Phosphorus ylides structure

Keywords Carbanion Stabilization / Phosphorus Ylides / Structure Determinations / Silyl Group Effects... 

Due to the presence of a heterocumulene unit, sulphines may be considered as a group of compounds which are able to undergo cycloaddition reactions. Reaction of sulphines with enamines and phosphorus ylides reported by Sheppard217 and Trippett218 and their coworkers may be considered formally as an example of [2 + 2] cycloaddition. In fact, Sheppard and Dickman217 obtained a 1 1 adduct from thiofluorenone S-oxide and 1-morpholinocyclohexene to which they assigned the dipolar sulphoxide structure 168. 

Further, Wasserman and coworkers developed a direct acylation of stabilized phosphonium ylides by carboxylic acids in presence of the EDCI/DMAP (way c). This last method allows the introduction of a-aminoacid structures into the resulting P-oxo phosphorus ylides [19-25],opening the way to the total synthesis of depsipeptide elastase inhibitors [22,24] or cyclic peptidic protease inhibitor EurystatinA [20]. 

This reaction may be visualized as proceeding by nucleophilic attack of tervalent phosphorus at the carbonyl group to give an intermediate such as (15). The structure of (16) was deduced from the fact that it was hydrolysed to the known phosphine oxide (17). Methylenephosphoranes (phosphorus ylides) may also be converted into monophosphazenes by reaction with benzonitrile ... 

Trzcinskabancroft, B., Knachel, H., Dudis, D., Delord, T.J. and Marler, D.O. (1985) Experimental And Theoretical-Studies Of Dinudear Gold(I) And Gold(II) Phosphorus Ylide Complexes - Oxidative Addition, Halide Exchange, And Structural-Properties Including The Crystal And Molecular-Structures Of [Au (CH2)2PPh2]2 And [Au(CH2)2PPh2]2(CH3) Bri. Journal of the American Chemical Society, 107(24), 6908-6915. 

Organometallic betaines of type I can be considered as the closest structural analogs of carbon betaines of the (+ )P-C-C-X( ) type (IV), which were regarded for a long time as possible intermediates in classical reactions of carbonyl and thiocarbonyl compounds with phosphorus ylides (Wittig and Corey-Chaykovsky reactions and related processes,5,6 Scheme 1). Vedejs and coworkers7,8 proved unambiguously that oxapho-sphetanes (III) are true intermediates in the reactions of nonstabilized phosphorus ylides with carbonyl compounds. The formation of oxabetaines (+)p-c-c-o(-> was detected only in the form of their adducts with lithium salts.9,10... 

The first kinetically stable dibenzosilafulvene (7), whose structure and properties should more correctly be described by the resonance hybrid 7a 7b with a great contribution of the ylide form 7a, reacts with phosphorus ylide to form betaine (8), which is rearranged, under thermodynamically controlled conditions, into the salt (9) (Scheme 4).24,25... 

Heterocyclic phosphorus ylides are a rather diverse and little known class of compounds. A variety of such structures are now known and in some cases these are of considerable synthetic value. In this chapter we have attempted to review all heterocyclic compounds containing one or more exocyclic phosphorus ylide functions, i.e. of general structure 1. It should be noted that in many cases these exist predominantly in the phosphonium ylide (P+—C ) form but for simplicity they are represented in the ylene form 1. Cyclic ylides 2 and 3 in which the phosphorus atom is within the ring are not included. 

In the course of investigations on a-sulphanyl carbenes, a new cleavage reaction was found. After irradiation the phosphorus ylide 53 yields nearly quantitatively the thiaazaphosphetane 54, in addition to triphenylphosphine (55) (equation 20)98. An insertion of the corresponding carbene intermediate in to the methine—CH bond is plausible. The structure of 54 was confirmed by single-crystal X-ray structure analysis98. 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

A second resonance structure can be written for a phosphorus ylide with a double bond between phosphorus and carbon. As a third-row element, phosphorus can have more than 8 electrons in its valence shell. 

Albanese, J. A. Staley, D. L. Rheingold, A. L. Burmeister, J. L. Synthesis and molecular structure of a dinuclear phosphorus ylide complex /t-dichlorobis[chloro(benzoylmeth-ylenetri-u-butylphosphorane) palladium(I I). 

Crystal structure determinations have not been carried out for a simple peralkylated phosphorus ylide, but a whole series of arylated species and more complicated systems have been carefully investigated (2, 6-8, 82, 8Jf, 85, 94). The results of these studies can be summarized as follows ... 

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