Electron transfer and bond breaking. Examples of passage from a sequential to a concerted mechanism in the electrochemical reductive cleavage of arylmethyl halides

A systematic investigation of the kinetics of the electrochemical reduction of a series of 11 arylmethyl halides in acetonitrile and N,N' dimethylformamide reveals a striking change in the reductive cleavage mechanism as a function of the energy of the π* orbital liable to accept the incoming electron. With ring-substituted nitrobenzyl chlorides and bromides, a stepwise mechanism involving the intermediacy of the anion radical takes place. When slightly less electron-withdrawing substituents, such as nitrile or ester groups, are involved, the reaction occurs via a concerted electron transfer-bond breaking mechanism