Electro-organic reactions. Part 17. The competition between cleavage and ylid formation in the cathodic reduction of benzyl-, allyl-, cinnamyl-, and polyenyl-phosphonium salts

Abstract

In aprotic solvents benzyl-, allyl-, cinnamyl-, and polyenyl-phosphonium salts undergo overall one-electron reduction with the formation of up to 50% of the corresponding ylid. In cyclic voltammetric experiments peaks for the reduction of ylids have been clearly identified and the mechanism of formation is found to involve initial two-electron cleavage to a benzylic or allylic carbanion which abstracts proton from a second molecule of phosphonium salt. Benzyltriphenylphosphonium nitrate, in NN-dimethylformamide, is converted into the ylid by the dianion generated cathodically at –1.50 V versus saturated calomel electrode from dicyano(fluoren-9-ylidene)methane and in situ reaction with benzaldehyde gives stilbene in 74% yield. Competitive electrolyses in acidic solution of p-methoxycarbonylbenzyltriphenylphosphonium and triethylammonium salts, and p-methoxycarbonylbenzyl naphthalene-1-carboxylate give for these two-electron cleavages a qualitative order of leaving group ability of Et₃N > Ph₃P RCO₂^–.