Electrochemical reduction of 2-chloro-N-phenylacetamides at carbon and silver cathodes in dimethylformamide

Abstract

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the direct electrochemical reduction of 2-chloro-N-methyl-N-phenylacetamide (1a), 2-chloro-N-ethyl-N-phenylacetamide (1b), and 2-chloro-N-phenylacetamide (1c) at carbon and silver cathodes, as well as the catalytic reduction of these compounds by electrogenerated nickel(I) salen, in dimethylformamide (DMF) containing 0.050M tetramethylammonium tetrafluoroborate (TMABF4). Cyclic voltammograms for reduction of 1a and 1b show a single irreversible cathodic peak for cleavage of the carbon–chlorine bond, but two irreversible cathodic peaks are observed in cyclic voltammograms for reduction of 1c. Controlled-potential reduction of 1a and 1b gives mixtures of dechlorinated amide and N-alkyl-N-phenylaniline, whereas bulk electrolyses of 1c afford N-phenylacetamide in almost quantitative yield. In addition, bulk electrolyses of 1a and 1b result in the formation of very small amounts of dimeric species that arise from coupling of the radical intermediate formed by one-electron cleavage of the carbon–chlorine bond. On the basis of the coulometric n values and product distributions, together with computations based on density functional theory, we propose mechanistic pictures for the reduction of 1a and 1b that involve radical intermediates, whereas reduction of 1c involves carbanion intermediates.