Electrochemical reduction of N-(arylthio)succinimides: the effect of aryl substituents.

The electrochemical reduction of a series of N-(arylthio)succinimides (1a-e) was investigated using cyclic voltammetry and constant-potential electrolysis. These results in conjunction with a computational investigation and application of electron transfer theories allowed elucidation of the initial electron transfer mechanisms, global reaction mechanisms, rationalization of trends and differences, and determination of the effect of the substituent on the aromatic ring on the reduction reactions. The study also revealed the effect of the leaving group through comparison with investigations of similar structures with other leaving groups. The study shows that for N-(arylthio)succinimides with an electron donating or a weakly withdrawing group, the first electron transfer is concerted with dissociation of the S-N chemical bond, and produces the disulfide, through a one-electron process. A totally different behavior was observed for the nitro-substituted derivative (1e), where the initial transfer follows a stepwise mechanism, involving the intermediate formation of a radical anion, yielding the 4-nitrophenyl thiolate instead of the disulfide, through a two-electron process. For the latter compound (1e), the reduction mechanism involves an interesting autocatalysis process, where the parent molecule is competitively consumed at the electrode and in solution. The occurrence of the autocatalytic mechanism depends on the effect of the substituent on the reduction potentials of the parent molecule as well as on the corresponding disulfide and is affected by the concentration and the scan rate.