Synthesis and Characterization of Molecules and Π

SYNTHESIS AND CHARACTERIZATION OF MOLECULES AND π-

CONJUGATED MATERIALS CONTAINING LOW-COORDINATE PHOSPHORUS

XUFANG CHEN

Submitted in partial fulfillment of the requirements

For the degree of Doctor of Philosophy

Thesis Advisor: Dr. John D. Protasiewicz

Department of Chemistry

CASE WESTERN RESERVE UNIVERSITY

January, 2005

CASE WESTERN RESERVE UNIVERSITY

SCHOOL OF GRADUATE STUDIES

We hereby approve the dissertation of

______

candidate for the Ph.D. degree *.

(signed)______(chair of the committee)

______

(date) ______

*We also certify that written approval has been obtained for any proprietary material contained therein. ii

Table of Contents

Dedication ………………………………..………………………………………... i

Table of Contents ………………………………………………………………….. ii

List of Tables ……………………………………………………………………… v

List of Figures ……………………………………………………………………... vi

List of Schemes ……………………………………………………………………. x

Acknowledgements ………………………………………………………………... xiv

List of Abbreviations ……………………………………………………………… xv

Abstract ……………………………………………………………………………. xvii

Chapter 1. Introduction..………………………………………………………... 1

1.1 Compounds with Multiple Bonds of Heavier Main Group Elements 1

1.1.1 Short History……………………………………………………….. 1

1.1.2 Recent Developments………………………………………………. 6

1.2 Bonding in Heavier Main Group Elements………………………… 10

1.3 Carbon-Phosphorus Analogy……………………………………….. 13

1.4 Proposed Work……………………………………………………… 18

1.4.1 Reactivity study of phosphanylidene-σ4-phosphoranes…………….. 18

1.4.2 Conjugated oligomers and polymers containing

low-coordinate phosphorus centers…………………………………. 20

1.4.3 Synthesis of DmpPF2 and its reactivity study………………………. 26

1.5 References…………………………………………………………... 28

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4 Chapter 2. Cycloaddition of Phosphanylidene-σ -phosphoranes ArP=PMe3

and Quinones to Yield 1,3,2-dioxaphospholanes…………………… 37

2.1 Introduction…………………………………………………………. 37

2.2 Results and Discussion……………………………………………… 40

2.3 X-ray Structural Analysis…………………………………………… 44

2.4 NMR Spectroscopic Analyses……………………………………… 47

2.5 Possible Mechanisms……………………………………………….. 53

2.6 Conclusions…………………………………………………………. 55

2.7 Experimental Section……………………………………………….. 56

2.8 References………………………………………………………….. 60

Chapter 3. A Cyclic Diphosphinite by a Formal [4+4] Cycloaddition Reaction

of β-phosphaenone………………………………………………….. 62

3.1 Introduction…………………………………………………………. 62

3.2 Results and Discussion……………………………………………... 63

3.2.1 Synthesis of a Cyclic Diphosphinite………………………………... 63

3.2.2 X-ray Structural Analysis of the Cyclic Diphosphinite 6…………... 64

3.3 NMR Spectroscopic Studies………………………………………... 67

3.4 Possible Mechanism………………………………………………… 74

3.5 Conclusions…………………………………………………………. 75

3.6 Experimental Section……………………………………………….. 76

3.7 References…………………………………………………………... 78

Chapter 4. Synthesis of meta-Terphenyl Iodo and Dichlorophosphine

Derivatives Bearing Methoxy Groups……………………………… 80

4.1 Introduction…………………………………………………………. 80

4.2 Results and Discussion……………………………………………… 81

4.2.1 Synthesis and NMR Spectroscopic Analysis of m-Terphenyl

Iodides Bearing Methoxy Groups………………………………….. 81

4.2.2 X-ray Crystal Structures of m-Terphenyl Iodides Bearing

Methoxy Groups…………………………………………………… 84

4.2.3 Synthesis of m-terphenyl Dichlorophosphines Bearing

Methoxy Groups……………………………………………………. 86

4.3 Conclusions…………………………………………………………. 92

4.4 Experimental Section……………………………………………….. 93

4.5 References…………………………………………………………... 99

Chapter 5. Synthesis and Characterization of 2,6-Dimesitylphenyl

Difluorophosphine………………………………………………….. 101

5.1 Introduction…………………………………………………………. 101

5.2 Results and Discussion……………………………………………… 101

5.2.1 Synthesis and NMR Spectroscopic Studies of DmpPF2……………. 101

5.2.2 Reactivity Studies of DmpPF2………………………………………. 104

5.2.3 X-ray Crystal Structure of DmpPF2………………………………… 106

5.3 Conclusions…………………………………………………………. 108

5.4 Experimental Section……………………………………………….. 110

5.5 References…………………………………………………………... 113

Chapter 6. Synthesis and Characterization of Phosphaalkene Polymers……….. 115

6.1 Introduction…………………………………………………………. 115

6.2 Results and Discussion……………………………………………... 120

6.3 Synthesis of New Tetraarylphenyl Difunctional Ligands………….. 129

6.4 Conclusions………………………………………………………… 131

6.5 Experimental Section……………………………………………….. 133

6.6 References………………………………………………………….. 138

Chapter 7. Synthesis and Characterization of Phosphaalkenes and

Phosphaalkynes……………………………………………………... 143

7.1 Introduction…………………………………………………………. 143

7.1.1 General Introduction of Phosphaalkynes…………………………… 143

7.1.2 Synthesis of Phosphaalkynes……………………………………….. 145

7.1.3 Reactivity and Coordination Chemistry of Phosphaalkynes……….. 146

7.2 Results and Discussion……………………………………………… 149

7.2.1 Synthesis and Characterization of m-Terphenyl

Phosphaalkenes and Phosphaalkynes………………………………. 149

7.2.2 X-ray Crystal Structure Analysis of DmpC≡P……………………... 156

7.2.3 Reactivity of Dibromophosphaalkenes and Phosphaalkynes………. 157

7.3 Proposed Mechanism for the Formation of Phosphaalkynes………. 159

7.4 Synthesis of Br2C=PC6(p-t-BuPh)4P=CBr2………………………… 162

7.5 Conclusions………………………………………………………… 167

7.6 Experimental Section………………………………………………. 169

7.7 References………………………………………………………….. 174

Bibliography ………………………………………………………………………. 178

List of Tables

Table 1 Relative energies (kcal/mol) of σ and π bonds in group 14 and 15

homonuclear double bonds ……………………………………………. 12

Table 2 Selected bond lengths and angles for DmpPF2…………………………. 107

Table 3 Selected IR data of simple phosphaalkenes……………………………. 126

Table 4 Selected phosphaalkynes synthesized by elimination of

hexamethyldisiloxane and their physical properties ………………….. 145

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List of Figures

Figure 1 Schematic representation of the s-p energy separation and orbital

sizes for 2nd and 3nd row elements………………………………….. 12

Figure 2 Schematic diagram for band gaps in alkenes, phosphaalkenes……... 13

Figure 3 Highest occupied molecular orbitals of imine and phosphaethylene.. 14

Figure 4 Representations of singlet and triplet states of phosphinidenes

and carbenes………………………………………………………… 16

Figure 5 Schematic cartoon of the band gaps in conjugated

alkene and phosphaalkene systems………………………………….. 22

Figure 6 Structural diagram for 3,4,5,6-tetrachlorobenzo-2-(2,6-dimesityl-

phenyl)-1,3,2-dioxaphospholane, 3a……………………………….. 45

Figure 7 Packing diagram for 3,4,5,6-tetrachlorobenzo-2-(2,6-dimesityl-phenyl)

-1,3,2-dioxaphospholane illustrating π-stacking in the crystal……... 46

Figure 8 Structural diagram for 3,5-di-tert-buytlbenzo-2-(2,6-dimesityl-

phenyl)-1,3,2-dioxaphospholane, 4a……………………………….. 46

Figure 9 31P NMR spectrum of 3,4,5,6-tetrachlorobenzo-2-(2,6-dimesityl-

phenyl)-1,3,2-dioxaphospholane, 3a in CDCl3…………………….. 47

Figure 10 31P NMR spectrum of 3,4,5,6-tetrachlorobenzo-2-(2,4,6-tri-

tert-butylphenyl) -1,3,2-dioxaphospholane, 3b in CDCl3………….. 48

Figure 11 31P NMR spectrum of 3,5-di-tert-buytlbenzo-2-(2,6-dimesityl-

phenyl)-1,3,2-dioxaphospholane, 4a in CDCl3…………………….. 48

Figure 12 31P NMR spectrum of 3,5-di-tert-butylbenzo-2-(2,4,6-tri-

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tert-butylphenyl)-1,3,2-dioxaphospholane, 4b in CDCl3…………... 49

1 Figure 13 H NMR spectrum of 1,3,2-dioxaphospholane, 3a in CDCl3………. 50

1 Figure 14 H NMR spectrum of 1,3,2-dioxaphospholane, 4a in CDCl3………. 52

Figure 15 Low field (aromatic range) of 1H NMR spectrum of

1,3,2-dioxaphospholane, 4a in CDCl3……………………………… 52

Figure 16 High field (methyl range) of 1H NMR spectrum of

1,3,2-dioxaphospholane, 4a in CDCl3……………………………… 53

31 Figure 17 P NMR spectrum for the reaction mixture of DmpP=PMe3

with acenaphthoquinone in CHCl3…………………………………. 64

Figure 18 ORTEP drawing of one of the independent molecules of

cyclic diphosphinite 6………………………………………………. 65

Figure 19 Structure of the eight-membered ring in 6 and schematic structure

of the lowest energy configuration of 1,5-cyclooctadiene………….. 66

Figure 20 The π – π interactions between two molecules in the crystal unit cell 66

31 Figure 21 P NMR spectrum of cyclic diphosphinite 6 in CDCl3……………. 69

1 Figure 22 H NMR spectrum of cyclic diphosphinite 6 in CDCl3…………….. 69

1 1 Figure 23 H- H COSY spectrum of cyclic diphosphinite 6 in CD3COCD3…. 70

1 Figure 24 Variable temperature H NMR spectra for compound 6 in C6D5Br.. 73

Figure 25 1H NMR spectrum of 2,6-di(2,6-dimethoxyphenyl)iodobenzene

in CDCl3……………………………………………………………. 83

Figure 26 1H NMR spectrum of 2,6-di(2,4,6-trimethoxyphenyl)iodobenzene

in CDCl3……………………………………………………………. 83

Figure 27 ORTEP drawing of 2,6-di(2,6-dimethoxyphenyl)iodobenzene, 7….. 85

Figure 28 ORTEP drawing of 2,6-di(2,4,6-trimethoxyphenyl)iodobenzene, 8... 86

Figure 29 31P NMR spectrum of 2,6-di(2,6-dimethoxyphenyl)

phenyldichlorophosphine in CDCl3……………………………….. 88

Figure 30 2,6-di(2,6-dimethoxyphenyl)phenyldichlorophosphine……………. 91

31 1 Figure 31 P { H} NMR spectrum of DmpPF2 in CDCl3……………………. 103

1 Figure 32 H 600 MHz NMR spectrum of DmpPF2 in CDCl3………………... 103

1 Figure 33 H 300 MHz NMR spectrum of DmpPCl2 in CDCl3……………….. 104

Figure 34 X-ray crystal structure of DmpPF2…………………………………. 108

Figure 35 Two possible orientations for PF2 unit in DmpPF2……………….... 108

Figure 36 UV-vis Absorption spectra of soluble polymer 10d and its

oligomer A in CHCl3……………………………………………….. 124

Figure 37 Fluorescence spectra of soluble polymer 10d and its oligomer A

(CHCl3) relative to (E)-stilbene…………………………………….. 125

Figure 38 IR spectrum of green phosphaalkene polymer 10a…………………. 127

Figure 39 IR spectrum of brick red phosphaalkene polymer 10b……………... 127

Figure 40 IR spectrum of brown phosphaalkene polymer 10c...... 128

Figure 41 IR spectrum of orange phosphaalkene polymer 10d……………….. 128

31 Figure 42 P NMR spectrum of DmpP=CBr2 in CDCl3……………………… 151

1 Figure 43 H NMR spectrum of DmpP=CBr2 in CDCl3………………………. 151

31 Figure 44 P NMR spectrum of DxpP=CBr2 in CDCl3………………………. 152

1 Figure 45 H NMR spectrum of DxpP=CBr2 in CDCl3……………………….. 152

31 Figure 46 P NMR spectrum for the reaction of DmpP=CBr2 with zinc in THF 153

31 Figure 47 P NMR spectrum of DmpC≡P in CDCl3…………………………. 155

1 Figure 48 H NMR spectrum of DmpC≡P in CDCl3………………………….. 155

31 Figure 49 P NMR spectrum of DxpC≡P in CDCl3………………………….. 156

Figure 50 The X-ray crystal structure of DmpC≡P……………………………. 156

31 Figure 51 P NMR spectrum for the reaction of DmpP=CBr2 with

PdCl2(PhCN)2………………………………………………………. 157

31 Figure 52 P NMR spectrum of Br2C=PC6(p-t-BuPh)4P=CBr2 in CDCl3…… 164

1 Figure 53 H NMR spectrum of Br2C=PC6(p-t-BuPh)4P=CBr2 in CDCl3……. 165

List of Schemes

Scheme 1 Synthesis of compounds containing As=As and As=P double bonds 4

Scheme 2 Synthesis of the first stable disilene………………………………… 5

Scheme 3 Synthesis of the first plumbanediyl dimmer………………………… 7

Scheme 4 Homonuclear double-bonded compounds of Sb and Bi…………….. 7

Scheme 5 Homonuclear double-bonded compounds of group 15……………... 8

Scheme 6 UV-vis absorption maxima for stilbene and selected phosphorus

Analogues…………………………………………………………… 13

Scheme 7 The Wittig reaction…………………………………………………. 16

Scheme 8 The Wittig-Horner reaction………………………………………… 17

Scheme 9 The phospha-Wittig reaction .……………………………………… 17

Scheme 10 The phospha-Wittig reaction ……………………………………….. 17

Scheme 11 The phospha-Wittig reaction ……………………………………….. 18

Scheme 12 Synthesis of phosphaalkenes and proposed diphosphaalkenes

by phospha-Wittig reactions………………………………………… 19

Scheme 13 Low-coordinate phosphorus compounds and their carbon analgues... 21

Scheme 14 Proposed phosphaalkyne-transition metal hybrid polymer………… 24

Scheme 15 Proposed method for the synthesis of phosphaalkynes…………….. 25

Scheme 16 Proposed method for the synthesis of diphosphaalkynes…………… 25

Scheme 17 The generation of free phosphinidene from phosphirane and

its trapping reaction………………………………………………… 26

Scheme 18 The reaction of 2,6-dimesitylphenyldichlorophosphine with

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styrene and active magnesium……………………………………… 27

Scheme 19 Synthesis of m-terphenyl phosphirane with DmpPF2………………. 27

Scheme 20 Resonance structure of phospha-Wittig and Wittg reagents……….. 37

Scheme 21 Synthesis of stable phosphanylidene-σ4-phosphoranes…………….. 37

Scheme 22 The phospha-Wittig reaction……………………………………….. 38

Scheme 23 Synthesis of 1,3,2 dioxaphospholanes 3 and 4……………………... 40

4 Scheme 24 Reaction of phosphanylidene-σ -phosphoranes ArP=PMe3

with 1,2-naphthalenedione…………………………………………. 42

Scheme 25 Proposed mechanisms for the formation of 1,3,2-dioxaphospholanes 54

Scheme 26 Reactions of ortho-quinones with Wittig reagents to form

1,3-oxolanes………………………………………………………… 55

Scheme 27 Sythesis of some phosphaenones…………………………………… 63

Scheme 28 The formation of β-phosphaenones and their resonance structures... 68

Scheme 29 A possible mechanism for the formation of 6……………………… 74

Scheme 30 Synthesis of 2,6-di(2,6-dimethoxyphenyl)benzene.………………... 81

Scheme 31 Synthesis of 2,6-di(2,6-dimethoxyphenyl) iodobenzene, 7 and

2,6-di(2,4,6-trimethoxyphenyl) iodobenzene, 8……………………. 82

Scheme 32 Attemped synthesis of dichlorophosphine derivatives of 7 and 8….. 87

Scheme 33 Synthesis of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine 90

Scheme 34 The reduction reaction of 2,6-di(2-methoxyphenyl)

phenyl dichlorophosphine with potassium…………………………. 92

Scheme 35 Synthesis of 2,6-dimesitylphenyldifluorophosphine……………….. 102

Scheme 36 The reaction of 2,6-dimesitylphenyldihalophosphine with

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styrene and active magnesium……………………………………… 105

Scheme 37 The reactivity studies of DmpPF2………………………………….. 106

Scheme 38 Synthesis of poly(arylphosphine)s…………………………………. 118

Scheme 39 Synthesis of poly(methylenephosphine)……………………………. 119

Scheme 40 Synthesis of poly(p-phenylenephosphaalkene)…………………….. 119

Scheme 41 Phosphaalkene polymers based on phospha-Wittig reaction……….. 121

Scheme 42 Synthesis of 1,4-diiodo-2,3,5,6-tetraarylbenzenes…………………. 130

Scheme 43 Synthesis of tetramesityl benzene via the reaction of Grignard

reagent and hexabromobenzene…………………………………….. 131

Scheme 44 Protonation reaction of tetrakis(p-n-butylphenyl)-1,4-diiodobenzene 131

Scheme 45 Synthesis of phosphaalkynes by elimination of hexamethyldisiloxane 145

Scheme 46 η1 coordination complexes of phosphaalkynes……………………… 148

Scheme 47 Synthesis of DmpP=CBr2 and DxpP=CBr2………………………… 150

Scheme 48 Synthesis of DmpC≡P and DxpC≡P……………………………….. 154

Scheme 49 Synthesis of a phosphaalkyne via lithium carbenoid……………….. 159

Scheme 50 Synthesis of phosphaalkyne via transition metal complex………….. 160

Scheme 51 The mechanism for the reaction of transition metal complex

with RP=CX2……………………………………………………….. 160

Scheme 52 The Fritsch-Buttenberg-Wiechell rearrangement…………………… 161

Scheme 53 An example of Fritsch-Buttenberg-Wiechell rearrangement………. 161

Scheme 54 The possible mechanism for the formation of m-terphenyl

Phosphaalkynes…………………………………………………….. 162

Scheme 55 Synthesis of Br2C=PC6(p-t-BuPh)4P=CBr2………………………… 164

xiv

Scheme 56 The reaction of Br2C=PC6(p-t-BuPh)4P=CBr2 with magnesium

or n-BuLi…………………………………………………………… 166

Scheme 57 The transmetallation reaction of a lithium carbenoid………………. 166

Acknowledgements

Professor John D. Protasiewicz (Ph.D. Thesis Advisor)

For guidance, support, inspiration, and advice

Professor Fred L. Urbach, Professor Anthony J. Pearson, Professor Philip P. Garner and

Professor Christoph Weder for being on my Ph.D. committee and for their guidance

Professor Malcolm E. Kenney for use of his IR instrument

Professor Tong Ren and his student Weizhong Chen for solving X-ray crystal structures

Professor Thomas Gray for assistance

Professor Michael W. Justik for help in learning organic chemistry

Dr. Dale Ray for assistance with collection of NMR data and analysis

Labmates: Shashin Shah, Bindu, Meprathu, Rhett C. Smith, Ben Sherry, Glen Alliger,

Thirupathi Natesan, Liqing (Sam) Ma, Robert Woloszynek, Lisa Beth Gleason, Vittal

Baba Gudimetla for assistance, discussion and sharing the life in this group

Professor Lawrence M. Sayre, Professor John E. Stuehr, Patricia Eland, Zedeara Diaz,

John Hays, James Sill, Dee D'Angelo for assistance

National Science Foundation and Department of Chemistry at Case Western Reserve

University for funding

My family for support and encouragement

Many friends who ever helped me

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List of Abbreviations

Ar – Aryl n-Bu – n-Butyl t-Bu – t-Butyl

Calcd. – Calculated

DBU – 1,8-Diazabicyclo[5,4,0]undec-7-ene

DFT – Density Functional Theory

DME – Dimethyl Ether

DMF – N,N-dimethylformide

Dmp – 2,6-dimesitylphenyl

Dxp – 2,6-di(meta-xylyl)phenyl

EL – Electroluminescence

EPR –Electron Paramagnetic Resonance

Et – Ethyl

FAB – Fast Atom Bombardment

HOMO – Highest Occupied Molecular Orbital

HRMS – High Resolution Mass Spectrometry

IR – Infra Red

LED – Light Emitting Diode

LUMO – Lowest Unoccupied Molecular Orbital

Me – Methyl

Mes – Mesityl

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Mes* – 2,4,6-Tri-tert-butylphenyl (Supermesityl)

Naph – Napthalene

NMR – Nuclear Magnetic Resonance

Ph – phenyl

PPE – Poly(phenyleneethynylene)

PPV – Poly(phenylenevinylene)

R – Alkyl

RT – Room Temperature

THF – tetrahydrofuran

Trip – 2,4,6-tri-iso-propylphenyl

UV-vis – Ultraviolet-visible

VT – Variable Temperature

xviii

Synthesis and Characterization of Molecules and π-Conjugated Materials Containing

Low-Coordinate Phosphorus

Abstract

XUFANG CHEN

There are many similarities between phosphaalkenes and phosphaalkynes, low-

coordinate phosphorus compounds containing RP=CR2 and RC≡P functional groups, and

4 olefins (R2C=CR2), their unsaturated carbon analogues. Phosphanylidene-σ - phosphoranes have proven to be phospha-Wittig reagents that react with aldehydes to generate phosphaalkenes. Using the phospha-Wittig reaction, phosphaalkene oligomers and polymers were synthesized and characterized and shown to exhibit bathochromic shifts in comparison with related olefins. meta-Terphenyl phosphaalkynes have also been prepared. These materials were prepared for possible synthesis of transition metal complexes of phosphaalkynes. In addition, phosphanylidene-σ4-phosphoranes react with

ortho-quinones to form 1,3,2-dioxaphospholanes or β-phosphaenones depending on the

nature of the quinone. Furthermore, meta-terphenyls containing methoxy groups were investigated which have different properties from those having alkyl substitutents. 2,6-

Dimesitylphenyl difluorophosphine was synthesized and characterized and found to be relatively inert to reduction by active magnesium.

Chapter 1. Introduction

1.1 Compounds with multiple bonds of heavier main group elements

It is well-known that the second row elements carbon, nitrogen, and oxygen can

form multiple bonds with each other or with other elements, to form such materials as

alkenes, alkynes, carbonyl compounds, nitrile, azo compounds, azides. Multiple-bonded

compounds containing the heavier congeners of the second row elements have only been

developed over last three decades. The preparation and isolation of multiple-bonded

compounds of heavier main group elements were thwarted in the early stages of

investigation by the increased reactivity of these compounds.

1.1.1 Short history

Silicon was once believed to be analogous to carbon in its ability to form

compounds having stable E=E’ (E = Si, E’ = Si, C) bonds. But all early attempts failed to

generate compounds having Si=Si and Si=C double bonds; polymeric and cyclic

structures were obtained instead.1 For example, the reaction of diphenyldichlorosilane

2-3 with sodium was claimed to have produced Ph2Si=SiPh2; but it was subsequently

4 proved to be octaphenylcyclotetrasilane, Si4Ph8. Similar difficulties were encountered in the attempted preparations of compounds containing P=P or P=C double bonds. The first report on “phosphobenzene” Ph-P=P-Ph, for example, turned out to be wrong.5 Later

studies showed the prodcut to be a mixture of oligomers of (PhP)5 and (PhP)6 by X-ray structure analysis.6-10 This phenomenon was observed in related attempts to prepare

diarsenes (RAs=AsR).11-12

The first theoretical studies on the stabilities of multiple bonds between elements

of the second and subsequent rows were performed by Pitzer,13 who postulated that the

repulsive interactions of the 3px orbital electrons of one atom (e.g. phosphorus) with the

inner shell electrons of the other phosphorus atom in the diatomic molecules containing

P=P, P≡P constitute an important repulsive force (inner shell repulsion). In nitrogen, the

inner shell contains only two electrons and the inner shell repulsion is much smaller than

phosphorus which has ten electrons. This explained the weakness of P=P and P≡P bonds

compared to N=N, N≡N. Coulson published a monograph14 on the role of repulsion

energy in weakening multiple bonds. Mulliken presumed that the higher row atoms were

not loosening of multiple bonds but less tightening for multiple bonds than for single

bonds based on his calculation on overlap integrals of atomic orbitals for nsσ-nsσ, npσ- npσ, and npπ- npπ (n = 2, 3) bonds.15 Mulliken found that the repulsions postulated by

Pitzer are not larger for second row than that for first row atoms. Mulliken’s calculation

indicated that the overlap integrals, indicators of bond strength, were larger for the single

bonds between the third row elements compared to the second row. Later studies proved

that single P-P and Si-Si bonds are weaker than the C-C bonds.16

The relative absence of stable multiple bonds in compounds of the heavier main group elements led to the so-called “double-bond rule”, which states that “elements having a principal quantum number greater than 2 should not be able to form a pπ-pπ bond with themselves or with other elements”.17

Further experimental studies on multiple-bonded systems containing second and

higher rows disproved this theoretical prediction. Breakthroughs have been made for the

preparation of stable multiple bonds containing heavier main group elements since 1960s.

Unsaturated systems with multiple bonds formed by heavier main group elements

were first obtained for phosphorus. The first phosphaalkyne (HC≡P) was synthesized and experimentally identified by Gier in 1961 which is reactive colorless gas, stable to storage in the pure state only at temperature below its triple point of -124 ±2o.18 Märkl

reported the first stable phosphabenzene I in 1966.19 Bickelhaupt and coworkers reported the first kinetically stabilized phosphorus-carbon double bond compound II in 1978.20

Becker and coworkers reported the first stable phosphaalkyne III in 1981.21 In the same

year, Yoshifuji and coworkers reported the first stable phosphorus-phosphorus double

bond compound IV.22

t-Bu t-Bu

t-Bu P t-BuC P PP t-Bu

P t-Bu

t-Bu IIIIII IV

Diarsenes and compounds with phosphorus-arsenic double bonds were made by

two independent groups using a DBU-promoted condensation reaction 23-24 or t-BuLi

induced coupling reaction of the respective dichlorides (Scheme 1).25

CH(SiMe3)2 t-Bu t-Bu As As DBU t-Bu AsH2 + (Me3Si)2CHAsCl2 THF t-Bu t-Bu t-Bu

CH(SiMe3)2 t-Bu t-Bu PAs DBU t-Bu PH2 + (Me3Si)2CHAsCl2 THF t-Bu t-Bu t-Bu

C(SiMe3)3 t-BuLi (Me3Si)3C AsCl2 As As

(Me3Si)3C

C(SiMe3)3 t-BuLi (Me3Si)3CAsClPCl2 + (Me3Si)3C 2 PAs

(Me3Si)3C

Scheme 1. Synthesis of compounds containing As=As and As=P double bonds

Compounds having multiple bonds between heavier Group 14 elements

[(Me3Si)2CH]2E=E[CH(SiMe3)2]2 (E = Ge, Sn) were first prepared by Lappert and coworkers.26 These compounds are monomeric in the vapor phase, but dimeric and feature

double bonding between the E elements in the solid state.

The first stable disilene (Scheme 2) was discovered by West et al. via the

27 photochemical elimination of Me6Si2 from (Mes)2Si(SiMe3)2 in 1981. Subsequently,

Masamune published a related compound having 2,6-dimethylphenyl as substituents.28

SiMe hυ 254 nm Si 3 Si Si SiMe3 - 2 Me3SiSiMe3

Scheme 2. Synthesis of the first stable disilene

The first compounds having stable Si=C V,29 Ge=C VI,30 and Sn=C VII31 bonds were prepared using bulky substituents to stabilize low-coordinate heavier main group elements.

CMe3 Me3Si OSiMe3 (Me3Si)2HC B SiMe3 Si C Ge C Sn C C

Me3Si C10H15 (Me3Si)2HC B SiMe3 OEt2 CMe3

VVIVII

The first compounds featuring double bonds between phosphorus and Group 14

elements, P=Si VIII,32 P=Ge IX,33 and P=Sn X34 have been prepared using bulky

substituents to protect reactive sites.

(Me3Si)2HC

Sn P t-Bu Si P t-Bu Ge P t-Bu (Me3Si)2HC t-Bu t-Bu t-Bu t-Bu t-Bu t-Bu VIII IX X

Compounds having double bonds of the heavier main-group elements are often

unstable and favor formation of oligomers due to thermodynamic preference for two σ-

bonds rather than one σ- and one π-bond. The use of sterically demanding substituents is

a common way to thwart the oligomerization tendency of the double bond compounds.

The stabilization is both kinetic and thermodynamic since the steric interactions among

the bulky substituents are less in diphosphenes or disilenes than the cyclic products.

Many stable multiple-bonded compounds have been successfully synthesized using

encumbering substituents as protective groups.

1.1.2 Recent developments

After almost three decades study on the multiple bonds of heavier main group

elements and their compounds, great progress has been made on understanding multiple

bonding of heavier main group elements. Much of the work in this field has been

summarized in several review papers.35-37 Some recent important developments are

mentioned in this section.

The first plumbanediyl dimer was obtained from monomers by ligand exchange

38 (Scheme 3). Two years later the first stable Pb-Pb compound XI containing 2,6-C6H2-

39 2,4,6-i-Pr3 as a protective ligand was reported. This compound is considered as a

diplumbylene with a Pb-Pb single bond and a lone pair of electrons at each lead atom.

f f R SiR3 R SiR Sn Pb Sn + Pb 3 f + f R SiR3 R3Si R

SiR3 SiR3 f Rf f f R Sn Sn + R Pb Pb R R Si R3Si 3

F3C f R = CF3

F3C

Scheme 3. Synthesis of the first plumbanediyl dimer

Pb Pb

Tokitoh et al. reported stable distibenes and dibismuthenes by using the very

40 bulky –C6H2-2,4,6-{CH(SiMe3)2}3 ligand (Scheme 4). One year later, a series of stable

heavier element dipnictenes bearing meta-terphenyl ligands by the reduction of their

dihalides with potassium or magnesium were reported (Scheme 5).41

Tbt Se E 3(Me2N)3P Tbt Tbt E Se EE - 3(Me2N)3P=Se Se E Tbt Tbt

E = Sb, Bi

Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl

Scheme 4. Homonuclear double-bonded compounds of Sb and Bi

K or Mg Ar ArECl2 E E Ar

E = P, As, Sb, Bi

Ar = 2,6-di(mesityl)phenyl, 2,6-di(2,4,6-triisopropylphenyl)phenyl

Scheme 5. Homonuclear double-bonded compounds of group 15

Compounds having triple bonds of heavier main group elements have been

developed recently. The first stable disilyne XII has been prepared42 and it was supported

by NMR spectroscopy and DFT calculations.43 Digermyne44 and distannyne45 XIII were reported in the same year and they were structurally characterized. Triple bonds between

5 group 14 elements and transition metal atoms [ArGe≡M(CO)2(η -C5H5)] (M = Cr, Mo, W)

were also reported .46

(t-Bu3Si)2MeSi Si Si SiMe(Sit-Bu3)2 XII

Ar MM Ar XIII M = Ge, Sn

Ar = 2,6-di(2,6-diisopropylphenyl)phenyl

Compounds having multiple-bonded group 13 elements are scarce compared to

those having group 14 and 15 elements. One reason is that the absence of electrons to

form multiple bonds due to the electron deficiency of this group. The electrons in the

orbitals comprising the E-E bond become increasingly s in character as the group is

descended and less electron density is available to undergo p orbital overlap to form π

9 bonds.35 Addition of electrons to p orbitals of low-coordinate group 13 compounds results in strengthened bonds of E-E. The first compound containing a B-B double bond XIV has a B-B distance ~12% shorter than a single bond.47 A gallium compound proposed to contain a Ga≡Ga triple bond XV was reported in 1997,48 which spurred a debate about the bond order and even raised questions to challenge widely accepted bonding theories.35, 49

OEt2 Na Li

BB Ga Ga Li Na OEt2

XIV XV

In conclusion, multiple bonding involving the heavier group 13, 14 and 15 elements has been an important theme in organometallic chemistry. The chemical bonding and properties of multiple bonds between these heavier elements are quite different from their second row congeners. For heavier group 14 and 13 elements (Sn, Pb,

In, Tl), the bond distances between main group elements are not much shorter than a single bond and can dissociate to monomers in solution. Chemists are still arguing about the real bond order for Ga-Ga in Na2[{Ga(C6H3-Trip2-2,6)}2] which has bond length similar to some Ga-Ga single bonds.

All group 15 elements have stable molecules with formula RE=ER (E = P, As, Sb,

Bi) in which the E=E distance is shorter than that of a single bond (about 6 % for E = Bi).

Data suggest that dibismuthene does not dissociate into monomers in solution.35, 40b, 41

Silicon and some germanium compounds also have double bond character. On the other hand, although they have multiple bonds, the bond strength of the heavier elements is very weak in comparison with the second row congeners.

As commented by Grützmacher and Fässler on the heavier main group elements:

“The classical multiple bond indicators—bond lengths and bond strengths—have no meaning for multiple bonds in which elements from the higher period are involved.

However, they are valid for an exceptional element: carbon.”50

1.2 Bonding in heavier main group elements

The second row elements differ from higher rows in the Periodic Table in that each only has an s shell below the valence shell. On the other hand, in the heavier elements the core orbitals consist of fully occupied s and p shells; a nitrogen atom in its ground state has the electronic configuration: 1s22s22p3, while a phosphorus in its ground state has the configuration: 1s22s22p63s23p3. The core orbitals affect the valence s and p orbitals. The first row elements experience Pauli repulsion and the heavier elements experience this repulsion together with angular momentum part of the potential energy resulting in a different space extension of s and p valence orbitals.51 Since the s orbitals have no nodal properties at the nucleus, they can penetrate through inner shells more effectively to experience greater attraction of the nucleus. Heavier main group elements have a greater valence s orbital contraction as compared with that of the valence p orbital.

The s pair of electrons is thus relatively “inert” and reluctant to participate in the bonding.

The “inert pair effect” is more obvious for bottom elements than top ones in the Periodic

Table of elements. This gives us a better understanding about the properties of heavier main group elements by evaluating the differences in electronic structures of the elements.

Heavier main group elements have orbital non-hybridization due to valence s- orbital contraction. Two parameters are important for the formation of hybrid orbitals: the energy separation between the interacting orbitals and the extent of the s-p orbital overlap.

The separation energy (Esp) decreases down a row, which would increase the hybridization. On the other hand, 2s and 2p orbitals have similar sizes and they overlap very well. The p orbitals are much more diffuse than s orbitals for heavier elements, and thus s-p overlap decreases down a row (Figure 1). The contribution from the overlap term dominates for the formation of hybrid orbitals making s-p hybridization less effective for heavier elements.52

2p 3p ∆Esp ∆Esp 3s 2s

Figure 1. Schematic representation of the s-p energy separation and orbital sizes for 2nd and 3nd row elements

Multiple bonds of heavier main group elements are much weaker than their second row congeners. The measured strength of the P=C bond is about 58% of that for

C=C bond and similar results was obtained by calculation which found that P=C bond as 60-

70% of that for C=C bond.53 The π-bond energies for the third row elements is less than

50% of that of the second row, for instance, the π-bond energy for C=C and N=N is 62 kcal/mol and 94 kcal/mol, respectively, and that for Si=Si and P=P is 28 and 34 kcal/mol, respectively. π-Bond energies are bigger for group 15 elements than those of group 14

(Table 1).

Table 1. Relative energies (kcal/mol) of σ and π bonds in group 14 and 15 homonuclear double bonds 35

14 Group π/σ π/σ (%) 15 Group π/σ π/σ (%)

C-C 62/81 77 N-N 94/38 247

Si-Si 28/47 60 P-P 34/48 71

Ge-Ge 26/39 67 As-As 28/35 80

Sn-Sn 11/35 31 Sb-Sb 20/31 65

Pb-Pb NA/23(33) - Bi-Bi 10/21 48

Multiple-bonded compounds of heavier main group elements have lower HOMO and LUMO energy separations than carbon analogues. The HOMO-LUMO gaps for C=C and P=C systems are portrayed in Figure 2. The UV-vis absorption maxima of stilbene and its phosphaalkene and diphosphene analogues are given in Scheme 6. The absorption maxima are red shifted from alkene to phosphaalkene to diphosphene.54-55 Diphosphenes and phosphaalkenes are more easily reduced to stable anions than their all-carbon counterparts owing to relatively low-lying LUMOs in these materials.56-57

LUMO π∗

HOMO π

C=C P=C

Fiugre 2. Schematic diagram for band gaps in alkenes, phosphaalkenes

P P P

(nm) 317 334 372

Scheme 6. UV-vis absorption maxima for stilbene and selected phosphorus analogues

1.3 Carbon-phosphorus analogy

Low-coordinate phosphorus chemistry has witnessed much research since the late of 1970s. The unifying theme of these studies is the carbon-phosphorus analogy.58 Low- coordinate phosphorus chemistry is more similar to its carbon analogue rather than nitrogen analogue, which upholds the concept of diagonal relationship in the Periodic

Table of the elements.

The difference in electronegativity of carbon and phosphorus (C: 2.5 vs P: 2.2) is much smaller than that of carbon-silicon (C: 2.5 vs Si: 1.7) and nitrogen-phosphorus (N:

3.1 vs P: 2.2). The diagonal analogy of carbon-phosphorus is determined by electronegativity, which controls the ability of an atom to accept or release electrons. The big difference in radii (C, 0.77 Å and P, 1.10 Å), however, means there will be some differences.

H H H H CN CP H H

H H H -10.62 eV H CN CP -10.30 eV H H nN πC=P

H H H H CN -12.49 eV CP -10.70 eV H H nP πC=N

Figure 3. Highest occupied molecular orbitals of imine and phosphaethylene

π Bond energies provided by UV photoelectron spectroscopic measurements give other evidence for understanding the different reactivities in main group elements and their second row congeners. The best model is comparison of simple π-bonds in ethylene, imine and phosphaethylene.58 The HOMOs and HOMO (-1)s of imine and phosphaethylene are shown in Figure 3. The HOMO of imine corresponds to the lone pair, while that of phosphaethylene corresponds to π bond. The π bond lies much lower in energy (∆E = 1.87 eV) in imine and the lone pair in phosphaethylene is only 0.40 eV more stable. Consequently, the lone pair dominates the reactivity of imines with electrophiles. In phosphaethylene, both the π bond and the lone pair can be involved in reactions with electrophiles.

Although the σ electronegativity of phosphorus is somewhat lower than that of carbon (2.2 versus 2.5), UV absorption and magnetic circular dichroism studies of phosphabenzene showed even higher π electronegativity than that of carbon.59 P=C bonds have conjugative ability similar to C=C by calculations53 and the P=C double bond is relatively non-polar.60

As the data shown in Table 1, although the π bond of P=P (34 kcal/mol) is much weaker than that of C=C (62 kcal/mol), the π/σ ratio for carbon (77%) is much closer to phosphorus (71%) than silicon (60%). The π/σ ratio for nitrogen (247%) is considerably higher than that of phosphorus.

Two types of species of low-coordinate phosphorus and unsaturated carbon are shown below to further display the similarity.

1) Phosphinidenes vs carbenes

Phosphinidenes [RP], like carbenes [R2C], have either a singlet or triplet ground state (Figure 4). The triplet-singlet gap can be affected by substituents. Recent experimental and ab initio studies find a triplet-singlet separation gap of ~22 kcal/mol for free phosphinidenes, such as [PH], [MeP] and [PhP].61 The corresponding difference in energies for carbenes are relatively small, for example, the triplet-singlet separation gap

62 in [H2C] is 9 kcal/mol. Phosphinidenes prefer a triplet ground state, however, the singlet ground state can be induced by substituents having π-type lone pair electrons such as amino or phosphido groups or metal complex as indicated by some theoretical calculations.63 Such approaches have been successfully applied to carbenes as illustrated by some recent papers.64

Phosphinidenes [RP]

R P R P

lowest triplet state lowest singlet state

Carbenes [R1R2C]

R2 R2 C C R1 R1

lowest triplet state lowest singlet state

Figure 4. Representations of singlet and triplet states of phosphinidenes and carbenes

2) The Wittig reaction vs the phospha-Wittig reaction

The Wittig reaction is a well-known reaction for the synthesis of alkenes from aldehydes or ketones (Scheme 7).65

R1 C + R'RCPR''3 R2R1C CRR' + OPR"3 R 2 alkene

Scheme 7. The Wittig reaction

The Wittig-Horner reaction is another variant of the Wittig reaction. It uses a phosphonate ester instead of a phosphine (Scheme 8).66

O O base "RRHC P(OR)2 "RRC CR1R2 + HO P(OR)2 R1 OC R2

Scheme 8. The Wittig-Horner reaction

Similar reactions were also discovered that formed new P=C, rather than C=C bonds. Mathey and Marinetti published the first example of phospha-Wittig reactions that involve deprotonation of transition-metal phosphorylphosphine complexes to give phosphorylphosphide complexes that, in turn, react with aldehydes and ketones to afford phosphaalkene complexes (Scheme 9).67

R" O OEt R' O OC OEt RP P R' RP C OEt + O P (OC) M R" OEt 5 M(CO)5

M = W, Mo

Scheme 9. The phospha-Wittig reaction

Unstable phospha-Wittig ylides are stabilized by complexation of the ylide to

transition metal centers, such as pentacarbonyl tungsten.68 These materials react with aldehydes to produce transition metal stabilized phosphaalkenes (Scheme 10).

