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  • The chemistry of organophosphorus compounds, Volume 3, Phosphonium salts, ylides and phosphoranesEdited by Frank R. Hartley. 1994 John Wiley & Sons, Ltd. ISBN: 0-471-93057-1

  • Contents

    1. Structure and bonding in phosphonium ylides, salts and phosphoranes 1 D. G. Gilheany

    2. Preparation, properties and reactions of phosphonium salts 45 H.-J. Cristau and F. Plenat

    3. Preparation, properties and reactions of phosphoranes 185 R. Burgada and R. Setton

    4. Structure, bonding and spectroscopic properties of phosphonium ylides 273 S. M. Bachrach and C. I. Nitsche

    5. Electrochemistry of ylides, phosphoranes and phosphonium salts 303 K. S. V. Santhanam

    6. Photochemistry of phosphonium salts, phosphoranes and ylides 325 M. Dankowski

    7. Chemical analysis of organophosphorus compounds 347 H. Feilchenfeld

    Author index 391

    Subject index 433

    XI

  • CHAPTER 1

    Structure and bonding in phosphonium ylides, salts and phosphoranes

    D. G. GILHEANY Department of Chemistry, St. Patricks College, Maynooth, Co-Kildare, Ireland

    I. INTRODUCTION 2 II. PHOSPHONIUM YLIDES 2

    A. Structure 3 1. Bond lengths 8 2. Bond angles 10 3. Conformation 10 4. sp ylides 12

    B. Bonding 13 1. Other experimental observations 13

    a. Bond energies and reactivity 13 b. Infrared spectroscopy 14 c. Dipole moments 14 d. Ultraviolet-visible spectroscopy 14 e. Nuclear magnetic resonance spectroscopy 15 f. Photoelectron spectroscopy 19

    2. Survey of theoretical calculations on phosphonium ylides 19 3. The two alternative views of the bonding in methylidenephosphorane 22

    a. Backbonding/negative hyperconjugation 23 b. Two bonds ( bonds/banana bonds) 23

    C. Summary 24 III. PHOSPHONIUM SALTS 24

    A. Structure 25 B. Bonding 25

    IV. PHOSPHONIUM SALT-PHOSPHORANE ISOMERISM 29 V. PHOSPHORANES 29

    A. Structure 30

    The chemistry of organophosphorus compounds, Volume 3, Phosphonium salts, ylides and pho Edited by Frank R. Hartley. 1994 John Wiley & Sons, Ltd. ISBN: 0-471-93057-1

    1

  • 2 D. G. Gilheany

    B. Bonding 31 1. Detailed studies of bonding and apicophilicity 35 2. Other studies 36

    VI. ADDENDUM 37 VII. REFERENCES 37

    I. INTRODUCTION The general features of bonding to phosphorus have been discussed in the first two volumes of this series1'2, including likely bonding schemes1, the strengths of single and double bonds to phosphorus1 and the non-involvement of virtual d orbitals in any of the three possible current descriptions of the phosphoryl bond2. This latter point is also relevant for the systems discussed here and, once again, as we shall see, there are more powerful alternative descriptions of the bonding than those involving d orbitals. Because there is a strong similarity between the structure and bonding in tertiary phosphine oxides and phosphonium ylides, the latter are discussed first here, followed by the less controversial salts and finally the pentacoordinate phosphoranes.

    II. PHOSPHONIUM YLIDES In phosphonium ylides, as we shall see in Section II.A, the phosphorus atom is tetracoordi-nate and the unique (anionic) carbon is tricoordinate. For exactly the same reasons as in the oxides2, all previous discussions3"14 of the C bond have been in terms of a resonance hybrid between a dipolar form 1A and a double bond form IB:

    R3PCR2 < R 3 P=CR 2 (1A) (IB)

    For the purposes of chemical reactivity, the dipolar form 1A is considered the more important3. Structure IB is meant to indicate dTr- bonding involving back-donation of electron density from a doubly occupied 2p orbital of the ylidic (anionic) carbon into vacant phosphorus 3d orbitals in an overlap scheme such as that in Figure 1. This dTi- bonding has been invoked to explain a number of the properties of phosphonium ylides (see Section II.B.l), particularly the fact that they are more stable than their nitrogen analogues, for which such a stabilizing interaction is not possible, nitrogen not having the requisite low-energy vacant orbitals315. Contrariwise, many authors have also taken these properties as evidence for dTr- bonding so that the structure and bonding of phosphonium ylides are part of the general controversy about dTr- bonding (see ref. 2 for a more general discussion). For these reasons, many studies in the past 20 years have addressed the two fundamental problems associated with phosphonium ylides, namely the extent (if any) of the contribution of structure IB and the related question of the geometry (i.e. configuration) at phosphorus and the anionic carbon.

