Elimination and addition reactions. Part 29. Ylide rearrangement in adducts from allenic sulphonium salts and malonic esters

Abstract

Addition of malonic esters to allenic sulphonium salts gives isolable allylic adducts. When the adducts are treated with ethanolic sodium ethoxide they undergo 2,3-sigmatropic rearrangement of the derived ylide. When the central malonyi carbon atom bears an alkyl group, rearrangement occurs directly, but when this atom bears a hydrogen atom, ylide rearrangement is preceded by a 1,3-prototropic shift and occurs exclusively by way of the conjugated tautomer.

Rearrangement in the conjugated tautomer is regiospecific and occurs entirely through an S-methyl rather than an S-ethyl group. A by-product of the addition–rearrangement sequence appears to be derived from the competitive addition of alkoxide ion followed by displacement of the sulphonium group by malonate ion

When reactions were carried out in the probe of an n.m.r. spectrometer no emission signals were detected, showing that the alternative (Stevens) rearrangement pathway is probably unimportant.