å) and NF4 (1.29Å) to the as yet unknown NF5 – 109 (equatorial: 1.41Å, axial: 1.57Å) and NF6 (1.58Å), probably largely due to steric conges- 107 – tion (despite this, calculations indicate the possible stability of NF6 ). Experimental ap- + – + – proaches to the formation of NF5, such as the metathesis between Me4N F and NF4 BF4 , have provided results consistent with this prediction.109 In this reaction, the two salts, as solutions in either MeCN or CHF3, were mixed at low temperature, whereupon quantitative evolution of NF3 was detected. The researchers believed that nitrogen pentafluoride, present + – either as NF5 or, more likely, as NF4 F , decomposed into NF3 and F2, even at –142˚C. An- 105 other approach, when NF3 was exposed to atomic fluorine, gave a white solid, presumed + – to be NF4 F , which decomposed above –143˚C to give NF3 (Scheme 4.8).

_ + + _ Me4N F + NF4 BF4 F F F + ? or N F N NF + F F 3 2 F F _ F NF3 + 2F F F

+ – Scheme 4.8: Attempts to synthesise NF5 (or NF4 F ). Chapter 4: Introduction to Part 2. 46

Similarly, computations were carried out in which H3NF2 was compared to the known 110 H3PF2. The results were that the axial N-F bonds in H3NF2 were quite long, equal to the calculated P-F bond length in the corresponding H3PF2, with the axial 3-centre-4-electron (3c-4e) (i.e., F…N…F) bond quite weak, unlike its phosphorus analogue. The apparent lack of ability of nitrogen to form five normal electron pair bonds simultaneously can be put down to its small size (covalent radii: N 0.70 Å, P 1.10 Å)1 and possibly its compact AOs (due to its substantial electronegativity). The specification of “five normal electron pair bonds” is important, as nitrogen (as well as oxygen, carbon, and boron) is able to form in excess of four delocalised bonds. An example is where the nitrogen atom is encapsulated inside a polyhedron of metal atoms (Figure 4.9).111 In these kinds of compounds, the number of electrons in the immediate vicinity of the encapsulated atom does not exceed eight, as only four orbitals – 107 (2s, 2px,2py,2pz) are available on that central atom. Also, as predicted for the NF6 anion, – the six N-Co bonds are quite long (1.93Å to 1.96Å). Unlike NF6 however, the hexa-cobalt cluster is stabilised by additional bonding between neighbouring cobalt atoms.

OC Co CO OC Co Co WhereCo Co equals Co N C O OC Co Co Co Co CO OC

– Figure 4.9: The complex [Co6(CO)15N] contains an octahedrally coordinated nitrogen atom. (The dotted lines indicate both metal-metal bonding and bridging CO molecules.)

4.2 The stereochemical properties of penta-coordinated phosphorus.

Unique among the geometric varieties exhibited by phosphorus in various oxidation states, molecules containing penta-coordinated phosphorus (e.g. PX5, where X = F, Cl, Ph) are observed to be stereochemically non-rigid, or fluxional.112 The preferred geometry for an 103 acyclic phosphorane like PF5 has been shown both by calculations and by X-ray crystal structure101 to be a trigonal bipyramid, with three short (1.522Å) equatorial bonds arranged in an equilateral triangle about the phosphorus atom, and two longer (1.580Å) axial bonds above and below the trigonal plane. Because of the difference in P-F bond length between the equatorial and axial groups, even when all five groups are identical (as in PF5), the three equatorial groups are chemically distinct from the two axial groups. A second geometry of high sym- Chapter 4: Introduction to Part 2. 47 metry is the square pyramid, where four basal groups form a square plane, with the phosphorus atom lying slightly above the centre, and a single apical group situated directly above the phosphorus atom. Additionally, the four basal bonds are predicted to be longer than the unique apical bond (Figure 4.10). The square pyramid may represent the ground state geometry for some cyclic phosphoranes.113,114

Equatorial Apical

P P P P

Axial Basal Trigonal Bipyramidal Square Pyramidal

Figure 4.10: Two alternative structures for PX5, the trigonal bipyramid (with three equivalent equatorial X atoms and two equivalent axial X atoms) in which the phosphorus atom lies in the geometric centre, and the square pyramid, with a single apical X atom and four equivalent basal X atoms, with the phosphorus atom lying above the basal plane.

The energy difference between the trigonal bipyramid and the square pyramid is small, with virtually no energy barrier between the two structures, allowing facile interconversion of the two geometries to occur with only a very small energy input.103,112 As a result of this, many phosphoranes are found to maintain their fluxional behaviour down to very low temp- 23 19 eratures. In the case of PF5, F-NMR shows all five fluorine atoms to be chemically equivalent, even down to –197˚C.112 The observed equivalence of the five fluorines in the 19F- NMR spectrum is consistent with the complete exchange of all five fluorine atoms occurring rapidly on the NMR timescale. Although the trigonal bipyramidal geometry has been shown to be the geometric ground state for most penta-coordinated molecules possessing five inde- 26 pendent ligands, a notable exception is a crystalline modification of Ph5Sb, which exhibits a distorted square pyramidal geometry. There have been several mechanisms proposed to account for the exchange of ligands on penta-coordinated molecules like phosphoranes.115,116 In 1969, Whitesides and Mitchell showed that the exchange of fluorine atoms in (Me2N)PF4 only occurs through a pairwise exchange.117 Of all the non-dissociative mechanisms proposed to date,115,116 only two result in the exchange of two equatorial ligands with the axial pair, leaving the remaining equatorial ligand unchanged at the completion of the transformation.118 In addition, angular momentum is conserved in both mechanisms,118 provided all 5 ligands are groups of identical mass (i.e. comprised of the same isotopes). The two mechanisms are the Berry Pseudo-Rotation (BPR) and the Turnstile Rotation (TR), and both will now be described. Chapter 4: Introduction to Part 2. 48

The mechanism119 proposed by Berry in 1960 involves the interconversion between the trigonal bipyramidal and square pyramidal geometries. Starting with a trigonal bipyramid, bending the two axial bonds away from one of the equatorial bonds (the pivot) while at the same time widening the angle between the two other equatorial bonds (moving them both towards the pivot), will result in the formation of a square pyramid.119 During the transformation, the two axial bonds shorten while bending away from the pivot, while the two equatorial bonds lengthen as they bend towards the pivot (Figure 4.11), until all 4 bonds are equivalent by symmetry.

E E E

B C B C P P P A A D D C D A B

Figure 4.11: The reaction coordinate of the Berry Pseudo-Rotation (BPR) mechanism, where the axial pair {A, B} and two of the equatorial ligands {C, D} of the trigonal bipyramid distort, becoming the four basal ligands of the square pyramidal transition state. The third equatorial ligand E (called the pivot) becomes the single apical ligand in the square pyramid.

As the square pyramid represents the transition state geometry in the BPR mechanism, it will rapidly relax back to a ground state trigonal bipyramidal structure.119 This may happen in one of two ways: either the molecule reverses its transformation, returning the two originally axial bonds back into the axial positions (and the second pair back to their original equatorial positions), or a pairwise ligand exchange may occur. When this happens, the two bonds that were originally axial become equatorial, with the pair of bonds previously equatorial now becoming axial. In either situation, the pivot remains equatorial in both trigonal bipyramids, becoming apical in the square pyramidal transition state. In a molecule like PF5, where all five groups are chemically equivalent, the identity of the pivot can change from one transformation to the next, allowing the complete scrambling of all five fluorine atoms between the two axial and three equatorial positions after only a few transformations. In addition, because the BPR mechanism interchanges the trigonal bipyramidal and square pyramidal geometries, the mechanism is equally valid in describing the scrambling of ligands in a square pyramidal ground state passing through a trigonal bipyramidal transition state. The second mechanism, the Turnstile Rotation, also involves the exchange of a pair of equatorial groups with the axial pair.118 However, the square pyramidal geometry is not part of the reaction coordinate of the TR mechanism, making this mechanism only applicable to the scrambling of ligands in trigonal bipyramidal molecules. Starting from a trigonal bipyramidal molecule, the five ligands divide themselves into two groups: a ligand couple made up of one equatorial and one axial bond, and a ligand triple composed of the remaining two Chapter 4: Introduction to Part 2. 49 equatorial bonds and the single axial bond. Firstly, a distortion occurs whereby the angle between the bonds in the couple compresses from 90˚ to about 70˚, with an adjustment of the bond lengths so as to make both bonds equivalent. Simultaneously, the three bonds in the triple also become virtually equivalent by bond length and bond angle adjustment (the overall symmetry is low at this point, and true equivalence within the ligand groups is strictly impossible; all that is implied is that the bonds become very similar within each ligand group, so as to be effectively indistinguishable from one another).

At this point in time, the pseudo-C3 axis for the ligand triple has become co-linear with the pseudo-C2 axis for the ligand couple. A twisting of the two ligand groups about their respective (common) axes then occurs, with the ligand triple moving through an arc of 24˚, while the ligand couple moves through 36˚ in the opposite direction.118 This rotation then brings the molecule to the same geometry as it was before the twist, and relaxing back to a trigonal bipyramid results in an overall pairwise exchange: the two ligands in the couple exchange, with the group previously axial becoming equatorial, and vice versa, whereas one of the two equatorial ligands of the triple remains equatorial, with the remaining two ligands ex- changing positions. The transition state for the transformation occurs halfway through the twist (Figure 4.12).

D D C E CC DE (i) P P

AB A AB

(ii)

C C E ED DE DE (iii) P P

BA B B A

Figure 4.12: The Turnstile Rotation (TR) mechanism, involving the interconversion of two trigonal bipyramidal isomers, involves three steps: (i) the trigonal bipyramidal molecule distorts into a

Cs geometry, dividing the ligands into the two groups {A, B} and {C, D, E}; (ii) the couple {A, B} twists by 36˚, while the triple {C, D, E} twists by 24˚ in the opposite direction, giving a new Cs isomer; (iii) relaxation of the strained Cs isomer into the new trigonal bipyramidal isomer then occurs. Chapter 4: Introduction to Part 2. 50

Because the transition state of the TR mechanism is calculated to lie higher in energy than that for the BPR mechanism when there are five independent ligands,120 the activation energy of the TR mechanism will also be higher. As a result, the rate of the BPR mechanism is expected to be much faster. Additionally, X-ray data for a number of cyclic phosphoranes shows them to lie on the “Berry coordinate,” a smooth distortion from the idealised trigonal bipyramidal to the idealised square pyramidal geometry,114 suggesting that distortion along the “Turnstile coordinate” is not favoured in most cases. Therefore, it will be assumed that BPR is the only mechanism operating on the phosphoranes with regard to the work presented here. (As the overall result is the same, i.e. the pairwise exchange of two equatorial ligands for the two axial ligands, the precise details of the molecular transformations are considered unimportant, and assuming one or other mechanism does not affect any conclusions drawn.) Because in either the trigonal bipyramidal or square pyramidal geometries there are two distinct ligand environments (either axial and equatorial, or basal and apical, respectively), phosphoranes possessing two or more different ligands will have different energies when a given ligand moves from one environment to another. It has been predicted118,121 by calculations, and confirmed by experiment,23 that the more electronegative (V-acidic) ligands prefer to occupy the axial positions, while the more electropositive (V-basic) ligands prefer the equatorial positions (the “Polarity Rule”). For ligands with S-orbitals, the axial positions are preferentially occupied by the S-acidic ligands, whereas the S-basic ligands prefer the equatorial positions.121 For the square pyramidal geometry, the basal sites behave in a similar way to the axial positions on the trigonal bipyramid, in being preferentially occupied by both V- and S-acidic ligands, with the V- and S-basic ligands preferring to occupy the solitary apical position. For ligands with opposing characteristics (e.g. fluorine, which is both highly electronegative as well as a good S-donor), the overall positional preference will result from a combination of the two effects, and also on the nature of other ligands present. As described above, in the BPR mechanism, one of the equatorial ligands of the initial trigonal bipyramid becomes the pivot during the transformation, passing through the apical site in the square pyramidal transition state before returning to an equatorial position in the new trigonal bipyramid.119 Because both V- and S-donors prefer to occupy the equatorial positions in the trigonal bipyramidal geometry, and also prefer the apical site in the square pyramidal geometry, these ligands will tend to be the pivot during such transformations. This can have a profound effect upon the observed fluxional behaviour of the molecule, as shown by the series of fluorophosphoranes RnPF5-n (for n = 0 to 5), which will now be discussed.

For PF5 (n = 0), all five ligands are equivalent, and each one is equally likely to adopt the role of the pivot during a single BPR transformation. The result is that all five fluorine atoms appear chemically equivalent by 19F-NMR down to –197˚C, as already described.112 An amino nitrogen atom behaves as a strong S-donor because of its lone pair of electrons.

Therefore, when one of the fluorine atoms in PF5 is replaced by a dimethylamino group, giv- Chapter 4: Introduction to Part 2. 51

ing (Me2N)PF4, the single amino group preferentially occupies an equatorial position and becomes the permanent pivot, allowing the four fluorine atoms (two axial, two equatorial) to undergo rapid pairwise exchange.117 This results in the complete scrambling of all four F atoms, making them appear chemically equivalent, as has been observed using 19F-NMR (see 31 117 Figure 4.13). In the P-NMR spectrum at –100˚C, (Me2N)PF4 exhibits a triplet of triplets (because of 31P-19F coupling), indicating that the fluorine atoms are in the ratio of 2:2 (axial: equatorial), with no evidence for dynamic ligand exchange (DLE). Upon warming the solution to –50˚C, a single 1:4:6:4:1 quintet is observed (due to four equivalent fluorine atoms), indicating that DLE occurs rapidly at this temperature.

F' F F F' Me2N P Me2N P F F' F' F

Figure 4.13: Using the amino group as a pivot, the axial fluorine atoms of (Me2N)PF4 (initially marked as F') can rapidly exchange with the equatorial fluorine atoms (initially marked as F).

When two of the fluorine atoms have been replaced, as in H2PF3, both H atoms prefer to occupy equatorial sites, and will share the role of pivot between them. Now, during any BPR transformation, an unfavourable conformation must occur: firstly, one H atom must be made to occupy a basal position in the transient square pyramidal transition state; and secondly, any new trigonal bipyramid formed must contain one axial hydrogen atom. This has the effect of slowing dramatically the rate of scrambling of the three fluorine atoms, which in fact can be frozen out at –46˚C122 (Figure 4.14), as shown by 19F-NMR.

F' H H F F' H P H P F' H P H F' F F' F' F

Figure 4.14: The isomerisation of H2PF3 via the BPR mechanism.

When there are three substitutions, as in Me3PF2 (Figure 4.15), the fluorine atoms will occupy the axial positions because of their higher electronegativity.23 As each ligand is already in its preferred place, no scrambling need occur.

F F Me Ph Me P Ph P Me Ph F Ph

Figure 4.15: No isomerisation need occur for Me3PF2 or Ph4PF via the BPR mechanism at RT. Chapter 4: Introduction to Part 2. 52

Only one kind of fluorine is observed by 19F-NMR, which would be expected anyway (either from both axial fluorine atoms being chemically equivalent by symmetry, or through 19 1 scrambling). Additionally, observation of F- H coupling in R3PF2 species rules out the oc- 23 currence of DLE. In a similar way, a compound like Ph4PF would also show only a single fluorine environment, whether scrambling is occurring, or (more likely) the geometry is fixed with the single fluorine atom in the axial position.

4.3 The relationship between penta- and hexa-coordinated phosphorus.

The penta-coordinated phosphorane and the octahedral phosphoranate anion are related to one another, as well as to the tetrahedral phosphonium cation, by all being forms of pentavalent phosphorus (i.e. there are no lone pairs located on the phosphorus atom). They differ only in their respective coordination numbers, and therefore they are readily interconvertible via Lewis acid-base equilibria (Figure 4.16). The electrophilicity of the phosphon- + – ium cation A4P towards the nucleophile B largely determines the value of the equilibrium constant K1 for the formation of the phosphorane A4PB; the electrophilicity of the phosphor- – ane towards the nucleophile B then determines the value of the equilibrium constant K2 for – the formation of the phosphoranate A4PB2 . In each case, the effective electrophilicity of the + central phosphorus atom in both A4P and A4PB increases with the number of electron-withdrawing groups (both V and S), and decreases with larger substituents.

_ + K1 K2 A __B B + B A + B A A P _ P A _ P A – B – B A A A A A A B

+ – Figure 4.16: Reaction of A4P with the Lewis base B gives the phosphorane A4PB. Addition of – – a second B anion gives the anionic phosphoranate A4PB2 .

For example, in the series PhnPCl5-n, the observed electrophilicity of the central phosphorus atom towards addition of chloride ion is defined as the equilibrium constant for:

– – PhnPCl5-n + Cl PhnPCl6-n (where n = 0 to 5).

As n increases, the electrophilicity decreases, until, at n = 4, even the phosphonium cation + Ph4P cation is not electrophilic enough to favour the formation of covalent Ph4PCl, and the 23 + – compound exists as an ionic salt, Ph4P Cl . Another series shows the effects of factors due to both steric size and electronegativity: in the dihalides Ph3PX2 (X = F, Cl, Br, I), the electrophilicity of the phosphorane decreases with increasing size (and decreasing electronegativity) of the halogen. Chapter 4: Introduction to Part 2. 53

In contrast to the fluxional nature of penta-coordinated phosphorus, hexa-coordinated phosphorus is both stereochemically rigid and found exclusively in only one geometry, where the six bonding electron pairs are disposed in an octahedral arrangement about the central phosphorus atom.123 As an alternative geometry of high symmetry, the trigonal prism has never been observed for phosphorus, although some transition metals2 exhibit this coordination geometry (Figure 4.17). The two geometries are related by a twist of one trigonal face relative to the other.

P P

Octahedral Trigonal Prismatic

Figure 4.17: Two possible geometries of high symmetry for hexa-coordination, octahedral and trigonal prismatic. In each case, the phosphorus atom lies in the geometric centre of the polyhedron.

It is interesting to note that, for a given set of six identical groups, the total orbital energies of the phosphorus atom in both the octahedral and trigonal prismatic geometries are identical (the spherically symmetric s orbital on the central atom is necessarily unchanged in energy, while both the p-orbitals124 and d-orbitals2 have been shown to be unaffected by the geometry change when bond lengths remain constant). Hence, the prohibitively high energy barrier to rotation of one trigonal face with respect to the other is presumably entirely steric in origin (i.e. the barrier to interconversion is due to inter-ligand non-bonded repulsions). Iso- merisation, by passing through the high-energy trigonal prismatic geometry, is believed not to occur, with any apparent fluxional behaviour125 of octahedral phosphorus explainable by bond breakage. The intermediate phosphorane can then isomerise through DLE processes before accepting the previously expelled group back, regenerating the hexa-coordinated species with a loss of optical activity.

An additional feature of penta-coordinated phosphorus (i.e. phosphoranes, PX5) is the reductive-elimination of X2 to give the corresponding trivalent compound PX3 (an example is 87 the dissociation of PCl5 into PCl3 and Cl2 in the gas phase ). It has been shown from MO theory that the concerted elimination of X2 from PX5 is an allowed process if both departing

X groups are either both axial or both equatorial in the trigonal bipyramidal PX5 molecule, with loss of one equatorial and one axial X group forbidden in concerted reductive elimina- Chapter 4: Introduction to Part 2. 54

121 126 tions. Calculations on PH5, AsH5, SbH5 and BiH5 have, with the exception of BiH5, confirmed the equatorial-equatorial elimination of H2 (BiH5 forms a zwitterionic transition +… – state BiH4 H which can then lose H2 by deprotonation). The barrier to loss of H2 from –1 EH5 (where E = P, As, Sb, Bi) was calculated to be about 30 kcal.mol , suggesting that all four pentahydrides may be capable of isolation. Thermodynamically, the process for the reverse oxidative-addition reaction can be described as follows: (i) cleavage of the X-X bond in X2 must occur, (ii) “re-hybridisation” of phosphorus, giving PX3* (where * denotes an excited valence state), and (iii) formation of 127 126 PX5 from PX3* and 2X. The thermodynamic instability of PH5 compared to the known halides PF5, PCl5 and PBr5 relates instead to the strong H-H bond, shifting the equilibrium 127 strongly in favour of H2. A similar scheme has been used to describe the formation of 128 AsCl5 from AsCl3 and Cl2 at low temperatures (arsenic pentachloride has been the focus of interest, as it is very unstable, in contrast to both PCl5 and SbCl5). Four steps are postulated to explain the reaction of Cl2 with AsCl3, involving a single electron transfer from AsCl3 to a chlorine atom:

x (i) Cl2 o 2Cl

x x+ – (ii) AsCl3 + Cl o AsCl3 + Cl

x+ x + (iii) AsCl3 + Cl o AsCl4 + – (iv) AsCl4 + Cl o AsCl5

Because the As-Cl bonds in AsCl5 are found to be of comparable energy to P-Cl bonds 128 in PCl5, and the Cl-Cl bond strength must naturally remain constant, the proposed reason for the instability of AsCl5 is the higher ionisation potential of AsCl3 compared to PCl3 (i.e., 11.7 and 10.5 eV, respectively).

4.4 Organic derivatives of penta- and hexa-coordinated phosphorus.

This section provides a brief overview of known examples of phosphoranes of the – type Rn(H)PX4-n and phosphoranates of the type [Rn(H)PX5-n] (R = H, alkyl, aryl; X = F, Cl, 129a Br, OR, SR, NR2, CN, CF3). In neither case has the parent compound PH5 (phosphorane) – or PH6 (phosphoranate) yet been reported, with H3PF2 representing the phosphorane with the largest number of hydrogen atoms. In order to limit the number of compounds discussed, and because Part 2 mainly examines the reaction of hydride donors to produce the hydrophospho- – ranes (R4PH and R3PH2) and -phosphoranates (R4PH2 ), only those possessing at least one hydrogen atom directly attached to the phosphorus atom will be described in any detail. The examples discussed are by no means exhaustive, but are chosen so as to cover as many different kinds of compounds within the defined group as possible. The phosphoranes will be discussed first, followed by the hexa-coordinated phosphoranates. Chapter 4: Introduction to Part 2. 55

Many examples of phosphoranes of the type RPX4 are known when R is an organic group and X = F, Cl or CN. The tetrafluorophosphoranes RPF4 (R = Me, Et, Ph) have been shown to undergo rapid exchange of the fluorine sites (equatorial and axial fluorine atoms rapidly interconvert), the evidence for which is the observed equivalence of all four fluorine atoms in both the 31P- and 19F-NMR spectra. Presumably, the BPR mechanism is responsible

(refer to the discussion of (Me2N)PF4 above). The physical and chemical properties of such compounds closely resemble the pentahalides PF5 and PCl5 in being violently hydrolysed by – water, as well as behaving as strong Lewis acids (e.g. addition of halide ion X to RPX4 gives – the hexa-coordinated anion RPX5 ). Some examples of different kinds of phosphoranes 130 containing hydrogen (i.e. HPX4) are shown below (Figure 4.18). In addition, HPF4 is ob- 1 1 served at GP –45.6 ppm, with JP-H = 1089.3 Hz and JP-F = 982.2 Hz. As the four X groups 1 become less electronegative, the observed magnitude of the JP-H decreases (e.g. from 1090 Hz when X = F down to 621 Hz when X = N).

O O Me Me Me Me N O O O O N S O N O H P H P H P H P H P H H H N S N N O N O O O O Me Me Me Me Me Me O O O [Ref 131] [Ref 132] [Ref 133] [Ref 134] [Ref 135] GP = –54.5 ppm GP = +8 ppm GP = –47 ppm GP = –75 ppm GP = –51 ppm 1 1 1 1 1 JP-H = 621 Hz JP-H = 711 Hz JP-H = 885 Hz JP-H = 816 Hz JP-H = 920 Hz

Figure 4.18: Examples of HPX4 phosphoranes (where X = S, N, O, F).

The trihalophosphoranes R2PX3 are similar to the tetrahalides (RPX4) described above – in that they are easily hydrolysed, form addition compounds with halide ion to give R2PX4 , 31 19 and the fluoro derivatives R2PF3 undergo DLE at RT, as shown by both P- and F-NMR.

Shown below (Figure 4.19) are four phosphoranes of the type RP(H)X3 (where R = H, alkyl, aryl; X = F, O, N). Again, the observed magnitude of the 1-bond P-H coupling constant decreases with the decreasing electronegativity of the remaining four groups.

The difluorophosphoranes R3PF2 are covalent molecules like the trifluorophosphor- 19 23 anes R2PF3, and exhibit only one fluorine environment in the F-NMR. Because both F atoms will presumably prefer to be axial, DLE does not need to be postulated here; however, it does not rule-out such processes. Shown below (Figure 4.20) are 3 phosphoranes of the type R2(H)PX2 (R = H, alkyl, aryl; X = F, O). Chapter 4: Introduction to Part 2. 56

F F O O H H H H P F P NH2 P N P N H H H Ph F F O O

[Ref 130] [Ref 136] [Ref 129a] [Ref 137]

GP = –19.2 ppm GP = –57.3 ppm GP = –61.8 ppm GP = –44.8 ppm 1 1 1 1 JP-H = 865.1 Hz JP-H = 754.4 Hz JP-H = 655 Hz JP-H = 705 Hz 1 1 JP-F = 816.7 Hz (ax) JP-F = 598.6 Hz 1 JP-F = 963.3 Hz (eq)

F F F CF3 H H H H P F P F P F P CF3 Me nBu Ph Me F F F CF3

[Ref 138] [Ref 139] [Ref 139] [Ref 140]

GP = –7.7 ppm GP = –7.1 ppm GP = –30.2 ppm GP = –103.7 ppm 1 1 1 1 JP-H = 860 Hz JP-H = 840 Hz JP-H = 863 Hz JP-H = 607.9 Hz 1 1 1 JP-F = 805, 968 Hz JP-F = 843.4, 986.7 Hz JP-F = 822, 955.5 Hz

Figure 4.19: Phosphoranes of the type R(H)PX3 (where R = H, alkyl, aryl; X = N, O, F).

F F Me Me H Me P H P H H Me O F F [Ref 141]

P H GP = –60.1 ppm [Ref 130] [Ref 138] 1J = 680 Hz O P-H GP = –62.5 ppm GP = –31.7 ppm 1 1 JP-H = 752.4 Hz JP-H = 733 Hz 1 1 Me JP-F = 532.1 Hz JP-F = 535 Hz Me

Figure 4.20: Phosphoranes of the type R2(H)PX2 (where R = H, alkyl, aryl; X = O, F).

+ – The equilibrium for R4PX lies far to the side of the ionised phosphonium salt R4P X , + with the tetraorganophosphonium cation R4P showing virtually no tendency to accept halide ion into its coordination sphere to form a fifth covalent bond. Despite this, the fluorophos- 142 143 phoranes Ph4PF and Et4PF can exist as molecular phosphoranes. Ph4PF has been characterised by both IR (P-F stretch at 817 cm–1) and mass spectroscopy (m/z 358 amu, corres- + 31 ponding to the molecular ion Ph4PF ), while for Et4PF, the P-NMR shows a signal at GP –60 144 ppm. In addition, five alkoxyphosphoranes have been characterised: Me4POR (R = Me, i GP –88 ppm; R = Et, GP –91 ppm) and Ph3P(Me)OR (GP –67 ppm for R = Me, Et, Pr). Only two hydrophosphoranes of the type R3(H)PX have been characterised so far (Figure 4.21). Chapter 4: Introduction to Part 2. 57

H F C 3 CF3 P O + _ P CF3SO3 P Ph Ph H

[Ref 64] [Ref 145]

No data given GP = –49.3 ppm 1 JP-H =266Hz

Figure 4.21: Phosphoranes of the type R3(H)PX (where R = H, alkyl, aryl; X = O, F).

One is a tricyclic phosphonium hydrophosphorane, made from the reaction of LiBH4 with the 1,6-diphosphoniumtricyclo[4.4.4.0]tetradecane dication 46 (see Section 1.4). The second has been shown by an X-ray crystal structure to possess an axial H atom, opposite the oxygen atom.145

There are a number of phosphoranes of the type R5P, all containing aromatic R groups, either as phenyl groups (or methylated phenyl groups such as p-tolyl), or as 2,2'-biphenyl (L) groups (and methylated derivatives), some examples being Ph5P, LPPh3 and L2PPh. The only phosphoranes of the type HPR4 that have been reported are derived from the compound shown below146 (Figure 4.22), in which the aromatic rings have been alkylated to varying ex- tents.

H P

[Ref 146]

GP = –112 ppm 1 JP-H = 482 Hz GH = +9.33 ppm

Figure 4.22: The spirocyclic hydrophosphorane bis(2,2'-diphenyl)phosphorane.

Unlike the penta-coordinated phosphoranes, which had a low barrier to ligand exchange making isolation of geometric isomers impossible, the hexa-coordinated phosphoranate anions are stereochemically rigid. Because of this, geometric isomers, where possible, – can be distinguished. Phosphoranates of the type RPX5 can be formed by the addition of the – Lewis base X to the Lewis acid RPX4, as previously described. Where all X groups are Chapter 4: Introduction to Part 2. 58

– identical (e.g. as in MePF5 ), only one geometric isomer is possible. The single signal appearing in the 31P-NMR, and the two signals in the 19F-NMR, observed to be in the ratio of 4:1 (for F cis and trans to Me), are consistent with the octahedral geometry assigned to the anion. The only examples of hydrogenated species are shown below in Figure 4.23. Com- 1 1 parison with similar phosphoranes indicate that both JP-H and JP-F decrease on going from the penta-coordinated phosphorane to the hexa-coordinated phosphoranate.129a,b This may be due to an increase in both P-H and P-F bond lengths in the sterically crowded phosphoranate.

_ _ _ Me Me Me Me O O O N O X O X O MeO O P P P O H O H O H O O O O Me Me

[Ref 129b][Ref 134] [Ref 135]

X = OMe GP = –97.2 ppm X = OMe GP = –110 ppm GP = –107 ppm 1 1 1 JP-H = 800 Hz JP-H = 765 Hz JP-H =830 Hz t X = OC6H4-o-OH GP = –99 ppm X = O Bu GP = –112 ppm 1 1 JP-H = 802 Hz JP-H = 740 Hz __ H F F F F H P P F F F C F F N

[Ref 147] [Ref 123]

GP = –141.5 ppm GP = –157 ppm 1 1 JP-H = 945 Hz JP-H = 832 Hz 1 1 JP-F =730,820Hz JP-F = 695, 739, 847 Hz – Figure 4.23: Examples of hydrophosphoranates HPX5 (X = F, OR, NR2, CN).

– When there are two R groups present, as in R2PX4 , both cis and trans isomers (des- – cribing the relationship of the two R groups) are possible. The same is true for R4PX2 , – where there are four organic groups attached to phosphorus, an example being Ph4PF2 , form- + 142 ed along with Ph4P from the disproportionation of Ph4PF. Examples of geometric iso- – – merism has been demonstrated with the cyanochlorides (NC)2PCl4 and (NC)4PCl2 , where – both cis and trans isomers are resolvable. Obviously, the trans isomer of A4PB2 has four Chapter 4: Introduction to Part 2. 59 equivalent A groups, whereas the cis isomer has two distinct pairs of A groups by symmetry. Both the cis and trans isomer has equivalent B groups (Figure 4.24).

_ _ B B A B A A P P A A A A A B

cistrans

– Figure 4.24: The two geometric isomers for A4PB2 , labeled as cis and trans with respect to the two B substituents.

– In the case of R3PX3 , there are also two possible isomers (when all three X groups, and all three R groups, are identical): these are labeled mer (where all three groups of a given kind are situated around a meridional line of an imaginary sphere upon which the six groups lie) and fac (where all three groups of a given kind lie on one trigonal face of the octahedron). Both isomers are shown in Figure 4.25.

_ _ B B A B A B P P B A A B A A

mer fac

– Figure 4.25: The two geometric isomers for A3PB3 , labeled mer and fac.

– Phosphoranates of the type R6P have been characterised, where two R groups correspond to a bidentate 2,2'-biphenyl group, forming a tris-bidentate anion148 (see Figure 4.26). 3+ Such species, possessing pseudo-octahedral geometry like [Co(2,2'-bipyridyl)3] , a complex of Co(III), are enantiomeric (comprised of / and ' isomers).

_ _ P P

/ ' Figure 4.26: The / and ' enantiomers of the tris(2,2'-biphenyl)phosphoranate anion. Chapter 4: Introduction to Part 2. 60

4.5 The discovery of hydrophosphoranes and hydrophosphoranates.

Chapters 5 to 8 describe the discovery of the hydrophosphoranes R4PH and R3PH2 and – the hydrophosphoranates R4PH2 (R = alkyl, aryl), intermediates in the hydride reduction of quaternary phosphonium salts. Chapter 5 discusses deuterium incorporation in the benzene and toluene recovered from the reductive cleavage of phenylbenzylphosphonium salts using

LiAlD4 in THF. In Chapters 6-8, the discoveries of the phenylhydrophosphoranes Ph4PY and – PH3PY2, and the corresponding phosphoranate Ph4PY2 (where Y = H, D), are described; and finally, in Chapter 9, reactions are described between phosphonium salts and some nucleophiles other than hydride. It should be noted that, unless otherwise stated, the exact stereochemistry of both the penta-coordinated phosphoranes and the hexa-coordinated phosphoranates discussed in the following chapters is not known, and where diagrams are presented, no implication about the stereochemistry is implied. As already discussed (Section 4.2), the phosphoranes are expected to undergo facile isomerisation (DLE) at RT, and so any geometric isomer is possible for those with five independent ligands attached to phosphorus. Chapter 5: Incorporation of Deuterium into Benzene and

Toluene cleaved using LiAlD4.

This chapter discusses the discovery of deuterium incorporation into the benzene or + – toluene cleaved from the phosphonium salts [PhnPBn4-n] Br (n = 0 to 4) by LiAlD4 in re- 149 fluxing THF. Since the first reported preparation of LiAlH4 in 1947, its use as a con- 150 venient reducing agent has become universal. The ability of LiAlH4 to reductively cleave both benzyl79 and phenyl151 groups from quaternary benzylphosphonium salts has been reported. Tertiary phosphine oxides152 can also be converted to the corresponding phosphines by LiAlH4 in refluxing THF; see Scheme 5.1. In both cases, loss of optical purity in the recovered phosphine69,153-155 has also been observed in these reductions.

_ Bn X O + LiAlH P LiAlH P 4 4 P R R R R R R R R R

+ – Scheme 5.1: The reduction of either R3PBn X or R3P=O with LiAlH4 giving R3P.

It has been suggested that these racemisations of the product phosphine from both chiral phosphonium salts69,153 and chiral phosphine oxides152,155 are the result of the formation of an intermediate phosphorane (Scheme 5.2). Once formed, the phosphorane may undergo the dynamic ligand exchange (DLE) processes described in Chapter 4, thereby scrambling all stereochemical information.

_ Bn X O + P P R R R R R R LiAlH4 LiAlH4 _ H H AlH3 R R DLEP DLE P Bn P O – PhCH R R – “H AlOH ” R 3 R 3 R R R

Scheme 5.2: Phosphorane intermediates in reductions with LiAlH4. Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 62

In contrast to the formation of tertiary phosphines (Schemes 5.1 and 5.2), the reaction of diphenyl(1-phenylallyl)phosphine oxide 101 with LiAlH4 gave significant quantities of diphenylphosphine in addition to the diphenyl(1-phenylallyl)phosphine expected.156 One possible explanation157 for this surprising result is shown below (Scheme 5.3). After formation of the intermediate phosphorane 102, a concerted rearrangement through a 5-membered cyclic transition state with the elimination of 1-phenylpropene 103 gives the aluminium phosphi- 156 nite 104, which, on reaction with additional LiAlH4, would then give Ph2PH as observed.

_ _ O H P LiAlH4 Ph LiAlH4 P H3AlO O P Ph Ph Ph P H Ph Ph Ph AlH3 Ph Ph 104 Ph + 101 102 Me Ph 103

Scheme 5.3: Proposed mechanism for conversion of the allylphosphine oxide 101 into a secondary phosphine via the intermediate phosphorane 102.

The mechanism proposed above (in Scheme 5.3) strongly resembles that for another concerted transformation, the interconversion between the allylphosphine oxide 105 and its isomeric allylphosphinite 106 (Scheme 5.4). This reaction is an equilibrium, with the reverse reaction (phosphinite into phosphine oxide) able to be thermally induced158,159 (presumably the driving force here is the formation of the P=O bond). Again, a cyclic 5-membered transition state is postulated.

O O

P P R R R R

105 106

Scheme 5.4: Rearrangement through a cyclic 5-membered transition state.

What if the process shown to occur when the allylic phosphine oxide 101 is reacted with LiAlH4 (see above, Scheme 5.3) can also occur for allylphosphonium salts (i.e. with the formation of an intermediate hydrophosphorane which then expels propene to give a phosphine)?157 The benzyl group, like the allyl group, has a formal carbon-carbon double bond at the carbon beta to phosphorus. As loss of benzyl from phosphonium salts treated with 157 LiAlH4 occurs readily, perhaps a similar process is occurring here as well. If the reaction of a benzylphosphonium salt was carried out using LiAlD4, the deuterium would be ex- Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 63 pected to be incorporated into the benzyl moiety, with (±)-6-deuteriomethylenecyclohexa-2,4- diene 107 expelled from the intermediate phosphorane 108 (Scheme 5.5). Rapid rearoma- 160,161 tisation of 107 would then give a mixture of PhCH2D 109 and 2-DC6H4CH3 110.

CH2D

H2C Bn H LiAlD – R P 109 + 4 D 3 P R D R CH3 R R P R D R 108 107 110

+ Scheme 5.5: The proposed mechanism for the reaction between R3PBn and LiAlD4.

The remainder of this chapter describes the isolation and analysis of the cleaved hy- + – + – drocarbon fragments (i.e. benzene from Ph4P Br and toluene from [PhnPBn4-n] Br (where + – n = 0 to 3), obtained from the reaction of [PhnPBn4-n] Br (n = 0 to 4) with LiAlD4 in refluxing THF.

+ – 5.1 Toluene cleaved from Ph3PBn Br with LiAlD4.

+ – 79 The reduction of Ph3PBn Br was carried out under standard conditions, with the phosphonium salt being refluxed with an excess of LiAlD4 in anhydrous THF for a mini- mum of 4 hours. The recovered toluene (which was found to be only partially deuterated) was analysed using 1H-, 2H-, and 13C{1H}-NMR, and these spectra will now be discussed. The 2H-NMR spectrum of the toluene sample just described is shown below in Figure

5.6 (page 64), and exhibits only two signals: a doublet at GD +6.88 ppm (J = 1.0 Hz), and a 2 1 1:2:1 triplet at GD +2.10 ppm (J = 2.2 Hz). The corresponding H{ H}-NMR spectrum showed both signals as singlets, indicating two facts: firstly, the aromatic doublet must be due to splitting of the deuteron by one proton, which therefore means that the aromatic deuteron must be located ortho to the methyl group; and secondly, the triplet structure of the methyl resonance implies coupling to two equivalent protons. The magnitude of the coupling is consistent with geminal coupling to the methyl protons (i.e. there is one deuteron on the methyl group). Hence, it can be deduced that the two deuteron environments are the ortho position on the aromatic ring and the alpha position on the methyl group (with only one D-deuterium). 2 Therefore, besides the non-deuterated isomer PhCH3 (which cannot be detected using H- NMR), the toluene sample must consist of a mixture of any one or more of the following five isomers: PhCH2D 109, 2-DC6H4CH3 110, 2,6-D2C6H3CH3 111, 2-DC6H4CH2D 112, and 2,6-

D2C6H3CH2D 113. Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 64

CH2D CH3 CH3 CH2D CH2D D D D D D D

109 110 111 112 113

7.0 6.9 6.8 2.2 2.1 2.0 (ppm) (ppm) 3 2 (a) Aromatic region. JH-D = 1.0 Hz. (b) Methyl region. JH-D = 2.2 Hz.

Figure 5.6: The 2H-NMR spectrum (at 76.7734 MHz) of the toluene sample obtained from the re- + – ductive cleavage of Ph3PBn Br by LiAlD4 in THF at reflux (not to scale).

Shown below (Figure 5.7) is the methyl region of the 1H-NMR spectrum of the same toluene sample just described. The downfield singlet at about GH +2.12 ppm is due to a non- deuterated methyl group (-CH3), while the 1:1:1 triplet at GH +2.10 ppm (J = 2.2 Hz) is due to the -CH2D group in PhCH2D 109. The upfield isotopic shift observed when hydrogen is substituted by deuterium has been well documented.162-165 The aromatic region was found to be qualitatively identical to the aromatic region of PhCH3 itself, and so no definite conclusions about deuteration around the ring could be made.

2.14 2.12 2.10 2.08 (ppm)

1 2 Figure 5.7: The H-NMR spectrum (at 500.113 MHz), methyl region. JH-D = 2.2 Hz. Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 65

Figure 5.8 shows the 13C{1H}-NMR spectrum of the same sample of toluene. The methyl region exhibits a singlet at GC +21.30 ppm, a 1:1:1 triplet at GC +21.24 ppm (where J = 0.5 Hz), and a 1:1:1 triplet at GC +21.02 ppm (J = 19.4 Hz), and these correspond to the methyl carbons of PhCH3, 2-DC6H4CH3 110 and PhCH2D 109, respectively (therefore, the recovered toluene is not completely deuterated; this will be discussed in Chapter 6). As there is no further splitting of the signals due to PhCH2D 109, it can be concluded that the recovered toluene is a mixture of only those three isomers just listed, with no poly-deuterated species present. The values of the respective carbon-deuterium splittings are consistent with the observed carbon-hydrogen splittings for the methyl carbon: 1-bond (methyl H) J = 126 Hz and 3-bond (ortho H) J = 4 Hz (i.e. the C-D splittings are found to be about 1/6 the magnitude 1 of the C-H splittings, equal to the ratio of the respective gyromagnetic ratios JH:JD = 6.51 ).

129.0 128.5 128.0 21.3 21.2 21.1 21.0 20.9 20.8 (ppm) (ppm)

(a) Aromatic region. (b) Methyl region. Figure 5.8: The 13C{1H}-NMR spectrum (at 125.758 MHz) of the toluene sample described above in Figures 5.6 and 5.7. (The peak marked * is due to a trace of benzene.)

