Dichotomous ylide formation from an alkyl(benzyl)phosphonium salt leading to stereoselective alkene syntheses

Abstract

Condensation reactions between the tributyl-(4-methoxy-2,6-dimethylbenzyl)phosphonium salt (1b) and several aryl alkyl ketones, in the presence of methylsulphinylmethanide ion, are shown to lead to mixtures of stilbene and styrene products, resulting from dichotomous ylide formation. By contrast, with p-anisaldehyde, the salt (1b) produces solely the stilbene (10), and condensation between the tributyl-(4-methoxybenzyl)phosphonium salt (8) and 4′-methoxyacetophenone produces only the stilbene (9). The olefin-forming reactions between (1b) and aryl alkyl ketones are completely stereoselective, leading to Z-stilbenes [e.g.(3)] and E-styrenes [e.g.(6)]. Explanations for the stereoselectivities are discussed.