3d-orbital resonance in divalent sulphide—X : The effects of α-aryl and α-alkylmercapto groups on the rate of decarboxylation of α-substituted carboxylic acids

Abstract

Several α-mercaptoacetic and related acids were synthesized and the rates of the amine-catalysed decarboxylation were measured. In the carbanion-forming reaction, the α-mercapto group exhibits a dramatic rate-enhancement, due to the 3d-orbital resonance effect of the divalent sulphide group. For example, the rate of the decarboxylation of diphenylmercaptoacetic acid was 8·8 × 10 3 fold greater than that of the oxygen analog, diphenoxyacetic acid; while the substitution of one phenylmercapto group at α-position of phenylmercaptoacetic acid was found to accelerate the rate by 1·7 × 10 3. These results can be correlated with those of the base-catalysed HD exchange reactions of various sulphides previously reported 1 and explained on the basis of 3d orbital resonance involving the S atom in divalent sulphides.