The aromatic region shows a signal due to the ortho carbon at GC +128.92 ppm, composed of two singlets (from PhCH3 and 2-DC6H4CH3 110) and a 1:1:1 triplet (due to coupling with the methyl deuteron in PhCH2D 109), in addition to a very low intensity 1:1:1 triplet at

GC +128.61 ppm (J = 23.9 Hz, isotopic shift 'G = –0.31 ppm) corresponding to the ortho carbon directly bound to the aromatic deuteron in 2-DC6H4CH3 110. Additionally, there is a low intensity singlet (marked as * in Figue 5.8; see also Figure 5.14, page 70) at GC +128.24 ppm due to a trace of benzene, a singlet due to the meta carbons that are not adjacent to an aromatic deuteron at GC +128.14 ppm, and a smaller singlet at GC +128.03 ppm due to the meta carbon which is adjacent to the aromatic deuteron (with a 2-bond isotopic upfield shift of 0.11 ppm). The observed isotopic shifts agree with those reported elsewhere.163 Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 66

In the same way, the ipso carbon also exhibits a clear separation of the three isomers, 13 1 as the C{ H}-NMR spectrum shows (Figure 5.9). PhCH3 gives rise to the downfield singlet at GC +137.36 ppm, the upfield singlet at GC +137.27 ppm is due to the ortho-D isomer 2- 110 DC6H4CH3 (with 'G = –0.090 ppm), while the 1:1:1 triplet at GC +137.325 ppm (J = 0.9

Hz) is due to PhCH2D 109 ('G = –0.035 ppm). Also shown below (Figure 5.9) for com- 13 parison is the proton-coupled C-NMR spectrum of the ipso carbon of PhCH3, acquired at 75.480 MHz. The signal is an almost perfect binomial 1:5:10:10:5:1 sextet, indicating that the ipso carbon is coupled to five approximately equivalent protons, with J = 6.4 Hz: in addition to the methyl group, both meta protons also couple166 (but not the ortho protons, due to 2 the fact that JC-H § 1 Hz).

Because deuterium splittings are only about 1/6 the value of the corresponding hydrogen splittings, no coupling between the ipso carbon and the ortho deuteron is resolved (if the deuteron was in the meta position then 3-bond carbon-deuterium splitting of the order of 1 Hz would be seen). The various carbon-hydrogen coupling constants for the aromatic parent benzene are given below in Figure 5.10.

137.35 137.30 137.25 138.4 138.0 137.6 137.2 (ppm) (ppm) (a) (b)

Figure 5.9: The ipso carbon region in (a) the 13C{1H}-NMR spectrum (at 125.758 MHz) of the 13 toluene sample, and (b) the C-NMR spectrum (at 75.480 MHz) of PhCH3 (J = 6.4 Hz), clearly showing the 1:5:10:10:5:1 sextet arising from coupling to both the methyl and meta protons. (Not to scale.)

H H H H H H H H H H H H

J § 160 Hz J § 1.1 Hz J § 7.6 Hz J § 1.2 Hz

Figure 5.10: 13C-1H coupling constants in aromatic systems (the 2-, 3- and 4-bond values taken from Ref. 166). Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 67

The Heteronuclear Single Quantum Coherence (HSQC) 2D-NMR shown in Figure 5.11 correlates the 13C satellites of the proton spectrum (horizontal axis) with the corresponding signals of the carbon spectrum (vertical axis). Because the peaks are essentially just the 13C satellites of the proton signals, they appear duplicated symmetrically about a vertical line intersecting the proton signal from which they are derived. Hence, the 1-bond 13C-1H coupling constant can be obtained directly from the observed separation between the two satellites, which in this case are ca 125 Hz for methyl C-H splittings.

(ppm)

20.9

21.0

21.1

21.2

21.3

(ppm) 2.2 2.1 2.0

Figure 5.11: The 2D-1H/13C{1H} HSQC spectrum of the toluene sample, showing the methyl region for both the 1H-NMR (horizontal axis) and 13C{1H}-NMR (vertical axis). The two most downfield peaks in the 13C{1H}-NMR spectrum are both associated with the downfield singlet in the 1H-NMR spectrum, as shown by the arrows on the diagram.

On the vertical (carbon) axis, it may be seen that both the singlet at GC +21.30 ppm and the 1:1:1 triplet at GC +21.24 ppm (corresponding to the methyl groups of both PhCH3 13 and 2-DC6H4CH3 110, respectively) are associated with C satellites which are centred on the downfield singlet at GH +2.12 ppm in the proton spectrum. As expected, the 1:1:1 triplet at GH +2.10 ppm in the proton spectrum is found to correlate only with the upfield 1:1:1 triplet at GC +21.02 ppm in the carbon spectrum, while the singlet at GH +2.12 ppm correlates with both the 1:1:1 triplet at GC +21.24 ppm and the singlet at GC +21.30 ppm. Hence, integration of the downfield proton singlet will give a combined measure of the -CH3 groups of both PhCH3 Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 68

and 2-DC6H4CH3 110, while integration of the upfield 1:1:1 triplet will give a proportional measure of the -CH2D group in PhCH2D 109 (and so will integrate for 2H instead of 3H). The same (HSQC) technique was used to obtain the aromatic region of the toluene sample shown below in Figure 5.12. As before, the vertical axis shows the 13C{1H}-NMR spectrum, while the horizontal axis shows the 1H-NMR; the signals appearing in the carbon spectrum are (from bottom to top) the ortho, meta, meta* and para/para* carbon signals (the small signal between the ortho and meta carbon signals is due to benzene).

(ppm)

125.5

126.0 CH2X

126.5

127.0

CH3 127.5 D * 128.0 * 128.5

129.0 (ppm) 7.1 7.0 6.9 6.8 6.7

(a) (b)

Figure 5.12: (a) The 2D-1H/13C{1H} HSQC spectrum of the toluene sample, showing the aromatic region for both the 1H-NMR (horizontal axis) and 13C{1H}-NMR (vertical axis); (b) The 3 toluene 109 110 isomers PhCH3 (X = H), PhCH2D (X = D), and 2-DC6H4CH3 (along with its labeling system, with the meaning of the terms ipso and para retaining their normal definitions with respect to the methyl group). The two pairs of unidentified signals in the centre of the spectrum (see box) do not originate from the toluene.

The naming system used here refers to the terms ipso, ortho, meta and para in their normal sense with respect to the methyl group. The terms ortho* and meta* are defined for

2-DC6H4CH3 110 (Figure 5.12b). Because of rapid rotation of the methyl group on the NMR timescale, an effective mirror plane exists for the isomers PhCH3 (where X = H) and PhCH2D 109 (X = D), perpendicular to the plane of the aromatic ring, in which lies the ipso-methyl C-C bond. This makes both ortho carbons equivalent, as well as both meta carbons. Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 69

1 166 In aromatic systems, JC-H is of the order of 160 Hz (see Figure 5.10), which corresponds to the main splitting separating the satellites (see Figure 5.12). The secondary 3 splittings are due to JH-H coupling, which is of the order of 6 Hz (Figure 5.13). The mul- tiplicities of secondary couplings support the assignment of a deuterium atom in the ortho position. For the satellites from the ortho carbon, the secondary splitting is into a doublet, from the single adjacent proton on the meta carbon. The meta satellites are split into a 1:2:1 triplet (from the neighbouring ortho and para protons), while the meta* satellites are split by the para proton into a doublet (the ortho proton has been replaced by a deuteron), and the para satellites are split into 1:2:1 triplets from coupling to both adjacent meta protons. Be- cause the HSQC experiment only correlates a given carbon with its bound proton, the ortho* carbon (i.e. the aromatic C-D) does not appear in Figure 5.12.

H H H H H H H H H

J § 7.7 Hz J § 1.4 Hz J § 0.6 Hz

Figure 5.13: 1H-1H coupling in aromatic systems (taken from Ref. 166).

In the carbon spectrum, both the ortho and para signals also exhibit additional 1:1:1 triplets due to the effect of the deuteron on the ortho* carbon, but they lie very close to the main signal, and so are not clearly resolved on the 2D spectrum (in the case of 2-DC6H4CH3

110: 'Go = 0 ppm (J = 0.7 Hz), 'Gm = –0.111 ppm (J = 0 Hz), and 'Gp = –0.011 ppm (J = 1.2 Hz), where 'G < 0 indicates an upfield shift). These isotopic shifts compare well with literature values.163 Figure 5.14 (next page) is just an expansion of Figure 5.12, along with an increase in signal intensity, showing the relation of the small singlet at +128.24 ppm to the corresponding singlet in the 1H-NMR at +7.08 ppm, downfield from the other aromatic signals. The respective chemical shifts, and the observed 1-bond 13C-1H coupling constant of J = 162 Hz, strongly suggests that this signal is due to a trace of benzene in the mixture. This may possibly arise from a very small proportion of phenyl cleavage in the reac- + – tion of Ph3PBn Br with LiAlD4, but no additional supporting evidence has yet been obtained

(i.e. no Ph2PBn has yet been detected as a product of the reductive cleavage). From these spectroscopic results, the following conclusions can been drawn: the toluene is a mixture of three isomers (PhCH3, PhCH2D 109 and 2-DC6H4CH3 110); and integration of the deuterium signals can give the ratio of PhCH2D 109 to 2-DC6H4CH3 110, integration of the hydrogen Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 70

methyl signals can give the ratio of combined PhCH3 and 2-DC6H4CH3 110 to PhCH2D 109, and integration of the carbon methyl signals can give the ratio of all three isomers.

(ppm)

128.0

128.1

128.2

128.3

(ppm) 7.2 7.1 7.0 6.9 6.8 6.7

Figure 5.14: An expansion of Figure 5.12 in the region of the meta carbon signal. The intensity

has been increased in order to observe the doublet centred at GH +7.07 ppm, GC +128.24 ppm in the lower left of the spectrum (J = 160 Hz; see arrows), corresponding to benzene (marked as *).

+ – 5.2 Toluene cleaved from PhnPBn4-n Br (n = 0, 1, 2, 3) with LiAlD4.

The reductive cleavage was carried out with LiAlD4 in refluxing THF for the series of + – phosphonium salts [PhnPBn4-n] Br (n = 0 to 4), following the procedure described previously in Section 3.2, and the hydrocarbon (benzene for n = 4, toluene for n < 4) recovered in each case. The distribution of deuterium in the toluene samples (between the three isomers 1 2 1 13 1 PhCH3, PhCH2D 109 and 2-DC6H4CH3 110) was analysed by H-, H{ H}- and C{ H}-NMR spectroscopy using long relaxation delays to allow reliable integration. In order to get reliable integrals, complete relaxation of the nuclei must occur. This is the situation only after the amplitude of the free induction decay (FID) of the signal has reached zero. The T1 value of a given nucleus is a measure of how fast that nucleus is observed to relax back to its magnetic ground state, and is defined167 as:

–D/T1 Mz = M0(1 – e ), Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 71 where Mz is the observed magnetisation of the spin population along the z-axis (where the signal for the FID is detected), M0 is the magnetisation of the spin population at equilibrium, and D is the delay between pulses. Setting D equal to 5uT1:

–5T1/T1 –5 Mz = M0(1 – e ) = M0(1 – e ) = 0.9933M0.

In other words, 99.33% of the sample would have reached equilibrium if the delay between each pulse (D) equals 5uT1. Both deuterium sites (ortho and alpha) were examined by 2H-NMR, while for 1H- and 13C{1H}-NMR, only the methyl region was examined because, in both cases, the methyl region was simple, and clear of signals from impurities, such as THF. The 13C{1H}-NMR spectra were run inverse-gated (i.e. the proton decoupler was only operated during the acquisition time, allowing the proton spin population to relax completely between each pulse), with a 75 1 second relaxation delay, which assumes a methyl carbon T1 of 15 seconds. Both the H- and 2H{1H}-NMR spectra were run using a relaxation delay of 20 seconds, assuming the longest

T1 to be about 4 seconds for the methyl protons. The T1 times of both the ortho and alpha deuterium atoms are expected to be quite short, due to the efficient electric quadrupolar relax- 1 1 ation mechanisms of nuclei with I > /2 (for deuterium, I = 1). This is especially true when the quadrupolar nucleus is in an unsymmetrical environment, such as in a terminal position, as is the case here, due to the resulting electric field gradient. The short T1 time, combined with the large separation in chemical shifts between the 2 different kinds of deuterium atoms present, makes integration of the 2H{1H}-NMR spectra very reliable. Mass Spectroscopy was considered unsuitable for analysis of toluene because of the +x 168 facile expulsion of a hydrogen atom from the molecular ion [C7H8] in the gas phase. Shown below in Figure 5.15 (see next page) is the process that is believed to occur in the gas phase upon ionisation of toluene. Initially, irreversible ionisation to the toluene radical cation 114 occurs, after which a dynamic equilibrium is set-up169,170 between 114 and the 1,3,5- cycloheptatrienyl radical cation 115 (possibly passing through the radical cation of methylenecyclohexa-2,4-diene 116171 or a symmetrical isomer170,172), with the equilibrium thought to favour 114.168 Either of these isomeric molecular radical cations can expel a hydrogen atom directly, giving either the benzyl cation 117 or the aromatic tropylium cation 118, which are themselves in equilibrium (with the tropylium cation 118 strongly favoured.168) Due to the dynamic nature of the 1,3,5-cycloheptatrienyl radical cation 115, in which all eight hydrogen atoms are scrambled by what are presumably 1,2-sigmatropic hydrogen shifts, the deuterium atom (wherever its initial location on the toluene molecule) can move around the ring to any position (Scheme 5.16, next page).170,173,174 This, along with the mechanism of Hx loss (which is also applicable to loss of Dx) described in Scheme 5.15, ensures that all useful information in regard to deuterium content of the toluene sample is lost. Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 72 + CH3 CH3 CH2

_ _ _ e H

114 117 ?

+ + CH2 H H H H ? _H

116 115 118

Figure 5.15: Processes toluene is believed to undergo in the Mass Spectrometer.

+ + CH3 CH2D D other other isomers isomers 120a 120b

+ H H + H H D D

119a119b

Figure 5.16: Rearrangement of the 2-DC6H4CH3 radical cation to the 1-deuterio-1,3,5-cycloheptatrienyl radical cation in the gas phase.

At any stage, isomerisation of the deuterated 1,3,5-cycloheptatrienyl radical cation 119 back to the deuterated toluene radical cation 120 can occur.168 By this process, all possible locations of the deuterium atom of each of the two isomers will be in equilibrium. If there is an energetic preference for the deuterium atom occupying one (or more) positions at the expense of other positions, because of the dynamic equilibrium, these isomers will increase in concentration relative to the less favoured isomers. Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 73

If this process tends to place the deuterium atom in the methyl group of 120 or the methylene group of 119, and loss of a chemically equivalent hydrogen is not preferred, then the probability that the deuterium atom will be expelled will increase, resulting in an under- estimation of the percentage deuteration in the toluene sample. Conversely, the probability that the deuterium atom will be expelled will decrease, resulting in an over-estimation of the percentage deuteration in the original sample; hence, Mass Spectrometry was not used to ana- lyse the toluene samples. Shown below in Tables 5A to 5C are the raw data obtained from the integration of 1H-, 2H- and 13C{1H}-NMR spectra, respectively, for the sample of toluene + – recovered from reductive cleavage of the benzylphosphonium salts [PhnPBn4-n] Br (n: 0–3). The data in each table is presented as percentage ratios for each type of spectrum (i.e. 1H-, 2H{1H}- and 13C{1H}-NMR). Because of the overlap in the 1H-NMR spectrum of the methyl peaks for both non-deuterated and ortho-deuterated toluene, the figures given in Table

5A in the column marked “PhCH3 and 2-DC6H4CH3” represent a combined integral for these two isomers. Table 5A: Integral Values of the Methyl Region from 1H-NMR. Recovered Deuterated Toluenes

Phosphonium Salt PhCH3 and 2-DC6H4CH3 PhCH2D*

+ – Ph3PBn Br 56% 44%

+ – Ph2PBn2 Br 35% 65%

+ – PhPBn3 Br 17% 83%

+ – Bn4P Br 12% 88% * The integral values for PhCH2D (2H) have been multiplied by a factor of 1.5.

Table 5B: Proportions of ortho- and alpha-Isomers from 2H-NMR. Recovered Deuterated Toluenes

Phosphonium Salt 2-DC6H4CH3 PhCH2D

+ – Ph3PBn Br 44% 56%

+ – Ph2PBn2 Br 31% 69%

+ – PhPBn3 Br 10% 90%

+ – Bn4P Br 7% 93% Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 74

Table 5C: Integral Values of the Methyl Region from 13C{1H}-NMR. Recovered Deuterated Toluenes

Phosphonium Salt PhCH3 2-DC6H4CH3 PhCH2D

+ – Ph3PBn Br 31% 31% 38%

+ – Ph2PBn2 Br 11% 27% 62%

+ – PhPBn3 Br 7% 9% 84%

+ – Bn4P Br 9% 5% 86%

1 The integration value of the -CH2D group in the H-NMR spectrum (Table 5A) has been multiplied by a factor of 1.5 so as to be proportional to 3H like the signal from the -CH3 group. The data in each table is presented as percentage ratios for each type of spectrum (i.e. 1H-, 2H{1H}-, and 13C{1H}-NMR). As the data shown in Tables 5A to 5C are derived from the same sample series, it is interesting to compare the results obtained from different nuclei, in order to see if there are any obvious discrepancies. The relative proportions for all three toluene isomers are available from the 13C{1H}-NMR data given in Table 5C, whereas Table 5B (2H{1H}-NMR) gives 1 only the proportions between 2-DC6H4CH3 110 and PhCH2D 109, and Table 5A ( H-NMR) gives the ratio between PhCH2D 109 and the combined signals for PhCH3 and 2-DC6H4CH3 110. To checkthe 2H{1H}-NMR data (Table 5B), the figures were scaled up by assuming 13 1 that the proportion of PhCH3 from Table 5C (i.e. the C{ H}-NMR data) is correct. When this was done, the relative proportions of 2-DC6H4CH3 110 and PhCH2D 109 derived from Table 5B were found not to deviate with those derived from Table 5C by more than 1% (see Table 5D below). The 1H-NMR data given in Table 5A gives the relative proportion of 13 1 PhCH2D 109 directly, and comparison with the figures from Table 5C ( C{ H}-NMR) shows a fairly good fit, with the observed deviation being no more than 5%. The most likely reason for the relatively poor fit of the 1H-NMR data is the partial overlap of the two methyl signals ('G = 0.06 ppm; Figure 5.7), preventing an exact integral from being taken. The large separation in the 2H{1H}-NMR spectrum ('G = 4.78 ppm; see Figure 5.6) allows the integrals of each signal to be taken without interference from the other signal, making the obtained proportions more reliable. The excellent fit between the data derived from 2H{1H}-NMR (Table 5B) and 13C{1H}-NMR (Table 5C) suggests that the 13C- {1H}-NMR data accurately reflects the actual proportions of the three toluene isomers (i.e.,

PhCH3, PhCH2D 109, and 2-DC6H4CH3 110). Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 75

Table 5D: Relative Proportions of the Three Toluene Isomers. % Recovered Toluene Isomers

Phosphonium PhCH3 2-DC6H4CH3 PhCH2D Salt 13C 2H* 13C 1H 2H* 13C

+ – Ph3PBn Br 31% 30% 31% 44% 39% 38%

+ – Ph2PBn2 Br 11% 28% 27% 65% 61% 62%

+ – PhPBn3 Br 7% 9% 9% 83% 84% 84%

+ – Bn4P Br 9% 6% 5% 88% 85% 86%

* Calculated as described above.

+ – 5.3 Benzene cleaved from Ph4P Br with LiAlD4.

+ – + – In addition to [PhnPBn4-n] Br (n = 0 to 3), Ph4P Br was also reacted with LiAlD4 under identical conditions. The products from this reaction were Ph3P and benzene (which was recovered as a mixture of PhH and PhD, as shown in Figure 5.17). Because the deuterium atom in PhD is bound to an aromatic carbon atom, there can be no protons also bound to this carbon, which therefore gives a very low-intensity signal in the 13C{1H}-NMR spectrum (refer to Figure 5.8a in Section 5.1, which shows the ortho carbon directly bound to the aromatic deuteron as a low-intensity signal at GC +128.61 ppm, J = 23.9 Hz).

(a) (b) Figure 5.17: The two benzene isomers: (a) benzene (PhH), and (b) deuteriobenzene, (PhD).

The 1H-NMR spectrum shows only aromatic hydrogen signals, giving no clear spectral separation between the two isomers, while the 2H{1H}-NMR spectrum is unable to compare between the two isomers, as only PhD is detected. This makes NMR spectroscopy (1H-, 2H{1H}-, and 13C{1H}-NMR) unsuitable for determination of the relative proportions of the two isomers, and so another technique was required. In contrast to toluene, the benzene seemed suited for Electron Impact Mass Spectros- +x +x copy (EI-MS), as the molecular ions [C6H6] and [C6H5D] are reasonably stable towards Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 76 fragmentation, thus allowing precise determination of their respective proportions. A 15 eV ionising potential was used, giving a molecular ion of relatively low internal energy, preventing fragmentation. This gave clean spectra, in which peaks due to the molecular ion were the only observed signals for both the PhH standard and the PhH/PhD mixture (added as a solution in THF). For the purposes of the following discussion, the term “normal,” as applied to either benzene or toluene, will be used to refer to the isomer containing only 12C and 1H, whereas “natural” will refer to a mixture of isomers containing the proportions of 13C and 2H that would be predicted from their natural abundance (i.e. natural abundance of 13C = 1.10%, while that of 2H is 0.015%1). For example, commercial toluene consists of a “natural” mixture of isomers, and can therefore be referred to as “natural” toluene, whereas “normal” benzene refers to a particular isomer (i.e. hexa-carbon-12-hexa-hydrogen-1-benzene). +x 12 1 The molecular ion [M] for “normal” benzene (i.e. C6H6 containing only C and H) is observed at 78 amu (atomic mass units), with the observed mass for isomers containing either 13C or 2H increasing by 1 amu for each substitution. Given that the natural abundances of 13C and 2H are 1.10% and 0.015% respectively1, and assuming that the natural abundances of the radioactive isotopes 14C and 3H are both so low as to be effectively 0%, the abundances of the common isotopes 12C and 1H are therefore 98.90% and 99.985%, respectively. Hence, the pattern expected for “natural” benzene (i.e. benzene with a distribution of carbon and hydrogen isotopes matching the natural abundance of those isotopes) can be calculated. The difference between the calculated mass spectrum and that observed for the benzene obtained + – from cleavage of Ph4P Br with LiAlD4 enables the determination of the relative proportion of the isomers PhH and PhD. As “normal” benzene contains six 12C nuclei and six 1H nuclei, the relative “natural” fraction f0 of this isomer is:

6 6 f 0 09890 u 099985 = 93.52% § 93.5%, the fraction with one 13C nucleus is:

6f0 0.0110 f = = 6.241% 6.2%, C 0.9890 § whereas the fraction with one 2H nucleus is: 6f 0.00015 f = 0 = 0.08418% § 0.1%. D 0.99985 The proportion containing two or more 2H nuclei is negligible, as is the fraction containing one 2H nucleus and one 13C nucleus. However, the fraction with two 13C nuclei is: 2 30f0 0.0110 f = = 0.3471% § 0.3% 2C 0.98902 The proportions of isomers containing more than two 13C nuclei are vanishingly small, and will not be considered. The sum of the calculated percentages is: Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 77

6 = 93.5 + 6.2 + 0.1 + 0.3 = 100.1% § 100%.

The figure for “normal” benzene, as represented by f0 = 93.5%, will be the sole contributor to the peak at 78 amu, whereas the peak at 79 amu will be the sum of fC and fD (6.3%).

Lastly, a large proportion of the peak at 80 amu will be made up from f2C (0.3%). Scaling these figures so that the peak at 78 amu reads as 100.0%, the others then become 6.7% (at 79 amu) and 0.3% (at 80 amu). Table 5E shows the results obtained for benzene (AR grade, representing “natural” benzene) and the mixture of benzenes PhH and PhD obtained from the reductive cleavage of + – Ph4P Br with LiAlD4. There is a good correspondence between the observed and calculated mass spectrum of “normal” benzene, suggesting that the figures can be taken to have a precision of ± 1%. Because the peak at 78 amu represents only the proportion of “normal” benzene, this fraction can be calculated to be 69% ± 1% (data from “Benzene Mixture” row in Table 5E), making the percentage proportion of PhD in the mixture 31% ± 1%.

Table 5E: EI Mass Spectrum of Benzene Sample.* Mass of Molecular Ion [M]+x (amu) Sample 78 79 80 %PhD

PhH 100.0% 6.2% 0.0% 0%

Calc. for 100.0% 6.7% 0.3% 0% PhH† Benzene 100.0% 40.2% 5.1% 31%‡ Mixture * Percentages are calculated so that the largest signal in each run = 100.0%. † Figures expected for “natural” benzene. ‡ Estimated error ± 1%.

5.4 Comparison of the percentage deuteration in the benzene and toluenes.

The data obtained for the toluenes and benzene recovered from the reductive cleavage + – of the phenylbenzylphosphonium salts [PhnPBn4-n] Br (n = 0 to 4) with LiAlD4 in refluxing THF, from both NMR and EI-MS (given separately in Tables 5A to 5C and Table 5E) is presented below in Table 5F. It is immediately apparent that the relative proportion of non- deuterated hydrocarbon recovered from the cleavage decreases dramatically with an increase in the number of benzyl groups from 0 to 4. As means of a comparison between phosphorus and nitrogen, the same procedure was carried out using N-benzyl-N,N-dimethylanilinium bromide 121. There is literature precedence for the removal of methyl groups from quaternary 175-177 methylammonium salts using LiAlH4, as well as of the reductive cleavage of benzylic Chapter 5: Incorporation of Deuterium into Benzene and Toluene cleaved using LiAlD4. 78

178 groups in the presence of methyl groups using NaBH3CN in HMPA. The toluene recovered from the reaction between LiAlD4 and 121 was found to be exclusively alpha-deu- 13 1 terated (i.e. PhCH2D 109) by C{ H}-NMR, with PhNMe2 122 isolated from the reaction (Scheme 5.18).

In addition, for a control experiment, “natural” toluene was refluxed with LiAlD4 in THF for 4 hours, with no incorporation of deuterium. What this shows is that the deuterated isomers PhCH2D 109 and 2-DC6H4CH3 110 do not originate from exchange with LiAlD4.

Table 5F: Proportion of Deuterated Hydrocarbons Recovered from

LiAlD4 Reductive Cleavage of Phosphonium Salts in Refluxing THF. Phosphonium Level of Deuteration of the Hydrocarbon Salt Used non-deuterated mono-deuterated Benzene Isomer: PhH PhD + – * Ph4P Br 69% 31%

Toluene Isomer: PhCH3 2-DC6H4CH3 PhCH2D

+ – † Ph3PBn Br 31% 31% 38%

+ – † Ph2PBn2 Br 11% 27% 62%

+ – † PhPBn3 Br 7% 9% 84%

+ – † Bn4P Br 9% 5% 86%

* Benzene, as PhH + PhD; percentages determined from EI-MS; the error is estimated to be ± 1%. † Toluene percentages determined from 13C{1H}-NMR; the error is estimated to be ± 1%.

_ Br Bn CH2D + LiAlD N 4 N Ph Me + Me Me Ph Me

121 122 109

121 109 Scheme 5.18: The reductive cleavage of gives PhCH2D as the only toluene isomer. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride.

The spectroscopic characterisation of the three isomeric toluenes (PhCH3, PhCH2D

109, and 2-DC6H4CH3 110) and two isomeric benzenes (PhH and PhD) which were presented in Chapter 5 are consistent with the formation of an intermediate phosphorane (Scheme 6.1) + – in the reaction of LiAlD4 with the phosphonium salt R3PR' Br (where R, R' = Ph or Bn).

R' D R + LiAlD4 – R'D P P P R R R R R R R R R'

+ Scheme 6.1: The formation of an intermediate phosphorane in the LiAlD4 reduction of R3PR' .

The isolation of the spirocyclic phosphorane 123 by Hellwinkel146 in 1966 from the reaction of the corresponding spirocyclic phosphonium salt 124 with LiAlH4 gives strong support to the mechanism proposed in Chapter 5 (Scheme 6.2). The observed phosphorus chemical shift (GP –112 ppm) agrees well with other organophosphoranes (e.g. Ph5P is ob- 129a served at GP –89 ppm ).

H + LiAlH4 P P

_ I

124 123

Scheme 6.2: The formation of Hellwinkel’s spirocyclic phosphorane 123.

100 Although previous attempts to prepare the tetraphenyl analogue Ph4PH had failed, + – 31 1 it was decided to observe the reaction between Ph4P Br and LiAlD4 by P{ H}-NMR + – spectroscopy. The main reason for choosing Ph4P Br was that phenyl is known to be a poorer leaving group than benzyl,69 thereby increasing the likelihood that the intermediate phosphorane will be observable directly. In addition, deuterium (as LiAlD4) was chosen so Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 80 as to enable the use of the proton decoupler while at the same time allowing phosphorus- deuterium coupling information to be obtained. Deuterium (2H) has a nuclear spin of I = 1,1 and will therefore give characteristic multiplets in the 31P{1H}-NMR spectrum if there is any phosphorus-deuterium coupling occurring. For instance, the formation of the desired Ph4PD would be expected to give a 1:1:1 31 1 triplet in the P{ H}-NMR spectrum at about GP –100 ppm, about where the spirocyclic phosphorane 123 is observed146 (Figure 6.3). In the same way, coupling to two equivalent deuterium atoms would give a 1:2:3:2:1 quintet, whereas a 1:3:6:7:6:3:1 septet would be observed for three equivalent deuterium atoms.

(a) (b) (c)

Figure 6.3: The expected multiplets for coupling to (a) one, (b) two equivalent, and (c) three equivalent deuterium atoms.

Section 6.1 describes the experiments undertaken in an attempt to observe directly + – the intermediate phosphoranes postulated for the reaction between LiAlD4 and Ph4P Br . In 31 1 addition to the sought after Ph4PD, two other species were observed by P{ H}-NMR, as- – signed as the phosphorane Ph3PD2 and the hexa-coordinated anionic phosphoranate Ph4PD2 . In the remainder of the chapter, a discussion of these compounds and their hydro-derivatives – (Ph4PH, Ph4PH2, and Ph4PH2 ) is given, as well as the results of further reactions using either

LiAlH4 or LiAlD4 with other mono-phosphonium salts.

+ – 6.1 The reaction of Ph4P Br with LiAlD4 in THF at RT.

To decrease the chance of thermal decomposition of any phosphoranes formed, the + – reaction was carried out at RT by adding an excess of LiAlD4 to a suspension of Ph4P Br in dry THF. Examination of the solution by 31P{1H}-NMR immediately after addition of the 1 deuteride revealed a 1:1:1 triplet at GP –87.3 ppm, with an observed splitting of JP-D = 49 Hz. In addition, three other signals were observed (Figure 6.4): a singlet at GP –4.4 ppm, 1 corresponding to Ph3P, and two 1:2:3:2:1 quintets at GP –70.0 ppm ( JP-D = 51 Hz) and GP 1 –187.2 ppm ( JP-D = 69 Hz). The magnitude of the observed splittings were consistent with Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 81 directly bound deuterium atoms, with the quintet structure of the multiplets indicating that there were two equivalent deuterium atoms in each case (compare with Figure 6.3).

-68 -70 -72 -86 -88 -90 -185 -187 -189 (ppm) (ppm) (ppm)

– (a) Ph3PD2 (GP –70.0 ppm) (b) Ph4PD (GP –87.3 ppm) (c) Ph4PD2 (GP –187.2 ppm)

31 1 + – Figure 6.4: The P{ H}-NMR spectrum of the reaction between Ph4P Br and LiAlD4 in THF.

The triplet at GP –87.3 ppm was assigned to the sought after phosphorane Ph4PD, and 129a corresponds well to the chemical shifts of the two similar compounds Ph5P and the spirocyclic phosphorane 123146 (the observed phosphorus-hydrogen coupling constant for 123 is 1 1 1 JP-H = 482 Hz). Because the nuclear gyromagnetic ratio of H is about 6.5 times that of 2 1 H, the predicted JP-D coupling constant for Hellwinkel’s phosphorane 123 is 74 Hz, which is of a comparable magnitude to 49 Hz, observed for Ph4PD. One of the quintets resonated in this region at GP –70.0 ppm, and on this basis was assigned the structure of Ph3PD2. The second quintet, which resonated 100 ppm upfield of these signals at GP –187.2 ppm, was presumed not to be another phosphorane, but because of its similarity in chemical shift to the 148 anion tris(2,2'-biphenyl)phosphoranate 125 (GP –181 ppm), it was assigned the structure of – Ph4PD2 . In each case, no splitting of the lines was observed, as would be expected to occur if geometric isomers were present. Hence, each species either consists of only one isomer, or that the isomers present interconvert rapidly on the NMR timescale, probably through DLE (dynamic ligand exchange), as discussed in Chapter 4. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 82

125

Repeating the reaction using LiAlH4 instead of LiAlD4 gave the corresponding hydro- 1 1 species Ph4PH (GP –86.6 ppm, JP-H = 323 Hz), Ph3PH2 (GP –68.4 ppm, JP-H = 334 Hz), and – 1 31 Ph4PH2 (GP –185.6 ppm, JP-H = 447 Hz), with the P-NMR spectrum showing a doublet for – 31 1 Ph4PH and 1:2:1 triplets for both Ph3PH2 and Ph4PH2 . A series of P{ H}-NMR spectra + – was acquired of the reaction between Ph4P Br and LiAlH4 in a timed experiment, over the course of several hours. Presented below is a stacked plot of the results (Figure 6.5).

60 20 -20 -60 -100 -140 -180 -220 (ppm)

31 1 + – Figure 6.5: P{ H}-NMR stacked plot of the reaction between Ph4P Br and LiAlH4 in THF at RT;

the signals are (from left): Bn3P=O at GP +45 ppm (external: a capillary of ethanol solution), Ph3P – at GP –4 ppm, Ph3PH2 at GP –68 ppm, Ph4PH at GP –86 ppm and Ph4PH2 at GP –186 ppm. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 83

From the above 3-dimensional stacked plot, it is immediately apparent that the signal intensities (and hence the concentrations) of both Ph3P and Ph3PH2 steadily increase over the – course of the reaction, while that of Ph4PH2 increases initially before declining over time.

The signal due to Ph4PH remains almost constant. The four species can be viewed indi- vidually along the time axis (Figure 6.6), showing the trends in concentration more clearly, from which it becomes apparent that the concentration of Ph4PH rises initially before slowly decreasing over time.

δ δ (a) Ph3 P at P – 4 ppm. (b) Ph3 PH2 at P – 68 ppm.

δ – δ (c) Ph4P PH at – 86 ppm. (d) Ph4 PH2 at P –186 ppm.

Figure 6.6: The trend observed by 31P{1H}-NMR of product peak intensities over time for the RT + – reaction between Ph4P Br and LiAlH4 in THF (taken from the data shown in Figure 6.5).

– The curves defined by the peak heights for both Ph3PH2 and Ph4PH2 can be modeled reasonably well by exponential functions, from which the rates of formation or decomposition, respectively, can be determined (Figures 6.7 and 6.8). The equations take the form:179

–k.t – h = a.e + b for exponential decay (of Ph4PH2 ), and –k.t h = a(1–e ) + b for exponential growth (of Ph3PH2), where h is the peak height in mm as measured from Figure 6.6, t is the elapsed time (in units of 980 s) from the first spectrum, a and b are in units of mm, and k is in units of s–1. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 84

Peak Height (mm) Height Peak 15

0 01234567 Elapsed Time (in units of 980 s)

Figure 6.7: The observed build up of signal intensity due to the formation of Ph3PH2 can be fitted to an exponential growth function h = a(1–e–k.t)+b, where h is the peak height in mm, t is the elapsed time in units of 980 seconds, with t = 0 for the first spectrum: a = 53.0 ± 1.1 mm, b = 3.8 ± 0.5 mm, and k = 0.279 ± 0.014 ms–1 (ms = milliseconds), with r2 = 0.99935 and SSE = 1.14537.

30 25

Peak Height (mm) Height Peak 15

10 5

1234567 E lapsed Time (in units of 980 s)

– Figure 6.8: The observed decay in signal intensity due to the decomposition of Ph4PH2 into – –k.t Ph3PH2 and Ph can be fitted to an exponential decay function h = a.e +b, where h is the peak height in mm, t is the elapsed time in units of 980 seconds, with t = 0 for the first spectrum: a = 53.3 ± 3.2 mm, b = 4.9 ± 4.7 mm, k = 0.290 ± 0.071 ms–1 (ms = milliseconds). In addition, r2 = 0.98849 and SSE = 9.82981 (as the decay begins at spectrum #2, the first spectral point has been omitted from the calculations). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 85

The parameters a, b, and k obtained from the graphs shown above (Figures 6.7 and – 6.8) relate to different aspects of the dynamic behaviour of Ph4PH2 and Ph3PH2, respectively. Parameter b represents the baseline concentration for each species, i.e. the concentration of – Ph3PH2 at t = 0 and the concentration of Ph4PH2 at t = (i.e., t is much greater than for # – spectrum 8). Because of its observed decay, b(Ph4PH2 ) § 0 (i.e., 4.9 ± 4.7), whereas b(Ph3PH2) = 3.8 ± 0.5 mm. The non-zero value of b(Ph3PH2) simply indicates that there was some Ph3PH2 present in the reaction mixture at t = 0 (which was defined as the start of the acquisition of spectrum #1, even though the reaction was mixed before the sample could be placed into the spectrometer, allowing the formation of a small amount of Ph3PH2 to occur before t = 0). The sum of a and b represents the maximum concentration of each species – (i.e., the concentration of Ph3PH2 at t = , and of Ph4PH2 at t = 0) in terms of the measured peak height (in mm). Parameter k is the rate constant for the reaction: the observed values of –1 k were found to be equal to within experimental error (i.e. k(Ph3PH2) = 0.279 ± 0.014 ms – –1 and k(Ph4PH2 ) = 0.290 ± 0.071 ms ; where ms = milliseconds), supporting the proposal that – the rate of formation of Ph3PH2 is directly related to the rate of decomposition of Ph4PH2 – (i.e. Ph4PH2 decomposes into Ph3PH2 through a unimolecular mechanism by expulsion of a phenyl anion). For the unimolecular decomposition of a species A, the half-life is independent of the concentration of A, and is given by:179 ln2 1 t2 = k where ln 2 = 0.693, and from above, k = 0.279 ± 0.014 ms–1, so therefore the half-life for the – unimolecular decomposition of Ph4PH2 , i.e.

– k – 4 1 Ph4PH2 o Ph3PH2 + Ph t2 = (4.95 ± 0.25) u 10 s.

The half-life is 49,500 ± 2,500 s, or 13 hours and 45 minutes (± about 45 minutes). – Direct comparisons between the a values for Ph3PH2 and Ph4PH2 are only valid if the

T1 values for each species are fairly similar, as the spectra from which the timed data described above are derived were acquired with a short relaxation delay (0.6 s). This means that the observed signal intensity will be lower than if all the nuclei were allowed to relax com- 167 pletely (i.e. with a relaxation delay 5uT1). The species with the longest T1 values will suffer the largest truncation of signal intensity (this topic was dealt with in Section 5.2). Additionally, the spectra were acquired with proton decoupling, thus enabling an enhancement of signal intensity due to cross polarisation from the proton spin population. This effect is related to the Nuclear Overhauser Effect (NOE), which for each proton diminishes with increasing distance from the phosphorus nucleus.167 The observed enhancement of sig- – nal intensity would be expected to be similar for Ph3PH2 and Ph4PH2 as they both have two protons directly bonded to phosphorus, with the phenyl protons having a comparatively minor effect due to their relatively greater distance. However, both would be expected to experi- Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 86

ence a much larger signal intensity enhancement than would be expected for Ph3P, which only contains the relatively distant phenyl protons.

The a value for Ph3PH2 is found to be 53.0 ± 1.1 mm, which is identical to within – experimental error to the value found for Ph4PH2 , which is 53.3 ± 3.2 mm (Figures 6.7 and 6.8). The corresponding b value is 4.9 ± 4.7 mm, which is rather vague with the large error, but seems to be consistent with the expected value of b = 0 at t = (i.e. all of the phos- – phoranate Ph4PH2 should have decayed by t = ). Shown below in Figure 6.9 is a graph of – the sum of the peak heights (in mm) for Ph3PH2 and Ph4PH2 plotted against the elapsed time (in units of 980 s). There is an initial sharp rise in total peak height between spectrum #1 (t = 0 s) and spectrum #2 (t = 980 s), followed by an approximately horizontal relationship, defined by:

– h(Ph3PH2) + h(Ph4PH2 ) § constant,

– which can be derived as follows, assuming that k(Ph3PH2) = k(Ph4PH2 ):

– –k.t –k.t h(Ph3PH2) + h(Ph4PH2 ) = a.e + b + a(1 – e ) + b = a.e–k.t + b + a – a.e–k.t + b = a + 2b.

40 T otal P eak H eight (mm)

30 01234567 E lapsed Time (in units of 980 s)

– Figure 6.9: A plot of the sum of the peak heights (in mm) of Ph3PH2 and Ph4PH2 versus the elapsed time (in units of 980 seconds).

– Because both a and b are constants, h(Ph3PH2) + h(Ph4PH2 ) will also have a constant value, which indicates that the reaction had reached a state of pseudo-equilibrium by the time Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 87 spectrum #2 was acquired at t = 980 s. This justifies the omission of the datum from spec- # – trum 1 in the calculation of the rate of decay of Ph4PH2 into Ph3PH2 (Figure 6.8). + – From the data described above, an overall scheme for the reaction between Ph4P Br and LiAlH4 in THF is proposed below in Scheme 6.10. The main features of the scheme are as follows: the initial step involves the transfer of a hydride anion from dissolved LiAlH4 to + the phosphonium cation Ph4P , which is present to a small extent in solution, forming the phosphorane Ph4PH (with rate constant k1). In the next step, Ph4PH accepts another hydride – from dissolved LiAlH4 with a rate constant of k2, giving the phosphoranate anion Ph4PH2 . The phosphoranate anion then undergoes the unimolecular decomposition as just described, giving the second phosphorane Ph3PH2, where the observed rate constant is k3 = 0.279 ± 0.014 ms–1 (see the discussion following Figure 6.8).