RP PBu3 RP CHR' + R'CHO Bu3P=O + W(CO)5 W(CO)5

Scheme 10. The phospha-Wittig reaction

4 Stable phospha-Wittig reagents, phosphanylidene-σ -phosphoranes (ArP=PMe3) can be prepared using bulky groups such as Dmp (2,6-dimesitylphenyl) or Mes* ( 2,4,6-tri-t- butylphenyl) to protect the low-coordinate phosphorus center.69-70 Phosphanylidene-σ4- phosphoranes react with aldehydes to produce phosphaalkenes (Scheme 11).70

O P Ar

RC + ArP PMe3 RC + OPMe3 H H

phosphaalkenes

Scheme 11. The phospha-Wittig reaction

1.4 Proposed work

1.4.1 Reactivity study of phosphanylidene-σ4-phosphoranes

Phosphanylidene-σ4-phosphoranes act as phospha-Wittig reagents to form phosphaalkenes upon reaction with aldehydes. No related reaction with ketones, however, has been successfully performed.

Ortho-diphosphaalkenes are now attracting attention as a new class of chelate ligands for transition metal catalysts.71 Some sterically protected diphosphinidenecyclobutenes XVI, XVII have been reported by several groups.72

Palladium complexes XVIII, XIX of these materials have been used as catalysts for various coupling reactions, ethylene polymerization and amination reactions.71d,73

Ar Ar H2 P R P C

(CH2)n P P R C Ar Ar H2 XVI XVII

R = H, SiMe3, Ph, Ph-CH2, t-Bu

Ar = 2,4,6-t-Bu3C6H2

Ar Ar H2 P R P C

Cl2Pd Cl2Pd (CH2)n P P R C Ar Ar H2

XVIII XIX

Seeking to extend this variant of the phospha-Wittig reaction69,74 to new systems, it was reasoned that reaction of phosphanylidene-σ4-phosphoranes with more reactive C=O bonds of o-quinones might be more favorable. In particular, reaction of o-quinones might lead to easy synthesis of ortho-diphosphaalkenes (Scheme 12)

H ArP=PMe3 Ar H O P - O=PMe R 3 R

Ar O P ArP=PMe3

- O=PMe3 ? O P Ar

Scheme 12. Synthesis of phosphaalkenes and proposed diphosphaalkenes by phospha-

Wittig reactions

1.4.2 Conjugated oligomers and polymers containing low-coordinate phosphorus centers

Conjugated polymers are interesting materials due to their potential applications in light emitting diodes (LEDs), electronic and optical devices, and sensors.75 PPVs, poly(phenylenevinylene)s, one class of conjugated polymers, have attracted significant attention76c since Friend’s report of organic polymeric LEDs.76 The conductivity of a material is indicated by its band gap. Band gap for metals is negligible, and band gaps range from about 0.1 eV77 to 3.0 eV for semiconductors (such as Si or Ge). Photon energies of 1.5 eV to 3.0 eV correspond to the visible range of the electromagnetic spectrum.77 PPV has a band gap of ~2.4 eV.78 Electroluminescence refers to the luminescence produced by some materials when exposed to an electric field, which is mainly observed in semiconductors. The electric field excites electrons in the material which then emit the excess energy as photons (light). LEDs are the most well known example of electroluminescence.

Incorporation of low-coordinate heavier main group elements into conjugated materials would be a very interesting project which may lead to new materials with different properties as compared to their PPVs or give better understanding of properties of PPVs.

PPP

alkenes phosphaalkenes diphosphenes

CC CP

alkynes phosphaalkynes

Scheme 13. Low-coordinate phosphorus compounds and their carbon analogues

Low-coordinate phosphorus compounds and their carbon analogues are shown in

Scheme 13. The specific aims of my research are to prepare oligomers and polymers containing low-coordinate phosphorus in conjugated π-systems, such as phospha-PPVs or phospha-PPEs [PPE, poly(phenyleneethynylenes)].

PPV n

P Phospha-PPV n

P Phospha-PPE n

Compounds containing P=C units have conjugation ability similar to that of alkenes.53 However, P=C double bonds are weaker than C=C bonds and require bulky substituents to stabilize low-coordinate phosphorus centers. Many results have shown that molecules containing P=C and P=P bonds are easily reduced.57,58 Polymers containing P=C units along the main chain might have smaller HOMO/LUMO gaps than all-carbon materials as shown in Figure 5.

π* π*

∆E ∆E

π π

C=Cn = 1 2 4 P=C n = 124

Figure 5. Schematic cartoon of the band gaps in conjugated alkene and phosphaalkene systems

P P P P P P

Chart 1. A diphosphaalkene and a bisdiphosphene containing tetraarylphenyl linker

Tetraarylphenyls have been shown to be effective bridging and protective groups for the formation of stable diphosphaalkenes and bisdiphosphenes (Chart 1).79 New phosphaalkene polymers (Chart 2) could be synthesized using this linker as a difunctional building block. Such polymers are part of the proposed studies.

( P P Linker n

Chart 2. Proposed phosphaalkene polymers

Another important class of organic polymers is poly(phenyleneethynylenes)

(PPEs). PPEs have showed many potential applications in sensors,75d,80 display technologies81 and nonlinear optics.82 Recent developments in PPEs were given in a review paper.83

Phospha-PPEs might have advantages over PPV-based systems due to the conjugation ability with an aromatic ring without considering particular orientations of

P≡C bonds with respect to the ring. The large bulky substituents used in phospha-PPV can hinder adoption of orientations that maximize π-conjugation between aromatic rings and P=C (or P=P) groups.84 A system containing P≡C bond, however, is able to undergo conjugation with an aromatic ring without regard to any particular orientation of that ring with respect to P≡C π system.

A phosphaalkyne might use the phosphorus lone pair of electrons to form a σ bond with carbon. Thus, phospha-PPEs are cationic polymers. Phosphaalkynes can also use the lone pair of electrons on phosphorus to coordinate with transition metals. If diphosphaalkynes could be made, they can add to transition metal complexes with two coordination sites on trans positions to form linear inorganic/organic hybrid polymers.

These polymers have low-coordinate phosphorus and transition metal centers along the main chain which may offer different properties and more redox centers (Scheme 14).

RPR R

P n n PPEs Phospha-PPEs

L L

PMCPC n L L

Scheme 14. Proposed phosphaalkyne-transition metal hybrid polymer

In order to study the properties of phosphaalkyne polymers, monophosphaalkynes can be prepared to construct oligomers.

A general and optimized approach for the synthesis of phosphaalkynes involves the elimination of hexamethyldisiloxane from suitably substituted phosphaalkenes. Most of the reported phosphaalkynes can be synthesized using this method. Examples of aryl phosphaalkynes are quite limited. The single most studied material is Mes*C≡P. Besides the above discussed general approach, it can also be synthesized from its lithium carbenoid or by the reaction of dihalophosphaalkene with transition metal complexes

M(PL3)4 (M = Pd, Pt, L = Ph, Et). More details for these studies are given in Chapter 7.

Bromide has been proven to be a better leaving group and trans-ArP=C(X)Li undergoes easier elimination of LiBr than does the chloro-analogue.85 New

25 phosphaalkynes containing m-terphenyl substituents will be synthesized by reduction of the corresponding dibromophosphaalkene counterparts by metals (Scheme 15).

Diphosphaalkynes can be synthesized following the same route using tetrakis(t- butylphenyl)phenyl ligand as a linker (Scheme 16).

ArP CBr2 + M ArC P + MBr2

Ar =

Scheme 15. Proposed method for the synthesis of phosphaalkynes

2M Br2C PCBrP 2 P CPC

Scheme 16. Proposed method for the synthesis of diphosphaalkynes

1.4.3 Synthesis of DmpPF2 and its reactivity study

The chemistry of phosphinidenes is still scarce compared to carbenes and nitrenes.

One reason is that there are few precursors for the generation of free phosphinidenes. A turning-point in phosphinidene chemistry is Gaspar and coworkers’ report on the EPR spectrum of [MesP] from irradiation of phosphirane, and trapping reactions of phosphinidene with acetylenes and olefins to form three-membered ring structures phosphirenes and phosphiranes, respectively (Scheme 17).86

Mes P

Mes Et Et Et Et P Mes P

[MesP] +

C2H4 Mes Mes P +

Scheme 17. The generation of free phosphinidene from phosphirane and its trapping reaction

m-Terphenyl phosphiranes might be used as precursors for the investigation of free phosphinidenes by photolysis. During study of the reaction of DmpPCl2 with styrene and active magnesium, phosphirane (δ = -190.6 ppm) and diphosphene (δ = 493.7 ppm) were observed by 31P NMR with a ratio of 4:1 in THF solution (Scheme 18). However, it is difficult to isolate phosphirane from the mixture. DmpPF2 is a proposed precursor in the exploration of an alternative route for the synthesis of phosphiranes. Because MgF2

has a higher lattice energy (-704 kcal/mol) than MgCl2 (-601 kcal/mol), it might be easier for ArP(X)MgX to eliminate MgF2 than MgCl2 to form higher yields of phosphirane

(Scheme 19).

P PCl2 + + Mg* P + -MgCl2 P

Scheme 18. The reaction of 2,6-dimesitylphenyldichlorophosphine with styrene and active magnesium

PF2 ++Mg* P -MgF2

Scheme 19. Synthesis of m-terphenyl phosphirane with DmpPF2

1.5 References

1. Gusel’nikov, L. E.; Nametkin, N. S. Chem. Rev. 1979, 79, 529.

2. Kipping, F. S.; Sands, J. E. J. Chem. Soc. (Abstr.) 1921, 119, 830.

3. Kipping, F. S.; Sands, J. E. J. Chem. Soc. (Abstr.) 1921, 119, 848.

4. Kipping, F. S. J. Chem. Soc. (Abstr.) 1927, 2719.

5. Kohler, H.; Michaelis, A. Ber. Dtsch. Chem. Ges. 1877, 10, 807.

6. Kuchen, W.; Buchwald, H. Chem. Ber. 1958, 91, 2296.

7. Daly, J. J.; Maier, L. Nature 1964, 203, 1167.

8. Daly, J. J. J. Chem. Soc., Suppl. 1964, 6147.

9. Daly, J. J. J. Chem. Soc., Abs. 1965, 4789.

10. Daly, J. J.; Maier, L. Nature 1965, 208, 383.

11. (a) Ehrlich, P.; Bertheim, A. Ber. Dtsch. Chem. Ges. 1911, 44, 1265. (b) Ehrlich,

P.; Bertheim, A. Ber. Dtsch. Chem. Ges. 1912, 45, 761.

12. (a) Hedberg, K.; Hughes, E. W.; Waser, J. Acta Cryst. 1961, 14, 369. (b)

Rheingold, A. C.; Sullivan, P. J. Organometallics 1983, 2, 327.

13. Pitzer, K. S. J. Am. Chem. Soc. 1948, 70, 2140.

14. Coulson, C. A. “Valence”, Oxford University Press, London, 1961.

15. Mulliken, R. S. J. Am. Chem. Soc. 1950, 72, 4493.

16. Schmidt, M. W.; Truong, P. N.; Gordon, M. S. J. Am. Chem. Soc. 1987, 109, 5217.

17. Jutzi, P. Angew Chem. Int. Ed. Engl. 1975, 14, 232.

18. Gier, T. E. J. Am. Chem. Soc. 1961, 83, 1769.

19. (a) Märkl, G. Angew Chem. Int. Ed. Engl. 1966, 5, 846. (b) Ashe, A. J. J. Am.

Chem. Soc. 1971, 93, 3293.

20. Klebach, T. C.; Lourens, R.; Bickelhaupt, F. J. Am. Chem. Soc. 1978, 100, 4886.

21. Becker, G.; Gresser, G.; Uhl, W. Z. Naturforsch., B 1981, 36, 16.

22. Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am. Chem. Soc.

1981, 103, 4587.

23. (a) Cowley, A. H.; Lasch, J. G.; Norman, N. C.; Pakulski, M. J. Am Chem. Soc

1983, 103, 5506. (b) Cowley, A. H.; Norman, N. C.; Pakulski, M. J. Chem. Soc.,

Dalton Trans. 1985, 383.

24. Cowley, A. H.; Lasch, J. G.; Norman, N. C.; Pakulski, M.; Whittlesey, B. R. J.

Chem. Soc., Chem. Commun. 1983, 881.

25. Couret, C.; Escudie, J.; Madaule, Y.; Ranaivonjatovo, H.; Wolf, J.-G.

Tetrahedron Lett. 1983, 24, 2769.

26. (a) Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas, K. M. J. Chem. Soc.,

Chem. Commun. 1976, 261. (b) Davidson, P. J.; Harris, D. H.; Lappert, M. F. J.

Chem Soc., Dalton Trans. 1976, 2268. (c) Fjeldberg, T.; Lappert, M. F.; Schilling,

B. E. R.; Seip, R.; Thorne, A. J. Chem. Soc., Chem. Commun. 1982, 1407. (d)

Hitchcock, P. B.; Lappert, M. F.; Miles, S. J.; Thorne, A. J. J. Chem. Soc., Chem.

Commun. 1984, 480.

27. West, R.; Fink, M. J.; Michl, J. Science (Washington, D. C.) 1981, 214, 1343.

28. Masamune, S.; Hanzawa, Y.; Murakami, S.; Bally, T.; Blount, J. F. J. Am. Chem.

Soc. 1982, 104, 1150.

29. (a) Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M.

R. J. Chem. Soc., Chem. Commun. 1981, 191. (b) Brook, A. G.; Nyburg, S. C.;

Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M. R.; Poon, Y. C.;

Chang, Y. M.; Wong-Ng, W. J. Am. Chem. Soc. 1982, 104, 5667.

30. Couret, C.; Escudie, J.; Satge, J.; Lazraq, M. J. Am. Chem. Soc. 1987, 109, 4411.

31. Meyer, H.; Baum, G.; Massa, W.; Berger, S.; Berndt, A. Angew Chem. Int. Ed.

Engl. 1987, 26, 546.

32. Smit, C. N.; Lock, F. M; Bickelhaupt, F. Tetrahedron Lett. 1984, 25, 3011.

33. Escudie, J.; Couret, C.; Satge, J.; Andrianarison, M.; Andriamizaka, J.-D. J. Am.

Chem. Soc. 1985, 107, 3378.

34. Couret, C.; Escudie, J.; Satge, J. J. Am. Chem. Soc. 1985, 107, 8280.

35. Power, P. P. J. Chem. Soc., Dalton Trans. 1998, 2939.

36. Power, P. P. Chem. Rev. 1999, 99, 3463.

37. Clyburne, J. A. C.; McMullen, N. Coord. Chem. Rev. 2000, 210, 73.

38. Klinkhammer, K. W.; Fässler, T. F.; Grützmacher, H. Angew. Chem. Int. Ed. 1998,

37, 124.

39. Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524.

40. (a) Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase, S.; Uekusa, H.;

Ohashi, Y. J. Am. Chem. Soc. 1998, 120, 433. (b) Tokitoh, N.; Arai, Y.; Okazaki,

R.; Nagase, S. Science 1997, 277, 78.

41. Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.

1999, 121, 3357.

42. Wiberg, N.; Niedermayer, W.; Fischer, G.; Nöth, H.; Suter, M. Eur. J. Inorg.

Chem. 2002, 1066.

43. Takagi, N.; Nagase, S. Eur. J. Inorg. Chem. 2002, 2775.

44. Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem. Int. Ed.

2002, 41, 1785.

45. Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.

2002, 124, 5930.

46. Pu, L.; Twamley, B.; Haubrich, S. T.; Olmstead, M. M.; Mork, B. V.; Simons, R.

S.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 650.

47. Moezzi, A.; Olmstead, M. M; Power, P. P. J. Am. Chem. Soc. 1992, 114, 2715.

48. Su, J.; Li, X.-W.; Crittendon, R. C.; Robinson, G. H. J. Am. Chem. Soc. 1997, 119,

5471.

49. Robinson, G. H. Chem. Comunn. 2000, 2175.

50. Grützmacher, H.; Fässler, T. F. Chem. Eur. J. 2000, 6, 2317.

51. Multiple bonds and Low Coordination in Phosphorus Chemistry; Regitz, M.,

Scherer, O. J., Eds.; Thieme Verlag: Stuttgart, 1990.

52. Bouhadir, G.; Bourissou, D. Chem. Soc. Rev. 2004, 33, 210.

53. Nyulaszi, L.; Veszpremi, T.; Reffy, J. J. Phys. Chem. 1993, 97, 4011.

54. Shah, S.; Protasiewicz, J. D. Chem. Commun. 1998, 1585.

55. Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem. 2000,

39, 3860.

56. (a) Jouaiti, A.; Geoffroy, M.; Terron, G.; Bernardinelli, G. J. Chem. Soc., Chem.

Commun. 1992, 155. (b) Jouaiti, A.; Geoffroy, M.; Terron, G.; Bernardinelli, G. J.

Am. Chem. Soc. 1995, 117, 2251. (c) Al Badri, A.; Jouaiti, A.; Geoffroy, M.

Magn. Reson. Chem. 1999, 37, 735. (d) Geoffroy, M.; Jouaiti, A.; Terron, G.;

Cattani-Lorente, M.; Ellinger, Y. J. Phys. Chem. 1992, 96, 8241.

57. (a) Cetinkaya, B.; Hudson, A.; Lappert, M. F.; Goldwhite, H. J. Chem. Soc.,

Chem. Commun. 1982, 609. (b) Bard, A. J.; Cowley, A. H.; Kilduff, J. E.; Leland,

J. K.; Norman, N. C.; Palkuvski, M. J. Chem. Soc., Dalton Trans. 1987, 249. (c)

Culcasi, M.; Grouchi, G.; Escudie, J.; Couret, C.; Pujol, L.; Tordo, P. J. Am.

Chem. Soc. 1986, 108, 3130.

58. Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy From

Organphosphorus to Phospha-organic Chemistry; Wiley, New York, 1998.

59. Waluk, J.; Klein, H.-P.; Ashe III, A. J.; Michl, J. Organometallics, 1989, 8, 2804.

60. Schoeller, W. W. J. Chem. Soc., Chem. Commun. 1985, 334.

61. (a) Zittel, P. F.; Lineberger, W. C. J. Chem. Phys. 1976, 65, 1236. (b) Kim, S.-J.;

Hamilton, T. P.; Schaefer, H. F. III J. Phys. Chem. 1993, 97, 1872. (c) Nguyen, M.

T.; Creve, S.; Vanquickenborne, L. G. J. Chem. Phys. 1996, 105, 1922. (d)

Hamilton, T. P.; Willis, A. G.; Williams, S. D. Chem. Phys. Lett. 1995, 246, 59. (e)

Nguyen, M. T.; Van Keer, A.; Eriksson, L. A.; Vanquickenborne, L. G. Chem.

Phys. Lett. 1996, 254, 307.

62. (a) Leopold, D. G.; Murray, K. K.; Miller, A. E. S.; Lineberger, W. C. J. Chem.

Phys. 1985, 83, 4849. (b) Jensen, P.; Bunker, P. R. J. Chem. Phys. 1988, 89, 1327.

63. (a) Nguyen, M. T.; Van Keer, A.; Vanquickenborne, L. G. J. Org. Chem. 1996, 61,

7077. (b) Ehlers, A. W.; Lammertsma, K.; Baerends, E. J. Organometallics 1998,

17, 2738. (c) Creve, S.; Pierloot, K.; Nguyen, M. T. Chem. Phys. Lett. 1998, 285,

429. (d) Creve, S.; Pierloot, K.; Nguyen, M. T.; Vanquickenborne, L. G. Eur. J.

Inorg. Chem. 1999, 107.

64. (a) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913. (b) Buron, C.; Guerret, O.;

Gabbaï, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39. (c) Solé, S.; Gornitzka, H.;

Schoeller, W. W.; Boruissou, D.; Bertrand, G. Science 2001, 292, 1901. (d) Kato,

T.; Gornitzka, H.; Baceiredo, A.; Savin, A.; Bertrand, G. J. Am. Chem. Soc. 2000,

122, 998. (e) Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science,

2000, 288, 834.

65. Selected references: (a) Wittig, G.; Schollkopf, U. Chem. Ber. 1954, 87, 1318. (b)

Johnson, A. W. Ylide Chemistry; Academic Press: New York, 1966. (c) Gosney,

I.; Rowley, A. G. In Organophosphorus Reagents in Organic Synthesis; Cadogan,

J. I. G., Ed.; Academic Press: New York, 1979; pp 17-153. (d) Walker, B. J. In

Organophosphorus Reagents in Organic Synthesis; Cadogan, J. I. G., Ed.;

Academic Press: New York, 1979; pp 155-205. (e) Pommer, H.; Thieme, P. C.

Top. Curr. Chem. 1983, 109, 165. (f) Maryanoff, B. E.; Reitz, A. B. Chem. Rev.

1989, 89, 863.

66. (a) Horner, L.; Hoffmann, H.; Wippel, H. G. Chem. Ber. 1958, 91, 61. (b) Horner,

L.; Hoffmann, H.; Wippel, H. G.; Klahre, G. Chem. Ber. 1959, 92, 2499. (c)

Wadsworth, W. S.; Emmons, W. D. J. Am. Chem. Soc. 1961, 83, 1733. (d)

Boutagy, J.; Thomas, R. Chem. Rev. 1974, 74, 87.

67. Marinetti, A.; Mathey, F. Angew Chem. Int. Ed. Engl. 1988, 27, 1382.

68. (a) Le Floch, P.; Marinetti, A.; Ricard, L.; Mathey, F. J. Am. Chem. Soc. 1990,

112, 2407. (b) Le Floch, P.; Mathey, F. Synlett. 1990, 171.

69. Shah, S.; Yap, G. P. A.; Protasiewicz, J. D. J. Organomet. Chem. 2000, 608, 12.

70. Shah, S.; Protasiewicz, J. D. Chem. Commun. 1998, 1585.

71. Some recent reports: (a) Ionkin, A. S.; Marshall, W. Chem. Commun. 2003, 710.

(b) Ionkin, A. S.; Marshall, W. Heteroat. Chem. 2002, 13, 662. (c) Minami, T.;

Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.; Yoshifuji, M. Angew. Chem. Int.

Ed. 2001, 40, 4501. (d) Ikeda, S.; Ohhata, F.; Miyoshi, M.; Tanaka, R.; Minami,

T.; Ozawa, F.; Yoshifuji, M. Angew. Chem. Int. Ed. 2000, 39, 4512. (e) Yoshifuji,

M. Bull. Chem. Soc. Jpn. 1997, 70, 2881.

72. (a) Appel, R.; Winkhaus, V.; Knoch, F. Chem. Ber. 1987, 120, 243. (b) Märkl, G.;

Kreitmeier, P.; Nöth, H.; Polborn, K. Angew. Chem. Int. Ed. Engl. 1990, 29, 927.

Hirotsu, K.; Nagase, S. Chem. Lett. 1990, 2195. (d) Toyota, K.; Tashiro, K.;

Yoshifuji, M.; Nagase, S. Bull. Chem. Soc. Jpn. 1992, 65, 2297. (e) Toyota, K.;

Tashiro, K.; Yoshifuji, M. Angew. Chem. Int. Ed. Engl. 1993, 32, 1163.

73. (a) Toyota, K.; Masaki, K.; Abe, T.; Yoshifuji, M. Chem. Lett. 1995, 221. (b)

Yamada, N.; Abe, K.; Toyota, K.; Yoshifuji, M. Organic Lett. 2002, 4, 569. (c)

Daugulis, O.; Brookhart, M.; White, P. S. Organometallics 2002, 21, 5935. (d)

Gajare, A. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F. Chem. Commun. 2004, 1994.

74. Shah, S.; Protasiewicz, J. D. Coord. Chem. Rev. 2000, 210, 181.

75. (a) Brédas, J. L.; Silbey, R. (Eds.) Conjugated Polymers: The Novel Science and

Technology of Highly Conducting and Non-linear Optically Active Materials,

Kluwer Academic, Dordecht, 1991. (b) Skotheim, T. A., Ed.; Handbook of

Conducting Polymers, Marcel Dekker, New York, 1986. (c) Kraft, A.; Grimsdale,

A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998, 37, 402. (d) Swager, T. M. Acc.

Chem. Res. 1998, 31, 201.

76. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.;

Friend, R. H.; Burn, P. L.; Holmes A. B. Nature 1990, 347, 539.

77. The electronic structure and chemistry of solids; Cox, P.A., Ed.; New York:

Oxford University Press, 1987.

78. Gaudin, O.; Jackman, R. B.; Nguyen, T.-P.; Le Rendu, P. J. Appl. Phys. 2001, 90,

4196.

79. Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem. 2000,

39, 3860.

80. (a) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (b) Lavigne, J.

J.; Broughton, D. L.; Wilson, J. N.; Erdogan, B.; Bunz, U. H. F. Macromolecules

2003, 36, 7409. (c) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593.

81. (a) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P. Science 1998,

279, 835. (b) Montali, A.; Bastiaansen, C.; Smith, P.; Weder, C. Nature 1998, 392,

261. (c) Sato, T.; Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1999, 121, 10658. (d)

Hirohata, M.; Tada, K.; Kawai, T.; Onoda, M.; Yoshino, K. Synth. Met. 1997, 85,

1273.

82. (a) Moroni, M.; Le Moigne, J.; Pham, T. A.; Bigot, J.-Y. Macromolecules 1997,

30, 1964. (b) Kondo, K.; Okuda, M.; Fujitani, T. Macromolecules 1993, 26, 7382.

83. Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.

84. Dutan, C.; Shah, S.; Smith, R. C.; Choua, S.; Berclaz, T.; Geoffroy, M.;

Protasiewicz, J. D. Inorg. Chem. 2003, 42, 6241.

85. Ito S.; Toyato, K.; Yoshifuji, M. Chem. Commun. 1997, 1637.

86. (a) Li, X.; Weissman, S. I.; Lin, T. S.; Gaspar, P. P.; Cowley, A. H.; Smirnov, A. I.

J. Am. Chem. Soc. 1994, 116, 7899. (b) Li, X.; Lei, D.; Chiang, M. Y.; Gaspar, P.

P. J. Am. Chem. Soc. 1992, 114, 8526.

4 Chapter 2. Cycloaddition of Phosphanylidene-σ -phosphoranes ArP=PMe3 and

Quinones to Yield 1,3,2-Dioxaphospholanes

2.1 Introduction

Neutral phosphanylidene-σ4-phosphoranes, adducts of phosphinidenes and

phosphines, resemble Wittig reagents in their resonance structures (Scheme 20).

ArP PMe3 ArP PMe3 R'RC PR'3 R'RC PR'3

4 phosphanylidene-σ -phosphorane alkylidenephosphorane

Scheme 20. Resonance structure of phospha-Wittig and Wittig reagents

4 Stable phosphanylidene-σ -phosphoranes (ArP=PMe3) can be isolated in good

yields when bulky groups, such as Dmp (2,6-dimesitylphenyl) or Mes* (2,4,6-tri-t-

butylphenyl), protect the low-coordinate phosphorus center. Phosphanylidene-σ4- phosphoranes were prepared by the reduction of dichlorophosphines with zinc dust in the presence of excess of trimethylphosphine (Scheme 21).1,2

Zn ArPCl2 + PMe3 ArP=PMe3 + ZnCl2 toluene

Ar =

Dmp Mes*

95% 88%

Scheme 21. Synthesis of stable phosphanylidene-σ4-phosphoranes

As mentioned earlier, phosphanylidene-σ4-phosphoranes react with aldehydes to produce phosphaalkenes, thereby acting as phospha-Wittig reagents (Scheme 22).1

O P Ar RC ArP PMe RC + 3 + OPMe3 H H

phosphaalkenes

Scheme 22. The phospha-Wittig reaction

Using bulky substituents, oligomers and polymers containing P=C double bonds

have been synthesized via the phospha-Wittig reaction (Chart 3, 4).3 Phosphanylidene-σ4- phosphoranes can form diphosphenes or inserted products by photolysis, possibly via phosphinidene intermediates.4

During this study, reagents 1 and 2 were found to be unreactive towards ketones

at ambient conditions. It seems reasonable to examine possible phospha-Wittig reactions

with ortho-quinones, materials that are more active carbonyl compounds. This chemistry may allow easy access to 1,2-diphosphaalkenes that might be good ligands for transition

metals (See Chapter 1, 4.1).5

O P P P P O

P P P

C D

Chart 3. Oligomers synthesized by the phospha-Wittig reaction

( P P Linker ) n

O Linker ) P n

P ( O

Chart 4. Polymers synthesized using the phospha-Wittig reaction

2.2 Results and discussion

Cl Cl OCl O Cl ArP=PMe3 Ar P - PMe O Cl 3 O Cl Cl Cl

3a, 3b

O ArP=PMe3 O Ar P - PMe3 O O

4a, 4b

Ar = 2,6-dimesitylphenyl, 3a, 4a; 2,4,6-tri-tert-butylphenyl, 3b, 4b

Scheme 23. Synthesis of 1,3,2 dioxaphospholanes 3 and 4

Reaction of 1 and 2 with either tetrachloro-o-benzoquinone or 3,5-di-tert-butyl-o-

benzoquinone over a 1 hour time period in toluene yielded pale yellow–green solutions.

While the reaction mixtures displayed 31P NMR resonances between 194.3 and 230.1

ppm, well within the range of values determined for phosphaalkenes,6 the resonance for

PMe3, not the anticipated product O=PMe3, was observed. Furthermore, the reaction

stoichiometry was found to be 1 : 1 for ArP=PMe3 : o-quinone (excess ArP=PMe3 was

left unreacted). The major products in these reactions were determined to be 1,3,2

dioxaphospholanes. From the reaction of 3,5-di-tert-butyl-o-benzoquinone isolated yields of 1,3,2-dioxaphospholanes are quite excellent (4a, 94.1%; 4b, 98.0%; Scheme 27).

Reactions of 1 and 2 with tetrachloro-o-benzoquinone gave somewhat lower yields of the cycloadducts (40–45%). Best results for 3a were obtained for reactions performed in toluene at reduced temperatures (-35 °C). Compound 3b has been previously isolated in

16% yield by reaction of tetrachloro-o-benzoquinone with the diphosphene

Mes*P=PMes*.7 The dried crystalline solids of 1,3,2 dioxaphospholanes are stable in the air for weeks and no changes were observed by 31P NMR spectroscopy. However,

solutions of these compounds are reactive in the presence of air.

4 The reactions of phosphanylidene-σ -phosphoranes ArP=PMe3 with other

quinones, such as 1,2-naphthalenedione, benzil, and 1,4-benzoquinone were also

investigated. DmpP=PMe3 reacts with 1,2-naphthalenedione in THF to produce a 1,3,2-

dioxaphospholane (31P NMR: δ 195.3 ppm) after 1.0 h. The reaction in benzene is relatively slow with a lot of decomposition product, DmpP=PDmp. The reaction of

Mes*P=PMe3 with 1,2-naphthalenedione in CHCl3 also generates 1,3,2-

dioxaphospholanes as main product with 31P NMR chemical shift at 193.1, 195.4 ppm

(3:1).

Different solvents such as tetrahydrofuran, toluene, chloroform and acetonitrile,

have been used for this reaction. The 31P NMR yields of 1,3,2-dioxaphospholanes is

about 80 % and 60 % for 1 and 2, respectively. 1,3,2-dioxaphospholanes were not isolated from these reactions (Scheme 24).

O 31 O Yield ( P NMR) toluene P = PMe3 + ~80 %

O O toluene ~60 % P = PMe3 +

4 Scheme 24. Reaction of phosphanylidene-σ -phosphoranes ArP=PMe3 with 1,2-

naphthalenedione

No new related compounds were observed from 31P NMR spectroscopy when

DmpP=PMe3 was allowed to react with benzil at room temperature in THF, except

decomposition of the starting material to diphosphene. Two new small doublets appeared

at 208 ppm (d, J = 677 Hz) and 67 ppm (d, J = 674 Hz) when Mes*P=PMe3 reacted with

benzil in toluene at room temperature for one day. There was, however, much starting

material left.

1,4-Benzoquinone reacted with DmpP=PMe3 in THF. The color changed from

yellow to orange. Two new peaks at 167 and 166 ppm were observed by 31P NMR

spectroscopy after 20 min addition of two equivalents of 1, longer reaction time leading

to diphosphene.

Benzil is less reactive than the four quinones mentioned above. Mes*P=PMe3 is more reactive than DmpP=PMe3. Mes*P=PMe3 reacts with benzil at room temperature,

while there is no reaction observed for DmpP=PMe3 with benzil. The reaction for

Mes*P=PMe3 is still very slow, however.

4 The reaction rate of phosphanylidene-σ -phosphoranes ArP=PMe3 with ortho-

quinones depends on the properties of the quinone. General speaking, tetrachloro-o-

benzoquinone reacts very fast at room temperature and better yields could be obtained at

low temperature. 3,5-Di-tert-butyl-o-benzoquinone is a mild reagent and almost quantitative yields were obtained at room temperature. The reaction of 1,2- naphthalenedione is relatively slow and much decomposition is observed in the reaction

mixture. There is almost no reaction with benzil. Furthermore, these reactions are also

dependent on the solvent. Toluene or benzene is the best solvent to form 1,3,2-

dioxaphospholanes for the former two ortho-quinones with ArP=PMe3. For 1,2-

naphthalenedione, polar solvents (THF and CHCl3) afforded better NMR yields. The

4 relative rates for the reaction of phosphanylidene-σ -phosphoranes ArP=PMe3 with ortho-quinones are listed in Chart 5.

Cl O O OO Cl O O > > >> Cl O O Cl

Chart 5. Relative rate order for the reaction of phosphanylidene-σ4-phosphoranes

ArP=PMe3 with ortho-quinones

2.3 X-ray structural analysis

An ORTEP diagram representing the results of crystallographic determination for

3a is provided in Figure 6. Immediately striking is the disposition of the electron

deficient ring above and parallel to one of the two more electron rich mesityl rings on the

Dmp unit. This intramolecular π– π stacking is evinced by a distance between rings of

3.23 Å. The structural data for 3a can be contrasted to that found in 3b,7 where longer P–

O and P–C bond distances indicate the greater steric presence of the Mes* compared to the Dmp unit. An additional feature of interest for 3a is the manner in which molecules aggregate in the solid state by additional π-stacking between electron deficient and electron rich aromatic rings (Figure 7) along the c axis of the unit cell, with an intermolecular distance of 3.70 Å between these rings. The intramolecular π-stacking is not sufficient to strongly inhibit rotation about the P–C bond in solution, as the mesityl rings are equivalent by both 1H and 13C NMR spectroscopy. The results of a single crystal

X-ray diffraction study of 4a are shown in Figure 8. In addition to lacking the electronic

disparity of the two sets of aromatic rings in 3a, steric repulsions between tert-butyl

groups and the mesityl groups presumably also discourage the type of intramolecular π-

stacking observed in 3a. As in 3a, the C(2)–C(1)–P(1) and C(6)–C(1)–P(1) bond angles

of 128.8(4) and 111.9(4)°, respectively, are significantly different, and might indicate

Menshutkin-type interactions between the phosphorus atom and the opposite mesityl ring,

as have been invoked in the related meta-terphenyls 2,6-Ar2–C6H3ECl2 (E = As, Bi or

Sb).8 While the sum of angles at phosphorus (302.7°) for 3a is slightly larger than that of compounds 4a (297.7°) and 3b (290.8°), each phosphorus center is pyramidal.

Figure 6. Structural diagram for 3,4,5,6-tetrachlorobenzo-2-(2,6-dimesityl-phenyl)-1,3,2- dioxaphospholane, 3a. Selected bond distances (Å) and angles (°): P(1)–C(1), 1.824(8),

P(1)–O(1) 1.675(6); P(1)–O(1) 1.674(6); O(1)P(1)O(2), 93.5(3); C(1)P(1)O(1), 104.5(3);

C(1)P(1)O(2), 104.7(3)

Figure 7. Packing diagram for 3,4,5,6-tetrachlorobenzo-2-(2,6-dimesityl-phenyl)-1,3,2-

dioxaphospholane illustrating π-stacking in the crystal

Figure 8. Structural diagram for 3,5-di-tert-buytlbenzo-2-(2,6-dimesityl-phenyl)-1,3,2- dioxaphospholane, 4a. Selected bond distances (Å) and angles (°): P(1)–C(1), 1.851(6),

P(1)–O(1) 1.673(4); P(1)–O(1) 1.656(4); O(1)P(1)O(2), 93.4(2); C(1)P(1)O(1), 100.6(2) ;

C(1)P(1)O(2) = 103.7(2)

2.4 NMR spectroscopic analyses

The 31P NMR chemical shifts of 1,3,2-dioxaphospholanes depend on the electronic nature of ortho-quinines. 31P NMR of 3a and 3b resonate at lower field than 4a and 4b because of four electron withdrawing chlorine atoms in the aromatic rings making the phosphorus atom deshielded. 31P NMR spectra of 1,3,2-dioxaphospholanes 3 and 4

are shown in Figures 9-12.

Cl Cl O P O Cl Cl

Figure 9. 31P NMR spectrum of 3,4,5,6-tetrachlorobenzo-2-(2,6-dimesityl-phenyl)-1,3,2-

dioxaphospholane, 3a in CDCl3

Cl Cl O P O Cl

Figure 10. 31P NMR spectrum of 3,4,5,6-tetrachlorobenzo-2-(2,4,6-tri-tert-butylphenyl)-

1,3,2-dioxaphospholane, 3b in CDCl3

O P O

Figure 11. 31P NMR spectrum of 3,5-di-tert-buytlbenzo-2-(2,6-dimesityl-phenyl)-1,3,2- dioxaphospholane, 4a in CDCl3

O P O

Figure 12. 31P NMR spectrum of 3,5-di-tert-butylbenzo-2-(2,4,6-tri-tert-butylphenyl)-

1,3,2-dioxaphospholane, 4b in CDCl3

The 1H NMR spectrum of 3a shows only two types of methyl protons which correspond to four ortho and two para methyl groups in the Dmp unit, and one resonance

for the equivalent aromatic protons (* Chart 6) in the mesityl groups (Figure 13). This assignment is also consistent with the 13C NMR spectrum. Although there is a close intermolecular interaction between one of the mesityl groups and the 3,4,5,6-tetrachloro-

1,2-benzenediol unit in this molecule in the solid state, as shown in the X-ray crystal structure, there is free rotation about the P-C bond in solution, as indicated by equivalent mesityl groups.

1 Figure 13. H NMR spectrum of 1,3,2-dioxaphospholane, 3a in CDCl3

The 1H NMR spectrum of 4a, however, shows five resonances for the methyl

protons and two broad singlet resonances for the aromatic protons in the mesityl groups

(Figure 14-16). Steric interactions between the mesityl groups and the 3,5-di-tert-butyl-

1,2-benzenediol group in 4a do not allow the π-stacking observed in 3a, thus both groups

directed away from each other. The NMR data suggest the P-C bond still can undergo

moderate rotation in the solution. The protons move to their corresponding positions by

rotation of P-C bond at 180o: a→b, b→a’, c→c’, e→f’, f→e’ as shown in Chart 6. There

are three situations for the 1H NMR spectrum: 1) If the rotation is slow, six methyl

protons and four aromatic protons in the mesityl groups should be observed from the 1H

NMR spectrum; 2) If the rotation is moderate, three broad methyl proton resonances and two broad aromatic proton resonances should be observed for mesityl groups; 3) if the rotation is very fast, three sharp aromatic proton resonances and two sharp aromatic proton resonances should be observed in the mesityl groups. The rotation rate in 4a is between 2) and 3), three sharp methyl proton resonances and two broad aromatic proton

resonances were observed in the mesityl groups from the 1H NMR spectrum. However,

the carbon c, and c’ resonances are different in the 13C NMR spectrum which showed

four methyl carbon resonances and two aromatic CH resonances (one is relatively broad

compared to the other) in mesityl groups.

* * Cl Cl O P O Cl Cl * *

3a a a' b' b f e e' f ' P P c c' c o c' f b OO b' f ' rotation 180 e' a' OOa e around P-C

4a 4a

Chart 6.

1 Figure 14. H NMR spectrum of 1,3,2-dioxaphospholane, 4a in CDCl3

Figure 15. Low field (aromatic range) of 1H NMR spectrum of 1,3,2-dioxaphospholane,

4a in CDCl3

Figure 16. High field (methyl range) of 1H NMR spectrum of 1,3,2-dioxaphospholane,

4a in CDCl3

1H and 13C NMR spectra of 3b and 4b are relatively simple since t-butyl groups

does not change much after rotation around P-C bonds, which show two singlet peaks for

methyl protons of tert-butyl groups and one singlet for aromatic protons in Mes* group.

