Ab-initio MO study of the peracid oxidation of dimethyl thiosulfinate

The mechanism of oxidation of dimethyl thiosulfinate with performic acid was investigated theoretically at MO ab-initio level. Equilibrium geometries and transition states were optimized with two different basis sets (3-21G* and 6-31G*) and the inclusion of dynamic correlation correction at MP2 level. Along the possible reactions paths three different transition states were characterized, namely, those leading to the formation of the diastereomeric forms (RR/SS or RS/SR) of alpha-disulfoxide and that leading to the thiosulfonate. The calculated values of activation energies and free energies of activation, in the gas phase, fail to reproduce the observed chemoselectivity of oxidation reactions. However, when the formation of reaction clusters, or better, the inclusion of solvent effects (evaluated via SCFR method) is taken into account, one obtains a correct behavior that fits the experimental trend. The reaction path was also analyzed by employing the intrinsic reaction coordinate methodology in order to examine the transitory character of alpha-disulfoxide. The effect of acids in depressing the reactivity was also demonstrated.