Different mechanisms for the reaction of disubstituted aromatic esters and thioic S-esters with electrochemically generated superoxide

Dialkyl pyridine and benzene dicarboxylates and dicarbothioates (dithioic S,S'-diesters) react with electrochemically generated superoxide to form the monocarboxylate anions in almost 100% yield (step 1) before being converted to the dicarboxylate dianions (step 2), This result indicates that the rate of reaction with superoxide to form the anion (rate constant k(1)) is considerably faster than the rate to form the dianion (rate constant k(2)) (k(1) much greater than k(2)), However, the number of moles/equivalent of superoxide needed to bring about the conversion of the diesters to the carboxylate anions is 2.0 +/- 0.1 for each step, while for the dithioic S-esters only 1.7 +/- 0.2 moles/equivalent are needed for each step, The reason for this difference between the diesters and dithioic S-esters is most likely due, in the case of the dithioic S,S'-diesters, to homolytic rather than heterolytic bond cleavage occurring, where the C(O)-S bond is cleaved to form the thiyl radical ((SPr)-S-.), which either dimerises to form PrSSPr or further reacts with O-2(.-) to form -SPr. Previously postulated mechanisms do not account for the difference between esters and thioic S-esters. From a synthetic perspective, this work provides a useful route for the preparation of mixed carboxylates-thioic S-esters from dithioic S-esters.