Mechanism of Ni-Catalyzed Oxidations of Unactivated C(sp³)–H Bonds

The Ni-catalyzed oxidation of unactivated alkanes, including the oxidation of polyethylenes, by <i>meta</i>-chloroperbenzoic acid (<i>m</i>CPBA) occur with high turnover numbers under mild conditions, but the mechanism of such transformations has been a subject of debate. Putative, high-valent nickel-oxo or nickel-oxyl intermediates have been proposed to cleave the C-H bond, but several studies on such complexes have not provided strong evidence to support such reactivity toward unactivated C(sp³)-H bonds. We report mechanistic investigations of Ni-catalyzed oxidations of unactivated C-H bonds by <i>m</i>CPBA. The lack of an effect of ligands, the formation of carbon-centered radicals with long lifetimes, and the decomposition of <i>m</i>CPBA in the presence of Ni complexes suggest that the reaction occurs through free alkyl radicals. Selectivity on model substrates and deuterium-labeling experiments imply that the <i>m</i>-chlorobenzoyloxy radical derived from <i>m</i>CPBA cleaves C-H bonds in the alkane to form an alkyl radical, which subsequently reacts with <i>m</i>CPBA to afford the alcohol product and regenerate the aroyloxy radical. This free-radical chain mechanism shows that Ni does not cleave the C(sp³)-H bonds as previously proposed; rather, it catalyzes the decomposition of <i>m</i>CPBA to form the aroyloxy radical.