2.5 Possible mechanisms

Several possible mechanisms are proposed for the formation of 1,3,2-

dioxaphospholanes. Such reactions may involve single electron transfer steps similar to

proposals made for the mechanisms in other reactions of ortho-quinones.7,9-10 Another possibility invokes the formation of free phosphinidenes that react with ortho-quinones

by [2+1] cycloaddition reactions (Scheme 25, 1). For example, aryloxyphosphinidenes

have been assumed as intermediates for the reaction of 2,6-di-tert-butyl-4-methylphenyl

dichlorophosphine (ArOPCl2) with 1,2-diketones in the presence of Mg metal to form

1,3,2-dioxaphospholanes.11 Free phosphinidenes have been proposed as intermediates

4 12,13 during cleavage of phosphanylidene-σ -phosphoranes (CF3P=PMe3, ArP=PMe3 ).

Photolysis of Mes*P=PMe3 and 2.6-(Trip)2C6H3P=PMe3 produced intramolecular C-H

and C-C insertion products, respectively, suggesting the formation of free

phosphinidenes.4 Free phosphinidenes have been assumed to be intermediates during the

reduction of dihaloorganophosphines by metals.8,14 The mechanism for the formation of

1,3,2-dioxaphospholanes 3 and 4 can also be postulated by nucleophilic attack of

electron-rich phosphorus to oxygen, then the other oxygen attack the phosphorus with releasing PMe3 (Scheme 25, 2).

X O Y - PMe3 PPMe3 Ar P Ar P Ar ortho-quinones O X Y

PMe P PMe3 PPMe3 3 Ar Ar Ar P OO OO OO Y X Y X Y X X Y X Y X Y Ar P O X - PMe3 O Y Y X X = Y = Cl or X = t-Bu, Y = H

Scheme 25. Proposed mechanisms for the formation of 1,3,2-dioxaphospholanes

2.6 Conclusions

The present method for preparing 1,3,2-dioxaphospholanes adds to the large number of reactions of ortho-quinones with phosphorus compounds,13 but distinguishes itself in that reactions to produce P(III) compounds (via low coordinate phosphorus compounds) are not often as simple or high yielding. One can also compare the current reactions to a limited set of reactions of these ortho-quinones with non-carbonyl

15 stabilized Wittig reagents R’2C=PR3 that yield 1,3-dioxolanes (Scheme 26).

CMe3 CMe3 15a O O + PhCH=PPh3 Ph R O R O R1 R1

1 1 R = CMe3, R = H; R = H, R = CMe3

X X 15b,15c OY Y O + PhCH=PPh3 Ph O X X O Y Y R 15b,15c X Y OY X O + R R O X Y O PPh3 Y X

X = Y = H, Cl, Br, X = t-Bu, Y = H R R = H, Br Me 15d Me R

+ Ph3P=CCOOEt O O O O C O O R COOEt

Scheme 26. Reactions of ortho-quinones with Wittig reagents to form 1,3-dioxolanes

2.7 Experimental Section

General All manipulations were performed in a MBraun 150 M dry box under an atmosphere of N2. Compounds DmpP=PMe3 and Mes*P=PMe3 were synthesized as

previously reported.1 Tetrachloro-o-benzoquinone and 3,5-di-tert-butyl-o-benzoquinone

were purchased from Aldrich and taken into the dry box for storage and use. 1,2-

naphthalenedione and 1,4-benzoquinone were obtained from Adrich. 1,2-

naphthalenedione was recrystallized from ethyl alcohol and dried in vacuo before using.

1,4-Benzoquinone was sublimed in vacuo and taken into the dry box for use. Benzil was purchased from Aldrich and dried in vacuo prior to use. Dry chloroform was purchased from Acros and transferred via syringe. Acetonitrile was distilled from CaH2 under

nitrogen; THF, toluene, hexane and benzene were purified by distillation from purple sodium/benzophenone solutions under nitrogen prior to use. NMR spectra were recorded

on a Varian Gemini instrument operating at 300 MHz (1H), 121.5 MHz (31P) and 75 MHz

or 50 MHz (13C). 1H NMR spectra are referenced to residual proton solvent signal of

31 CDCl3, while P NMR spectra are referenced to external 85% H3PO4. High resolution

mass spectrometry was performed at Michigan State University mass spectrometry

facility. Elemental analyses were performed by Quantitative Technologies Inc.

The X-ray crystal structures were solved by Rhett C. Smith and Dr. John D.

Protasiewicz at Department of Chemistry, Case Western Reserve University. The X-ray

intensity data were measured on a Bruker P4 X-ray diffractometer system equipped with

a Mo-target X-ray tube (λ = 0.71073 Å) operated at 273 K.

3,4,5,6-Tetrachlorobenzo-2-(2,6-dimesitylphenyl)-1,3,2-dioxaphospholane (3a)

o To a chilled (-35 C) solution of DmpP=PMe3 (0.230 g, 0.547 mmol) in 5 mL

toluene was added dropwise a chilled (-35 oC) solution of tetrachloro-o-benzoquinone

(0.135 g, 0.547 mmol) in 5 mL toluene. The yellow solution changed to greenish brown over 30 minutes. The solvent was removed under reduced pressure and the residue was

extracted with hexane and filtered. White crystalline 3a was obtained from hexane at –35

o o 1 C. Yield: 0.146 g (45.1 %). m. p. 234-235 C. H NMR (CDCl3): δ = 2.03 (s, 12H), 2.26

3 4 (s, 6H), 6.81 (s, 4H), 7.12 (dd, 2H, JHH = 7.6 Hz, JPH = 2.0 Hz), 7.66 (t, 1H, J = 7.6 Hz).

13 1 C { H} NMR (50 MHz, CDCl3): δ = 21.04 (s), 21.16 (d, J = 1.1Hz), 127.96 (s), 130.05

(d, J = 1.8 Hz), 134.14 (s), 135.46 (d, J = 8.0 Hz), 136.15 (d, J = 2.2 Hz), 137.59 (s),

31 1 137.80 (s), 138.59 (s), 144.52 (s), 147.29 (s), 147.81 (s). P { H} (CDCl3): δ = 230.1.

Anal. Calcd for C30H25PO2Cl4 (590.31): C, 61.04; H 4.27. Found: C, 61.26; H, 4.40.

+ HRMS (FAB) Calcd for C30H26O2PCl4 (MH ) 589.0425, found 589.0424.

3,4,5,6-Tetrachlorobenzo-2-(2,4,6-tri-tert-butylphenyl)-1,3,2-dioxaphospholane (3b)

To a solution of 0.375 g (1.06 mmol) Mes*=PMe3 in 6 mL toluene was added

dropwise a red solution of 0.262 g (1.06 mmol) tetrachloro-o-benzoquinone in 6 mL

toluene with stirring. The yellow solution changed to greenish brown upon mixing and

was allowed to stir for 30 minutes. The solvent was removed under reduced pressure and

the residue was extracted by hexane. A white crystalline solid was obtained after two recrystallizations from hexane. Yield: 0.222 g (40.0 %). m.p. 154-156oC. 1H NMR

31 1 (CDCl3): δ = 1.21(s, 9H), 1.54 (d, 18H, J = 1.1 Hz), 7.13 (d, 2H, J = 1.1 Hz). P { H}

NMR (CDCl3): δ = 217.5.

3,5-Di-tert-butylbenzo-2-(2,6-dimesitylphenyl)-1,3,2-dioxaphospholane (4a)

To a solution of DmpP=PMe3 (0.361 g, 0.858 mmol) in 7 mL toluene was added

dropwise a solution of 3,5-di-tert-butyl-o-benzoquinone (0.189 g, 0.858 mmol) in 7 mL

toluene with stirring. The yellow solution became lighter over a period of 1.0 hour. The

solvent was evaporated and the residue was extracted with hexane and filtered. White

crystalline 4a was obtained after solvent was removed under reduced pressure. Crystals

suitable for X-ray studies were grown from Et2O/CH3CN. Yield: 0.456 g (94.1 %). m. p.

o 1 154-156 C. H NMR (CDCl3): δ = 1.23 (s, 9H), 1.25 (s, 9H), 1.76 (s, 6H), 2.14 (s, 6H),

2.36 (s, 6H), 5.87 (d, 1H, J = 2.0 Hz), 6.73 (d, 1H, J = 2.1 Hz), 6.78 (s, 2H), 6.94 (dd, 2H,

3 4 13 1 JHH = 7.6Hz, JPH = 1.8 Hz), 6.99(s, 2H), 7.47 (t, 1H, J = 7.6 Hz). C { H} NMR (50

MHz, CDCl3): δ = 20.75 (s), 20.86 (s), 21.33 (s), 21.67 (s), 29.89 (s), 31.63 (s), 34.36 (s),

34.45 (s), 108.07 (d, J = 1.6 Hz), 116.34 (s), 127.86 (s), 128.28 (s), 130.07 (s), 131.56 (s),

134.01 (s), 135.94 (d, J = 2.0 Hz), 136.64 (s), 136.86 (d, J = 1.7 Hz), 137.32 (d, J = 4.6

31 1 Hz), 143.69 (s), 144.30 (s), 144.69 (s), 146.94 (s), 147.06 (s). P { H} NMR (CDCl3): δ

= 194.3. Anal. Calcd for C38H45PO2 (564.75): C, 80.81; H, 8.04. Found: C, 80.20; H, 8.17.

+ HRMS (FAB) calcd for C38H46O2P (MH ) 565.3235, found 565.3236.

3,5-Di-tert-butylbenzo-2-(2,4,6-tri-tert-butylphenyl)-1,3,2-dioxaphospholane (4b)

To a solution of Mes*P=PMe3 (0.401 g, 1.14 mmol) in 8 mL toluene was added

dropwise a solution of 3,5-di-tert-butyl-o-benzoquinone (0.251 g, 1.14 mmol) in 8 mL

toluene with stirring. The yellow solution became lighter after 45 minutes. The solvent

was evaporated and the residue was extracted with 10 mL heptane. Removal of the solvent in vacuo yielded white crystalline 4b. Yield: 0.555 g (98.0 %). m.p. 100-101oC.

1 H NMR (CDCl3): δ = 1.12 (s, 9H), 1.20 (s, 9H), 1.24 (s, 9H), 1.52 (s, 18H), 6.65 (d, 1H,

J = 1.9 Hz), 6.75 (d, 1H, J = 2.2 Hz), 7.01 (d, 2H, J = 1.0 Hz). 13C {1H} NMR (75 MHz,

CDCl3): δ = 30.23 (s), 31.02 (s), 31.57 (s), 34.14 (d, J = 9.2 Hz), 34.36 (s), 34.44 (s),

34.54 (s), 39.49 (d, J = 2.7 Hz), 108.03 (s), 115.70 (s), 121.21 (s), 134.22 (s), 141.28 (s),

142.41 (s), 144.00 (s), 146.94 (d, J = 7.4 Hz), 149.54 (s), 156.62 (d, J = 6.8 Hz). 31P {1H}

NMR (CDCl3): δ = 195.9. Anal. Calcd for C32H49PO2 (496.71): C, 77.38; H, 9.94. Found:

+ C, 77.47; H, 10.16. HRMS (FAB) calcd for C32H50O2P (MH ) 497.3549, found 497.3548.

2.8 References

1. Shah, S.; Protasiewicz, J. D. Chem. Commun. 1998, 1585.

2. Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem. 2000,

39, 3860

3. (a) Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem.

2000, 39, 3860. (b) Smith, R. C.; Chen, X.; Protasiewicz, J. D. Inorg. Chem. 2003,

42, 5468. (c) Smith, R. C.; Protasiewicz, J. D. Eur. J. Inorg. Chem. 2004, 998. (d)

Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2004, 126, 2268.

4. Shah, S.; Simpson, M. C.; Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc.

2001, 123, 6925.

5. Some recent reports: (a) Ionkin, A. S.; Marshall, W. Chem. Commun. 2003, 710.

(b) Ionkin, A. S.; Marshall, W. Heteroat. Chem. 2002, 13, 662. (c) Minami, T.;

Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.; Yoshifuji, M. Angew. Chem. Int.

Ed. 2001, 40, 4501. (d) Ikeda, S.; Ohhata, F.; Miyoshi, M.; Tanaka, R.; Minami,

T.; Ozawa, F.; Yoshifuji, M. Angew. Chem. Int. Ed. 2000, 39, 4512. (e) Yoshifuji,

M. Bull. Chem. Soc. Jpn. 1997, 70, 2881.

6. Appel, R.; Knoll, F.; Ruppert, I. Angew. Chem. Int. Ed. 1981, 20, 731.

7. Freytag, M.; Jones, P. G.; Schmutzler, R.; Yoshifuji, M. Heteroat. Chem. 2001,

12, 300.

8. Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.

1999, 121, 3357.

9. Ogata, J.; Yamashita, M. J. Org. Chem. 1973, 38, 3423.

10. Van der Knaap, T. A.; Bickelhaupt, F. Tetrahedron 1983, 39, 3189.

11. Chasar, D. W. Phosphorus, Sulfur Relat. Elem. 1987, 34, 149.

12. (a) Burg, A. B.; Mahler, W. J. Am. Chem. Soc. 1961, 83, 2388. (b) Cowley, A. H.;

Cushner, M. C. Inorg. Chem. 1980, 19, 515. (c) Schmidpeter, A. Multiple Bonds

and Low Coordination in Phosphorus Chemistry; Regitz, M. and Schere, O. J. Ed.;

Thieme Verlag: Stuttgard, 1990, 338. (d) Fritz, G.; Scheer, P. Chem. Rev. 2000,

100, 3341. (e) Shah, S.; Simpson, M. C.; Smith, R. C.; Protasiewicz, J. D. J. Am.

Chem. Soc. 2001, 123, 6925.

13. Osman, F. H.; Al-Samahy, F. A. Chem. Rev. 2002, 102, 629.

14. Smith, R. C.; Shah, S.; Protasiewicz, J. D. J. Organomet. Chem. 2002, 646, 255.

15. For example: (a) Voleva, V. B.; Zhorin, V. A.; Khristyuk, A. L.; Ershov, V. V.;

Enikolopyan, N. S. Bull. Acad. Sci. USSR Div. Chem. Sci. (Engl. Transl.) 1983,

32, 402. (b) Sidky, M. M.; Boulos, L. S. Phosphorus, Sulfur Relat. Elem. 1984, 19,

27. (c) Abdou, W. M. Phosphorus, Sulfur Silicon Relat. Elem. 1992, 66, 285. (d)

Fylaktakidou, K. C.; Gautam, D. R.; Hadjipavlou-Litina, D. J.; Kontogiorgis, C.

A.; Litinas, K. E.; Nicolaides, D. N. J. Chem. Soc., Perkin Trans. 1 2001, 3073.

Chapter 3. A Cyclic Diphosphinite by a Formal [4+4] Cycloaddition Reaction of β-

Phosphaenone

3.1 Introduction

The reactions of Wittig reagents with ortho-quinones have been investigated and

it has been shown that the products of the reactions are dependent on the structure of the

Wittig reagents and 1,2-dicarbonyl compounds. They can undergo bis- or mono- Wittig

reaction to form 1,2-dialkenes or enones1-3 or by cycloaddition to form 1,3-dioxolanes.4-5

These three kinds of products are either final products or intermediates that can undergo further reaction with nearby functional groups.

ortho-Diphosphaalkenes and phosphaenones are examples of phosphorus analogues of 1,2-dialkenes and enones. Because of reduced stability of low-coordinate phosphorus compounds, bulky groups were introduced for purposes of stabilization. ortho-Diphosphaalkenes have been prepared and some of these have been used as chelate ligands for transition metal catalysis (See Chapter 1, 4.1).6-10 The examples of

phosphaenones are usually synthesized via chloro(phosphinidene)

methyllithium/magnesium reagents with carbonyl chlorides (Scheme 27).11-13

Cl Cl Cl Ar Ar n-BuLi Ar RC(O)Cl P P P R = t-Bu, Ph, OEt, NHPh o THF, -110oC O Cl THF, -110 C Li R

MgBr2

Cl Ar Cl Ar Cl(Me)C=O NH4Cl (sat) P P o RT MgBr THF, -5 C O Me Ar = 2,4,6-tri-tert-butylphenyl

Scheme 27. Sythesis of some phosphaenones

4 Like Wittig reagents, phosphanylidene-σ -phosphoranes (ArP=PMe3) act as

phospha-Wittig reagents to prepare phosphaalkenes from aldehydes.14 In an extension of

the reactivity studies of phosphanylidene-σ4-phosphoranes, we have recently reported

that they can react with ortho-quinones to yield 1,3,2-dioxaphospholanes.15 However, the

reaction with acenaphthenequinone was quite different from previous one. In this chapter,

full characterization of a new eight-membered cyclic diphosphinite is described.

3.2 Results and discussion

3.2.1 Synthesis of a cyclic diphosphinite

4 The phosphanylidene-σ -phosphorane DmpP=PMe3 1 reacts rapidly with

acenaphthenequinone (1:1) in CHCl3 at room temperature. Analysis of the reaction

mixture by 31P NMR reveals formation of new materials 5 (31P NMR: δ = 304.6 (s) and

290.8 (s), about 4:1, as indicated by * in Figure 17) and trimethylphosphine oxide (31P

NMR: δ = 39.2 (s), indicated by ** in the Figure 17). The solution also was noted to change from yellow to red. After removal of solvent under reduced pressure, the residue was extracted with hexane and filtered. Orange crystals of a new compound 6 were

obtained via slow evaporation of hexane at room temperature for several days in the dry

box (yield: 39%). The crystals are not stable and change from red orange to yellow

orange and lose transparency after about 2.0 hours in the air.

31 Figure 17. P NMR spectrum for the reaction mixture of DmpP=PMe3 with acenaphthoquinone in CDCl3

3.2.2 X-ray structural analysis of the cyclic diphosphinite 6

An ORTEP drawing of the crystal structure of 6 is shown in Figure 18. The crystal belongs to the space group P-1. The molecule of 6 has pseudo C2 symmetry

around the rotation axis passing through the center of the eight-membered ring. Each

asymmetric unit contains two independent molecules which are enantiomers (RR and SS)

and one hexane molecule. The shape of the eight-membered heterocyclic ring is similar

to the lowest energy configuration of 1,5-cyclooctadiene (Figure 19).16 The acenaphthylene groups from two molecules in the unit cell are parallel to each other with the distance of 3.60 Å, which suggest π-π stacking interaction between the two moieties

(Figure 20).

Figure 18. ORTEP drawing of one of the independent molecules of cyclic diphosphinite,

6. Some selected bond lengths (Å) and bond angles (o): P(1)-O(1),1.697(8); P(2)-O(2),

1.661(9); O(1)-C(25), 1.434(14); O(2)-C(61), 1.375(14); P(1)-C(62), 1.789(14); P(2)-

C(26) 1.877(13); P(1)-C(1), 1.880(13); P(2)-C(37), 1.790(12); O(1)-P(1)-C(62), 99.7(6);

O(2)-P(2)-C(26), 98.7(5); O(1)-P(1)-C(1), 94.8(5); O(2)-P(2)-C(37), 97.0(5); C(25)-

O(1)-P(1), 111.4(7); C(61)-O(2)-P(2), 116.4(8); C(62)-P(1)-C(1), 106.8(6); C(37)-P(2)-

C(26), 106.7(6); C(26)-C(25)-O(1), 123.4(12); O(2)-C(61)-C(62), 126.0(13); C(25)-

C(26)-P(2), 127.6(11); C(61)-C(62)-P(1), 127.7(10)

C7 C8 C6

C1 C5 C3 C4 C2

1,5-cyclooctadiene

Figure 19. Structure of the eight-membered ring in 6 and schematic structure of the lowest energy configuration of 1,5-cyclooctadiene

Figure 20. The π – π interactions between two molecules in the crystal unit cell

The atoms, C(1), P(1), O(1), C(25) and C(37), P(2), O(2), C(61) are almost on the

same planes and having dihedral angles C(1)-P(1)-O(1)-C(25) and C(37)-P(2)-O(2)-C(61)

of 175.6(9) and 174.9(9)o, respectively. Two Dmp units move away from each other in

order to decrease steric interactions. Two central phenyl rings in the Dmp units are

distorted with distortion angle of 14(2)o and -14(3)o in C(2), C(1), C(6), C(5) and C(4),

C(5), C(6), C(1), and -10(2)o and 8(2)o in C(38), C(37), C(51), C(50) and C(38), C(48),

C(49), C(50), respectively. The mean plane angles between mesityl groups and central

phenyl rings are 73.3o and 79.6o in one Dmp unit and 81.4o and 71.2o in the other.

3.3 NMR spectroscopic studies

The 31P NMR chemical shifts for the two isomers are within the broad range of

values (δ 257-335 ppm) reported for β-phosphaenones. These materials have 31P NMR

shifts that show deshielding of the phosphorus nucleus compared to non-conjugated

phosphaalkenes.11-13 31P NMR spectra showed two isomers (E, Z) of the phosphaenone.

Other phosphaenones have also been reported to exist as two isomers.11-12 The Z-isomer resonates at a lower field than the E-isomer. This is probably due to steric effect of Ar and acyl group causing phosphorus lone pair of electrons close to acenaphthylene group and influenced by ring current (in the deshielding region of the ring current). In the E- isomer, the phosphorus lone pair of electrons is closer to the acyl moiety because of the steric interaction between the hydrogen atom of the acenaphthylene group and the Ar group, causing the electron density on the phosphorus atom to be higher than that for the

Z-isomer. The presence of Z/E isomers may be attributed to the contribution of the

zwitterionic structure and repulsion between the bulky substituents (Scheme 28). This mechanism explained Z/E isomerization of other β-phosphaenones.11

Ar O O P O P O Ar P PMe3 + + Ar

5 E 5 Z

Ar Ar P P O O O Ar P O P Ar H

Ar = Dmp, 2,6-dimesitylphenyl E Z

Scheme 28. The formation of β-phosphaenones and their resonance structures

No matter how much phosphanylidene-σ4-phosphorane was used (2 equivalents

or excess), β-phosphaenones are the main products observed from 31P NMR spectra.

Addition of one Dmp group hinders approach of the next DmpP=PMe3 and 1,2-

diphosphaalkene could not form. The same phenomenon was observed in Wittig reaction.

Enones were formed even two equivalents phosphorane were used.1-3 β-phosphaenones

31 are not stable in CHCl3 and slowly decompose, which was observed from P NMR

spectrum taken after 2.0 days in chloroform.

31P NMR chemical shift for compound 6 is located at δ 138.8 ppm (Figure 21).

This value is similar to chemical shifts observed in ten-membered heterocyclic

phosphorochloridites17-18 and N-substituted chlorodiazadiphosphetidine derivatives.19

31 Figure 21. P NMR spectrum of cyclic diphosphinite 6 in CDCl3

1 Figure 22. H NMR spectrum of cyclic diphosphinite 6 in CDCl3 (Hexanes as co-solvent

in the crystal were observed as indicated by *)

1 1 Figure 23. H- H COSY spectrum of cyclic diphosphinite 6 in CD3COCD3

g e h f e'

d d' C1 a c a' b P b' c' 2 1 1 O 3

4 O 5 6 P

6" inversion

g g a f h e' h f a' e b C d' 1 b' C1 d e e' a' c d c' a P d' c' b' b c 2 1 1 P1 O 2 1 rotation O 3 3

4 4 O O 5 6 P 5 6 P

66'

Chart 7. Compound 6 and its conformations 6’ and 6” after rotation about P-C bond for

180o and inverstion

Although there might be steric interactions among Dmp groups and acenaphthylene groups, Dmp groups can rotate about the P(1)-C(1) and P(2)-C(37) bonds in the solution. The 1H NMR spectrum of compound 6 showed broad signals for the aromatic protons and methyl protons in the Dmp groups. For the methyl protons, two

broad and overlapped signals are observed at δ 2.3 and 1.9 ppm in chloroform-d (Figure

22). 1H NMR spectroscopy showed a triplet for aromatic proton g in the central phenyl

ring of Dmp groups, as well as two doublets of doublets and four doublets for

acenaphthylene groups (1-6). The triplet for g and doublet for nearby aromatic protons of

acenaphthylene overlap together in chloroform-d but are distinct in acetone-d6 (Figure

23). This assignment was confirmed by 2D 1H-1H COSY NMR spectroscopy in acetone-

d6 (Figure 23).

Considering the structure of compound 6, after rotation of 180o along P(1)-C(1), it is converted to 6’ in which protons move to their corresponding positions: a→e’, b→d’, c→c’, d→b’, e→a’, f →g, vice versa (Chart 7). If the rotation is slow, six methyl proton resonances and seven aromatic proton resonances should be observed for Dmp groups in the 1H NMR spectrum; If the rotation is moderate, there would be three broad methyl

proton resonances and three broad aromatic proton resonances and one triplet observed for Dmp groups in the spectrum; if the rotation is very fast, there would be three sharp methyl proton resonances and three sharp aromatic proton resonances and one triplet observed for Dmp groups. Another possibility is that the chiral phosphorus center can do inversion (Chart 7). Trigonal pyramidal phosphorus compounds have higher inversion barrier than that of their nitrogen analogues.20 However, the bulky aryl substituents can

make inversion easier by steric interaction.21 Compound 6 can invert to 6” in which

process the protons go to their corresponding position: a→e, b→d, a’→e’, b’→d’, and vice versa. Using the approach mentioned above, the proton resonances in the 1H NMR spectrum can be predicted by considering the inversion rate. From the spectrum observed, compound 6 might do rotation and inversion at moderate rate. Thus there would be two

broad resonances (δ 2.3, 1.9 ppm) for methyl protons (c, c’ and a, e, a’ e’) and one broad resonance (δ 7.0 ppm) for f, h protons in the central phenyl ring which were observed in

Figure 18. One broad peak for b, d, b’, d’ protons indeed appeared at δ 6.4 ppm in CDCl3 at 55 oC, at which temperature, the broad peak at δ 7.0 ppm became sharp and shifted to δ

6.9 ppm.

A single broad resonance is observed at δ 2.2 ppm for methyl protons in

bromobenzene-d5 at room temperature (Figure 24). As the temperature of compound 6

o was increased to 110 C in bromobenzene-d5 solution, the broad peak for methyl groups

in Dmp unit became two distinct peaks with increasing temperature as shown in Figure

20. The integration for the two peaks is 1:2 which matches the molecular structure. The

peaks in the aromatic region became broad with increasing temperature.

1 Figure 24. Variable temperature H NMR spectra for compound 6 in C6D5Br

(Sigals at 7.15, 6.88, 6.80 indicated by * are residual solvent protons in C6D5Br, and signal at 1.20 ppm is due to solvent impurities. Signal at 1.40 and 1.05 indicated by **

are due to hexane co-solvent from crystals of compound 6· (n-C6H14)0.5)

OO Ar PO Ar -O=PMe3 PPMe3 +

+ -- ArPO Ar P O P Ar 22 O Ar P O

Scheme 29. A possible mechanism for the formation of 6

3.4 Possible mechanism

A mechanism that could account for the formation of this compound is shown in

Scheme 29. Acenaphthenequinone reacts with the first equivalent of the

phosphanylidene-σ4-phosphorane by a phospha-Wittig reaction to give phosphaenone 5

and trimethylphosphine oxide. Dimerization of the spectroscopically detected phosphaenone intermediate 5 by a formal [4+4] cycloaddition reaction affords 6. It would be logical to expect isomerization to 5Z is required for the dimerization reaction to occur.

The equilibrium shifts to 5Z driven by the consumption of 5Z to form a more stable dimer. Similar mechanisms were proposed for some enones which dimerized by Diels-

Alder type reactions.1,3, 22

3.5 Conclusions

An eight-membered cyclic diphosphinite was synthesized by the reaction of

4 acenaphthenequinone and phosphanylidene-σ -phosphorane DmpP=PMe3. Its structure was established by X-ray analysis and NMR spectroscopy. Acenaphthenequinone behaves differently from tetrachloro-o-benzoquinone and 3,5-di-tert-butyl-o- benzoquinone towards phosphanylidene-σ4-phosphoranes. This is also the case for the

reaction of acenaphthenequinone with a Wittig reagent.2 An enone and its dimer were

isolated from the reaction when two equivalents of ylide was used. However, the β- phosphaenones formed in this reaction are not stable and isolation of the phosphaenone failed. A relatively stable cyclic diphosphinite was obtained instead. The significance of this study is that the establishment of a new method for the synthesis of cyclic diphosphinite by a formal [4+4] cycloaddition reaction of β-phosphaenones.

4 When the phosphanylidene-σ -phosphorane Mes*P=PMe3 2 (Mes* = 2,4,6-tri-t-

butylphenyl) reacts with acenaphthenequinone (1:1) in CHCl3 or THF at room

temperature, β-phosphaenones (31P NMR δ = 300 and 287 ppm) and trimethylphosphine

oxide were observed from 31P NMR spectroscopy. However, there are much

Mes*P=PMes* and other unidentified species. This reaction was not pursued further to isolate the phosphaenones.

3.6 Experimental section

General All manipulations were performed in a MBraun 150 M dry box under an

13a atmosphere of N2. DmpP=PMe3 was synthesized following the literature procedure.

Acenaphthenequinone was purchased from Aldrich. n-Hexane was distilled from sodium/benzophenone prior to use. Extra dry chloroform was purchased from Acros and transferred via syringe. Bromobenzene-d5 was purchased from Aldrich and deoxygened

before moving to the dry box. NMR spectra were recorded on a Varian Gemini

instrument operating at 300 MHz (1H), 121.5 MHz (31P) and 50 MHz (13C). The 2D 1H-

1 1 H (COSY) spectrum was recorded at 600 MHz in acetone-d6. H variable-temperature

(VT) NMR experiments were performed on a Varian Gemini 300 MHz instrument from

room temperature to 110 oC (10 oC increments) allowing ~15 min equilibration at each

temperature setting. Bromobenzene-d5 was used as solvent for the VT experiments.

Proton, carbon and phosphorus spectra are referenced to residual solvent signals and 85

% phosphoric acid, respectively.

The X-ray crystal structure was solved by Professor Tong Ren and his student

Weizhong Chen at Miami University. Here is the experimental section for solving the structure: The X-ray intensity data were measured at 300 K on a Bruker SMART 1000

CCD-based X-ray diffractometer system equipped with a Mo-target X-ray tube (λ =

0.71073 Å) operated at 2000 watts power. A total of 1271 frames were collected with a

final resolution of 0.75 Å. About 9% decay was indicated by the recollection of the first

50 frames at the end of data collection. The frames were integrated with the Bruker

SAINT© software package using a narrow-frame integration algorithm, which also

corrects for the Lorentz and polarization effects. No absorption corrections were done to

the data. The structure was solved and refined in the space group P-1 using the Bruker

SHELXTL© (Version 5.1) Software. All non-hydrogen atoms were derived from the

direct method solution. With all non-hydrogen atoms being anisotropic and all hydrogen

atoms being isotropic the structure was refined to convergence by least squares method

on F2, SHELXL-93, incorporated in SHELXTL.PC V 5.03.

Synthesis of cyclic phosphinite

To a solution of acenaphthoquinone (0.0910 g, 0.50 mmol) in 5.0 mL CHCl3 in 25 mL flask was added dropwise a solution of DmpP=PMe3 (0.2100 g, 0.50 mmol) in 5 mL

CHCl3 with stirring. The color changed from yellow to red gradually over 30 minutes’

31 period. P NMR spectroscopy showed the disappearance of DmpP=PMe3 and the formation of phosphaenone and trimethylphosphine oxide. The solvent was removed under reduced pressure and the residue was extracted with hexane and filtered. Orange crystals of the cyclic diphosphinite were isolated from the filtrate via slow evaporation at room temperature to afford 0.10 g (39%) of the product.

o 31 1 m.p. 197 C (dec.). P NMR (CDCl3): δ = 138.8; H NMR (CDCl3) δ: 7.46 (t, J = 8.0 Hz,

2H), 7.47 (d, J = 8.0 Hz, 2H), 7.32 (d, J = 8.3, 2H), 7.19 (dd, J = 7.6 Hz, 2H), 7.0 (s,

broad, 12H), 6.90 (dd, J = 7.5 Hz, 2H), 6.57 (d, J = 6.8 Hz, 2H), 6.41 (d, J = 6.9 Hz, 2H),

13 1 2.30 (s, broad, 12H), 1.80 (s, broad, 24H). C { H} NMR (CDCl3) δ: 158.8 (s), 136.3 (s)

133.6 (t, J = 2.54), 130.5 (s), 129.7 (s), 127.8 (s), 127.1 (s), 127.0 (s), 126.7 (s), 126.4 (s),

124.3 (s), 122.7 (t, J = 3.0 Hz), 120.7 (t, J = 4.2 Hz), 21.9 (s, broad), 21.0 (s).

3.7 References

1. Soliman, F. M.; Khalil, Kh. M.; Abd-El-Naim, G. Phosphorus and Sulfur and the

Related Elements 1988, 35, 41.

2. Boulos, L. S.; Hennawy, I. T. Phosphorus, Sulfur, and Silicon and the Related

Elements 1993, 84, 173.

3. Abdou, W. M.; Ganoub, N. A. F.; Abdel-Rahman, N. M. Phosphorus and Sulfur

and the Related Elements 1991, 61, 91.

4. Fylaktakidou, K. C.; Gautam, D. R.; Hadjipavlou-Litina, D. J.; Kontogiorgis, C.

A.; Litinas, K. E.; Nicolaides, D. J. Chem. Soc., Perkin Trans. 1 2001, 3073.

5. (a) Voleva, V. B.; Zhorin, V. A.; Khristyuk, A. L.; Ershov, V. V.; Enikolopyan, N.

S. Bull. Acad. Sci. USSR Div. Chem. Sci. (Engl. Transl.) 1983, 32, 402. (b) Sidky,

M. M.; Boulos, L. S. Phosphorus, Sulfur Relat. Elem. 1984, 19, 27. (c) Abdou, W.

M. Phosphorus, Sulfur Silicon Relat. Elem. 1992, 66, 285.

6. Appel, R.; Winkhaus, V.; Knoch, F. Chem. Ber. 1987, 120, 243.

7. Märkl, G.; Kreitmeier, P.; Nöth, H.; Polborn, K. Angew. Chem. Int. Ed. Engl.

1990, 29, 927.

8. (a) Yoshifuji, M.; Toyota, K.; Murayama, M.; Yoshimura, H.; Okamoto, A.;

Hirotsu, K.; Nagase, S. Chem. Lett. 1990, 2195. (b) Toyota, K.; Tashiro, K.;

Yoshifuji, M.; Nagase, S. Bull. Chem. Soc. Jpn 1992, 65, 2295. (c) Toyota, K.;

Tashiro, K.; Yoshifuji, M. Angew. Chem. 1993, 105, 1256. (d) Yamada, N.; Abe,

K.; Toyota, K.; Yoshifuji, M. Organic Lett. 2002, 4, 569.

9. Maerkl, G.; Hennig, R.; Noeth, H.; Schmidt, M. Tetrahedron Lett. 1995, 36, 6429.

10. Ikeda, S.; Ohhata, F.; Miyoshi, M.; Tanaka, R.; Tatsuya, M.; Fumiyuki, O.;

Yoshifuji, M. Angew. Chem. Int. Ed. 2000, 39, 24.

11. Yoshifuji, M.; Ito, S.; Toyato, K.; Yasunnami, M. Bull. Chem. Soc. Jpn. 1995, 68,

1206.

12. Van der Sluis, M.; Bickelhaupt, F.; Veldman, N.; Kooijman, H.; Spek, A. L.;

Eisfeld, W.; Regitz, M. Chem. Ber. 1995, 128, 465.

13. Van der Sluis, M.; Wit, J. B. M.; Bickelhaupt, F. Organometallics 1996, 15, 174.

14. (a) Shah, S.; Protasiewicz, J. D. Chem. Commun. 1998, 1585. (b) Smith, R. C.;

Chen, X.; Portasiewicz, J. D. Inorg. Chem. 2003, 42, 5468. (c) Smith, R. C.;

Protasiewicz, J. D. J. Am. Chem. Soc. 2004, 126, 2268. (d) Smith, R. C.;

Protasiewicz, J. D. Europ. J. Inorg. Chem. 2004, 998.

15. Chen, X.; Smith, R. C.; Protasiewicz, J. D. Chem. Commun. 2004, 146.

16. Hovis, J. S.; Hamers, R. J. J. Phys. Chem. B 1997, 101, 9581.

17. Balakrishna, M. S.; Panda, R.; Mague J. T. Inorg. Chem. 2001, 40, 5620.

18. Balakrishna, M. S.; Panda, R. Phosphorus, Sulfur, and Silicon and the Related

Elements 2003, 178, 1391.

19. Vijjulatha, M.; Kumaraswamy, S.; Kumara Swamy, K. C.; Engelhardt U.

Polyhedron 1999, 18, 2557.

20. Baechler, R. D.; Andose, J. D.; Stackhouse, J.; Mislow, K. J. Am. Chem. Soc.

1972, 94, 8060.

21. Baechler, R. D.; Mislow, K. J. Am. Chem. Soc. 1970, 92, 3090.

22. Abd El-Rahman, N. M.; Boulos, L. S. Molecules 2002, 7, 81.

Chapter 4. Synthesis of meta-Terphenyl Iodo and Dichlorophosphine Derivatives

Bearing Methoxy Groups

4.1 Introduction

Bulky meta-terphenyl groups have been proven to be good protecting groups for

the stabilization of diphosphenes and their heavier main group analogues.1 The introduction of methoxy groups near the metal center has significant influence in late transition-metal catalysis.2,3 Introduction of methoxy groups into the meta-terphenyls

confers different electronic properties on the aromatic systems in comparison with alkyl

substituted analogues.

m-Terphenyls have been synthesized via two successively formed aryne intermediates starting from trihalobenzene or 1,3-dichlorobenzene.4,5 Methoxy- substituted terphenyls can be synthesized following the same route. Iokin and Marshall published the synthesis of 2,6-di(2-methoxyphenyl)iodobenzene through the tandem aryne addition of the Grignard reagent prepared from 1-bromo-2-methoxy-benzene to the

1,3-dichloro-2-iodobenzene. However, they did not obtain 2,6-di(2,6-dimethoxyphenyl)

iodobenzene using this route, and 2,6-di(2,6-dimethoxyphenyl) benzene was formed

instead (Scheme 30).6 2,6-Di(2,6-dimethoxyphenyl)benzene could also be prepared by the treatment of 1,3-dichloro-2-iodobenzene with vinylmagnesium bromide, followed by

2,6-dimethoxyphenyl magnesium bromide under refluxing, followed by aqueous quench.7 Here we present the synthesis of 2,6-di(2,6-dimethoxyphenyl)iodobenzene, 7,

2,6-di(2,4,6-trimethoxyphenyl)iodobenzene, 8 and their dichlorophosphine derivatives.

MgI I O O O O Cl Cl 1)

2) I2 O O

Scheme 30. Synthesis of 2,6-di(2,6-dimethoxyphenyl) benzene6

4.2 Results and discussion

4.2.1 Synthesis and NMR spectroscopic analysis of meta-terphenyl iodides bearing

methoxy groups

2-Bromo-1,3-dimethoxybenzene was prepared by slight modification of the

procedure published by Hayashi et al.8 with a 69% optimized yield. The reaction of 2-

bromo-1,3-dimethoxybenzene with activated magnesium turnings in THF generated the

Grignard reagent, 1,3-dimethoxyphenylmagnesium bromide under nitrogen. 1,3-

dichlorobenzene was treated with n-BuLi in THF solution at -78 oC for 1.0 hour,

followed by the Grignard reagent addition. The reaction mixture was slowly warmed to

room temperature and refluxed overnight, then quenched with excess iodine at room

temperature. After workup of the mixture by aqueous wash, 2,6-di(2,6-dimethoxyphenyl) iodobenzene 7 was obtained in 32 % yield (Scheme 36). 2,6-Di(2,4,6-trimethoxyphenyl)

iodobenzene 8 can be synthesized in a similar way with a yield of 35 % (Scheme 31).

Cl Cl n-BuLi ArMgBr Li -78oC -78oC r. t. Cl Cl

reflux I2 r. t. Ar Ar I 7, 8

MeO OMe MeO OMe Ar =

OMe

Scheme 31. Synthesis of 2,6-di(2,6-dimethoxyphenyl) iodobenzene, 7 and 2,6-di(2,4,6- trimethoxyphenyl) iodobenzene, 8

The 1H NMR spectrum of compound 7 showed one resonance for the methoxy

groups, and two triplet resonances and two doublet resonances for aromatic CH protons.

So the two pendant aromatic rings are equivalent. These resonances from low field to

high field correspond to d, b, c, a aromatic protons as indicated by the integration of the

resonances (Figure 25). This assignment agrees with the 13C NMR spectrum which showed four carbon resonances for aromatic CH groups and four carbon resonances for aromatic 4o carbons and one resonance for methoxy carbon.

