Electrochemical reduction of pyridine- and benzene-substituted n-alkyl esters and thioic S-esters in acetonitrile

Bulk controlled potential electrolysis experiments have been performed on a wide range of n-alkyl-substituted esters and thioic S-esters of pyridine and benzene in dry acetonitrile with tetraalkylammonium salts as the supporting electrolyte. In most cases, the bulk one-electron reduction of oxygen esters results in unstable or semistable radicals being formed that decompose via loss of the alkyl radical to leave the carboxylate anion in high yield (ca. 70-100%). Benzoate and dinicotinate esters are the exception to this where the final decomposition products are numerous and complicated. For the thioic S-esters, two types of decomposition mechanism have been identified as operating depending on the stability of their anion radicals. Thioic S-ester radical anions that are very unstable (lifetimes in the order of several milliseconds) decompose with loss of the thiolate ion to leave a neutral acyl radical that undergoes aromatic substitution reactions with other acyl radicals to form, among other products, gamma-lactones. Thioic S-esters radical anions that are stable for many minutes to hours ultimately decompose via reaction with molecular oxygen to form carboxylate anions.