Water oxidation by Brønsted acid-catalyzed in situ generated thiol cation: dual function of the acid catalyst leading to transition metal-free substitution and addition reactions of S–S bonds

Abstract

An unprecedented water oxidation reaction by a small organic molecule, i.e., the thiol cation generated in situ by Brønsted acid-catalyzed heterolytic cleavage of S–S bond of a disulfide, is observed for the first time. The control reactions and the literature clearly support the key transformations in this interesting reaction, such as the generation of thiol and thiol cation from the disulfide, formation of the unstable sulfenic acid intermediate from the reaction of thiol cation and water, production of H₂O₂ from water, and regeneration of the starting disulfide. DFT calculation of the reaction mechanism also demonstrated that the stepwise conversions of disulfide into thiol and thiol cation, water oxidation/thiol cation reduction to sulfenic acid, and disproportionation of sulfenic acid to H₂O₂ and disulfide, are, on a whole, a thermodynamically favorable process (ΔG < 0). Therefore, by readily oxidizing water into H₂O₂, the thiol cation is in turn reduced to the starting disulfide, through which the corresponding thiol can be continuously generated and well utilized in the subsequent Brønsted acid-catalyzed direct nucleophilic substitution and addition reactions with primary, secondary, and tertiary alcohols, and substituted alkenes and alkynes. With the Brønsted acid catalyst playing dual functions in the foregoing S–S bond cleavage step and the latter substitution or addition reactions, this new method can provide several transition metal-free, green, and facile ways for the synthesis of diverse thioethers starting from advantageous and more easily disposable disulfides.