Vinyl cation formation by decomposition of vinyl-lead triacetates. The reactions of vinylmercury and vinyltin compounds with lead tetra-acetate

Abstract

Vinylmercury compounds and vinylstannanes undergo rapid metal–metal exchange with lead tetra-acetate in chloroform to generate vinyl-lead triacetates, unstable compounds which undergo a thermodynamically favourable reductive elimination of lead(II) acetate. In the presence of mercury(II) salts, vinyl-lead triacetates collapse to the corresponding enol acetate, but in the absence of mercury(II) they yield either the enol acetate or an acetylene depending on the substitution at the double bond. Evidence for the formation of vinyl cations in the collapse of a number of vinyl-lead triacetates has been obtained. Strong support for such intermediates comes from the decomposition of (E)-o-methoxystyryl-lead triacetate, where the nature of the participation by the neighbouring methoxy group excluded the possibility of involvement of an alkylidenecarbene. Attempted trapping of an alkylidenecarbene by cyclohexane in the decomposition of 2-methylprop-1-enyl-lead triacetate was also unsuccessful, indicating the absence of such an intermediate.