Abstract
Mechanistic studies are performed on the alkane hydroxylation with m-CPBA (m-chloroperbenzoic acid) catalyzed by nickel(II) complexes, NiII (L). In the oxidation of cycloalkanes, NiII (TPA) acts as an efficient catalyst with a high yield and a high alcohol selectivity. In the oxidation of adamantane, the tertiary carbon is predominantly oxidized. The reaction rate shows first-order dependence on [substrate] and [NiII (L)] but is independent on [m-CPBA]; vobs =k2 [substrate][NiII (L)]. The reaction exhibited a relatively large kinetic deuterium isotope effect (KIE) of 6.7, demonstrating that the hydrogen atom abstraction is involved in the rate-limiting step of the catalytic cycle. Furthermore, NiII (L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the NiII (L) in the catalytic cycle. Based on the kinetic analysis and the significant effects of O2 and CCl4 on the product distribution pattern, possible contributions of (L)NiII -O. and the aroyloxyl radical as the reactive oxidants are discussed.
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