+ Ph3P + H Ph4PH

(iv) (k4) (i) LiAlH4 (k1)

(ii) LiAlH4 (k2) + Ph4P

– Ph3PH2 Ph4PH2

Ph– (iii)

(k3)

Scheme 6.10: The pathway proposed to explain the appearance of the various species observed in 31 1 + – the P{ H}-NMR spectrum of the reaction between Ph4P Br and LiAlH4.

+ Finally, Ph3PH2 can react with another equivalent of dissolved Ph4P (with rate constant k4) to produce the ultimate product of the reaction, Ph3P (with the phosphonium cation accepting a hydride ion to form Ph4PH; the second hydrogen atom from Ph3PH2 presumably + comes off as H , which, besides reaction with LiAlH4, can be quenched by either Ph4PH, – – Ph4PH2 , or Ph , as well as catalysing the decomposition of Ph3PH2 into Ph3P and H2 (Scheme 6.11, next page). + + The reaction between Ph3PH2 and Ph4P to give Ph4PH and Ph3PH (Scheme 6.10, step iv) is a specific example of a more general hydride transfer reaction between hydrophosphoranes (R4PH) and phosphonium cations, which normally would establish an equilibrium between the four possible species (Scheme 6.12, next page). Because Ph3PH2 possesses two + hydrogen atoms, the phosphonium cation produced (Ph3PH ) has an acidic hydrogen, which then reacts with any of the several types of hydridic hydrogen available, producing H2, and Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 88 thereby pushing the equilibrium completely over to the right. A parallel reaction is that be- – tween a hydrophosphoranate anion R5PH and a phosphonium cation, giving two phosphoranes. Because of the apparent stability observed for Ph4PH and Ph3PH2 at RT in the absence of Lewis acids, the equilibrium for this second reaction would be expected to lie well over to the right (away from the phosphoranate).

+ – H2 + (i) H + Ph4PH o Ph4P

+ – H2 + + (ii) H + Ph3PH2 o Ph3PH o Ph3P + H

+ – – H2 (iii) H + Ph4PH2 o Ph4PH

(iv) H+ + Ph– o PhH

+ + Scheme 6.11: The proposed fate of the H produced from the reaction of Ph3PH2 with Ph4P (other than reaction with aluminium hydride species).

+ + + (i) R4PH + R'4P R4P + R'4PH

– + (ii) R5PH + R'4P R5P+ R'4PH

Scheme 6.12: The proposed equilibria set up between phosphonium cations and either hydrophosphoranes or hydrophosphoranates, where R = alkyl, aryl, H.

+ – To test this equilibrium, the following procedures were carried out. When Ph4P Br was added slowly to a large excess of LiAlH4 in THF at RT, examination of the solution by 31 1 – P{ H}-NMR showed that Ph4PH2 was the only product. When left to itself, this species was observed to decay into Ph3PH2 (with very slow formation of Ph3P), as described above. + – However, if additional Ph4P Br is added to the solution, immediately the signal corresponding to Ph4PH is produced. Ph4PH cannot be formed by the unimolecular decomposition of – Ph4PH2 , but must instead be formed from the reversal of the established equilibrium between + – Ph4P Br and LiAlH4.

_ Ph H H _ H Ph Ph + LiAlH4 LiAlH4 Ph Ph – Ph P + P Ph + P P Ph Ph H Ph P Ph Ph Ph Ph Ph 4 Ph Ph H H

+ – Scheme 6.13: There exists a dynamic equilibrium between Ph4P , Ph4PH and Ph4PH2 , which can + + be driven forward by addition of LiAlH4, or reversed by addition of Ph4P (or H ).

+ – A further test of the equilibrium between Ph4P and Ph4PH2 (shown above in Scheme + – 6.13) is the slow addition of LiAlH4 to a suspension of an excess of Ph4P Br in THF. The Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 89

31 1 equilibrium predicts that only Ph4PH should be formed, this being confirmed using P{ H}-

NMR, which also showed Ph4PH to be apparently indefinitely stable at RT. Subsequent ad- – dition of LiAlH4 to this solution produced Ph4PH2 (along with Ph3PH2 and Ph3P arising from the unimolecular decomposition of the phosphoranate anion), consistent with the equilibrium proposed in Scheme 6.12 (see previous page). + – Now that the mechanism for the reaction between Ph4P Br and LiAlH4 has been deduced, an explanation for the origin of both PhH and PhD in the benzene isolated from the + – reaction of Ph4P Br with LiAlD4 in refluxing THF can be proposed. An additional factor to consider is the temperature under which the reaction occurs: the temperature of refluxing THF is 67˚C, while RT can be assumed to be 27˚C (or 300K; the temperature at which the 31 1 P{ H}-NMR spectra were acquired), making 'T = 40˚C. At this higher temperature, Ph4PD and Ph3PD2 are no longer found to be thermally stable, and both can decompose into Ph3P 121,126 through elimination either of PhD or D2, respectively (Scheme 6.14). Hence, this represents one source of the PhD detected in the recovered benzene.

+ Ph3P + D Ph3P + PhD Ph4PD

(k5) (k4) LiAlD4 (k1)

(k6) Ph3P + D2 LiAlD4 (k2) + Ph4P

– Ph3PD2 Ph4PD2

Ph–

(k3)

+ – Scheme 6.14: The proposed pathway for the reaction between Ph4P Br and LiAlD4 in THF at re-

flux: the two thermally induced steps are the reductive-elimination of either PhD from Ph4PD (k5),

or D2 from Ph3PD2 (k6), to give Ph3P.

The mechanism of the elimination of XY from R3P(X)Y giving R3P is believed to occur through a concerted process.121 The concerted reductive-elimination of XY from a trigonal bipyramidal phosphorane R3P(X)Y is symmetry allowed when X and Y are either both axial or both equatorial, but is symmetry forbidden when one is axial and the other equatorial.121 Clearly, two axial groups will be much further apart than two equatorial groups, thereby making the diequatorial elimination appear more likely (the pentahydrides PH5, AsH5 and

SbH5 are all predicted to eliminate H2 through an equatorial-equatorial departure of the two H Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 90 atoms126). Similar reactions with silicon reagents also indicate the formation of penta-coor- 180 dinated intermediates in the addition of hydride to silanes. For instance, when SiH4 reacts – – – – with H in the gas phase, SiH3 is detected. Repeating the reaction using D gives both H – – – and SiH2D in addition to SiH3 (SiH3 is the only ionic product expected if the reaction in- 180 – volves a simple deprotonation). The formation of SiH2D can be explained by assuming – – that the initial step is the nucleophilic addition of D to SiH4, forming SiH4D , which presumably exists as a trigonal bipyramidal ion. DLE processes could then scramble the D atom between the axial and equatorial positions, after which reductive elimination of H2 will give – – the observed SiH2D (analogous elimination of HD will give SiH3 , also observed). However, direct diaxial elimination cannot be ruled out if the phosphorane is allowed to distort towards a square pyramidal geometry (Scheme 6.15). If the BPR mechanism is assumed to operate, both situations are equivalent in the sense that the two trigonal bipyramidal geometric isomers (in which X and Y are either both axial or both equatorial) are interconvertible via a single BPR process.

X R DLE R X R P R P R Y Y R

R R X X RP R P Y Y R R Square Pyramidal R P R R + XY

Scheme 6.15: The concerted reductive-elimination of XY from R3P(X)Y to give R3P can occur in two ways: (i) X and Y are both axial, and bend towards each other, passing through a square pyramidal TS structure, where X and Y both become basal (and diagonally opposite), or (ii) X and Y

are both equatorial, and bend towards each other, passing through a pseudo-C2v structure.

Another source of PhD is the quenching of the expelled phenyl anion by the D+ pro- + + duced from the reaction between Ph3PD2 and Ph4P (giving Ph3PD and Ph4PD). The observed PhH can arise from the analogous quenching of phenyl anion by a source of H+, either upon work-up of the mixture, or by deprotonation of the solvent.181 The fact that the ob- Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 91 served proportion of PhD in the recovered benzene is only 31% (Section 5.3) suggests that direct reductive elimination of PhD (k5) is at most a minor pathway, with the majority of the intermediate Ph4PD accepting a second deuteride ion from LiAlD4 (k2) to give the phos- – phoranate anion Ph4PD2 (which then decomposes into Ph3PD2 by expulsion of phenyl anion, giving Ph3PD2). Presumably, phenyl anion is rapidly quenched at 67˚C by deprotonation of 181 THF, which would then be expected to lose ethylene giving the enolate anion of CH3CHO (as shown below in Scheme 6.16).

_ O O H _ – PhH H Ph – H2C=CH2

Scheme 6.16: The proposed quenching of phenyl anion by an THF solvent molecule.

+ – 6.2 The reaction of PhnPBn4-n Br (n = 0, 1, 2, 3) with LiAlD4 in THF at RT.

The origin of the three toluene isomers PhCH3, PhCH2D 109, and 2-DC6H4CH3 110, + – recovered from the reductive cleavage of [PhnPBn4-n] Br (where n = 0 to 3) with LiAlD4 in + – refluxing THF (as described in Chapter 5), will be discussed using Ph3PBn Br as an ex- + – ample (i.e., n = 3). When LiAlD4 was added to a suspension of Ph3PBn Br in anhydrous THF at RT, two signals were detected in the 31P{1H}-NMR spectrum: a singlet corresponding to Ph3P at GP –4.4 ppm, and a 1:2:3:2:1 quintet corresponding to Ph3PD2 at GP –70.0 ppm + – (J = 51 Hz). Assuming that the reaction of Ph3PBn Br with LiAlD4 parallels the reaction + – deduced for Ph4P Br with LiAlD4 (Scheme 6.14), a similar reaction scheme can be con- structed (Scheme 6.17).

+ Ph3P + D Ph3P + BnD* Ph3P(Bn)D 126

(k5) (k4) LiAlD4 (k1)

(k6) Ph3P + D2 LiAlD4 (k2) + Ph3PBn

– Ph3PD2 Ph3P(Bn)D2 127 Bn–

(k3)

+ – * Scheme 6.17: The proposed mechanism for the reaction of Ph3PBn Br with LiAlD4 ( BnD is a 109 110 mixture of PhCH2D and 2-DC6H4CH3 ). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 92

+ – + As in the reaction of Ph4P Br with LiAlD4, the scheme for the reaction of Ph3PBn – Br with LiAlD4 predicts the existence of six rate constants k1 to k6. Of these, only k6 is + – + – identical for both Ph4P Br and Ph3PBn Br because it defines the rate for the reductive- elimination of D2 from Ph3PD2, and hence is independent of whether the previously expelled group was Ph or Bn. The rate constant k1 for the initial step (i.e. formation of Ph3P(Bn)D 126) will be proportional to both the electrophilicity and concentration (i.e. the solubility) of the phosphonium cation, and inversely proportional to its steric hindrance. As the solubility of both salts is very low, the concentrations may be taken as being similar (i.e. both are approximately zero), and so this factor may be ignored. Substituting a benzyl group for one of + – the phenyl groups in Ph4P Br should increase steric hindrance around the phosphorus atom, and would also be expected to decrease the electrophilicity of the phosphonium cation; hence, k1[R = Bn] < k1[R = Ph]; using similar arguments, it is expected that k2[R = Bn] < k2[R = Ph]. – The rate constant k3 represents the unimolecular expulsion of a carbanion R from the – – phosphoranate anion (either Ph4PD2 or Ph3P(Bn)D2 127). Because the benzyl anion, unlike the phenyl anion, is able to delocalise its negative charge through resonance (see Figure 2.7, page 22), it functions as a far better leaving group than phenyl in this reaction, thus making + – k3[R = Bn] > k3[R = Ph], and, as in the case of phenyl cleavage from Ph4P Br , this step is the source of the non-deuterated hydrocarbon fraction (PhCH3), characterised in the toluene + – recovered from the cleavage reaction between Ph3PBn Br and LiAlD4 in refluxing THF.

CH2D CH3 D

107 D Ph 109 110 P Ph D Ph Ph Ph 126 P P Ph Ph Ph Ph

126 Scheme 6.18: The alternative mechanism for the decomposition of Ph3P(Bn)D via a cyclic

transition state (the phosphine fragment Ph3P rapidly relaxes from a T-shaped excited state to the ground state pyramidal geometry).

The factors affecting the k4 are the same as those listed for k1: the magnitude of k4 is expected to be proportional to both the concentration and electrophilicity of the phosphonium cation, but inversely proportional to the steric hindrance around the phosphorus atom, making k4[R = Bn] < k4[R = Ph]. The rate constant labeled k5 is actually composed of two rate constants, k5a and k5b, as there are two mechanisms believed to be operating in this step. The Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 93

first mechanism, the rate of which is defined by k5a, is simply the reductive-elimination of

PhCH2D 109 from Ph3P(Bn)D 126 in the same way as described above (Scheme 6.15) for the reductive-elimination of XY from R3P(X)Y. The second mechanism, corresponding to k5b, does not occur in Ph4PD, and involves a 5-membered cyclic TS structure. The deuterium atom is transferred from phosphorus to the ortho carbon of the benzyl group, with simultaneous movement of the S-electrons out of the aromatic ring into the ipso- methylene bond, breaking the methylene-phosphorus bond in the process (an example of a cheletropic182 rearrangement, in which two V-bond to a single atom are either made or broken in a concerted process). Hence, the benzyl group comes off as (±)-6-deuterio-5-methylenecyclohexa-1,3-diene 107, which was not isolated due to rapid rearomatisation by transfer of either the D or H atom on C6 to the methylene group, forming PhCH2D 109 or 2-DC6H4CH3 110, respectively.160,161 Therefore, the source of the three isomeric toluenes are as follows: – PhCH3 is derived from the benzyl anion expelled from the phosphoranate Ph3P(Bn)D2 127, which either reacts with the solvent at reflux or is protonated upon work-up, giving PhCH3.

PhCH2D 109 is the only product expected from the reductive-elimination of Ph3P(Bn)D 126, giving also Ph3P, whereas both PhCH2D 109 and 2-DC6H4CH3 110 are expected products from the cyclic decomposition of the phosphorane described above (Scheme 6.18, page 92).

In fact, it is hard to imagine the formation of the ortho isomer 2-DC6H4CH3 110 by any other route that does not also require at least a small (if not a large) amount of the para isomer, 4-DC6H4CH3 128, of which none was ever detected. Direct nucleophilic attack of – AlD4 (or its equivalent) on the methylene of the benzyl group should give exclusive PhCH2D 109 (as was the case with N-benzyl-N,N-dimethylanilinium bromide 121). Alternatively, if attack were to occur on the ring, then the para position should be more accessible than the ortho positions, giving rise to p-deuteriotoluene (4-DC6H4CH3 128). However, because direct attack precludes the formation of the phosphoranate intermediate 127 and hence Ph3PD2, and because no trace of the para isomer 128 has ever been detected in the toluene mixtures, – this mechanism (i.e. direct nucleophilic attack of AlD4 on the benzyl group) can be ruled out.

CH3 CH3

109 128 D

Scheme 6.17 (see page 91) showed that there are two steps where direct competition between alternative reactions occurs: firstly, for the hydrophosphorane Ph3P(Bn)D 126, the – transfer of a second deuteride ion from LiAlD4 to form the phosphoranate Ph3P(Bn)D2 127 Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 94

(k2) competes with the elimination of “BnD” (which consists of a mixture of both PhCH2D

109 and 2-DC6H4CH3 110) to form Ph3P (k5), while secondly, for the dihydrophosphorane + Ph3PD2, elimination of D2 (k6) competes with deuteride transfer to Ph3PR (k4). Clearly, increasing the concentration of LiAlD4 should increase k2 while decreasing k5, due to the reduction in concentration of available Ph3P(Bn)D 126. Also, by reducing the concentration of + available Ph3PBn , k4 is decreased (with k6 unaffected). What this means is that, if a small + – amount of Ph3PBn Br is added slowly to an excess of LiAlD4 in THF at RT, then decomposition of the intermediate phosphorane Ph3P(Bn)D 126 into Ph3P and “BnD” should be inhibited, hence favouring instead further reaction with LiAlD4, giving the phosphorane – Ph3PD2 via the intermediate phosphoranate Ph3P(Bn)D2 127. When the reaction is carried 31 1 out in this manner, Ph3PD2 is the only species observed using P{ H}-NMR, supporting the mechanism proposed (Scheme 6.17 on page 91) for the reaction of quaternary phosphonium salts with lithium aluminium hydride in THF.

To explore the possibility of preparing Ph2PH3, PhPH4, and PH5, the reaction was + – + – + – repeated with the salts Ph2PBn2 Br , PhPBn3 Br and Bn4P Br . In each case, when either

LiAlH4 or LiAlD4 was added to a suspension of the salt in dry THF, the only product observed using 31P{1H}-NMR was the phosphine obtained from exclusive loss of benzyl (i.e.

Ph2PBn, PhPBn2, and Bn3P, respectively); no direct observation of any intermediate phosphoranes was obtained. With the reverse addition of the salt into a solution of either LiAlH4 + – + – or LiAlD4 in THF, both PhPBn3 Br and Bn4P Br behaved as before (i.e. only PhPBn2 and

Bn3P, respectively, were observed).

_ Bn D D Bn + LiAlD4 LiAlD4 Bn Ph P P Ph P Ph Bn Ph Ph Bn Bn Ph D 130 131 _ – Bn P Ph D – D2 Ph – BnD* D D Ph Ph P D P Bn Ph _ Ph _ LiAlD – Bn 4 D D D 129 Bn Ph P D Ph D 132

+ – Scheme 6.19: Mechanism proposed for the reaction between Ph2PBn2 Br and LiAlD4. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 95

+ – However, when Ph2PBn2 Br was added to a solution of LiAlD4 in THF, in addition to the major product of the reaction (Ph2PBn), a very small amount of a species giving a 1 129 1:2:3:2:1 quintet at GP –79.3 ppm ( JP-D = 51 Hz) was observed, assigned as Ph2P(Bn)D2 , 1 along with Ph2PD, observed as a 1:1:1 triplet at GP –41.4 ppm ( JP-D = 34 Hz). There are + – several ways of producing Ph2PD from the reaction of Ph2PBn2 Br with excess LiAlD4 consistent with the mechanism put forward in Schemes 6.14 and 6.18, and they are shown above (Scheme 6.19, previous page). + – + – + – As in the case of Ph4P Br and Ph3PBn Br , Ph2PBn2 Br will react with LiAlD4 to give the phosphorane Ph2P(D)Bn2 130 (which is not observed directly), which will then itself – react with more LiAlD4 to give the phosphoranate anion Ph2PBn2D2 131 (also not observed). Expulsion of benzyl anion from phosphoranate 131 would then give the phosphorane 129, 31 1 observed to be present in small amount by P{ H}-NMR. Ph2PD could then be formed by the concerted elimination of “BnD” (i.e. a mixture of the toluene isomers PhCH2D 109 and 2-

DC6H4CH3 110) from phosphorane 129. Alternatively, Ph2P(Bn)D2 129 can react with more – LiAlD4 to give a second unobserved phosphoranate Ph2P(Bn)D3 132, which could then expel benzyl anion to give the trideuteriophosphorane Ph2PD3 (also unobserved). Loss of D2 from

Ph2PD3 would give Ph2PD.

– 6.3 Attempts to isolate Ph4PH, Ph3PH2 and Ph4PH2 .

– Because each of the three hydrido species Ph4PH, Ph3PH2, and Ph4PH2 could be made as a relatively clean solution, as characterised using 31P{1H}-NMR, a more detailed examination could be undertaken, as well as the attempted isolation of the two phosphoranes Ph4PH and Ph3PH2 (Scheme 6.20).

Ph H Ph + LiAlH4 P P Ph Ph (limiting) Ph Ph Ph Ph

31 1 Scheme 6.20: Ph4PH can be made as a clean solution (by P{ H}-NMR) by the slow addition of + – LiAlH4 to an excess of a stirred suspension of Ph4P Br in THF at RT.

A solution of Ph4PH was prepared from the careful addition of powdered LiAlH4 to a + – stirred suspension of Ph4P Br in THF at RT, keeping the phosphonium salt in excess at all 31 1 times. P{ H}-NMR showed Ph4PH (the only product) as a singlet at GP –86.6 ppm, which was observed to split into a doublet of nonets in the 31P-NMR spectrum due to coupling with both the single directly bound proton (1J = 323 Hz) and the eight equivalent ortho protons (3J = 14 Hz), respectively. Additional fine splitting due to the twelve approximately equivalent meta and para protons was also observed (4,5J = 3 Hz). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 96

The solution containing Ph4PH had no tendency to decompose into Ph3P when kept at or below RT. The solvent was removed under vacuum at RT, yielding a colourless solid, which appeared very reluctant to redissolve in either THF or Et2O. However, enough was extracted into benzene to run both a 1H-NMR and a 13C{1H}-NMR spectrum, which were consistent with spectra acquired in d8-THF, described later. The solid was observed to melt at 160˚C, much higher than expected for such a small molecule, consistent with the formation of a polymer (Figure 6.21). This may be possible as Ph4PH possesses both an electron-rich H atom and a vacant coordination site.

H Ph Ph Ph Ph Ph Ph Ph H H Ph P P P P H H Ph Ph Ph Ph Ph Ph Ph Ph

Scheme 6.21: Removal of the solvent from solutions of Ph4PH results in the precipitation of an intractable white solid, possibly indicating that the phosphorane had polymerised.

148 146 144 142 140 138 136 134 132 130 128 (ppm) 13 1 Figure 6.22: The 75.480 MHz C{ H}-NMR spectrum of Ph4PH in d8-THF, showing the 4 phenyl signals (from left are the ipso, ortho, para and meta carbons), each split by phosphorus.

+ – Because of possible interference from the aluminium salts (such as Li AlBr4 ), it was decided to pass the THF solution down a column of silica, which should trap the aluminium salts while possibly allowing the phosphorane to pass through. A short column of silica was Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 97

prepared, pre-moistened with anhydrous THF, and the Ph4PH solution was forced down under pressure of dry argon. Immediately, bubbles of gas were observed to form, disrupting the packing of the silica column. This was presumably due to the phosphorane Ph4PH behaving as a hydride donor towards a proton source (e.g. traces of water in the silica), producing H2 + 31 1 with Ph4P being regenerated (as determined by P{ H}-NMR). The procedure was repeated using a column prepared from basic alumina with the same result, i.e. decomposition of the phosphorane by reaction with traces of acid. Although Ph4PH could not be isolated, it was 1 prepared as a solution in d8-THF by the method described above, from which good H- and 13C{1H}-NMR spectra could be obtained. Shown above (Figure 6.22, previous page) is the 13 1 C{ H}-NMR spectrum of Ph4PH in d8-THF, showing all four phenyl carbons, each split into doublets due to phosphorus coupling. Because no further splitting of the signals was observed, the phenyl carbons must be equivalent on the NMR timescale. This could arise either by isomerisation of the phosphorane (i.e. through DLE processes such as the Berry Pseudo-Rotation), or because the phenyl groups actually are equivalent by symmetry. Shown below (Scheme 6.23) are the four possible geometric isomers of Ph4PH when only the trigonal bipyramidal and square pyramidal geometries are considered. They are interconvertible through the mechanism of BPR (Berry Pseudo-Rotation), where the trigonal bipyramidal isomers 133a and 133c are assumed to be of lower energy than the two square pyramidal isomers 133b and 133d. Therefore, the isomers 133a and 133c need only interconvert through a TS corresponding to isomer 133b, without the need to form isomer 133d. If the rate of interconversion between these isomers is fast compared to the NMR timescale, then all four phenyl groups will appear equivalent.

H Ph Ph H Ph Ph Ph P Ph P Ph H P Ph P Ph Ph Ph H Ph Ph Ph Ph Ph

133a 133b 133c 133d

Scheme 6.23: Isomerisation between the four postulated structural isomers of Ph4PH.

If Ph4PH is undergoing rapid exchange of the phenyl groups on the NMR timescale, this could possibly be verified by cooling the solution, whereupon the observed NMR signal should first broaden, and then separate into two or more signals corresponding to the number of geometric isomers “frozen out.” A series of low-temperature 31P{1H}-NMR spectra of

Ph4PH in THF solution were run, but no broadening or splitting of the singlet at GP –86.6 ppm was observed down to a temperature of –110˚C, below which the experiment had to be abandoned due to solidification of the solvent. An alternative explanation is that the molecule is fixed in one geometry, with all four phenyl groups equivalent by symmetry. Isomer 133d, a Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 98 square pyramid with all the phenyl groups basal and the single hydrogen apical, satisfies this requirement, assuming free rotation about the P-C V-bonds. The 1H-NMR spectrum showed the expected aromatic proton signals, as well as a 1 doublet at G+ +7.85 ppm ( J = 323.1 Hz) which integrated to 1H, and was assigned to the proton directly bound to phosphorus. The chemical shift of this proton is much further downfield than its hydridic nature would suggest, and the possible reasons for this will be discussed later in Section 6.5 (compare Hellwinkel’s phosphorane 123, in which the hydrogen bound to phosphorus is observed at GH +9.33 ppm). + – If Ph4P Br is added slowly to an excess of LiAlH4 in THF, the only signal observed 31 1 – using P{ H}-NMR is that due to the phosphoranate anion Ph4PH2 (Scheme 6.24). Because this octahedral anion undergoes unimolecular decomposition at RT, expelling phenyl anion to give Ph3PH2, the signal intensity is observed to decrease over time, replaced by the signal due to Ph3PH2 (Section 6.1).

_ Ph H H Ph + LiAlH4 LiAlH4 Ph Ph P P Ph P Ph (excess) (excess) Ph Ph Ph Ph Ph Ph H

– 31 1 Scheme 6.24: Ph4PH2 can be made as a clean solution (by P{ H}-NMR) by the slow addition of + – Ph4P Br to an excess of LiAlH4 in THF at RT.

As there seemed no way around this stability problem, it was decided not to attempt to isolate the phosphoranate, but instead to try to characterise it more completely spectro- – scopically. Toward this aim, a solution of Ph4PH2 was prepared in d8-THF, by a procedure similar to that just described, and shown below (Figure 6.25, next page) is the 1H-NMR spectrum. The doublet, centred at G+ +8.77 ppm (J = 447.1 Hz), corresponds to two equivalent hydrogens directly bound to phosphorus. 13 1 – The C{ H}-NMR spectrum of Ph4PH2 showed all four phenyl carbons to be doublets (due to splitting by phosphorus), with no additional splitting of the signals, which means that all four phenyl groups are equivalent on the NMR timescale. Unlike the penta-coordinated phosphoranes, the phosphoranate anion, which is presumed to exhibit an octahedral disposition of groups around the phosphorus, is not expected to be able to isomerise without dissociation occurring (see Figure 6.26, next page). As the unimolecular decomposition of – Ph4PH2 to give Ph3PH2 proceeds by the dissociation of an anionic phenyl group, clearly this represents an irreversible reaction for this system. Hence, it may be concluded that no interconversion between geometric isomers can occur for as long as the phosphoranate anion is in existence, thereby inferring that the observed equivalence of the phenyl groups is due to – Ph4PH2 adopting only the trans geometry. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 99

* *

9.5 9.0 8.5 8.0 7.5 7.0 6.5 (ppm) 1 – Figure 6.25: The H-NMR of Ph4PH2 in d8-THF; the downfield peaks at GH +9.51 ppm and GH +8.02 ppm are the doublet due to the directly bound hydrogen atoms centred on GH +8.77 ppm (J = 447.1 Hz). The two small peaks appearing at GH +7.87 and +6.76 ppm (marked *) are a doublet – (J = 333.9 Hz) due to trace amounts of Ph3PH2, formed from the decomposition of Ph4PH2 .

_ _ _ H Ph Ph Ph Ph Ph Ph H P P P Ph Ph H H Ph H Ph Ph H Ph

trans octahedral trigonal prismatic cis octahedral

– Figure 6.26: The proposed trans geometry of Ph4PH2 , which is thought to be unable to racemise to the cis geometry via the trigonal prismatic geometry due to large, inter-ligand repulsions.

+ – Slow addition of Ph3PBn Br to an excess of LiAlH4 in THF produces Ph3PH2 as the 31 1 only product detected using P{ H}-NMR. As with Ph4PH, the phosphorane showed virtually no tendency to decompose into Ph3P when kept at RT, but was sensitive to both increases in temperature and the presence of traces of acids, including solvents such as H2O, MeOH and EtOH. Attempts to isolate the phosphorane by removal of the solvent gave a similar insoluble white solid to that obtained for Ph4PH, indicating possible polymer formation: Ph3PH2 has two electron rich donor P-H bonds and one vacant coordination site, so may Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 100

be able to form a branched polymer (Scheme 6.27 below). Solutions of Ph3PH2 were observed to decompose (i.e. an evolved gas was observed) when passed down columns of either silica or basic alumina, as was observed with Ph4PH, with formation of Ph3P, characterised using 31P{1H}-NMR.

H Ph H P Ph Ph H Ph Ph H Ph Ph Ph Ph H H Ph P P P P H H Ph H Ph H Ph Ph Ph H H Ph Ph H P P Ph Ph Ph Ph H H

Scheme 6.27: Ph3PH2 may be able to polymerise through the formation of hydride bridges.

The behaviour of both Ph4PH and Ph3PH2 upon removal of solvent is similar to that of 87 metal hydrides like BeH2 or BH3 (see Scheme 6.28). In each of the phosphoranes, a polymeric structure is formed by hydrides bridging between two electron-deficient “metal” atoms.

The driving force is similar to that for the dimerisation of borane (BH3) into diborane, B2H6 (Scheme 6.28a), in that vacant orbitals localised on the central atom (Be or B) can participate in bonding by accepting some of the electron density from the electron rich V-bonds. The reasoning for the phosphoranes Ph4PH and Ph3PH2 is similar in that they each have a formally vacant orbital (or coordination site) and either one or two electron rich P-H V-bonds, respectively, allowing Ph4PH to form a linear polymer (Scheme 6.21, page 96), whereas Ph3PH2 would be able to form a branched polymer (Scheme 6.27).

H H H H H (a) H B B H B B H H H H H

H H H H (b) HBeH Be Be Be Be H H H H n

Scheme 6.28: (a) the dimerisation of borane (BH3) into diborane (B2H6), and (b) the polymerisation

of beryllium hydride (BeH2). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 101

31 Shown below in Figure 6.29 is the P-NMR spectrum of Ph3PH2 in THF. Clearly visible is the main 1:2:1 triplet structure due to the 1-bond coupling of phosphorus to two equivalent hydrogens (J = 334 Hz), with additional smaller couplings to the aromatic protons (18 Hz for the six ortho protons, and 3 Hz for both the six meta and three para protons). These splittings indicate that all three phenyl groups are equivalent on the NMR timescale, 13 1 and this is confirmed by the C{ H}-NMR spectrum (acquired in d8-THF), which shows only four signals due to phenyl carbons, with each signal split into a doublet because of coupling to phosphorus.

-66 -68 -70 -72-67.5 -68.0 -68.5 -69.0 (ppm) (ppm) (a) (b)

31 Figure 6.29: The proton-coupled P-NMR of Ph3PH2 in d8-THF; (a) the 1:2:1 triplet structure of the directly bound protons (J = 334 Hz), and (b) an expansion of the central peak of the triplet.

As for Ph4PH, the apparent equivalence of the phenyl groups in Ph3PH2 may either be due to rapid interconversion between different geometric isomers or that Ph3PH2 exists frozen into a highly symmetrical isomer. Ph3PH2 can conceivably adopt three distinct trigonal bipyramidal isomers in addition to three different square pyramidal isomers (Scheme 6.30, next page), which could be interconverted via the BPR mechanism. Alternatively, if the molecule was to adopt the structure of isomer 134a, then all three phenyl groups will be equivalent by symmetry, assuming free rotation about the P-C V-bonds. As in the case of Ph4PH, there is not yet enough evidence to decide between the alternatives. The stability of both Ph4PH and 183 Ph3PH2 at RT suggests that a fairly large barrier to reductive-elimination must exist.

Hydrophosphoranes such as R4PH have been postulated as intermediates in the reduc- 22,69,153 152,154,155 tion of both phosphonium salts and phosphine oxides with LiAlH4. The Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 102 term “meso-phosphorane” has been used to describe these intermediates because of the plane of symmetry passing through the three equatorial groups.184,185 Formation of such a species from a chiral salt or oxide will necessarily erase all optical activity in both the phosphorane and the recovered phosphine without the need for intermediate isomerisation via DLE processes such as the BPR mechanism (Scheme 6.31).

H Ph Ph Ph H Ph P H P Ph Ph P Ph Ph H H H Ph 134a 134b 134c

Ph H H Ph Ph P H H P Ph P Ph Ph H Ph Ph H Ph 134f 134e 134d

Scheme 6.30: Isomerisation between the six postulated structural isomers of Ph3PH2.

_ Bn Bn H R + LiAlH4 2 LiAlH4 R R P R P 1 P 2 1 Bn R R2 R 3 R1 R 3 3 H H

_ – Bn

_ O H3AlO H LiAlH4 R2 LiAlH4 R2 – H2 P P R1 P R1 P R2 R2 R1 R R1 R R3 R3 3 3 H H racemic meso-phosphorane phosphine

Scheme 6.31: Reduction with LiAlH4 of either a chiral phosphonium salt or chiral phosphine oxide may form a meso-phosphorane intermediate, explaining the loss of optical activity in the product.

– The analogous reductive-elimination of H2 from the phosphoranate Ph4PH2 would – give the phosphoranide ion Ph4P 135, which, presumably, could then immediately expel a phenyl anion to give Ph3P (see Scheme 6.32, next page). As traces of Ph3P are observed in – mixtures containing Ph4PH2 , this pathway cannot be ruled out as a side-reaction, although clearly the direct expulsion of phenyl anion (see Section 6.1) to give Ph3PH2 is much more important. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 103

_ _ _ H Ph H _ P Ph Ph Ph – Ph P P Ph P Ph Ph Ph Ph Ph Ph Ph Ph Ph Ph H H 135

– Scheme 6.32: Reductive-elimination of H2 from the phosphoranate Ph4PH2 (shown for the trans – 135 isomer) would produce the intermediate tetraphenylphosphoranide anion Ph4P , which, pre-

sumably, could then expel phenyl anion rapidly to give Ph3P.

– The barrier to reductively-eliminating H2 from trans-Ph4PH2 is expected to be prohibitively high due to the severe distortion necessary to bring the two hydrogen atoms close enough for bond formation to begin. The cis isomer, with adjacent hydrogen atoms, would be expected to have a much lower barrier to elimination of H2, because presumably much less distortion of the phosphoranate is required. Therefore, the apparent lack of direct reductive- – – elimination of H2 from Ph4PH2 is consistent with the suggestion that only trans-Ph4PH2 is formed from the addition of hydride to Ph4PH. – Another explanation for the equivalence of all four phenyl groups in Ph4PH2 on the – NMR timescale (see discussion after Scheme 6.24) is hydride exchange with AlH4 (or with – AlH3 or its equivalent). It seems unlikely that the phosphorus atom in Ph4PH2 has enough space around it to fit an additional atom, even one as small as hydrogen, so formation of 2– traces of the hepta-coordinated dianion Ph4PH3 is assumed not to occur. However, an – equilibrium has been shown to exist between Ph4PH and Ph4PH2 (Section 6.1). – Hence, if minute quantities of Ph4PH are able to be formed from Ph4PH2 in solution, then the equivalence of the phenyl groups is explained, regardless of whether Ph4PH undergoes DLE or is fixed in the symmetrical square pyramidal geometry (i.e. 133d with the single hydrogen atom apical; see Scheme 6.23 and accompanying discussion). That the exchange – of the hydrogen atoms in Ph4PH2 does indeed occur was demonstrated by careful addition of

LiAlD4 to a solution of Ph4PH in THF (prepared as previously described), in an attempt to – prepare the mixed isotope phosphoranate Ph4P(D)H . Examination of the solution using 31 1 – P{ H}-NMR showed a mixture of species, including a 1:2:3:2:1 quintet due to Ph4PD2 in addition to the expected 1:1:1 triplet. If the possibility of increasing the coordination number of phosphorus to seven is discounted, the only way that the dideuterated phosphoranate – – anion Ph4PD2 could have formed is if the initially formed Ph4P(D)H then exchanged the hydrogen atom for a deuterium, presumably going via Ph4PD as an intermediate.

– 6.4 The multi-nuclear NMR spectroscopy of Ph4PH, Ph3PH2 and Ph4PH2 . In this section, a comparison of the 1H-, 2H-, 13C{1H}-, and 31P{1H}-NMR spectra of – Ph4PH, Ph3PH2, and Ph4PH2 will be made in an attempt to determine the possible bonding situation in each species. All three compounds were prepared as described in Section 6.3; 2 31 1 the H- and P{ H}-NMR spectra were acquired from solutions in THF, using external D2O Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 104

for deuterium locking, while solutions prepared using d8-THF as the solvent were used to obtain both the 1H- and 13C{1H}-NMR spectra. (a) The 31P{1H}-NMR spectra: As discussed in Section 6.1, the 31P{1H}-NMR spectra reveal that the phosphoranes Ph4PH and Ph3PH2 resonate upfield of 85% H3PO4 at GP –86.6 – and –68.4 ppm, respectively, whereas the anionic phosphoranate Ph4PH2 is observed about

100 ppm further upfield at GP –185.6 ppm. These chemical shifts are in the region of the spectrum normally associated with high electron density on phosphorus. However, they should be interpreted with care, as the chemical shifts of the both Ph4PH and Ph3PH2 compare 129a well with the observed chemical shift for PF5 and PCl5 (GP –80 and –82 ppm, respectively), each of which contains five strongly electron withdrawing groups. Even more surprising, the – 129b phosphoranate appears significantly downfield from PCl6 (which appears at –282 ppm ) despite the fact that chlorine would be expected to withdraw much more electron density from the phosphorus than either phenyl or hydrogen. A consistent upfield shift of about 0.8 ppm is observed for each directly bound hydrogen atom replaced by deuterium, and the ratio of the 1-bond P-H couplings to their deuterium analogues consistently gives a ratio1 of about 6.5, as expected (see Table 6A). In the case of 31 Ph3PH2, the P-NMR spectrum shows a binomial triplet of septets of decets at GP –68.4 ppm (1J = 334 Hz, 3J = 18 Hz and 4,5J = 3 Hz), corresponding to coupling from the two directly bound protons, the six ortho protons and the nine approximately equivalent meta and para protons, respectively. As for Ph4PH, all phenyl groups appear equivalent on the NMR timescale.

Table 6A: Observed Isotopic Shifts and Coupling Ratios.

GP (ppm) 'Shift JP-Y (Hz) Ratio

Compound Y = H Y = D 'GP (ppm) Y = H Y = D JP-H/JP-D

Ph4PY –86.6 –87.3 –0.7 323 49 6.6

Ph3PY2 –68.4 –70.0 –1.6 334 51 6.5

+ – Li Ph4PY2 –185.6 –187.2 –1.6 447 69 6.5

31 – The P-NMR spectrum of Ph4PH2 , like Ph3PH2, shows a large 1:2:1 triplet, in this case centred at GP –185.6 ppm, with further splitting of each peak split into a binomial nonet (1J = 447 Hz and 3J = 13 Hz), corresponding to coupling from the two directly bound protons Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 105 and the eight ortho protons, respectively (with no splitting from the meta and para protons being observed). (b) The 1H-NMR spectra: the 1H-NMR data for the three compounds are presented in

Table 6B. As expected, Ph4PH and Ph3PH2 show similarities in the observed values of GH 1 and JP-H, whereas the P-H protons of the phosphoranate exhibit both a downfield shift and a 1 larger value of JP-H by comparison. Besides the aromatic protons attached to the phenyl groups, all three compounds contain either one or a pair of equivalent hydrogen atoms directly bound to phosphorus, with both types of proton being clearly visible in the 1H-NMR spectra (e.g. Figure 6.25). The aromatic protons appear in the usual range of about GH +6.5 to +8.3 ppm, with the ortho protons separated from the others, appearing downfield. The protons directly bound to phosphorus appear much further downfield than expected for hydridic hydrogens (i.e. around GH +7.5 ppm for Ph4PH and Ph3PH2, and GH +8.77 – ppm for Ph4PH2 ). As a comparison, the hydrogen bound directly to phosphorus in Hell- winkel’s spirocyclic phosphorane 123 resonates at GH +9.33 ppm It should also be noted that

LiAlH4, also observed in these spectra, appeared at about GH +4 ppm, much further downfield than would be expected from its behaviour as a hydride donor. The magnitudes of the one- – bond P-H coupling constants follow the order: Ph4PH < Ph3PH2 << Ph4PH2 (Table 6A, previous page). Where other hydrohalophosphoranes and -phosphoranates have been characterised, it was found that the magnitude of the one-bond phosphorus-hydrogen couplings de- – creases when going from the phosphorane HPX4 to the corresponding phosphoranate HPX5 129b (where X = F, Cl). This is the opposite trend to that observed for Ph4PH, Ph3PH2 and – Ph4PH2 . Possible reasons for these differences will be presented during the discussion of the 13C{1H}-NMR spectra (next page).

