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 k₁) is considerably faster than the rate to form the dianion (rate constant k₂) (k₁ ≫ k₂). 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), which either dimerises to form PrSSPr or further reacts with O₂^·- 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.