The 1H NMR spectrum of compound 8 shows one triplet resonance, one doublet resonance, one singlet resonance in aromatic range which correspond to aromatic CH groups in central phenyl ring (1, 2) and pendant rings (3) (Figure 26). Two methyl proton resonances at higher field correspond to methoxy methyl groups. This assignment is

consistent with 13C NMR spectrum.

d O c c O a a b b a O I O a

1 Figure 25. H NMR of 2,6-di(2,6-dimethoxyphenyl)iodobenzene in CDCl3

1 O 2 2 O 3 3

O O 3 O I O 3

1 Figure 26. H NMR of 2,6-di(2,4,6-trimethoxyphenyl)iodobenzene in CDCl3

4.2.2 X-ray crystal structures of meta-terphenyl iodides bearing methoxy groups

Compounds 7 and 8 were isolated as white crystalline solids. Facile recrystallization was achieved in THF at -6.3 oC. X-ray structure analyses of compounds

7 and 8 reveal the expected molecular structures (Figure 27, 28). The phenyl rings show

no significant deviation from planarity. The bond lengths and angles are close to normal

phenyl rings. The angles between mean planes of the pendant aromatic rings with central

phenyl ring are 72.3o, 87.6o in compound 7 and 76.7, 91.8o in compound 8 and the angles

between two pendant aromatic rings are 62.0o in 7 and 55.3o in 8. The C-I bond lengths in

7 and 8 are 2.109 (3) and 2.07 (2) Å, respectively. These bond lengths are comparable to

those in the structurally characterized terphenyl iodides. For 2,6-Trip2-C6H3I, a reported

9 value was 2.102 (6) Å in C-I distance. 2,6-Mes2C6H3I (Mes = 2,4,6-trimethylphenyl) has

10 C-I distance of 2.122(4) Å and 2.127 (13) Å in poly-chlorinated terphenyl iodide ArICl2

11 (Ar = 2,6-bis(3,5-dichloro-2,4,6-trimethylphenyl)benzene). For 2,6-Mes2-4-MeC6H2I

12 and 2,6-Ph2-4-MeC6H2I, the C-I bond lengths are 2.135(5) and 2.105(2) Å respectively.

The C-I distances in 7, 8 are also close to the C-I distances in three 1,4-diiodo-2,3,5,6- tetraarylbenzenes that are 2.118(3), 2.116(7) and 2.108(6) Å.13

Figure 27. ORTEP drawing of 2,6-di(2,6-dimethoxyphenyl) iodobenzene, 7. Thermal

ellipsoids are shown at 50 % probability. Selected bond lengths [Å]: I(1)-C(1), 2.109(3);

C(1)-C(2), 1.403(5); C(2)-C(7), 1.488(5); C(13)-(6), 1.491 5); O(1)-C(8), 1.373(6); O(2)-

C12), 1.359(5); O(3)-C(14), 1.355(5); O(4)-C(18), 1.367(5). Selected bond angles [o]:

C(2)-C(1)-I(1), 119.5(3); C(6)-C(1)-I(1), 118.6 (2); C(6)-C(1)-C(2), 122.0(3); C(8)-C(7)-

C(2), 121.6(4); O(1)-C(8)-C(7), 114.8(4); O(1)-C(8)-C(9), 124.9(4)

Figure 28. ORTEP drawing of 2,6-di(2,4,6-trimethoxyphenyl) iodobenzene, 8. Thermal

ellipsoids are shown at 50 % probability. Selected bond lengths [Å]: I(1)-C(1), 2.07(2);

C(1)-C(2), 1.45(2); C(2)-C(7), 1.43(3); O(1)-C(8), 1.40(3); O(2)-C(12), 1.39(3); O(3)-

C(15), 1.44(3); O(4)-C(21), 1.38(3); O(5)-C(22), 1.45(3); O(6)-C(23), 1.44(3). Selected

bond angles [o]: C(2)-C(1)-I(1), 116.3(12); C(6)-C(1)-I(1), 122.0(12); C(6)-C(1)-C(2),

121.7(18); C(7)-C(2)-C(1), 125.0(19); C(2)-C(7)-C(8), 123(2); O(1)-C(8)-C(9),

125.8(18); O(1)-C(8)-C(7), 109.1(19)

4.2.3 Synthesis of meta-terphenyl dichlorophosphines bearing methoxy groups

The synthesis of 2,6-di(2,6-dimethoxyphenyl)phenyldichlorophosphines was

1a attempted using the protocol for the DmpPCl2 synthesis. 2,6-Di(2,6-dimethoxyphenyl)

iodobenzene is not soluble in hexanes and Et2O. Its solubility in THF is also very limited,

which may slow down the reaction and make the lithiation reaction difficult to complete

if the solubility of the lithiation product is very limited in THF at low temperature.

n-BuLi PCl3 Ar Ar Ar Ar I PCl2

MeO OMe Ar = MeO OMe

OMe

Scheme 32. Attempted synthesis of dichlorophosphine derivatives of 7 and 8

2,6-Di(2,6-dimethoxyphenyl)iodobenzene, 7 was treated with one equivalent of n-

BuLi in THF solution at -78 oC. The reaction mixture was stirred at -78 oC for 1.0 hours,

31 followed by the addition of PCl3 (Scheme 32). After workup of the reaction mixture, P

NMR spectrum showed the formation of 2,6-di(2,6-dimethoxyphenyl)phenyl

dichlorophosphine by a peak at 161 ppm (Figure 29). However, the 1H NMR spectrum

indicated that it was a mixture of 2,6-di(2,6-dimethoxyphenyl)phenyldichlorophosphine and 2,6-di(2,6-dimethoxyphenyl) iodobenzene in the ratio of 2:1. It was very difficult to isolate the pure 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine from the mixture.

Increase of the lithiation reaction time 7.5 hours did not improve the yield of the product.

The reaction was also conducted in dilute THF and diethyl ether solution, which failed to produce 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine.

Since the 2,6-di(2-methoxyphenyl)phenyl dichlorophosphine and 2,6-Di(2- methoxyphenyl)-4-methylphenyl dichlorophosphine were synthesized in hexanes at room temperature,6,14 hexanes was used as solvent for this reaction. To a solution of 7 in hexanes was added n-BuLi at room temperature. The resultant mixture was stirred at room temperature for 22.5 hours, followed by addition of PCl3 and stirring for additional

2.0 hours. 31P NMR spectroscopy showed the formation of 2,6-di(2,6-

88 dimethoxyphenyl)phenyl dichlorophosphine. 1H NMR spectroscopy indicated that the reaction was not complete. There was still large amount of 7 present. The reaction of 7 with activated magnesium under refluxing or lithium metal agitated under ultrasound did not give desirable results either.

Figure 29. 31P NMR spectrum of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine in CDCl3

The solubility of 8 is even less than 7 in hexanes, Et2O and THF. 2,6-Di(2,4,6- trimethoxyphenyl)phenyl dichlorophosphine was indeed observed from 31P NMR spectrum with a resonance at 165.6 ppm following the similar procedure using THF as solvent (Scheme 32). However, the yield is less than 10 % and no pure 2,6-di(2,4,6- trimethoxyphenyl)phenyl dichlorophosphine was isolated. Elongation of the reaction time for the lithiation reaction to 12.0 hours did not work well either. To a solution of 8 in hexanes was added n-BuLi, and then the mixture was refluxed for 3.0 hours. To this mixture was added PCl3 at room temperature. The resultant solution was stirred 2.0 hours at room temperature. After workup of the reaction mixture the formation of 2,6-di(2,4,6- trimethoxyphenyl)phenyl dichlorophosphine was not indicate by 31P NMR spectroscopy.

There are two factors which might prevent formation of the expected products.

Most importantly, the solubility of 7 and 8 is very low in THF, Et2O and hexanes. If the

solubility of the lithiated product is also very limited in the aforementioned solvents, it

may take a very long time for the reaction to reach equilibrium. On the other hand,

methoxy groups are very good ortho-directing substituents in the metalation reactions

with n-butyllithium.15 Thus n-butyllithium can attack iodine in the m-terphenyl or ortho

positions of the methoxy groups (Chart 8, arrows indicated). The yields for iodine

substitution are relatively low due to ortho-directing substitution of methoxy groups.

O O O O

I I O O O O O O

Chart 8. The possible positions of 7 and 8 for the metalation reactions with n- butyllithium

The synthesis of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine was attempted by the protocol that is used for the synthesis of m-terphenyls, except the

o Grignard reagent was quenched with PCl3 at -78 C rather than iodine or acid (Scheme

33).4,5 Pure 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine was prepared and isolated using this method. However, the yield is only ~ 5% based on 1,3- dichlorobenzene.

Cl Cl n-BuLi ArMgBr Li -78oC -78oC r. t. Cl Cl

reflux PCl3 Ar Ar r. t. -78oC PCl2

Ar = MeO OMe

Scheme 33. Synthesis of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine

31P NMR chemical shifts of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine and 2,6-di(2,4,6-trimethoxyphenyl)phenyl dichlorophosphine are at δ

161.4 ppm and δ 165.6 ppm, respectively, which are very close to those reported for other meta-terphenyl dichlorophosphines: 2,6-(Mes)2C6H3PCl2 at δ 160.1 ppm, 2,6-(m-

Xyl)2C6H3PCl2 at δ 159.6 ppm, TripPCl2 at 156.5 ppm and 2,6-(t-BuC6H4)2C6H3PCl2 at δ

151.3 ppm.

The 1H NMR spectrum of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine is similar to the iodo derivative except the meta-CH groups (c protons) which appear a

4 doublet of doublets with JPH of 2.9 Hz (Figure 30). The long range coupling of

phosphorus with meta position aromatic protons is observed in other meta-terphenyl

4 4 dichlorophosphines: 2,6-(Mes)2C6H3PCl2, JPH = 3.1 Hz; 2,6-(m-Xyl)2C6H3PCl2, JPH =

3.1 Hz.

d O c O a a'

b b' O P O Cl Cl

Figure 30. 2,6-di(2,6-dimethoxyphenyl)phenyldichlorophosphine (The proton resonances

from THF were indicated by *)

The reaction of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine with

potassium was very slow and no reaction was observed from 31P NMR spectroscopy after

one day. The reaction of 2,6-di(2-methoxyphenyl)phenyl dichlorophosphine with

potassium in THF formed diphosphene, F and inserted product, G. However, the only

product isolated from this reaction was 1-methoxy-6-(2-methoxy-phenyl)-5-oxo-5H-5λ5- dibenzophosphol-5-ol, H (Scheme 34).6

MeO OMe O MeO Cl O K P + P hexanes P PCl2 MeO OMe MeO

δ = 160.7, 160.6 δ = 510.4, 505.7 δ = 129.4, 127.6

MeO O Cl P

MeO

δ = 36.3

Scheme 34. The reduction reaction of 2,6-di(2-methoxyphenyl)phenyl dichlorophosphine with potassium6

4. 3. Conclusions

Two new m-terphenyl iodides containing methoxy groups were prepared. They were fully characterized by NMR, elemental analysis and X-ray crystal structure analysis.

However, the synthesis of their dichlorophosphine derivatives was problematic. The dichlorophosphine products were observed by 31P NMR, but the yields were not good and only pure 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine was isolated with relatively low yield.

4.4 Experimental section

General All air sensitive reactions were performed in a MBraun 150 M dry box or

Schlenk line under dry N2. Magnesium turnings used in Grigard reagent preparation were

activated by heating and stirring overnight under dry nitrogen. 1,3-Dimethoxybenzene

and 1,3,5-trimethoxybenzene were purchased from Aldrich, deoxygened and taken into

dry box for storage and use. n-BuLi (2.5 M hexanes solution) was purchased from

Aldrich. PCl3 was purified by vacuum transfer and degassed in vacuo. Acetonitrile was

distilled from CaH2, and THF, hexane, and diethyl ether were distilled from

sodium/benzophenone prior to use. NMR spectra were recorded on a Varian Gemini

instrument operating at 300 MHz (1H), 200 MHz (1H), 121.5 MHz (31P) and 50 MHz

(13C). Proton, carbon and phosphorus spectra are referenced to residual solvent signals

and 85 % phosphoric acid, respectively.

The X-ray crystal structures were solved by Rhett C. Smith and Dr. John D.

Protasiewicz at Department of Chemistry, Case Western Reserve University. The X-ray

intensity data were measured on a Bruker P4 X-ray diffractometer system equipped with

a Mo-target X-ray tube (λ = 0.71073 Å) operated at 273 K.

Synthesis of 2-Bromo-1,3-dimethoxybenzene

This compound was synthesized by modifying the procedure reported by Hayashi

et al.9 To a stirred solution of 9.938 g (71.9 mmol) 1,3-dimethoxybenzene in 300 mL

diethyl ether in a 1.0 L round-bottom flask was added a solution of 30.2 mL (75.5 mmol)

n-BuLi (2.5 M hexanes solution) via syringe at 0 oC. The resultant mixture was stirred for

1.0 hour and then warmed to room temperature to form a white suspension. The

suspension was refluxed for 3.0 hours and then cooled to -78 oC. To the chilled reaction

mixture was added 3.6 mL (70.3 mmol) bromine via syringe. The reaction mixture was slowly warmed to room temperature and stirred for 1.0 hour. The mixture was quenched with 200 mL saturated sodium thiosulfate solution followed by extraction with 300 mL diethyl ether. The organic layer was washed with brine (2x50 mL), dried over magnesium sulfate and filtered. The solvent was reduced to ~30 mL under reduced pressure followed by filtration to yield 10.83 g of 2-bromo-1,3-dimethoxybenzene (69.4 %) as an off-white

o 1 crystalline solid. m.p. 91 C. H 300 MHz NMR (CDCl3): δ 3.90 (s, 6H), 6.58 (d, 2H, J =

8.34 Hz), 7.24 (t, 1H, J = 8.42 Hz).

Synthesis of 2,6-di(2,6-dimethoxyphenyl) iodobenzene (7)

A solution of 5.06 g (23.3 mmol) 2-bromo-1,3-dimethoxybenzene in 25 mL THF

was added to 1.13 g (46.5 mmol) of activated magnesium turnings via cannula with

stirring. The resultant dark brown mixture was stirred at room temperature for 1.0 hour.

To a chilled (-78 oC) solution of 1.37 g (9.32 mmol) 1,3-dichlorobenzene in 30 mL THF in a 100 mL flask was added slowly a solution of 4.1 mL (10.25 mmol) n-BuLi (2.5 M hexanes solution) via syringe with stirring. The resultant mixture was stirred at -78 oC for

1.0 hour. To this solution was added slowly the prepared Grignard reagent via cannula at

-78 oC. The reaction mixture was stirred for additional 1.0 hour at -78 oC, and refluxed

overnight to form dark brown solution. The dark brown solution was quenched with 4.40

g (17.3 mmol) iodine at room temperature and stirred for 20 minutes. Excess iodine was

consumed by aqueous sodium sulfite. The mixture was extracted with diethyl ether, and

the organic layer was washed three times with water. The organic layer contained a small

amount of white precipitate. The solvent was reduced to one third volume under reduced

pressure followed by filtration and rinse with diethyl ether to yield 1.42 g of pure 7 as

o 1 off-white powder (32 %). m.p. 240-241 C. H 300MHz NMR (CDCl3): δ 3.77 (s, 12H),

6.66 (d, 4H, J = 8.37 Hz), 7.14 (d, 2H, J = 7.59 Hz), 7.34 (t, 2H, J = 8.34 Hz), 7.43 (t, 1H,

13 1 J = 7.53 Hz). C { H} NMR (CDCl3): δ 56.2(s), 104.5 (s), 109.6 (s), 123.9 (s), 127.5 (s),

129.2 (s), 129.3 (s), 133.2(s), 141.0 (s), 157.7 (s). Anal. Calcd for C22H21IO4 (476.31): C,

55.48; H, 4.44. Found: C, 55.59; H, 4.44.

Synthesis of 2,6-di(2,6-dimethoxyphenyl)phenyl dichlorophosphine

To a solution of 1.32 g (2.8 mmol) 2,6-di(2,6-dimethoxyphenyl) iodobenzene in

40 mL THF was added slowly a solution of 1.2 mL (3.0 mmol) n-BuLi (2.5 M hexanes solution) via syringe with stirring at -78 oC. The resultant mixture was stirred at -78 oC for 1.0 hours. To the mixture was added quickly 1.0 mL (11.5 mmol) of PCl3 via syringe.

The mixture was reacted for 1.0 hour and then warmed to room temperature. The solution was taken into dry box, where the solvent was removed under reduced pressure. The residue was rinsed with acetonitrile and extracted with hot diethyl ether. Removal of solvent yielded an off-white solid. Only one peak was observed at 161.3 ppm from 31P

NMR spectrum. However, 1H NMR indicated that it was a mixture of 2,6-di(2,6- dimethoxyphenyl) phenyl dichlorophosphine and 2,6-di(2,6-dimethoxyphenyl) iodobenzene with the ratio of 2:1. It was very difficult to isolate the pure 2,6-di(2,6- dimethoxyphenyl)phenyl dichlorophosphine by recrystallization.

This compound was also synthesized in the following manner: A solution of 4.08 g (18.8 mmol) 2-bromo-1,3-dimethoxybenzene in 25 mL THF was added to 1.45 g (59.7 mmol) activated magnesium turnings via cannula. The resultant dark brown solution was stirred at room temperature for 1.0 hour. To a solution of 1.164 g (7.9 mmol) m- dichlorobenzene in 30 mL THF was added slowly a hexane solution of n-BuLi (3.2 mL,

8.0mmol, 2.5 M) via syringe at -78 oC. The reaction mixture was stirred for 1.0 hour at -

78 oC. To the resultant mixture was added slowly the prepared Grignard reagent via

cannula. The reaction mixture was stirred 1.0 hour at -78 oC and then refluxed overnight.

o To the mixture was added quickly 2.5 mL of PCl3 via syringe at -78 C. The mixture was

warmed to room temperature slowly and stirred overnight. The solution contained white

precipitate, which was filtered. The solid obtained from filtration was dissolved in THF and filtered again. The solvent was reduced to one third volume under reduced pressure.

White crystals were obtained from THF solution at -32oC (0.1650 g, 4.5 %). 1H 200 MHz

NMR (CDCl3): 7.58 (t, J = 7.76 Hz, 1H), 7.34 (t, J = 8.33 Hz, 2H), 7.21 (dd, JHH = 7.64

13 1 Hz, JPH = 2.91 Hz, 2H), 6.61 (d, J = 8.28 Hz, 4H), 3.73 (s, 12H). C { H}, NMR (CDCl3):

δ 258.3 (d, J = 1.8 Hz), 139.7 (s), 139.1 (s), 131.8 (s), 131.7(s), 129.7(s), 117.1 (s), 103.4

31 (s), 55.6 (s). P NMR (CDCl3): δ 161.4.

Synthesis of 2-bromo-1,3,5-trimethoxybenzene

To a solution of 11.14 g (66.2 mmol) 1, 3, 5-trimethoxybenzene in 200 mL

dichloromethane in 1.0 L round-bottom flask covered with foil was added dropwise 3.22

mL (62.8 mmol) bromine in 200 mL dichloromethane by addition funnel over a period of

2.0 hours with stirring. The resultant mixture was stirred 2 hours at room temperature,

quenched by addition of 200 mL 5% aqueous sodium sulfite. The organic layer was

washed three times with water. The solvent was removed under reduced pressure and the

residue was recrystallized from heptane, then 50% aqueous ethyl alcohol to yield 13.0 g

o 1 of a off-white solid (80 %). m.p. 95-97 C. H NMR (CDCl3): δ 6.18 (s, 2H), 3.88 (s, 6H),

3.83 (s, 3H).

Synthesis of 2,6-di(2,4,6-trimethoxyphenyl) iodobenzene (8)

A solution of 10.30 g (41.7 mmol) 2-bromo-1,3,5-trimethoxybenzene in 40 mL

THF was added to 2.20 g (90.5 mmol) of activated magnesium turnings via cannula with

stirring. The resultant dark brown mixture was stirred at room temperature for 1.0 hour.

To a chilled (-78 oC) solution of 1.37 g (9.32 mmol) 1,3-dichlorobenzene in 30 mL THF in a 100 mL flask was added slowly a solution of 7.33 mL (18.3 mmol) n-BuLi (2.5 M hexanes solution) via syringe with stirring. The resultant mixture was stirred at -78 oC for

1.0 hour. To this solution was added slowly the prepared Grignard reagent via cannula at

-78 oC. The reaction mixture was stirred for 1.0 hour at -78 oC, and refluxed overnight to

form dark brown solution. The dark brown solution was quenched with 8.46 g (33.3

mmol) iodine at room temperature and stirred for 20 minutes. Excess iodine was

consumed by aqueous sodium sulfite. The mixture was extracted with diethyl ether, and

the organic layer was washed three times with water. The organic layer contained a small

amount of white precipitate. The solvent was reduced to ~20 mL under reduced pressure

followed by addition of 30 mL diethyl ether and filtration, and rinse with diethyl ether to

yield 3.07 g of pure 8 as off-white powder (34.4 %). m.p. 271-273 oC. 1H 300 MHz NMR

(CDCl3): δ 3.74 (s, 12H), 3.87 (s, 6H), 6.22 (s, 4H), 7.11 (d, 2H, J = 7.62 Hz), 7.39 (t, 1H,

13 1 J = 7.48 Hz). C { H} NMR (CDCl3): δ 55.35 (s), 56.13 (s), 91.08 (s), 102.49 (s), 117.24

(s), 127.37 (s), 129.64 (s), 140.96 (s), 158.30(s), 161.01 (s). Anal. Calcd for C24H25IO6

(536.36): C, 53.74; 4.70. Found: C, 53.72; H, 4.61. HRMS calcd for C24H25IO6 (M):

536.0696, found, 536.0690.

Synthesis of 2,6-di(trimethoxylphenyl)phenyl dichlorophosphine

To a solution of 3.07 g (6.3 mmol) 2,6-di(trimethoxylphenyl) iodobenzene in 100

mL THF was added slowly a solution of 2.8 mL (6.9 mmol) n-BuLi (2.5 M hexane

solution) via syringe with stirring at -78 oC. The resultant mixture was stirred at -78 oC

for 1.5 hours. To the mixture was added quickly 1.7 mL of PCl3 via syringe. The mixture

was stirred for 1.0 hour at -78 oC and then warmed to room temperature. The solution was taken into dry box, where the solvent was removed under reduced pressure. The residue was washed with acetonitrile and filtered. The solid in the filter was 2,6- di(trimethoxylphenyl) iodobenzene. The products in acetonitrile filtrate was the mixture of 2,6-di(trimethoxylphenyl) iodobenzene and 2,6-di(trimethoxylphenyl)phenyl dichlorophosphine with about 1:1 ratio indicated by 1H NMR spectrum. It is difficult to

isolate 2,6-di(trimethoxyphenyl)phenyl dichlorophosphine by recrystallization. 31P NMR

(CDCl3): δ 165.6.

4.5 References:

1. (a) Urnezius, E.; Protasiewicz, J. D. Main Group Chem. 1996, 1, 369. (b) Shah, S.;

Burdette, S. C.; Swavey, S.; Urbach, F. L.; Protasiewicz, J. D. Organometallics

1997, 16, 3395. (c) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J.

Am. Chem. Soc. 1999, 121, 3357.

2. Schmid, M.; Eberhardt, R.; Klinga, M.; Leskelae, M.; Rieger, B. Organometallics

2001, 20, 2321.

3. Preparation of phosphide intermediates for polymerization catalysts; Neth Appl

1989, 28, NL 8800770.

4. Du, C.-J. F.; Hart, H.; Ng, K.-K. D. J. Org. Chem. 1986, 51, 3162.

5. Saednya, A.; Hart, H. Synthesis, 1996, 1455.

6. Ionkin A. S.; Marshall, W. J. Heteroatom Chem. 2003, 14, 360.

7. Grewal, R. S.; Hart, H.; Vinod, T. K. J. Org. Chem. 1992, 57, 2721.

8. Kamikawa, T.; Hayashi, T. Tetrahedron, 1999, 55, 3455.

9. Twamley, B.; Hardman, N. J.; Power, P. P. Acta Cryst. 2000, C56, e514.

10. Niemeyer, M. Organometallics 1998, 17, 4649.

11. Protasiewicz, J. D. J. Chem. Soc. Chem. Commun. 1995, 1115.

12. Dickie, D. A.; Abeysekera, D.; McKenzie, I. D.; Jenkins, H. A.; Clyburne, J. A. C.

Crystal Engineering 2003, 6, 79.

13. Shah, S.; Eichler, B. E.; Smith, R. C.; Power, P. P.; Protasiewicz, J. D. New J.

Chem. 2003, 27, 442.

100

14. Rigon, L.; Ranaivonjatovo, H.; Escudie, J. Phosphorus, Sulfur Silicon Relat. Elem.

1999, 152, 153.

15. Slocum, D. W.; Jennings, C. A. J. Org. Chem. 1976, 41, 3653.

101

Chapter 5. Synthesis and Characterization of 2,6-Dimesitylphenyl

Difluorophosphine

5.1. Introduction

Aryldifluorophosphines are less common compared to aryldichlorophosphines

because of their instability.1-2 Aryldifluorophosphines can undergo spontaneous

transformation to form bisarylfluorophosphines and phosphorus trifluoride, or redox

disproportionation to form aryltetrafluorophosphoranes and cyclopolyphosphines.3

Relatively stable aryldifluorophosphines can be obtained with highly electronegative substituents which increase Lewis acidity of the trivalent phosphorus compounds and discourage disproportion reactions.4

Syntheses of alkyl, alkenyl and alkynyl difluorophosphines by the reaction of

organo-lithium, -mercury, or -magnesium compounds with halodifluorophosphines, XPF2

(X = Cl, Br) between -78 oC and room temperature have been reported.3,5 Ortho- substituted phenyl difluorophosphines have been synthesized by a chlorine-fluorine exchange reaction on the appropriate dichlorophosphines.1 The synthesis and

characterization of a very stable meta-terphenyl difluorophosphine is presented in this chapter.

5.2 Results and discussion

5.2.1 Synthesis and NMR spectroscopic studies of DmpPF2

DmpPF2 was synthesized via a chloro-fluorine exchange reaction on the

dichlorophosphine (Scheme 35). The reaction mixture of DmpPCl2, 10 equivalents of

NaF and tetramethylene sulfone was heated to 120-130 oC for 45 minutes. The products

102

were extracted with hexanes, followed by recrystallization to yield white crystals (74.5

%).

S OO + + PCl2 2 NaF 120-130 oC PF2 2 NaCl

Scheme 35. Synthesis of 2,6-dimesitylphenyldifluorophosphine

31 1 The P NMR spectrum showed a triplet at δ 213.3 ppm ( JPF = 1170 Hz) in

31 CDCl3 (Figure 31). The P NMR chemical shift and coupling constant are close to 2,6-

1 3,6 dimethoxyphenyldifluorophosphine, δ 212.8 (t, JPF = 1160 Hz) in CDCl3. The shift is

in the range of other published difluorophosphine compounds δ 197.1-216.2 ppm.3,6 The

1 variation in the reported values of JPF for aryl difluorophosphines range from 1129 to

3 1 1199 Hz. The H NMR spectrum of DmpPF2 is similar to DmpPCl2 except that the

splitting of the aromatic CH protons (indicated by * in Figure 32) by coupling with phosphorus is not obvious in DmpPF2 possibly due to the greater electronegativity of

fluorine than chlorine (Figure 32). It is clear doublet of doublets in DmpPCl2 due to

4 coupling with phosphorus ( JPH = 3.1 Hz) (Figure 33).

103

31 1 Figure 31. P { H} NMR spectrum of DmpPF2 in CDCl3

* PX 2

1 Figure 32. H 600 MHz NMR spectrum of DmpPF2 in CDCl3

104

1 Figure 33. H 300 MHz NMR spectrum of DmpPCl2 in CDCl3

5.2.2 Reactivity studies of DmpPF2

The pure dry crystalline DmpPF2 solid is relatively stable in the air for several weeks without decomposition. This material, however, is not stable in chloroform and a proposed disproportion reaction occurs within several hours as indicated by the observation of a doublet of doublets, at 32.5 ppm and 27.3 ppm (J = 1018 Hz) in the 31P

NMR spectrum. This compound showed a doublet at 29.5 ppm with J = 1016 Hz in the decoupled 31P NMR spectrum. The compound was tentatively assumed to be DmpPHF.

The reaction of DmpPCl2 with styrene and active magnesium produces

DmpP=PDmp (31P NMR: δ 493.7) and 2,6-dimesitylphenylphosphirane (31P NMR: δ -

190.6 ppm) with a ratio of 1:4 in THF at room temperature (Scheme 36). This reaction may involve free phosphinidene intermediate which was trapped by styrene or another molecule of phosphinidene to form the observed products. m-Terphenyl phosphiranes

105

might be utilized as precursors for the generation of free phosphinidenes by photolysis.

However, attempted isolation of the desired phosphirane from the mixture was found to

be difficult.

P PX2 + + Mg* P + -MgX2 P

Scheme 36. The reaction of 2,6-dimesitylphenyldihalophosphine with styrene and active

magnesium

DmpPF2 reacted with activated magnesium in the presence of 20 equivalents of

styrene in THF solution. Small multiple resonances were observed at δ 23.3 ppm from

31P NMR spectrum. The mixture was stirred 1.0 day further and mainly the starting

31 material was observed from P NMR spectrum. DmpPF2 reacted with only activated

magnesium to form a small multiple peaks at δ 18.9 ppm. The mixture was stirred for two

days further and mainly the starting material shown in the 31P NMR spectrum. No

reaction occurred between DmpPF2 and activated magnesium in the presence of PPh3.

DmpPF2 reacted slowly with PMe3 and Zn in THF solution to form four singlet

resonances at δ 35.8, 30.9, 27.6 and 22.6 ppm and heating the reaction mixture in a sealed

tube at 60 oC for 1.5 hours showed increased peak at 30.9 with two singlet resonance at

35.6, 18.0 ppm and a doublet at δ 22.4 (J = 62.4 Hz). The mixture of DmpPF2, zinc dust and PPh3 in THF solution produced many small peaks between 35.0 and 15 ppm. The

reaction with sodium naphthalenide gave a mixture. (Scheme 37)

106

31P NMR

almost no r x n DmpPF2 + Mg* + Styrene

almost no r x n DmpPF2 + Mg*

DmpPF2 + Mg* + PPh3 no r x n

DmpPF2 + Zn + PMe3 four resonances between δ 36 and δ 18

DmpPF2 + Zn + PPh3 many small peaks between δ 35 to δ 15

DmpPF2 + Na[Naph] a mixture

Scheme 37. The reactivity studies of DmpPF2

5.2.3 X-ray crystal structure of DmpPF2

Crystals suitable for X-ray analysis of DmpPF2 were grown from hexanes. The X-

ray crystal structure of DmpPF2 was shown in Figure 34. DmpPF2 crystallizes in the orthorhombic space group Pbca. There is positional disorder about the PF2 unit. Two

possible orientations were observed with occupancies at 65% and 35% (Figure 35). The

two mesityl groups are perpendicular to the central phenyl ring as observed in most of

compounds with Dmp units.7-8 The bond lengths for P(1)-F(1) and P(1)-F(2) are different with 1.551 (3) and 1.488 (3) Å on one orientation and 1.338 (9) and 1.650 (14) Å on the other. The average bond lengths of P-F are close to P-F bond length of 1.5814 (10) Å in

9 10 MesPF2 and 1.572 and 1.581 (2) Å in anthracene-9-PF2 dimer, which are much shorter

than the P-Cl bond in dichlorophosphines (in the range of 2.06-2.09 Å).11-13 The P-C bond lengths of 1.898 (3) and 1.785 (5) Å in the two orientations are comparable with the

P-C bond length of 1.8116 (17) Å found for MesPF2. They are also consistent with those

found in other terphenyl-substituted phosphines, for instance, 2,6-(p-t-BuC6H4)2C6H3PCl2

107

[1.837 (2), 1.833 (2) Å].13 The bond angle of F(2)-P(1)-F(1) is 103.4 (2)o and 84.8 (6)o in

9 the two orientations. The average bond angle is close to the angles in MesPF2 and the

10 o o anthracene-9-PF2 dimer which are 95.95 (8) , 96.3 (1) respectively.

The bond angles of C(2)-C(1)-P(1) and C(6)-C(1)-P(1) are 114.71 (15) and

125.29 (17) in one orientation and 128.85 (18) and 111.16 (18)o in the other. The quite

different angles may indicate weak interactions of phosphorus with one of the ortho

13 aromatic rings, as was observed in 2,6-(p-t-BuC6H4)2C6H3PCl2 . This so-called

Menshutkin interaction,14 seems to be a common feature of terphenyl-substituted group

15 dihalides15 or group 14 monohalides16.

Table 2. Selected bond lengths and angles for DmpPF2.

bond lengths/Å angles/o P(1)-F(2) 1.488(3) F(2)-P(1)-F(1) 103.4(2) P(1)-F(1) 1.551(3) F(2)-P(1)-C(1) 100.86(14) P(1)-C(1) 1.898(3) F(1)-P(1)-C(1) 97.86(14) P(1')-F(1') 1.338(9) F(1')-P(1')-F(2') 84.8(6) P(1')-F(2') 1.650(14) F(1')-P(1')-C(1) 104.8(4) P(1')-C(1) 1.785(5) F(2')-P(1')-C(1) 95.9(5) C(2)-C(1)-P(1') 128.85(18) C(6)-C(1)-P(1') 111.16(18) C(2)-C(1)-P(1) 114.71(15) C(6)-C(1)-P(1) 125.29(17) P(1')-C(1)-P(1) 14.58(10)

108

Figure 34. X-ray crystal structure of DmpPF2

Figure 35. Two possible orientations for PF2 unit in DmpPF2.

5. 3 Conclusions

A relatively stable 2,6-dimesitylphenyl difluorophosphine was synthesized by a chlorine-fluorine exchange reaction. It was characterized by X-ray crystal analysis which showed two possible orientations for the PF2 unit. DmpPF2 is not active to Mg* and

styrene or Mg* and triphenylphosphines in comparison to dichlorophosphines. The

reaction with zinc dust and trimethylphosphine and triphenylphosphine is very slow.

109

DmpPF2 can be reduced by sodium naphthalenide to form DmpP=PDmp and other unidentified products.

110

5.4 Experimental section

General All sensitive materials were handled in a MBraun 150 M dry box or Schlenk

line under an atmosphere of N2. DmpPCl2 was synthesized following the literature method.7 NaF was degassed in vacuo and taken into dry box for use. Tetramethylene sulfone was purchased from Acros. Hexanes were distilled from sodium/benzophenone

prior to use. NMR spectra were recorded on a Varian Gemini instrument operating at 600

MHz (1H), 121.5 MHz (31P). Proton and phosphorus spectra are referenced to residual

solvent signals and 85 % phosphoric acid, respectively.

Synthesis of 2,6-(Mes)2C6H3PF2

A 50 mL round bottom flask was charged with 0.500 g (1.2 mmol) DmpPCl2 and

0.543 g (12.9 mmol) sodium fluoride and equipped with a sealed condenser in the dry box, and then taken out of the dry box. To the mixture was added 5.2 mL of tetramethylene sulfone via syringe. The mixture was heated for 45 min with stirring and the temperature raised from room temperature to 130 oC. The mixture was cooled to

room temperature to form a white gel-like material. The flask was taken into a dry box,

where the mixture was extracted with three portions of hexanes. The clear solution was

transferred to another clean flask. The solvent was evaporated under reduced pressure to

one third volume, and the residue was recrystallized from hexanes to yield 0.48 g white

crystalline solid (75.4 %). The single crystals suitable for X-ray diffraction analysis were

o 1 grown in hexane solution by slow evaporation. m.p. 213-215 C. H NMR (CDCl3): 7.64

(t, J = 7.5 Hz, 1H), 7.12 (d, J = 6.6 Hz, 2H), 6.93 (s, 4H), 2.35 (s, 6H), 2.02 (s, 12H). 31P

1 { H} NMR (CDCl3) δ: 213.3 (t, J = 1169.9 Hz).

111

Preparation of Mg* Turnings

A batch of magnesium turnings was stirred with a crystal of naphthalene in 5-10

mL THF solution overnight. The surface of the activated magnesium turnings appears

much darker and a suspension of black powder appears in the solution. The activated

magnesium turnings may be stored in THF in the dry box in a capped vial or flask for

extended periods of time. For use in reactions, the required amount of magnesium turnings is weighed by removing them from the capped vial or flask with a spatula and

THF is removed by brief contact with Kimwipes. The weighed amount is then quickly

transferred to the reaction vessel.

Reaction of 2,6-(Mes)2C6H3PF2 with Styrene and Mg*

A 5 mL round-bottom flask was charged with 0.107 g (0.28 mmol) DmpPF2,

0.575 g (5.5 mmol) styrene, 0.01 g (0.41 mmol) activated magnesium and 1.0 mL THF in a dry box. The mixture was stirred at room temperature for 2 hours and 10 minutes. No reaction occurred, indicated by 31P NMR spectroscopy. To the mixture was added more

activated magnesium and the mixture was stirred further for 2.0 days. Small multiple

peaks were observed at 23.3 ppm from 31P NMR spectrum, however, a large amount of

unreacted starting material remained.

Reaction of 2,6-(Mes)2C6H3PF2 with Mg*

A 5 mL round-bottom flask was charged with 0.03 g (0.078 mmol) DmpPF2,

0.026 g (1.0 mmol) activated magnesium and 0.7 mL THF in a dry box. The mixture was stirred in the dry box for 2.0 days. Small multiple resonances were observed at 18.9 ppm, however, mainly unreacted starting material remained.

112

Reaction of 2,6-(Mes)2C6H3PF2 with PPh3 and Mg*

A 5 mL round-bottom flask was charged with 0.03 g (0.078 mmol) DmpPF2,

0.026 g (1.0 mmol) activated magnesium, 0.04 g (0.15 mmol) PPh3 and 0.7 mL of THF in

a dry box. The reaction mixture was stirred at room temperature for 2.0 days and no

reaction was detected by 31P NMR spectrum.

Reaction of 2,6-(Mes)2C6H3PF2 with PMe3 and Zn

A 5 mL round-bottom flask was charged with 0.03 g (0.078 mmol) DmpPF2, 0.01

g (0.15 mmol) zinc dust, 0.0597 g (0.78 mmol) PMe3 and 0.7 mL of THF in a dry box.

The reaction mixture was stirred at room temperature for 4.0 hours. The 31P NMR spectrum showed four singlet resonances at 35.8, 30.9, 27.6 and 22.6 ppm. The mixture was stirred for 14 hours at room temperature and then heated at 60 oC in a sealed NMR tube for 1.5 hours. 31P NMR spectrum showed increased peak at 30.9 with two singlet

resonances at 35.6, 18.0 ppm and a doublet at 22.4 with J value of 62.4 Hz, but a large

amount of starting material remained.

Reaction of 2,6-(Mes)2C6H3PF2 with PMe3 and Zn

The similar procedure was used for the reaction of DmpPF2 with Zn and PPh3.

The mixture of DmpPF2, zinc dust and PPh3 in THF solution produced tow relatively

large peaks at 19.4, 15.5 ppm and many small peaks between 35.0 and 21.6 ppm. But a

large amount of unreacted starting material remained.

Reaction of 2,6-(Mes)2C6H3PF2 with Na[Naph]

Following the similar procedure, the reaction of DmpPF2 with sodium

naphthalene gave a mixture and no starting material left.

113

5.5 References

1. Heuer, L.; Sell, M.; Schmutzler, R.; Schomburg, D. Polyhedron 1987, 6, 1295.

2. (a) Kosolapoff, G. M.; Maier, L. Organic Phosphorus Compounds, Wiley-Inter-

Science, 1972, Vol.4, p.75. (b) Seel, F.; Rudolph, K. H. Z. Anorg. All. Chem.

1968, 363, 233. (c) Ang, H. G.; Schmutzler, R. J. Chem. Soc. (A) 1969, 702. (d)

Schmutzler, R.; Stelzer, O.; Liebman, J. F. J. Fluorine Chem. 1984, 25, 289.

3. Leuer, L.; Schmutzler, R. J. Fluorine Chem. 1988, 39, 197.

4. Fild, M.; Schmutzler, R. J. Chem. Soc. (A) 1969, 840.

5. (a) Lines, E. L.; Centofanti, L. F. Inorg. Chem. 1973, 12, 598. (b) Lines, E. L.;

Centofanti, L. F. Inorg. Chem. 1974, 13, 1517. (c) Lines, E. L.; Centofanti, L. F.

Inorg. Chem. 1974, 13, 2796.

6. Heuer, L.; Schomburg, D.; Schmutzler, R. Phosphorus, Sulfur Silicon Relat. Elem.

1989, 45, 217.

7. Urnezius, E.; Protasiewicz, J. D. Main Group Chem. 1996, 1, 369.

8. Chen, X.; Smith, R. C.; Protasiewicz, J. D. Chem. Commun. 2004, 146.

9. Jones, P. G.; Heuer, L.; Schmutzler, R. Acta. Cryst. 2002, E58, o999.

10. Heuer, L. 1989, PhD dissertation, Technical University of Braunschweig,

Germany.

11. Al-Juaid, S. S.; Dhaher, D. M.; Eaborn, C.; Hitchcock, P. B.; McGeary, C. A.;

Smith, J. D. J. Organomet. Chem. 1989, 366, 39.

