Electrochemical reduction mechanism of organic sulphides. Diphenylmethyl p-nitrophenyl sulphide

Abstract

The electrochemical reduction mechanism of diphenylmethyl p-nitrophenyl sulphide (DNPS) in dimethylformamide has been investigated by direct current polarography, potential sweep voltammetry, controlled potential electrolysis, and e.s.r. spectrometry. This compound presents several kinetic reduction waves, the first of which corresponds to a reversible one-electron transfer, leading to the radical anions of DNPS, as revealed by the e.s.r. spectrum of the solution electrolysed intra muros at low temperature. These radical anions undergo irreversible first-order decay, with fission of the activated C–S bond, giving rise to an e.c.e. electrode reaction. The nature and yields of the electrolysis products and the coulometric results indicate that DNPS radical anions dissociate into p-nitrothio-phenolate anions and diphenylmethyl radicals. The latter decay through two fast parallel reactions: electrode reduction to diphenylmethane at the formation potential and dimerization to tetraphenylethane, while p-nitrothio-phenolate anions undergo a further reversible one-electron transfer at more negative potential (second reduction step) with formation of relatively stable radical dianions. The rate constant for the dissociation of the DNPS radical anions, which is the rate-determining step of the overall electrode process, has been evaluated by kinetic analysis of the voltammetric results. The third reduction step corresponds to a direct two-electron transfer of DNPS, leading to unstable dianions which undergo a very fast dissociation into p-nitrothiophenolate anions and diphenylmethyl carbanions. The intermediate formation of the latter is confirmed by the presence, among the electrolysis products, of the adducts of their nucleophilic attack on DNPS.