9 - Reaction of Ylides with Saturated or α,β-Unsaturated Carbonyl Compounds

Abstract

It is doubtful as to whether the negative charge on the carbanionic C atom of P and S ylides is indeed stabilized by partial double bond formation to the heteroatom, as is expressed by the ylene resonance form. Probably, the formal charge at the carbanionic center of these ylides is rather stabilized to a considerable extent by two other effects: electron withdrawal due to the –I effect of the heteroatom and stabilization of the electron pair on the carbanionic carbon atom by an anomeric effect. P and S ylides are useful reagents in organic synthesis: P ylides condense with saturated and unsaturated carbonyl compounds to give olefins, S ylides react with α,β-unsaturated esters to give cyclopropanes, sulfoxonium ylides also react with α,β-unsaturated carbonylcompounds to give cyclopropanes, S ylides convert saturated carbonyl compounds into epoxides, and sulfonium ylides convert α,β-unsaturated carbonyl compounds into epoxides, but sulfoxonium ylides cannot do this because they react to form cyclopropanes. Although the Horner–Wadsworth–Emmons reaction is not an ylide reaction, it represents methodologically an important supplement to the Wittig reaction.