114

12. Plack, V.; Goerlich, J. R.; Fischer, A.; Thönnessen, H.; Jones, P. G.; Schmutzler,

R. Z. Anorg. Allg. Chem. 1995, 621, 1080.

13. Wehmschulte, R. J.; Khan, M. A.; Hossain, S. I. Inorg. Chem. 2001, 40, 2756.

14. Mootz, D.; Händler, V. Z. Anorg. Allg. Chem. 1986, 533, 23.

15. Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc.

1999, 121, 3357.

16. Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics 1997, 16,

1920.

115

Chapter 6. Synthesis and Characterization of Phosphaalkene Polymers

6.1 Introduction

The 2000 Nobel Prize in Chemistry was awarded to MacDiarmid, Heeger and

Shirakawa for their pioneering work and great discoveries on electrically conducting polymers.1 Conjugated polymers have attracted great interest due to their wide applications in opto-electronic devices and sensors.2 Some common conjugated polymers receiving attention are shown in Chart 9.2c,3 The conductivity of some conjugated polymers can be modified by chemical or electrochemical doping. Doped conjugated polymers have carriers that can move along the polymer chain because of the attraction of the nuclei in the neighboring units. Charge-carrier mobility is extended into three dimensions through interchain electron transfer and is limited by disorder. Good conducting polymers can be obtained by improving structural order, hence achieving higher mobility. Polyaniline was the first π-conjugated polymer to be easily doped by oxidation (chemically or electrochemically) of the leuco-emeraldine base or by protonation of the emeraldine base by acid-base chemistry. Its mechanism for the conversion from semiconductor into metal by acid-base reaction has been well- documented4 and its processability into electronic devices has been reported.5

116

n n n polyacetylene poly(para-phenylene) poly(phenylenevinylene)

H N N n H S n n polypyrrole polythiophene polyaniline

Chart 9. Representative conjugated polymers

Non-substituted conjugated polymers, such as poly(phenylenevinylene) (PPV), are often insoluble, intractable and infusible.6 Incorporation of moderately long flexible side chains

onto the monomer can increase the solubility and decrease the interchain contact of

conjugated polymers. These modifications can also improve the polymers’ fluorescence

and electroluminescence quantum yields. Soluble PPV derivatives have been prepared

and fabricated into light emitting diodes (LEDs)7-9 following the discovery of

electroluminescence (EL) in PPV.10 Dialkoxy substituted PPV derivatives (Chart 10)

have very good solubility in organic solvents and high EL efficiency.11-18 The color of the emission light is shown below the structure. The color changes from red to red orange, to green with widening the band gap. A single layer device with high external EL efficiency

(2.1%) was made by Philips using emission of red light polymer in Chart 10.18

117

O O Si

n MeO n MeO MeO n

red-orange red green

Chart 10. PPV derivatives with high EL efficiency and processability

Conjugated polymers containing heteroatoms can offer opportunities for the lone pair of electrons to participate in conjugation along the path of conjugation and to modulate the electronic properties of the polymers such as polythiophene and polyaniline.

There has been increasing interest in the incorporation of phosphorus into the backbone of conjugated polymers. Representative phosphorus-containing polymers are shown in Chart 11. 19, 20

R R R P P n n PN n P Fe n R R

poly(para-phenylenephosphine) polyphosphole poly(ferrocenylphosphine) polyphosphazene

Chart 11. Selected phosphorus containing polymers

The most widely studied phosphorus-containing inorganic polymers are polyphosphazenes. Polyphosphazenes are usually prepared by thermal ring-opening polymerization (ROP) of cyclic monomers.20-26 An account was reported in 2002 on the synthesis of polyphosphazene and hybrid systems which incorporate phosphorus and

118 other main group elements in the polymer main chain.27 Last year (2003) a report was published on the development of new polyphosphazenes and their potential applications in fuel cells, non-linear optical materials, and biodegradable non-toxic materials.28

Lucht reported the synthesis of poly(p-phenylenephosphine)s with high molecular weights via palladium catalyzed cross coupling of difunctional monomers.29 The synthesis of poly(arylphosphine)s by palladium catalyzed carbon-phosphorus bond formation and nickel mediated carbon-carbon bond formation was also reported (Scheme

38). UV-vis-NIR spectroscopy and cyclic voltammetry suggested electron delocalization along the polymer chain through phosphorus.

Pd cat R Pd cat R Br I Br PH P RPH2 n

R = i-Bu, 2,4,4-trimethylpentyl

Pd cat R Br I Br P Br RPH2

R Ni cat P n

R = i-Bu, 2,4,4-trimethylpentyl

Scheme 38. Synthesis of poly(arylphosphine)s

The preparation of poly(methylenephosphine) by addition polymerization of a phosphaalkene has recently been published (Scheme 39).30

119

Ph [init] Ph PC o PC Mes 150 C Ph MesPh n

Scheme 39. Synthesis of poly(methylenephosphine)

These polymers have π systems using phosphorus lone pair of electrons that are different from those in the organic conjugated materials which are formed by overlap of two p orbitals. It is questionable that double bond resonance structures can exist in these polymers and they are not as suitable for conjugation as those based on the overlap of two p orbitals. Conjugated materials having low-coordinate phosphorus (P=C or P=P entities) along the main chain serve as interesting analogues. The first π-conjugated polymer containing P=C functional groups in a PPV-type main chain polymer was prepared recently by a condensation polymerization strategy (Scheme 40).31

O O

Cl Cl ( P OSiMe3 85oC + -2 Me SiCl Me3SiO P 3 n

(Me3Si)2P P(SiMe3)2

Scheme 40. Synthesis of poly(p-phenylenephosphaalkene)

As was mentioned in Chapter 1.3, P=C bonds are relatively nonpolar and they offer the potential to show similar conjugative ability to systems having only C=C bonds.

P=C double bonds, however, are weaker and more reactive than C=C bonds.

Phosphaalkenes can be stabilized by conjugation, steric hindrance or complexation. The most successful method, however, is to use steric bulk to protect the reactive sites.32

120

PPV and its derivatives are interesting electroluminescent materials and the incorporation of phosphorus multiple bonds into the PPV main chain may lead to new materials with their own unique electronic and optical properties. The HOMO-LUMO gap for phosphaalkenes or diphosphenes is predicted to be different from those of alkenes, and in fact has been proven by UV-vis spectroscopy and EPR investigations. 33-34 We are interested in conjugated polymers containing P=C units in the backbone of PPV structure, so-called “phospha-PPVs” (Chart 12).

alkenes phosphaalkenes

R ( P n ( n or R PPVs n PP (

poly(phosphaalkenes)

Chart 12. Phosphorus-based analogues of alkenes and PPVs

6.2 Results and discussion

Phosphaalkene polymers were synthesized via a one-pot phospha-Wittig reaction.

Phosphanylidene-σ4-phosphoranes formed in these reactions are not stable and can not be isolated, thus generated in situ by the methodology shown in Scheme 41. The only by- product, O=PMe3, is easily removed by filtration.

121

O ( + Zn, PMe3 Cl2P PCl2 Linker P P O -ZnCl2, O=PMe3 Linker )n

9 10 a-d

O O O Linker = S O O O 11a 11b Me3P P P O OC H PMe3 O 6 13 Fe

O C6H13O O 11c 11d di(phospha-Wittig) reagent

Scheme 41. Phosphaalkene polymers based on phospha-Wittig reaction

The syntheses of polymers 10a-10c were carried out by adding 1.00 equivalent of

9 over about 30 minutes to a rapidly stirred solution of 12.0 equivalents of trimethylphosphine, 2.20 equivalents of Zn dust, and 1.05 equivalents of the requisite aldehyde 11a-c in THF (Scheme 41). The resultant precipitate was collected by filtration after stirring overnight. Highly colored materials: green (10a), brick red (10b) and brown

(10c) were obtained. Efforts to dissolve the resultant solids in numerous solvents at room temperature or with heating failed to visibly dissolve any of the materials. Following the same procedure, polymer 10d was synthesized by slowly adding 9 at a fixed time interval.

The solution became orange in color after adding. The resultant mixture was stirred for an additional 8 hours followed by filtration and removal of volatiles in vacuo. The residue was taken up in hexanes and filtered, followed by removal of solvent under

122 reduced pressure. The resulting solid was rinsed with acetonitrile and dried in vacuo to yield 10d (24.8%) as an orange powder.

31P NMR spectroscopy of the soluble fraction of reaction mixtures of the first three insoluble polymerization showed only the presence of O=PMe3 that suggested that the expected coupling reaction had occurred. As mentioned above, the dialkoxy side chains are good substituents to increase the solubility for PPV derivatives.11-18 In order to increase the solubility of the polymers, dialdehyde 11d, having similar solubility groups, was used for polymer synthesis. 31P NMR spectroscopic analysis of the reaction mixture showed only trimethylphosphine oxide and two broad peaks at 273-269 ppm and 241-254 ppm which were in the range of phosphaalkenes and assigned to 10d. The two broad peaks represent internal and end group P=C units. Hexane soluble poly(p- phenylenephosphaalkene) 10d was isolated as stable orange powder in 24.8 % yield. End group analysis (1H NMR integrations of phosphaalkene versus aldehydic protons) indicated that the average degree of polymerization is 6, corresponding to a material with an average of 12 (P=C) units per chain with a modest Mn estimated at 6500 g mol-1.

The absorption maximum of 445 nm for 10d represents a substantial red shift

31 compared with the only other known poly(p-phenylenephosphaalkene) (λmax = 338 nm,

35 Z/E = 1.14) ) (Scheme 40) and a 19 nm red shift versus E-PPV (λmax = 426 nm) , despite the modest conjugation length in 10d. The absorption maximum for polymer 10d is identical to that for model compound A (Figure 36); thus, the increased color of the former material arises from the broader absorption that extends further into the visible region compared to the latter. These data can also be compared to absorption data for other structurally related diphosphaalkenes B, C and J (Chart 13) that display λmax values

123 of 349, 411 nm and 326 nm, respectively.33,36 The lower energy transition (presumably π-

π*) for C may be ascribed in part to the fact that better conjugation of the two P=C units occurs across the less hindered phenylene linker. The bulky Mes* groups in the oligomer

J make the P=C double bonds and phenyl ring linker not on the same plane and less conjugated and have high transition energy. The significant bathochromic shift (34 nm) in the π- π* transition of A compared to that of C is largely attributable to the electronic effects due to substitution of the central ring in A with strongly electron-donating alkoxy groups. Similar effects are well established in PPV systems,37 as demonstrated by the progressively lower energy π- π* transition upon comparing unsubstituted PPV (426 nm), poly(phenylenevinylene-alt- 2,5-di-n-alkoxyphenylenevinylene)s (459 nm),38 and poly-

(2,5-dimethoxyphenylenevinylene)s (474 nm).39

O P P P P

λmax = 445 nm 349

P P

CJ λmax = 411 326

Chart 13. Diphosphaalkene oligomers and their UV-vis absorption maxima

124

Figure 36. UV-vis absorption spectra of soluble polymer 10d and its oligomer A in

CHCl3

Fluorescence was observed for both 10d and A (Figure 37), with the fluorescence intensity per P=C unit in 10d observed to be a factor of 3.3 greater than that of A and approximately 8% that of E-stilbene. The weaker fluorescence intensity in 10d versus its all-carbon analogues may be due to quenching by the phosphorus lone pair, or simply a heavy atom effect. The steric bulk employed in our choice of linker also endows 10d with a good degree of air stability, with no change being observed by 31P NMR after the solid was allowed to stand open to air for one week. The steric factors in 10d thus aid thermal stability compared to poly(p-phenylenephosphaalkene) published by Gates. However, the stabilization is not complete as solutions of this material still undergo reaction with air and moisture.

125

Figure 37. Fluorescence spectra of soluble polymer 10d and its oligomer A (CHCl3) relative to (E)-stilbene

Reports on the IR spectra of phosphaalkenes are limited. Gas-phase infrared spectra of the unstable phosphaalkenes CF2=PH, CF2=PCF3, CH2=PCl have been reported by Ohno and coworkers.40b Ohno found that the C=P stretching bands are in the wide range of 840-1371 cm-1. The C=P stretching wave numbers depend on both the value of the C=P stretching force constant, which gives an intrinsic wave number of approximately 980 cm-1, and the magnitude of the coupling with nearby vibrations of the same symmetry. Some IR data for simple phosphaalkene molecules are shown in Table 3.

-1 The stretching vibration of the P=C bond (νP=C) in MesP=C(Ph)2 is at 917 cm . This assignment is supported by 13C labeling experiment.41 The vibration frequency for the

-1 -1 -1 P=C bond is at 1090cm , 1046 cm , 1015 cm for MesP=CX2, (X = Cl, Br, I), respectively.42

126

IR analysis of the polymers 10a-d was performed. IR spectra showed one strong and sharp peak at 1298 cm-1, one broad peak at 1110-1140 cm-1 for all four polymers

(Figure 38-41). A very strong peak was observed at 945-980 cm-1 for 10a and 10b, 954 cm-1 for 10c, 945 cm-1 for 10d which was tentatively assigned to the stretching vibration of the P=C bonds in the polymers. The vibration spectra for the four compounds are similar in the range of 1700-600 cm-1, which indicates that they have similar characteristic stretching vibration (P=C bonds) in the structures.

Table 3. Selected IR data of simple phosphaalkenes

-1 Phosphaalkenes νP=C (cm ) Ref.

CH2=PH 850 40a

CF2=PH 1349.5 40b

CH2=PCl 979.7 40b

CF2=PCF3 1365.3 40b

CH2=PCH=CH2 978 40c

CH2=CHCH=PH 968 40c

CH2=CHCH=PMe 968 40c

CH2=C=PMe 869 40d

Me2NC(F)=PH 1302 40e

Et2NC(F)=PH 1323 40e

Pr2NC(F)=PH 1302 40e

(CH2)5NC(F)=PH 1292 40e

127

Figure 38. IR spectrum of green phosphaalkene polymer 10a

Figure 39. IR spectrum of brick red phosphaalkene polymer 10b

128

Figure 40. IR spectrum of brown phosphaalkene polymer 10c

Figure 41. IR spectrum of orange phosphaalkene polymer 10d

129

6. 3 Synthesis of new tetraarylphenyl difunctional ligands

Synthesis of new difunctional tetraarylphenyl ligands were attempted in order to increase the solubility of the phosphaalkene polymers.

Hart and coworkers reported a convenient method for the synthesis of tetraarylbenzenes and 1,4-dibromo-2,3,5,6-tetraarylbenzenes 20 years ago.43 Thereafter, tetraarylbenzene derivatives have been utilized in many ways such as cyclometalation- resistant ligands to metal centers44 and difunctional analogues of meta-terphenyls in main group group and transition metal chemistry.45 Our group has prepared 1,4-diiodo-2,3,5,6- tetraarylbenzenes adapting Hart’s procedure which have been used as bridging ligand between two low-coordinate phosphorus centers to construct new materials.33,46-47 Herein synthesis of new 1,4-diiodo-2,3,5,6-tetraarylbenzenes is presented.

Syntheses of three new 1,4-diiodo-2,3,5,6-tetraarylbenzenes were attempted by modifying the one pot procedure published by Hart and coworkers (Scheme 42). Ligands containing floppy long alkyl chain substituents were chosen in order to increase the solubility of the materials. The Grignard reagents were prepared by the reaction of p- octyldimethylsilylbromobenzene, p-propyldimethylsilylbromobenzene or p-n-butyl- bromobenzne with activated magnesium in THF solution under nitrogen.

Hexabromobenzene was added as a solid to the requisite Grignard reagents slowly in a dry box and the mixtures were stirred at room temperature under nitrogen for 24 hours.

After quenching with iodine and workup of the mixture, only 14 was obtained in good yield (38.5%). Pure 12 and 13 could not be isolated even after distillation and column chromatography. They are viscous oily liquids, which have the p- octyldimethylsilyliodobenzene and p-propyldimethylsilyliodobenzene as impurity in 12

130 and 13, respectively, since an excess of Grignard reagent was used in order to obtain higher yield of the tetraaryldiiodobenzene. The long chain substituents also make purification of the 2,3,5,6-tetraaryl diiodobenzene more difficult.

Br Br

THF 8 ArMgBr

Si Si Si Si

I I I I I I

Si Si Si Si

12 13 14

Scheme 42. Synthesis of 1,4-diiodo-2,3,5,6-tetraarylbenzenes

Aryne cycloaddition to the aromatic ring of the arylmagnesium bromide can also introduce other impurities which were observed to be a common problem in the preparation of 2,3,5,6-tetraarylbenzene derivatives.46,48 An example of tetraarylbenzene

131 synthesis was shown in Scheme 43. These side reactions limit the yield of the desired compounds.

Br CH3 Br Br Br Mes Mes 1. THF + H3C Br + 2. H O+ Br Br 3 Mes Mes CH Br 3 Br

30-55 % 20 %

Scheme 43. Synthesis of tetramesitylbenzene via the reaction of Grignard reagent and hexabromobenzene

Compound 14 was characterized by 1H and 13C NMR spectroscopy and it is very difficult to crystallize due to the four n-butyl chains on the phenyl groups. Tetrakis(p-n- butylphenyl)-1,4-diiodobenzene was treated with n-BuLi, then proton donor to produce

2,3,5,6-tetrakis(n-butylphenyl)benzene in quantitative yield (Scheme 44).

n-BuLi H+ I I H H

Scheme 44. Reduction reaction of tetrakis(p-n-butylphenyl)-1,4-diiodobenzene

6.4 Conclusions

In summary, a soluble (E)-poly(p-phenylenephosphaalkene) with modest air stability, imparted by sterically encumbering ligands, has been prepared by a phospha-

Wittig reaction. This material exhibits a bathochromic shift with respect to unsubstituted

132

E-PPV and the only other known phosphaalkene polymer. Fluorescence study of a poly(p-phenylenephosphaalkene) showed that this material does exhibit fluorescence, though with modest intensity relative to carbon-based analogues. In addition, tetra(p-n- butylphenyl)-1,4-diiodobenzene and tetra(p-n-butylphenyl)benzene were synthesized, via the reaction of Grignard reagent with hexabromobenzene, and these compounds might be used as linkers for oligomer or polymer synthesis.

133

6.5 Experimental section

General All sensitive materials were handled in a MBraun 150 M dry box or modified

Schlenk line under dry N2. Magnesium turnings used in Grignard reagent preparation were activated by heating and stirring overnight under dry nitrogen. THF were distilled from sodium benzophenone prior to use. DMF was dried with CaH2 before using. NMR spectra were recorded on a Varian Gemini instrument operating at 300 MHz (1H), 200

MHz (1H), 121.5 MHz (31P) and 75 MHz (13C). Proton, carbon and phosphorus spectra are referenced to residual solvent signals and 85 % phosphoric acid, respectively.

Synthesis of 1,4-dibromo-2,5-bis-hexyloxybenzene

To a solution of 22.95 g (82.4 mmol) 1,4-dihexyloxybenzene in 200 mL CH2Cl2 in the 500 mL three-neck flask covered with foil was added dropwise 10.55 mL (206.0 mmol) bromine in 60 mL of CH2Cl2 by addition funnel over a period of 5.0 hours. The reaction mixture was stirred for 25 hours at room temperature. To the reaction mixture was added 10% NaOH solution, followed by 5% aqueous sodium sulfite (150 mL). The organic layer was washed three times with water. Removal of the solvent under reduced pressure yielded 30.1 g of an off-white powder (84%). m.p. 61oC. 1H NMR 200 MHz

(CDCl3): 7.09 (s, 2H), 3.95 (t, J = 6.5 Hz, 4H), 1.81 (quintet, J = 6.4 Hz, 4H), 1.50 (m,

4H), 1.35 (m, 8H), 0.91 (t, J = 6.6 Hz, 6H).

Synthesis of 2,5-bis(hexyloxy)-1,4-benzenedicarboxaldehyde

A solution of 2.20 g (5.0 mmol) 1,4-dibromo-2,5-bis-hexyloxybenzene in 10 mL

THF was added to 0.56 g (20.6 mmol) of activated magnesium turnings via cannula with stirring. The resultant yellow brown suspension was stirred at ambient temperature

134 overnight. To the yellow brown suspension was added 1.6 mL (20.0 mmol) of DMF to form a gel-like solid in an exothermic reaction. The reaction mixture was quenched with saturated NH4Cl and then extracted with diethyl ether. The mixture was washed at least 5 times with water. The volatiles were removed under reduced pressure. The residue was recrystallized from diethyl ether to yield 0.5 g yellow crystals (30 %). mp. 75-76oC.

1 H NMR 300 MHz (CDCl3): 10.53 (s, 1H), 7.44 (s, 2H), 4.10 (t, J = 6.3 Hz, 4H), 1.85 (m,

4H), 1.48 (m, 4H), 1.36 (m, 8H), 0.92 (t, J = 6.9 Hz, 6H).

Synthesis of insoluble polymers 10a-c

The syntheses of polymers 10a-10c were carried out by adding 1.00 equivalent of

9 (0.50 g) over about 30 minutes to a rapidly stirred solution of 12.0 equivalents of trimethylphosphine, 2.20 equivalents of Zn dust, and 1.05 equivalents of the requisite aldehyde 11a-c in THF. After stirring overnight the resultant precipitate was collected by filtration. Efforts to dissolve the resultant solids in numerous solvents at room temperature or with heating failed to visibly dissolve any of the materials. The impure solid materials did exhibit fascinating green (10a), brick red (10b) and brown (10c) colors, however.

Synthesis of polymer 10d

To a rapidly stirring mixture of Zn dust (0.121 g, 1.85 mmol), PMe3 (0.288 g,

3.79 mmol), and 11d (0.301 g, 0.476 mmol) in THF (10 mL) at room temperature, was added 9 (0.192 g, 0.237 mmol) as a solid in ~6 mg portions over 36 minutes, followed by addition of another portion of 9 (0.192 g, 0.237 mmol) in ~5 mg portions over 1 hour, during which time the solution becomes orange in color. The resultant mixture was stirred for an additional 8 hours followed by filtration and removal of volatiles in vacuo.

135

The residue was taken up in hexanes and filtered, followed by removal of solvent under reduced pressure. The resulting solid was rinsed with acetonitrile and dried in vacuo to

1 yield 10d (0.114 g, 24.8%) as an orange powder. H 300 MHz NMR (CDCl3): δ 0.89-

1.31 (br, 58H), 3.40-3.90 (br, 4H), 6.50-7.20 (br, 18H), 8.40-8.80 (br, 2H), 10.32 (br,

31 1 0.17H per repeat unit). P { H} NMR (CDCl3): δ 250.5 (br, internal P=C), 270.6 (s, end group P=C).

Synthesis of tetrakis(p-n-butylphenyl)-1,4-diiodobenzene

A solution of 30.92 g (145.1 mmol) 4-bromo-n-butylbenzene in 145 mL THF was added to 7.430 g (305.6 mmol) of activated magnesium turnings via cannula with stirring.

The resultant dark brown mixture was stirred at ambient temperature for 1.0 hour. This solution was then taken into a dry box, where the solution was decanted into a clean 500 mL flask (leaving excess magnesium), and to this solution was added 9.97 g (18.1 mmol) hexabromobenzene as a solid over a period of 30 mins with rapid stirring. The reaction mixture was stirred for additional 24 hours in the dry box to form a grey suspension, which was then removed from the dry box, quenched with 20.00 g (78.8 mmol) iodine and stirred for 2.0 hours. Excess iodine was consumed by addition of sodium sulfite and water. The products were extracted with diethyl ether. The organic layer was washed three times with distilled water. Addition of more diethyl ether to the organic layer precipitated a white solid, which was filtered and dried. This step was repeated two times

1 to obtain more off-white powder, 5.97 g (38.5 %). H NMR 300 MHz (CDCl3): 6.95 (d, J

= 8.4 Hz, 8H), 6.91 (d, J = 8.3 Hz, 8H), 2.50 (t, J = 7.7 Hz, 8H), 1.49 (m, 8H), 1.23 (m,

13 1 8H), 0.877 (t, J = 7.3 Hz, 12H). C { H} NMR 75 MHz (CDCl3): 146.5, 143.1, 141.3,

136

129.7, 127.3, 108.4, 35.3, 33.4, 22.1, 14.0. Anal. Calcd for C46H52I2 (858.22): C, 64.34; H,

6.10. Found: C, 64.40; H, 5.68.

Synthesis of tetrakis(p-n-butylphenyl)benzene

To a solution of 2.20 g (2.6 mmol) tetrakis(p-n-butylphenyl)-1,4-diiodobenzene in

30 mL THF was added 4.2 mL (10.5 mmol) n-BuLi (2.5 M hexanes solution) at -78 oC.

The resultant mixture was stirred for 1.0 hour at -78 oC, and then methanol (0.5 mL) was added. The reaction mixture was warmed to room temperature. The solvent was removed under reduced pressure. The residue was washed with acetonitrile and trace hexanes, followed by filteration. The solid obtaied was dried in vacuo to yield 1.34 g of an off-

o 1 white powder (98 %). m.p. 142-143 C. H NMR 200 MHz (CDCl3): 7.50 (s, 2H), 7.13 (d,

J = 8.2 Hz, 8H), 7.03 (d, J = 8.0 Hz, 8H), 2.59 (t, J = 7.6 Hz, 8H), 1.60 (qunit, J = 7.3 Hz,

8H), 1.45-1.25 (m, 8H), 0.93 (t, J = 7.2 Hz, 12H).

Synthesis of tetrakis(p-octyldimethylsilylphenyl)-1,4-diiodobenzene

A solution of 2.08 g (6.35 mmol) p-octyldimethylsilylbromobenzene in 10 mL

THF was added to 0.437 g (18.00 mmol) of activated magnesium turnings via cannula with stirring. The resultant dark brown mixture was stirred at ambient temperature for 1.0 hour. This solution was then taken into a dry box, where the solution was decanted into a clean 50 mL flask (leaving excess magnesium), and to this solution was added 0.584 g

(1.06 mmol) hexabromobenzene as a solid over a period of 5 mins with rapid stirring.

The reaction mixture was stirred for 24 hours in the dry box to form a grey suspension.

The slurry was then removed from dry box, quenched with 1.1 g (4.24 mmol) iodine and stirred for 15 minutes. Excess iodine was destroyed by addition of sodium sulfite and water. The products were extracted with diethyl ether. The organic layer was washed

137 three times with distilled water, dried with anhydrous magnesium sulfate. The solvent was removed by rotatory evaporation to give yellow-brown viscous oil. Purification using column chromatography failed to get pure tetraaryl diiodobenzene.

Synthesis of tetrakis(p-propyldimethylsilylphenyl)-1,4-diiodobenzene

A similar procedure was used to synthesize tetrakis(p-propyldimethylsilylphenyl)-

1,4-diiodobenzene. The solvent was removed by rotatory evaporation to give a yellow- brown liquid. A pink liquid was removed by distillation from the reaction mixture and the remaining liquid was purified further by column chromatography. The final product was still not pure enough from 1H NMR.

138

6.6 References

1. (a) Heeger, A. J. Angew. Chem. Int. Ed. 2001, 40, 2591. (b) MacDiarmid, A. G.

Angew. Chem. Int. Ed. 2001, 40, 2581. (c) Shirakawa, H. Angew. Chem. Int. Ed.

2001, 40, 2574.

2. (a) Brédas, J. L.; Silbey, R. (Eds.) Conjugated Polymers: The Novel Science and

Technology of Highly Conducting and Non-linear Optically Active Materials,

Kluwer Academic, Dordecht, 1991. (b) Skotheim, T. A. (Ed.) Handbook of

Conducting Polymers, Marcel Dekker, New York, 1986. (c) Kraft, A.; Grimsdale,

A. C.; Holmes, A. B. Angew. Chem. Int. Ed. 1998, 37, 402. (d) Swager, T. M. Acc.

Chem. Res. 1998, 31, 201.

3. (a) Yamamoto, T. Bull. Chem. Soc. Jpn. 1999, 72, 621. (b) Feast, W. J.; Friend, R.

H. J. Mater. Sci. 1990, 25, 3796.

4. Salaneck, W. R.; Lundstrom, I.; Haung, W. S.; MacDiarmid, A. G. Synth. Met.

1986, 13, 291.

5. Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91.

6. Nalwa, H. S. (Ed.) Handbook of Conductive Materials and Polymers, John Wiley

and Sons: New York, 1997.

7. Doi, S.; Kuwabara, M.; Noguchi, T.; Ohnishi, T. Synth. Met. 1993, 57, 4174.

8. Braun, D.; Heeger, A. J. Appl. Phys. Lett. 1991, 58, 1982.

9. Braun, D.; Heeger, A. J.; Kroemer, H. J. Electron. Mater. 1991, 20, 945.

10. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.;

Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539.

139

11. Heeger, A. J.; Braun, D. (UNIAX); WO-B 92/16023, 1992 [Chem. Abstr. 1993,

118, 157401j].

12. Wudl, F.; Allemand, P. M.; Srdanov, G.; Ni, Z.; McBranch, D. ACS Symp. Ser.

1991, 455; Wudl, F. (University of California), US-B 5189136, 1990 [Chem.

Abstr. 1993, 118, 255575p].

13. Höger, S.; McNamara, J. J.; Schricker, S.; Wudl, F. Chem. Mater. 1994, 6, 171.

14. Hwang, D.-H.; Shim, H.-K.; Lee, J.-I.; Lee, K.-S. J. Chem. Soc. Chem. Commun.

1994, 2461.

15. Zhang, C.; Höger, S.; Pakbaz, K.; Wudl, F.; Heeger, A. J. J. Electron. Mater.

1994, 23, 453.

16. Hwang, D.-H.; Kang, I.-N.; Jang, M.-S.; Shim, H.-K.; Zyung, T. Polym. Bull.

1996, 36, 383.

17. Kim, S. T.; Hwang, D.-H.; Li, X.-C.; Grüner, J.; Friend, R. H.; Holmes, A. B.;

Shim, H.-K. Adv. Mater. 1996, 8, 979.

18. Salbeck, J. Berichte der Bunsen-Gesellschaft 1996, 100, 1667.

19. (a) Deschamps, E.; Ricard, L.; Mathey, F. Angew. Chem. Int. Ed. Engl. 1994, 33,

1158. (b) Honeyman, C. H.; Foucher, D. A.; Dahmen, F. Y.; Rulkens, R.; Lough,

A. J. Organometallics 1995, 14, 5503. (c) Peckham, T. J.; Massey, J. A.;

Honeyman, C. H.; Manners, I. Macromolecules 1999, 32, 2830. (d) Kuryla, W. C.;

Papa, A. J. Flame Retardancy of Polymeric Materials Vol. 1-5, Marcel Dekker,

New York, 1973. (e) Allcock, H. R. Adv. Mater. 1994, 6, 106. (f) Manners, I.

Angew. Chem. Int. Ed. Engl. 1996, 35, 1602. (g) Ghassemi, H.; McGrath, J. E.

Polymer 1997, 38, 3139. (h) Bergbreiter, D. B.; Liu, Y. Tetrahedron Lett. 1997,

140

38, 7843. (i) Allcock, H. R.; Olmeiger, D. L.; O’Connor, J. M. Macromolecules

1998, 31, 753. (j) Kanbara T.; Takase, S.; Izumi, K.; Kagaya, S.; Hasegawa, K.

Macromolecules 2000, 33, 657.

20. Mark J. E.; Allcock, H. R.; West, R. Inorganic Polymers, Prentice Hall,

Englewood Cliffs, New Jersey, 1992.

21. Allcock, H. R.; Kugel, R. L. J. Am. Chem. Soc. 1965, 87, 4216.

22. Allcock, H. R. Chem. Eng. News 1985, 63, 22.

23. Allcock, H. R. Phosphorus-Nitrogen Compounds; Academic Press: New York,

1972.

24. Allcock, H. R. Angew. Chem. Int. Ed. Engl. 1977, 16, 147.

25. Zeldin, M.; Wynne, K. J.; Allcock, H. R. (Eds.) Inorganic and organometallic

Polymers; ACS Symposium Series 360; American Chemical Society: Washington,

DC, 1988, 250.

26. Manners, I.; Allcock, H.R.; Renner, G.; Nuyken, O. J. Am. Chem. Soc. 1989, 111,

5478.

27. McWilliams, A. R.; Dorn, H.; Manners, I. Topics in Current Chemistry, 2002,

220, 141.

28. Gates, D. P. Annu. Rep. Prog. Chem., Sect. A 2003, 99, 453.

29. (a) Jin, Z.; Lucht, B. L. J. Organomet. Chem. 2002, 653, 167. (b) Lucht, B. L.;

Onge, N. O. St. Chem. Commun. 2000, 2097.

30. Tsang, C.-W.; Yam, M.; Gates, D. P. J. Am. Chem. Soc. 2003, 125, 1480.

31. Vincent, A. W.; Gates, D. P. Angew. Chem. Int. Ed. 2002, 41, 2389.

32. Markovski, L. N.; Romanenko, V. D. Tetrahedron 1989, 45, 6019.

141

33. Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem. 2000, 39,

3860.

34. (a) Dutan, C.; Shah, S.; Smith, R. C.; Choua, S.; Berclaz, T.; Geoffroy, M.;

Protasiewicz, J. D. Inorg. Chem. 2003, 42, 6241. (b) Jouaiti, A.; Geoffroy, M.;

Terron, G.; Bernardinelli, G. J. Chem. Soc., Chem. Commun. 1992, 155. (c)

Jouaiti, A.; Geoffroy, M.; Terron, G.; Bernardinelli, G. J. Am. Chem. Soc. 1995,

117, 2251. (d) Al Badri, A.; Jouaiti, A.; Geoffroy, M. Magn. Reson. Chem. 1999,

37, 735. (e) Geoffroy, M.; Jouaiti, A.; Terron, G.; Cattani-Lorente, M.; Ellinger,

Y. J. Phys. Chem. 1992, 96, 8241. (f) Cetinkaya, B.; Hudson, A.; Lappert, M. F.;

Goldwhite, H. J. Chem. Soc., Chem. Commun. 1982, 609. (g) Bard, A. J.; Cowley,

A. H.; Kilduff, J. E.; Leland, J. K.; Norman, N. C.; Palkuvski, M. J. Chem. Soc.,

Dalton Trans. 1987, 249. (h) Culcasi, M.; Grouchi, G.; Escudie, J.; Couret, C.;

Pujol, L.; Tordo, P. J. Am. Chem. Soc. 1986, 108, 3130.

35. Simmons, W. W. The Stadtler Handbook of Ultraviolet Spectra; Stadtler

Research Laboratories: Philadelphia, 1979.

36. Kawanami, H.; Toyota, K.; Yoshifuji, M. J. Organomet. Chem. 1997, 535, 1.

37. (a) Askari, S. H.; Rughooputh, S. D.; Wudl, F. Synth. Met. 1989, 29, E129. (b)

Han, C.-C.; Elsenbaumer, R. L. Mol. Cryst. Liq. Cryst. 1990, 189, 183. (c) Sonoda,

Y.; Kaeriyama, K. Bull. Chem. Soc. Jpn. 1992, 65, 853. (d) Sarnecki, G. J.; Burn,

P. L.; Kraft, A.; Friend, R. H.; Holmes, A. B. Synth. Met. 1993, 55-57, 914. (e)

Hamaguchi, M.; Yoshino, K. Jpn. J. Appl. Phys. 1994, 33, L1478. (f) Dehong, H.;

Yu, J.; Barbara, P. F. J. Am. Chem. Soc. 1999, 121, 6936.

38. Bao, Z.; Chen, Y.; Cai, R.; Yu, L. Macromolecules 1993, 26, 5281.

39. Jin, J.-I.; Kim, J.-C.; Shim, H.-K. Macromolecules 1992, 25, 5519.

142

40. (a) Pellerin, B.; Guenot, P.; Denis, J. M. Tetrahedron Lett. 1987, 28, 5811. (b)

Ohno, K.; Kurita, E.; Kawamura, M.; Matsuura, H. J. Am. Chem. Soc. 1987, 109,

5614. (c) Guillemin, J. C.; Cabioch, J. L.; Morise, X.; Denis, J. M.; Lacombe, S.;

Gonbeau, D.; Pfister-Guillouzo, G.; Guenot, P.; Savignac, P. Inorg. Chem. 1993,

32, 5021. (d) Guillemin, J. C.; Janati, T.; Denis, J. M.; Guenot, P.; Savignac, P.

Tetrahedron Lett. 1994, 35, 245. (e) Grobe, J.; Le Van, D.; Lüth, B.; Hegemann,

M. Chem. Ber. 1990, 123, 2317.

41. Van der Knaap, T. A.; Klebach, T. C.; Bickelhaupt, F. Tetrahedron 1984, 40, 765.

42. Boldeskul, I. E.; Tukhvatullin, F. Kh.; Ismailov, Sh. S. Ukrains'kii Fizichnii

Zhurnal 2000, 45, 662.

43. Harada, K.; Hart, H.; Du, C.-J. F. J. Org. Chem. 1985, 50, 5524.

44. (a) Lockwood, M. A.; Fanwick, P. E.; Eisenstein, O.; Rothwell, I. P. J. Am. Chem.

Soc. 1996, 118, 2762; (b) Lockwood, M. A.; Clark, J. R.; Parkin, B. C.; Rothwell,

I. P. Chem. Commun. 1996, 1973; (c) Deacon, G. B.; Fanwick, P. E.; Gitlits, A.;

Rothwell, I. P.; Skelton, B. W.; White, A. H.; Eur. J. Inorg. Chem. 2001, 1505.

45. (a) Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organomet. Chem. 1999, 1.

(b) Clyburne, J. A. C.; McMullen, N. Coord, Chem. Rev. 2000, 210, 73.

46. Shah, S.; Eichler, B. E.; Smith, R. C.; Power, P. P.; Protasiewicz, J. D. New J. Chem.

2003, 27, 442.

47. Smith, R. C.; Chen, X.; Protasiewicz, J. D. Inorg. Chem. 2003, 42, 5468.

48. Hart, H.; Du, C.-J., F.; Mohebalian, J. J. Org. Chem. 1988, 53, 2720.

49. Balavoine, G. G. A.; Doisneau, G.; Fillebeen-Khan, T. J. Organomet. Chem. 1991,

412, 381.

143

Chapter 7. Synthesis and Characterization of Phosphaalkenes and Phosphaalkynes

7.1 Introduction

7.1.1 General introduction of phosphaalkynes

Multiple bonds involving phosphorus have been of great interest in the past three

decades. Due to low stability of low-coordinate phosphorus compounds, sterically

demanding substituents have been used to kinetically stabilize unsaturated phosphorus

compounds. Phosphaalkynes have played an important role in low-coordinate phosphorus

chemistry. The first phosphaalkyne, HC≡P was discovered by Gier in 1961 through the

1 decomposition of PH3 in an electric arc between graphite electrodes. The chemistry of

phosphaalkynes has been well developed since the first stable phosphaalkyne, t-BuC≡P,

was reported in 1981 by Becker.2 Many organophosphorus and phospha-organometallic

and coordination complexes have been prepared using it as a starting material.3 There is no evidence for the formation of its isomers, free isophosphaalkynes so far.4 Their

nitrogen homologues, cyanides (R-C≡N) and isocyanides (R-N≡C) are very useful and versatile organic reagents (Chart 14).

R CC R alkynes

R CN R NC nitriles isocyanides

R CP

phosphaalkynes

Chart 14. Isoelectronic compounds featuring C, N, P triple bonds

144

Heavier main group elements have orbital non-hybridization which has been mentioned in Chapter 1.2. Although they have smaller energy separation between s and p orbitals, the radius for p is much bigger than that of s. The overlap between s and p obitals is too small to form strong hybrid orbitals. Obital non-hybridization plays an important role in phosphorus chemistry. Its inert s orbital prefers to have two nonbonding electrons as shown in linear white phosphorus (:P≡P:). This bonding situation also exists in phosphaalkynes (R-C≡P:), while in isophosphaalkynes (R-P≡C), phosphorus is forced to participate in bonding with the neighboring carbon atom and undergo unfavorable sp hybridization. This may give qualitative explanation for the instability of isophosphaalkynes. Quantum chemical calculations for the HP≡C and HC≡P systems supported this explanation and indicated that they correspond to the energy maximum and energy minimum, respectively, on the energy surface of the singlet ground state. The energy difference between HP≡C and HC≡P is 83.9 kcal/mol5 and that between HC≡N

and HN≡C is only 14 kcal/ mol by calculation6.

Free isophosphaalkynes have been proposed to be intermediates for the formation

of phosphaalkynes. However, there is still no evidence to support the existence of free

isophosphaalkynes during the reactions.7-10 Transition metal coordinated phosphaalkyne

(C≡P-R) and cyaphide [µ-C≡P]- complexes have been structurally characterized11 and it has been demonstrated that steric protection is not necessary for the coordinated cyaphide ion. Another thing to mention is that the chemistry of phosphaalkynes is close to alkynes

rather than cyanides based on the basis of their polarity, their MO sequences and their

reactivity.

