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Aspects of Reductive Methods in Organophosphorus Chemistry

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Aspects of Reductive Methods in Organophosphorus Chemistry 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 hy- dride 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 dem- onstrated 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]oc- tane 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 ter- tiary 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 li- thium 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 in- terest 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.
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