AB-INITIO MO STUDY OF THE MECHANISM OF OXIDATION OF DISULFIDES BY HYDROGEN-PEROXIDE

A theoretical study of the mechanism of the first oxidation step of disulfides to thiosulfinates was performed on hydrogen disulfide and methyl disulfide with hydrogen peroxide. Ab initio molecular orbital calculations were carried out with the 6-31G** basis set. Key equilibrium geometries and transition states were optimized at the MP2 level. The reaction paths were checked in accordance with the Intrinsic Reaction Coordinate (IRC) model to and fro along the reaction coordinate. The reaction turned out to be a two-step process: the first step is a high-barrier 1,2-hydrogen shift involving hydrogen peroxide to. yield water oxide. The second step is a low-barrier oxygen shift to one of the sulfur atoms. Of the molecular complexes found between the reagents the most stable was a six-membered ring with two hydrogen bonds. Different activated complexes involving hydrogen disulfide in the formation of water oxide and corresponding to first-order saddle points were located. They correspond to transition states lower than that occurring in the isolated molecule of hydrogen peroxide. The lowest transition state is the result of an intermolecular proton exchange between hydrogen peroxide and hydrogen disulfide, which acts as a catalyst for the process. The formation of molecular complexes along the reaction path indicates the selectivity of the nucleophilic attack of one of the lone pairs of the sulfur atom on the oxygen atom. In the asymmetrically substituted disulfide, the reaction mechanism is very similar to that found for the simple disulfide. The attack on the sulfur atoms is regioselective; oxidation of the most nucleophilic sulfur atom bonded to the methyl group is strongly preferred.