Electrochemical Reduction of 1-Bromomethyl-2-oxocycloalkane-1-carboxylates at Silver Cathodes in Dimethylformamide: One-Carbon Ring-Expansion Reactions

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been employed to investigate the separate electrochemical reductions of methyl 1-bromomethyl-2-oxocyclopentane-1-carboxylate (1) and ethyl 1-bromomethyl-2-oxocyclohexane-1-carboxylate (2) at silver cathodes in dimethylformamide (DMF) containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4). One-electron reductive cleavage of the carbon-bromine bond of each substrate yields a radical intermediate that undergoes a ring-expansion reaction, followed by hydrogen-atom abstraction from the solvent, to afford methyl 3-oxocyclohexane-1-carboxylate (3a) and ethyl 3-oxocycloheptane-1-carboxylate (3b), respectively, in good yield. Each substrate gives rise to three other products: (a) a debrominated analogue of each starting material, (b) a dimeric species formed via radical coupling, and (c) a species possessing an ester group extended by one carbon atom. Electrolyses of 1 and 2 done in the presence of D2O have revealed that carbanion intermediates result in small amounts from two-electron cleavage of carbon-bromine bonds. A mechanistic scheme, involving both radicals and carbanions, is proposed to account for the formation of the various products. (C) The Author(s) 2014. All rights reserved.