Ylides and Related Compounds

Overall levels of innovation in the area this year are disappointingly low. All the various forms of phosphorus-based olefination continue to be used widely in synthesis and perhaps the relative paucity of new phosphorus chemistry is a reflection on the extent to which these methods have been developed. One area which does continue to develop, and where there is still considerable potential, is the use of phosphorus stabilised anions in enantioselective and asymmetric synthesis. Warren s continuing use of phosphine oxides and Denmark s excellent contributions to this area are especially worthy of mention. 

In volume 29 this chapter will, for the first time since volume 11, have a new author. Much of the ylide chemistry discussed in volume 11 would not be out of place in the current volume. In volume 28 an even wider application in synthesis, helped by improvements in available bases and other reagents and conditions, is apparent. A better understanding of mechanism, particularly of olefin synthesis, is available and this has enabled improved control of stereochemistry in all its forms, although improved stereo-control has also derived from a greater use of phosphonate and phosphine oxide anions. However, the opportunities offered by the enormous improvements in spectroscopic techniques, particularly NMR and mass spectrometry, and chiral and preparative HPLC and the increased demands of modem biology and medicine have perhaps had the greatest influence. 

The chemistry of phosphonium salts, ylides and phosphoranes is covered in volume 3 of The Chemistry of Organophosphorus Compounds series.  

Preparation.—A historical account of the development of the synthesis of methylenephosphoranes has been given. Details have appeared - of the use of epoxides in the generation of ylides from phosphonium salts. Weakly acidic salts are deprotonated by the anions such as XCHaCH20 formed by attack of the phosphonium covmter-anion on epoxide, but strongly acidic phosphonium salts are deprotonated more rapidly than these species are produced. The base in these cases must be either epoxide or original anion. Side-reactions in olefin syntheses using epoxides as base include acetal formation from aldehyde and epoxide, cyclopropane formation from ylide and epoxide, and decomposition of quinquecovalent phosphoranes, e.g. (1), formed from phosphonium salt and 

Reactions.—Carbonyls. Mechanism. Additional evidence is appearing that 1,2-oxaphosphetans are formed directly from ylides and carbonyl compounds. Further studies on the kinetics of the reactions of phenacylidenetriphenyl-phosphoranes with substituted benzaldehydes in non-polar aprotic solvents and in glycols support the concept of a highly oriented transition state of low polarity. n.m.r. spectroscopy on solutions in which ylides and carbonyl compounds had been allowed to react at — 70 °C showed the presence of only 

2-oxaphosphetans. Orthogonal approach of the reactants, i.e. 2 + n2 , has been suggested. This would lead to the more sterically hindered 1,2-oxaphosphetan and hence to c/j-olefin, as found in general under conditions in which the first stage is irreversible. 

CNDO-MO calculations suggest that the Wittig olefin synthesis proceeds via 1,2-oxaphosphetans, which undergo P—C bond cleavage considerably in advance of P—O bond cleavage. 

A fine balance between steric and electronic effects in attempted olefin synthesis in protic solvents has been revealed in experiments involving furyl-and pyrrolyl-phosphonium salts. Thus although the salt (2 R = Ph) gave a high yield of styrene with benzaldehyde in ethanolic ethoxide solution, the t-butyl salt (2 R = Bu ) under the same conditions gave the styrylphosphine 

An improved synthesis of dichloromethylenetriphenylphosphorane has been reported.2 It can be isolated from the reaction of (dichloromethyl)triphenylphos-phonium chloride with an excess of bis(triphenylphosphoranylidene)methane provided a solvent is used which is unable to donate a proton to the strongly basic ylide.3 

A number of preparations of cumulated ylides from methylenetriphenylphos-phorane have been described. Reaction with geminal dihalogenoalkenes,4 thiophos-gene,5 and dichloro-imines8 (ClaC=NR) gave (1), (2), and (3) respectively. 

Ketenylidenetriphenylphosphorane and its thio-analogue (2) can be obtained from the corresponding ester ylides by treatment with sodium bis(trimethylsilyl)-amide (4).7 Salt-free solutions of alkylidenetriphenylphosphoranes can be conveni- 

Ph3P=CHCXMe + NaN(SiMe3)2------ Ph3P=C=C=X + NaXMe + HN(SiMe3)a 

Me3P=CHSi Me3 + MePF2— [Me3P=CHPMe3]+ F- + Me3SiF 

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