Identification of a cobalt(IV)-oxo intermediate as an active oxidant in catalytic oxidation reactions

Abstract

We report the catalytic oxidation of organic substrates by a cobalt complex, [Co-III(TAML)](-) (TAML = tetra-amido macrocyclic ligand), and meta-chloroperbenzoic acid (m-CPBA) under the mild reaction conditions and the identification of a cobalt(IV)-oxo intermediate, [Co-IV(TAML)(O)](2-), as an active oxidant in the catalytic oxidation reactions. In the hydroxylation of cyclohexane, cyclohexanol is the sole product and the hydroxylation reaction occurs via an oxygen non-rebound mechanism. Kinetic isotope effect value of 3.1, obtained in the oxidation of cyclohexane-h(12)/d(12), indicates that a hydrogen atom abstraction from cyclohexane by the cobalt(IV)-oxo complex is the rate-determining step. The catalytic oxidation of cyclohexene prefers the CC bond epoxidation to the C-H bond activation, yielding cyclohexene oxide as a sole product. In the epoxidation of cis- and trans-stilbenes, cis- and trans-stilbene oxides, respectively, are yielded stereoselectively. The present study reports a highly selective catalytic oxidation of organic substrates by a cobalt complex and m-CPBA and the involvement of a cobalt(IV)-oxo intermediate as an active oxidant in the catalytic oxidation reactions.