Electrochemical reduction and carboxylation of halobenzophenones

The electrochemical reduction of a series of halogenated benzophenones X-C6H4COC6H4-Y( 1) was studied in aprotic solvents, in the absence and presence of CO2, by cyclic voltammetry (CV) and controlled-potential electrolysis. The mechanism of electroreduction in dimethylformamide (DMF) has been investigated in some detail. Most of the compounds undergo reductive carbon-halogen bond cleavage in the time window of CV. The radical anions derived from difluorobenzophenones (X = Y = 4-F; X = 2-F, Y = 4-F) as well as from the 3-chloro derivative are quite stable with a cleavage rate constant (kc)of the order of 0.01 s-1 or less. With 4-chloro-, 2-chloro-, 4,4'-dichloro- and 2,4'-dichorobenzophenones, kc of 1 radical anion has values in the range 4 - 254 s-1 in DMF. In the presence of CO2 all radical anions react quite rapidly with CO2, carboxylation of 1 radical anion being much faster than dehalogenation. Controlled-potential electrolyses were carried out in DMF, MeCN and N-methyl-2-pyrrolidinone (NMP) in an undivided cell using a compact graphite cathode and an aluminium sacrificial anode. The principal product under these conditions was always the a-hydroxyacid X-C6H4C(OH)(CO2H)C6H4-Y. In NMP under 1 atm of CO2, acid yields around 90% were obtained except in the case of 2-chloro- and 2,4'-dichlorobenzophenone, where acid yields of 58 and 68% were obtained, respectively.