145

7.1.2 Synthesis of phosphaalkynes

O O

+ P(SiMe ) C C 3 3 R P(SiMe ) R Cl 3 2

SiMe Me3SiO 3 Me3SiO C P C P R R SiMe 3 -Me3SiOSiMe3

RC P

Scheme 45. Synthesis of phosphaalkynes by elimination of hexamethyldisiloxane

The kinetically stable phosphaalkyne, t-BuC≡P, was synthesized by the elimination reaction of hexamethyldisiloxane from suitably substituted phosphaalkenes

(Scheme 45).2 This approach was generalized and optimized by Regitz et al.12 and was

routinely used for the preparation of phosphaalkynes. Table 3 shows selected

phosphaalkynes synthesized using this method.3b Many other methods have been reported

for the formation of phosphaalkynes in the review paper.3b

Table 3. Selected phosphaalkynes synthesized by elimination of hexamethyldisiloxane

and their physical properties

Phosphaalkyne Yield/ % 31P NMR / ppm C≡P bond length/Å Ref. t-Bu-C≡P 76, 96 -69.2 1.536 (2) 2,13,14 Ad-C≡P 83 -66.9 - 15 Mes- C≡P 43 2.5 - 16 Mes*-C≡P 10-15 34.4 1.516(13) 17, 18 P≡C(Tript)C≡P 67 -15.7 1.532 19

146

Ad Mes* Tript

Phosphaalkynes with C≡P bond directly connected to aryl substituents are very limited, only two examples, MesC≡P and Mes*C≡P being known. Mesityl phosphaalkyne was synthesized using the approach shown in Scheme 45, which is a colorless viscous oil and decomposes at room temperature. Mes*C≡P is the most studied arylphosphaalkyne.

7.1.3 Reactivity and coordination chemistry of phosphaalkynes

The C≡P bond is usually polarized due to higher electronegativity of carbon than phosphorus (2.5 vs 2.2): Cδ--Pδ+. Protonation reaction of phosphaalkynes occurs exclusively at the carbon center.20 They react with nucleophiles to form phosphaalkenes and 1,3-diphosphabutadienes.18 Addition reaction is a common feature for alkynes. The addition behavior of the C≡P bond has been investigated as well. 1,2-Addition of phosphaalkynes with halogen compounds, organolithium derivatives, enophiles or Sn-H,

Zr-H bonds, and cycloaddition reactions of phosphaalkynes including [2+1], [2+2], [2+3],

[2+4] and homo Diels-Alder reactions have been reported in a review paper.3b Last year, a supplementary review was published on the same topic.21

Phosphaalkynes can utilize both the triple bond and the phosphorus lone pair electrons to form a variety of inorganic and organometallic compounds. The possible coordination modes in mono- and dinuclear metal systems have been established for C≡P

147 triple-bond compounds (Chart 15). More details for the ligation behaviors of phosphaalkynes were given in the review papers of Nixon.3a, 3d

M RRR P P C C C M M M M M M C C P P P

R R M M A B CD E

Chart 15. The coordination modes for the phosphaalkynes

Side-on η2 coordination mode dominates in the coordination chemistry of phosphaalkynes due to the π-type of HOMO and higher π-n separation than in the corresponding nitrile.22 It is also in agreement with the high ionization energy of the phosphorus lone pair: In H-C≡P, first ionization potential (π MO), 10.79 eV, second

23 ionization potential (nσ MO) 12.86 eV; in t-BuC≡P, first ionization potential (π MO),

24 1 9.70 eV, second ionization potential (nσ MO) 11.45 eV. End-on η coordination can be achieved in certain complexes with narrow coordination sites which only allow ligation of linear molecules.25 Phosphaalkyne molybdenum complexes with η1 coordination are

1 shown in Scheme 46 and trans-[Mo(η -P≡C-Ad)2(Et2PPCH2CH2PPEt2)2] has been structurally characterized. The C≡P bond distance is not elongated compared with average values of 1.540(4) Å observed in the free RC≡P ligands (R = H, F, Me, or t-Bu) and with side-on complexes. 3a, 3b, 3d, 23, 26 In the η1-phosphaalkyne iron (II) complex, the

C≡P bond is further shortened (1.512(5) Å).25d

148

N C R'2 R'2 R'2 R'2 P N P P P P 2 RC P M M -N2 P N P P P P R' R'2 R' R' 2 N 2 C 2 R

MRR'

Mo t-Bu Et

Mo t-Bu Ph

Mo t-Bu p-Tolyl

Mo Ad Et

W t-Bu Ph

Scheme 46. η1 coordination complexes of phosphaalkynes

Phosphaalkynes bearing aryl substituents have special electronic properties in comparison with alkyl phosphaalkynes due to conjugation between C≡P and aromatic systems without considering the orientations of the substituents (See Chapter 1, 4.2). m-

Terphenyl groups have been proven to be good protective groups for stabilization of low- coordinate phosphorus compounds.27, 28 They must be also good protective groups to stabilize molecules containing C≡P triple bond. Synthesis and characterization of new phosphaalkenes and phosphaalkynes are discussed in this chapter.

149

7.2 Results and Discussion

7.2.1 Synthesis and characterization of new m-terphenyl phosphaalkenes and phosphaalkynes

Bromide has been reported to be a better leaving group than chloride during the

29 elimination of LiX from lithium carbenoids. ArP=CBr2 (Ar = Dmp, 2,6-dimesitylphenyl;

Dxp, 2,6-di(m-xylyl)phenyl) were chosen as precursors to convert to molecules containing C≡P triple bond, which were synthesized using the method for the synthesis of

30 Mes*P=CBr2.

To a solution of ArPCl2 and CBr4 in THF was added a solution of n-BuLi in hexanes (2.5 M) at -110 oC. The resultant solution was stirred for 30 minutes, followed by warming to room temperature slowly. After the solvent was evaporated, the residue was extracted with hexanes and the extract was filtered. Removal of the solvent under reduced pressure followed by recrystallization from hexanes yielded pale yellow and white crystalline solids of pure DmpP=CBr2 and DxpP=CBr2 with yields of 60 % and 55

% , respectively (Scheme 47).

31 The P NMR chemical shifts of ArP=CBr2 are δ 271.5 and 270.3 ppm in CDCl3

31 30 which are very close to that of Mes*P=CBr2 ( P NMR: δ 271) (Figure 42, 44). In the

1H NMR spectra, broad peaks were observed for the o-methyl protons and CH aryl protons in mesityl or m-xylyl groups (Figure 43, 45). The broad resonances for o-methyl carbons and aryl CH carbons in pendant rings were also observed in 13C NMR. This suggests moderate rotation of the (Ar)C-P bonds in the solution. The molecule of

ArP=CBr2 have C2 symmetry axis along the center of the central phenyl ring and phosphorus. Rotation 180o along the symmetry axis make the protons move to their

150 corresponding positions: a →b’, b →a’, c →c’, d →e’ and e →d’, vice versa (Chart 16).

Protons in a and b, a’ and b’, d and e, d’ and e’ are equal. If the rotation rate is slow, three methyl proton resonances and two aromatic proton resonances in mesityl groups can be observed; if the rotation rate is moderate, one broad and one sharp methyl proton resonances and one broad aromatic proton resonances in mesityl groups can be observed; if the rotation is very fast, two sharp methyl proton resonance and one sharp aromatic proton resonances in mesityl groups can be observed. So the 1H NMR spectra of

ArP=CBr2 belong to the second case.

b' a a' b d e' P d' P e c o c' e C c' C rotation 180 d' c b Br Br b' e' a' Br Br a d

Chart 16.

-110 oC ArPCl2 + CBr4 + 2 n-BuLi ArP=CBr2 + 2 n-BuBr + 2 LiCl THF

Ar =

Dmp Dxp

31 P NMR {CDCl3}: δ = 271.5 ppm δ = 270.3 ppm

Scheme 47. Synthesis of DmpP=CBr2 and DxpP=CBr2

151

31 Figure 42. P NMR spectrum of DmpP=CBr2 in CDCl3

1 Figure 43. H NMR spectrum of DmpP=CBr2 in CDCl3 (Signals indicated by * are due to hexanes molecule from the solvent)

152

31 Figure 44. P NMR spectrum of DxpP=CBr2 in CDCl3

1 Figure 45. H NMR spectrum of DxpP=CBr2 in CDCl3

31 The reaction of DmpP=CBr2 with zinc dust in THF produced DmpC≡P ( P NMR:

δ -10) and the other product (31P NMR: δ 144) in almost 1:1 ratio after 1.0 hour as indicated by the 31P NMR spectrum with some starting material remaining. After the

153 mixture was heated at 90 oC for 13 hours, the 31P NMR spectrum showed one singlet at

144 ppm and a doublet at 88 ppm (J = 221 Hz) (Figure 46).

31 Figure 46. P NMR spectrum for the reaction of DmpP=CBr2 with zinc in THF

A phosphaalkyne was indeed observed during the reactions. However, it is not stable under these conditions and reacted further to form other products. Magnesium was commonly used as the reductant for the generation of diphosphenes.31 The reduction of dibromophosphaalkenes with magnesium turnings was studied.

A round-bottom flask charged with DmpP=CBr2, excess magnesium turnings and

THF was agitated under ultrasound for 2.0-4.0 hours, at which time the solution became dark brown. Once the reaction was complete, the flask was taken into a dry box, where the solvent was removed under reduced pressure and the residue was extracted with hexanes and filtered. Removal of the volatiles under reduced pressure followed by recrystallization from hexanes yielded a pale yellow solid of DmpC≡P (40%) (Scheme

48). DxpC≡P can be synthesized following the same procedure with a yield of 27%.

154

sonication ArP CBr2 + Mg ArC P + MgBr2 2.0 h

Ar =

Dmp Dxp

31 P NMR (CDCl3): δ = −8.9 ppm δ = −8.5 ppm

Scheme 48. Synthesis of DmpC≡P and DxpC≡P

31P NMR spectra of DmpC≡P and DxpC≡P display a singlet (δ -8.9, -8.5) in

3c 1 CDCl3 (Figure 47, 49) which is in the normal region for phosphaalkynes. The H NMR spectrum of DmpC≡P is very similar to DmpI (Figure 48). In addition, an alkynic carbon resonance appears as a singlet at δ 107.6 ppm in its 13C NMR spectrum. This is quite different from the reported value for the alkynic carbon resonance of P≡CC(Tript)CC≡P,

1 19 which appears as a doublet at δ 164.0 ( JCP = 47.1 Hz).

155

31 Figure 47. P NMR spectrum of DmpC≡P in CDCl3

1 Figure 48. H NMR of DmpC≡P in CDCl3 (Signals indicated by * are due to hexanes molecule from the solvent)

156

31 Figure 49. P NMR of DxpC≡P in CDCl3

7.2.2 X-ray crystal structure analysis of DmpC≡P

Figure 50. The X-ray crystal structure of DmpC≡P

Yellow crystals of DmpC≡P suitable for X-ray analysis were grown in THF via slow evaporation (Figure 50). The C≡P triple bond length is 1.507(3) Å in DmpC≡P and is comparable with the distances of 1.516(13) Å in Mes* C≡P,18 1.548(1) Å in t-Bu

C≡P,32 and average 1.532 Å in P≡C(Trip)C≡P.19 P(1), C(1), C(2) are on the same line

157

(180o) with the angle P(1), C(1), C(2), C(3) of 129o. The aromatic rings are almost planar and two mesityl groups are perpendicular to the central phenyl ring.

7.2.3 Reactivity of dibromophosphaalkenes and phosphaalkynes

DmpP=CBr2 reacted with one equivalent of PdCl2(PhCN)2 in THF solution in a dry box at room temperature. The reaction was monitored by 31P NMR spectroscopy which showed a singlet at δ 232.5 ppm and a doublet at δ 2.7 ppm in 30 minutes (Figure

31 51). The P chemical shift of trivalent phosphorus in trans-(Br)(PEt3)2Pt[C(=PMes*)Br], trans-(Cl)(PEt3)2Pt[C(=PMes*)Cl] and cis-(Cl)(PPh3)2Pt[C(=PMes*)Cl] are at δ 234.2,

223.3 and 234.6 ppm respectively,33 which indicate that the compound characterized by the peak at 232.5 ppm might have similar structure.

31 Figure 51. P NMR spectrum for the reaction of DmpP=CBr2 with PdCl2(PhCN)2

Two molybdenum dinitrogen complexes, Mo(N2)2(PPh2CH2CH2PPh2)2 and

34 Mo(N2)2(PPh2Me)4 were synthesized following the published procedure. To a solution of DmpC≡P in THF was added slowly a solution of Mo(N2)2(PPh2CH2CH2PPh2)2 in THF in a dry box at room temperature. The mixture was transferred into a NMR tube and taken out of the dry box, then agitated under ultrasound for 8.0 hours, at which time the color became bright red. 31P NMR spectrum showed mainly phosphaalkyne, free

158 bis(diphenylphosphino)ethane and small broad peaks at 37 and 31 ppm. The same reaction was conducted in toluene solution and there was still some starting material left after agitating under ultrasound for 8.0 h. After reacting under ultrasound overnight, the color changed to brown. 31P NMR showed mainly phosphaalkyne, free bis(diphenylphosphino)ethane and a new compound at 26 ppm. The molybdenum complex might have decomposed. If the steric effect of this reaction is considered, it is possible that the bulky phenyl substituents in the molybdenum complex and the mesityl groups in Dmp unit make coordination difficult.

In order to decrease steric interactions, Mo(N2)2(PPh2Me)4 was chosen to react with DmpC≡P. Mo(N2)(PPh2Me)4 is not stable in solution and decomposes slowly if the solvent is not sufficiently pure. To a pale yellow solution of two equivalents of DmpC≡P in toluene was added one equivalent of Mo(N2)2(PPh2Me)4 in toluene dropwise in a dry box at room temperature. The solution was quickly changed from yellow to orange, then

31 dark red with the addition of Mo(N2)(PPh2Me)4. However, P NMR spectroscopy showed only phosphaalkyne (-8.0 ppm) and free PPh2Me (-26 ppm). The same result was observed if THF was used as solvent. The same reaction was conducted in toluene at -78 oC, as there was no color change at that temperature. When the reaction mixture was slowly warmed up, the color gradually changed from orange to red. It became dark red very quickly with increasing temperature. 31P NMR spectrum was taken immediately after it was removed from the cold bath. A broad multiple resonance at 19 ppm and a triplet at 52 ppm were observed which were tentatively assigned to the molybdenum

31 diphosphaalkyne complex. For trans-Mo(Et2PCH2CH2PEt2)2(AdC≡P)2, P NMR chemical shifts are at 10.0 ppm (quint) and 55.0 ppm (triplet). The dark red solution

159 became brown when it was maintained at -7 oC for above two days. 31P NMR spectrum indicated the disappearance of all the peaks that was previously observed and only a small peak at 34.5 appeared. Further experiments will be conducted to confirm and isolate phosphaalkyne complexes.

7.3 Proposed mechanism for the formation of phosphaalkynes

So far, the mechanisms for the formation of phosphaalkynes are unclear. The reported possible mechanisms for the formation of phosphaalkynes are summarized in the following paragraphs.

E-2-chloro-1-(2,4,6-tri-t-butyl/pentylphenyl)-1-phosphaethylene reacts with s- or t-butyllithium at -78 oC to form phosphavinylidene carbenoids which convert to phosphaalkynes upon warming to room temperature (Scheme 49).8c, 9 Free isophosphaalkynes are assumed to be intermediates for the formation of phosphaalkynes.

The transient isophosphaalkynes rearrange very quickly via migration of the Ar group from phosphorus to carbon to form their stable isomers, phosphaalkynes. However, no intermediate was detected by 31P NMR spectroscopy even at low temperature. A similar reaction was observed when Mes*P=C(Br)Cl was a precursor.7

Ar H s or t-BuLi Ar Li O PC PC ArP C ArC P Cl Cl

Scheme 49. Synthesis of a phosphaalkyne via lithium carbenoid

The reaction of Mes*P=CCl2 with Pd(PPh3)4 at room temperature gave phosphaalkyne in 87 % yield together with the formation of PdCl2(PPh3)2 (Scheme 50). It was proposed to be a multi-step reaction with transient formation of isophosphaalkyne which rearranged very quickly to form phosphaalkyne.10

160

(PPh3)3 *Mes Pd Cl *Mes *Mes PC + Pd(PPh3)4 Cl Pd(PPh3)2Cl Cl -PPh PC -PPh PC 3 Cl 3 Cl

ArP CO ArC P -Pd(PPh3)2Cl2

Scheme 50. Synthesis of phosphaalkyne via transition metal complex

Angelici and co-workers studied the mechanism of the transition metal (Pd, Pt) promoted conversion of RP=CX2 (X= Cl and Br) to RC≡P, in which non-aromatic intermediate was isolated and characterized by X-ray analysis (Scheme 51). No free

33,35 isophosphaalkyne is involved in this transformation. So the reaction of Mes*P=CCl2 with Pd(PPh3)4 cannot involve isophosphaalkyne as intermediate as shown in Scheme 55.

- - X L X X L L M L CP X M CP X M CP X R L L

X X L L P L P X M X + CP X M C X M C L L L

(Isolated intermediate) M = Pt, Pd

L = PEt3

X = Cl, Br

Scheme 51. The mechanism for the reaction of transition metal complex with RP=CX2

The Fritsch-Buttenberg-Wiechell rearrangement36 of β-aryl-substituted vinylic halides is the first reaction of carbenoid that provides a synthetic approach to alkynes by

1,2-migration of an aryl group which is trans to the halide (Scheme 52). 37-40

161

Ar H Ar M + BaseM Ar' Ar - BaseH -MX Ar' X Ar' X

Scheme 52. The Fritsch-Buttenberg-Wiechell rearrangement

In the Fritsch-Buttenberg-Wiechell rearrangements, (R’-substituted) aryl group migrates through a three-membered ring intermediate without complete elimination of lithium halide, finally leading to alkyne (Scheme 53).40

R' R'

Li δ +

Cl RLiCC δ −Cl R

-LiCl RR'CC

R, R' = H, CH3, OCH3

Scheme 53. An example of Fritsch-Buttenberg-Wiechell rearrangement

Based on the above studies, the mechanism for formation of m-terphenyl phosphaalkynes was postulated through a Fritsch-Buttenberg-Wiechell type rearrangement (Scheme 54). Magnesium inserts into the C-Br bond to form a carbenoid.

Bromide departure is assisted by migration of the phenyl group from phosphorus to carbon to form a three-membered ring intermediate with positive charge on the phenyl ring which could be stabilized through conjugation with mesityl groups. The P-C(Ar) bond breaks with the loss of MgBr2 to form the phosphaalkyne. This process circumvents the formation of high energy free isophosphaalkyne intermediate.

162

R R R R

Mg P δ − P P δ + Br C P sonication C -MgBr2 CBr CBr MgBr Br MgBr

R R R R

R = CH3, H

Scheme 54. The possible mechanism for the formation of m-terphenyl phosphaalkynes

1H NMR spectra indicate that the (Ar)C-P bond in the dibromophosphaalkene can rotate in the solution at room temperature. On the other hand, the mesityl groups on the

Dmp unit might conjugate partially with the central phenyl ring to stabilize the cation intermediate. There were no other products detected by 31P NMR spectroscopy except phosphaalkenes and phosphaalkynes during the reaction.

7.4 Synthesis of Br2C=PC6(p-t-BuPh)4P=CBr2

Diphosphaalkynes could be useful building blocks for the synthesis of organophosphorus and coordination polymers. P≡C(Tript)C≡P is the first diphosphaalkyne reported by Jones in 2003 (Chart 17).19 It was synthesized by elimination of hexamethyldisiloxane from corresponding diphosphaalkene by strong base

(KOH) in dimethyl ether (DME).

163

P C

C P

Chart 17. The first diphosphaalkyne

2,3,5,6-Tetrakis(t-butylphenyl)phenyl is good bulky ligand for the formation of phosphalkenes and diphosphenes.28,41 Phosphaalkene polymers have been constructed using it as building block.42 The synthesis of a new diphosphaalkyne has been attempted using this framework as protecting ligand.

Bis(dibromophosphaalkene), Br2C=PC6(p-t-BuPh)4P=CBr2 was synthesized

31 following the same procedure as DmpP=CBr2 (Scheme 55). The P NMR spectrum showed two singlets (δ 269.0 and 268.5) in THF solution (Figure 52) which are assumed to be cis and trans isomers of the diphosphaalkene (Chart 18). If this is the case, there would be four pairs of doublets for the aromatic protons and four singlets for t-butyl

1 1 groups in the H NMR spectrum. The H NMR spectrum in CDCl3 shows broad peaks for aromatic protons and two singlets (δ 1.23, 1.22) for methyl protons of the t-butyl groups, and it is presumed that some peaks overlap. Integration of the aromatic peaks and methyl peaks indicates that the ratio of aromatic protons to methyl protons is 8:18 which fits very well with the proposed structures (16:36) (Figure 53). Broad resonances for aromatic carbons were also observed in the 13C NMR spectrum.

164

o -110 C Br2C=P P=CBr2 Cl2P PCl2 + n-BuLi + CBr4

Scheme 55. Synthesis of Br2C=PC6(p-t-BuPh)4P=CBr2

31 Figure 52. P NMR spectrum of Br2C=PC6(p-t-BuPh)4P=CBr2 in CDCl3

165

1 Figure 53. H NMR spectrum of Br2C=PC6(p-t-BuPh)4P=CBr2 in CDCl3

Br2C P P P P

CBr2 Br2C CBr2

trans cis Chart 18.

Bisdibromophosphaalkene, Br2C=PC6(p-t-BuPh)4P=CBr2 reacted with magnesium in THF under ultrasound irradiation for 2.0-4.0 hours, at which time the color changed from pale yellow to dark brown. However, no phosphorus signal was observed

166 for the reaction mixture monitored by 31P NMR spectroscopy. A similar phenomenon was observed when the same substrate was reacted with n-BuLi in THF solution at -78 oC

(Scheme 56).

ultrasound No phosphorus signal observed! Br2C=P P=CBr2 + Mg

o Br C=P P=CBr -78 C No phosphorus signal observed! 2 2 + n-BuLi

Scheme 56. The reaction of Br2C=PC6(p-t-BuPh)4P=CBr2 with magnesium or n-BuLi

Mes* MgX Cl 2 Mes* Cl P P o Li THF, -110 C MgX

0.5 MgX Mes* 2 Cl P o THF, -110 C Mg

Cl Mes* Scheme 57. The transmetallation reaction of a lithium carbenoid

167

Bickelhaupt studied the transmetallation reactions of the (phophaalkenyl)lithium carbenoid to magnesium carbenoid and found that magnesium carbenoids are not stable and slowly decompose when the temperature is raised to 15 oC (Scheme 57). Neither phosphaalkynes nor any other phosphorus-containing product could be detected in the 31P

NMR spectrum during this process. The products of decomposition are unknown.30

Trapping reactions to deduce the transient intermediate and final products failed.

New diphosphaalkyne systems could be developed using a longer chain and bulky ligand (Chart 19). This bridging ligand has been used to construct phosphalkene polymers.43 The expected diphosphaalkyne compound could be synthesized following the similar procedure as for m-terphenyl phosphaalkynes.

O C P

P C O

Chart 19. A conjugated diphosphaalkyne

7.5 Conclusions

New phosphaalkenes and phosphaalkynes containing m-terphenyls have been synthesized and characterized. The reaction of phosphaalkynes with molybdenum dinitrogen complexes has been explored and it was found that the coordination of

168

Mo(N2)2(PPh2CH2CH2PPh2)2 with DmpC≡P was prevented by steric bulk. However, the reaction of Mo(N2)2(PPh2Me)4 with DmpC≡P is very fast at room temperature and the metal complexes formed in this reaction are not stable and easily decompose at room temperature. The mechanism for the formation of phosphaalkynes was also discussed. A new bis(dibromophosphaalkene) was synthesized and its reaction with magnesium and n-

BuLi failed to generate diphosphaalkyne. Its decomposition products are not yet characterized. m-Terphenyls are good protective groups for the formation of phosphaalkynes. However, they are not bulky enough to allow the formation of isophosphaalkynes.

169

7.6 Experimental Section

General All manipulations were performed in a MBraun 150 M dry box or Schlenk line under dry N2. Acetonitrile was distilled from CaH2 and THF and hexanes were distilled from sodium benzophenone prior to use. NMR spectra were recorded on a Varian Gemini instrument operating at 300 MHz (1H), 121.5 MHz (31P) and 50.3 MHz (13C). Proton, carbon and phosphorus spectra are referenced to residual solvent signals and 85 % phosphoric acid, respectively.

X-ray Crystal structure was solved by Professor Tong Ren and his student

Weizhong Chen at Miami University. Here is the experimental section for solving the structure: The X-ray intensity data were measured at 300 K on a Bruker SMART 1000

CCD-based X-ray diffractometer system equipped with a Mo-target X-ray tube (λ =

0.71073 Å) operated at 2000 watts power. The detector was placed at a distance of 4.997 cm from the crystal. Used for X-ray crystallographic analysis is a light yellow block of approximate dimensions 0.13x 0.10 x 0.04 mm3 wedged in a 0.1mm quartz capillary filled with deoxygenated mineral oil. Data were measured using omega scans of 0.3o per frame for 20 seconds such that a hemisphere was collected. A total of 1271 frames were collected with a final resolution of 0.75 Å. No decay was indicated by the recollection of the first 50 frames at the end of data collection. The frames were integrated with the

Bruker SAINT© software package using a narrow-frame integration algorithm, which also corrects for the Lorentz and polarization effects. Absorption correction was not applied. The structure was solved and refined in the space group Fdd2 using the Bruker

170 independent molecule. All non-hydrogen atoms were derived from the direct method solution. With all non-hydrogen atoms being anisotropic and all hydrogen atoms being isotropic the structure was refined to convergence by least squares method on F2,

SHELXL-93, incorporated in SHELXTL.PC V 5.03.

Synthesis of 2,6-(Mes)2C6H3P=CBr2

A 100 mL of round bottom flask was charged with 0.6694 g (1.6 mmol) DmpPCl2,

0.5345 g (1.6 mmol) CBr4, 10 mL THF and a magnetic stir bar in a dry box. The solution was removed from the dry box, chilled to -110 oC. To the solution was added a solution of n-BuLi in hexanes (1.3 mL, 2.5 M, 3.22 mmol) via syringe with stirring. The mixture was stirred at -110 oC for 30 minutes and then warmed to room temperature slowly to form a pale yellow solution. The solution was taken into a dry box, where the solvent was removed under reduced pressure and the residue was extracted with n-hexane and filtered.

Evaporation of the solvent from the filtrate followed by recrystallization of the residue

o from hexanes at -32 C yielded 0.500 g pale yellow crystalline solid of pure DmpP=CBr2

o 1 (60.1 %). m.p. 121 C (dec.). H NMR 300 MHz (CDCl3): δ 7.57 (t, J = 7.8 Hz, 1H), 7.19

13 1 (dd, JHH = 7.5 Hz, JPH = 1.2 Hz, 2H), 6.95 (s, 4H), 2.34 (s, 6H), 2.17 (br s, 12H). C { H}

NMR (CDCl3): δ 144.5 (s), 144.3 (s), 137.2 (s), 136.8 (d, J = 2.8 Hz), 130.2 (s), 129.5 (s),

31 128.8 (s), 127.9 (s), 126.2 (s), 21.3 (br s), 21.1 (s). P (CDCl3): δ 271.5. Anal. Calcd for

C25H25PBr2 (516.26): C, 58.16; H, 4.88. Found: C, 58.55; H, 4.92.

Synthesis of 2,6-(Mes)2C6H3C≡P

A 50 mL flask was charged with 0.3302 g (0.64 mmol) DmpP=CBr2 and 0.1450 g

(6.0 mmol) magnesium turnings and 10 mL THF in a dry box. The mixture was removed from the dry box and agitated under ultrasound for 4.0 hours, at which time, the solution

171 became dark brown. The mixture was moved into a dry box, where the solvent was removed under reduced pressure and the residue was extracted with n-hexnae and filtered to give brown filtrate. Evaporation of the solvent from the filtrate followed by recrystallization from hexane at -32 oC yielded yellow crystals of pure DmpC≡P (0.100 g,

43.8 %). Crystals suitable for X-ray analysis were grown from THF solution of DmpC≡P

o 1 via slow evaporation. m.p. 172 C. H NMR 300 MHz (CDCl3): δ 7.48 (t, J = 7.5 Hz, 1H),

13 1 7.10 (d, J = 7.5 Hz, 2H), 6.99 (s, 4H), 2.37 (s, 6H), 2.01(s, 12H). C { H} NMR (CDCl3):

δ 147.1(s), 142.1 (s), 137.2 (s), 135.6 (s), 135.4 (s), 128.8 (s), 128.1 (s), 127.8(s), 107.6

31 (s), 21.2 (s), 20.2 (s). P (CDCl3): δ -8.9. Anal. Calcd for C25H25P (356.45): C, 84.24; H,

+ 7.07. Found: C, 84.07; H, 7.20. HRMS (FAB) calcd. for C25H25P (MH ) 357.1772, found

357.1774.

Synthesis of 2,6-(m-Xyl)2C6H3P=CBr2

A 100 mL of round bottom flask was charged with 1.3487 g (3.48 mmol)

DxpPCl2, 1.1549 g (3.48 mmol) CBr4, 60 mL THF and a magnetic stir bar in a dry box.

The solution was removed from the dry box, chilled to -110 oC. To the solution was added a solution of n-BuLi in hexanes (2.8 mL, 2.5 M, 7.0 mmol) via syringe with stirring. The mixture was stirred at -110 oC for 30 minutes and warmed to room temperature to form a pale yellow solution. The solution was taken into a dry box, where the solvent was removed under reduced pressure and the residue was extracted with n- hexane and filtered. Evaporation of the solvent from the filtrate followed by recrystallization of the residue from hexanes at -32 oC yielded white crystalline solid of

o 1 pure DxpP=CBr2 (0.927 g, 54.5 %). m.p. 140 C (dec.). H 300 MHz NMR (CDCl3): 7.60

(t, J = 7.8 Hz, 1H), 7.22 (dd, JHH = 7.6 Hz, JPH = 1.6 Hz, 2H), 7.29 (t, J = 7.3 Hz, 2H),

172

13 1 7.12 (br d, J = 7.0 Hz, 4H), 2.20 (br s, 12H). C { H} NMR (CDCl3): 144.5 (s), 144.4 (s),

139.6 (d, J = 2.7 Hz), 130.4(s), 129.4 (s), 128.5 (s), 128.0 (s), 127.9 (s), 126.8 (s), 21.5

31 (br s). P {1H} NMR {CDCl3}: 270.2. Anal. Calcd for C23H21PBr2 (488.20): C, 56.59; H,

4.34. Found: C, 56.90; H, 4.21.

Synthesis of 2,6-(m-Xyl)2C6H3C≡P

A 50 mL flask was charged with 0.1960 g (0.4 mmol) DxpP=CBr2, 0.0776 g (3.19 mmol) magnesium turnings and 10 mL THF in a dry box. The mixture was removed from the dry box and agitated under ultrasound for 30 minutes, at which time, the solution became dark brown. The mixture was moved into a dry box, where the solvent was removed under reduced pressure and the residue was extracted with n-hexane and filtered to give brown filtrate. Evaporation of the solvent from the filtrate followed by recrystallization of the residue from hexanes at -32 oC yielded pale yellow solid (0.035 g,

1 26.5 %). H 200 MHz NMR (CDCl3): 7.47 (t, J = 7.6 Hz, 1H), 7.24-7.09 (m, 8H), 2.08 (s,

31 12H). P NMR (CDCl3): -8.5.

Synthesis of Br2C=PC6(p-t-BuPh)4P=CBr2

t A 100 mL of flask was charged with 0.3960 g (0.49 mmol) Cl2PC6(p-Bu Ph)4PCl2,

0.3248 g (0.98 mmol) tetrabromomethane, 45 mL THF and a magnetic stir bar in a dry box. The solution was removed from the dry box and chilled to -110oC. To the chilled solution was added a solution of n-BuLi in hexanes (0.82 mL, 2.5 M, 2.1 mmol) via syringe. The reaction mixture was stirred at -110oC for 30 minutes and then warmed to room temperature slowly to become a yellow solution. The solution was taken into a dry box, where the solvent was removed under reduced pressure and the residue was washed with acetonitrile and filtered, followed by rinse with small amount of hexanes to yield a

173

1 off white powder of the expected product (0.295 g, 60.0 %). H NMR 300 MHz (CDCl3):

δ 7.18 (br, 4H), 7.10 (br, 6H), 7.03 (br d, 2H), 6.87 (br d, 4H), 1.23 (s, 18H), 1.22(s, 18H).

13 1 C { H} NMR (CDCl3): δ 150.1 (d, J = 5.2 Hz), 149.9(br, s), 148.8 (s), 148.7 (s), 143.8

(s), 143.7 (s), 135.9 (s), 135.7 (s), 129.9 (br, s), 129.2 (br, s),124.6 (br, s), 124.2 (br,s),

31 1 124.1 (br, s), 34.5 (br, s), 31.2 (s), 31.0 (br, s). P { H} NMR (CDCl30: δ 268.3, 268.1.

174

7. 7 References

1. Gier, T. E. J. Am. Chem. Soc. 1961, 83, 1769.

2. Becker, G.; Gresser, G.; Uhl, W. Z. Zeitschrift fuer Naturforschung, Teil B:

Anorganische Chemie, Organische Chemie 1981, 36, 16.

3. (a) Nixon, J. F. Chem. Rev. 1988, 88, 1327. (b) Regitz, M. Chem. Rev. 1990, 90,

191. (c) Gaumont, A. C.; Denis, J. M. Chem. Rev. 1994, 94, 1413. (d) Nixon, J. F.

Coord. Chem. Rev. 1995, 145, 201.

4. Weber, L. Eur. J. Inorg. Chem. 2003, 1843.

5. Pearson, P. K.; Schäfer, H. F.; Wahlgren, U. J. Chem. Phys. 1975, 62, 350.

6. Nguyen, M. T.; Ha, T.-K. Theochem 1986, 32, 145.

7. Appel, R.; Immenkeppel, M. Z. Anorg. Allg. Chem. 1987, 553, 7.

8. (a) Goede, S. J.; Bickelhaupt, F. Chem. Ber. 1991, 124, 2677. (b) Markovskii, L.

N.; Koidan, G. N.; Marchenko, A. P.; Romanenko, V. D.; Povolotskii, M. I.;

Pinchuk, A. M. Zh. Obshch. Khim. 1989, 59, 2133. (c) Yoshifuji, M.; Niitsu, T.;

Inamoto, N. Chem. Lett. 1988, 1733.

9. Yoshifuji, M.; Kawanami, H.; Kawai, Y.; Toyota, K.; Yasunami, M.; Niitsu, T.;

Inamoto, N. Chem. Lett. 1992, 1053.

10. Romanenko, V. D.; Sanchez, M.; Sarina, T. V.; Mazieres, M. R.; Wolf, R.

Tetrahedron Lett. 1992, 21, 2981.

11. (a) Jun, H.; Young, Jr., V. G.; Angelici, R. J. J. Am. Chem. Soc. 1991, 113, 9379.

(b) Jun, H.; Angelici, R. J. Organometallics 1994, 13, 2454. (c) Weber, L.;

Schumann, I.; Schmidt, T. Stammler, H.-G.; Neumann, B. Z. Anorg. Allg. Chem.

175

1993, 619, 1759. (d) Weber, L.; Schumann, I.; Stammler, H.-G.; Neumann, B.

Chem. Ber. 1994, 127, 1349. (e) Konze, W. V.; Young, Jr., V. G.; Angelici, R. J.

Organometallics 1999, 18, 258. (f) Agapie, T.; Diaconescu, P. L.; Cummins, C. C.

J. Am. Chem. Soc. 2002, 124, 2412.

12. Rösch, W.; Allspach, T.; Bergsträsser, U.; Regitz, M. Multiple Bonds and Low

Coordination in Phosphorus Chemistry (Eds.: Rigitz, M; Scherer, O. J.), Thieme,

Stuttgart, 1990, pp.11-16.

13. Rösch, W.; Hees, U.; Regitz, M. Chem. Ber. 1987, 120, 1645.

14. Oberhammer, H.; Becker, G.; Gresser, G. J. Mol. Struct. 1981, 75, 283.

15. Allspach, T.; Regitz, M.; Becker, G.; Becker, W. Synthesis 1986, 31.

16. Mack, A.; Pierron, E.; Allspach, T.; Bergsträsser, U.; Regitz, M. Synthesis 1998,

1305.

17. Maerkl, G.; Sejpka, H. Tetrahedron Lett. 1986, 27, 171.

18. Arif, A. M.; Barron, A. R.; Cowley, A. H.; Hall, S. W. J. Chem. Soc., Chem.

Commun. 1988, 171.

19. Brym, M.; Jones, C. Dalton Trans. 2003, 3665.

20. Laali, K. K.; Geissler, B.; Regitz, M.; Houser, J. J. J. Org. Chem. 1995, 60, 6362.

21. Mathey, F. Angew. Chem. Int. Ed. 2003, 42, 1578.

22. Burckett-St. Laurent, J. C. T. R.; King, M. A.; Kroto, H. W.; Nixon, J. F.; Suffolk,

R. J. J. Chem. Soc., Dalton Trans. 1983, 755.

23. Frost, D. C.; Lee, S. T.; McDowell, C. A. Chem. Phys. Lett. 1973, 23, 472.

24. Solouki, B.; Bock, H.; Appel, R.; Westerhaus, A.; Becker, G.; Uhl, G. Chem. Ber.

1982, 115, 3747.

176

25. (a) Hitchcock, P. B.; Maah, M. J.; Nixon, J. F.; Zora, J. A.; Leigh, G. J.; Bakar, M.

A. Angew. Chem. Int. Ed. Engl. 1987, 26, 474. (b) Hitchcock, P. B.; Johnson, J.

A.; Lemos, M. A. N. D. A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A. J. L. J.

Chem. Soc., Chem. Commun. 1992, 345. (c) Hitchcock, P. B.; Lemos, M. A. N. D.

A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A. J. L. J. Organomet. Chem. 1991,

402, C23. (d) Meidine, M. F.; Lemos, M. A. N. D. A.; Pombeiro, A. J. L.; Nixon,

J. F.; Hitchcock, P. B. J. Chem. Soc., Dalton Trans. 1998, 3319.

26. (a) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy, Wiley,

New York, 1998, p366. (b) Nixon, J. F. Chem. Soc. Rev. 1995, 319. (c) Nixon, J.

F. Chem. Ind. 1993, 404. (d) Nixon, J. F. Endeavour 1991, 15, 49. (e) Regitz, M.

J. Heterocycl. Chem. 1994, 31, 663. (f) Regitz, M. Heteroatom Chemistry, Ed.

Block, E., VCH, New York, 1990, p.295. (g) Regitz, M.; Binger, P. Multiple

Bonds and Low Coordination in Phosphorus Chemistry, Eds. Regitz, M.; Scherer,

O. J.; Georg Thieme, Stuttgart, 1990, ch.2.

27. Selected references: (a) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P.

P. J. Am. Chem. Soc. 1999, 121, 3357. (b) Rigon, L.; Ranaivonjatovo, H.; Escudie,

J. Phosphorus, Sulfur Silicon Relat. Elem. 1999, 152, 153. (c) Shah, S.;

Protasiewicz, J. D. Chem. Commun. 1998, 15, 1585. (d) Smith, R. C.; Shah, S.;

Protasiewicz, J. D. J. Organomet. Chem. 2002, 646, 255. (e) Tsuji, K.; Sasaki, S.;

Yoshifuji, M. Tetrahedron Lett. 1999, 40, 3203.

28. Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. Inorg. Chem. 2000,

39, 3860.

29. Ito S.; Toyato, K.; Yoshifuji, M. Chem. Commun. 1997, 1637.

177

30. Van der Sluis, M.; Wit, J. B. M.; Bickelhaupt, F. Organometallics 1996, 15, 174.

31. (a) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am. Chem.

Soc. 1981, 103, 4587. (b) Smith, R. C.; Shah, S.; Protasiewicz, J. D. J. Organomet.

Chem. 2002, 646, 255. (c) Tsuji, K.; Sasaki, S.; Yoshifuji, M. Tetrahedron Lett.

1999, 40, 3203.

32. Antipin, M. Y.; Chernega, A. N.; Lysenko, K. A.; Struchkov, Y. T.; Nixon, J. F. J.

Chem. Soc., Chem. Commun. 1995, 505.

33. Jun, H.; Young, Jr., V. G.; Angelici, R. J. Organometallics 1994, 13, 2444.

34. George, T. A.; Noble, M. E. Inorg. Chem. 1978, 17, 1678.

35. Jun, H.; Angelici, R. J. Organometallics 1993, 12, 4265.

36. (a) Fritsch, P. Liebigs Ann. Chem. 1894, 279, 319. (b) Buttenberg, W. P. Liebigs

Ann. Chem. 1894, 279, 324. (c) Wiechell, H. Liebigs Ann. Chem. 1894, 279, 337.

37. Bothner-By, A. A. J. Am. Chem. Soc. 1955, 77, 3293.

38. (a) Curtin, D. Y.; Flynn, E. W. J. Am. Chem. Soc. 1959, 81, 4714. (b) Curtin, D.

