Mechanism of the Electrochemical Carboxylation of Aromatic Ketones in Dimethylformamide

The mechanism of the electrochemical carboxylation of several benzophenones (X-C6H4COC6H5; X = 4-OCH3, 4-CH3, H, 3-Cl, 3-CF3, 4-CF3 and 4-CN) and several ring-substituted acetophenones (Y-C6H4COCH3; Y = 4-OCH3, H, 3-OCH3, 3-Cl, 3-CF3, 4-CF3, 3-CN and 4-CN) has been investigated by cyclic voltammetry in dimethylformamide. In the presence of CO2, all compounds exhibit a single irreversible peak representing a 2e- reduction process. The reaction mechanism has been analysed using the dependence of the peak potential Ep on various experimental parameters such as the concentrations of the reacting species, the scan rate and the temperature as a mechanistic tool. Also the kinetics of the electrocarboxylation reaction has been examined. The whole set of results has been carefully analysed in the framework of an ECE-DISP mechanism. It has been found that, under the experimental conditions employed, the electrocarboxylation reaction is always under a mixed ECE-DISP1 kinetic control. The first step of the reaction is an oxygen centred attack of the electrogenerated ketyl radical anion RR'CO•- at CO2. Further reduction of the carbonate-like adduct stemming from such an attack followed by a second carboxylation reaction gives a 2-arylcarboxylic acid.