Electro-organic reactions. Part 20. Electrogenerated bases, ylide formation, and Wittig alkene synthesis

Abstract

Dicyano(fluoren-9-ylidene)methane (1a), dicyano(2,7-dibromofluoren-9-ylidene)methane (1b), and ethyl 2-cyano-2-fluoren-9-ylideneacetate (1c) may be cathodically reduced in N,N-dimethylformamide to give dianions which efficiently convert phosphonium salts into ylides. The potentials required are sufficiently modest to allow the electrogeneration of the bases in the presence of several phosphonium salts and of several aldehydes to give a convenient method of carrying out the Wittig alkene synthesis. This method has been explored for the synthesis of stilbene, 1,4-diphenylbutadiene, and vitamin A acetate. The basicity of the dianions varies according to the choice of electrolyte cation (Bu₄N⁺ or Li⁺). The cations also have a marked effect on the stereochemical course of the reaction and, in particular, mixtures of alkenes are obtained in which the cis-isomer predominates in the presence of lithium ion; in the vitamin A acetate synthesis, the 11 -cis-isomer constitutes 76% of the product. The results are consistent with recent hypotheses concerning the mechanism of the Wittig reaction.