Y.; Flynn, E. W.; Nystrom, R. F. J. Am. Chem. Soc. 1958, 80, 4599.

39. Simonetta, M.; Carra, S. Tetrahedon 1963, 19, 467.

40. (a) Koebrich, G.; Trapp, H. Z. Naturforsch. 1963, 186, 1125. (b) Köbrich, G.

Angew. Chem. Int. Ed. Engl. 1965, 4, 49.

41. Smith, R. C.; Protasiewicz, J. D. Eur. J. Inorg. Chem. 2004, 5, 998.

42. Smith, R. C.; Chen, X.; Protasiewicz, J. D. Inorg. Chem. 2003, 42, 5468.

43. Smith, R. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 2004, 126, 2268.

178

Bibliography

Abd El-Rahman, N. M.; Boulos, L. S. “The Behavior of 1,2,3-Indantrione Towards Wittig-Horner Reagents” Molecules 2002, 7, 81.

Abdou, W. M. “Further Studies on the Behavior of 3,5-Di-tert-butyl-1,2-benzoquinone Toward Phosphorus Ylides” Phosphorus, Sulfur Silicon Relat. Elem. 1992, 66, 285.

Abdou, W. M.; Ganoub, N. A. F.; Abdel-Rahman, N. M. “Reaction of Phosphonium Ylides with 3,5-Di-tert-butyl-1,2-benzoquinone” Phosphorus, Sulfur Silicon Relat. Elem. 1991, 61, 91.

Agapie, T.; Diaconescu, P. L.; Cummins, C. C. “Methine (CH) Transfer via a Chlorine Atom Abstraction/Benzene-Elimination Strategy: Molybdenum Methylidyne Synthesis and Elaboration to a Phosphaisocyanide Complex” J. Am. Chem. Soc. 2002, 124, 2412.

Al Badri, A.; Jouaiti, A.; Geoffroy, M. “Spin Delocalization in Radical Anions Containing Two Phosphaalkene Groups: an EPR Study” Magn. Reson. Chem. 1999, 37, 735.

Al-Juaid, S. S.; Dhaher, D. M.; Eaborn, C.; Hitchcock, P. B.; McGeary, C. A.; Smith, J. D. “Some [Tris(trimethyl)- and Tris(dimethylphenyl)silyl]methyl Derivatives of Phosphorus. The Crystal Structure of [Tris(dimethylphenylsilyl)methylphosphonous Dichloride” J. Organomet. Chem. 1989, 366, 39.

Allcock, H. R. “Poly(organophosphazenes) - Unusual New High Polymers” Angew. Chem. Int. Ed. Engl. 1977, 16, 147.

Allcock, H. R. “Inorganic Macromolecules” Chem. Eng. News 1985, 63, 22.

Allspach, T.; Regitz, M.; Becker, G.; Becker, W. “Unusually Coordinated Phosphorus Compounds. 7. Adamant-1-ylmethylidynephosphine, a New, Stable Phosphaalkyne” Synthesis 1986, 31.

Allcock, H. R. “Inorganic-Organic Polymers” Adv. Mater. 1994, 6, 106.

Allcock, H. R.; Kugel, R. L. “Synthesis of High-Polymeric Alkoxy and Aryloxy Phosphonitriles” J. Am. Chem. Soc. 1965, 87, 4216.

Allcock, H. R.; Olmeiger, D. L.; O’Connor, J. M. “Cation Complexation and Conductivity in Crown Ether Bearing Polyphosphazenes” Macromolecules 1998, 31, 753.

Allcock, H. R. Phosphorus-Nitrogen Compounds; Academic Press: New York, 1972.

179

Ang, H. G.; Schmutzler, R. “Phosphorus-Fluorine Chemistry. XX. Redox Disproportionation of Difluorophosphines, RPF2, with Electronegative Substituents R” J. Chem. Soc. (A) 1969, 702.

Antipin, M. Y.; Chernega, A. N.; Lysenko, K. A.; Struchkov, Y. T.; Nixon, J. F. “Electron Distribution in tert-Butyl Phosphaethyne ButC.tplbond.P” J. Chem. Soc., Chem. Commun. 1995, 505.

Appel, R.; Immenkeppel, M. “Low-coordinated Phosphorus Compounds. 58. Synthesis and Reactivity of 2,4,6-Tri-tert-butyphenyl(halomethylene)phosphines” Z. Anorg. Allg. Chem. 1987, 553, 7.

Appel, R.; Knoll, F.; Ruppert, I. “Phospha-alkenes and Phospha-alkynes, Genesis and Properties of the (p-p) π-Multiple Bond” Angew. Chem. Int. Ed. 1981, 20, 731.

Appel, R.; Winkhaus, V.; Knoch, F. “Low-Coordinate Phosphorus Compounds. 56. Valence Isomerization of a 3,4-Diphospha-1,5-hexadiyne with Formation of a 3,4- Bis(phosphamethylene)cyclobutene” Chem. Ber. 1987, 120, 243.

Arduengo, A. J. “Looking for Stable Carbenes: The Difficulty in Starting Anew” Acc. Chem. Res. 1999, 32, 913.

Arif, A. M.; Barron, A. R.; Cowley, A. H.; Hall, S. W. “Reaction of the Phosphaalkyne ArC.tplbond.P (Ar = 2,4,6-(Me3C)3C6H2) with Nucleophiles: a New Approach to 1,3- Diphosphabutadiene Synthesis” J. Chem. Soc., Chem. Commun. 1988, 171.

Ashe, A. J. “Phosphabenzene and Arsabenzene” J. Am. Chem. Soc. 1971, 93, 3293.

Askari, S. H.; Rughooputh, S. D.; Wudl, F. “Soluble Substituted-Polyphenylenevinylene (PPV) Conducting Polymers: Spectroscopic Studies” Synth. Met. 1989, 29, E129.

Baechler, R. D.; Andose, J. D.; Stackhouse, J.; Mislow, K. “Linear Free Energy Correlations of Barriers to Pyramidal Inversion” J. Am. Chem. Soc. 1972, 94, 8060.

Baechler, R. D.; Mislow, K. “Effect of Structure on the Rate of Pyramidal Inversion of Acyclic Phosphines” J. Am. Chem. Soc. 1970, 92, 3090.

Balakrishna, M. S.; Panda, R. “Syntheses and Characterization of Ten-Membered Cyclic and Large Bite Acyclic Bis(phosphines)” Phosphorus, Sulfur Silicon Relat. Elem. 2003, 178, 1391.

Balakrishna, M. S.; Panda, R.; Mague J. T. “New 10-Membered Inorganic Heterocyclic t t Diphosphanes, PhN(PX)2{(-OC6H2( Bu)2)(µ-S)(( Bu)2C6H2O-)} (X = Cl, F). Synthesis and Transition Metal Complexes (Molybdenum(0), Ruthenium(II), Palladium(II), and Platinum(II)) of Heterocyclic Diphosphanes. Crystal and Molecular Structures of the t t Chloro Derivative, PhN(PCl)2{(-OC6H2( Bu)2)(µ-S)(( Bu)2C6H2O-)}, and of a

180

3 Molybdenum(0) Complex of the Fluoro Derivative, [Mo(CO)3{η -PhN(PF)2{(- t t OC6H2( Bu)2)(µ-S)(( Bu)2C6H2O-)}- P, P, S}]” Inorg. Chem. 2001, 40, 5620.

Balavoine, G. G. A.; Doisneau, G.; Fillebeen-Khan, T. “An Improved Synthesis of Ferrocene-1,1-dicarbaldehyde” J. Organomet. Chem. 1991, 412, 381.

Bao, Z.; Chen, Y.; Cai, R.; Yu, L. “Conjugated Liquid-Crystalline Polymers - Soluble and Fusible Poly(phenylenevinylene) by the Heck Coupling Reaction” Macromolecules 1993, 26, 5281.

Bard, A. J.; Cowley, A. H.; Kilduff, J. E.; Leland, J. K.; Norman, N. C.; Palkuvski, M. “Oxidation and Reduction of Phosphorus-Phosphorus and Arsenic-Arsenic Double Bonds. An Electrochemical Study of Two Diphosphenes and a Diarsene” J. Chem. Soc., Dalton Trans. 1987, 249.

Becker, G.; Gresser, G.; Uhl, W. Z. “Acyl- and Alkylidenephosphines. XV. 2,2- Dimethylpropylidynephosphine, a Stable Compound with a Phosphorus Atom of Coordination Number 1” Zeitschrift fuer Naturforschung, Teil B: Anorganische Chemie, Organische Chemie 1981, 36, 16.

Bergbreiter, D. B.; Liu, Y. “Water-soluble Polymer-Bound, Recoverable Palladium(0)- Phosphine Catalysts” Tetrahedron Lett. 1997, 38, 7843.

Boldeskul, I. E.; Tukhvatullin, F. Kh.; Ismailov, Sh. S. “Vibrational Spectra and Normally Coorinated Analysis of Unsaturated Phosphorus Compounds” Ukrains'kii Fizichnii Zhurnal 2000, 45, 662.

Bouhadir, G.; Bourissou, D. “Unusual Geometries in Main Group Chemistry” Chem. Soc. Rev. 2004, 33, 210.

Boulos, L. S.; Hennawy, I. T. “Studies on Phosphonium Ylides. XIX. Reactions of Cyanomethylenetriphenylphosphorane with Certain Dicarbonyl Compounds” Phosphorus, Sulfur Silicon Relat. Elem. 1993, 84, 173.

Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. “Stable Carbenes” Chem. Rev. 2000, 100, 39.

Boutagy, J.; Thomas, R. “Olefin Synthesis with Organic Phosphonate Carbanions” Chem. Rev. 1974, 74, 87.

Braun, D.; Heeger, A. J. “Visible Light Emission from Semiconducting Polymer Diodes” Appl. Phys. Lett. 1991, 58, 1982.

Braun, D.; Heeger, A. J.; Kroemer, H. “Improved Efficiency in Semiconducting Polymer Light-Emitting Diodes” J. Electron. Mater. 1991, 20, 945.

181

Brédas, J. L.; Silbey, R. (Eds.); Conjugated Polymers: The Novel Science and Technology of Highly Conducting and Non-linear Optically Active Materials, Kluwer Academic, Dordecht, 1991.

Brook, A. G.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M. R. “A Solid Silaethene: Isolation and Characterization” J. Chem. Soc., Chem. Commun. 1981, 191.

Brook, A. G.; Nyburg, S. C.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M. R.; Poon, Y. C.; Chang, Y. M.; Wong-Ng, W. “Stable solid Silaethylenes” J. Am. Chem. Soc. 1982, 104, 5667.

Bothner-By, A. A. “Stereochemistry of the Rearrangement Involving Migration between Multiply-bonded Carbons” J. Am. Chem. Soc. 1955, 77, 3293.

Boutagy, J.; Thomas, R. “Olefin Synthesis with Organic Phosphonate Carbanions” Chem. Rev. 1974, 74, 87.

Brym, M.; Jones, C. “Synthesis, Characterisation and Reactivity of the First Diphosphaalkyne” Dalton Trans. 2003, 3665.

Bunz, U. H. “Poly(aryleneethynylene)s: Syntheses, Properties, Structures, and Applications” Chem. Rev. 2000, 100, 1605.

Burckett-St. Laurent, J. C. T. R.; King, M. A.; Kroto, H. W.; Nixon, J. F.; Suffolk, R. J. “Photoelectron Spectra of the Phosphaalkynes: 3,3-Dimethyl-1-phosphabutyne, Me3CC.tplbond.P, and Phenylphosphaethyne, PhC.tplbond.P” J. Chem. Soc., Dalton Trans. 1983, 755.

Burg, A. B.; Mahler, W. “Bicoordinate Phosphorus: Base-PCF3 Adducts” J. Am. Chem. Soc. 1961, 83, 2388.

Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. “Stable Versions of Transient Push-Pull Carbenes: Extending Lifetimes from Nanoseconds to Weeks” Science, 2000, 288, 834.

Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. “Light-Emitting Diodes Based on Conjugated Polymers” Nature 1990, 347, 539.

Cao, Y.; Smith, P.; Heeger, A. J. “Counter-Ion Induced Processibility of Conducting Polyaniline and of Conducting Polyblends of Polyaniline in Bulk Polymers” Synth. Met. 1992, 48, 91.

182

Cetinkaya, B.; Hudson, A.; Lappert, M. F.; Goldwhite, H. “Generation and ESR Spectra of Some New Phosphorus-Centered Radicals .bul.P2Ar2X, .bul.P(Ar)X, .bul.P(OAr)2, . bul.PAr2(:O),.bul.PAr[N(SiMe3)2](:NSiMe3), and [P2Ar2].bul.- Derived From the Bulky Group C6H2(CMe3)3-2,4,6 (= Ar)” J. Chem. Soc., Chem. Commun. 1982, 609.

Chasar, D. W. “Synthesis of 1,3,2-Dioxaphospholenes. Evidence for an Aryloxyphosphinidene Intermediate” Phosphorus, Sulfur Silicon Relat. Elem. 1987, 34, 149.

Chen, X.; Smith, R. C.; Protasiewicz, J. D. “Cycloaddition of Phosphanylidene-σ4- phosphoranes ArP=PMe3 and Quinones to Yield 1,3,2-Dioxophospholanes” Chem. Commun. 2004, 146.

Clybrune, J. A. C.; McMullen, N. “Unusuall Structures of Main Group Organometallic Compounds Containing m-Terphenyl Ligands” Coord. Chem. Rev. 2000, 210, 73.

Coulson, C. A. “Valence”, Oxford University Press, London, 1961.

Couret, C.; Escudie, J.; Madaule, Y.; Ranaivonjatovo, H.; Wolf, J.-G. “The First Stable Diarsene: Bis[tris(trimethylsilyl)methyl]diarsene” Tetrahedron Lett. 1983, 24, 2769.

Couret, C.; Escudie, J.; Satge, J. “A Compound with a Tin-Phosphorus Double Bond: The First Stable Stannaphosphene” J. Am. Chem. Soc. 1985, 107, 8280.

Couret, C.; Escudie, J.; Satge, J. “A Compound with a Tin-Phosphorus Double Bond: the First Stable Stannaphosphene” J. Am. Chem. Soc. 1985, 107, 8280.

Couret, C.; Escudie, J.; Satge, J.; Lazraq, M. “The First Stable Germene: A Compound with a Germanium-Carbon Double Bonds” J. Am. Chem. Soc. 1987, 109, 4411.

Cowley, A. H.; Cushner, M. C. “Exchange Reactions in (CH3)3PPCF3, Phosphinidene Analog of a Wittig Reagent” Inorg. Chem. 1980, 19, 515.

Cowley, A. H.; Lasch, J. G.; Norman, N. C.; Pakulski, M. “Synthesis and Structure of a Diarsene: the First Compound with an Unsupported Arsenic-Arsenic Double Bond” J. Am Chem. Soc 1983, 103, 5506.

Cowley, A. H.; Lasch, J. G.; Norman, N. C.; Pakulski, M.; Whittlesey, B. R. “Synthesis of a Phospha-arsene and a Phosphastibene: The First Compounds with Phosphorus- Arsenic and Phosphorus-Antimony Double Bonds” J. Chem. Soc., Chem. Commun. 1983, 881.

Cowley, A. H.; Norman, N. C.; Pakulski, M. “Purification and X-ray Crystal Structure of Bis[tris(trimethylsilyl)methyl]diarsene” . J. Chem. Soc., Dalton Trans. 1985, 383.

183

Creve, S.; Pierloot, K.; Nguyen, M. T. “Stabilization of Phosphinidenes by Metal Complexation: a Theoretical Study of Cr(CO)5-PH” Chem. Phys. Lett. 1998, 285, 429.

Creve, S.; Pierloot, K.; Nguyen, M. T.; Vanquickenborne, L. G. “Phosphinidene Transition Metal Complexes: A Combined Ab Initio MO-DFT Study of Cr(CO5)-PR” Eur. J. Inorg. Chem. 1999, 107.

Culcasi, M.; Grouchi, G.; Escudie, J.; Couret, C.; Pujol, L.; Tordo, P. “Electrochemical and ESR Characterization of the Redox Behavior of Bis[tris(trimethylsilyl)methyl]diphosphene, (Me3Si)3CP:PC(SiMe3)3 (TsiP:PTsi)” J. Am. Chem. Soc. 1986, 108, 3130.

Curtin, D. Y.; Flynn, E. W. “Reaction of 1,1-Diaryl-2-haloethylenes with Butyllithium” J. Am. Chem. Soc. 1959, 81, 4714.

Curtin, D. Y.; Flynn, E. W.; Nystrom, R. F. “Reaction of Stereoisomeric 14C-Labeled 1- Bromo-2,2-diarylethylenes and β-Bromostyrenes with Butyllithium” J. Am. Chem. Soc. 1958, 80, 4599.

Daly, J. J.; Maier, L. “Molecular Structure of Phosphobenzene” Nature 1964, 203, 1167.

Daly, J. J.; Maier, L. “Structure of Phosphobenzene” Nature 1965, 208, 383.

Daly, J. J. “The Crystal and Molecular Structure of Phosphobenzene A, (PC6H5)5” J. Chem. Soc., Suppl. 1964, 6147.

Daly, J. J. “The Crystal and Molecular Structure of a Trigonal Form of Phosphobenzene B, (PC6H5)6” J. Chem. Soc., Abs. 1965, 4789.

Daugulis, O.; Brookhart, M.; White, P. S. “Phosphinidine-Palladium Complexes for the Polymerization and Oligomerization of Ethylene” Organometallics 2002, 21, 5935.

Davidson, P. J.; Harris, D. H.; Lappert, M. F. “Subvalent Group 4B Metal Alkyls and Amides. Part 1. The Synthesis and Physical Properties of Kinetically Stable Bis[bis(trimethylsilyl)methyl]germanium(II), -tin(II), and -lead(II)” J. Chem Soc., Dalton Trans. 1976, 2268.

Deacon, G. B.; Fanwick, P. E.; Gitlits, A.; Rothwell, I. P.; Skelton, B. W.; White, A. H. “Organoamido- and Aryloxo-lanthanoids, 25. Rare Earth Complexes of Bulky 2,6- Diphenylphenolates Containing Additional, Potentially Buttressing 3,5-Substituents” Eur. J. Inorg. Chem. 2001, 1505.

Dehong, H.; Yu, J.; Barbara, P. F. “Single-Molecule Spectroscopy of the Conjugated Polymer MEH-PPV” J. Am. Chem. Soc. 1999, 121, 6936.

184

Deschamps, E.; Ricard, L.; Mathey, F. “Toward Polyphospholes: Synthesis and Structure of an α-Linked Quaterphosphole” Angew. Chem. Int. Ed. Engl. 1994, 33, 1158.

Dickie, D. A.; Abeysekera, D.; McKenzie, I. D.; Jenkins, H. A.; Clyburne, J. A. C. [Crystallographic Studies on Substituted m-Terphenyls: Identification of Weak [CH3ּּּI“ Interactions” Crystal Engineering 2003, 6, 79.

Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy, Wiley, New York, 1998.

Doi, S.; Kuwabara, M.; Noguchi, T.; Ohnishi, T. “Organic Electroluminescent Devices Having Poly(dialkoxy-p-phenylene vinylenes) as a Light Emitting Material” Synth. Met. 1993, 57, 4174.

Du, C.-J. F.; Hart, H.; Ng, K.-K. D. “A One-Pot Synthesis of m-Terphenyls, via a Two- Aryne Sequence” J. Org. Chem. 1986, 51, 3162.

Dutan, C.; Shah, S.; Smith, R. C.; Choua, S.; Berclaz, T.; Geoffroy, M.; Protasiewicz, J. D. “Sterically Encumbered Diphosphaalkenes and a Bis(diphosphene) as Potential Multiredox-Active Molecular Switches: EPR and DFT Investigations” Inorg. Chem. 2003, 42, 6241.

Ehlers, A. W.; Lammertsma, K.; Baerends, E. J. “Phosphinidene Complexes M(CO)5-PR: A Density Functional Study on Structures and Electronic States” Organometallics 1998, 17, 2738.

Ehrlich, P.; Bertheim, A. “Arseno-benzene Derivatives” Ber. Dtsch. Chem. Ges. 1912, 45, 761.

Ehrlich, P.; Bertheim, A. “Diaminodioxyarsenobenzene” Ber. Dtsch. Chem. Ges. 1911, 44, 1265.

Escudie, J.; Couret, C.; Satge, J.; Andrianarison, M.; Andriamizaka, J.-D. “2,2-Dimesityl- 1-(2,4,6-tri-tert-butylphenyl)germaphosphene: The First Stable Compound with a Germanium-Phosphorus Double Bond” J. Am. Chem. Soc. 1985, 107, 3378.

Feast, W. J.; Friend, R. H. “Synthesis and Material and Electronic Properties of Conjugated Polymers” J. Mater. Sci. 1990, 25, 3796.

Fild, M.; Schmutzler, R. “Phosphorus-Fluorine Chemistry. XXI. (Pentafluorophenyl) fluorophosphines and (Pentafluorophenyl)fluorophosphoranes” J. Chem. Soc. (A) 1969, 840.

Filippou, A. C.; Portius, P.; Philippopoulos, A. I.; Rohde, H. “Triple Bonding to Tin: Synthesis and Characterization of the Stannylyne Complex trans-[Cl(PMe3)4W Sn C6H3-2,6-Mes2]” Angew. Chem. Int. Ed. 2003, 42, 445.

185

Fjeldberg, T.; Lappert, M. F.; Schilling, B. E. R.; Seip, R.; Thorne, A. “A New Method for the Preparation of Bis(bistrimethylsilylmethyl)tin(II) and its Molecular Structure Determined by Gas Electron Diffraction. Prediction of Nonplanar, trans Folded Ground State Structures for Digermene and Distannene” J. Chem. Soc., Chem. Commun. 1982, 1407.

Freytag, M.; Jones, P. G.; Schmutzler, R.; Yoshifuji, M. “Reaction of (E)-Bis(2,4,6-tri- tert-butylphenyl)diphosphene with Tetrachloro-o-benzoquinone” Heteroat. Chem. 2001, 12, 300.

Fritz, G.; Scheer, P. “Silylphosphanes: Developments in Phosphorus Chemistry” Chem. Rev. 2000, 100, 3341.

Frost, D. C.; Lee, S. T.; McDowell, C. A. “Photoelectron Spectrum of Methiophosphide and Comments on the First Photoelectron Band of Hydrogen Cyanide” Chem. Phys. Lett. 1973, 23, 472.

Fylaktakidou, K. C.; Gautam, D. R.; Hadjipavlou-Litina, D. J.; Kontogiorgis, C. A.; Litinas, K. E.; Nicolaides, D. N. “Reactions of 4-Methylchromene-2,7,8-trione with Phosphonium Ylides. Synthesis and Evaluation of Fused 1,3-Dioxolocoumarins as Antioxidants and Antiinflammatories” J. Chem. Soc., Perkin Trans. 1 2001, 3073.

Gajare, A. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F. “Application of a Diphosphinidenecyclobutene Ligand in the Solvent-free Copper-catalysed Amination Reactions of Aryl Halides” Chem. Commun. 2004, 1994.

Gates, D. P. “Inorganic and organometallic polymers” Annu. Rep. Prog. Chem., Sect. A 2003, 99, 453.

Gaudin, O.; Jackman, R. B.; Nguyen, T.-P.; Le Rendu, P. “Determination of Traps in Poly(p-phenylene vinylene) Light Emitting Diodes by Charge-Based Deep Level Transient Spectroscopy” J. Applied Phys. 2001, 90, 4196.

Gaumont, A. C.; Denis, J. M. “Preparation, Characterization, and Synthetic Potential of Unstable Compounds Containing Phosphorus-Carbon Multiple Bonds” Chem. Rev. 1994, 94, 1413.

Geoffroy, M.; Jouaiti, A.; Terron, G.; Cattani-Lorente, M.; Ellinger, Y. “Phosphaalkene Radical Anions: Electrochemical Generation, Ab Initio Predictions, and ESR Study” J. Phys. Chem. 1992, 96, 8241.

George, T. A.; Noble, M. E. “A Direct One-step Preparation of Bis(dinitrogen) Complexes of Molybdenum(0) from Molybdenum(V) Chloride” Inorg. Chem. 1978, 17, 1678.

186

Ghassemi, H.; McGrath, J. E. “Synthesis of Poly(arylene phosphine oxide) by Nickel- Catalysed Coupling Polymerization” Polymer 1997, 38, 3139.

Gier, T. E. “HCP, a Unique Phosphorus Compound” J. Am. Chem. Soc. 1961, 83, 1769.

Goede, S. J.; Bickelhaupt, F. “Synthesis and Reactions of P-supermesityl-C- halophosphaalkenes” Chem. Ber. 1991, 124, 2677.

Goldberg, D. E.; Harris, D. H.; Lappert, M. F.; Thomas, K. M. “A New Synthesis of Divalent Group 4B Alkyls M[CH(SiMe3)2 ]2 (M = Ge or Sn), and the Crystal and Molecular Structure of the Tin Compound” J. Chem. Soc., Chem. Commun. 1976, 261.

Gosney, I.; Rowley, A. G. In Organophosphorus Reagents in Organic Synthesis; Cadogan, J. I. G., Ed.; Academic Press: New York, 1979; pp 17-153.

Grewal, R. S.; Hart, H.; Vinod, T. K. “Oxacyclophanes Based on a m-Terphenyl Framework” J. Org. Chem. 1992, 57, 2721.

Grobe, J.; Le Van, D.; Lüth, B.; Hegemann, M. “Reactive E:C(p,p) π-Systems. XXVI. Simple Synthesis of Methylenephosphines (phosphaalkenes) HP:C(F)NR2 and the Phosphaalkyne P.tplbond.CN(Pr-iso)2” Chem. Ber. 1990, 123, 2317.

Grützmacher, H.; Fässler, T. F. “Topographical Analyses of Homonuclear Multiple Bonds Between Main Group Elements” Chem. Eur. J. 2000, 6, 2317.

Guillemin, J. C.; Cabioch, J. L.; Morise, X.; Denis, J. M.; Lacombe, S.; Gonbeau, D.; Pfister-Guillouzo, G.; Guenot, P.; Savignac, P. “Unsubstituted 1- and 2- Phosphabutadienes: Preparation and Spectroscopic Characterization” Inorg. Chem. 1993, 32, 5021.

Guillemin, J. C.; Janati, T.; Denis, J. M.; Guenot, P.; Savignac, P. “Unstabilized 1- Phosphaallenes: Synthesis and Characterization” Tetrahedron Lett. 1994, 35, 245.

Gusel’nikov, L. E.; Nametkin, N. S. “Formation and Properties of Unstable Intermediates Containing Multiple pπ- pπ Bonded Group 4B Metals” Chem. Rev. 1979, 79, 529.

Hamaguchi, M.; Yoshino, K. “Liquid Crystalline Behavior and Optical Properties of Poly(2,5-dinonyloxy-p-phenylenevinylene)” Jpn. J. Appl. Phys. 1994, 33, L1478.

Hamilton, T. P.; Willis, A. G.; Williams, S. D. “Ab Initio Predictions of the Lowest Electronic States, Structures Vibrational Frequencies of Phenylphosphinidene” Chem. Phys. Lett. 1995, 246, 59.

Han, C.-C.; Elsenbaumer, R. L. “Conveniently Processible Forms of Electrically Conductive Poly(dibutoxyphenylene vinylene)” Mol. Cryst. Liq. Cryst. 1990, 189, 183.

187

Harada, K.; Hart, H.; Du, C.-J. F. “Reaction of Aryl Grignard Reagents with Hexahalobenzenes: Novel Arenes via Multiple Aryne Sequence” J. Org. Chem. 1985, 50, 5524.

Hart, H.; Du, C.-J. F.; Mohebalian, J. “Aryne Cycloaddition to the Aromatic Ring of Mesitylmagnesium Bromide. An Anion-Assisted Diels-Alder Reaction?” J. Org. Chem. 1988, 53, 2720.

Heeger, A. J.; Braun, D. “Visible-light-emitting diodes fabricated from soluble semiconducting polymers and their fabrication” WO-B 92/16023, 1992 [Chem. Abstr. 1993, 118, 157401j].

Hedberg, K.; Hughes, E. W.; Waser, J. “The Structure of Arsenobenzene” Acta Cryst. 1961, 14, 369.

Heeger, A. J. “Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials” Angew. Chem. Int. Ed. 2001, 40, 2591.

Heuer, L. 1989, PhD dissertation, Technical University of Braunschweig, Germany.

Heuer, L.; Schomburg, D.; Schmutzler, R. “Formation of a 1,2-Diorgano-1,2- dichlorodiphosphine” Phosphorus, Sulfur Silicon Relat. Elem. 1989, 45, 217.

Heuer, L.; Sell, M.; Schmutzler, R.; Schomburg, D. “Preparation of ortho-Substituted Aryldifluorophosphines, 2-XC6H4PF2 (X = MeO or Me2N), and Some of Their Derivatives. X-ray Crystal Structure Determination of (2-MeOC6H4P)4 and of the Platinum Complex, Cl2Pt(2-MeOC6H4PF2)2” Polyhedron 1987, 6, 1295.

Hirohata, M.; Tada, K.; Kawai, T.; Onoda, M.; Yoshino, K. “Electroluminescence in Conducting Polymers Based on Poly(phenylene ethynylene)” Synth. Met. 1997, 85, 1273.

Hitchcock, P. B.; Johnson, J. A.; Lemos, M. A. N. D. A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A. J. L. “Novel Synthesis of Phosphinidene Oxide-κP (RP:O, R = tert-BuCH2) Complex of Rhenium(I) from a Phosphaalkyne Precursor. Crystal and Molecular Structure of [ReCl(Ph2PCH2CH2PPh2)2{P(O)CH2Bu-tert}]” J. Chem. Soc., Chem. Commun. 1992, 345.

Hitchcock, P. B.; Lappert, M. F.; Miles, S. J.; Thorne, A. J. “Chemistry, Including the X- ray Structure, of Bis[bis(trimethylsilyl)methylgermanium(II)], a Stable Compound Having a Metal-Metal Double Bond, a Dimetallene” J. Chem. Soc., Chem. Commun. 1984, 480.

Hitchcock, P. B.; Lemos, M. A. N. D. A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A. J. L. “Synthesis of the η1-Phosphaalkyne Complex trans-[FeH(P.tplbond.CBu- 1 tert)(dppe)2][BF4] and its Conversion to an η -Fluorophosphaalkene Complex. Crystal

188

Structure of trans-[FeH(PF:CHBu-tert)(dppe)2][FeCl2F2]” J. Organomet. Chem. 1991, 402, C23.

Hitchcock, P. B.; Maah, M. J.; Nixon, J. F.; Zora, J. A.; Leigh, G. J.; Bakar, M. A. “First Examples of Complexes Containing η1-Bonded Phosphaalkynes - Synthesis and Crystal and Molecular Structure of trans-[Mo(AdC≡P)2(Et2PCH2CH2PEt2)2] (Ad = adamantyl)” Angew. Chem. Int. Ed. Engl. 1987, 26, 474.

Höger, S.; McNamara, J. J.; Schricker, S.; Wudl, F. “Novel Silicon-Substituted, Soluble Poly(phenylenevinylene)s: Enlargement of the Semiconductor Bandgap” Chem. Mater. 1994, 6, 171.

Honeyman, C. H.; Foucher, D. A.; Dahmen, F. Y.; Rulkens, R.; Lough, A. J. “Thermal Ring-Opening Polymerization (ROP) of Strained, Ring-Tilted Phosphorus-Bridged [1]Ferrocenophanes: Synthesis of Poly(ferrocenylphosphines) and Poly(ferrocenylphosphine sulfides)” Organometallics 1995, 14, 5503.

Horner, L.; Hoffmann, H.; Wippel, H. G. “Phosphorus Organic Compounds. XII. Phosphine Oxides as Reagents for the Olefin Formation” Chem. Ber. 1958, 91, 61.

Horner, L.; Hoffmann, H.; Wippel, H. G.; Klahre, G. “Phosphorus Organic Compounds. XX. Phosphine Oxides as Reagents for Olefin Formation” Chem. Ber. 1959, 92, 2499.

Hovis, J. S.; Hamers, R. J. “Structure and Bonding of Ordered Organic Monolayers of 1,5-Cyclooctadiene on the Silicon (001) Surface” J. Phys. Chem. B 1997, 101, 9581.

Hwang, D.-H.; Kang, I.-N.; Jang, M.-S.; Shim, H.-K.; Zyung, T. “Green Electroluminescent Diode from Poly(2-trimethylsilyl-1,4-phenylenevinylene)” Polym. Bull. 1996, 36, 383.

Hwang, D.-H.; Shim, H.-K.; Lee, J.-I.; Lee, K.-S. “Synthesis and Properties of Multifunctional Poly(2-trimethylsilyl-1,4-phenylenevinylene): A Novel, Silicon- Substituted, Soluble PPV Derivative” J. Chem. Soc. Chem. Commun. 1994, 2461.

Ikeda, S.; Ohhata, F.; Miyoshi, M.; Tanaka, R.; Minami, T.; Ozawa, F.; Yoshifuji, M. “Synthesis and Reactions of Palladium and Platinum Complexes Bearing Diphosphinidenecyclobutene Ligands: a Thermally Stable Catalyst for Ethylene Polymerization” Angew. Chem. Int. Ed. 2000, 39, 4512.

Ionkin A. S.; Marshall, W. J. “2,2''-Dimethoxy-1,1':3',1''-terphenyl as a Novel Protective Group for Low-Coordinated Phosphorus Compounds: The Case of Diphosphenes” Heteroatom Chem. 2003, 14, 360.

Ionkin, A. S.; Marshall, W. “New Thermally Stable Cationic η3-Allyl(1,4-diphospha- (1,3)-butadiene)nickel Complexes for Ethylene Polymerization” Chem. Commun. 2003, 710.

189

Ionkin, A. S.; Marshall, W. “2,4,6-Tri-tert-butylphenyl and 2,4-di-tert-butyl-6- methylphenyl groups: look similar, react differently” Heteroat. Chem. 2002, 13, 662.

Ito S.; Toyato, K.; Yoshifuji, M. “Formation of 6,8-di-tert-butyl-3,4-dihydro-4,4- dimethyl-1-phosphanaphthalene by the Reaction of 2,2-dibromo-1-(2,4,6-tri-tert- butylphenyl)-1-phosphaethene with Butyllithium” Chem. Commun. 1997, 1637.

Jensen, P.; Bunker, P. R. “The Potential Surface and Stretching Frequencies of ~X3B1 Methylene (CH2) Determined from Experiment Using the Morse Oscillator-Rigid Bender Internal Dynamics Hamiltonian” J. Chem. Phys. 1988, 89, 1327.

Jin, J.-I.; Kim, J.-C.; Shim, H.-K. “Synthesis and Electrical Properties of Poly(2-bromo- 5-methoxy-1,4-phenylenevinylene) and Copolymers” Macromolecules 1992, 25, 5519.

Jin, Z.; Lucht, B. L. “Transition Metal Mediated Routes to Poly(arylphosphine)s: Investigation of Novel Phosphorus Containing Conjugated Polymers” J. Organomet. Chem. 2002, 653, 167.

Johnson, A. W. Ylide Chemistry; Academic Press: New York, 1966.

Jones, P. G.; Heuer, L.; Schmutzler, R. “Difluoro(mesityl)phosphine” Acta. Cryst. 2002, E58, o999.

Jouaiti, A.; Geoffroy, M.; Terron, G.; Bernardinelli, G. “The Benzodiphosphaalkene Ligand and Its PdII and PtII Complexes: Their Synthesis, Structure, and an ESR Study of Their Reduction Products” J. Am. Chem. Soc. 1995, 117, 2251.

Jouaiti, A.; Geoffroy, M.; Terron, G.; Bernardinelli, G. “Synthesis, Structure and Ligand- Centered Reduction of an Orthometalated Complex of Palladium Containing Two Phosphaalkene Groups” J. Chem. Soc., Chem. Commun. 1992, 155.

Jun, H.; Angelici, R. J. “Bridging Cyaphide (C≡P-) and Bridging Aryl Isocyaphide (C≡P- R) Ligands: Synthesis and Characterization of (X)(PEt3)2Pt(µ-C=P)Pt(PEt3)2 and (X)(PEt3)2Pt(µ-C=P-R)Pt(PEt3)2(X) (X = Cl, Br; R = 2,4,6-Tri-tert-butylphenyl)” Organometallics 1994, 13, 2454.

Jun, H.; Angelici, R. J. “Mechanism of the Platinum-Promoted Conversion of Cl2C:PR to RC.tplbond.P, where R = 2,4,6-tri-tert-butylphenyl” Organometallics 1993, 12, 4265.

Jun, H.; Young, Jr., V. G.; Angelici, R. J. “A Phosphorus Analog of a Semibridging Aryl Isocyanide Ligand: Synthesis and Structure of Diplatinum Complex (Et3P)PtCl((µ- C:PR)Pt(PEt3)2(Cl)” J. Am. Chem. Soc. 1991, 113, 9379.

Jun, H.; Young, Jr., V. G.; Angelici, R. J. “Synthesis, Reactions, and Rearrangement of X(PR'3)2M[C(:PR)X] (M = Pt, Pd; X = Cl, Br; R' = Et, Ph; R = 2,4,6-tri-tert-butylphenyl):

190

Mechanism of the Transition Metal Promoted Conversion of X2C:PR to RC.tplbond.P” Organometallics 1994, 13, 2444.

Jutzi, P. “New Element-Carbon (p-p)π Bonds” Angew Chem., Int. Ed. Engl. 1975, 14, 232.

Kamikawa, T.; Hayashi, T. “Enantioposition-Selective Arylation of Biaryl Ditriflates by Palladium-Catalyzed Asymmetric Grignard Cross-Coupling” Tetrahedron 1999, 55, 3455.

Kanbara, T.; Takase, S.; Izumi, K.; Kagaya, S.; Hasegawa, K. “Palladium-Catalyzed Polycondenstion of Diiodobenzenes with 1,3-Bis(phenylphosphino)propane and Preparation of Polymer Tansition-Metal Complexes” Macromolecules 2000, 33, 657.

Kano, T.; Ohyabu, Y.; Saito, S.; Yamamoto, H. “Asymmetric Carbon-Carbon Coupling of Phenols or Anilines with Aryllead Triacetates” J. Am. Chem. Soc. 2002, 124, 5363.

Kato, T.; Gornitzka, H.; Baceiredo, A.; Savin, A.; Bertrand, G. “On the Electronic Structure of (Phosphino)(silyl)carbenes: Single-Crystal X-ray Diffraction and ELF Analyses” J. Am. Chem. Soc. 2000, 122, 998.

Kawanami, H.; Toyota, K.; Yoshifuji, M. “Preparation and Photoisomerization of 2- Phosphaethenylbenzenes Having More Than One Phosphorus-Carbon Double Bond” J. Organomet. Chem. 1997, 535, 1.

Kim, S.-J.; Hamilton, T. P.; Schaefer, H. F. III “Methylphosphinidene (CH3P) and its Rearrangement to Phosphaethylene (CH2PH): Toward the Observation of Ground-State Triplet CH3P” J. Phys. Chem. 1993, 97, 1872.

Kim, S. T.; Hwang, D.-H.; Li, X.-C.; Grüner, J.; Friend, R. H.; Holmes, A. B.; Shim, H.- K. “Efficient Green Electroluminescent Diodes Based on Poly(2-dimethyloctylsilyl-1,4- phenylenevinylene)” Adv. Mater. 1996, 8, 979.

Kipping, F. S. “Organic Derivatives of Silicon. XXXIII. An Amorphous Variety of Octaphenyl-Cyclosilicotetrane” J. Chem. Soc.(Abstr.) 1927, 2719.

Kipping, F. S.; Sands, J. E. “Organic Derivatives of Silicon. XXV. Saturated and Unsaturated Silicohydrocarbons, Si4Ph8” J. Chem. Soc. (Abstr.) 1921, 119, 830.

Kipping, F. S.; Sands, J. E. “Organic Derivatives of Silicon. XXVI. Piperidine as an Analytical Reagent” J. Chem. Soc. (Abstr.) 1921, 119, 848.

Klebach, T. C.; Lourens, R.; Bickelhaupt, F. “Synthesis of Mesityldiphenylmethylenephosphine: a Stable Compound with a Localized Phosphorus=Carbon Bond” J. Am. Chem. Soc. 1978, 100, 4886.

191

Klinkhammer, K. W.; Fässler, T. F.; Grützmacher, H. “The Formation of Heteroleptic Carbene Homologues by Ligand Exchange - Synthesis of the First Plumbanediyl Dimer” Angew. Chem. Int. Ed. 1998, 37, 124.

Köbrich, G. “Eliminations from Olefins” Angew. Chem. Int. Ed. Engl. 1965, 4, 49.

Koebrich, G.; Trapp, H. “The Existence of Halo- and Alkali-Metal-Substituted Carbons. Proof and α-elimination of Metalated 1,1-Diaryl-2-chloroethylenes” Z. Naturforsch. 1963, 186, 1125.

Kohler, H.; Michaelis, A. “Isolation of Phosphobenzene” Ber. Dtsch. Chem. Ges. 1877, 10, 807.

