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 b y transition metal peroxide complexes is studied using a theoretical a pproach. First, the oxygen transfer from a lithium peroxide complex to H2S is investigated, with eta2-Li as a model for a bidentate-coordina ted peroxide to a transition metal. Then, oxygen transfer is investiga ted for a titanium peroxide complex, eta2-(O2H)Ti(X)3, to H2S. These c alculations show that the sulfur atom of the sulfide coordinates first to the metal, followed by a slipping motion towards the peroxygen whi ch is going to be transferred to the sulfur atom, leading to the trans ition state for the reaction. The pre-coordination of the sulfide to t he metal and the transition state for the reaction are described using a frontier-orbital approach and total-energy considerations. The lith ium and titanium peroxide systems give nearly similar results for the oxygen-transfer step to the sulfide. The results obtained are discusse d in relation to the experimental data for the oxidation of sulfides t o sulfoxides by different oxidation systems containing peroxygen-perox ygen 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 ox idation of sulfides to sulfoxides catalysed by titanium tartrate compl exes. These considerations lead to a model for the oxygen-transfer ste p with an enantioselectivity for the oxidation of the prochiral sulfid e in accordance to the experimental data.