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?

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 NiII–oxyl species. Instead, an aroyloxy radical (mCBA·) and a NiIII–hydroxyl species formed by a rate-limiting O–O homolysis of a NiII–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 NiIII–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 NiII–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.