Mechanistic Study for the Reaction of B12 Complexes with m-Chloroperbenzoic Acid in Catalytic Alkane Oxidations

The oxidation of alkanes with m-chloroperbenzoic acid (mCPBA) catalyzed by the B12 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 firstorder dependence on the [catalyst] and [mCPBA] while being independent of [cyclohexane]; Vobs = k2[catalyst][mCPBA]. 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 mCPBA 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 center dot) and the m-chlorobenzoyloxyl radical. Radical trapping experiments using Ntert-butyl-alpha-phenylnitrone and CCl3Br, 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.

Automated summary

The oxidation of alkanes with m-chloroperbenzoic acid (mCPBA) catalyzed by the B12 derivative, heptamethyl cobyrinate, was studied under various conditions. The reaction rate shows a first-order dependence on the catalyst and mCPBA concentrations, but is independent of the alkane concentration. The kinetic deuterium isotope effect suggests that substrate hydrogen atom abstraction is not the rate-determining step. The formation of a cobalt(III)acylperoxido complex and its role in the oxidation reaction were investigated through experimental and theoretical studies.