Titanium(IV)-(R,R,R)-tris(2-phenylethoxy)amine-alkylperoxo complex mediated oxidations: The biphilic nature of the oxygen transfer to organic sulfur compounds

The enantioselective sulfoxidation of methyl p-tolyl sulfide by Ti(IV)/ N(CH2CHPhO)3(OiPr), 1, and cumyl hydroperoxide yields both the corresponding sulfoxide and sulfone with comparable reaction rates (kS/kSO=3.2). Such a behavior stands in contrast with the remarkably high selectivity usually exhibited by d0 transition metal peroxocomplexes. Hammett correlations show that peroxocomplex 2, derived from the addition of cumylhydroperoxide to 1, has a biphilic nature, behaving as an electrophilic oxidant towards sulfides (rho=-0.60) while a nucleophilic pathway dominates the oxidation of sulfoxides (rho =+0.45). Kinetic evidence and theoretical calculations [RHF/3-21G(*)] indicate that the mechanistic modification of the reactivity is triggered by coordination of the sulfoxide to the metal center occurring with a calculated stabilization energy of 19.3 Kcal/mol. According to this mechanistic picture, the Lewis acid Ti(IV) metal center activates the hydroperoxide and the sulfoxide in a template intramolecular oxidative process.