Table 6B: Observed Proton Chemical Shift and Coupling Data.

hydridic ortho meta and para

a a a b a b Compound GH (ppm) J (Hz) GH (ppm) J (Hz) GH (ppm) J (Hz)

Ph4PH +7.85 323.1 7.50-7.58 14 7.33-7.43 3

Ph3PH2 +7.32 333.9 8.14-8.28 18 7.15-7.27 3

+ – Li Ph4PH2 +8.77 447.1 6.55-6.81 13 7.24-7.59 < 1

a Taken from the 1H-NMR spectra. b Taken from the 31P-NMR spectra. N/R = Not Resolved. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 106

2 (c) The H-NMR spectra: For comparison, the three deuterated phophoranes Ph4PD, – Ph3PD2, and Ph4PD2 were prepared in the same manner as solutions in THF, and examined using 2H-NMR. As the phenyl groups were non-deuterated, the spectra showed only the phosphorus bound deuterium atoms, which were observed to appear about 0.35 to 0.40 ppm downfield to that of the their hydrogenated isomers (i.e. Ph4PD: GD +8.23 ppm; Ph3PD2: GD – +7.69 ppm; Ph4PD2 : GD +9.17 ppm), although the 1-bond phosphorus-deuterium couplings were all about 6.5 times smaller, as expected.1 The 2H-NMR spectra were calibrated with 1 CDCl3, which was set to GD +7.26 ppm, the observed chemical shift of CHCl3 using H- NMR, hence making the observed discrepancy in chemical shifts a real phenomenon. As to whether the difference lies in the three compounds described here (i.e., Ph4PY, Ph3PY2 and – Ph4PY2 , where Y = H, D), or with the CHCl3/CDCl3 pair, is not known, making the discrepancy between the 1H- and 2H-NMR chemical shifts inconclusive. (d) The 13C{1H}-NMR spectra: In each of the three compounds, all four phenyl carbons are equivalent on the NMR timescale (e.g. Figure 6.22). The chemical shifts for the ortho, meta and para carbons are unexceptional, but the ipso carbons are significantly down- – field from the others (Table 6C); the ipso carbon of Ph4PH2 appears the furthest downfield.

Table 6C: Phenyl Carbon Chemical Shifts.

Aromatic Carbon atom (in ppm)

Compound* ipso ortho meta para

Ph4PH +147.2 +135.5 +129.4 +131.1

Ph3PH2 +142.7 +133.6 +130.4 +134.5

+ – Li Ph4PH2 +170.6 +127.4 +129.8 +124.2

+ – Ph4P Br +117.0 +134.0 +130.5 +135.5

* + – Solvent used to acquire spectra was d8-THF, except for Ph4P Br , where CDCl3 was used.

+ – The phosphorus-carbon coupling constants are shown in Table 6D, with Ph4P Br in- cluded for comparison. As expected, there is a general decrease in the observed magnitude of the splittings as the number of intervening bonds between phosphorus and the carbon in + – question increases. The data for Ph4P Br is representative of quaternary phenylphosphonium salts in general (see Chapter 10 for analogous data on similar compounds used in this project), with a large splitting observed for the ipso carbon, along with a small increase in the coupling constant on going from the ortho to the meta carbons, and a small splitting of the Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 107

+ para carbon. As the data shows, both Ph4PH and Ph3PH2 are qualitatively similar to Ph4P – Br , although the splitting of the ortho carbons in Ph3PH2 are moderately larger in magnitude.

Table 6D: The Observed Phosphorus-Carbon Couplings. Aromatic Carbon atom coupled to Phosphorus Compound* ipso ortho meta para

Ph4PH 79 Hz 13 Hz 13 Hz 3 Hz

Ph3PH2 83 Hz 19 Hz 14 Hz 3 Hz

+ – Li Ph4PH2 40 Hz 10 Hz 14 Hz < 1 Hz

+ – Ph4P Br 89 Hz 10 Hz 12 Hz 3 Hz

* + – Solvent used to acquire spectra was d8-THF, except for Ph4P Br , where CDCl3 was used.

– In the case of the phosphoranate Ph4PH2 however, both the ipso and para splittings are found to be substantially smaller in magnitude, although the ortho and meta splittings are + – similar to those found in Ph4P Br . A possible explanation for these trends will be presented – in the next section, when the qualitative bonding situation for Ph4PH, Ph3PH2, and Ph4PH2 will be discussed.

– 6.5 The bonding in Ph4PH, Ph3PH2 and Ph4PH2 .

The following discussion will be based largely on the 13C{1H}-NMR data, with supporting evidence taken from the 1H-, 2H{1H}-, and 31P{1H}-NMR where needed. Because all three compounds possess only phenyl carbons, it is convenient to present the 13C chemical shifts using benzene as a suitable model compound. The parameter ' (in units of ppm) will be defined as:

¨X = (GX – GC[PhH]) ppm,

where GC[PhH] = 128.3 ppm, the observed chemical shift of benzene in CDCl3 solution, and X is the aromatic carbon atom considered (i.e. ipso, ortho, meta or para). Positive values of ' for a particular signal indicate that the signal lies downfield from benzene, whereas signals upfield from benzene give negative values of ¨. Before the results for the three compounds are considered, a discussion about V- and S- type donors and acceptors is appropriate. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 108

For a mono-substituted benzene PhX, when X is a S-donor, electron density is pushed into the ring and accumulates at the ortho and para positions (Figure 6.33). This has the effect of shifting these carbons upfield from benzene (i.e. ¨ is negative). The arguments are reversed when X is a S-acceptor (i.e. the ortho and para carbons shift downfield). Some examples are shown below in Table 6E.

+++ X X X X XG+

_ _ G G

_ G Figure 6.33: The resonance contributors of PhX when X is a S-donor.

Table 6E: Observed Phenyl Carbon ¨ Values for PhX. Aromatic Carbon atom ¨ values (in ppm)186 X ipso ortho meta para V – Li +58.1 +15.2 –3.8 +5.4 S+ V – MgBr +35.8 +11.4 +2.7 +4.0 S+ V + NO +20.6 –4.3 +1.3 +6.2 S+ 2 V – CH +9.3 +0.7 –0.1 –3.0 S– 3 V + NH +20.2 –14.1 +0.6 –12.2 S– 2 V + OH +26.6 –12.8 +1.6 –7.1 S– V + F +35.1 –14.3 +0.9 –4.4 S– V + Cl +6.4 +0.2 +1.0 –2.0 S– V – Br –5.9 +3.0 +1.5 –1.5 S– V – I –32.3 +9.9 +2.6 –0.4 S–

All ¨ values were calculated from benzene, as given (GC[PhH] = +128.5 ppm). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 109

+ – As a comparison, the ' values for the three compounds and Ph4P Br ar e given below + – in Table 6F. Bo th phospho ranes and Ph4P Br exhibit positive values for 'ortho and 'para, suggesting that the phosphorus atoms are functioning as S-acceptors. This is expected for + – Ph4P Br , where the phosphorus atom bears a positive charge, and is also consistent with the pnolarisatio of negativ e charge onto the axial substitu ents of a trigo nal bipyramida l phosphorane (or the ba sal subs tituents of a squ are pyramidal phosphorane) . As the elec tron density moves on to th ese axial (or b asal) grou ps,the phosph orus atom wi ll bec ome elec tron deficient, and therefore a better S-acceptor.

Table 6F: Phenyl Carbon Chemical Shifts (from Benzene). Aromatic Carbon atom¨ value s (in ppm) Compound* ipso ortho meta para

Ph4PH +18.9 +7.2 +1.1 +2.8

Ph3PH2 +14.4 +5.3 +2.1 +6.2

+ – Li Ph4 PH2 +42.3 –0.9 +1.5 –4.1

+– Ph4P Br –11.3 +5.7 +2.2 +7.2

* +– Solvent used to acquire spectra was d8-THF, except for Ph4P Br , where CDCl3 was used.

– In contrast, the anionic phosphoranate Ph4PH2 exhibits negative val ues for both 'ortho and 'para, indicating that in this case the phosphorus atom is behaving as a S-donor. This is not surprising, not only because the species bears a negative charge, but also because of its octahedral geometry. If d-orbitals on phosphorus are assumed to play only a minor role in chemical bonding then all three phosphorus 3p-orbitals will be involved in 3c-4e bonds to two substituents each (with the 3s orbital overlapping with all six substituents). As only the p-orbitals can be involved in S-overlap, they must necessarily behave as S-donors as they are already electron rich. Hence, apart from the ipso carbon chemical shifts, the 13C{1H}-NMR – data for each of the three compounds Ph4PH, Ph3PH2, and Ph4PH2 can be explained in terms of the resonance model shown above in Figure 6.33. The counter-intuitive upfield shifts observed for the ipso carbons of phenylphosphonium salts have been noted previously.187 For comparison, the ipso carbon chemical shifts of some tetraphenylated species were determined,187 from which an interesting trend was discovered (see Table 6G, next page). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 110

Table 6G: The ipso Carbon Chemical Shifts of Ph4E (from Benzene).

a 187 n+ The Obs erved valu es of ¨ipso (ppm) in Ph4E (n = –1, 0, + 1).

Group 13 ïpso Gr oup 14 ïpso Group 15 ïpso

– + b Ph4B36 +Ph+Ar4C 18 4 N +3

– + Ph4A31l +Ph+Ph4Si 5 4P –11

– not + Ph4Ga examine d Ph4Ge +Ph7 4 As –9 a b All ¨ values calculated using the listed ch emical shift of benzene (GC[PhH] = +129 ppm); T he spiro- + + cyclic ammonium salt [N(b iph)2] was used in plac e of Ph4N .

+ Firstly, the upfield shift of the ipso carbons in Ph4P appears to be a general trend in the Group 15 cationic tetraphenyl derivatives, and is matched by an equally counter-intuitive downfiel d shift (¨ > 0) of the ipso carbons in the Group 13 anionic tetraphenyl derivatives, with the uncharged derivatives of Group 14 exhibiting intermediate shifts. Secondly, there are trends within each group across a given period, with the derivatives of the first period – + (Ph4B , Ph4C and (biph)2N ) showing a large downfield shift compared to derivatives of the second and third periods.

An explanation for the observed ¨ipso values (Table 6G) is presented in Figure 6.34

(next page). Compared with the uncharged silicon atom in Ph4Si, the negatively charged – aluminium atom in Ph4Al is expected to be a much better V-donor, thereby increasing the electron density in the Ph-Al V-bond relative to the Ph-Si bond in Ph4Si. Because the delocalised S-electrons in the phenyl rings are the most polarisable electrons in the molecule, they are quite easily distorted by changes in the V-electron density at the ipso carbon. Therefore, – the increase in V-electron density at the ipso carbons in Ph4Al relative to Ph4Si would be expected to repel the aromatic S-cloud away from the ipso carbon towards the para carbon (see Figure 6.34). The decrease in S -electron density at the ipso carbon results in the observed – 187 downfield shift for Ph4Al (i.e. ïpso > 0). The reverse argument can be used to explain the upfield shift (i.e., negative values of + ïpso) for the phosphonium salt Ph4P . The positively charged phosphorus is a weaker V- donor than the uncharged silicon atom in Ph4Si, and tends to withdraw the V-electron density away from the ipso carbon. Hence, the aromatic S-cloud experiences less repulsion from the + phosphorus substituent Ph3P - than it does from the analogous silicon substituent Ph3Si-, enabling it to partially expand into the region of space occupied by the Ph-P+ V-bond, thereby increasing the S-electron density at the ipso carbon. This results finally in the observed upfield shift for ïpso. A general plot of these effects is shown below (Figure 6.35, next page). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 111

_ + (a)Ph3Al (b) Ph3P

– + Figure 6.34: The postulated polaris ation of the arom at ic S-elect rons in (a) Ph4Al ,a nd (b) Ph4P .

30 Group 13

² (ppm) 10 Group 14

Group 15 -10

1 2 3 Period

st nd rd Figure 6.35: A plot of the observed values of ¨ipso versus the period (1 ,2 or 3 ) for the Groups – + – 13 (Ph4E ), 14 (Ph4E) and 15 (Ph4E ). Ph4Ga was not examined.

The reverse is true for the situation where the X atom of PhX is electron rich. This is – the case for both Ph4PH and Ph3PH2, and especially Ph4PH2 , where this time the V-electrons in the C-P bond are pushed away from phosphorus towards the ipso carbon. The S-electrons, being more mobile and polarisable than the V-electrons, are repelled by the C-P V-electrons, and drift away from the ipso carbon towards the para carbon. Again, the effect of diminish- ing S-electron density at the ipso carbon outweighs the effect of increasing V-electron density, and hence it appears downfield from benzene in the 13C{1H}-NMR spectrum.

The positive values of ¨para for both Ph4PH and Ph3PH2 (¨para = +2.8 and +6.2 ppm, respectively) suggests that the phenyl groups are behaving as a S-donors in these compounds, as has been discussed. This is intuitively reasonable as Ph4PH is known to function as a – – Lewis acid towards H with formation of the phosphoranate anion Ph4PH2 . Because the exact geometries of both Ph4PH and Ph3PH2 are not known with certainty, a precise discussion concerning their respective ¨para values is not possible. Despite this, a general discussion considering the expected V and S properties of both Ph and H is possible. Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 112

As was discussed in Chapter 4, the trigonal bipyramidal geometry is usually found to be the ground state structure for phosphoranes containing five independent attached groups.

In this geometry, for a phosphorane with five identical groups (e.g. PF5), the three equatorial bonds are equivalent to one another, but are found to be shorter than the two equivalent axial bonds.102 In general, both S-acidic and electronegative groups (i.e. V-acceptors) prefer to occupy the axial positions, with groups which are either S-basic or V-donors (electropositive groups) occupying the equatorial positions.113,121,188 The two sites are also non-equivalent sterically, with the axial positions tending to be more crowded than the equatorial positions, possibly explaining the observed lengthening of the axial bonds183 relative to the equatorial bonds for any given group. Because the electronegativity of hydrogen is similar to carbon, the relative stabilisa- tion of the various isomers are likely to be dominated by both the steric requirements of the ligand and whether it functions as a S-donor or S-acceptor. Hydrogen does not possess any low-lying orbitals of S-symmetry, and so it cannot function as either a S-donor or a S-acceptor. In contrast, a phenyl group is able to behave as either a S-donor or a S-acceptor (Fig- ure 6.33, page 108). Hence, neither H or Ph are expected to show a large preference for either the axial or equatorial sites in a phosphorane PhnPH5-n (n = 1 to 4). Therefore, the small size of hydrogen should favour its placement in the axial positions of such a phosphorane. How- ever, both the energy difference between such isomers and the barrier between them are likely to be small, and scrambling of the ligand positions at RT is expected to occur readily.

H Ph Ph Ph (a) P Ph P H Ph Ph Ph Ph

133a 133c

H Ph H Ph H Ph (b) P Ph P Ph P H Ph H Ph H Ph Ph

134a134c 134e

Figure 6.36: The possible geometric isomers of (a) Ph4PH and (b) Ph3PH2, assuming that both phosphoranes exist as trigonal bipyramids.

A comparison of the data presented in Table 6F for Ph4PH and Ph3PH2 can only be made if the geometry of the phosphoranes are identical (e.g. both trigonal bipyramidal). If the geometry of both phosphoranes is assumed to be predominantly trigonal bipyramidal then five isomers are possible, two for Ph4PH and three for Ph3PH2 (see Figure 6.36 above). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 113

Phenyl groups can behave as S-donors when situated equatorially due to overlap of the aromatic S-system with the phosphorus 3pz orbital. The large positive value of ¨para for Ph3PH2 (+6.2 ppm) indicates substantial S-electron movement from the phenyl rings, consistent with the bonding scheme presented (Figure 6.37). Comparison with other monosubstituted benzenes PhX, where X has S-acid character, show similar values of ¨para: +5.4, +6.2 and +3.8 ppm for X = Li, NO2 and C(O)Ph, respectively, suggesting that the larger shift observed for

Ph3PH2 is due to S-donation from the phenyl rings to the phosphorus. Hence, the NMR data presented in Section 6.4 is consistent with both hydrogen atoms remaining axial in Ph3PH2 (isomer 134a), possibly due to both the steric bulk and the S-donor character of the phenyl rings.

Figure 6.37: The proposed interaction between filled orbitals of S-symmetry on equatorial phenyl

rings (shown for the ipso carbons) and the 3pz orbital on phosphorus in Ph3PH2.

Figure 6.38: Overlap between the vacant S* orbitals on the axial phenyl group of Ph4PH (shown

at the ipso carbon) with the filled P 3px (or 3py).

Ph4PH, with ¨para = +2.8 ppm, shows a much reduced S-donor effect for the phenyl groups compared to Ph3PH2. Only three of the four phenyl groups can be equatorial at any one time, with the remaining phenyl group taking up the axial position (isomer 133a), where it will accept more electron density from the phosphorus, either through the P-C V-bond, or Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 114 via overlap of the vacant aromatic S* orbitals with the filled equatorial P-C V-bonds (Figure 6.38, previous page).

Presumably, the three equatorial phenyl groups will behave in the same way as in Ph3PH2, with the observed differences in the NMR data being due to an averaging of the various effects with the axial phenyl group. As has been already discussed, both S-acceptors and electronegative groups prefer to occupy the axial positions of a trigonal bipyramidal phosphorane. Because phenyl groups can function either S-donors or S-acceptors depending on the attached substituent, the axial phenyl group would be expected to behave as a S-acceptor, which should produce a negative value for ¨para. The expected build-up of electron density in the axial P-C V-bond due to the locali- sation of negative charge on the axial groups of a trigonal bipyramidal phosphorane should 187 appear as a downfield shift of the ipso carbon (i.e. ¨ipso should become more positive).

The ipso carbon of Ph4PH, which should contain at least one axial phenyl group, is observed to resonate downfield from the corresponding ipso carbon of Ph3PH2 (i.e. ¨ipso = +18.9 ppm for Ph4PH, compared to ¨ipso = +14.4 ppm for Ph3PH2), consistent with the proposal that the phenyl groups of Ph4PH have more axial character (i.e. they spend more time in the axial positions) than those in Ph3PH2.

Figure 6.39: The square pyramidal geometry proposed for Ph4PH, with the single hydrogen atom apical. The vacant orbitals of S-symmetry on the phenyl rings (shown at the ipso carbons) can possibly accept electron density from the P-H V-bond (i.e. hyperconjugation).

An alternative explanation, which was suggested in Section 6.3, is that Ph4PH may exist predominantly in the square pyramidal geometry (see above in Figure 6.39). The four phenyl rings are predicted to prefer a horizontal orientation for good overlap between the aromatic S* orbitals on the phenyl rings with th e central P-H bonding MO (which is ma de up largely from the phosphorus 3pz AO). Electron density would then be transferred from the

P-H V-bond to the phenyl rings, thereby reducing the value of ¨para, which is observed to be the case. Additionally, this model predicts electron withdrawal from the P-H bond, which should result in a downfield shift of the P-H signal in the 1H-NMR spectrum, as is observed

(compare GH[Ph4PH] = +7.85 ppm with GH[Ph3PH2] = +7.32 ppm). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 115

If all four phenyl groups occupy the four basal positions in a square pyramidal phosphorane, they would be expected to accept more electron density from the phosphorus atom, both as a simple V-acceptor due to the preference of V-acidic ligands in the basal positions, and as a S-acceptor via hyperconjugation from the electron rich P-H bond. The observed 187 downfield shift of the ipso carbon and upfield shift of the para carbon of Ph4PH relative to

Ph3PH2 supports this view. The arguments presented about the geometries of, and the nature of the bonding in, the phosphoranes Ph4PH and Ph3PH2 are necessarily speculative. The fact that all five methyl groups in the analogous phosphorane penta-p-tolylphosphorane are found to remain equivalent in the 1H-NMR down to –60˚C suggests that dynamic ligand exchange is still occurring even at low temperatures.23 Because the difference in electronegativity between hydrogen and carbon is small, it is likely that both Ph4PH and Ph3PH2 undergo DLE processes at RT.

In contrast to the S-acidic phosphorus atom in the phosphoranes Ph4PH and Ph3PH2, – the negative value of ¨para (–4.1 ppm) for the phosphoranate Ph4PH2 suggests that in this case the phenyl groups are functioning as S-acceptors (i.e. the phosphoranate phosphorus atom is S-basic). This can be understood from examination of the MOs for the simple model – compound PF6 , which is an octahedral anion in which the central phosphorus atom partici- pates in three mutually perpendicular F-P-F 3c-4e bonds (Figure 6.40).

F F–

z F– – F 5+ y P F– – x F

F– F

(a) (b)

– 5+ Figure 6.40: The octahedral PF6 anion can be considered to be a P cation surrounded by six fluoride anions: (a) the basic geometry, and (b) the spacial location of the AOs.

Each of the three phosphorus 3p orbitals is involved with V-bonding in the linear F-P-F groups, but also has S-symmetry with respect to the lone pairs of the other four fluorine atoms. Because of the large difference in electronegativity between phosphorus and fluorine, – – 5+ the PF6 anion can be approximated as six F ions coordinating a central P cation. Starting with this model, each of the 3p orbitals on phosphorus can function as V-acceptors to the two fluoride ions lying along its axis (i.e. the P 3pz orbital can accept V-electron density from the two fluoride ions lying along the z axis). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 116

At the same time, the P 3p orbitals can also behave as S-acceptors to the four fluoride ions lying in its nodal plane. The 3p orbitals on phosphorus are able to function as S-acceptors to the lone pairs in the fluorine 2p orbitals because the high electronegativity of fluorine – reduces the flow of V-electron density into the P 3p orbitals. In the phosphoranate Ph4PH2 , the groups bound to phosphorus have a much lower electronegativity than fluorine, and so more electron density from the 3c-4e V-bonds flows into the P 3p orbitals, lowering their S- acidity so much that they can then behave as S-donors, thereby pushing electron density into the phenyl rings, resulting in the observed negative value for ¨para. The phosphorus-carbon coupling constants presented in Table 6D are reproduced below in Table 6H. Of the four compounds shown, the observed splittings for the phosphor- – anate Ph4PH2 are of a comparatively small magnitude. This may be taken as an indication of + a weakening of the P-C bonds in the phosphoranate relative to the phosphoranes and Ph4P .

Table 6H: The Observed Phosphorus-Carbon Couplings. Aromatic Carbon atom coupled to Phosphorus Compound* ipso ortho meta para

Ph4PH 79 Hz 13 Hz 13 Hz 3 Hz

Ph3PH2 83 Hz 19 Hz 14 Hz 3 Hz

+ – Li Ph4PH2 40 Hz 10 Hz 14 Hz < 1 Hz

+ – Ph4P Br 89 Hz 10 Hz 12 Hz 3 Hz

* + – Solvent used to acquire spectra was d8-THF, except for Ph4P Br , where CDCl3 was used.

– The phosphorus atom in Ph4PH2 is expected to be quite crowded, probably due to non-bonded repulsions between the ortho hydrogens on neighbouring phenyl rings. The very – 1 small 1-bond P-C splitting observed for the ipso carbon of Ph4PH2 ( JP-C = 40 Hz) suggests a reduction of the s-character of the P-C bonds (which is consistent with a lengthening of these bonds), and the highly positive value observed for ¨ipso (+42.3 ppm) is of a similar magnitude and direction to that observed for phenyl lithium (+58.1 ppm),186 where a substantial degree of negative charge is expected to be localised at the ipso carbon (Table 6E, page 108). An – interpretation of this information is that the phosphorus atom in Ph4PH2 is sterically crowded, and because of non-bonded repulsions between the ortho protons of neighbouring phenyl groups, the 4 P-C bonds are stretched, resulting in a partial heterolytic cleavage in the sense of G+P…CG–. This is consistent with the unimolecular decomposition via the expulsion of a Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 117 phenyl anion (described in Section 6.1). The large magnitude of the one-bond phosphorus- – 1 hydrogen coupling constant for Ph4PH2 ( JP-H = 447.1 Hz) is consistent with a strengthening of the P-H bonds, presumably made possible by both the low steric requirements of hydrogen, and the stretching of the four P-C bonds, thereby reducing the steric interactions between the two phosphorus-bound hydrogen atoms and the ortho hydrogens on the four phenyl rings.

6.6 Reaction of other mono-phosphonium salts.

The RT reaction with LiAlD4 was repeated using six other mono-phosphonium salts: + – + – n + – + – + – Ph3PMe I , Ph2PMe2 I , Ph3P Bu I , Ph3PCn Cl , Bn3PCn Cl , and 1,1-diphenylphosphorinanium perchlorate 136. The observations made on their RT reaction with LiAlD4 in THF will be discussed in order.

Ph _ ClO + 4 P Ph

136

+ – When powdered LiAlD43 was added to a suspension of Ph PMe I in anhydrous THF, 31 1 the P{ H}-NMR spectrum showed a singlet at GP + 22.9 ppm cor responding to unreacted + 129c Ph3PMe cation, a singlet at GP –4.4 ppm corresponding to Ph3P, a singlet at GP –26.2 ppm correspond ing to Ph2PMe , a1:2:3:2:1 q uintet at GP –7 0.0ppm (J = 51Hz) corresp onding to

Ph3PD2, and t w o new si gnals,oneatGP –103.6 ppm ( 1:1:1 triplet, J = 54 Hz) assigned to the 137 pePhosphoran h3P(Me)D , the other at GP –106. 4 ppm (1:2:3: 2:1 quintet, J = 48 Hz) assigned tothe phosphor ane Ph2P(Me)D2 138. These results can be explained usin g the same + – mechanism as that proposed for the reaction of Ph4P Br with LiAlD4 (Section 6.1), a s shown – in Scheme 6.41 (next page). The phosphoranate anion Ph3P(Me)D2 139 is not observed, presum ably because its rate of decomposition is too high to allow its concentration to exceed the threshold of detection of the 31P{1H}-NMR spectrometer. However, the observation of both Ph3PD2 and Ph2P(Me)D2 138 strongly support the existence of the phosphoranate as an intermediate. – + The initial step is the transfer of deuteride from AlD4 to Ph3PMe giving Ph3P(Me)D + – + 137. Despite the fact that Ph3PMe I appears to be much more soluble in THF than Ph4P – + Br (Ph3PMe : GP +22.9 ppm; no signal corresponding to unreacted phosphonium cation was + – observed in the reaction of Ph4P Br with LiAlD4), the reaction proceeded at a much lower + – + – rate. Interestingly, as in the case of Ph4P Br , a suspension of Ph3PMe I in anhydrous THF showed no signal in the 31P{1H}-NMR spectrum, suggesting that the initial step in this + – reaction may be the formation of a soluble ion pair Ph3PMe AlD4 , which can then slowly Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 118

– – decompose into Ph3P(Me)D 137 and AlD3Br , with AlD3Br presumably able to dispropor- – tionate back to AlD4 , as follows:

– – – 4AlD3Br o 3AlD4 + AlBr4 .

The mechanism postulated in Scheme 6.41 also predicts the production of benzene (as a mixture of PhH and PhD) and methane (as MeH and MeD) as by-products, but they were neither isolated or detected.

Me _ + I P Ph Ph Ph

LiAlD4

D P – MeDPh – PhD P Ph P Me Ph Ph Ph Ph Ph Me Ph 137

– D2 LiAlD4 – D2

_ D __D D Ph Ph – Me Ph Ph – Ph P Ph P P Me Ph Ph Me Ph D D D 139138

+ – Scheme 6.41: The proposed mechanism for the RT reaction of Ph3PMe I with LiAlD4 in THF.

When the reaction was repeated using powdered LiAlH4, the unreacted phosphonium 31 salt and both Ph3P and Ph2PMe were observed using P-NMR, along with the corresponding 140 hydrido derivative Ph3P(Me)H at GP –102.9 ppm (doublet, J = 343 Hz). However, neither Ph3PH2 or Ph2P(Me)H2 141 were observed. Because Ph3PH2 is known to be quite stable, enough for observation by 31P-NMR spectroscopy, its absence from the spectrum indicates that it was never formed in the first place, which is probably also true for Ph2P(Me)H2 141. – Therefore, either the phosphoranate Ph3P(Me)H2 142 was never formed, or it decomposed via a different pathway, giving the phosphine products directly (Scheme 6.42, next page). If phosphoranate 142 was never formed, this suggests that either the observed phosphorane Ph3P(Me)H 140 reacts at a much lower rate with LiAlH4 than Ph3P(Me)D 137 does with LiAlD4, and that had the mixture been allowed to react for a longer time, the two missing dihydrophosphoranes Ph3PH2 and 141 may have been observed. This could be because either Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 119

– – AlH4 is less nucleophilic than AlD4 , or Ph3P(Me)H 140 is less electrophilic than its deuterio isomer Ph3P(Me)D 137. Alternatively, it could mean that the rate of direct decomposition of

Ph3P(Me)H 140 to either Ph3P or Ph2PMe is much higher than the rate of addition of a second hydride from LiAlH4.

Me _ + I P Ph Ph Ph

LiAlH4

H P – MeHPh – PhH P Ph P Me Ph Ph Ph Ph Ph Me Ph 140

– H2 LiAlH4 – H2

_ H __H H Ph Ph – Me Ph Ph – Ph P Ph P P Me Ph Ph Me Ph H H H 142 141

+ – Scheme 6.42: The proposed mechanism for the RT reaction of Ph3PMe I with LiAlH4 in THF.

A similar result was obtained when powdered LiAlD4 was added to a suspension of + – 31 1 Ph2PMe2 I in dry THF. Observation by P{ H}-NMR showed a singlet at GP +22.5 ppm + 129c corresponding to unreacted Ph2PMe2 cation, a singlet at GP –26.2 ppm due to Ph2PMe, a singlet at GP –45.3 ppm corresponding to PhPMe2, a 1:2:3:2:1 quintet at GP –106.4 ppm 138 (J = 48 Hz) corresponding to Ph2P(Me)D2 , and two new signals, one at GP –116.9 ppm 143 (1:1:1 triplet, J = 56 Hz) assigned to the phosphorane Ph2P(D)Me2 , another at GP –143.7 ppm (1:2:3:2:1 quintet, J = 42 Hz) assigned to Me2P(Ph)D2 144. Again, formation of these species is easily accounted for by the same mechanism that has been proposed previously + – (Scheme 6.43, next page); as in the case of the reaction of Ph3PMe I described above, it – is assumed that the stability of the phosphoranate anion, in this case Ph2Me2PD2 145, is too low to allow its concentration to rise above the level of detection for the spectrometer. How- ever, formation of the two observed dideuterophosphoranes 138 and 144 imply the existence of phosphoranate 145 as an intermediate. This time, all three corresponding hydrophosphoranes were observed with 31P{1H}-

NMR when the reaction was repeated using LiAlH4: Ph2P(Me)H2 was observed at GP –104.7 Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 120

ppm, Ph2P(H)Me2 at GP –116.2 ppm, and Me2P(Ph)H2 at GP –142.3 ppm. Only Ph2P(H)Me2 had a long enough half-life to allow the time necessary to resolve the doublet in the 31P-NMR spectrum (J = 389 Hz).

Me _ + I P Ph Ph Me

LiAlD4

D P – MeDMe – PhD P Ph P Ph Me Ph Me Me Me Ph Ph 143

– D2 LiAlD4 – D2

_ D __D D Ph Me – Me Ph Ph – Ph P Me P P Ph Ph Me Me Me D 138 D 145D 144

+ – Scheme 6.43: The proposed mechanism for the RT reaction of Ph2PMe2 I with LiAlD4 in THF.

+ – + – For the reactions of both Ph3PMe I and Ph2PMe2 I with either LiAlH4 or LiAlD4 in

THF at RT, the proportion of phosphines produced in the reaction (Ph3P and Ph2PMe from + – + – Ph3PMe I , Ph2PMe and PhPMe2 from Ph2PMe2 I ) was observed to be far greater than the + – + – proportion of Ph3P derived from either Ph4P Br or Ph3PBn Br . A interpretation of these results is that the methylated phosphoranes are less stable than their phenylated analogues, possibly due to the greater electron density localised on phosphorus, due to the V-donor in- ductive effects of methyl groups relative to phenyl groups. n + – When Ph3P Bu I was added to a solution of LiAlD4 in dry THF at RT, and the 31 1 mixture examined using P{ H}-NMR, the major signal was a 1:1:1 triplet at GP –90.1 ppm n 146 (J = 49 Hz), assigned the structure of Ph3P( Bu)D , as well as a singlet at GP +24.8 ppm n + due to unreacted Ph3P Bu cation (observed at GP +25.1 ppm in EtOH solution), a second n singlet at GP –4.4 ppm (Ph3P), and a third at GP –15.7 ppm (Ph2P Bu). As the reaction pro- n n ceeded, the concentration of Ph2P Bu increased at the expense of Ph3P( Bu)D 146, while the signal due to Ph3P increased only slowly. No evidence for the formation of the phosphor- n – anate intermediate Ph3P( Bu)D2 147 was obtained, either by direct observation, or by its de- n 31 1 composition products (i.e. neither Ph3PD2 or Ph2P( Bu)D2 148 were observed in the P{ H}- Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 121

n NMR spectrum). This suggests that Ph3P( Bu)D 146 decomposes directly, presumably eli- n n minating either BuD or PhD, to give Ph3P or Ph2P Bu, respectively.

_ D D D Ph Ph Ph Ph P Bun P P Bun Ph Bun Ph Ph Ph D D

146 147 148

n The reason for the apparent inability of the phosphorane Ph3P( Bu)D 146 to accept a second deuteride ion is possibly due to the steric requirements of the n-butyl group: although the carbon bound to phosphorus is a methylene group, and so relatively unhindered, the other three carbons in the chain would be expected to move around in solution, thereby blocking – the close approach of a second AlD4 anion. However, this explanation seems unlikely, as evidence for the ready formation of both phosphoranes and phosphoranates for the benzyl- + – phosphonium salts R3PBn Br (R = Ph, Bn) has been presented in Section 6.1. + – + – Two cinnamylphosphonium salts, Ph3PCn Cl and Bn3PCn Cl , were also examined. + – To a suspension of Ph3PCn Cl in dry THF was added powdered LiAlD4, the only product 31 1 observed by P{ H}-NMR was Ph3P (at GP –4.4 ppm). The order of addition was reversed + – (i.e. Ph3PCn Cl was added to a solution of LiAlD4 in dry THF) and again Ph3P was the only product observed by 31P{1H}-NMR. The E-cinnamyl group is expected to be at least as good a leaving group as benzyl, and possibly much better, as the E-cinnamyl anion has both benzylic and allylic character. – Hence, if the phosphoranate Ph3P(Cn)D2 149 is formed at all, rapid expulsion of E- cinnamyl anion should occur leading only to Ph3PD2. Because no trace of Ph3PD2 is observed to form, even when the source of deuteride ion is kept in excess at all times by the slow addition of the salt to a solution of LiAlD4 in THF, it was concluded that the phosphoranate is not formed during the reaction. This, and the fact that no direct observation 31 1 of the intermediate phosphorane Ph3P(Cn)D 150 was made using P{ H}-NMR, suggest that either the decomposition of this phosphorane is extremely rapid, or that it is never formed in the first place, with the E-cinnamyl group being cleaved by an alternative mechanism. + – The hydrocarbon by-product from reaction between Ph3PCn Cl and LiAlD4 in Et2O at 20˚C has been characterised, and was found to be exclusively (±)-3-deuterio-3-phenylpro- 189 – pene 151. The authors suggested that this results from direct attack of AlD4 on the alkenyl carbon D to the phenyl group of the E-cinnamyl group (Scheme 6.44). The resulting movement of the S-electrons causes the expulsion of Ph3P, giving the deconjugated phenyl- alkene 151. An alternative explanation is a 6-electron cheletropic rearrangement,182 analogous to that postulated to occur in the case of Ph3P(Bn)D 126. The deuterium atom attached to phosphorus moves across to the alkenyl carbon J to the phosphorus atom, with ul- Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 122

timate loss of Ph3P. The absence of any 3-deuterio-1-phenylpropene 152 recovered from the reaction mixture189 indicates that the simple reductive-elimination of the E-cinnamyl and deuterium groups from the postulated phosphorane Ph3P(Cn)D 150 does not occur. Also ex- – cluded is the direct attack of AlD4 upon the methylene carbon of the E-cinnamyl group on + the phosphonium cation Ph3PCn .

Ph + P Ph Ph Ph D – AlD3 (a) Ph _ – Ph3P DAl D 151 D D

– Ph3P Ph Ph + P Ph Ph – AlD3 (b) Ph D D _ Ph DAl P D Ph Ph 150 D CH2D

109

CH3 – Ph3P D (c) D D Ph P 107 110 Ph Ph 126 Scheme 6.44: (a) A possible pathway to (±)-3-deuterio-3-phenylpropene 151; (b) an alternative 150 pathway involving phosphorane Ph3P(Cn)D ; (c) the analogous mechanism proposed for the 126 decomposition of Ph3P(Bn)D .

+ – The failure to observe either Ph3P(Cn)D 150 or Ph3PD2 in the reaction of Ph3PCn Cl with LiAlD4 is consistent with a much higher rate of decomposition of the postulated phosphorane Ph3P(Cn)D 150 compared to its benzyl analogue Ph3P(Bn)D 126 (even though

Ph3P(B n)D 126 is also not directly observable in the spectrum, the observation of Ph3PD2 – indicates the previous formation of both Ph3P(Bn)D 126 and Ph3P(Bn)D2 127). This is also consistent with the mechanisms shown in parts (b) and (c) of Scheme 6.44: the decomposition of Ph3P(Bn)D requires the disruption of the aromaticity of the benzyl group, forming the intermediate (±)-6-deuterio-5-methylenecyclohexa-1,3-diene 107, whereas the similar decomposition of Ph3P(Cn)D 150 requires only the deconjugation of the carbon-carbon double bond, with the aromaticity of the pendant phenyl ring remaining undisturbed. Hence, the energy barrier to decomposition would be expected to be significantly greater for the elimination Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 123

from the benzylphosphorane Ph3P(Bn)D 126 compared to analogous elimination from E-cin- namylphosphorane Ph3P(Cn)D 150, if the mechanism proposed above occurs. In order to test this idea, it was decided to prepare a phosphonium salt possessing both E-cinnamyl and benzyl groups, to observe the direct competition between them. Due to the + – availability of a commercial sample of Bn3P, the salt Bn3PCn Cl was prepared. A pure sample was not obtained because of competitive oxidation of Bn3P, despite deoxygenation of the solvent (toluene) with argon before and during the reaction. + Regardless of the order of addition, the reaction of the mixture containing Bn3PCn – Cl with LiAlD4 in dry THF at RT gave only one signal at GP –11.4 ppm, corresponding to

Bn3P. In both cases, no direct indication of phosphorane formation was obtained from the 31 1 P{ H}-NMR spectra, and the signal corresponding to Bn3P=O remained unchanged. From these results, it is clear that the E-cinnamyl group functions as a better leaving group than both phenyl and benzyl. Finally, the reaction of the cyclic salt 1,1-diphenylphosphorinanium perchlorate 136 was examined. The salt was suspended in THF and powdered LiAlD4 was added. Initially, 31 1 the P{ H}-NMR spectrum showed only two signals, the first a singlet at GP +17.7 ppm corresponded to the unreacted phosphonium salt,129c the second a 1:1:1 triplet (J = 48 Hz) at

GP –111.8 ppm corresponded to the cyclic phosphorane 153. The reaction was allowed to proceed, and more signals appeared, among them three singlets: the first one at GP +38.4 ppm was unidentified, another one at GP –15.5 ppm corresponding to what was presumably the ring-opened phosphine (5-deuteriopentyl)diphenylphosphine 154, while the third appeared at

GP –32.6 ppm corresponding to the cyclic phosphine 1-phenylphosphorinane 155.

Ph _ ClO4 D + P Ph2P Ph P Ph P Ph Ph D 136 153 154 155

This case was interesting, as no evidence was obtained to indicate that any phosphoranate 156 was formed, even though LiAlD4 was kept in excess at all times (see Scheme 6.45, next page). In addition, the cyclic phosphorane 153 was observed to form extremely readily, much faster than observed for the formation of its diaryldialkyl analogue Ph2P(D)Me2

143, or even Ph3P(D)Me 137, from their respective phosphonium salts. The answer may be due to the cyclic nature of both the salt and the phosphorane. Because in each case the phosphorus atom is part of a 6-membered ring, ring-strain will play an important role in determining whether a particular derivative is stabilised or destabilised. The apparent ease of formation of the cyclic phosphorane 153 from the reaction of cation 136 with LiAlD4 suggests that the ring-strain present in the cation may be at least Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 124 partially relieved upon formation of the phosphorane. In contrast to this, it may be argued that the apparent failure to form the corresponding phosphoranate anion 156 by addition of a second deuteride ion possibly indicates that the ring-strain in this species is much greater than it is for either the cation 136 or the phosphorane 153. If phosphoranate 156 were to form, the two P-CH2 ring bonds could not occupy trans positions in the octahedral species.

Hence, the cis disposition of the ring bonds at phosphorus (i.e. the angle CH2-P-CH2 | 90˚) is presumably the only geometric possibility. Therefore, if ring-strain is thought to disfavour the formation of phosphoranate 156, then either the cis ring arrangement must also be too strained, or addition of the second deuteride ion would, for stereochemical reasons, force a trans arrangement of the ring, and is therefore impossible without P-C bond breakage. If it is the case that an approximately 90˚ angle in the 6-membered ring at phosphorus is too strained in the phosphoranate 156, then it is also reasonable to assume that the same may be true for the cyclic phosphorane 153. Assuming a trigonal bipyramidal geometry for the phosphorane, there are three theoretically possible ring configurations: the two ring methylenes bound to phosphorus are either diaxial, axial-equatorial, or diequatorial. Clearly, the diaxial arrangement is far too strained, being equivalent to the already discounted trans arrangement in an octahedral geometry. The axial-equatorial arrangement requires an angle of about 90˚ at phosphorus, being equivalent to the cis arrangement in an octahedral geometry, which is assumed to be too strained. This leaves the diequatorial arrangement, with a C-P-C angle of about 120˚, as the preferred conformation (Scheme 6.45).

_ _ Ph D D ClO4 + LiAlD4 LiAlD4 Ph P Ph P P Ph Ph Ph D

136 153 156

Scheme 6.45: Phosphoranate 156 is not formed from phosphorane 153.

If this is the case, then both the BPR and TR mechanisms would be expected not to operate, as in both cases there is no possible way of achieving dynamic ligand exchange without changing the diequatorial configuration of the ring.

F F P F P F F F

157 158

Ring strain has been shown23 to influence the barrier to DLE in the two trifluorophos- phoranes 157 and 158. The 19F-NMR spectrum of 157 shows a single resonance at RT, indi- Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 125 cating that all three fluorine atoms are equivalent on the NMR timescale. When the solution is cooled down to –70˚C, the spectrum shows two signals due to axial and equatorial fluorine atoms in the ratio of 2:1.23 These observations are consistent with rapid exchange of the fluorine atoms through interconversion between the isomers 157a and 157b at RT, presumably via the BPR mechanism, using one of the carbon atoms attached to phosphorus as a pivot (Scheme 6.46). Cooling the sample down to –70˚C “freezes out” the phosphorane into isomer 157a, which possesses 1 equatorial and 2 axial fluorine atoms.