Kolodyazhnyi, O. I. “Phosphenes” Zh. Obshch. Khim. 1980, 50, 230.

Kondo, K.; Okuda, M.; Fujitani, T. “Synthesis and Nonlinear Optics of Soluble poly(phenyleneethynylene)” Macromolecules 1993, 26, 7382.

Konze, W. V.; Young, Jr., V. G.; Angelici, R. J. “Functionalization of 1, 2-Bridging Cyaphide (C≡P)- Ligands: Trinuclear η1,η1,η2-Bridging Cyaphide and Dinuclear Bridging Isocyaphide (C≡PR) Complexes of Platinum” Organometallics 1999, 18, 258.

Kosolapoff, G. M.; Maier, L. Organic Phosphorus Compounds, Vol.4, Wiley-Inter- Science, 1972, p.75.

Kraft, A.; Grimsdale, A. C.; Holmes, A. B. “Electroluminescent Conjugated Polymers- Seeing Polymers in a New Light” Angew. Chem. Int. Ed. 1998, 37, 402.

Kuchen, W.; Buchwald, H. “Organophosphorus Compounds. I. Tetraphenylcyclo- tetraphosphine” Chem. Ber. 1958, 91, 2296.

Kuryla, W. C.; Papa, A. J. Flame Retardancy of Polymeric Materials, Vol. 1-5, Marcel Dekker, New York, 1973.

Laali, K. K.; Geissler, B.; Regitz, M.; Houser, J. J. “C-Protonation of Adamantylphosphaacetylene (1-AdC.tplbond.P) and tert-Butylphosphaacetylene (tBuC.tplbond.P) in Superacids: Phosphavinyl Cation Generation and Trapping To Form Phosphaalkenes, Formation of Isomeric Boron-Containing Spirocyclic Betaines by Reaction of 1-AdC.tplbond.P with B(OTf)3, and Theoretical Studies on Protonation of MeC.tplbond.P” J. Org. Chem. 1995, 60, 6362.

Lammertsma, K. “Phosphinidenes” Top. Curr. Chem. 2003, 229, 95.

Lavigne, J. J.; Broughton, D. L.; Wilson, J. N.; Erdogan, B.; Bunz, U. H. F. “‘Surfactochromic’ Conjugated Polymers: Surfactant Effects on Sugar-Substituted PPEs” Macromolecules 2003, 36, 7409.

192

Le Floch, P.; Marinetti, A.; Ricard L.; Mathey, F. “Synthesis, Structure, and Reactivity of (Phosphoranylidenephosphine)pentacarbonyltungsten Complexes. Another Access to the Phosphorus-Carbon Double Bond” J. Am. Chem. Soc. 1990, 112, 2407.

Le Floch P.; Mathey, F. “A Simplified Version of the Phospha-Wittig Synthesis of Phosphorus-Carbon Double Bonds from Carbonyl Compounds” Synlett 1990, 171.

Leopold, D. G.; Murray, K. K.; Miller, A. E. S.; Lineberger, W. C. “Methylene: a Study of the ~X3B1 and ~a1A1 States by Photoelectron Spectroscopy of CH2- and CD2-” J. Chem. Phys. 1985, 83, 4849.

Leuer, L.; Schmutzler, R. “A New Approach to the Synthesis of Aryldifluorophosphines. Formation of cis-Dichlorobis(aryldifluorophosphine)platinum(II) Complexes” J. Fluorine Chem. 1988, 39, 197.

Li, X.; Lei, D.; Chiang, M. Y.; Gaspar, P. P. “General Approaches to Phosphinidenes via Retroadditions” J. Am. Chem. Soc. 1992, 114, 8526.

Li, X.; Weissman, S. I.; Lin, T. S.; Gaspar, P. P.; Cowley, A. H.; Smirnov, A. I. “Observation of a Triplet Phosphinidene by ESR Spectroscopy” J. Am. Chem. Soc. 1994, 116, 7899.

Lines, E. L.; Centofanti, L. F. “Basicity of Substituted Difluorophosphines” Inorg. Chem. 1974, 13, 2796.

Lines, E. L.; Centofanti, L. F. “Preparation and Characterization of Vinyldifluorophosphine” Inorg. Chem. 1974, 13, 1517.

Lines, E. L.; Centofanti, L. F. “Synthesis of Propynyldifluorophosphine” Inorg. Chem. 1973, 12, 598.

Lockwood, M. A.; Clark, J. R.; Parkin, B. C.; Rothwell, I. P. “Intramolecular Activation of Aromatic C-H Bonds by Tantalum Alkylidene Groups: Evaluating 'Cyclometalation Resistant' Aryloxide Ligation” Chem. Commun. 1996, 1973.

Lockwood, M. A.; Fanwick, P. E.; Eisenstein, O.; Rothwell, I. P. “Mechanistic Studies of the Facile Four-Electron Reduction of Azobenzene at a Single Tungsten Metal Center” J. Am. Chem. Soc. 1996, 118, 2762.

Lucht, B. L.; Onge, N. O. St. “Synthesis and Characterization of Poly(p- phenylenephosphine)s” Chem. Commun. 2000, 2097.

Lutsenko, I. F.; Prishchenko, A. A.; Borisenko, A. A.; Novikova, Z. S. “Synthesis and Rearrangements in a Series of Element-substituted Phosphoethylenes” Dokl. Akad. Nauk, SSSR, 1981, 256, 1401.

193

MacDiarmid, A. G. “‘Synthetic Metals’: A Novel Role for Organic Polymers” Angew. Chem. Int. Ed. 2001, 40, 2581.

Mack, A.; Pierron, E.; Allspach, T.; Bergsträsser, U.; Regitz, M. “Organophosphorus Compounds. Part 129. Mesitylphosphaacetylene. Synthesis and Reactivity Studies of a New Phosphaalkyne” Synthesis 1998, 1305.

Maerkl, G.; Hennig, R.; Noeth, H.; Schmidt, M. “1λ3σ2,6λ3σ2-Diphospha-1,2,4,5- Hexatetraen aus 1,6-Di-H-1,6-diphosphino-2,4-hexadiin” Tetrahedron Lett. 1995, 36, 6429.

Maerkl, G.; Sejpka, H. “2-(2,4,6-Tri-tert-butylphenyl)-1-phosphaethyne and 1,4- Bis(trimethylsiloxy)-1,4-bis(2,4,6-tri-tert-butylphenyl)-2,3-diphosphabutadiene” Tetrahedron Lett. 1986, 27, 171.

Manners, I.; Allcock, H. R.; Renner, G.; Nuyken, O. “Poly(carbophosphazenes): A New Class of Inorganic-Organic Macromolecules” J. Am. Chem. Soc. 1989, 111, 5478.

Manners, I. “Polymers and the Periodic Table: Recent Developments in Inorganic Polymer Science” Angew. Chem. Int. Ed. Engl. 1996, 35, 1602.

Marinetti, A.; Mathey, F. “A Novel Entry to the PC-Double Bond: the Phospha-Wittig Reaction” Angew Chem. Int. Ed. Engl. 1988, 27, 1382.

Mark J. E.; Allcock, H. R.; West, R. Inorganic Polymers, Prentice Hall, Englewood Cliffs, New Jersey, 1992.

Märkl, G. “2,4,6-Triphenylphosphabenzene” Angew Chem. Int. Ed. Engl. 1966, 5, 846.

Märkl, G.; Kreitmeier, P.; Nöth, H.; Polborn, K. “1,6-Diphospha-1,2,4,5-hexatetraene: Synthesis and Ring Closure Reaction to Cyclobutenes” Angew. Chem. Int. Ed. Engl. 1990, 29, 927.

Markovskii, L. N.; Koidan, G. N.; Marchenko, A. P.; Romanenko, V. D.; Povolotskii, M. I.; Pinchuk, A. M. “(2,2,6,6-Tetramethylpiperidino)phosphaethyne” Zh. Obshch. Khim. 1989, 59, 2133.

Markovski, L. N.; Romanenko, V. D. “Phosphaalkynes and Phosphaalkenes” Tetrahedron 1989, 45, 6019.

Maryanoff, B. E.; Reitz, A. B. “The Wittig Olefination Reaction and Modifications involving Phosphoryl-stabilized Carbanions. Stereochemistry, mechanism, and selected synthetic aspects” Chem. Rev. 1989, 89, 863.

194

Masamune, S.; Hanzawa, Y.; Murakami, S.; Bally, T.; Blount, J. F. “Cyclotrisilane (R2Si)3 and Disilene (R2Si:SiR2) System: Synthesis and Characterization” J. Am. Chem. Soc. 1982, 104, 1150.

Mathey, F. “Phospha-Organic Chemistry: Panorama and Perspectives” Angew. Chem. Int. Ed. 2003, 42, 1578.

Mathey, F.; Tran Huy, N. H.; Marinetti, A. “Electrophilic Terminal-Phosphinidene Complexes: Versatile Phosphorus Analogues of Singlet Carbenes” Helv. Chim. Acta 2001, 84, 2938.

McWilliams, A. R.; Dorn, H.; Manners, I. “New Inorganic Polymers Containing Phosphorus” Topics in Current Chemistry 2002, 220, 141.

Meidine, M. F.; Lemos, M. A. N. D. A.; Pombeiro, A. J. L.; Nixon, J. F.; Hitchcock, P. B. “Stepwise Reduction of a Phosphaalkyne P≡C Bond to a Phosphaalkene and a Phosphine 1 at the FeH(dppe)2 Centre. Crystal and Molecular Structure of the η -co-ordinated 1 t Phosphaalkyne Complex trans-[FeH(η -P≡CBu )(dppe)2][BPh4]” J. Chem. Soc., Dalton Trans. 1998, 3319.

Meyer, H.; Baum, G.; Massa, W.; Berger, S.; Berndt, A. “A Stable Stannaethene” Angew Chem. Int. Ed. Engl. 1987, 26, 546.

Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.; Yoshifuji, M. “(η3- Allyl)palladium Complexes Bearing Diphosphinidenecyclobutene Ligands: Highly Active Catalysts for the Hydroamination of 1,3-Dienes” Angew. Chem., Int. Ed. 2001, 40, 4501.

Moezzi, A.; Olmstead, M. M; Power, P. P. “Boron-Boron Double Bonding in the Species 2- [B2R4] : Synthesis and Structure of [{(Et2)Li}2{Mes2BB(Mes)Ph}], a Diborane (4) Dianion Analogue of a Substituted Ethylene” J. Am. Chem. Soc. 1992, 114, 2715.

Montali, A.; Bastiaansen, C.; Smith, P.; Weder, C. “Polarizing Energy Transfer in Photoluminescent Materials for Display Applications” Nature 1998, 392, 261.

Mootz, D.; Händler, V. “Crystal Structure of the Menshutkin Complex Benzene- Antimony Trichloride (1:2)” Z. Anorg. Allg. Chem. 1986, 533, 23.

Moroni, M.; Le Moigne, J.; Pham, T. A.; Bigot, J.-Y. “Rigid Rod Conjugated Polymers for Nonlinear Optics. 3. Intramolecular H Bond Effects on Poly(phenyleneethynylene) Chains” Macromolecules 1997, 30, 1964.

Mulliken, R. S. “Overlap Integrals and Chemical Binding” J. Am. Chem. Soc. 1950, 72, 4493.

195

Nalwa, H. S. (Ed.) Handbook of Conductive Materials and Polymers, John Wiley and Sons: New York, 1997.

Nguyen, M. T.; Creve, S.; Vanquickenborne, L. G. “Properties of Phosphorus Compounds by Density Functional Theory: CH3P Species as a Test Case” J. Chem. Phys. 1996, 105, 1922.

Nguyen, M. T.; Ha, T.-K. “Ab Initio Study of Structures and Relative Stabilities of RCP (R = H,F) and Their Energetically Higher-Laying Isomers RPC” Theochem 1986, 32, 145.

Nguyen, M. T.; Van Keer, A.; Eriksson, L. A.; Vanquickenborne, L. G. “Some Calculated Properties of Phenylphosphinidene (C6H5P)” Chem. Phys. Lett. 1996, 254, 307.

Nguyen, M. T.; Van Keer, A.; Vanquickenborne, L. G. “In Search of Singlet Phosphinidenes” J. Org. Chem. 1996, 61, 7077.

Niemeyer, M. “σ-Carbon versus π-Arene Interactions in the Solid-State Structures of Trimeric and Dimeric Copper Aryls (CuAr)n (n = 3, Ar = 2,6-Ph2C6H3; n = 2, Ar = 2,6- Mes2C6H3)” Organometallics 1998, 17, 4649.

Nixon, J. F. “Coordination Chemistry of Compounds Containing Phosphorus-Carbon Multiple Bonds” Chem. Rev. 1988, 88, 1327.

Nixon, J. F. “Phosphaalkynes: New Building Blocks in Organic, Inorganic, and Organometallic Chemistry” Endeavour 1991, 15, 49.

Nixon, J. F. “Phospha-alkynes, RC≡P: New Building Blocks in Inorganic and Organometallic Chemistry” Chem. Soc. Rev. 1995, 319.

Nixon, J. F. “The Fascinating Organometallic Chemistry of Phosphaalkynes” Chem. Ind. 1993, 404.

Nixon, J. F. “ Recent Developments in the Organometallic Chemistry of Phosphaalkynes, RC≡P” Coord. Chem. Rev. 1995, 145, 201.

Nyulaszi, L.; Veszpremi, T.; Reffy, J. “A New Look at the Similarities of the Conjugative Ability and Reactivity of Phosphorus-Carbon and Carbon-Carbon Double Bonding” J. Phys. Chem. 1993, 97, 4011.

Oberhammer, H.; Becker, G.; Gresser, G. “Molecular Structures of Phosphorus Compounds. Part IX. Gas-Phase Structure of 2,2-dimethylpropylidynephosphine, (CH3)3C-C.tplbond.P” J. Mol. Struct. 1981, 75, 283.

Ogata, J.; Yamashita, M. “Kinetics of the One-electron Transfer Reaction of Trimethyl Phosphite with Quinones” J. Org. Chem. 1973, 38, 3423.

196

Ohno, K.; Kurita, E.; Kawamura, M.; Matsuura, H. “Gas-Phase Infrared Spectra of the Unstable Phosphaalkenes CF2:PH, CF2:PCF3, and CH2:PC: the C:P Stretching Vibration and Force Constant” J. Am. Chem. Soc. 1987, 109, 5614.

Osman, F. H.; Al-Samahy, F. A. “Reactions of -Diketones and o-Quinones with Phosphorus Compounds” Chem. Rev. 2002, 102, 629.

Pearson, P. K.; Schäfer, H. F.; Wahlgren, U. “Potential Energy Surface for the Model Unimolecular Reaction Hydrogen Isocyanide.far.Hydrogen Cyanide” J. Chem. Phys. 1975, 62, 350.

Peckham, T. J.; Massey, J. A.; Honeyman, C. H.; Manners, I. “Living Anionic Polymerization of Phosphorus-Bridged [1]Ferrocenophanes: Synthesis and Characterization of Well-Defined Poly(ferrocenylphosphine) Homopolymers and Block Copolymers” Macromolecules 1999, 32, 2830.

Pellerin, B.; Guenot, P.; Denis, J. M. “Methylidenephosphine” Tetrahedron Lett. 1987, 28, 5811.

Pellon, P.; Hamelin, J. “New access to Functionalized Dichlorophosphines: Synthesis of two Coordinated Phosphorus Heterocycles” Tetrahedron Lett. 1986, 27, 5611.

Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. “Synthesis and i Characterization of 2,6-Dipp2-H3C6SnSnC6H3-2,6-Dipp2 (Dipp = C6H3-2,6-Pr 2): A Tin Analogue of an Alkyne” J. Am. Chem. Soc. 2002, 124, 5930.

Pitzer, K. S. “Repulsive Forces in Relation to Bond Energies, Distances and Other Properties” J. Am. Chem. Soc. 1948, 70, 2140.

Plack, V.; Goerlich, J. R.; Fischer, A.; Thönnessen, H.; Jones, P. G.; Schmutzler, R. “Organophosphorus Compounds with Tertiary Alkyl Substituents. V. Synthesis and Reactions of Triphenylmethyl-Substituted Dihalophosphines TrtPX2 (Trt = triphenylmethyl-, X = F, Cl, Br); X-Ray-Crystal Structures of Triphenylmethyl Dichlorophosphine TrtPCl2 and Triphenylmethylthiophosphonic Difluoride TrtP(:S)F2” Z. Anorg. Allg. Chem. 1995, 621, 1080.

Pommer, H.; Thieme, P. C. “Industrial Applications of the Wittig Reaction” Top. Curr. Chem. 1983, 109, 165.

Power, P. “π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements” Chem. Rev. 1999, 99, 3463.

Power, P. “Homonuclear Multiple Bonding in Heavier Main Group Elements” J. Chem. Soc., Dalton Trans. 1998, 2939.

197

Preparation of Phosphide Intermediates for Polymerization Catalysts. Neth Appl 1989, 28, NL 8800770.

Protasiewicz, J. D. “Reduction of Intermolecular Association in the Sterically Encumbered (Dichloroiodo)arene ArICl2 [Ar = 2,6-bis(3,5-dichloro-2,4,6- trimethylphenyl)benzene]” J. Chem. Soc. Chem. Commun. 1995, 1115.

Pu, L.; Twamley, B.; Haubrich, S. T.; Olmstead, M. M.; Mork, B. V.; Simons, R. S.; Power, P. P “Triple Bonding to Germanium: Characterization of the Transition Metal 5 Germylynes (η -C5H5)(CO)2M≡Ge-C6H3-2,6-Mes2 (M = Mo, W; Mes = -C6H2-2,4,6-Me3) 5 and (η -C5H5)(CO)2M Ge-C6H3-2,6-Trip2 (M = Cr, Mo, W; Trip = -C6H2-2,4,6-i-Pr3) 5 and the Related Single Bonded Metallogermylenes ( -C5H5)(CO)3M-Ge-C6H3-2,6-Trip2 (M = Cr, W)” J. Am. Chem. Soc. 2000, 122, 650.

Pu, L.; Twamley, B.; Power, P. P. “Synthesis and Characterization of 2,6- Trip2H3C6PbPbC6H3-2,6-Trip2 (Trip = C6H2-2,4,6-i-Pr3): A Stable Heavier Group 14 Element Analogue of an Alkyne” J. Am. Chem. Soc. 2000, 122, 3524.

Regitz, M.; Binger, P. Multiple Bonds and Low Coordination in Phosphorus Chemistry, Eds. Regitz, M.; Scherer, O. J.; Georg Thieme, Stuttgart, 1990, ch.2.

Regitz, M. Heteroatom Chemistry, Ed. Block, E., VCH, New York, 1990, p.295.

Regitz, M. “Organophosphorus Compounds. 75. Phosphaalkynes - New Building Blocks in Heterocyclic Chemistry” J. Heterocycl. Chem. 1994, 31, 663.

Regitz, M. “Phosphaalkynes: New Building Blocks in Synthetic Chemistry” Chem. Rev. 1990, 90, 191.

Regitz, M.; Scherer, O. J. Multiple Bonds and Low Coordination in Phosphorus Chemistry Thieme, Stuttgart, 1990.

Rheigold, A. C.; Sullivan, P. J. “Crystal and Molecular Structure of Hexaphenylcyclohexaarsine, c-(AsPh)6” Organometallics 1983, 2, 327.

Rigon, L.; Ranaivonjatovo, H.; Escudie, J. “Novel Isolable C,C-Dichlorophosphaalkenes RP:CCl2 Owing to the Use of New Huge Stabilizing Groups” Phosphorus, Sulfur Silicon Relat. Elem. 1999, 152, 153.

Robinson, G. H. “On the Organometallic Chemistry of Gallium and the Dynamic of Ga- Ga Bond Formation” Chem. Commun. 2000, 2175.

198

Romanenko, V. D.; Sanchez, M.; Sarina, T. V.; Mazieres, M. R.; Wolf, R. “A New Synthesis of a 1-Phosphaalkyne via a Phosphaalkene Palladium(0) Complex Reaction” Tetrahedron Lett. 1992, 21, 2981.

Rösch, W.; Allspach, T.; Bergsträsser, U.; Regitz, M. Multiple Bonds and Low Coordination in Phosphorus Chemistry (Eds.: Rigitz, M; Scherer, O. J.), Thieme, Stuttgart, 1990, p.11-16.

Rösch, W.; Hees, U.; Regitz, M. “Unusually Coordinated Phosphorus Compounds. 19. 1,2,4-Diazaphospholes by [3 + 2] Cycloaddition of Diazo Compounds to a Stable Phosphaalkyne” Chem. Ber. 1987, 120, 1645.

Saednya, A.; Hart, H. “Two Efficient Routes to meta-Terphenyls from 1,3- Dichlorobenzenes” Synthesis, 1996, 1455.

Saito, S.; Kano, T.; Ohyabu, Y.; Yamamoto, H. “Direct Coupling of Anilines with Aryllead Triacetates” Synlett 2000, 1676.

Salaneck, W. R.; Lundstrom, I.; Haung, W. S.; MacDiarmid, A. G. “A Two- Dimensional-Surface 'State Diagram' for Polyaniline” Synth. Met. 1986, 13, 291.

Salbeck, J. “Electroluminescence with Organic Compounds” Berichte der Bunsen- Gesellschaft 1996, 100, 1667.

Sarnecki, G. J.; Burn, P. L.; Kraft, A.; Friend, R. H.; Holmes, A. B. “The Synthesis and Characterization of Some Poly(2,5-dialkoxy-1,4-phenylenevinylenes)” Synth. Met. 1993, 55-57, 914-917.

Sato, T.; Jiang, D.-L.; Aida, T. “A Blue-Luminescent Dendritic Rod: Poly(phenyleneethynylene) within a Light-Harvesting Dendritic Envelope” J. Am. Chem. Soc. 1999, 121, 10658.

Schmid, M.; Eberhardt, R.; Klinga, M.; Leskelae, M.; Rieger, B. “New C2v- and Chiral C2-Symmetric Olefin Polymerization Catalysts Based on Nickel(II) and Palladium(II) Diimine Complexes Bearing 2,6-Diphenyl Aniline Moieties: Synthesis, Structural Characterization, and First Insight into Polymerization Properties” Organometallics 2001, 20, 2321.

Schmidpeter, A. “Multiple Bonds and Low Coordination in Phosphorus Chemistry” Regitz, M. and Schere, O. J. Ed.; Thieme Verlag: Stuttgard, 1990, 338.

Schmidt, M. W.; Truong, P. N.; Gordon, M. S. “π Bond Strengths in the Second and Third Periods” J. Am. Chem. Soc. 1987, 109, 5217.

199

Schmutzler, R.; Stelzer, O.; Liebman, J. F. “Catalytic and Autocatalytic Disproportionation Reactions of Fluorophosphines and Related Lower Valence Nonmetal Fluorides” J. Fluorine Chem. 1984, 25, 289.

Schoeller, W. W. “The (4 + 2) Cycloaddition Properties of Heteroatom Double Bond Systems. A Frontier Orbital Approach to Reactivity” J. Chem. Soc., Chem. Commun. 1985, 334.

Seel, F.; Rudolph, K. H. “Dimethylfluorophosphine and Dimethyldifluorophosphorane” Z. Anorg. All. Chem. 1968, 363, 233.

Shah, S.; Burdette, S. C.; Swavey, S.; Urbach, F. L.; Protasiewicz, J. D. “Alkali Metal Induced Rupture of a Phosphorus-Phosphorus Double Bond. Electrochemical and EPR Investigations of New Sterically Protected Diphosphenes and Radical Anions [ArPPAr]·-” Organometallics 1997, 16, 3395.

Shah, S.; Concolino, T.; Rheingold, A. L.; Protasiewicz, J. D. “Sterically Encumbered Systems for Two Low-Coordinate Phosphorus Centers” Inorg. Chem. 2000, 39, 3860.

Shah, S.; Eichler, B. E.; Smith, R. C.; Power, P. P.; Protasiewicz, J. D. “Synthesis and Solid State Structures of Increasingly Sterically Crowded 1,4-Diiodo-2,3,5,6- tetraarylbenzenes. A New Series of Bulky Benzenes and Aryls” New J. Chem. 2003, 27, 442.

Shah, S.; Protasiewicz, J. D. “"Phospha-Variations" on the Themes of Staudinger and Wittig: Phosphorus Analogs of Wittig Reagents” Coord. Chem. Rev. 2000, 210, 181.

Shah, S.; Protasiewicz, J. D. “'Phospha-Wittig' Reactions Using Isolable Phosphoranylidenephosphines ArP:PR3 (Ar = 2,6-Mes2C6H3 or 2,4,6-t-Bu3C6H2)” Chem. Commun. 1998, 15, 1585.

Shah, S.; Simpson, M. C.; Smith, R. C.; Protasiewicz, J. D. “Three Different Fates for Phosphinidenes Generated by Photocleavage of Phospha-Wittig Reagents ArP=PMe3” J. Am. Chem. Soc. 2001, 123, 6925.

Shah, S.; Yap, G. P. A.; Protasiewicz, J. D. “Crystal Structure of the Phosphanylidene-σ4- phosphorane DmpP=PMe3 (Dmp=2,6-Mes2C6H3) and Peactions with Plectrophiles” J. Organomet. Chem. 2000, 608, 12.

Shirakawa, H. “The Discovery of Polyacetylene Film: The Dawning of an Era of Conducting Polymers” Angew. Chem. Int. Ed. 2001, 40, 2574.

Sidky, M. M. Boulos, L. S. “Phosphonium Ylides, IV. The Reaction of Tetrahalo-o- benzoquinones with Diphenylmethylenetriphenylphosphorane and Fluorenylidene- triphenylphosphoranes. An Anomalous Behavior of the Wittig Reagent” Phosphorus Sulfur Relat. Elem. 1984, 19, 27.

200

Simonetta, M.; Carra, S. “Molecular Orbital Calculations for the Fritsch Rearrangement Reaction” Tetrahedon 1963, 19, 467.

Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. “Synthesis and Characterization of the Monomeric Diaryls M{C6H3-2,6-Mes2}2 (M = Ge, Sn, or Pb; Mes = 2,4,6-Me3C6H2-) and Dimeric Aryl-Metal Chlorides [M(Cl){C6H3-2,6-Mes2}]2 (M = Ge or Sn)” Organometallics 1997, 16, 1920.

Simmons, W. W. The Stadtler Handbook of Ultraviolet Spectra; Stadtler Research Laboratories: Philadelphia, 1979.

Skotheim, T. A. (Ed.) Handbook of Conducting Polymers, Marcel Dekker, New York, 1986.

Slocum, D. W.; Jennings, C. A. “Directed Metalation Reactions. 6. Competition of Substituents for Ortho Direction of Metalation in Substituted Anisoles” J. Org. Chem. 1976, 41, 3653.

Smit, C. N.; Lock, F. M; Bickelhaupt, F. “2,4,6-Tri-tert-butylphenylphosphene- Dimesitylsilene; the First Phosphasilaalkene” Tetrahedron Lett. 1984, 25, 3011.

Smith, R. C.; Chen, X.; Protasiewicz, J. D. “A Fluorescent (E)-Poly(p- phenylenephosphaalkene) Prepared by a Phospha-Wittig Reaction” Inorg. Chem. 2003, 42, 5468.

Smith, R. C.; Protasiewicz, J. D. “Conjugated Polymers Featuring Heavier Main Group Element Multiple Bonds: A Diphosphene-PPV” J. Am. Chem. Soc. 2004, 126, 2268.

Smith, R. C.; Protasiewicz, J. D. “Systematic Investigation of PPV Analogue Oligomers Incorporating Low-Coordinate Phosphorus Centers” Eur. J. Inorg. Chem. 2004, 5, 998.

Smith, R. C.; Shah, S.; Protasiewicz, J. D. “A Role for Free Phosphinidenes in the Reaction of Magnesium and Sterically Encumbered ArPCl2 in Solution at Room Temperature” J. Organomet. Chem. 2002, 646, 255.

Solé, S.; Gornitzka, H.; Schoeller, W. W.; Boruissou, D.; Bertrand, G. “(Amino)(aryl)carbenes: Stable Singlet Carbenes Featuring a Spectator Substituent” Science 2001, 292, 1901.

Soliman, F. M.; Khalil, Kh. M.; Abd-El-Naim, G. “The Reaction of Phosphonium Ylides with 1,2-Benzo[a]phenazine-8,9-dione, Naphtho[2,1-b]furan-1,2-dione, Benzo[b] thiophene-2,3-dione and 1,2,3-Indantrione” Phosphorus and Sulfur and the Related Elements 1988, 35, 41.

201

Solouki, B.; Bock, H.; Appel, R.; Westerhaus, A.; Becker, G.; Uhl, G. “Gas-Phase Reactions. 31. Photoelectron Spectra of Methylidynephosphines RC.tplbond.P” Chem. Ber. 1982, 115, 3747.

Sonoda, Y.; Kaeriyama, K. “Preparation of Poly(2,5-diheptyl-1,4-phenylenevinylene) by Sulfonium Salt Pyrolysis” Bull. Chem. Soc. Jpn. 1992, 65, 853.

Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. “Synthesis and Characterization of a Digermanium Analogue of an Alkyne” Angew. Chem. Int. Ed. 2002, 41, 1785.

Su, J.; Li, X.-W.; Crittendon, R. C.; Robinson, G. H. “How Short is a -Ga Ga- Triple Bond? Synthesis and Molecular Structure of Na2[Mes*2C6H3-Ga Ga-C6H3Mes*2] (Mes* = 2,4,6-i-Pr3C6H2): The First Gallyne” J. Am. Chem. Soc. 1997, 119, 5471.

Swager, T. M. “The Molecular Wire Approach to Sensory Signal Amplification” Acc. Chem. Res. 1998, 31, 201.

Takagi, N.; Nagase, S. “Theoretical Study of an Isolable Compound with a Short Silicon- Silicon Triple Bond, (tBu3Si)2MeSiSi≡SiSiMe(SitBu3)2” Eur. J. Inorg. Chem. 2002, 2775.

Tokitoh, N.; Arai, Y.; Okazaki, R.; Nagase, S. “Synthesis and Characterization of a Stable Dibismuthene: Evidence for a Bi-Bi Double Bond” Science 1997, 277, 78.

Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase, S.; Uekusa, H.; Ohashi, Y. “A Unique Crystalline-State Reaction of an Overcrowded Distibene with Molecular Oxygen: The First Example of a Single Crystal to a Single Crystal Reaction with an External Reagent” J. Am. Chem. Soc. 1998, 120, 433.

Toyota, K.; Masaki, K.; Abe, T.; Yoshifuji, M. “Preparation and Structure of Dichloropalladium(II) Complexes of Sterically Protected Diphosphinidenecyclobutenes and their Synthetic Application as Homogeneous Catalysts” Chem. Lett. 1995, 221.

Toyota, K.; Tashiro, K.; Yoshifuji, M. “Synthesis and Structure of a Diphospha[4]radialene” Angew. Chem. Int. Ed. Engl. 1993, 32, 1163.

Toyota, K.; Tashiro, K.; Yoshifuji, M.; Nagase, S. “Preparation and Isomerization of 3,4- Bis(2,4,6-tri-tert-butylphenylphosphinidene) Cyclobutenes” Bull. Chem. Soc. Jpn. 1992, 65, 2297.

Tsang, C.-W.; Yam, M.; Gates, D. P. “The Addition Polymerization of a P:C Bond: A Route to New Phosphine Polymers” J. Am. Chem. Soc. 2003, 125, 1480.

Tsuji, K.; Sasaki, S.; Yoshifuji, M. “Synthesis and Redox Properties of a Diphosphene Carrying a Redox-Active Sterically Protecting Group” Tetrahedron Lett. 1999, 40, 3203.

202

Twamley, B.; Hardman, N. J.; Power, P. P. “A Terphenyl Halide Series: 2,6-Trip2-H3C6X (Trip = 2,4,6-triisopropylphenyl; X = Cl, Br, I)” Acta Cryst. 2000, C56, e514.

Twamley, B.; Haubrich, S. T.; Power, P. P. “Element Derivatives of Sterically Encumbering Terphenyl ligands” Adv. Organomet. Chem. 1999, 1.

Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. “Homologous Series of Heavier Element Dipnictenes 2,6-Ar2H3C6E=EC6H3-2,6-Ar2 (E = P, As, Sb, Bi; Ar = Mes = C6H2-2,4,6-Me3; or Trip = C6H2-2,4,6-iPr3) Stabilized by m-Terphenyl Ligands” J. Am. Chem. Soc. 1999, 121, 3357.

Urnezius, E.; Protasiewicz, J. D. “Synthesis and Structural Characterization of New Hindered Aryl Phosphorus Centers (aryl = 2,6-dimesitylphenyl)” Main Group Chem. 1996, 1, 369.

Van der Knaap, T. A.; Bickelhaupt, F. “The Reaction of a Phosphaalkene with Ortho- quinones” Tetrahedron 1983, 39, 3189.

Van der Knaap, T. A.; Klebach, T. C.; Bickelhaupt, F. “Synthesis and Structure of Aryl- Substituted Phosphaalkenes” Tetrahedron 1984, 40, 765.

Van der Sluis, M.; Bickelhaupt, F.; Veldman, N.; Kooijman, H.; Spek, A. L.; Eisfeld, W.; Regitz, M. “Synthesis and Characterization of β-Phosphaenones. An Investigation on the Conjugative Properties of the P=C Bond” Chem. Ber. 1995, 128, 465.

Van der Sluis, M.; Wit, J. B. M.; Bickelhaupt, F. “Reactivity of (Phosphaalkenyl)metal Species: Transmetalations and Reactions with Carbonyl Compounds” Organometallics 1996, 15, 174.

Vijjulatha, M.; Kumaraswamy, S.; Kumara Swamy, K. C.; Engelhardt U. “Synthesis and Structures of New Bicyclic 1,3-Di-t-butyl-1,3,2-λ3,4-λ3-Diazadiphosphetidines Containing Ten- and Eleven-membered Rings” Polyhedron 1999, 18, 2557.

Vincent, A. W.; Gates, D. P. “Poly(p-phenylenephosphaalkene): A π-Conjugated Macromolecule Containing P=C Bonds in the Main Chain” Angew. Chem. Int. Ed. 2002, 41, 2389.

Voleva, V. B.; Zhorin, V. A.; Khristyuk, A. L.; Ershov, V. V.; Enikolopyan, N. S. “Reaction of o-Benzoquinones with Benzylidenetriphenylphosphorane under Simultaneous Conditions of High Pressure and Shearing Deformation” Bull. Acad. Sci. USSR Div. Chem. Sci. (Engl. Transl.) 1983, 32, 402.

Wadsworth, W. S.; Emmons, W. D. “The Utility of Phosphonate Carbanions in Olefin Synthesis” J. Am. Chem. Soc. 1961, 83, 1733.

203

Walker, B. J. In Organophosphorus Reagents in Organic Synthesis; Cadogan, J. I. G., Ed.; Academic Press: New York, 1979; pp 155-205.

Waluk, J.; Klein, H.-P.; Ashe III, A. J.; Michl, J. “Effective Electronegativities of Phosphorus, Arsenic, and Antimony in a π System. Evidence from Magnetic Circular Dichroism” Organometallics, 1989, 8, 2804.

Weber, L. “The Quest for Isophosphaalkynes (Isophosphocyanides) C≡P−R — Still an Elusive Class of Compounds” Eur. J. Inorg. Chem. 2003, 1843.

Weber, L.; Schumann, I.; Schmidt, T. Stammler, H.-G.; Neumann, B. “Transition Metal Substituted Acylphosphines and Phosphaalkenes. 17. Synthesis and Structure of the µ- 5 Isophosphaalkyne Complexes [(η -C5H5)2(CO)2Fe2(µ-CO)(µ-C:PC6H2R3)] (R = Me, iso- Pr, tert-Bu)” Z. Anorg. Allg. Chem. 1993, 619, 1759.

Weber, L.; Schumann, I.; Stammler, H.-G.; Neumann, B. “Transition-Metal-Substituted Acylphosphines and Phosphaalkenes. XXI. Isophosphaalkynes as µ3-ligands in Transition Metal Complexes” Chem. Ber. 1994, 127, 1349.

Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P. “Incorporation of Photoluminescent Polarizers into Liquid Crystal Displays” Science 1998, 279, 835.

Wehmschulte, R. J.; Khan, M. A.; Hossain, S. I. “Reaction of m- Terphenyldichlorophosphanes with Sodium Azide: Synthesis and Characterization of Stable Azidocyclophosphazenes” Inorg. Chem. 2001, 40, 2756.

Weidenbruch, M. “Triple Bonds of the Heavy Main-Group Elements: Acetylene and Alkylidyne Analogues of Group” Angew. Chem. Int. Ed. 2003, 42, 2222.

West, R.; Fink, M. J.; Michl, J. “Tetramesityldisilene, a Stable Compound Containing a Silicon-Silicon Double Bond” Science (Washington, D. C.) 1981, 214, 1343.

Wiberg, N.; Niedermayer, W.; Fischer, G.; Nöth, H.; Suter, M. “Synthesis, Structure and Dehalogenation of the Disilene RClSi=SiClR [R = (tBu3Si)2MeSi]” Eur. J. Inorg. Chem. 2002, 1066.

Wittig, G.; Schollkopf, U. “Triphenylphosphinemethylene as an Olefin-Forming Reagent. I” Chem. Ber. 1954, 87, 1318.

Wudl, F.; Allemand, P. M.; Srdanov, G.; Ni, Z.; McBranch, D. “Polymers and an Unusual Molecular Crystal with Nonlinear Optical Properties” ACS Symp. Ser. 1991, 455.

Wudl, F. “Conducting Polymer Formed of Poly[2-methoxy-5-(2-ethylhexyloxy)-p- phenylene Vinylene]” (University of California), US-B 5189136, 1990 [Chem. Abstr. 1993, 118, 255575p].

204

Yamada, N.; Abe, K.; Toyota, K.; Yoshifuji, M. “Preparation of Sterically Protected 3,4- Bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene Derivatives Having Ring- Fused Structures” Organic Lett. 2002, 4, 569.

Yamaguchi, Y.; Schaefer, H. F. III “An Ab Initio Study on the Four Electronically Lowest-Lying States of CH2 Using the State-Averaged Complete Active Space Second- Order Configuration Interaction Method” Chem. Phys. 1997, 225, 23.

Yamamoto, T. “π-Conjugated Polymers Bearing Electronic and Optical Functionalities. Preparation by Organometallic Polycondensations, Properties, and Their Applications” Bull. Chem. Soc. Jpn. 1999, 72, 621.

Yang, J.-S.; Swager, T. M. “Fluorescent Porous Polymer Films as TNT Chemosensors: Electronic and Structural Effects” J. Am. Chem. Soc. 1998, 120, 11864.

Yoshifuji, M. “Coordination Chemistry of Some Low Coordinate Organophosphorus Compounds of Coordination Number 2” Bull. Chem. Soc. Jpn. 1997, 70, 2881.

Yoshifuji, M.; Ito, S.; Toyato, K.; Yasunnami, M. “Preparation of Some Multifunctionalized Methylenephosphines by Reactions of Chloro[(2,4,6-tri-t- butylphenyl)phosphinidene]methyllithiums with Carbonyl Compounds” Bull. Chem. Soc. Jpn. 1995, 68, 1206.

Yoshifuji, M.; Kawanami, H.; Kawai, Y.; Toyota, K.; Yasunami, M.; Niitsu, T.; Inamoto, N. “Phosphaethynes from Dichlorophosphaethenes by Lithiation, Photoisomerization, and Relithiation Involving Migration” Chem. Lett. 1992, 1053.

Yoshifuji, M.; Niitsu, T.; Inamoto, N. Chem. Lett. “Formation of a Phosphaalkyne via Migration of the Phenyl Group from Phosphorus to Carbon” Chem. Lett. 1988, 1733.

Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. “Synthesis and Structure of Bis(2,4,6-tri-tert-butylphenyl)diphosphene: Isolation of a True Phosphobenzene” J. Am. Chem. Soc. 1981, 103, 4587.

Yoshifuji, M.; Toyota, K.; Murayama, M.; Yoshimura, H.; Okamoto, A.; Hirotsu, K.; Nagase, S. “Preparation of Sterically Protected 3,4-Bis(phosphinidene)cyclobutenes and their Isomerism” Chem. Lett. 1990, 2195.

Zeldin, M.; Wynne, K. J.; Allcock, H. R. (Eds.) Inorganic and organometallic Polymers; ACS Symposium Series 360; American Chemical Society: Washington, DC, 1988, 250.

Zhang, C.; Höger, S.; Pakbaz, K.; Wudl, F.; Heeger, A. J. “Improved Efficiency in Green Polymer Light-Emitting Diodes with Air-Stable Electrodes” J. Electron. Mater. 1994, 23, 453.

205

Zhou, Q.; Swager, T. M. “Fluorescent Chemosensors Based on Energy Migration in Conjugated Polymers: The Molecular Wire Approach to Increased Sensitivity” J. Am. Chem. Soc. 1995, 117, 12593.

Zittel, P. F.; Lineberger, W. C. “Laser Photoelectron Spectrometry of PO-, PH-, and PH2-” J. Chem. Phys. 1976, 65, 1236.