    As discussed in detail in Volume 22, it is now known that d orbitals are not involved in bonding in Main Group compounds. In particular, it is now clear that the requirement for

    FIGURE 1. The obsolete view of the p-type overlap in phosphonium ylides

  • CHAPTER 2

    Preparation, properties and reactions of phosphonium salts

    H. J. CRISTAU and F. PLENAT Laboratoire de Chimie Organique, URA 458, Ecole Nationale Superieure de Chimie, Universite de Montpellier, 8 rue de I'Ecole Nor male, 34053-Montpellier Cedex 1, France

    I. PROPERTIES OF PHOSPHONIUM SALTS 47 A. Introduction 47 B. Analysis and Purification 48 C. Spectrometry 48

    1. NMR spectrometry 48 a. 3 1P NMR spectra 48 b. *H NMR spectra 53 c. 13C NMR spectra 54

    2. Infrared spectra 58 3. UV-visible spectra 59 4. Mass spectrometry 60 5. X-ray spectra 60 6. Others 62

    D. Thermochemistry 62 E. Photochemistry 63 F. Electrochemistry 64 G. Stereochemistry 66 H. Biochemistry 67 I. Others 68

    II. PREPARATION OF PHOSPHONIUM SALTS 69 A. Synthesis of Phosphonium Salts by Alkylation of Phosphines 69 B. Synthesis of Phosphonium Salts by Arylation of Tertiary Phosphines . . . 71 C. Synthesis of ,-Unsaturated Phosphonium Salts 73

    1. Introduction of an alk-1-enyl or alk-1-ynyl chain on a tertiary phosphine 73 a. Addition of phosphine to activated acetylenic compounds 73 b. Substitution by the phosphine of an activated vinylic halogen . . 74 c. Alkenylation of phosphines complexed on rhodium (I)

    by ,-unsaturated acid chlorides 74

    The chemistry of organophosphorus compounds, Volume 3, Phosphonium salts, ylides and phosphoranes Edited by Frank R. Hartley. 1994 John Wiley & Sons, Ltd. ISBN: 0-471-93057-1

    45

  • 46 . J. Cristau and F. Plenat

    d. Electrochemical alkenylation of phosphines by cycloalkenes . . . . 74 e. Alkynylation of phosphines by alk-1-ynyliodonium salts 75

    2. Creation or modification of an unsaturation on a phosphonium group 75 a. Isomerization of alk-2-enyl into alk-1-enyl-phosphonium salts . . 75 b. Synthesis by addition to alk-1-ynylphosphonium salts 77 c. Synthesis by elimination from an alkylphosphonium salt 77 d. Synthesis by modification of phosphonium ylides 79

    3. Alkylation of ,-unsaturated phosphines 79 D. Functional Phosphonium Salts 79

    1. -Functionalized phosphonium salts 79 a. Alcohols and derivatives 79 b. Sulphurated or seleniated compounds 79 c. Nitrogenous function 82 d. -Halogenated salts 83 e. Carbonyl functions and derivatives 83

    2. /?-Functionalized phosphonium salts 84 a. Phosphine alkylation by a /?-functionalized alkyl halide 84 b. Addition of YH compounds, with mobile hydrogen, to ,-

    unsaturated phosphonium salts 84 c. Functional chain creation from a phosphonium ylide 85

    3. rc-functionalized phosphonium salts (n^y) 87 E. Synthesis of Polyphosphonium Salts 88

    1. Multiphosphonium synthesis 88 2. Synthesis of polymeric phosphonium salts 92

    F. Synthesis of Cyclic Phosphonium Salts 92 1. Synthesis with cyclization on the phosphorus 92 2. Synthesis by cyclization between two chains linked to phosphorus . . 94

    G. Heterophosphonium Salts 96 1. Reaction of tricoordinated phosphorus compounds with heteroatomic

    oxidizing agents 97 2. Heteroatomic ligand exchange in heterophosphonium salts 99 3. Alkylation or arylation of heterosubstituted tricoordinated phosphorus

    compounds 101 4. Reaction of R 3 P = Y compounds with electrophilic reagents 103

    H. Synthesis by Anion Exchange or Modification 105 1. Anion exchange 105 2. Modification of anion 107

    III. REACTIVITY OF PHOSPHONIUM SALTS 108 A. Acidity of Phosphonium Salts 108 B. Reactivity Towards Organolithium Compounds 110 C. Alkaline Hydrolysis of Phosphonium Salts I l l

    1. Main reaction in the hydrolysis of phosphonium salts: 5N(P) mechanism 113 a. Nature of the leaving group 113 b. Kinetic characteristics of the reaction 115 c. Stereochemistry of the reaction 117

    2. Secondary reactions in the alkaline hydrolysis of phosphonium salts . . . 122 a. Alkaline hydrolysis of bisphosphonium salts with fragmentation: Ep

    mechanism 122 b. Alkaline hydrolysis with migration: SN(P)mig mechanism 127 c. Hofmann elimination reactions:

    mechanism 132 d. Elimination with vinylphosphonium formation: EHa mechanism . . . 137

  • 2. Preparation, properties and reactions of phosphonium salts 47

    3. Application of alkaline hydrolysis of phosphonium salts 137 D. Reduction of Phosphonium Salts 138

    1. Reduction using hydrides 138 2. Reduction using metals 140 3. Others 141

    E. Reactivity of Functional Chains 142 1. Reactivity induced by the phosphonio group 142

    a. Reactions with formation of P=Element double bond (-proton abstraction and basic hydrolysis excluded) 142

    b. Reactions with formation of tricoordinated phosphorus 148 c. Michael-type addition 149 d. Others 152

    2. Reactivity consistent with the phosphonio group 152 a. Phosphonium salts containing keto groups 152 b. Phosphonium salts containing hydroxy groups 154 c. Phosphonium salts containing halo groups 154 d. Phosphonium salts containing ethers or thioethers 155

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