MECHANISTIC STUDY OF THE OXIDATION OF SULFIDES TO SULFOXIDES AND ASYMMETRIC SULFOXIDES CATALYZED BY TRANSITION-METAL PEROXIDE COMPLEXES

The mechanism for the oxidation and asymmetric oxidation of sulfides by transition metal peroxide complexes is studied using a theoretical approach. First, the oxygen transfer from a lithium peroxide complex to H2S is investigated, with eta2-Li as a model for a bidentate-coordinated peroxide to a transition metal. Then, oxygen transfer is investigated for a titanium peroxide complex, eta2-(O2H)Ti(X)3, to H2S. These calculations show that the sulfur atom of the sulfide coordinates first to the metal, followed by a slipping motion towards the peroxygen which is going to be transferred to the sulfur atom, leading to the transition state for the reaction. The pre-coordination of the sulfide to the metal and the transition state for the reaction are described using a frontier-orbital approach and total-energy considerations. The lithium and titanium peroxide systems give nearly similar results for the oxygen-transfer step to the sulfide. The results obtained are discussed in relation to the experimental data for the oxidation of sulfides to sulfoxides by different oxidation systems containing peroxygen-peroxygen bonds. The transition state geometry for the oxygen transfer from the titanium peroxide complex to the sulfide, and the precoordination of the sulfide to the metal, are then used to study the asymmetric oxidation of sulfides to sulfoxides catalysed by titanium tartrate complexes. These considerations lead to a model for the oxygen-transfer step with an enantioselectivity for the oxidation of the prochiral sulfide in accordance to the experimental data.