Mechanistic Study for the Reaction of B 12 Complexes with m -Chloroperbenzoic Acid in Catalytic Alkane Oxidations.

The oxidation of alkanes with m -chloroperbenzoic acid ( m CPBA) catalyzed by the B 12 derivative, heptamethyl cobyrinate, was investigated under several conditions. During the oxidation of cyclohexane, heptamethyl cobyrinate works as a catalyst to form cyclohexanol and cyclohexanone at a 0.67 alcohol to ketone ratio under aerobic conditions in 1 h. The reaction rate shows a first-order dependence on the [catalyst] and [ m CPBA] while being independent of [cyclohexane]; V obs = k 2 [catalyst][ m CPBA]. The kinetic deuterium isotope effect was determined to be 1.86, suggesting that substrate hydrogen atom abstraction is not dominantly involved in the rate-determining step. By the reaction of m CPBA and heptamethyl cobyrinate at low temperature, the corresponding cobalt(III)acylperoxido complex was formed which was identified by UV-vis, IR, ESR, and ESI-MS studies. A theoretical study suggested the homolysis of the O-O bond in the acylperoxido complex to form Co(III)-oxyl (Co-O • ) and the m -chlorobenzoyloxyl radical. Radical trapping experiments using N - tert -butyl-α-phenylnitrone and CCl 3 Br, product analysis of various alkane oxidations, and computer analysis of the free energy for radical abstraction from cyclohexane by Co(III)-oxyl suggested that both Co(III)-oxyl and the m -chlorobenzoyloxyl radical could act as hydrogen-atom transfer reactants for the cyclohexane oxidation.