Electro-organic reactions. Part 16. The preparative-scale cathodic hydrogenolysis and coupling of benzyl-, allyl-, cinnamyl-, and polyenylphosphonium salts

Abstract

An investigation of the cathodic reduction of benzyltriphenylphosphonium nitrate confirms that optimum yields of the product of coupling (bibenzyl) are formed at an aluminium cathode using high current densities and in dimethylformamide or hexamethylphosphoramide solution. The cathodic reduction of allylphosphonium salts gives mainly cleavage; e.g. α-farnesene (5) is obtained in 32% yield from the corresponding farnesylphosphonium salt. Moderately efficient coupling (ca. 30%) is observed for the electro-reduction of trans-cinnamyltriphenylphosphonium nitrate (6) and a C₁₅ polyenylphosphonium salt (7). The products of coupling have been fully characterised; the relative proportions of positional isomers are apparently dictated by steric factors.