Factors Controlling the Chemoselectivity in the Oxidation of Olefins by Nonheme Manganese(IV)-Oxo Complexes

Abstract

We report the oxidation of cyclic olefins, such as cyclohexene, cyclohexene-d(10), and cyclooctene, by mononuclear nonheme manganese(IV)-oxo ((MnO)-O-IV) and triflic acid (HOTf)-bound (MnO)-O-IV complexes. In the oxidation of cyclohexene, the (MnO)-O-IV complexes prefer the C-H bond activation to the C=C double bond epoxidation, whereas the C=C double bond epoxidation becomes a preferred reaction pathway in the cyclohexene oxidation by HOTf-bound (MnO)-O-IV complexes. In contrast, the oxidation of cyclohexene-d(10) and cyclooctene by the (MnO)-O-IV complexes occurs predominantly via the C=C double bond epoxidation. This conclusion is drawn from the product analysis and kinetic studies of the olefin oxidation reactions, such as the epoxide versus allylic oxidation products, the formation of Mn(II) versus Mn(III) products, and the kinetic analyses. Overall, the experimental results suggest that the energy barrier of the C=C double bond epoxidation is very close to that of the allylic C-H bond activation in the oxidation of cyclic olefins by high-valent metal-oxo complexes. Thus, the preference of the reaction pathways is subject to changes upon small manipulation of the reaction environments, such as the supporting ligands and metal ions in metal-oxo species, the presence of HOTf (i.e., HOTf-bound (MnO)-O-IV species), and the allylic C-H(D) bond dissociation energies of olefins. This is confirmed by DFT calculations in the oxidation of cyclohexene and cyclooctene, which show multiple pathways with similar rate-limiting energy barriers and depending on the allylic C-H bond dissociation energies. In addition, the possibility of excited state reactivity in the current system is confirmed for epoxidation reactions.