Catalysis of cobalt-Schiff base complexes in oxygenation of alkenes: on the mechanism of ketonization

A study showing a unique mechanism for a cobalt-Schiff base complex-catalyzed monooxygenation of alkenes is reported. Four-coordinate cobalt-(II)-Schiff base chelate complexes catalyze efficiently, in primary or secondary alcohols under atmospheric oxygen pressure at 60 °C, the oxygenation of alkenes substituted with an aromatic or an electron-withdrawing group, which results in ketonization without carbon-carbon bond cleavage. Kinetic studies on the oxygenation of styrene in benzyl alcohol show that the reaction proceeds as a co-oxidation of these substances with a 1:1 stoichiometry. The proposed mechanism involves rate-determining decomposition of a benzyl alcoholatocobalt(III) complex to benzaldehyde and a hydridocobalt species, followed by rapid addition of the hydride intermediate to styrene, and dioxygen incorporation into the resulting organocobalt complex.