P F P F F F F 157a 157b

Scheme 6.46: Isomerisation of 157 between isomers 157a and 157b via the BPR mechanism.

This behaviour may be explained by the competition between ring strain and site preference due to electronegativity. Because of the extreme electronegativity of fluorine, it will prefer to occupy the two axial positions, favouring isomer 157a. In direct opposition to this, the ring will experience more strain when it is diequatorial (isomer 157a) in comparison to isomer 157b, where it is axial-equatorial.23 The third possibility, where the ring is diaxial, cannot be realised with a 5-membered ring due to extreme ring strain, and therefore will not be considered. At low temperature (–70˚C), isomer 157a predominates, showing that in this case the preference for keeping the electronegative fluorine atoms axial exceeds the unfavourable ring strain. However, the situation with the 5-membered ring has the effect of de- stabilising the favoured isomer 157a while at the same time stabilising the unfavoured isom er 157b. This results in the observed energy barrier to interconversion being lower (ca 30 –1 –1 kJ.mol ) than is usual for phosphoranes R2PF3 (ca 60 kJ.mol ), allowing the scrambling of the fluorine atoms at RT.23 In the case of phosphorane 158, the 19F-NMR spectrum shows signals due to inequivalent axial and equatorial fluorine atoms up to +100˚C.23 This can be explained by the 6- membered ring experiencing little or no ring strain in either the diequatorial (isomer 158a) or axial-equatorial (isomer 158b) conformations. As in the case of 157, formation of the diaxial isomer is presumed not to occur due to a prohibitively high ring strain, and so can be ignored. Because now the ring has little or no preference for isomer 158a or isomer 158b, the axial preference of fluorine is unchallenged (Scheme 6.47), leading to 158 existing only as isomer 158a up to at least +100˚C23 (i.e. phosphorane 158 does not undergo DLE processes below +100˚C). Chapter 6: The Reaction of Mono-Phosphonium Salts with Lithium Aluminium Hydride. 126

P F P F F F F 158a 158b

Scheme 6.47: Phosphorane 158 is “frozen” into isomer 158a below +100˚C.

In the case of phosphorane 153, which also possesses a 6-membered ring, the effects of ring strain should be minimal. In addition, the apicophilicities of both hydrogen and phenyl are greater than alkyl groups, and therefore isomers 153a and 153d should be favoured over isomers 153b and 153c (Scheme 6.48).

D Ph

P Ph P D P Ph P D Ph Ph Ph Ph D Ph 153a 153b 153c 153d

Scheme 6.48: Possible trigonal bipyramidal isomers of phosphorane 153. Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride.

Because of the large number of interesting results obtained with the simple mono- phosphonium salts (Chapter 6), it was decided to apply the same methodology to some bis- phosphonium salts. As in the reactions with naphthalene radical anion described in Part 1, it is convenient to discuss the behaviour of these salts in terms of whether or not they possess an 2+ ethylene bridge (i.e. R3PCH2CH2PR3 ). Seven bis-phosphonium salts were examined, each being reacted with either LiAlH4 or LiAlD4 in THF. Two of these salts contain only propylene bridges, and will be discussed as a group in Section 7.2. The remaining five salts have at least one ethylene bridge, known to be easily lost (as ethylene gas,69 characterised as its dibromide by passing the evolved gasses through a solution of Br2 in CCl4) in reductions using lithium aluminium hydride in THF, and will be considered as a group in the following section.

7.1 Reactions with bis-phosphonium salts containing a 2-carbon bridge.

The five bis-phosphonium salts discussed in this section all contain at least one ethylene bridge separating the two phosphorus cations, and generally behaved differently from those bis-salts which lacked any ethylene bridges. The two cyclic bis-phosphonium salts 54 and 59 will be considered first, as the discussion of their reactions with LiAlH4 clarify the results obtained from the remaining three acyclic bis-salts.

_ _ 2Br Ph 2Br Ph Ph + Ph + P P P +P + Ph Ph Ph Ph 54 59 _ _ _ 2Br Ph 2Br nBu 2Cl Cn Ph nBu Ph Ph + nBu + Ph + P P P P P P + Ph + nBu + Ph Ph nBu Ph Ph nBu Cn 159 160 161

When powdered LiAlH4 was added to a suspension of the cyclic bis-salt 54 in dry THF, vigorous evolution of what was presumed to be gaseous ethylene69 was observed. The 31 1 P{ H}-NMR spectrum showed the bis-phosphine 56 at GP = –12.3 ppm as being the only Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 128 soluble product. Reversing the addition, by adding the salt slowly to an excess of 1.0 M 31 1 solution of LiAlH4 in THF, also resulted in vigorous evolution of gas, and the P{ H}-NMR spectrum this time showed two doublets (Figure 7.1), each with an observed splitting of J =

46 Hz (3-bond P-P coupling); one of the doublets was centred at GP –13.1 ppm, the other at 31 GP –79.3 ppm. The corresponding P-NMR spectrum of the mixture showed that the signal at GP –79.3 ppm had been split into a 1:2:1 triplet (J = 300 Hz), indicating that this signal was due to a phosphorane with two equivalent directly bonded hydrogen atoms.

-10 -15 -75 -80 (ppm) (ppm)

-12-14 -16 -18 -20 -78 -80 -82 -84 -86 (ppm) (ppm)

(a) phosphino signal. (b) phosphoranyl signal.

Figure 7.1: The 31P{1H}-NMR spectra of the phosphino-phosphorane 162 (insets show the corresponding 31P-NMR spectra).

The existence of phosphorus-phosphorus coupling in the second solution (when the bis-salt 54 was added to a large excess of LiAlH4) shows that the two phosphorus atoms are within coupling distance. As the bis-phosphine 56 was shown to be the ultimate product in the first reaction, it seems probable that an ethylene bridge also separates the two chemically inequivalent phosphorus atoms in the second reaction. The chemical shifts of the phosphorus atoms, and the fact that there are directly bound hydrogen atoms only on the upfield phos- Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 129

phorus signal strongly suggests the mixed phosphino-phosphorane Ph2PCH2CH2P(H2)Ph2 162 is the species under observation. A possible mechanism explaining the formation of mixed phosphino-phosphorane 162 is analogous to that proposed for the reductive-elimination of toluene (Scheme 7.2a) from the phosphorane intermediate Ph3P(Bn)D 126 (Section 6.2). Initially, the bis-salt 54 reacts with

2 equivalents of LiAlH4 to give the cyclic bis-phosphorane 163 (not directly observed), which then could undergo an intramolecular hydride shift from one phosphorus to the other (Scheme 7.2b), accompanied by elimination of one of the bridges as ethylene, giving the mixed phosphino-phosphorane 162. An intermolecular hydride shift cannot be ruled out, although this would seem less likely to occur.

CH2D

109

CH3 – Ph3P D (a) D D Ph P 126 107 110 Ph Ph

Ph Ph H H P – H C=CH P (b) H 2 2 Ph Ph P Ph Ph P 163 H 162 Ph Ph

Scheme 7.2: The proposed mechanisms for the elimination of (a) 107 (±)-6-deuterio-5-methylene- 126 163 cyclo-1,3-diene from Ph3P(D)Bn , and (b) ethylene from the cyclic bis-phosphorane , which 54 is formed from the reaction of LiAlH4 with the cyclic bis-salt in THF at RT.

_ 2Br Ph O Ph + Ph P excess Ph + P _ P P Ph OH Ph Ph Ph 54 164

Scheme 7.3: The reaction of bis-salt 54 with excess OH– gives the monoxide 164.

The reaction of the cyclic bis-salt 54 with excess LiAlH4 (giving the phosphino-phosphorane 162) is paralleled by its reaction76 with excess OH–, giving the bis-phosphine monoxide 164 (shown above in Figure 7.3). In both cases, a kind of disproportionation occurs: in 54, both phosphorus atoms have four bonds, with the observed product containing one Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 130 phosphorus with three bonds, the other having five bonds (with the P=O counted as a double bond). The cyclic 7-membered bis-salt 59 behaved in an analogous manner to its 6-membered analogue 54: addition of the salt to an excess of 1.0 M LiAlH4 in THF resulted in vigorous evolution of a gas, presumed to be ethylene.69 The 31P{1H}-NMR spectrum shows two singlets, one at GP –16.2 ppm corresponding to a Ph2P- group, the other at GP –83.4 ppm (Figure 7.4). The upfield signal was again observed to split into a 1:2:1 triplet (J = 307 Hz) when the 31P-NMR spectrum was acquired, and therefore corresponds to a dihydrophosphoranyl group:

-P(H2)Ph2.

-15 -20 -80 -85 (ppm) (ppm)

-16 -18 -20 -22 -24 -84 -86 -88 -90 (ppm) (ppm)

(a) phosphino signal. (b) phosphoranyl signal.

Figure 7.4: The 31P{1H}-NMR spectra of the phosphino-phosphorane 165 (insets show the corres- 31 60 ponding P-NMR spectra). The signal at GP –16.9 ppm is Ph2P(CH2)3PPh2 .

By analogy with the reaction of the 6-membered cyclic bis-salt 54, these results are consistent with the formation of another phosphino-phosphorane 165 (Scheme 7.5, next page). The coupling between the two phosphorus atoms is no longer observed simply because they are now separated by four bonds instead of three. Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 131

_ 2Br Ph H Ph Ph + LiAlH4 Ph P Ph P P +P Ph – H2C=CH2 Ph Ph H 59 165 Scheme 7.5: (3-Diphenylpropyl)dihydrodiphenylphosphorane 165, the product from the RT reac- 59 tion of the bis-salt with and excess of LiAlH4 in THF.

The linear bis-salt 159 was examined next. When the salt was slowly added to an excess of 1.0 M LiAlH4 in THF, evolution of gas was again observed. There were two signals 31 1 in the P{ H}-NMR spectrum, corresponding to Ph3P and Ph3PH2. Again, this is consistent with the mechanism proposed above for the elimination of ethylene.

When powdered LiAlD4 was added to a suspension of 160 in dry THF (Scheme 7.6), 31 1 several species were observed in the P{ H}-NMR spectrum: a singlet at GP –31.0 ppm cor- n responding to Bu3P (which was the major product), a 1:1:1 triplet at GP –70.5 ppm (J = 31 n Hz) corresponding to Bu2PD, and a 1:1:1 triplet at GP –97.8 ppm (J = 39 Hz), which was assigned as the bis-phosphorane 166 (Y = D).

P _ n n Bu 2Br nBu Y nBu Bu n nBu nBu Bu nBu + LiAlD4 P P nBu P P nBu + nBu nBu nBu nBu nBu Y P 160 166 nBu D nBu 160 166 Scheme 7.6: The reaction of the bis-salt with LiAlD4 in THF gave the bis-phosphorane (where Y = D).

n + – As in the case of the mono-phosphonium salt Ph3P Bu Br , there was no trace of n 31 1 either Bu3PD2 167 or the bis-phosphoranate 168 detected using P{ H}-NMR. Presumably, n Bu3P arises due to either direct reductive-elimination of the bridge methylene and the deuterium atom, or the same cyclic elimination of ethylene occurs, but this time much more slowly, perhaps because of the steric shielding provided by the three n-butyl groups attached to n each phosphorus. The small amount of Bu2PD observed could possibly arise from reductive-cleavage of nBuD from traces of 167. The reaction was repeated by adding powdered n LiAlH4 to a suspension of the salt in dry THF, and again both Bu3P and the corresponding n Bu2PH were observed; however, no signal corresponding to the bis-phosphorane 166 (Y = H) was observed. Finally, the bis-E-cinnamyl bis-salt will be discussed. Addition of 161 slowly to an excess of 1.0 M LiAlH4 in THF gave the bis-phosphine 56 as the only product detected by 31P-NMR. This was the only bis-phosphonium salt observed where the ethylene bridge was not lost, but instead exclusive cleavage of the E-cinnamyl groups occurred. Both the mono- Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 132

+ – + – phosphonium salts Ph3PCn Cl and Bn3PCn Cl (Section 6.5) also lost their E-cinnamyl groups exclusively, suggesting that this group is an excellent leaving group with regards to + – lithium aluminium hydride reductive cleavage. As in the case of both Ph3PCn Cl and + – 31 Bn3PCn Cl , no phosphorane or phosphoranate intermediates were observed by P-NMR, and so neither direct attack of hydride ion (or its equivalent) on the alkenyl carbon alpha to the cinnamyl phenyl group, or formation of a phosphorane intermediate which then can reductively-eliminate 3-phenylpropene, are able to be ruled out as the mechanism. The observed facile nature of the E-cinnamyl group in lithium aluminium hydride reductions of quaternary phosphonium salts suggests an application for the preparation of bicyclic heterocycles containing bridgehead phosphorus atoms, which was discussed earlier in Chapter 2. The synthesis of the model compound 1,4-diphosphabicyclo[2.2.2]octane 37 reported previously (Chapter 1) involved the synthetic pathway, shown again in Scheme 7.7. Although benzyl is a good leaving group, the new insights into the lithium aluminium hydride reductive cleavage that have been acquired from the observations presented in this chapter, particularly with regard to the facile expulsion of ethylene from bis-salts possessing an ethylene bridge, suggest a reason for the low yields achieved in the synthetic route to the bicyclic bridgehead bis-phosphine 1,4-diphosphabicyclo[2.2.2]octane 37 (see Scheme 7.7). It has been shown previously69 that the products of the reaction between ethane-1,2-bis(ben- zyldiphenylphosphonium) dibromide and LiAlH4 in refluxing THF are Ph2PBn and 56, which corresponds precisely with the behaviour of 159 described above. Hence, loss of the ethylene bridge is competitive even with loss of benzyl.

R R R P R BrCH2CH2Br R + LiAlH4 P P R +P P R R R _ R R R R 2Br BrCH2CH2Br

trans cis _ R R 2Br R R LiAlH R + P P 4 P P P P + R R R BrCH2CH2Br BrCH2CH2Br _ Polymeric 2Br Material LiAlH4 R P ++P R P P 37 Scheme 7.7: The synthetic route (R = Bn) used by Mann to prepare the bicyclic bridgehead bis- phosphine 1,4-diphosphabicyclo[2.2.2]octane 37; substituting R = Cn may give higher yields during the 6 step procedure. Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 133

In contrast to this, E-cinnamyl appears to be a much better leaving group under these conditions, with the model bis-salt 161 showing no loss of the ethylene bridge upon reaction with LiAlH4 in THF at RT. Therefore, a better route to the bicyclic bis-phosphine 1,4-diphosphabicyclo[2.2.2]octane 37 would possibly be a modified synthetic route, identical in principle to the route shown above (Scheme 7.7), except where all the benzyl groups have been replaced with E-cinnamyl groups. Tris(E-cinnamyl)phosphine (Cn3P) is not commercially available, but should be easily made from the reaction of the Grignard reagent CnMgCl with PCl3. Of course, only a bis-(E-cinnamyl)phosphonium salt possessing a single ethylene bridge has yet been investigated, so the success of the latter steps of the proposed synthesis are entirely speculative.

7.2 Reactions with bis-phosphonium salts containing no 2-carbon bridges.

The two bis-salts which have no ethylene bridges were found to behave differently from those just discussed, and are the subject of this section. _ _ 2Br Ph 2Br Ph Ph ++Ph + + P P P P Ph Ph Ph Ph Ph Ph 169 61

When the acyclic bis-salt 169 was slowly added to a suspension of excess LiAlD4 in dry THF, initially two signals were observed in the 31P{1H}-NMR spectrum. These were a singlet at GP +22.4 ppm corresponding to the unreacted salt, and a 1:1:1 triplet (J = 49 Hz) at GP –90.4 ppm, which was assigned the structure of the bis-phosphorane 170, the analogue of the postulated intermediate in the mechanism described above for the reaction of bis-salts containing ethylene bridges. The reaction was allowed to continue, and new signals were detected: two singlets at GP –15.7 ppm and GP –4.4 ppm, corresponding to the phosphines 60 and Ph3P, respectively, and a 1:2:3:2:1 quintet (J = 67 Hz) at GP –199.7 ppm assigned to the bis-phosphoranate dianion, 171 (the upfield chemical shift and the large magnitude for the – coupling constant agree well with that found for the mono-phosphoranate Ph4PD2 , i.e. GP –187.2 ppm and J = 69 Hz). Unlike those described in the previous section, the bis-phosphorane cannot decompose by eliminating the bridge, and so behaves in an analogous manner to the simple phosphorane Ph4PD, with each phosphorus atom accepting a second deuteride ion (see Scheme 7.8, next page). It is interesting that bis-phosphoranate 171 can be directly observed, as the analogous – n – triarylalkylphosphoranates Ph3P(Me)D2 139 and Ph3P( Bu)D2 147 were never able to be detected directly by 31P{1H}-NMR; only their decomposition products were ever seen (Section 6.6). The phosphine 60 could either have been formed by expulsion of two equivalents of phenyl anion from bis-phosphoranate 171 followed by loss of D2, or by elimination of two Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 134 molecules of PhD from the bis-phosphorane 170, as shown in Scheme 7.8. Curiously, the bis(dideuteriophosphorane) 172 was not observed.

D D Ph Ph ++ Ph Ph LiAlD4 P P Ph P P Ph (i) Ph Ph Ph Ph 169 Ph Ph 170 – 2PhD (ii) (iii) LiAlD4

_ D D 2 Ph Ph Ph Ph Ph Ph P P P P Ph Ph Ph Ph 60 D D _ 171 (v) – 2D2 – 2Ph (iv)

D D

Ph P P Ph Ph Ph D D 172

169 Scheme 7.8: The postulated intermediates in the reaction of LiAlD4 with the bis-salt .

The cyclic bis-salt 61 behaved differently from all the other bis-salts examined: when 31 1 the salt was slowly added to a suspension of excess LiAlD4 in THF, the P{ H}-NMR spectrum showed only one signal, a 1:1:1 triplet (J = 12 Hz) at GP –41.1 ppm. After about 10 minutes, this signal had begun to decay, with other signals appearing: one at GP –161.8 ppm, which rapidly disappeared, several at about GP –17 ppm, a 1:2:3:2:1 quintet at GP –84.5 ppm (J = 47 Hz), three singlets at GP –87.5 ppm, –87.7 and –87.8 ppm, and three 1:1:1 triplets, two overlapping at GP –93.1 and –93.3 ppm (J = 47 Hz) and one at GP –97.2 ppm (J = 40 Hz).

After 4 hours had passed, only the signals appearing at about GP –17 and –88 ppm remained. The interpretation of these results is as follows: there is an initial addition of deuteride ion to 61 to give the mono-cation 173, observed at GP –41.1 ppm (1:1:1 t: J = 12 Hz), which exhibits a deuterium atom bridging the two phosphorus atoms (Scheme 7.9). This species reacts slowly with excess LiAlD4, accepting a second deuteride ion to give the bis-bridged neutral compound 174, resonating upfield at GP –161.8 ppm. This species can presumably then rearrange to the symmetrical cyclic bis-(monodeute- riophosphorane) 175, which appears as a 1:1:1 triplet at GP –97.2 ppm. Two more singlets, at GP –16.7 and –17.2 ppm, were presumed to correspond to the two overlapping 1:1:1 triplets at GP –93.1 and –93.3 ppm, and were assigned the structure of cis and trans cyclic phosphino- Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 135 monodeuteriophosphoranes 176, presumably formed from the reductive-elimination of one equivalent of PhD from 175. Loss of a second equivalent of PhD would then give the cyclic bis-phosphine 1,5-diphenyl-1,5-diphosphocane 12 as a mixture of cis and trans isomers.

_ _ 2Br Br LiAlD4 LiAlD4 + + Ph Ph Ph Ph P P P + P P P Ph Ph D D D Ph Ph 61 Ph 173 Ph Ph Ph 174 LiAlH4 refluxing THF

Ph Ph Ph – PhD – PhD P P P P P P D Ph Ph Ph Ph D Ph Ph D cis- and trans-12 cis- and trans-176 175

61 Scheme 7.9: The reaction of cyclic bis-salt with LiAlD4.

An alternative rearrangement of the bis-bridged species 174 is that both deuterium atoms become attached to one of the phosphorus atoms (Scheme 7.10), with a corresponding shift of one of the phenyl groups to the other phosphorus atom, giving the cyclic bis-phos- 177 phorane containing one R3PD2 group (observed as the 1:2:3:2:1 quintet at GP –84.5 ppm) and one R5P group (a singlet at GP –87.5 ppm). Subsequent loss of D2 from this species gives the cyclic phosphino-phosphorane 178, which, because of the high inversion barrier at the phosphino phosphorus atoms, exists as cis and trans isomers. Both isomers appear as two singlets, one for each of the two inequivalent phosphorus atoms: the major isomer is observed at GP –16.8 ppm and –87.8 ppm, with the minor isomer at GP –16.2 ppm and –87.7 ppm. No coupling is observed between the inequivalent phosphorus atoms because they are separated by more than three bonds.

Ph D Ph Ph Ph P P P P D P P D Ph Ph D Ph Ph Ph Ph Ph Ph 174 177 cis- and trans-178

Figure 7.10: An alternate decomposition pathway for the bis-bridged compound 174.

The actual geometry of the bridged mono-cation 173 (Scheme 7.9) is unknown; however, predictions can be made about the likely geometry of the P…D…P bridge. In the reaction, the two phosphorus atoms each have a vacant orbital (as they are positively charged), Chapter 7: The Reaction of Bis-Phosphonium Salts with Lithium Aluminium Hydride. 136 while the deuteride ion has a pair of electrons. Therefore, the bridge contains a total of two + electrons, and so is formally isoelectronic to the H3 cation, which is calculated to have a symmetrical triangular geometry.124 (This is in contrast to a bridge containing four electrons, – 124 such as in the H3 anion, which is calculated to be linear. An example is the triiodide – 190 … … anion I3 , which has been shown to be linear. ) Hence, the P D P bridge in 173 is expected to be non-linear, with each phosphorus atom attaining penta-coordination, and the deuterium atom axial at each phosphorus. (A similar argument also applies to the bis-bridged compound 174, where both of the P…D…P bridges are expected to be bent, with the geometry at each phosphorus presumably approaching that of an octahedron.) The positive charge, which must be shared between both phosphorus atoms, goes some way to explaining the unusual downfield shift of this species (at GP –41.1 ppm, it is about 50 ppm downfield from other deuteriodiaryldialkylphosphoranes). Additionally, the small 1-bond phosphorus- deuterium coupling constant of 12 Hz indicates a possible weakening of the P-D bonds compared to other phosphoranes like Ph4PD, for which the 1-bond P-D coupling is J = 49 Hz.

This is not surprising, as the P-D bond in Ph4PD is a normal 2-electron bond, whereas the P…D…P bridge in 173 shares two electrons between the three atoms, thereby making each P…D linkage approximately a half-bond. In support of the postulated bent P…D…P bridge is a neutron diffraction study191 of – the analogous anionic species B2H7 179, which can be thought of as a complex of BH3 with – … … BH4 . The anion contains a B H B bridge, bent by an angle of 127.2˚, with the three atoms of the B…H…B bridge sharing two electrons, as in the case of the P…D…P bridge discussed above.

_ H H H H B B

H H H

179

191 – Additionally, the authors noted that the bridging hydrogen atom in B2H7 179 not only lies off the internuclear boron-boron axis (i.e. the bridge is bent), but that it also lies off 191 the local C3 axis of each boron atom. This suggests a significant amount of B-B bonding , in addition to B-H bonding, in the bridge. Unfortunately, because of the chemical equivalence of the two phosphorus atoms in the bridged mono-cation 173 described above, no phosphorus-phosphorus coupling can be expected to confirm a similar P-P bonding interaction. Chapter 8: Further Reactions using Metal Hydride Reagents.

This section discusses some reactions which do not fit neatly into the previously discussed categories, such as the reaction of lithium aluminium hydride with compounds other than phosphonium salts, and the reaction of phosphorus compounds (whether or not they are phosphonium salts) with metal hydride reagents other than lithium aluminium hydride (either lithium borohydride, i.e. LiBH4; sodium dihydrobis(2-methoxyethoxy)aluminate, Red-Al; or potassium hydride, KH). Of the three additional metal hydride reagents, both Red-Al and

LiBH4, like LiAlH4, are metal hydride complexes, where one of the ‘metals’ (either aluminium or boron) is bonded to all of the active hydrogen atoms, forming a complex anion (i.e. – – – [Al(OCH2CH2OCH3)2H2] or BH4 and AlH4 , respectively), with the alkali metal cation only present as a counter-ion (see Figure 8.1).

_ __ Me H H O H O B Al Al H H H O H H H H O Me

(a) (b) (c) – – – Figure 8.1: The structures of (a) BH4 , (b) AlH4 and (c) the [Al(OCH2CH2OCH3)2H2] anion.

This has the effect of spreading out the anionic charge, and making them reasonably 149 soluble in ethereal solvents like THF and Et2O, while the aluminium complex Red-Al, is supplied as a 3.4 M solution in toluene. In contrast, KH is a simple saline hydride, containing K+ cations and H– anions,87 and as such tends to only dissolve in solvents with which it reacts. In order to increase its solubility in solvents like THF, an excess of 18-crown-6 was + – added, presumably forming the complex [K(18-crown-6)2] H . The following sections discuss each of these reagents, and their behaviour towards various phosphorus compounds.

8.1 Additional reactions using lithium aluminium hydride in THF.

Of the other phosphorus(V) compounds (besides phosphonium salts) that were sub- jected to lithium aluminium hydride in THF solution, the phosphine chalcogenides Ph3P=X (where X = O, S, Se) will be discussed first. Each of the three compounds was freely soluble Chapter 8: Further Reactions using Metal Hydride Reagents. 138

in THF, and the reaction was carried out by addition of either LiAlH4 or LiAlD4 to a solution of the chalcogenide in dry THF.

Ph3P=O was found to react very slowly, over a period of several hours, to give Ph3P 31 1 and a very small amount of Ph3PD2, as detected using P{ H}-NMR. A mechanism, consistent with this observation, in addition to explaining the rapid racemisation of chiral phosphine oxides, has been previously suggested,155 and is shown below (see Scheme 8.2). The – reversible complexation of AlH4 with the phosphine oxide gives an intermediate phosphorane 180, which, either through dynamic ligand exchange (DLE), or a planar arrangement of the three groups R1, R2, and R3, all asymmetry is lost. The reverse reaction gives the phosphine oxide as a racemic mixture. Further reaction with LiAlH4 gives a meso-phosphorane, which also erases all asymmetry from the original oxide. Irreversible loss of H2 from the meso-phosphorane then gives the desired phosphine as a racemic mixture. Alternatively, the phosphorane 180 can expel a carbanion to give either the secondary phosphine 181 or its corresponding conjugate base, a phosphide.

O H3AlO H R R LiAlH4 2 LiAlH4 2 – H2 P P R1 P R1 P R2 R2 R1 R R1 R R3 R3 3 3 H H 180 – R1 meso phosphorane P _ + H AlO H R2 3 R3 181

Scheme 8.2: The mechanism proposed for the reaction of a tertiary phosphine oxide with LiAlH4 in THF.

The fact that no monodeuteriophosphoranes (analogous to 180) were observed in the reaction of Ph3P=O with LiAlD4 indicated that such species are probably very short-lived, either reverting back to the phosphine oxide, or reacting with additional LiAlD4 and giving

Ph3PD2, as observed. When the reaction was repeated with Ph2P(O)Bn, a small amount of 129 1 Ph2P(Bn)D2 was observed at GP –79.3 ppm as a 1:2:3:2:1 quintet, JP-D = 51 Hz (compare + – this to the reaction of Ph2PBn2 Br with LiAlD4 in Section 6.2). Of the remaining two phosphine chalcogenides, the 31P{1H}-NMR spectra showed that

Ph3P=S was found to remain completely unreacted over a twelve hour period, while the selen- + – ide reacted at a reasonably fast rate (although much slower than in the case of Ph4P Br ), being consumed within thirty minutes, the signal at GP –4.0 ppm (due to Ph3P) steadily building up at the expense of the signal at GP +36.0 ppm (Ph3P=Se). The apparent lack of reactivity of the sulphide may be explained by assuming that neither lithium nor aluminium complexes with the sulphur atom, and so the phosphorus atom remains relatively electron rich, and hence unreactive with respect to attack by hydride. Additionally, the larger size of sul- Chapter 8: Further Reactions using Metal Hydride Reagents. 139 phur relative to oxygen may shield the phosphorus atom from attack by hydride. The same factors would be expected to also operate in the selenide, preventing direct attack of hydride upon the phosphorus atom. In fact, there is no evidence for the formation of a phosphorane, but rather for the direct attack of hydride at the selenium atom (as shown in Scheme 8.3). This would not be unreasonable, as the selenium atom is both relatively large and polarisable. In addition, because it is a third period element, would be expected to exhibit some electrophilic character.87

_ Se H _ H _ + + "Li[H3AlSeH]" P Li Al P Ph Ph Ph Ph H H Ph Ph

Scheme 8.3: The mechanism proposed for the reaction between Ph3P=Se and LiAlH4 in THF.

The reaction between phosphoryl chloride (POCl3) and LiAlH4 was carried out in an attempt to detect either phosphine oxide (existing either as H3P=O or H2POH) or phosphorane 31 (PH5) by P-NMR. To a solution of POCl3 in dry THF was added powdered LiAlH4 at RT. 31 The P-NMR spectrum showed a complex symmetrical multiplet at GP –214.1 ppm (appear- 1 ing as a singlet when proton decoupled), and a 1:3:3:1 quartet at GP –244.0 ppm ( JP-H = 187

Hz) due to PH3 (Figure 8.4). The species corresponding to the multiplet at GP –214.4 ppm was not identified, but could conceivably be a poly-phosphorus hydride (PaHb) with all phosphorus atoms remaining chemically equivalent (because no phosphorus-phosphorus coupling is observed).

-211 -213 -215 -217 -240 -242 -244 -246 -248 (ppm) (ppm)

(a) the unidentified multiplet at GP –214.1 ppm (b) PH3 (GP –244.0 ppm, J = 187 Hz)

31 Figure 8.4: The P-NMR spectrum of the reaction between POCl3 and LiAlH4 in THF. Chapter 8: Further Reactions using Metal Hydride Reagents. 140

Finally, the reaction of triethylphosphate, (EtO)3P=O, with LiAlH4 was attempted: when either the powdered LiAlH4 was added to a solution of the ester in THF, or the liquid ester was added dropwise to a suspension of LiAlH4 in THF, the only product detected using 31 P-NMR was phosphine, PH3, as characterised by its distinctive splitting pattern and its ex- 1 treme upfield chemical shift (GP –244.4 ppm, JP-H = 187 Hz; see Figure 8.4). No further experiments involving lithium aluminium hydride were undertaken.

8.2 Reactions using LiBH4 in THF.

Lithium borohydride, being the first period’s equivalent of lithium aluminium hydride, was an obvious choice for a second hydride donor. It is known to be a weaker hydride donor than its aluminium analogue, being reasonably stable in alcoholic solution, but is still a strong + – reducing agent. The reaction between LiBH4 and Ph4P Br was attempted with both MeOH and THF as the solvent. In methanolic solution, examination by 31P{1H}-NMR showed that no reaction had occurred. This was probably due to the strong solvation of both the phosphonium cation and – the BH4 anion by methanol, preventing the close approach of the ions. This problem was never encountered in the reductions using LiAlH4, presumably because of the much weaker solvating power of THF towards both cations and anions. For this reason, the reaction was repeated in THF, with subsequent examination by 31P{1H}-NMR showing that a reaction had – occurred, with formation of Ph4PH. Neither Ph4PH2 or Ph3PH2, observed in the reaction of + – Ph4P Br with LiAlH4, were observed in this reaction, consistent with the known reduced hydride donor ability of LiBH4 relative to LiAlH4. + – An attempt to react Ph4P F with KBH4 in aqueous solution has been made previ- 192 + – ously, with the characterisation of the precipitated phosphonium borohydride Ph4P BH4 .

No mention was made of the formation of Ph4PH, but its behaviour towards protic solvents would prevent its isolation under these conditions (Section 6.1). The thermal decomposition + – 192 of Ph4P BH4 gave benzene, Ph3P and Ph3P-BH3. + – The reaction of LiBH4 in THF was repeated with Ph3PBn Br , but this time no reaction was observed by 31P{1H}-NMR. Evidently, the reduction in electrophilicity on going + + from Ar4P to Ar3PR (where Ar = aryl and R = alkyl) is more than the hydride donor ability – of BH4 can make up for. Because of this, no other phosphonium salts were examined.

As in the case of LiAlH4 (discussed above in Section 8.1), the reaction between a solu- 31 tion of (EtO)3P=O in dry THF and LiBH4 was attempted. The P-NMR spectrum showed the result to be identical to that described above, with PH3 as the only observed product. No further experiments using lithium borohydride were attempted. Chapter 8: Further Reactions using Metal Hydride Reagents. 141

8.3 Reactions using 1.6 M Red-Al in toluene.

Phosphorus pentachloride is known to exhibit complex solution chemistry, producing 193 either molecular solutions (PCl5) in solvents of low polarity, such as benzene, or else ionic + – 129c solutions (e.g. PCl4 PCl6 in MeCN ). Red-Al, NaH2Al(OCH2CH2OCH3)2, was originally chosen as a hydride donor because, as it is available as a solution in toluene, it would be ideal for the hydride reduction of molecular PCl5, with the possibility of detecting the as yet un- 127 characterised parent compound, phosphorane (PH5). No reactions involving phosphonium salts were attempted, due to the presumed insolubility of the salts in toluene.

When solid PCl5 was added to Red-Al, PH3 was produced, along with a small amount 1 of a second product (GP –122.6 ppm, 1:2:1 t: JP-H = 191 Hz). As the coupling constant 1 corresponds to trivalent phosphorus (e.g. for PH3, JP-H = 187 Hz), a possible assignment is

H2PCl, although no additional evidence exists to support this assignment. Identical results were obtained when the reactions were repeated at 0˚C. As the decomposition of PH5 into 126 PH3 and H2 has been calculated to be strongly exothermic, these results are consistent with the transient formation of phosphorane, although no evidence for its independent existence has been obtained from these results.

8.4 Reactions using KH in THF.

Because of the great insolubility of potassium hydride, all reactions were carried out in dry THF into which an excess amount of 18-crown-6 had already been dissolved. As in + – + – the case of LiBH4, the only salts examined were Ph4P Br and Ph3PBn Br . The reactions were carried out by suspending either the phosphonium salt or KH in dry THF to which excess 18-crown-6 had previously been dissolved, before adding the remaining reagent. No conclusive results were obtained, with the 31P-NMR showing only unreacted phosphonium salt in both cases, and so no further investigation of KH was undertaken. Chapter 9: Reaction of Phosphonium Salts with Other Nucleophiles.

Because of the large number of hydrophosphoranes and -phosphoranates discovered when complex metal hydrides (especially lithium aluminium hydride) were reacted with quaternary phosphonium salts, it was decided to repeat the reactions using different nucleophiles. Besides hydrogen, phosphorus is also known to form strong covalent single bonds with the first period elements fluorine, oxygen, nitrogen, and carbon, and examples of both phosphoranes and phosphoranates bearing groups bound through each of these atoms have been described (see Chapter 4). Additionally, if phosphoranes were formed, would these groups then migrate from phosphorus to the benzyl group, in a similar way to that previously described for the deuterium atom?

The sources of the nucleophiles to be used were KF, NaN3 and KCN (each suspended in THF, with or without the appropriate crown ether), or LiOD in D2O/THF. For each of + – + – these nucleophiles, either Ph4P Br , Ph3PBn Br , or the cyclic bis-salt 1,1,5,5-tetraphenyl- 1,5-diphosphocanium dibromide 61 were used as the phosphonium salt for the following reasons: + – (i) Ph4P Br was found to react with LiAlH4 faster than all the other monophosphonium salts examined, presumably because it has the most electrophilic cation. Because of this, it was thought to have the highest chance of forming a detectable phosphorane; + – (ii) Any hydrocarbon fragment recovered from Ph3PBn Br could be examined for indirect evidence of phosphorane formation; (iii) Because of the apparent ease of forming a bridging hydride complex 173 with bis-salt 61, the small fluoride ion190 was an obvious choice with which to repeat this reaction. The following sections describe the results obtained, with all reaction mixtures examined using 31P{1H}-NMR.

9.1 Reaction of deuteroxide ion in D2O/THF.

A 0.5 M solution of lithium deuteroxide was made by dissolving lithium metal in a + – solution of D2O in THF at RT. To the resulting clear solution was added Ph4P Br , which immediately dissolved. Examination of the solution by 31P{1H}-NMR showed the only product to be Ph3P=O, as expected, with no evidence for the formation of the intermediate deuteroxyphosphorane Ph4P-OD. This is perhaps not surprising, as ‘deprotonation’ of Ph4P-OD Chapter 9: Reaction of Phosphonium Salts with Other Nucleophiles. 143

– would give the anionic Ph4P-O , which would then be expected to rapidly expel phenyl anion to give the observed Ph3P=O. + – The reaction was repeated on a larger scale using Ph3PBn Br , which also immediately dissolved in the lithium deuteroxide solution. When the solution was examined by 31 1 P{ H}-NMR, again the only product was Ph3P=O. The toluene was isolated, and was ex- 13 1 amined using C{ H}-NMR, which clearly showed that PhCD3 was the only compound present (the methyl carbon appears as a 1:3:6:7:6:3:1 septet, with J = 20.7 Hz, indicating that it is attached to three equivalent deuterium atoms, as shown in Figure 9.1).

20.5 20.0 19.5 (ppm) + – – Figure 9.1: The toluene from the hydrolysis of Ph3PBn Br in OD /D2O (the methyl region of the 13 1 C{ H}-NMR spectrum, at 125.758 MHz), showing clearly the 1:3:6:7:6:3:1 septet of the CD3 group (J = 20.7 Hz).

A likely explanation for the complete deuteration of the methyl group is that revers- + ible deprotonation of the Ph3PBn cation by deuteroxide ion (giving the corresponding ylide species Ph3P=CHPh) occurs much faster than direct attack at phosphorus (leading to alkaline hydrolysis). The ylide thus formed is itself a reasonably strong base, and so is quickly quenched by D2O (Scheme 9.2, page 144). The isolation of PhCD3 as the only recovered toluene indicates that this (or another) exchange mechanism occurs at a much greater rate than the hydrolysis. Although this result was interesting, no direct evidence was obtained for the formation of a deuteroxyphosphorane, and so no further investigation undertaken. Chapter 9: Reaction of Phosphonium Salts with Other Nucleophiles. 144

D H Ph H Ph Ph H _ H C C C OD D2O + + P D2O/THF P THF P Ph Ph Ph Ph Ph Ph Ph Ph Ph ylide

CD3 D Ph D Ph D _ D O C C OD D2O + Ph P P D2O/THF P Ph THF Ph Ph Ph Ph Ph Ph Ph O _ D OD 109

Scheme 9.2: Ylide formation versus alkaline hydrolysis, for the reaction of deuteroxide ion with + – Ph3PBn Br in D2O.

9.2 Reaction of cyanide, azide and fluoride ions.

Because both cyanide ion and fluoride ion were available as their potassium salts, their solubility in THF could be increased by the addition of 18-crown-6. The solubility of sodium azide could similarly be increased by 15-crown-5, greatly increasing the possibility of reaction with the phosphonium cation. As in the case of deuteroxide ion, reactions using + – + – 64 both Ph4P Br and Ph3PBn Br were attempted. Fluoride has successfully been trans- – ferred to phosphorus from the anionic Ph3SnF2 .

Suspensions of KCN, NaN3, and KF in THF were prepared, and to each a phospho- + – + – nium salt was added, either Ph4P Br or Ph3PBn Br . In all cases, no reaction could be detected by 31P{1H}-NMR. Addition of the appropriate crown ether was observed only to increase the solubility of the phosphonium salt (as determined by a larger signal for the salt in the 31P{1H}-NMR), with no apparent interaction between the phosphonium cation and the nucleophiles. It seems likely that this is due to complexation of the phosphonium cations by the crown ethers, which both increased their solubility and protected them from nucleophilic attack.

_ 2Br Ph Bn Bn + + KCN P P P Bn Bn MeOH Ph Bn Bn Ph 182

Scheme 9.3: Exclusive loss of the bridge from bis-salt 182 upon treatment with KCN. Chapter 9: Reaction of Phosphonium Salts with Other Nucleophiles. 145

Cyanide and azide have previously been reacted with ethane-1,2-bis-phosphonium 75 salts dissolved in polar solvents like H2O, DMSO, and MeOH. In these reactions, exclusive elimination of the bridge occurs even in the presence of benzyl groups (e.g. reaction of

182 with KCN in MeOH gave PhPBn2 in quantitative yield, as shown above in Scheme 9.3). Part 3: The Experimental Details. Chapter 10: Experimental.

