Radical Formation in the [MeReO3]-Catalyzed Aqueous Peroxidative Oxidation of Alkanes: A Theoretical Mechanistic Study

Plausible mechanisms of radical formation in the catalytic system [MeReO3]/H2O2/H2O-CH3CN for the oxidation of alkanes to alcohols and ketones, via radical pathways, are investigated extensively at the density functional theory level. The most favorable route is based on the monoperoxo complex [MeReO2(O-2)(H2O)] and includes the formation of an H2O2 adduct, water-assisted H-transfer from H2O2 to the peroxo ligand, and generation of HOO center dot. The thus formed reduced Re-VI complex [MeReO2(OOH)(H2O)] reacts with H2O2, resulting, upon water-assisted H-transfer and O-OH bond homolysis, in the regeneration of the oxo-Re-VII catalyst and formation of the HO center dot radical that reacts further with the alkane. Water plays a crucial role by (i) stabilizing transition states for the proton migrations and providing easy intramolecular H-transfers in the absence of any N,O-ligands and (ii) saturating the Re coordination sphere what leads to a decrease of the activation barrier for the formation of HOO center dot. The activation energy of the radical formation calculated for [MeReO3] (17.7 kcal/mol) is compatible with that determined experimentally [Shul'pin et al. J. Chem. Soc., Perkin Trans. 2 2001, 1351.] for oxo-V-based catalytic systems (17 2 kcal/mol), and the overall type of mechanism proposed for such V catalysts is also effective for [MeReO3].