The elusive active species in nickel(II)-mediated oxidations of hydrocarbons by peracids: a NiII-oxyl species, an aroyloxy radical, or a NiII-peracid complex?

Abstract

Nonheme nickel(II)-mediated oxidations of hydrocarbons by meta-chloroperbenzoic acid (mCPBA) show promising activity and selectivity; however, the active species and the reaction mechanism of these reactions are still elusive after decades of efforts. Herein, a novel free radical chain mechanism of the Ni(II)-mediated oxidation of cyclohexane by mCPBA is investigated using density functional theory calculations. In this study, we rule out the involvement of a long speculated Ni^II-oxyl species. Instead, an aroyloxy radical (mCBA˙) and a Ni^III-hydroxyl species formed by a rate-limiting O-O homolysis of a Ni^II-mCPBA complex are active species in the C-H bond activation to form a carbon-centered radical R˙, where mCBA˙ is more robust than the Ni^III-hydroxyl species. The nascent R˙ radical either reacts with mCPBA to form a hydroxylated product and a mCBA˙ radical to propagate the radical chain or reacts with the solvent dichloromethane to form a chlorinated product. In addition, the Ni^II-mCPBA complex is found for the first time to be a robust oxidant in hydroxylation of cyclohexane, with an activation energy of 13.4 kcal mol^-1. These mechanistic findings support the free radical chain mechanism and enrich the mechanistic knowledge of metal-peracid oxidation systems containing transition metals after group 8 in periodic table of elements.