Revisiting Alkane Hydroxylation with m-CPBA (m-Chloroperbenzoic Acid) Catalyzed by Nickel(II) Complexes

Abstract

Mechanistic studies are performed on the alkane hydroxylation with m-CPBA (m-chloroperbenzoic acid) catalyzed by nickel(II) complexes, Ni^II (L). In the oxidation of cycloalkanes, Ni^II (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 [Ni^II (L)] but is independent on [m-CPBA]; v_obs =k₂ [substrate][Ni^II (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, Ni^II (L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the Ni^II (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)Ni^II -O^. and the aroyloxyl radical as the reactive oxidants are discussed.

Keywords: aroyloxyl radical; catalytic alkane hydroxylation; hydrogen atom abstraction; nickel oxyl; nickel(II) complex.