10.1 Introduction. The following sections describe the characterisation of starting materials used and products synthesised in the course of this project. These materials were characterised using 1H-, 13C{1H}-, and 31P{1H}-/31P-NMR, and melting point, where appropriate. Additionally, the partially deuterated benzene and toluene obtained from the reductive cleavage of the salts + – [PhnPBn4-n] Br (n = 0 to 4) with LiAlD4 in refluxing THF (Sections 5.2 and 5.3) were analysed by Electron Impact Mass Spectroscopy (EI-MS) and 1H-, 2H{1H}-, and 13C{1H}-NMR, respectively. New compounds were also characterised using MS and Microanalysis (at the Microanalytical Unit, Research School of Chemistry, ANU, Canberra). All NMR spectra were obtained on the following Bruker spectrometers: ACP-300 (31P at 121.490 MHz and 2H at 46.073 MHz); ACF-300 (1H at 300.163 MHz and 13C at 75.480 MHz); DMX-500 (1H at 500.113 MHz, 13C at 125.758 MHz and 2H at 76.7734 MHz). The 1 2 chemical shift references used were: CHCl3 for H (GH +7.26 ppm), CDCl3 for H (GD +7.26 13 31 ppm) and C (GC +77.00 ppm), and 85% H3PO4 for P (GP 0.00 ppm). All spectra were 31 acquired using a deuterium lock to either external D2O (for P) or an internal deuterated solvent (for 1H and 13C), except for the 2H spectra, which were acquired without a lock. Mass spectra were run by the Mass Spectrometry Unit, UNSW (Dr. Joe Brophy) on a VG Quattro Mass Spectrometer. The benzene samples were injected onto a GCMS DB-Wax column (35 to 220˚C, 3˚C/minute), and ionised by Electron Impact (EI) by a 200˚C ion source at 15 eV. Analysis of the phosphonium salts was achieved by Electrospray of a solution in 1:1 aqueous MeCN (source temperature 60˚, cone voltage 40 V, probe voltage 3,500 V). Melting points were determined using a heating-stage microscope, with the sample between two cover slips. Sublimation, when it occurred, was observed by crystal growth on the upper cover slip as the compound was heated. Tetrahydrofuran (THF) was used for all of the reactions requiring either the naphthalene radical anion, lithium aluminium hydride, or lithium metal as a reagent; it was obtained anhydrous and free from peroxides by distillation from sodium (or potassium) benzophenone ketyl under an atmosphere of dry argon or nitrogen immediately before use. Diethyl ether, which was used for all the Grignard reactions, was purified by distillation from CaH2 under an atmosphere of dry argon. Organic solvents were deoxygenated by bubbling dry argon through them for a few minutes. Water was deoxygenated by allowing to cool from reflux under an atmosphere of argon. Solvents for recrystallisation were usually AR grade, and used without further puri- Chapter 10: Experimental. 148

fication. Deuterated solvents for NMR were as follows: acetonitrile-d3 [2206-26-0], benzene-d6 [1076-43-3], chloroform-d1 [865-49-6], deuterium oxide [7789-20-0], and tetrahydrofuran-d8 [1693-74-9]. In the following sections, characterisation of the starting materials, obtained either from commercial outlets (Section 10.2), as research samples which had been synthesised previously by others (Section 10.3), or synthesised in the course of this work by standard methods (Sections 10.4 to 10.7), will be described. The compounds were characterised by 1H-, 13C{1H}-, and 31P-NMR, where appropriate. Where given, values of coupling constants were determined from the observed spectral separation of peaks (in Hz). For the 13C{1H}-NMR spectra of phosphorus compounds, P-C coupling constants are given as J values, with the number of bonds through which the coupling occurs being indicated by context (i.e. in the 1 2 3 4 case of phenyl groups: ipso J = JP-C, ortho J = JP-C, meta J = JP-C, para J = JP-C; for benzyl 1 2 3 4 5 groups: methylene J = JP-C, ipso J = JP-C, ortho J = JP-C, meta J = JP-C, para J = JP-C). Additionally, phosphorus-carbon coupling constants marked with an asterix (*) indicate a non-first order splitting pattern, making precise assignments (such as the number of bonds through which the atoms are coupled) difficult. In these cases, the numerical value given is from the separation between the two lines giving the most similar value to an unambiguous example. The key to peak multiplicity is as follows: s = singlet, d = doublet, t = triplet, q = quartet, qn = quintet, and sx = sextet.

10.2 Analysis of the commercial samples.

The following compounds were obtained as commercial samples, and used without further purification:

+ – Tetraphenylphosphonium bromide, Ph4P Br [2751-90-8].

1 Ph A colourless, crystalline solid; H-NMR (CDCl3): GAr +7.45 to +7.85 ppm _ + 13 1 P Br (m); C{ H}-NMR (CDCl3): Gipso +117.0 ppm (d: J = 89 Hz), Gortho Ph Ph Ph +134.0 ppm (d: J = 10 Hz), Gmeta +130.5 ppm (d: J = 12 Hz), Gpara +135.5 31 1 129c ppm (d: J = 3 Hz); P{ H}-NMR (EtOH): GP +24.1 ppm (s).

Triphenylphosphine, Ph3P [603-35-0].

1 P A colourless, crystalline solid; H-NMR (CDCl3): GAr +7.38 ppm (d: JP-H = 0.2 Ph Ph 13 1 Ph Hz); C{ H}-NMR (CDCl3): Gipso +136.7 ppm (d: J = 8 Hz), Gortho +133.7 ppm 31 1 (d: J = 19 Hz), Gmeta +128.6 ppm (d: J = 7 Hz), Gpara +128.5 ppm (s); P{ H}-NMR (THF): 129d GP –4.4 ppm (s) Chapter 10: Experimental. 149

Tribenzylphosphine, Bn3P [7650-89-7].

1 P A colourless, crystalline solid; H-NMR (CDCl3): GAr +7.15 to +7.30 ppm Bn Bn (15H, m), +2.78 ppm (6H, s); 13C{1H}-NMR (CDCl ): +137.7 ppm Bn GCH2 3 Gipso (d: J = 6 Hz), Gortho +129.2 ppm (d: J = 6 Hz), Gmeta +128.4 ppm (s), Gpara +125.8 ppm (d: J = 2 Hz), +34.1 ppm (d: J = 19 Hz); 31P{1H}-NMR (THF): –11.4 ppm129d (s). GCH2 GP

Ethane-1,2-bis(diphenylphosphine) 56, [1663-45-2].

1 1 A colourless, crystalline solid; H-NMR (CDCl3): G +7.32 ppm 2 PPh2 Ar 13 1 Ph2P (20H, s), +2.12 ppm (C1 = C2 4H, t: J = 4* Hz); C{ H}- GCH2 P-H

NMR (CDCl3): Gipso +138.0 ppm (dd: J = 7 Hz, J = 6 Hz), Gortho +132.7 ppm (qn: J = 19* Hz), +128.4 ppm (t: J = 7* Hz), +128.6 ppm (s), +23.8 ppm (C1 = C2 d: J = 3 Gmeta Gpara GCH2 31 1 129d Hz); P{ H}-NMR (CDCl3): GP –11.7 ppm (s).

Propane-1,3-bis(diphenylphosphine) 60, [6737-42-4].

1 3 1 A colourless, crystalline solid; H-NMR (CDCl3): GAr +7.30 to 2 3 Ph2P PPh2 +7.45 ppm (20H, m), +2.22 ppm (C1 = C3, 4H, t: J = 7 GCH2 H-H Hz), +1.62 ppm (C2, 2H, qn: 3J = 7 Hz); 13C{1H}-NMR (CDCl ): +138.5 ppm GCH2 H-H 3 Gipso (d: J = 12 Hz), Gortho +132.6 ppm (d: J = 18 Hz), Gmeta +128.4 ppm (d: J = 6 Hz), Gpara +128.3 ppm (s), +22.3 ppm (C1 = C3, t: J = 34* Hz), +29.6 ppm (C2, t: J = 25 Hz); GCH2 GCH2 31 1 129d P{ H}-NMR (CDCl3): GP –16.5 ppm (s).

Triphenylphosphine oxide, Ph3P=O [791-28-6].

1 O A colourless, crystalline solid; H-NMR (CDCl3): GAr +7.34 to +7.68 ppm 13 1 194 P (m); C{ H}-NMR (CDCl3): Gipso +132.3 ppm (d: J = 104 Hz), Gortho +131.9 Ph Ph Ph ppm (d: J = 10 Hz), Gmeta +128.3 ppm (d: J = 12 Hz), Gpara +131.8 ppm 31 1 194 (s); P{ H}-NMR (CDCl3): GP +29.7 ppm (s).

Diphenylphosphinous chloride, Ph2PCl [1079-66-9].

1 P A fuming, colourless liquid; H-NMR (CDCl3): GAr +7.62 to +7.70 ppm (4H, m) Ph Cl 13 1 Ph and GAr +7.41 to +7.50 ppm (6H, m); C{ H}-NMR (CDCl3): Gipso +138.7 ppm (d: J = 32 Hz), Gortho +131.6 ppm (d: J = 25 Hz), Gmeta +128.5 ppm (d: J = 7 Hz), Gpara +130.2 31 1 129e ppm (s); P{ H}-NMR (CDCl3): GP +82.5 ppm (s). Chapter 10: Experimental. 150

Phenylphosphonous dichloride, PhPCl2 [644-97-3].

1 P A fuming, colourless liquid; H-NMR (CDCl3): GAr +7.89 to +7.95 ppm (2H, m) Ph Cl 13 1 Cl and GAr +7.50 to +7.60 ppm (6H, m); C{ H}-NMR (CDCl3): Gipso +140.3 ppm

(d: J = 52 Hz), Gortho +130.0 ppm (d: J = 31 Hz), Gmeta +128.9 ppm (d: J = 8 Hz), Gpara +132.7 31 1 129e ppm (s); P{ H}-NMR (CDCl3): GP +161.7 ppm (s).

Phosphoryl chloride, POCl3 [10025-87-3].

31 O A fuming, colourless liquid; P-NMR (CDCl3): GP +4.8 ppm (s).

P Cl Cl Cl

+ – Phosphorus pentachloride, monomer PCl5 [10026-13-8]; ionic PCl4 Cl [19453-01-1].

31 Cl Cl _ A fuming, pale yellow solid; P-NMR (PhCH3): GP –80.6 Cl + Cl 129a 129c Cl P or P ppm (monomeric PCl5, s); (MeCN): GP +83.6 ppm Cl Cl Cl + – Cl (PCl4 Cl , s). Cl

Triethyl phosphate, (EtO)3P=O [78-40-0].

1 3 A colourless liquid; H-NMR (CDCl3): G +3.90 ppm (6H, dq: J = 7.9 O CH2 P-H Hz, 3J = 7.1 Hz), +1.13 ppm (9H, td: 3J = 7.1 Hz, 4J = 1.0 Hz); H-H GCH3 H-H P-H P 13 1 C{ H}-NMR (CDCl3): GCH +63.2 ppm (d: J = 6 Hz), GCH +15.6 ppm (d: J EtO OEt 2 3 EtO 31 129f 3 = 6 Hz); P-NMR (CDCl3): GP –0.1 ppm (binomial septet: JP-H = 8 Hz).

N,N-Dimethylaniline, PhNMe2 [121-69-7].

N 1 Me A colourless liquid; H-NMR (CDCl3): GAr +7.28 to +7.37 ppm (2H, m) and Ph Me +6.77 to +6.89 ppm (3H, m), +3.03 ppm (6H, s); 13C{1H}-NMR GAr GCH3

(CDCl3): Gipso +150.6 ppm (s), Gortho +112.6 ppm (s), Gmeta +129.0 ppm (s), Gpara +116.6 ppm (s), +40.5 ppm (s). GCH3

Cinnamyl chloride, CnCl [2687-12-9].

1 H A yellow liquid; H-NMR (CDCl3): GAr +7.30 to +7.45 ppm (5H, m), 2 Ph 3 1 GCH +6.69 ppm (C1, 1H, d: JH-H = 15.6 Hz), GCH +6.36 ppm (C2, 1H, CH2Cl 3 dt: 3J = 15.6 Hz, 3J = 7.2 Hz), +4.27 ppm (C3, 2H, dd: H H-H H-H GCH2 3 4 13 1 JH-H = 7.2 Hz, JH-H = 1.2 Hz); C{ H}-NMR (CDCl3): Gipso +135.8 ppm (s), Gortho +126.6 ppm (s), Gmeta +128.5 ppm (s), Gpara +128.2 ppm (s), GCH +134.0 ppm (C1, s), GCH +124.8 ppm (C2, s), +45.3 ppm (C3, s). GCH2 Chapter 10: Experimental. 151

Additionally, the following compounds were used without further purification: acetic acid (glacial) [64-19-7], ammonium bromide [12124-97-9], ammonium chloride [12125-02-9], benzyl bromide [100-39-0], benzyl chloride [100-44-7], butyl lithium (as 1.6 M in hexane) [109-72-8], 1,2-dibromoethane [106-93-4], hydrogen peroxide (as a 30% aqueous solution) [7722-84-1], lithium (metal) [7439-93-2], 95% enriched lithium-6 (metal) [14258-72-1], lithium aluminium hydride (powder; 1.0 M solution in tetrahydrofuran) [16853-85-3], lithium aluminium deuteride (powder) [14128-54-2], lithium borohydride (powder) [101947-37-9], magnesium (metal turnings) [7439-95-4], methyl iodide [74-88-4], naphthalene [91-20-3], phosphorus trichloride [7719-12-2], potassium cyanide [151-50-8], potassium fluoride [7789-23-3], potassium hydride [7693-26-7], selenium (grey) [7782-49-2], sodium (metal) [7440-23-5], sodium azide [26628-22-8], sodium bis(2-methoxyethoxy)aluminium dihydride (Red-Al, as 3.4 M in toluene) [22722-98-1], sulphur [7704-34-9] and trifluoroacetic acid [76-05-1].

10.3 Analysis of the research samples.

The following compounds were previously synthesised in the School of Chemistry, and were characterised by melting point and/or by the 1H-, 13C- and 31P-NMR, where appropriate.

+ – Dimethyldiphenylphosphonium iodide, Ph2PMe2 I [1017-88-5].

Me A colourless, crystalline solid (mp 250-52˚C; Lit. 248-52˚C195); 1H-NMR _ + I (CDCl ): +7.60 to +7.75 ppm (10H, m), +2.84 ppm (6H, d: 2J = P 3 GAr GCH3 P-H Ph Me 13 1 Ph 13.9 Hz); C{ H}-NMR (CDCl3): Gipso +120.3 ppm (d: J = 88 Hz), Gortho +132.2 ppm (d: J = 11 Hz), Gmeta +130.2 ppm (d: J = 13 Hz), Gpara +134.8 ppm (d: J = 2 Hz), +11.2 ppm (d: J = 56 Hz); 31P{1H}-NMR (EtOH): +22.0 ppm196 (s). GCH3 GP

n + – n-Butyltriphenylphosphonium iodide, Ph3P Bu I [22949-84-4].

1 Ph _ A colourless, crystalline solid (mp 243-5˚C); H-NMR (CDCl3): + I E G +7.50 to +7.53 ppm (15H, m), +3.46 ppm ( , 2H, m), P CH GAr GCH2 D Ph 3 D J +1.44 ppm ( , 4H, m), +0.68 ppm ( , 3H, m); Ph GCH2 EJ GCH3 G 13 1 C{ H}-NMR (CDCl3): Gipso +117.6 ppm (d: J = 86 Hz), Gortho +133.0 ppm (d: J = 10 Hz), +130.0 ppm (d: J = 12 Hz), +134.3 ppm (d: J = 2 Hz), +22.0 ppm ( , d: J = Gmeta Gpara GCH2 D 50 Hz), +23.1 ppm ( , d: J = 17 Hz), +24.0 ppm ( , d: J = 4 Hz), +13.1 ppm ( , GCH2 E GCH2 J GCH3 G 31 1 196 s); P{ H}-NMR (EtOH): GP +25.1 ppm (s). Chapter 10: Experimental. 152

1,1-diphenylphosphorinanium perchlorate 136. Ph _ A colourless, crystalline solid (mp 208-11˚C; Lit. 208˚C197); 1H- ClO 6 + 4 NMR (CDCl /MeOH): G +7.52 to +7.74 ppm (10H, m), G P 3 Ar CH2 3 Ph +2.87 ppm (C2 = C6, 4H, m), GCH +1.91 ppm (C3 = C5, 4H, m), 2 2 +1.72 ppm (C4, 2H, m); 13C{1H}-NMR (CDCl /MeOH): GCH2 3 Gipso +118.1 ppm (d: J = 83 Hz), Gortho +131.7 ppm (d: J = 10 Hz), Gmeta +130.4 ppm (d: J = 12 Hz), +134.7 ppm (d: J = 3 Hz), +18.8 ppm (C2 = C6, d: J = 49 Hz), +24.1 Gpara GCH2 GCH2 ppm (C3 = C5, d: J = 7 Hz), +21.2 ppm (C4, d: J = 6 Hz); 31P{1H}-NMR (EtOH): GCH2 GP +17.2 ppm129c (s).

Ethane-1,2-bis(tri-n-butylphosphonium) dibromide 160, [23054-04-8].

+ A colourless, crystalline solid (mp 218-20˚C; Lit. P J CH3 D E G 225˚C198a); 1H-NMR (CDCl ): +2.53 ppm (C1 = 1 3 _ 3 GCH2 2Br 2 C2, 4H, d: J = 6.2* Hz), +2.24 ppm ( , 12H, m), P-H GCH2 D P CH +1.51 ppm ( + , 24H, m), +0.96 ppm ( , 18H, + 3 GCH2 E J GCH3 G 3 1:2:1 t: 3J = 6.9 Hz); 13C{1H}-NMR (CDCl ): H-H 3 GCH2 +12.4 ppm (C1 = C2, t: J = 45* Hz), +17.9 ppm ( , qn: J = 47* Hz), +23.7 ppm ( , GCH2 D GCH2 E t: J = 16* Hz), +23.4 ppm ( , t: J = 5* Hz), +12.8 ppm ( , s); 31P{1H}-NMR GCH2 J GCH3 G (EtOH): GP +38.0 ppm (s).

Propane-1,3-bis(triphenylphosphonium) dibromide 169, [7333-67-7].

1 3 A colourless, crystalline solid (mp >315˚C; Lit. 334˚C199); 1H- ++2 Ph3P PPh3 NMR (CDCl ): +7.45 to +7.78 ppm (30H, m), +4.48 _ 3 GAr GCH2 2Br ppm (C1 = C3, 4H, m), +1.74 ppm (C2, 2H, m); 13C{1H}- GCH2

NMR (CDCl3): Gipso +117.3 ppm (d: J = 87 Hz), Gortho +133.6 ppm (qn: J = 10* Hz), Gmeta +130.0 ppm (qn: J = 12* Hz), +134.5 ppm (s), +21.8 ppm (C1 = C3, hx: J = 72* Gpara GCH2 Hz), +17.2 ppm (C2, s); 31P{1H}-NMR (EtOH): +24.6 ppm (s). GCH2 GP

1,1,5,5-Tetraphenyl-1,5-diphosphocanium dibromide 61, [31721-62-7].

4 2 8 A colourless, crystalline solid (mp >315˚C; Lit. 29 330-3˚C); 1H- + + P NMR (CDCl3/TFA): GAr +7.56 to +7.82 ppm (20H, m), GCH P 7 Ph 2 Ph _ +3.73 ppm (C2 = C4 = C6 = C8, 8H, m), GCH +2.54 ppm (C3 = Ph Ph 2Br 2 3 13 1 C7, 4H, 1:2:1 t: JP-H = 23.0 Hz); C{ H}-NMR (CDCl3/TFA):

Gipso +117.2 ppm (d: J = 85 Hz), Gortho +132.4 ppm (d: J = 10 Hz), Gmeta +130.7 ppm (d: J = 13 Hz), +135.3 ppm (d: J = 1 Hz), +20.4 ppm (C2 = C4 = C6 = C8, d: J = 49 Hz), Gpara GCH2 +14.4 ppm (C3 = C7, t: J = 5* Hz); 31P{1H}-NMR (EtOH/AcOH): +27.6 ppm29 (s). GCH2 GP Chapter 10: Experimental. 153

Benzyldiphenylphosphine oxide, Ph2P(O)Bn [2959-74-2].

72 1 O A colourless, crystalline solid (mp 194-5˚C; Lit. 193-4˚C ); H-NMR: GAr +7.08 to +7.72 ppm (15H, m), +3.65 ppm (2H, d: 2J = 13.9 Hz); P GCH2 P-H Ph Bn 13 1 Ph C{ H}-NMR: phenyl Gipso +132.3 ppm (d: J = 99 Hz), Gortho +131.1 ppm (d: J = 9 Hz), Gmeta +128.4 ppm (d: J = 12 Hz), Gpara +131.7 ppm (d: J = 3 Hz), benzyl Gipso not located, Gortho +130.1 ppm (d: J = 6 Hz), Gmeta +128.3 ppm (d: J = 2 Hz), Gpara +126.7 ppm (d: J = 3 Hz), +38.1 ppm (d: J = 67 Hz); 31P{1H}-NMR (CDCl ): +30.2 ppm200 (s). GCH2 3 GP

10.4 Preparation and reactions of lithium phosphides.

With one exception, all the lithium phosphides used in this project were prepared using the following procedure. In addition, lithium diphenylphosphide was also prepared by reaction of diphenylphosphinous chloride with lithium metal in THF, described on page 154.

Preparation of lithium phosphides from phenylphosphines. To a magnetically stirred solution of the phosphine in dry THF was added freshly hammered strips of lithium metal (1.0 eq) against a stream of dry argon at RT. Almost immediately, the lithium became a dull coppery-brown colour, indicating that the reaction had begun. Not long after (usually within a few minutes), the solution itself became a deep reddy-brown colour, and as the reaction proceeded, the lithium strips took on a shiny golden appearance. The reaction was deemed to have finished when no more lithium could be seen floating in the solution, usually after about three hours. The resulting phosphide was not isolated, but reacted immediately. Its purity was estimated using 31P{1H}-NMR.

The following describe the actual experimental details undertaken, all of the species were characterised using 31P-NMR except where stated.

Lithium diphenylphosphide, Ph2PLi [4541-02-0]: (i) reductive cleavage of triphenylphosphine with lithium in THF.

Li Ph P Ph PLi + PhLi 3 THF 2

As described above using Ph3P (5 gm, 19.1 mmol) in dry THF (50 mL). The crude product was shown to be free from soluble phosphorus-containing impurities using 31P{1H}- 129h NMR (THF): GP –21.6 ppm (s). Chapter 10: Experimental. 154

(ii) lithiation of diphenylphosphinous chloride with lithium in THF.

Li Ph PCl Ph PLi + LiCl 2 THF 2

The procedure closely follows that described above, except that in place of Ph3P,

Ph2PCl (5 gm, 22.7 mmol) was dissolved in dry THF (50 mL), and the flask chilled in ice- water before the strips of lithium (0.32 gm, 46.1 mmol) were added. The reaction tended to proceed to completion in a shorter amount of time than when Ph3P was used. The crude product was shown to be free from soluble phosphorus-containing impurities using 31P{1H}- 129h NMR (THF): GP –21.6 ppm (s).

Dilithium ethane-1,2-bis(phenylphosphide) 34, [60778-67-8].

Li PPh Li PPh 2 Li + 2PhLi Ph2P THF PhP 56 34 As described in (i), using ethane-1,2-bis(diphenylphosphine) 56 (10 gm, 25.1 mmol) in dry THF (100 mL). The crude product was shown to be free from soluble phosphorus- 31 1 129h containing impurities using P{ H}-NMR (THF): GP –59.3 ppm (s).

1,4-Diphenyl-1,4-diphosphorinane 7, cis = [125707-10-0]; trans = [125707-11-1].

Li PPh Br Li Ph P P Ph PhP THF 34 cis/trans 7 To a magnetically stirred solution of dilithium ethane-1,2-bis(phenylphosphide) 34 in dry THF (0.2 M, 250 mL, 50.0 mmol) chilled in ice-water was added vinyl bromide (ca 10 gm, 93.5 mmol, 1.87 eq) in small portions against a flow of dry argon. After the addition was complete, the solution was allowed to warm to RT before it was quenched with 1.0 M deoxygenated aqueous NH4Cl solution (1 M, 20 mL). The product was extracted with CH2Cl2

(3 u 50 mL), the organic layers combined and dried using MgSO4. Removal of the solvent under reduced pressure gave the crude product as a colourless crystalline solid, which was recrystallised from EtOH giving trans-1,4-diphenyl-1,4-diphosphorinane 7 (5.04 gm, 37% yield, mp 172-3˚C; Lit. 168-70˚C29); the cis isomer proved much harder to purify, apparently being less crystalline than the trans isomer. 1 H-NMR (CDCl3): GH +7.25 to +7.55 ppm (aromatic 10H, m), GH +2.16 ppm (C2 = C3 = C5 13 1 = C6 methylene 8H, s); C{ H}-NMR (CDCl3): Gipso +139.3 ppm (sx: J = 9* Hz), Gortho +130.9 ppm (qn: J = 16* Hz), +128.4 ppm (t: J = 6* Hz), +128.1 ppm (s), Gmeta Gpara GCH2 +22.2 ppm (C2 = C3 = C5 = C6 methylene, dd: J = 28 Hz, J = 14 Hz); 31P{1H}-NMR 29 29 (CDCl3): GP –28.9 ppm (s, cis-isomer), GP –30.0 ppm (s, trans-isomer). Chapter 10: Experimental. 155

10.5 Synthesis of the ammonium and phosphonium salts.

The following describes the preparation (and the subsequent characterisation by 1H-, 13C{1H}-, and 31P-, 31P{1H}-NMR, and melting point) of the compounds used in this project which were not already available. Two of the bis-salts, ethane-1,2-bis(E-cinnamyldiphenylphosphonium) dichloride and 1,1,4,4-tetraphenyl-1,4-diphosphepanium bis(tetrafluoroborate), are new compounds and so were also characterised by Electrospray MS and Microanalysis.

General Procedure for the Preparation of Ammonium and Phosphonium Halides. To a solution of the amine or phosphine in dry deoxygenated toluene (ca 0.25–1.0 M) was added the alkyl halide (ca 1.1 eq.), and the solution refluxed for at least three hours, under an atmosphere of argon for solutions containing phosphines. The crude product had usually crystallised out by this time and could then be collected by filtration and purified by recrystallisation.

+ – N-Benzyl-N,N-dimethylanilinium bromide, PhN(Bn)Me2 Br [23145-45-1].

Me _ N PhCH Br + Br Me 2 Ph N Me PhCH3 Ph Me Bn To a solution of N,N-dimethylaniline (5.0 gm, 41.3 mmol) in dry toluene (50 mL) was added benzyl bromide (8.0 gm, 46.8 mmol). After refluxing for four hours, the product had precipitated as colourless crystals, which were collected by filtration (crude yield was 10.0 + gm, 83%). The crude salt was recrystallised from MeOH/n-hexane, giving PhN(Bn)Me2 Br– as colourless, hygroscopic crystals (7.5 gm, 62% yield, mp 85-90˚C; Lit. 96-8˚C,201 143-5˚C,202 200-3˚C203); 1 H-NMR (CDCl3/MeOH): phenyl GAr +7.71 ppm (2H, m) and GAr +7.30 to +7.40 ppm (3H, m), benzyl +6.94 to +7.13 ppm (5H, m), +5.35 ppm (2H, s), +3.74 ppm (6H, s); GAr GCH2 GCH3 13 1 C{ H}-NMR (CDCl3/MeOH): phenyl Gipso +143.8 ppm (s), Gortho +121.3 ppm (s), Gmeta +129.9 ppm (s), Gpara +130.0 ppm (s), benzyl Gipso +130.0 ppm (s), Gortho +132.3 ppm (s), +128.1 ppm (s), +127.3 ppm (s), +72.4 ppm (s), +53.2 ppm (s). Gmeta Gpara GCH2 GCH3 Chapter 10: Experimental. 156

+ – Benzyltriphenylphosphonium bromide, Ph3PBn Br [1449-46-3].

Bn _ P PhCH2Br + Br Ph P Ph PhCH3 Ph Ph Ph Ph

To a solution of Ph3P (5 gm, 19.1 mmol) in dry toluene (50 mL) was added benzyl bromide (4.3 gm, 25.1 mmol). After refluxing for four hours, the product had precipitated as colourless crystals, which were collected by filtration (crude yield was 7.9 gm, 95%). The + – crude salt was recrystallised from MeOH/n-hexane giving Ph3PBn Br (7.2 gm, 87% yield, mp 287-8˚C; Lit. 280-1˚C204); 1 H-NMR (CDCl3): phenyl GAr +7.51 to +7.78 ppm (15H, m), benzyl GAr +6.95 to +7.20 ppm (5H, m), +5.23 ppm (2H, d: 2J = 15 Hz); 13C{1H}-NMR (CDCl ): phenyl +117.5 GCH2 P-H 3 Gipso ppm (d: J = 86 Hz), Gortho +134.2 ppm (d: J = 10 Hz), Gmeta +130.1 ppm (d: J = 12 Hz), Gpara +134.9 ppm (d: J = 3 Hz), benzyl Gipso +126.9 ppm (d: J = 8 Hz), Gortho +131.3 ppm (d: J = 6 Hz), +128.7 ppm (d: J = 4 Hz), +128.3 ppm (d: J = 4 Hz), +30.7 ppm (d: J = Gmeta Gpara GCH2 31 1 196,205 47 Hz); P{ H}-NMR (CH2Cl2): GP +23.5 ppm (s).

+ – Dibenzyldiphenylphosphonium bromide, Ph2PBn2 Br [77382-18-4].

Bn _ P PhCH2Br + Br Ph P Bn PhCH3 Ph Ph Ph Bn

To a magnetically stirred solution of Ph2PCl (5 gm, 22.7 mmol) in dry THF (50 mL) was added freshly hammered lithium (0.15 gm, 21.6 mmol) against a stream of dry argon. The solution became warm, with a brown colour emanating from the lithium metal, indicating the formation of a phosphide (Ph2PLi). After about one hour, all the lithium had dissolved, and to the red/brown solution (chilled in ice-water) was added benzyl bromide (8.5 gm, 49.7 mmol) dropwise. After refluxing for four hours, the product had precipitated as a colourless crystalline solid, and was collected by filtration (crude yield was 9.3 gm, 92%). The + – crude salt was recrystallised from MeOH/n-hexane giving Ph2PBn2 Br (7.5 gm, 74% yield, mp 265-6˚C; Lit. 252-4˚C206); 1 H-NMR (CDCl3): phenyl GAr +7.42 to +7.71 ppm (10H, m), benzyl GAr +6.91 to +7.12 ppm (10H, m), +4.90 ppm (4H, d: 2J = 14 Hz); 13C{1H}-NMR (CDCl ): phenyl +116.2 GCH2 P-H 3 Gipso ppm (d: J = 82 Hz), Gortho +134.1 ppm (d: J = 8 Hz), Gmeta +129.4 ppm (d: J = 12 Hz), Gpara

+134.6 ppm (d: J = 3 Hz), benzyl Gipso +127.3 ppm (d: J = 8 Hz), Gortho +130.5 ppm (d: J = 6 Hz), +128.5 ppm (d: J = 3 Hz), +127.9 ppm (d: J = 4 Hz), +29.3 ppm (d: J = Gmeta Gpara GCH2 31 1 200 46 Hz); P{ H}-NMR (CH2Cl2): GP +27.0 ppm (s). Chapter 10: Experimental. 157

+ – Tribenzylphenylphosphonium bromide, PhPBn3 Br [118214-14-5].

Bn _ P PhCH2Br + Br Bn P Bn PhCH3 Bn Ph Ph Bn Into a 500 mL RBF containing magnesium turnings (1.57 gm, 65.1 mmol) in dry

Et2O (50 mL) under an atmosphere of dry argon was added dropwise over the period of about one hour to a solution of benzyl chloride (7.6 gm, 60.0 mmol) in dry Et2O (20 mL). The mixture was allowed to stir at RT for another three hours, by which time the magnesium had been consumed. The solution was then diluted with additional dry Et2O (8 mL), and to this solution was added dropwise a solution of PhPCl2 (5 gm, 27.9 mmol) in dry Et2O (100 mL) over the course of about one hour (the rate of addition adjusted to maintain a steady reflux).

During the addition a white precipitate separated, presumably MgCl2. When all of the

PhPCl2 had been added, the solution was allowed to stir at RT for a further three hours, after which time it was washed with deoxygenated water (2 u 50 mL) by use of a cannula. The solution was then dried over MgSO4, and transferred to a second flask through a cannula. To this solution was added benzyl bromide (5.2 gm, 30.4 mmol), and the solution refluxed for four hours, after which time the product had separated as a colourless crystalline solid. The product was collected by filtration (crude yield was 10.5 gm, 82%), and recrystallised from + – MeOH/n-hexane, giving PhPBn3 Br (7.8 gm, 61% yield: mp of picrate 165-7˚C; Lit. 164.5-6.5˚C57); 1 H-NMR (CDCl3): phenyl GAr +7.50 to +7.75 ppm (5H, m), benzyl GAr +7.12 to +7.30 ppm (15H, s), +4.44 ppm (6H, d: 2J = 15 Hz); 13C{1H}-NMR (CDCl ): phenyl +118.0 GCH2 P-H 3 Gipso ppm (d: J = 76 Hz), Gortho +132.9 ppm (d: J = 8 Hz), Gmeta +129.6 ppm (d: J = 12 Hz), Gpara

+134.5 ppm (d: J = 3 Hz), benzyl Gipso +127.4 ppm (d: J = 8 Hz), Gortho +130.5 ppm (d: J = 6 Hz), +129.1 ppm (d: J = 3 Hz), +128.3 ppm (d: J = 4 Hz), +27.8 ppm (d: J = Gmeta Gpara GCH2 31 1 200 47 Hz); P{ H}-NMR (CH2Cl2): GP +22.2 ppm (s).

+ – Tetrabenzylphosphonium bromide, Bn4P Br [912-13-0].

Bn _ P PhCH2Br + Br Bn P Bn PhCH3 Bn Bn Bn Bn

To a solution of Bn3P (5 gm, 16.4 mmol) in dry toluene (50 mL) was added benzyl bromide (3.4 gm, 19.9 mmol). After refluxing for four hours, the product had precipitated as colourless crystals, and were collected by filtration (crude yield was 7.2 gm, 92%). The + – crude salt was recrystallised from MeOH/n-hexane giving Bn4P Br (6.3 gm, 81% yield, mp 223-4˚C; Lit. 215-6˚C207); 1H-NMR (CDCl ): +7.07 to +7.27 ppm (20H, m), +4.00 ppm (8H, d: 2J = 14 Hz); 3 GAr GCH2 P-H 13 1 C{ H}-NMR (CDCl3): Gipso +127.3 ppm (d: J = 8 Hz), Gortho +130.4 ppm (d: J = 6 Hz), Gmeta Chapter 10: Experimental. 158

+129.4 ppm (d: J = 3 Hz), +128.4 ppm (d: J = 4 Hz), +26.7 ppm (d: J = 43 Hz); Gpara GCH2 31 200 2 P-NMR (CH2Cl2): GP +24.5 ppm (nonet: JP-H = 14 Hz).

+ – Methyltriphenylphosphonium iodide, Ph3PMe I [2065-66-9].

Me _ P CH3I + I Ph P Ph PhCH3 Ph Ph Ph Ph

To a solution of Ph3P (5 gm, 19.1 mmol) in dry toluene (50 mL) was added methyl iodide (3.5 gm, 24.7 mmol). After refluxing for four hours, the product had precipitated as colourless crystals, which were collected by filtration (crude yield was 6.5 gm, 84%). The + – crude salt was recrystallised from MeOH/n-hexane giving Ph3PMe I (5.9 gm, 76% yield, mp 185-7˚C; Lit. 182-3˚C72); 1H-NMR (CDCl /MeOH): +7.50 to +7.70 ppm (15H, m), +3.00 ppm (3H, d: 2J = 3 GAr GCH3 P-H 13 1 13.1 Hz); C{ H}-NMR (CDCl3/MeOH): Gipso +118.3 ppm (d: J = 89 Hz), Gortho +132.8 ppm (d: J = 11 Hz), +130.1 ppm (d: J = 13 Hz), +134.8 ppm (d: J = 3 Hz), +10.8 Gmeta Gpara GCH3 31 1 196 ppm (d: J = 58 Hz); P{ H}-NMR (EtOH): GP +22.9 ppm (s).

+ – E-Cinnamyltriphenylphosphonium chloride, Ph3PCn Cl [1530-35-4].

_ Ph Cl P PhCH=CHCH2Cl + E Ph P Ph Ph PhCH3 Ph Ph Ph D J

To a solution of Ph3P (5 gm, 19.1 mmol) in dry toluene (50 mL) was added E-cinnamyl chloride (3.5 gm, 22.9 mmol). After refluxing for four hours, a colourless solid product had separated from the yellow solution, and was collected by filtration (crude yield + was 4.9 gm, 62%). The crude salt was recrystallised from MeOH/n-hexane giving Ph3PCn Cl– (3.6 gm, 45% yield, mp 221-4˚C; Lit. 224-6˚C208); 1 H-NMR (CDCl3): phenyl GAr +7.46 to +7.72 ppm (15H, m), cinnamyl GAr +6.96 to 7.08 ppm 4 3 (5H, m), GCH +6.56 ppm (J, 1H, dd: JP-H = 5.6 Hz, JH-H = 15.9 Hz), GCH +5.80 ppm (E, 1H, m), +4.76 ppm ( , 2H, dd: 2J = 15.4 Hz, 3J = 7.7 Hz); 13C{1H}-NMR (CDCl ): GCH2 D P-H H-H 3 phenyl Gipso +117.5 ppm (d: J = 85 Hz), Gortho +133.4 ppm (d: J = 10 Hz), Gmeta +129.9 ppm

(d: J = 13 Hz), Gpara +134.6 ppm (d: J = 3 Hz), cinnamyl Gipso +135.2 ppm (d: J = 4 Hz), Gortho +126.0 ppm (s), +128.1 ppm (s), +127.9 ppm (s), +27.5 ppm ( , d: J = 50 Gmeta Gpara GCH2 D 31 1 Hz), GCH +113.3 ppm (E, d: J = 11 Hz), GCH +139.6 ppm (J, d: J = 13 Hz); P{ H}-NMR 205 (EtOH): GP +22.5 ppm (s). Chapter 10: Experimental. 159

+ – E-Cinnamyltribenzylphosphonium chloride, Bn3PCn Cl . + 209,210 (For the cation Bn3PCn , the CA number is [152043-94-2]. )

_ Bn Cl P PhCH=CHCH2Cl + E Bn P Ph Bn PhCH3 Bn Bn Bn D J

To a solution of Bn3P (1 gm, 3.3 mmol) in dry toluene (35 mL) was added E-cinnamyl chloride (0.6 gm, 3.9 mmol). After refluxing for four hours, a colourless solid product had precipitated from the yellow solution, and was collected by filtration (crude yield was 1.2 gm, 80%). However, this product was shown by 31P{1H}-NMR to contain a significant 13 1 amount (ca 40%) of Bn3P=O, as determined both by C{ H}-NMR [(CDCl3): Gipso +131.6 ppm (d: J = 7 Hz), Gortho +129.7 ppm (d: J = 5 Hz), Gmeta +128.6 ppm (d: J = 3 Hz), Gpara +126.8 ppm (d: J = 3 Hz), +35.3 ppm (d: J = 61 Hz)] and by 31P{1H}-NMR by spiking GCH2 the solution with an authentic sample of Bn3P=O [(CDCl3): GP +39.1 ppm (septet J = 13.4

Hz)]. The origin of the contamination by Bn3P=O was presumed to be from oxidation of the + – Bn3P. The crude Bn3PCn Cl was not recrystallised, but instead used as it was; 1H-NMR: +7.18 to +7.39 ppm (m), +4.13 ppm (benzyl methylene salt 6H, d J = 14.4 GAr GCH2 4 3 Hz), GCH +6.58 ppm (J-methine 1H, dd: JP-H = 4 Hz, JH-H = 15 Hz), GCH +5.19 ppm ( -methine 1H, m), +3.32 ppm ( -methylene 2H, dd: 2J 15 Hz, 3J 8 Hz); 13C{1H}- E GCH2 D P-H H-H + – NMR (CDCl3): [Bn3PCn Cl ]; benzyl Gipso +127.8 ppm (d: J = 8 Hz), Gortho +130.4 ppm (d: J = 5 Hz), +129.4 ppm (d: J = 3 Hz), +128.5 ppm (d: J = 4 Hz), +26.8 ppm (d: J Gmeta Gpara GCH2 = 43 Hz), cinnamyl Gipso +135.2 ppm (d: J = 3 Hz), Gortho +126.3 ppm (d: J = 2 Hz), Gmeta not located, not located, +24.6 ppm (d: J = 46 Hz), +113.7 ppm ( , d: J = 10 Hz), Gpara GCH2 GCH E 31 1 + – GCH +138.7 ppm (J, d: J = 12 Hz); P{ H}-NMR (MeOH): [Bn3PCn Cl ]; GP +24.4 ppm (s).

Ethane-1,2-bis(triphenylphosphonium) dibromide 159, [1519-45-5].

Ph _ Ph + P BrCH2CH2Br + P 2Br Ph Ph Ph P Ph Ph Ph Ph 159

Ph3P (5.5 gm, 21.0 mmol) was dissolved in 1,2-dibromoethane (20 mL), and the solution brought to reflux, whereupon a colourless solid product precipitated. The crystals were collected by filtration (crude yield was 7.2 gm, 96%), and the crude salt was recrystallised from EtOH giving ethane-1,2-bis(triphenylphosphonium) dibromide 159 (6.2 gm, 83% yield, mp 275-88˚C; Lit. 280-95˚C208); 1H-NMR (CDCl /TFA): +7.62 to +7.83 ppm (30H, m), +3.80 ppm (4H, d: J = 5.1 3 GAr GCH2 13 1 Hz); C{ H}-NMR (CDCl3/TFA): Gipso +115.6 ppm (qn: J = 87* Hz), Gortho +133.8 ppm (t: J Chapter 10: Experimental. 160

= 11* Hz), +130.7 ppm (t: J = 14* Hz), +135.7 ppm (s), +17.5 ppm (qn: J = Gmeta Gpara GCH2 31 1 51* Hz); P{ H}-NMR (EtOH): GP +26.4 ppm (s).

Ethane-1,2-bis(E-cinnamyldiphenylphosphonium) dichloride 161.

