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 10−2 s−1 or less. With 4-chloro-, 2-chloro-, 4,4′-dichloro- and 2,4′-dichorobenzophenones, kc of 1− 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− 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 α-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.