Nickel(I)(salen)-electrocatalyzed reduction of benzyl chlorides in the presence of carbon dioxide

The electrochemical carboxylation of a series of substituted benzyl and benzylic-type chlorides Y-C6H4CH(Z)Cl (Z=H; Y=H, 4-CF3, 4-OCH3, 3-OCH3; and Y=H; Z=Ph, CH3), catalyzed by nickel(I)(salen), was investigated in acetonitrile by cyclic voltammetry and controlled-potential electrolysis. For all of the chlorides investigated, remarkable catalytic currents were measured under atmospheric CO2 pressure. For most chlorides, controlled-potential electrolysis yields the corresponding carboxylic acid as the main product, the other significant product being the substituted toluene. The results indicate that the extent of carboxylation is strongly influenced by the structure of the halide. Electron-withdrawing groups, either on the phenyl ring or on the benzylic carbon, favor the formation of the carboxylate. In the first step of the electrocatalytic process, the halide reacts with electrogenerated [NiI(salen)]− to form the benzyl radical. Further reduction of the radical to the corresponding carbanion is followed by electrocarboxylation in competition with protonation by residual water. The yield of the carboxylic acid is determined by the ease of reduction of the benzylic radical, which in turn reflects the nucleophilicity and basicity of the ensuing carbanion.