E Ph D J Ph + PhCH=CHCH2Cl P PPh2 Ph + Ph _ P Ph2P PhCH3 Ph 2Cl

56 161 Ph To a solution of ethane-1,2-bis(diphenylphosphine) 56 (10 gm, 25.1 mmol) in dry toluene (100 mL) was added E-cinnamyl chloride (4.6 gm, 30.1 mmol). After refluxing for four hours, a colourless solid product had precipitated, and was collected by filtration (crude yield was 14.3 gm, 81%). The crude salt was recrystallised from MeOH/n-hexane giving ethane-1,2-bis(E-cinnamyldiphenylphosphonium) dichloride (12.9 gm, 73% yield, mp 260- 2˚C dec.); 1 H-NMR (CDCl3/TFA): phenyl GAr +7.47 to +7.70 ppm (20H, m), cinnamyl GAr +6.97 to 3 +7.23 ppm (10H, m), GCH +6.47 ppm (J, 2H, d: JH-H = 15 Hz), GCH +5.58 ppm (E, 2H, m), +3.97 ppm ( , 4H, d: 3J = 6.4 Hz), +3.11 ppm (C1 = C2, 4H, s); 13C{1H}-NMR GCH2 D H-H GCH2

(CDCl3/TFA): phenyl Gipso +114.9 ppm (t: J = 84* Hz), Gortho +133.1 ppm (t: J = 9* Hz), Gmeta +130.8 ppm (t: J = 12* Hz), Gpara +136.2 ppm (s), cinnamyl Gipso +134.8 ppm (s), Gortho +126.6 ppm (s), +128.9 ppm (s), +129.4 ppm (s), +26.1 ppm ( , t: J = 48* Gmeta Gpara GCH2 D Hz), +110.9 ppm ( , t: J = 11* Hz), +141.1 ppm ( , t: J = 14* Hz), +14.9 ppm GCH E GCH J GCH2 31 1 (C1 = C2, t: J = 50* Hz); P{ H}-NMR (MeOH): GP +28.0 ppm (s). Microanalysis:

[C44H42P2Cl2] carbon (calc. 75.10%, found 74.85%), hydrogen (calc. 6.02%, found 6.11%). 2+ 2+ Electrospray MS data are shown in Table 10A (where M = [C44H42P2] ):

Table 10A: Electrospray Mass Spectral Data.

Cationic Fragment m/z Relative Intensity

[M2++Cl–] 667 21%

[M2+–Cn+] 515 14%

unidentified 329 100%

[M2+] 316 10%

[Cn+] 117 5% Chapter 10: Experimental. 161

1,1,4,4-Tetraphenyl-1,4-diphosphorinanium dibromide 54, [2316-28-1].

Ph _ Ph + + 2Br PPh2 BrCH2CH2Br P P Ph2P Ph

56 Ph 54 Ethane-1,2-bis(diphenylphosphine) 56 (20 gm, 50.2 mmol) was dissolved in 1,2-dibromoethane (100 mL), and the solution was heated on a steam bath overnight. The precipitated product was filtered off, and washed with diethyl ether (2 u 50 mL) to give 1,1,4,4- tetraphenyl-1,4-diphosphorinanium dibromide 54 (27.7 gm, 94% yield, mp >315˚C; Lit.76 324-5˚C); 1 H-NMR (CDCl3/MeOH): GAr +7.85 to +7.95 ppm (8H, m) and GAr +7.55 to +7.72 ppm (12H, m), +3.72 ppm (8H, d: 2J = 7 Hz); 13C{1H}-NMR (CDCl /MeOH): +115.0 ppm GCH2 P-H 3 Gipso (dd: J = 90 Hz, J = 4 Hz), Gortho +132.0 ppm (t: J = 11* Hz), Gmeta +130.8 ppm (t: J = 13* Hz), +135.7 ppm (s), +15.4 ppm (qn: J = 47* Hz); 31P{1H}-NMR (MeOH/EtOAc): Gpara GCH2 76 GP +16.0 ppm (s).

1,1,4,4-Tetraphenyl-1,4-diphosphepanium dibromide 59.

Ph _ BrCH CH Br Ph + + 2Br 2 2 P Ph P PPh P 2 2 Ph 60 Ph 59 Propane-1,3-bis(diphenylphosphine) 60 (10 gm, 24.2 mmol) was dissolved in 1,2-dibromoethane (50 mL), and the solution was heated with stirring overnight. The product was obtained as an oil, which would not crystallise in all attempts at recrystallisation. The oil formed a glassy solid upon standing overnight (11.3 gm, 78%), and finally crystallised of its own accord after about 3 months, giving 1,1,4,4-tetraphenyl-1,4-diphosphepanium dibromide 59. Attempts to recrystallise the glass only reproduced the oily product, but as this was of high purity (by 31P{1H}-NMR), it was used in the reactions described elsewhere. Finally, metathesis with NaBF4 gave a white solid which could be recrystallised from hot water, giving 1,1,4,4-tetraphenyl-1,4-diphosphepanium bis(tetrafluoroborate) (mp 235-7˚C); 1 H-NMR (CDCl3/MeOH): GAr +7.72 to +7.83 ppm (12H, m) and GAr +7.61 to +7.69 ppm (8H, m), +3.62 to +3.87 ppm (C2 = C3 and C5 = C7, 8H, m), +2.77 ppm (C6, 2H, 1:2:1 GCH2 GCH2 3 13 1 t: JP-H = 26.2 Hz); C{ H}-NMR (CDCl3/MeOH): Gipso +116.5 ppm (d: J = 85 Hz), Gortho

+132.2 ppm (d: J = 10 Hz), Gmeta +130.9 ppm (d: J = 13 Hz), Gpara +135.9 ppm (d: J = 3 Hz), +16.0 ppm (C2 = C3, dd: J = 51 Hz, J = 3 Hz), +22.1 ppm (C5 = C7, d: J = 48 Hz), GCH2 GCH2 +15.8 ppm (C6, 1:2:1 t: J = 11 Hz); 31P{1H}-NMR (MeOH): +29.6 ppm (s). GCH2 GP Chapter 10: Experimental. 162

Microanalysis: [C29H30P2B2F8] carbon (calc. 56.72%, found 56.25%), hydrogen (calc. 4.92%, 2+ 2+ found 4.94%). Electrospray MS data are shown in Table 10B (where M = [C29H30P2] ):

Table 10B: Electrospray Mass Spectral Data.

Cationic Fragment m/z Relative Intensity

2+ – [M +BF4 ] 527 26%

[M2+–H+] 439 27%

[M2+] 220 100%

10.6 Synthesis of the phosphine chalcogenides.

Triphenylphosphine sulphide, Ph3P=S [3878-45-3].

S P S Ph 8 P Ph Ph EtOAc Ph Ph Ph

Ph3P (5 gm, 19.1 mmol) was dissolved in EtOAc (50 mL), and sulphur (0.61 gm, 19.0 mmol). The crude product, obtained from removal of the solvent on a rotary evaporator, was 211 recrystallised from EtOH giving Ph3P=S (4.8 gm, 85% yield, mp 164-5˚C; Lit. 158-60˚C ); 1 H-NMR (CDCl3): GAr +7.68 to +7.78 ppm (6H, m) and GAr +7.38 to +7.54 ppm (9H, m); 13 1 194 C{ H}-NMR (CDCl3): Gipso +132.7 ppm (d: J = 85 Hz), Gortho +132.1 ppm (d: J = 11 Hz), 31 1 Gmeta +128.4 ppm (d: J = 12 Hz), Gpara +131.4 ppm (d: J = 3 Hz); P{ H}-NMR (THF): GP 194 33 3 1 1 +43.1 ppm (s; S (0.75% natural abundance, I = /2) satellites: 1:1:1:1 q, JP-S = 28 Hz).

Triphenylphosphine selenide, Ph3P=Se [3878-44-2].

Se P Se Ph P Ph Ph C H Ph 6 6 Ph Ph

To a solution of Ph3P (5 gm, 19.1 mmol) in benzene (50 mL) was added black selenium shot (1.74 gm, 22.0 mmol), and the mixture refluxed for about three hours, by which time all the phosphine had reacted. The residual selenium was filtered off, and the solvent was then removed under reduced pressure, giving crude crystalline Ph3P=Se (5.9 gm, 91% yield). The crude product was recrystallised from EtOH, (4.8 gm, 74% yield, sub. 165˚C, mp 186-8˚C; Lit. 187-8˚C212); 1 H-NMR (CDCl3): GAr +7.67 to +7.78 ppm (6H, m) and GAr +7.38 to +7.53 ppm (9H, m); 13 1 194 C{ H}-NMR (CDCl3): Gipso +131.6 ppm (d: J = 76 Hz), Gortho +132.5 ppm (d: J = 11 Hz), Chapter 10: Experimental. 163

31 1 Gmeta +128.4 ppm (d: J = 12 Hz), Gpara +131.5 ppm (d: J = 3 Hz); P{ H}-NMR (THF): GP 194 77 1 1 1 +36.0 ppm (s; Se (7.6% natural abundance, I = /2) satellites: d, JP-Se = 758 Hz).

Tribenzylphosphine oxide, [4538-55-0].

O P P BnMgCl H2O2 Cl Cl Bn Bn P Cl Et2O Bn CH Cl /MeOH Bn 2 2 Bn Bn BnMgCl was prepared as follows: to magnesium turnings (3.2 gm, 131.7 mmol) in dry

Et2O (10 mL) was added a small fraction (ca 3 mL) of benzyl chloride from a 250 mL pressure-equalising dropping funnel (containing benzyl chloride 15 gm, 118.5 mmol in Et2O 150 mL), with the reaction observed to begin immediately. Small amounts (ca 1 mL) of the remainder of the benzyl chloride were added at such a rate as to maintain a steady reflux of the solution. After addition was complete, the mixture was refluxed for 1 hour, before being diluted with more dry diethyl ether (100 mL). A solution of PCl3 (4.8 gm, 35.0 mmol) in dry

Et2O (100 mL) was added slowly in portions via the dropping funnel, with vigorous magnetic stirring to prevent coagulation of magnesium salts observed to precipitate. When all of the

PCl3 had been added, the mixture was washed with water (2 u 50 mL), to extract the solid

MgCl2, and air was bubbled through the ether layer overnight, forming Bn3P=O directly, which crystallised as colourless crystals. The product was collected, and recrystallised from

CH2Cl2/EtOAc (8.3 gm, 74% yield, sub. 190˚C, mp 215-7˚C; Lit. 211-2˚C); 1H-NMR (CDCl ): +7.22 to +7.33 ppm (15H, m), +3.03 ppm (6H, d: 2J = 13.7 3 GAr GCH2 P-H 13 1 Hz); C{ H}-NMR (CDCl3): Gipso +131.7 ppm (d: J = 7 Hz), Gortho +129.8 ppm (d: J = 6 Hz), +128.7 ppm (d: J = 2 Hz), +126.9 ppm (d: J = 3 Hz), +35.4 ppm (d: J = 62 Gmeta Gpara GCH2 31 Hz); P-NMR (CDCl3): GP +41.5 ppm (s).

Ethane-1,2-bis(diphenylphosphine)-P,P'-dioxide 81, [4141-50-8].

O Ph PPh H2O2 P 2 Ph Ph Ph P P 2 CH2Cl2/MeOH Ph 56 O 81 To a magnetically stirred solution of ethane-1,2-bis(diphenylphosphine) 56 (10 gm,

25.1 mmol) in CH2Cl2 (60 mL) was added an equal volume of MeOH, and the solution chilled in ice-water. To this solution was added dropwise 30% aqueous H2O2 (ca 1.3 eq), at such a rate that the solution did not become overheated. After addition was complete, the solution was allowed to stir for another two hours, then diluted with an equal volume of CH2Cl2 and the resulting solution washed with water (ca 50 mL) repeatedly until the aqueous washings showed a negative test for peroxides. The solvent was then removed under reduced pres- Chapter 10: Experimental. 164 sure, giving the crude product. Recrystallisation from EtOH gave ethane-1,2-bis(diphenylphosphine)-P,P-dioxide 81 (8.6 gm, 80% yield, sub. 245˚C, mp 275-6˚C; Lit. 276-8˚C213); 1H-NMR (CDCl /TFA): +7.45 to +7.75 ppm (20H, m), +4.28 ppm (C1 = C2, 4H, t: 3 GAr GCH2 13 1 JP-H = 13 Hz); C{ H}-NMR (CDCl3/TFA): Gipso +131.8 ppm (qn: J = 100 Hz), Gortho +130.6 ppm (t: J = 10* Hz), +128.7 ppm (t: J = 12* Hz), +131.9 ppm (s), +21.5 ppm Gmeta Gpara GCH2 31 1 129g (C1 = C2, t: J = 67* Hz); P{ H}-NMR (CH2Cl2): GP +32.5 ppm (s).

Ethane-1,2-bis(diphenylphosphine)-P,P'-disulphide, [7615-76-1].

S Ph PPh S8 P 2 Ph Ph Ph2P EtOAc P Ph 56 S

To a stirred solution of ethane-1,2-bis(diphenylphosphine) 56 (10 gm, 25.1 mmol) in toluene (75 mL) was added sulphur (0.8 gm, 24.9 mmol), and the mixture was allowed to stir at RT for a further three hours. The crude product, obtained from removal of the solvent under vacuum, was recrystallised from toluene giving colourless crystals of ethane-1,2-bis(diphenylphosphine)-P,P-disulphide (10.3 gm, 89% yield, mp 229-31˚C; Lit. 229-31˚C198b); 1H-NMR (CDCl ): +7.33 to 7.52 ppm (12H, m), +7.71 to 7.87 ppm (8H, m), 3 GAr GAr GCH2 13 1 +2.70 ppm (C1 = C2, 4H, d: J = 14.5 Hz); C{ H}-NMR (CDCl3): Gipso +131.7 ppm (m: J =

81* Hz), Gortho +131.0 ppm (t: J = 10* Hz), Gmeta +128.7 ppm (t: J = 12* Hz), Gpara +131.7 ppm (d: J = 1 Hz), +25.6 ppm (C1 = C2, qn: J = 54* Hz); 31P{1H}-NMR (CDCl ): GCH2 3 GP +45.1 ppm129g (s).

Ethane-1,2-bis(phenylphosphine oxide) 82, meso [73210-72-7], DL [80358-25-4]:

Ph P H2O2 Ph P H H MeOH/THF H H P Ph P Ph O

A solution of dilithium ethane-1,2-bis(phenylphosphide) 34 in THF was prepared from ethane-1,2-bis(diphenylphosphine) 56 (10 gm, 25.1 mmol) and lithium (0.36 gm, 52.0 mmol), as described in Section 10.4. The solution was quenched with the dropwise addition of glacial AcOH (3.6 gm, 60 mmol), and then chilled in ice-water. An excess of 30% H2O2 was added dropwise with rapid stirring, and the solvent removed under vacuum, to give the crude ethane-1,2-bis(phenylphosphine)-P,P-dioxide 82 (6.4 gm, 92% yield) as an oil. The dioxide was able to be recrystallised with difficulty from benzene,91 giving DL-ethane-1,2- bis(phenylphosphine)-P,P-dioxide 82a (3.2 gm, 46% yield, Lit. 105-6˚C91). The other dia- Chapter 10: Experimental. 165 stereomer, meso-ethane-1,2-bis(phenylphosphine)-P,P-dioxide 82b, was obtained as an oil; (1.9 gm, 27% yield, mp 141-9˚C; Lit. 156˚C91).

1 DL-isomer: H-NMR (CDCl3): GAr +7.21 to +7.70 ppm (10H, m), GP-H +7.34 ppm (2H, d: 1J = 476.6 Hz), +2.24 ppm (C1 = C2, 4H, s); 13C{1H}-NMR (CDCl ): +129.4 P-H GCH2 3 Gipso ppm (d: J = 99 Hz), Gortho +129.8 ppm (t: J = 11* Hz), Gmeta +129.1 ppm (t: J = 12* Hz), Gpara +132.9 ppm (s), +22.2 ppm (C1 = C2, qn: J = 62 Hz); 31P-NMR (EtOH): +25.4 ppm GCH2 GP 1 91 31 1 91 (d, JP-H = 477 Hz) ; meso-isomer: P-NMR (EtOH): GP +25.1 ppm (d, JP-H = 477 Hz) .

1,4-Diphenyl-1,4-diphosphorinane-1,4-dioxide 6, cis = [27771-03-5]; trans = [27771-04-6].

O O H2O2 Ph P P Ph P P CH2Cl2/MeOH Ph Ph cis/trans 7 cis/trans 6

To a solution of trans-1,4-diphenyl-1,4-diphosphorinane 7 (5.0 gm, 16.4 mmol) in 1:1

CH2Cl2:MeOH (25 mL) chilled in ice-water was added 30% H2O2 by dropwise addition. The crude product obtained from removal of the solvent was recrystallised from EtOH giving trans-1,4-diphenyl-1,4-diphosphorinane-1,4-dioxide 6 (4.6 gm, 92% yield, mp 284-5˚C (sub. 272˚C); Lit. 325˚C214); 1 H-NMR (CDCl3): GAr +7.37 to +7.50 ppm (aromatic 6H, m), GAr +7.62 to +7.78 ppm (aromatic 4H, m), 2.48 ppm (C2 = C3 = C5 = C6 methylene 8H, d: J = 2.1 Hz); 13C{1H}- GCH2

NMR (CDCl3): Gipso +131.8 ppm (qn: J = 101* Hz), Gortho +130.6 ppm (t: J = 9* Hz), Gmeta +128.7 ppm (t: J = 12* Hz), +132.0 ppm (s), +21.5 ppm (C2 = C3 = C5 = C6 Gpara GCH2 31 1 methylene, qn: J = 66* Hz); P{ H}-NMR (EtOH): GP +35.8 ppm (s, trans isomer).

10.7 The reaction of phosphine oxides with metallic lithium in THF.

The behaviour of Ph3P=O and ethane-1,2-bis(diphenylphosphine)-P,P'-dioxide 81 in THF towards lithium metal are described in this section.

Reaction of triphenylphosphine oxide.

+ Li H Ph P=O Ph P(O)H 3 THF MeOH 2

Into a dry 10 mm NMR tube was placed Ph3P=O (0.2 gm, 0.72 mmol). The tube was flushed with dry argon, and freshly distilled THF (2 mL) was added, dissolving the solid. To this solution was added a piece of freshly cut lithium (0.02 gm, 2.9 mmol), and the tube sonicated in an ultrasound bath at RT. Almost immediately, a dark brown colour was Chapter 10: Experimental. 166 observed to disperse from the surface of the lithium, indicating that a reaction was occurring. When the reaction was complete, the solution was quenched with excess ethanolic HCl (ca 1 31 1 mL). P-NMR: one signal at GP +23.3 ppm (d, JP-H = 497 Hz, Ph2P(O)H).

Reaction of ethane-1,2-bis(diphenylphosphine)-P,P-dioxide 81.

Li Ph P(O)CH CH P(O)Ph NO REACTION 2 2 2 2 THF

The procedure followed was that for the reaction of Ph3P=O with metallic lithium in THF; in this case, the dioxide 81 was completely insoluble in THF, and no reaction could be detected, either visually or by using 31P{1H}-NMR.

10.8 The reaction of phosphine chalcogenides with lithium naphthalenide.

General Method used to Prepare Solutions of Lithium Naphthalenide in THF.

Li Li THF

Naphthalene (0.4 gm, 3.1 mmol) was dissolved in freshly distilled THF (6 mL) in a 10 mL RBF at RT. The solution was flushed with dry argon, and magnetically stirred with a bare metal flea (made from a normal magnetic flea by cutting away the teflon). To this solution was added against a flow of dry argon freshly hammered lithium (0.02 gm, 2.9 mmol) which had been cut into strips approximately 3 mm u 20 mm, and about 1 mm thick. The characteristic green colour of the naphthalene radical anion was seen to form within 30 seconds, and all of the lithium had reacted within 1 hour with vigorous stirring.

General Procedure for the Reaction of Phosphine Chalcogenides with Lithium Naphthalenide. A solution of 0.5 M lithium naphthalenide in THF was prepared (as described above), and added to it against a flow of dry argon was the chalcogenide (0.3 eq). The deep green mixture was stirred for about 10 minutes, after which time a 31P{1H}-NMR spectrum was taken. The solution was quenched by adding solid ammonium chloride (1 eq) and then methanol until the colour completely dissipated. The resulting precipitate, consisting mainly of naphthalene, could be redissolved by addition of a small amount of CH2Cl2, after which another 31P-NMR was taken. Chapter 10: Experimental. 167

Reaction of triphenylphosphine oxide.

Li [C H ] H Ph P=O10 8 Ph P(O)H + PhH 3 THF MeOH 2

Ph3P=O (0.3 gm, 1.1 mmol) was reacted at RT with 0.5 M lithium naphthalenide (5 31 mL, 2.5 mmol), following the procedure described above. P-NMR: one signal at GP +23.3 1 ppm (d, JP-H = 497 Hz, Ph2P(O)H).

Reaction of triphenylphosphine sulphide.

+ _ + Li [C H ] H Ph P=S10 8 Ph P+ HS 3 THF MeOH 3 2

Ph3P=S (0.3 gm, 1.0 mmol) was reacted at RT with 0.5 M lithium naphthalenide (5 31 1 mL, 2.5 mmol), following the procedure described above. P{ H}-NMR: one signal at GP

–4.0 ppm (s, Ph3P); upon quenching, the characteristic odour of H2S was detected, with no change observed using 31P{1H}-NMR.

Reaction of triphenylphosphine selenide.

+ _ + Li [C H ] H O Ph P=Se10 8 Ph P+HSe 2 Se 3 THF MeOH 3 2 8

Ph3P=Se (0.4 gm, 1.2 mmol) was reacted at RT with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure described above. 31P{1H}-NMR: one signal observed at GP –4.0 ppm (s, Ph3P); upon quenching, the particularly horrible odour of H2Se was detected, with no change observed using 31P{1H}-NMR. Evidence to support this was obtained by the eventual formation of a deposit of red selenium around the mouth of the tube; this can be formed by exposure of the evolving H2Se gas to oxygen in the air.

Reaction of ethane-1,2-bis(diphenylphosphine)-P,P-dioxide.

O OLi _ O Li Ph P + Ph P Ph Ph Li [C10H8] P Ph P P Ph + P Ph THF Ph 81 Li O O 83 OLi OLi O O Li Ph P + Ph P H H H Ph P P Ph + P P Ph MeOH Ph Li O 83 OLi O 82

Ethane-1,2-bis(diphenylphosphine)-P,P-dioxide 81 (0.5 gm, 1.2 mmol) was reacted at RT with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure described 31 1 above. P{ H}-NMR: in addition to a broad signal spanning GP +85 ppm to GP +107 ppm, Chapter 10: Experimental. 168 two signals were observed at GP +98.3 ppm (s) and GP +90.8 ppm (1:1:1:1 q, J = 55 Hz), presumed to be due to the two diastereomers of dilithium ethane-1,2-bis(phenylphosphinite)

83. Other signals were also observed at GP +102 ppm and around GP +45 ppm. The mixture 31 was quenched with MeOH, and was found to contain only two signals by P-NMR: GP +25.4 1 1 ppm (d, JP-H = 477 Hz) and GP +25.1 ppm (d, JP-H = 477 Hz) due to the meso and DL diastereomers of ethane-1,2-bis(phenylphosphine)-P,P-dioxide 82.

To investigate the possibility that the 1:1:1:1 quartet observed at GP +90.8 ppm is indeed due to direct 31P-7Li spin-spin coupling, lithium-6 naphthalenide was prepared as described above using lithium-6 (95% isotopic purity, 0.10 gm, 16.6 mmol) and naphthalene (2.5 gm, 19.5 mmol) in THF (6 mL). The 31P{1H}-NMR showed that the 1:1:1:1 quartet previously observed had been replaced by a 1:1:1 triplet, also observed at GP +90.8 ppm, with 1 JP-6Li = 20 Hz, proving that these multiplets were indeed due to phosphorus-lithium spin-spin coupling.

Reaction of 1,4-diphenyl-1,4-diphosphorinane-1,4-dioxide.

_ O O + Li [C10H8] NO REACTION P P THF Ph Ph

trans-1,4-Diphenyl-1,4-diphosphorinane-1,4-dioxide 6 (0.3 gm, 1.0 mmol) was reacted at RT with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure described above, but only unreacted starting material was detected. 31P{1H}-NMR: one signal at GP +35.8 ppm.

10.9 The reaction of phosphonium salts with lithium naphthalenide.

General Procedure for the Reaction of Phosphonium Salts with Lithium Naphthalenide. A solution of 0.5 M lithium naphthalenide in THF was prepared (as described in Section 10.9), and chilled in a cold temperature bath. The salt (0.3 eq) was added against a flow of dry argon, and the deep green mixture stirred continuously for about 3 hours while as it warmed to RT. At this point a 31P-NMR was taken. As in the case of the chalcogenides, the solution was quenched by adding solid ammonium chloride (1 eq) and then methanol until the colour completely dissipated. The resulting precipitate of naphthalene could be redis- 31 solved by the addition of a small amount of CH2Cl2, after which another P-NMR was taken. Shown below are the experimental procedures for each phosphonium salt. The actual temperature used is given in each case, and any cleavage of an alkyl(aryl) group is assumed to produce the corresponding alkyl(aryl) lithium species, which are not listed. Chapter 10: Experimental. 169

Reaction of tetraphenylphosphonium bromide.

+ _ Li [C H ] Ph PBr10 8 PhP 4 THF 3 + – Ph4P Br (0.4 gm, 1.0 mmol) was reacted at RT with 0.5 M lithium naphthalenide (5 31 1 mL, 2.5 mmol), following the procedure described above. P{ H}-NMR: one signal at GP

–4.4 ppm (s, Ph3P).

Reaction of benzyltriphenylphosphonium bromide.

_ + + Li [C H ] H Ph PBn Br10 8 Ph P + Ph PBn 3 THF MeOH 3 2 + – Ph3PBn Br (0.5 gm, 1.2 mmol) was reacted at –23˚C (in a slurry of CCl4 and liquid nitrogen) with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure de- 31 1 scribed above. P{ H}-NMR: two signals at GP –4.4 ppm (s, Ph3P) and GP –8.9 ppm (s,

Ph2PBn), with the approximate ratio of 15:85 = Ph3P:Ph2PBn.

Reaction of dibenzyldiphenylphosphonium bromide.

_ + + Li [C H ] H Ph PBn Br 10 8 Ph PBn 2 2 THF MeOH 2 + – Ph2PBn2 Br (0.5 gm, 1.1 mmol) was reacted at –23˚C (in a slurry of CCl4 and liquid nitrogen) with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure de- 31 1 scribed above. P{ H}-NMR: one signal at GP –8.9 ppm (s, Ph2PBn).

Reaction of tribenzylphenylphosphonium bromide.

_ + + Li [C H ] H PhPBn Br10 8 PhPBn 3 THF MeOH 2 + – PhPBn3 Br (0.5 gm, 1.1 mmol) was reacted at –23˚C (in a slurry of CCl4 and liquid nitrogen) with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure de- 31 1 scribed above. P{ H}-NMR: one signal at GP –9.1 ppm (s, PhPBn2).

Reaction of tetrabenzylphosphonium bromide.

_ + + Li [C H ] H Bn P Br10 8 Bn P 4 THF MeOH 3 + – Bn4P Br (0.5 gm, 1.1 mmol) was reacted at –23˚C (in a slurry of CCl4 and liquid nitrogen) with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure de- 31 1 scribed above. P{ H}-NMR: one signal at GP –11.4 ppm (s, Bn3P). Chapter 10: Experimental. 170

Reaction of 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide.

Ph Ph + _ Li [C10H8] PP2Br Ph PPPh +PhPCH CH PPh THF 2 2 2 2 Ph Ph

The cyclic bis-salt 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide 54 (0.6 gm,

1.0 mmol) was reacted at –60˚C (in a slurry of CHCl3 and liquid nitrogen) with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure described above. The mixture was 31 1 quenched, and the P{ H}-NMR showed three signals of approximately equal proportion: GP –12.3 ppm (s, 56), GP –26.9 ppm (s, cis-7) and GP –27.6 ppm (s, trans-7).

Reaction of 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide.

_ Ph Ph + Li [C10H8] PP 2Br PPPh +PhP(CH ) PPh THF 2 2 3 2 Ph Ph Ph

1,1,5,5-Tetraphenyl-1,4-diphosphocanium dibromide (0.7 gm, 1.1 mmol) was reacted at –23˚C (in a slurry of CCl4 and liquid nitrogen) with 0.5 M lithium naphthalenide (5 mL, 2.5 mmol), following the procedure described above. The mixture was quenched, and the 31 1 P{ H}-NMR showed the following three signals: GP –16.5 ppm (s, 60) and GP –26.2 ppm (s, cis/trans-1229) in the approximate ratio 1:8. Oxidation to the dioxide with 30% aqueous

H2O2 gave signals at about GP +46 ppm corresponding to cis/trans-1,1,5,5-tetraphenyl-1,4- 29 diphosphocane-P,P'-dioxide (along with one at GP +36.2 ppm, presumably due to ethane- 1,2-bis(diphenylphosphine)-P,P'-dioxide 81).

10.10 Towards the 1,4,7-triphenyl-1,4,7-triphosphonane system.

Diphenylbis(2-diphenylphosphinoethyl)phosphonium bromide 35:

Reaction of 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide 54 with Ph2PLi.

Ph2P Ph Ph Ph2P Ph Ph PPh2 Ph Ph + + PhLi Ph2PLi P P THF P THF P Ph Ph + _ +_ _ Br Br 2Br 54 58 35

A 0.5 M solution of Ph2PLi in 50 mL THF was prepared (as described in Section 10.4) using Ph3P (6.5 gm, 24.8 mmol) and lithium metal (0.35 gm, 50.4 mmol). To this well stirred solution was added the bis-salt 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide 54 (14.0 gm, 23.9 mmol) in portions, allowing each portion of the bis-salt to dissolve before adding more. When addition was complete, the mixture was allowed to stir for another 30 minutes, Chapter 10: Experimental. 171

before being quenched with solid NH4Cl (ca 1 gm), followed by EtOH (10 mL). Preparation of Ph2PLi from Ph2PCl results in lower yields, with a significant amount of Ph2PH present (GP 1 –40.1 ppm, JP-H = 220 Hz). This supports the function of the PhLi (formed as a by-product in the preparation of Ph2PLi) in assisting the reaction by causing the ring-opening of the cyclic bis-salt 54. Attempts to isolate the compound gave only a viscous oil, which was easily oxidised by air. 31 P-NMR (THF/EtOH): GP +30.8 ppm (phosphonium P, 1:2:1 t) coupled to GP –12.4 ppm 3 (phosphino P, d): JP-P = 44 Hz.

Diphenylbis(2-diphenylthiophosphorylethyl)phosphonium bromide 75: Reaction of diphenylbis(2-diphenylphosphinoethyl)phosphonium bromide with sulphur.

Ph Ph Ph Ph S8 P Ph P Ph + THF Ph + Ph Ph2P _ PPh2 P _ P Br Br S S 35 75

To a solution of diphenylbis(2-diphenylphosphinoethyl)phosphonium bromide 35 in THF (50 mL, 23.9 mmol) was added sulphur (1.5 gm, 46.8 mmol, 1.96 eq.). The solution was allowed to stir at RT for 1 hour, and the crude product was obtained by removal of the solvent under reduced pressure. It was recrystallised from EtOH giving diphenylbis(P-thio- 2-diphenylphosphinoethyl)phosphonium bromide 75 (11.1 gm, 63% yield, mp 118-9˚C); 1 H-NMR: GAr +7.45 ppm (central phenyl 5H, s), GAr +7.57 to +7.75 ppm (terminal phenyl 8H, s), GAr +7.97 to +8.14 ppm (terminal phenyl 12H, s), GH +3.72 ppm (C1 methylene (4) H, tdd), 13 1 GH +3.26 ppm (C2 methylene (4) H, tdd); C{ H}-NMR: sulphide-phenyl Gipso +130.3 ppm 1 2 3 (d: JP-C = 82 Hz), Gortho +131.5 ppm (d: JP-C = 11 Hz), Gmeta +128.9 ppm (d: JP-C = 13 Hz), 4 1 Gpara +132.0 ppm (d: JP-C = 3 Hz), phosphonium-phenyl Gipso +116.8 ppm (d: JP-C = 83 Hz), 2 3 Gortho +133.4 ppm (d: JP-C = 10 Hz), Gmeta +130.4 ppm (d: JP-C = 13 Hz), Gpara +135.0 ppm (d: 4J = 4 Hz), +25.2 ppm (C1 methylene, dd: 1J = 53 Hz, 2J = 4 Hz), +16.1 P-C GCH2 P-C P-C GCH2 2 31 ppm (C2 methylene, d: JP-C = 50 Hz); P-NMR (EtOH/THF): GP +46.4 ppm (thiophosphoryl 3 P, d) coupled to GP +31.8 ppm (phosphonium P, 1:2:1 t): JP-P = 53 Hz. Microanalysis:

[C40H38P3S2Br + H2O] carbon (calc. 62.09%, found 61.61%), hydrogen (calc. 5.21%, found + + 5.31%). Electrospray MS data (where M = [C40H38P3S2] ) are shown in Table 10C: Chapter 10: Experimental. 172

Table 10C: Electrospray Mass Spectral Data.

Cationic Fragment m/z Relative Intensity

[M+] 675 80%

[M+–S] 643 3%

+ [PhPCH2CH2S ] 165 100%

unidentified 105 99%

1,1,4,4,7,7-Hexaphenyl-1,4,7-triphosphonanium tribromide: Reaction of diphenylbis(2-diphosphinoethyl)phosphonium bromide with 1,2-dibromoethane.

Ph Ph Ph Ph P+ _ BrCH2CH2Br P 3Br + MeOH Ph2P _ PPh2 + + Br Ph P P Ph 35 Ph Ph 36

A batch of diphenylbis(2-diphenylphosphinoethyl)phosphonium bromide 35 in THF (50 mL) was prepared as described above. The solvent was removed under reduced pressure to give a viscous oil, which was then dissolved in MeOH (30 mL) and a molar excess of 1,2- dibromoethane (6.6 gm, 35.1 mmol, 1.47 eq) was added. The solution was refluxed overnight, by which time a white solid had precipitated. This solid was found to be insoluble in boiling MeOH, but did dissolve in TFA, and was presumed to be the tris-salt 36: 31P-NMR

(TFA/MeOH): GP +32.6 ppm.

10.11 The reaction of mono-phosphonium salts with LiAlH4 in THF.

The following procedures, Methods A to D, are general experimental details followed for many of the specific reactions outlined below. Methods A, B and C describe the procedures followed to obtain a solution, suitable for characterisation by 31P-NMR, of the various phosphorus species discussed below, while Method D describes how the cleaved hydrocarbon fragment was worked up so that it could be characterised using either MS (for benzene) or 1H-, 2H- and 13C{1H}-NMR (for toluene).

Method A: The hydride added to the salt. Into a 10 mm NMR tube flushed with dry argon was placed the salt (ca 0.1 mmol) and dry THF (ca 2 mL) at RT. To this mixture was added the solid hydride source, either LiAlH4, Chapter 10: Experimental. 173

LiAlD4, LiBH4 or KH (ca 2 mmol), and any evolution of gas was allowed to cease before the tube was stoppered.

Method B: The salt added to the hydride. Into a 10 mm NMR tube flushed with dry argon was placed dry THF, to which the solid hydride source, either LiAlH4, LiAlD4, LiBH4 or KH (ca 2 mmol), was added, and the mixture shaken to allow as much of the solids to dissolve as possible. The salt (ca 0.1 mmol) was added, with any gas evolution being allowed to cease before the tube was stoppered.

Method C: The salt added to the hydride (modification of Method B). Into a 10 mm NMR tube flushed with dry argon was placed the hydride solution, either 1.0 M LiAlH4 in THF or 1.6 M Red-Al in toluene (2.0 mL, 2.0 mmol), and the salt (ca 0.1 mmol) was added slowly, with any gas evolution was allowed to cease before the tube was stoppered.

Method D: Procedure for the analysis of the hydrocarbon cleaved by LiAlD4. Into a 100 mL RBF was placed the salt (ca 20 mmol) and dry THF (ca 50 mL). The solvent was flushed with dry argon, and LiAlD4 (ca 24 mmol) was added, after which evolution of a small amount of gas was always observed. Boiling chips were added, and the mixture refluxed under an atmosphere of argon for at least 5 hours. The volatile products of the reaction were collected under high vacuum by use of a liquid nitrogen trap. The resulting solution was transferred to a 250 mL RBF, along with the Et2O rinsings of the trap, and the volume was carefully reduced down to between 5 and 10 mL on a rotary evaporator at RT. The resulting clear liquid was used without further modification for obtaining the 2H{1H}- 1 13 1 NMR, while for the H- and C{ H}-NMR, CD3CN was added for a lock.

The following describe the actual experimental details undertaken, all of the species characterised using 31P-NMR except where stated.

Reaction of tetraphenylphosphonium bromide.

_ D Ph D _ Ph Ph + LiAlD4 Ph D Ph P Br P Ph + P + P Ph + Ph P + D 4 THF Ph D 3 Ph Ph Ph Ph D

31 1 (i) Procedure as for Method A using LiAlD4. P{ H}-NMR: signals at GP –4.4 ppm 1 1 (s, Ph3P), GP –70.0 ppm (1:2:3:2:1 qn, JP-D = 51 Hz, Ph3PD2), GP –87.3 ppm (1:1:1 t, JP-D = 1 – 2 49 Hz, Ph4PD), and GP –187.2 ppm (1:2:3:2:1 qn, JP-D = 69 Hz, Ph4PD2 ); H-NMR: GD 1 1 +7.69 ppm (d, JP-D = 52 Hz, Ph3PD2), GD +8.23 ppm (d, JP-D = 50 Hz, Ph4PD), and GD +9.17 1 – ppm (d, JP-D = 69 Hz, Ph4PD2 ). [As the reaction was allowed to proceed, the signals due to – Ph3P and Ph3PD2 were observed to build up at the expense of Ph4PD and Ph4PD2 .] Chapter 10: Experimental. 174

Characterisation of the cleaved benzene PhD. The procedure followed as for Method D. Characterisation of the mixture was achieved using EI-MS: 78 (100.0%), 79 (40.2%), and 80 amu (5.1%). These results indicate that the benzene mixture contains 31% (±1%) PhD (see explanation in text, Section 5.3). 31 (ii) Procedure as for Method A using excess LiAlH4. P-NMR: four signals at GP 1 1 –4.4 ppm (s, Ph3P), GP –68.4 ppm (1:2:1 t, JP-H = 334 Hz, Ph3PH2), GP –86.6 ppm (d, JP-H = 1 3 323 Hz, Ph4PH), and GP –185.6 ppm (binomial triplet of nonets, JP-H = 447 Hz, JP-H =13 – Hz, Ph4PH2 ). 31 (iii) Procedure as for Method A using a limiting amount of LiAlD4. P-NMR: + – initially, three signals at GP +24.0 ppm (s, unreacted Ph4P Br ), GP –4.4 ppm (s, Ph3P), and GP 1 –87.3 ppm (1:1:1 t, JP-D = 49 Hz, Ph4PD) were observed, their intensities remaining proportional to one another (steady state). A new signal appeared, after about 1 hour, at GP +9.1 ppm (s, unidentified). 31 (iv) Procedure as for Method A using a limiting amount of LiAlH4. P-NMR: one 1 3 signal at GP –86.6 ppm (binomial doublet of nonets of tridecets, JP-H = 323 Hz, JP-H =14Hz, 4,5 JP-H = 3 Hz, Ph4PH). 31 (v) Procedure as for Method B using LiAlD4. P-NMR: initially, one signal at GP 1 – –187.2 ppm (1:2:3:2:1 qn, JP-D = 69 Hz, Ph4PD2 ), which then slowly decayed, replaced by a 1 signal at GP –70.0 ppm (1:2:3:2:1 qn, JP-D = 51 Hz, Ph3PD2) after about two hours, along with a trace of a signal at GP –4.4 ppm (s, Ph3P). 31 (vi) Procedure as for Method C. P-NMR: initially, one signal at GP –185.6 ppm 1 – (1:2:1 t, JP-H = 440.0 Hz, Ph4PH2 ), which then slowly decayed, replaced by a signal at GP 1 –68.4 ppm (1:2:1 t, JP-H = 334 Hz, Ph3PH2) after about two hours, along with a trace of a signal at GP –4.4 ppm (s, Ph3P).

Characterisation of the three toluenes.

CH 1H-NMR: +7.12 to +7.39 ppm (m), +2.41 ppm (s); 13C{1H}-NMR: 3 GAr GCH3 Gipso +137.7 ppm (s), Gortho +129.2 ppm (s), Gmeta +128.4 ppm (s), Gpara +125.6 ppm (s), +21.6 ppm (s). GCH3

CH 1H-NMR: +7.12 to +7.39 ppm (m), +2.41 ppm (s); 2H-NMR: +7.13 3 GAr GCH3 GD D 3 13 1 2 ppm (d, JD-H = 0.9 Hz); C{ H}-NMR: GC +137.6 ppm (C1, 1:1:1 t, JC-D = 0.9 1 Hz), GC +128.9 ppm (C2, 1:1:1 t, JC-D = 23.8 Hz), GC +128.3 ppm (C3, s), GC +125.6 ppm (C4, 1:1:1 t, 3J = 1.1 Hz), +128.4 ppm (C5, s), +129.2 ppm (C6, s), C-D GC GC GCH3 3 +21.5 ppm (1:1:1 t, JC-D = 0.7 Hz). Chapter 10: Experimental. 175

CH D 1H-NMR: +7.12 to +7.39 ppm (m), +2.40 ppm (1:1:1 t, 2J = 2.2 Hz); 2 GAr GCH3 D-H 2 2 13 1 H-NMR: GD +2.35 ppm (1:2:1 t, JD-H = 2.2 Hz); C{ H}-NMR: Gipso +137.6 ppm 3 (s), Gortho +129.2 ppm (1:1:1 t, JC-D = 0.8 Hz), Gmeta +128.4 ppm (s), Gpara +125.6 ppm (s), +21.3 ppm (1:1:1 t, 1J = 19.3 Hz). GCH3 C-D

Solubility of tetraphenylphosphonium bromide in dry THF. Into a 10 mm NMR tube con- + – 31 taining dry THF (3 mL) was added Ph4P Br (0.2 gm, 0.5 mmol). P-NMR: one signal at + – GP +24.1 ppm (s, Ph4P Br ).

Reaction of benzyltriphenylphosphonium bromide.

