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

The electrochemical carboxylation of a series of substituted benzyl and ben zylic-type chlorides Y-C6H4CH(Z)Cl (Z = H; Y = H, 4-CF3, 4-OCH3, 3-OCH3; an d Y = H; Z = Ph, CH3), catalyzed by nickel(I)(salen), was investigated in a cetonitrile by cyclic voltammetry and controlled-potential electrolysis. Fo r all of the chlorides investigated, remarkable catalytic currents were mea sured under atmospheric CO2 pressure. For most chlorides, controlled-potent ial electrolysis yields the corresponding carboxylic acid as the main produ ct, the other significant product being the substituted toluene. The result s 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 elect ro catalytic process, the halide reacts with el ectrogenerated [Ni-I(salen)](-) to form the benzyl radical, Further reducti on of the radical to the corresponding carbanion is followed by electrocarb oxylation in competition with protonation by residual water. The yield of t he carboxylic acid is determined by the ease of reduction of the benzylic r adical, which in turn reflects the nucleophilicity and basicity of the ensu ing carbanion. (C) 2001 Elsevier Science B.V. All rights reserved.