D CH3 CH2D CH3 _ Ph + LiAlD4 D Ph PBn Br P Ph + Ph P+ + + 3 THF 3 Ph D

31 (i) Procedure as for Method A using LiAlD4. P-NMR: two signals at GP –4.4 ppm 1 (s, Ph3P) and GP –70.0 ppm (1:2:3:2:1 qn, JP-D = 51 Hz, Ph3PD2). Characterisation of the cleaved toluene. The procedure followed was as for Method D, and the relative proportions of the three toluene isomers PhCH3, 2-DC6H4CH3 110 and PhCH2D 109 was determined by comparison of the integrals of the following sets of signals: for 1H-

NMR: the methyl: singlet (PhCH3 and 2-DC6H4CH3 110 together) and the methyl 1:1:1 triplet 2 (PhCH2D 109), for H-NMR: the ortho: doublet (2-DC6H4CH3 110) and the methyl: 1:2:1 13 1 triplet (PhCH2D 109), and for C{ H}-NMR: the methyl: singlet (from PhCH3), the methyl 3 110 1 1:1:1 triplet, JC-D = 0.7 Hz (2-DC6H4CH3 ), and the methyl 1:1:1 triplet, JC-D = 19.3 Hz

(PhCH2D 109). The results are presented in Table 10D, which shows the integral values obtained from the various spectra, and their corresponding proportions (as percentages).

+ – Table 10D: Raw Integrals for Toluene Isomers (Ph3PBn Br ). 1H-NMR 2H{1H}-NMR 13C{1H}-NMR

PhCH3 N/A N/A 1.0000 31% 1.0000 u 2 = 2.0000 56% 2-DC6H4CH3 1.0000 44% 0.9832 31%

PhCH2D 0.5142 u 3 = 1.5426 44% 1.2786 56% 1.1991 38% Total 3.5426 100% 2.2786 100% 3.1823 100%

For reasons outlined in the text (Table 5D, Section 5.2), the composition of the toluene mixture recovered was taken to be the proportions indicated by the 13C{1H}-NMR data, i.e.

31% PhCH3, 31% 2-DC6H4CH3 110 and 38% PhCH2D 109. Chapter 10: Experimental. 176

31 (ii) Procedure as for Method A using LiAlH4. P-NMR: two signals at GP –4.4 ppm 1 (s, Ph3P) and GP –68.4 ppm (1:2:1 t, JP-H = 334 Hz, Ph3PH2). 31 (iii) Procedure as for Method B using LiAlD4. P-NMR: one signal at GP –70.0 ppm 1 (1:2:3:2:1 qn, JP-D = 51 Hz, Ph3PD2). 31 (iv) Procedure as for Method B using LiAlH4. P-NMR: one signal at GP –68.4 ppm 1 3 4,5 (binomial triplet of septets of decets, JP-H = 334 Hz, JP-H = 18 Hz, JP-H = 3 Hz, Ph3PH2).

Solubility of benzyltriphenylphosphonium bromide in dry THF. Into a 10 mm NMR tube con- + – 31 taining dry THF (3 mL) was added Ph3PBn Br (0.2 gm, 0.5 mmol). P-NMR: no signals were detected.

Reaction of dibenzyldiphenylphosphonium bromide.

D CH3 CH2D CH3 Ph + _ LiAlD4 D Ph PBn Br P Bn + Ph PBn + Ph PD + + + 2 2 THF 2 2 Ph D

31 (i) Procedure as for Method A using LiAlD4. P-NMR: one signal at GP –8.9 ppm (s,

Ph2PBn). Characterisation of the cleaved toluene. The procedure followed was as for Method

D, and the relative proportions of the three toluene isomers PhCH3, 2-DC6H4CH3 110 and + – PhCH2D 109 were determined as in the case of the toluene cleaved from Ph3PBn Br described above. The results are presented in Table 10E.

+ – Table 10E: Raw Integrals for Toluene Isomers (Ph2PBn2 Br ). 1H-NMR 2H{1H}-NMR 13C{1H}-NMR

PhCH3 N/A N/A 1.0000 11% 1.0000 u 2 = 2.0000 35% 2-DC6H4CH3 1.0000 31% 2.3779 27%

PhCH2D 1.2566 u 3 = 3.7698 65% 2.2443 69% 5.6036 62% Total 5.7698 100% 3.2443 100% 8.9815 100%

These results indicate that the composition of the toluene mixture recovered was 11% PhCH3,

27% 2-DC6H4CH3 and 62% PhCH2D. 31 (ii) Procedure as for Method B using LiAlD4. P-NMR: three signals at GP –8.9 ppm 1 (s, Ph2PBn), GP –41.4 ppm (1:1:1 t, JP-D = 34 Hz, Ph2PD) and GP –79.3 ppm (1:2:3:2:1 qn, 1 129 JP-D = 51 Hz, Ph2PD2Bn ). Chapter 10: Experimental. 177

Reaction of tribenzylphenylphosphonium bromide.

CH3 CH2D CH3 _ + LiAlD4 D PhPBn Br PhPBn + + + 3 THF 2

31 (i) Procedure as for Method A using LiAlD4. P-NMR: one signal at GP –9.1 ppm (s,

PhPBn2). Characterisation of the cleaved toluene. The procedure followed was as for Method

+ – Table 10F: Raw Integrals for Toluene Isomers (PhPBn3 Br ). 1H-NMR 2H{1H}-NMR 13C{1H}-NMR

PhCH3 N/A N/A 1.0000 7% 0.2219 u 2 = 0.4438 17% 2-DC6H4CH3 1.0000 10% 1.3667 9%

PhCH2D 0.7351 u 3 = 2.2053 83% 8.6820 90% 12.4783 84% Total 2.6491 100% 9.6820 100% 14.8450 100%

These results indicate that the composition of the toluene mixture recovered was 7% PhCH3,

9% 2-DC6H4CH3 and 84% PhCH2D. 31 (ii) Procedure as for Method B using LiAlD4. P-NMR: one signal at GP –9.1 ppm (s,

PhPBn2).

Reaction of tetrabenzylphosphonium bromide.

CH3 CH2D CH3 _ + LiAlD4 D Bn PBr Bn P+ + + 4 THF 3

31 (i) Procedure as for Method A using LiAlD4. P-NMR: one signal at GP –11.4 ppm

(s, Bn3P). Characterisation of the cleaved toluene. The procedure followed was as for Method

D, and the relative proportions of the three toluene isomers PhCH3, 2-DC6H4CH3 110 and + – PhCH2D 109 were determined as in the case of the toluene cleaved from Ph3PBn Br described above. The results are presented in Table 10G. Chapter 10: Experimental. 178

+ – Table 10G: Raw Integrals for Toluene Isomers (Bn4P Br ). 1H-NMR 2H{1H}-NMR 13C{1H}-NMR

PhCH3 N/A N/A 1.0000 9% 0.1614 u 2 = 0.3228 12% 2-DC6H4CH3 1.0000 7% 0.6059 5%

PhCH2D 0.7575 u 3 = 2.2725 88% 14.3900 93% 9.5533 86% Total 2.5953 100% 15.3900 100% 11.1592 100%

These results indicate that the composition of the toluene mixture recovered was 9% PhCH3,

5% 2-DC6H4CH3 and 86% PhCH2D. 31 (ii) Procedure as for Method B using LiAlD4. P-NMR: one signal at GP –11.4 ppm

(s, Bn3P).

The following procedures describe the preparation of the species in d8-THF, enabling both 1H- and 13C{1H}-NMR to be obtained.

Tetraphenylphosphorane, Ph4PH.

31 + – H Observation by P-NMR: To a stirred suspension of Ph4P Br (0.63 gm, 1.5 Ph P Ph mmol) in dry d8-THF (1.0 mL) in a 10 mm NMR tube (under an atmosphere of Ph dry argon) was added LiAlH4 (0.01 gm, 0.3 mmol) at RT. The mixture was Ph shaken so as to dissolve as much solid as possible, and then left standing for a further five minutes to allow the residual solids to settle.

1 1 H-NMR (d8-THF): GP-H +7.85 ppm (1H, d: JP-H = 323.1 Hz), GAr +7.50 to +7.58 ppm (8H, 13 1 m) and GAr +7.33 to +7.43 ppm (12H, m); C{ H}-NMR (d8-THF): Gipso +147.2 ppm (d: J = 79 Hz), Gortho +135.5 ppm (d: J = 13 Hz), Gmeta +129.4 ppm (d: J = 13 Hz), Gpara +131.1 ppm 31 1 (d: J = 3 Hz); P-NMR (THF): GP –86.6 ppm (binomial doublet of nonets of tridecets, JP-H = 3 4,5 323 Hz, JP-H = 14 Hz, JP-H = 3 Hz, Ph4PH). + – Attempted isolation of Ph4PH: The procedure was followed on a larger scale, with Ph4P Br (5 gm, 11.9 mmol) suspended in dry deoxygenated THF (50 mL). To this rapidly stirred solution, LiAlH4 (0.11 gm, 3.0 mmol) was added in small portions, keeping the phosphonium salt in excess at all times. Passing the mother liquor down a short column of either silica or basic alumina under pressure of argon produced a gas, disrupting the packing of the column. Evaporation of the solvent under reduced pressure gave a solid which resisted attempts to redissolve it in THF, although enough dissolved in d6-benzene to confirm the presence of

Ph4PH by NMR. The solid was found to give a melting point of 156-68˚C, suggesting that it may have polymerised (see text, Section 6.3). Chapter 10: Experimental. 179

Triphenylphosphorane, Ph3PH2.

H Observation by 31P-NMR: Into a 10 mm NMR tube flushed with dry argon was Ph P Ph placed LiAlH4 (0.02 gm, 0.5 mmol), and d8-THF (1.0 mL) was then added. Ph + – Ph3PBn Br (0.13 gm, 0.23 mmol) was also added, and the tube was agitated H so as to dissolve as much solid as possible, and then left standing for a further five minutes to allow the residual solids to settle.

1 1 H-NMR (d8-THF): GP-H +7.32 ppm (2H, d: JP-H = 333.9 Hz), GAr +8.14 to +8.28 ppm (6H, 13 1 m) and GAr +7.15 to +7.27 ppm (9H, m); C{ H}-NMR (d8-THF): Gipso +142.7 ppm (d: J = 83 Hz), Gortho +133.6 ppm (d: J = 19 Hz), Gmeta +130.4 ppm (d: J = 14 Hz), Gpara +134.5 ppm (d: 31 1 J = 3 Hz); P-NMR (THF): GP –68.4 ppm (binomial triplet of septets of decets, JP-H = 334 3 4,5 Hz, JP-H = 18 Hz, JP-H = 3 Hz, Ph3PH2).

Attempted isolation of Ph3PH2: The procedure was followed on a larger scale, with LiAlH4 (0.5 gm, 13.2 mmol) suspended in dry deoxygenated THF (50 mL). To the rapidly stirred + – solution was added Ph3PBn Br (5 gm, 11.5 mmol) portion-wise against a stream of argon. After addition was complete, the solids were allowed to settle, and the mother liquor passed down a column of basic alumina, as in the case of Ph4PH. In this case, gas evolution was much greater, and only Ph3P was detected in the recovered fractions.

+ – Lithium tetraphenylphosphoranate, Li Ph4PH2 .

_ 31 Ph Observation by P-NMR: Into a 10 mm NMR tube flushed with dry + Ph H argon was placed LiAlH4 (0.02 gm, 0.5 mmol), and d8-THF (1.0 mL) Li P Ph H + – was added. Ph4P Br (0.10 gm, 0.24 mmol) was also added, and the Ph tube was agitated so as to dissolve as much solid as possible. Because – of the unimolecular decay of Ph4PH2 into Ph3PH2, the tube was placed in the spectrometer as soon as the gas evolution had decreased.

1 1 H-NMR (d8-THF): GP-H +8.77 ppm (2H, d: JP-H = 447.1 Hz), GAr +7.24 to +7.59 ppm (12H, 13 1 m) and GAr +6.55 to +6.81 ppm (8H, m); C{ H}-NMR (d8-THF): Gipso +170.6 ppm (d: J = 40 Hz), Gortho +127.4 ppm (d: J = 10 Hz), Gmeta +129.8 ppm (d: J = 14 Hz), Gpara +124.2 ppm (s); 31 1 3 P-NMR (THF): GP –185.6 ppm (binomial triplet of nonets, JP-H = 447 Hz, JP-H = 13 Hz, – Ph4PH2 ). Because of the nature of the phosphoranate (i.e. rapid unimolecular decay), attempts to isolate it were not made. Chapter 10: Experimental. 180

Reaction of N-benzyl-N,N-dimethylanilinium bromide.

_ + LiAlD4 PhN(Bn)Me Br PhNMe + CH D 2 THF 2 2 This reaction was done as a control, to compare the deuterium distribution in the toluene isolated from the cleavage of the benzylphosphonium salts with that of a benzylammonium salt. Characterisation of the cleaved toluene. Procedure as for Method D, Section 10.13: 13C- 1 1 { H}-NMR: methyl GC +19.3 ppm (1:1:1 septet: JC-D = 21.3 Hz). These results indicate that the toluene recovered was exclusively PhCH2D (i.e. no other isomer, including PhCH3, was detected). 1 Characterisation of the residual amine. H-NMR: GAr +7.23 to +7.33 ppm (2H, m), GAr +6.73 to 6.83 ppm (3H, m), +2.99 ppm (6H, s); 13C{1H}-NMR: +150.6 ppm (s), GCH3 Gipso +112.6 ppm (s), +129.0 ppm (s), +116.6 ppm (s), N-methyl +40.5 ppm Gortho Gmeta Gpara GCH3 (s).

Reaction of methyltriphenylphosphonium iodide.

D D D _ Ph Ph Ph + LiAlD4 Ph PMe I P Me + P Ph + P Me +PhP + Ph PMe 3 THF 3 2 Ph Ph Ph Ph D D

31 (i) Procedure as for Method A using LiAlD4. P-NMR: six signals at GP +22.9 ppm

(s, unreacted salt), GP –4.4 ppm (s, Ph3P), GP –26.2 ppm (s, Ph2PMe), GP –103.6 ppm (1:1:1 t, 1 1 JP-D = 54 Hz, Ph3PDMe), GP –70.0 ppm (1:2:3:2:1 qn, JP-D = 51 Hz, Ph3PD2), and GP –106.4 1 ppm (1:2:3:2:1 qn, JP-D = 48 Hz, Ph2PD2Me). 31 (ii) Procedure as for Method A using LiAlH4. P-NMR: four signals at GP +22.9 ppm

(s, unreacted salt), GP –4.4 ppm (s, Ph3P), GP –26.2 ppm (s, Ph2PMe), and GP –102.9 ppm (d, 1 JP-H = 343 Hz, Ph3PHMe).

Reaction of dimethyldiphenylphosphonium iodide.

D D D _ Me Ph Me + LiAlD4 Ph PMe I P Ph + P Me + P Ph +PhPMe + PhPMe 2 2 THF 2 2 Me Ph Me Ph D D

31 (i) Procedure as for Method A using LiAlD4. P-NMR: six signals at GP +22.5 ppm

(s, unreacted salt), GP –26.2 ppm (s, Ph2PMe), GP –45.3 ppm (s, PhPMe2), GP –106.4 ppm 1 1 (1:2:3:2:1 qn, JP-D = 48 Hz, Ph2PD2Me), GP –116.9 ppm (1:1:1 t, JP-D = 56 Hz, Ph2PDMe2), 1 and GP –143.7 ppm (1:2:3:2:1 qn, JP-D = 42 Hz, PhPD2Me2). 31 (ii) Procedure as for Method A using LiAlH4. P-NMR: six signals at GP +22.5 ppm

(s, unreacted salt), GP –26.2 ppm (s, Ph2PMe), GP –45.3 ppm (s, PhPMe2), GP –104.7 ppm 1 (Ph2PH2Me, P-H coupling not observed due to low signal intensity), GP –116.2 ppm (d, JP-H Chapter 10: Experimental. 181

= 389 Hz, Ph2PHMe2), and GP –142.3 ppm (PhPH2Me2, P-H coupling not observed due to low signal intensity).

Reaction of n-butyltriphenylphosphonium iodide.

D _ Ph + LiAlD4 n Ph PnBu I P nBu + Ph P + Ph P Bu 3 THF 3 2 Ph Ph

31 Procedure as for Method B, using LiAlD4. P-NMR: four signals at GP +24.8 ppm (s, n unreacted salt), GP –4.4 ppm (s, Ph3P), GP –15.7 ppm (s, Ph2P Bu), and GP –90.1 ppm (1:1:1 t, 1 n JP-D = 49 Hz, Ph3P( Bu)D). Initially, the phosphorane at GP –90.1 ppm was the major n species, but slowly decayed while the signal at GP –15.7 ppm (Ph2P Bu) grew.

Reaction of E-cinnamyltriphenylphosphonium chloride. _ + LiAlD4 Ph3PCn Cl Ph3P THF

31 (i) Procedure as for Method A using LiAlD4. P-NMR: GP –4.4 ppm (s, Ph3P). 31 (ii) Procedure as for Method B using LiAlD4. P-NMR: GP –4.4 ppm (s, Ph3P).

Reaction of E-cinnamyltribenzylphosphonium chloride.

+ _ LiAlH Bn PCn Cl4 Bn P 3 THF 3

31 (i) Procedure as for Method A using LiAlH4. P-NMR: GP –11.4 ppm (s, Bn3P). 31 (ii) Procedure as for Method B using LiAlH4. P-NMR: GP –11.4 ppm (s, Bn3P).

Reaction of 1,1-diphenylphosphorinanium perchlorate 136.

Ph Ph Ph D + Ph _ P P Ph P ClO4 LiAlD4 + + Ph P(CH ) CH D+ D THF 2 2 4 2

31 Procedure as for Method A using LiAlD4. P-NMR: initially, two signals were 1 observed at GP +17.7 ppm (s, unreacted salt 136) and GP –111.8 ppm (1:1:1 t, JP-D = 48 Hz,

153). As the reaction proceeded, additional signals appeared, first at GP +23.4 ppm (s, unidentified), GP –15.5 ppm (s, 154) and GP –32.6 ppm (s, 155), and finally another signal at

GP +38.4 ppm (s, unidentified). After a few hours, the only signals remaining were those at GP +38.4, –15.5 and GP –32.6 ppm. Chapter 10: Experimental. 182

10.12 The reaction of bis-phosphonium salts with LiAlH4 in THF.

Reaction of ethane-1,2-bis(triphenylphosphonium) dibromide 159.

D + + _ Ph LiAlD4 Ph PCH CH PPh 2Br P Ph + Ph P + H C=CH 3 2 2 3 THF 3 2 2 Ph D

31 Procedure as for Method C. P-NMR: two signals at GP –4.4 ppm (s, Ph3P) and GP 1 –68.4 ppm (1:2:1 t, JP-H = 334 Hz, Ph3PH2). An evolved gas, presumed to be ethylene, was observed but not characterised. Reaction of ethane-1,2-bis(E-cinnamyldiphenylphosphonium) dichloride 161.

Cn Ph + + P LiAlH4 PPh2 Ph Ph Ph P P Ph THF 2 _ Cn 2Cl 31 Procedure as for Method C. P-NMR: one signal at GP –12.2 ppm (s). The product of the reaction was characterised as the disulphide as follows: To a stirred suspension of ethane-1,2-bis(E-cinnamyldiphenylphosphonium) dichloride (5 gm, 7.1 mmol) in dry THF

(60 mL) at RT was added LiAlH4 (0.3 gm, 2.6 mmol, 4.5 eq) against a flow of dry argon. The white salt was observed to dissolve, and the solution became opaque and dark grey due to undissolved lithium and/or aluminium salts. After three hours at RT, sulphur (0.22 gm, 6.9 mmol) was added, and the mixture allowed to stir at RT for another two hours, after which time the undissolved solids were removed by filtration, and the solvent removed on the rotary evaporator. The crystalline residue was recrystallised from toluene, giving ethane- 1,2-bis(diphenylphosphine)-P,P'-disulphide as colourless crystals (1.79 gm, 56% yield, mp 228˚C; Lit. 229-31˚C197b); 1 H-NMR (CDCl3): GH +7.76 to +7.84 ppm (8H, m), GH +7.40 to +7.50 ppm (12H, m), GH 13 1 +2.73 ppm (C1 = C2, 4H, d: J = 2.2* Hz); C{ H}-NMR (CDCl3): Gipso +131.6 ppm (m: J = 86 Hz), Gortho +131.0 ppm (t: J = 10* Hz), Gmeta +128.7 ppm (t: J = 12* Hz), Gpara +131.7 ppm (s), +25.6 ppm (qn: J = 55* Hz); 31P-NMR (CDCl ): +45.0 ppm (s); GCH2 3 GP spiking the solution with and authentic sample prepared in Section 10.7 gave a single signal 13 1 at GP +45.1 ppm (s), and no observed splitting of the peaks in the C{ H}-NMR spectrum; this indicates that the product of the cleavage is ethane-1,2-bis(diphenylphosphine) 56. Also visible in the spectrum were two doublets, each with an identical splitting, presumed to be ethane-1,2-bis(diphenylphosphine)-P-oxide-P'-sulphide; 31 3 P-NMR (CDCl3): GP +45.5 ppm (P=S, d) and GP +33.7 ppm (P=O, d), JP-P = 58 Hz. Chapter 10: Experimental. 183

Reaction of ethane-1,2-bis(tri-n-butylphosphonium) dibromide 160.

D D + + _ nBu nBu n n LiAlD4 n n Bu3PCH2CH2P Bu3 2Br Bu3P + Bu2PD +P (CH2)2 P + H2C=CH2 THF nBu nBu nBu nBu

31 (i) Procedure as for Method A using LiAlD4. P-NMR: ten signals, including six unidentified singlets with the following chemical shifts: +53.9, +36.1, –22.9, –23.2, –26.4 and n –26.9 ppm, as well as GP +38.2 ppm (s, unreacted bis-salt), GP –31.0 ppm (s, Bu3P), GP –70.5 1 n 1 ppm (1:1:1 t, JP-D = 31 Hz, Bu2PD), and GP –97.8 ppm (1:1:1 t, JP-D = 39 Hz, n n Bu3(D)PCH2CH2P(D) Bu3 166). 31 (ii) Procedure as for Method A using LiAlH4. P-NMR: nine signals (six unidentified at GP +37.9, +36.1, –22.9, –23.2, –26.4, and –26.9 ppm), including GP +38.2 ppm (s, n 1 n unreacted bis-salt), GP –31.0 ppm (s, Bu3P), and GP –70.5 ppm (d, JP-H = 254 Hz, Bu2PH).

Reaction of propane-1,3-bis(triphenylphosphonium) dibromide 169. _ D D D D 2 D D + + _ Ph Ph LiAlD4 Ph P(CH ) PPh 2Br P (CH ) P + Ph P (CH ) P Ph 3 2 3 3 THF 2 3 2 3 Ph Ph Ph Ph Ph Ph Ph Ph

31 Procedure as for Method B using LiAlD4. P-NMR: two signals at GP +22.4 ppm (s, 1 unreacted bis-salt) and GP –90.4 ppm (1:1:1 t, JP-D = 49 Hz, bis-phosphorane), followed by signals at GP –4.4 ppm (s, Ph3P), GP –15.7 ppm (s, Ph2PCH2CH2CH2PPh2), GP –83.0 ppm (s, 1 unidentified), and GP –199.7 ppm (1:2:3:2:1 qu, JP-D = 67 Hz, bis-phosphoranate).

Reaction of 1,1,4,4-tetraphenyl-1,4-diphosphorinanium dibromide 54.

H Ph Ph Ph + + LiAlH4 excess PP H2C=CH2 +Ph2PCH2CH2 P Ph2PCH2CH2PPh2 THF Ph salt Ph _ Ph H 2Br 31 56 (i) Procedure as for Method A using LiAlD4. P-NMR: GP –12.3 ppm (s, ). An evolved gas, presumed to be ethylene, was observed but not characterised. 31 3 (ii) Procedure as for Method C. P-NMR: two signals (each a doublet, with JP-P = 1 44 Hz), appearing at GP –13.1 ppm and GP –79.3 ppm ( JP-H = 300 Hz, 163). An evolved gas, presumed to be ethylene, was observed but not characterised. Chapter 10: Experimental. 184

Reaction of 1,1,4,4-tetraphenyl-1,4-diphosphepanium dibromide 59.

H Ph Ph Ph + + LiAlH4 P P H2C=CH2 +Ph2PCH2CH2CH2 P THF Ph _ Ph Ph 2Br H

31 1 Procedure as for Method C. P-NMR: two singlets at GP –16.2 and –83.4 ppm ( JP-H = 308 Hz, 165). During the reaction, an evolved gas (presumed to be ethylene) was observed but not characterised.

Reaction of 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide 61.

Ph Ph Ph _ PhD _ PhD P P P P P P Ph D D Ph Ph Ph Ph Ph D cis- and trans-12 cis- and trans-176 175

LiAlH4 refluxing THF _ _ 2Br Br Ph Ph + LiAlD4 Ph LiAlD4 Ph + P P P P Ph P + P Ph D D Ph Ph Ph Ph Ph D Ph 61 173 174

Ph Ph D P P P P Ph Ph Ph Ph Ph D Ph cis- and trans-178 177

31 Procedure as for Method B using LiAlD4. P-NMR: initially one signal at GP –41.1 1 p:1:1t,pm (1 JP-D = 12 Hz, bridged cation). The signal intensity slowly decreased, to be replaced with the following signals: GP +31.2 ppm (s, unidentified), GP +25.2 ppm (s, unreacted salt), GP +11.2 ppm (m, unidentified), GP –16.7 ppm (s), GP –17.2 ppm (s), GP –21.5 ppm to GP –27.5 ppm (m, 12), GP –84.5 ppm (1:2:3:2:1 qn, J = 47 Hz), GP –87.5 ppm (s), GP –87.7 ppm (s), GP –87.8 ppm (s), two overlapping 1:1:1 triplets (two at GP –93.1 ppm and GP –93.3 ppm, both with J = 47 Hz, and a third 1:1:1 triplet at GP –97.2 ppm, J = 40 Hz). Initially, there was also a weak signal at GP –161.8 ppm. Chapter 10: Experimental. 185

10.13 The reaction of other phosphorus compounds with LiAlH4 in THF.

Reaction of triphenylphosphine, Ph3P.

LiAlH4 Ph P NO REACTION 3 THF

31 Procedure as described for Method A in Section 10.13, using LiAlH4. P-NMR: one signal at GP –4.4 ppm (s, unreacted Ph3P).

Reaction of triphenylphosphine oxide, Ph3P=O.

D Ph LiAlD4 Ph P=O P Ph + Ph P 3 THF 3 Ph D

31 (i) Procedure as described for Method A in Section 10.13, using LiAlD4. P-NMR: six unidentified signals at GP +86.0 to +88.6 ppm, with other signals at GP +39.4 ppm (s, unidentified), GP +32.1 ppm (s, unreacted Ph3P=O), GP –4.4 ppm (s, Ph3P) and GP –70.3 ppm 1 (1:2:3:2:1 qn, JP-D = 52 Hz, Ph3PD2). The reaction was very much slower than that observed with the phosphonium salts, presumably due to the decreased electrophilicity at phosphorus in + – Ph3P=O when compared to Ph4P Br . Only trace amounts of Ph3PD2 were observed, presumably due to loss of D2 by thermal decomposition. 31 (ii) Procedure as described for Method A in Section 10.13, using LiAlH4. P-NMR: four signals at GP +31.7 ppm, GP +31.5 ppm, (s, unreacted Ph3P=O), GP –4.4 ppm (s, Ph3P) 1 and GP –68.4 ppm (1:2:1 t, JP-H = 334 Hz, Ph3PH2).

Reaction of benzyldiphenylphosphine oxide, Ph2P(O)Bn.

D Ph LiAlD4 Ph P(O)Bn P Bn + Ph PBn 2 THF 2 Ph D

31 Procedure as described for Method A in Section 10.13, using LiAlD4. P-NMR: three signals at GP +34.6 ppm (s, unreacted Ph2P(O)Bn), GP –9.1 ppm (s, Ph2PBn), and GP 1 129 –79.3 ppm (1:2:3:2:1 qn, JP-D = 51 Hz, Ph2PD2Bn ).

Reaction of triphenylphosphine sulphide, Ph3P=S.

LiAlD4 Ph P=S NO REACTION 3 THF

31 Procedure as described for Method A in Section 10.13, using LiAlD4. P-NMR: one 33 3 1 signal at GP +43.1 ppm (s, unreacted Ph3P=S; S satellites (I = /2): 1:1:1:1 q, JP-S = 28 Hz) over a period of several hours. Chapter 10: Experimental. 186

Reaction of triphenylphosphine selenide, Ph3P=Se.

LiAlD4 Ph P=Se Ph P 3 THF 3

31 Procedure as described for Method A in Section 10.13, using LiAlD4. P-NMR: two 77 1 1 signals at GP +36.0 ppm (s, unreacted Ph3P=Se; Se satellites (I = /2): d, JP-Se = 758 Hz) and

GP –4.4 ppm (s, Ph3P). The reaction proceeded much faster than in the case of the oxide, but + – still a lot slower than for Ph4P Br . Here, the intensity of the signal due to Ph3P steadily increased over time, at the expense of the selenide.

Reaction of phosphoryl chloride, POCl3.

LiAlH4 POCl PH 3 THF 3

31 (i) Procedure as described for Method A in Section 10.13, using LiAlH4. P-NMR: four signals at GP +3.7 ppm (s, unreacted POCl3), GP –6.9 ppm (s, unidentified), GP –214.1 1 1 ppm (s { H} m, unidentified), and GP –244.0 ppm (1:3:3:1 q, JP-H = 187 Hz, PH3). 31 (ii) Procedure as described for Method A in Section 10.13, using LiAlD4. P-NMR: four signals at GP +3.6 ppm (s, unreacted POCl3), GP –6.9 ppm (s, unidentified), GP –217.2 1 1 ppm (m { H}, unidentified), and GP –246.3 ppm (1:3:6:7:6:3:1 septet, JP-D = 29 Hz, PD3).

Reaction of phosphorus pentachloride, PCl5.

LiAlH4 PCl PH 5 THF 3

31 Procedure as described for Method A in Section 10.13, using LiAlH4. P-NMR: one 1 signal at GP –244.0 ppm (1:3:3:1 q, JP-H = 187 Hz, PH3).

Reaction of triethyl phosphate, (EtO)3P=O.

LiAlH4 (EtO) P=O PH 3 THF 3

31 Procedure as described for Method A in Section 10.13, using LiAlD4. P-NMR: two 1 signals at GP –0.2 ppm (3:4:3 t, JP-H = 7.6 Hz [i.e. a 1:6:15:20:15:6:1 septet, only the three 1 inner peaks resolved], unreacted (EtO)3P=O) and GP –244.6 ppm (1:3:3:1 q, JP-H = 187 Hz,

PH3). Chapter 10: Experimental. 187

10.14 The reaction of phosphorus compounds with other hydride donors.

Reaction of tetraphenylphosphonium bromide with LiBH4 in THF. _ + LiBH4 Ph P Br Ph PH 4 THF 4 (i) Procedure as described for Method A in Section 10.13. 31P-NMR: three signals at + – 1 GP +30.1 ppm (s, Ph3P=O), GP +23.6 ppm (s, unreacted Ph4P Br ), and GP –86.6 ppm (d, JP-H

= 324 Hz, Ph4PH). (ii) Procedure as described for Method B in Section 10.13. 31P-NMR: two signals at + – 1 GP +23.6 ppm (s, unreacted Ph4P Br ), and GP –86.6 ppm (d, JP-H = 324 Hz, Ph4PH).

Reaction of benzyltriphenylphosphonium bromide with LiBH4 in THF. _ + LiBH4 Ph PBn Br Ph P 3 THF 3

31 Procedure as described for Method B in Section 10.13. P-NMR: one signal at GP 31 –4.4 ppm (s, Ph3P). To the solution was added MeOH; P-NMR revealed an additional + – signal at GP +23.9 ppm (s, unreacted Ph3PBn Br ).

Reaction of phosphorus pentachloride with LiBH4 in THF.

LiBH4 PCl PH 5 THF 3

31 Procedure as described for Method A in Section 10.13. P-NMR: one signal at GP 1 –244.0 ppm (1:3:3:1 q, JP-H = 187 Hz, PH3).

Reaction of triethylphosphate with LiBH4 in THF.

LiBH4 (EtO) P=O PH 3 THF 3

31 Procedure as described for Method A in Section 10.13. P-NMR: one signal at GP 1 –244.0 ppm (1:3:3:1 q, JP-H = 187 Hz, PH3).

Reaction of tetraphenylphosphonium bromide with NaBH4 in MeOH. _ + NaBH4 Ph P Br NO REACTION 4 MeOH

31 Procedure as described for Method A in Section 10.13. P-NMR: two signals at GP

+33.2 ppm (s, Ph3P=O) and GP +24.2 ppm (s, unreacted salt). Chapter 10: Experimental. 188

Reaction of tetraphenylphosphonium bromide with NaH in THF. _ + NaH Ph P Br Ph PH 4 THF 4 Procedure as described for Method A in Section 10.13; the mixture was sonicated be- 31 fore the spectrum was acquired. P-NMR: two signals at GP +24.2 ppm (s, unreacted salt) and GP –84.7 ppm (Ph4PH, signal too weak to obtain P-H splitting).

Reaction of tetraphenylphosphonium bromide with KH in THF.

_ + KH, 18-Crown-6 Ph P Br Ph PH 4 THF 3 2 + – (i) Modification of Method A in Section 10.13: Ph4P Br (0.3 gm, 0.7 mmol) was suspended in dry THF (3 mL), and KH (0.1 gm, 2.5 mmol) was added against a stream of dry argon. 31P-NMR: no signals detected. (ii) To the solution described in part (i) was added 18-crown-6 (0.2 gm, 0.8 mmol) 31 against a stream of dry argon. P-NMR: two signals at GP +24.0 ppm (s, unreacted salt) and

GP –86.1 ppm (Ph3PH2, signal too weak to determine P-H coupling).

Reaction of benzyltriphenylphosphonium bromide with Red-Al in toluene. _ + Red-Al Ph3PBn Br Ph3PH2 +Ph3P + Ph3P=CHPh PhCH3

31 Procedure as described for Method C in Section 10.13. P-NMR: three signals at GP 1 +8.5 ppm (s, Ph3P=CHPh), GP –4.6 ppm (s, Ph3P), and GP –68.8 ppm (1:1:1 t, JP-H = 669 Hz,

Ph3PH2).

Reaction of phosphorus pentachloride with Red-Al in toluene.

Red-Al PCl5 H2PCl3 ?? + PH3 PhCH3

31 Procedure as described for Method C in Section 10.13. P-NMR: two signals at GP 1 –122.6 ppm (1:2:1 t, JP-H = 191 Hz, unidentified, possibly H2PCl3) and GP –244.0 ppm 1 (1:3:3:1 q, JP-H = 187 Hz, PH3).

10.15 The reaction of phosphonium salts with miscellaneous nucleophiles.

This section deals with the attempted reaction of phosphonium salts with the first row – – – – nucleophiles CN , N3 , OD and F . A 0.5 M solution of deuteroxide ion was prepared from + a solution of deuterium oxide in dry THF, as described below. The phosphonium salts Ph4P – + – Br and Ph3PBn Br were found to be freely soluble in this solution. Chapter 10: Experimental. 189

For each of the remaining nucleophiles, obtained from the salts NaN3, KCN and KF, + – the solvent used was dry THF. In each case, the phosphonium salt used (either Ph4P Br or + – 31 Ph3PBn Br ) was found, using P-NMR, to be virtually insoluble, and no evidence for any reaction was obtained. In an attempt to increase the solubility of the inorganic salts, the reactions were repeated, adding the appropriate crown ether (15-crown-5 for Na+, 18-crown-6 for K+) to the mixture. Presumably, the solubility of the alkali salts had increased, but still no evidence of any reaction could be detected. The solubility of the phosphonium salts was observed (using 31P-NMR) to increase in the presence of either crown ether.

Preparation of 0.5 M lithium deuteroxide solution in D2O/THF.

D2O Li LiOD + D THF 2 A solution of ca 0.5 M lithium deuteroxide was prepared as follows: lithium metal (0.1 gm, 14.4 mmol) was hammered flat and cut into strips of the approximate dimensions 20mm u 5mm u 1mm. The pieces were then added against a stream of dry argon to a vig- orously stirred solution of D2O (15 mL) in dry THF (15 mL) at RT. After about an hour, the lithium had completely dissolved, and the slightly murky colourless solution was used immediately.

Reaction of tetraphenylphosphonium bromide with deuteroxide ion. _ _ + OD Ph4PBr Ph3P=O D2O/THF + – Ph4P Br (5.0 gm, 11.9 mmol) was added against a stream of dry argon to the stirred solution of 0.5 M lithium deuteroxide at RT. The salt quickly dissolved, giving a slightly yellowish clear solution. 31P-NMR showed that the only product containing phosphorus was

Ph3P=O at GP +30.1 ppm (s). The hydrocarbon by-product, presumed to be PhD, was not isolated.

Reaction of benzyltriphenylphosphonium bromide with deuteroxide ion. _ _ + OD Ph3PBn Br Ph3P=O + PhCD3 D2O/THF + – Ph3PBn Br (5.0 gm, 11.5 mmol) was added against a stream of dry argon to the + – stirred solution of 0.5 M lithium deuteroxide at RT. As before, in the case of Ph4P Br , the salt quickly dissolved. The resulting transparent solution had a pronounced yellow colour, 31 but P-NMR again showed that the only product was Ph3P=O at GP +30.1 ppm (s). Characterisation of the cleaved toluene. Procedure as for Method D, Section 10.13: 13C{1H}- NMR: methyl +19.87 ppm (1:3:6:7:6:3:1 septet: 1J = 20.71 Hz). Hence, the toluene GCD3 C-D recovered was exclusively PhCD3. Chapter 10: Experimental. 190

Reaction of tetraphenylphosphonium bromide with sodium azide. _ _ + N3 Ph PBr NO REACTION 4 THF (i) Without 15-crown-5: into a 10 mm NMR tube containing THF (3 mL) was added + – dry NaN3 (0.3 gm, 4.6 mmol) against a flow of dry argon. To this suspension dry Ph4P Br

(0.5 gm, 1.2 mmol) was added, followed by a 5 mm NMR tube containing D2O. The mixture, which contained undissolved solid, was then examined by 31P-NMR. Only one very weak signal was observed, appearing at GP +24.2 ppm (s, unreacted salt). (ii) With 15-crown-5: into the previously prepared solution described in part (i) above was added 15-crown-5 (0.2 gm, 0.9 mmol), which rapidly dissolved. Again, undissolved 31 solids remained in the mixture. Examination by P-NMR showed two signals at GP +24.8 + – ppm (s, unreacted salt) and GP +23.6 ppm (s, Ph4P Br complexed with 15-crown-5).

Reaction of benzyltriphenylphosphonium bromide with sodium azide. _ _ + N3 Ph PBn Br NO REACTION 3 THF (i) Without 15-crown-5: the same procedure as outlined in part (i) above for the + – + – reaction of Ph4P Br with NaN3 was undertaken, using instead Ph3PBn Br (0.2 gm, 0.5 31 mmol). Examination by P-NMR revealed only one very weak signal, which appeared at GP +23.8 ppm (s, unreacted salt). (ii) With 15-crown-5: as before, 15-crown-5 (0.2 gm, 0.8 mmol) was then added to the solution from part (i). 31P-NMR showed the same signal, which had increased significantly in intensity, at GP +23.6 ppm (s, unreacted salt).

Reaction of tetraphenylphosphonium bromide with potassium cyanide. _ _ + CN Ph PBr NO REACTION 4 THF + – (i) Without 18-crown-6: the procedure outlined above for the reaction of Ph4P Br with NaN3 was undertaken, using instead KCN (0.3 gm, 4.5 mmol). As before, examination 31 by P-NMR revealed only one very weak signal, which appeared at GP +24.8 ppm (s, unreacted salt). (ii) With 18-crown-6: to the mixture from part (i) was added 18-crown-6 (0.2 gm, 0.8 31 + – mmol). The P-NMR spectrum showed two signals at GP +24.7 ppm (s, Ph4P Br ) and GP +23.6 ppm (s, complex between salt and 18-crown-6). Chapter 10: Experimental. 191

Reaction of benzyltriphenylphosphonium bromide with potassium cyanide. _ _ + CN Ph PBn Br NO REACTION 3 THF + – (i) Without 18-crown-6: Ph3PBn Br (0.2 gm, 0.5 mmol) and KCN (0.2 gm, 3.1 mmol) were reacted, following the procedure described above. Examination by 31P-NMR revealed only one very weak signal, which appeared at GP +23.7 ppm (s, unreacted salt). (ii) With 18-crown-6: to the mixture from part (i) was added 18-crown-6 (0.2 gm, 0.8 mmol). The 31P-NMR spectrum showed the same signal, which had increased significantly in intensity, at GP +23.6 ppm (s, unreacted salt).

Reaction of tetraphenylphosphonium bromide with potassium fluoride. _ _ + F Ph PBr NO REACTION 4 THF + – (i) Without 18-crown-6: Ph4P Br (0.2 gm, 0.5 mmol) and KF (0.2 gm, 3.4 mmol) were reacted, following the procedure described above. Examination by 31P-NMR revealed only one very weak signal, which appeared at GP +24.7 ppm (s, unreacted salt). (ii) With 18-crown-6: to the mixture from part (i) was added 18-crown-6 (0.2 gm, 0.8 mmol). The 31P-NMR spectrum showed the same signal, which had increased significantly in intensity, at GP +24.6 ppm (s, unreacted salt).

Reaction of benzyltriphenylphosphonium bromide with potassium fluoride. _ _ + F Ph3PBn Br NO REACTION THF + – (i) Without 18-crown-6: Ph3PBn Br (0.2 gm, 0.5 mmol) and KF (0.2 gm, 3.4 mmol) were reacted, following the procedure described above. Examination by 31P-NMR revealed only one very weak signal, which appeared at GP +23.8 ppm (s, unreacted salt). (ii) With 18-crown-6: to the mixture from part (i) was added 18-crown-6 (0.2 gm, 0.8 mmol). The 31P-NMR spectrum showed the same signal, which had increased significantly in intensity, at GP +23.6 ppm (s, unreacted salt). 192

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