Unravelling the different pathways of cyclohexene oxidation via peroxyl radical generated from tert-butyl hydroperoxide (TBHP) by various metal salts

The selective oxidation of cyclohexene is rather challenging due to its allylic C-H and C=C bond two reactive centers. It is great significant to achieve highly selective oxidation towards the two reactive centers through regulating catalytic system. Herein, the selective oxidation towards allylic C-H and C=C bond reactive sites of cyclohexene was achieved by using CuCl2 and VCl3 as catalyst respectively in the presence of tert-butyl hydroperoxide (TBHP). CuCl2-catalyzed oxidation of cyclohexene mainly yielded oxidation products at allylic position, while VCl3-catalyzed oxidation of cyclohexene mainly yielded epoxidation products at C=C bond. The two different reaction mechanisms are mainly due to the different roles of t-BuOO• radical in the respective catalytic systems. From EPR (Electron Paramagnetic Resonance) characterizations, it could be known that the amount of t-BuOO• radicals in the CuCl2 catalytic system is much lower than that of in the VCl3 system. The two different mechanisms were proposed by means of 18O2 experiments, KIE kinetic and EPR. It revealed that CuCl2 catalyst could rapidly generate the t-BuOO• radical, which undergoes a bimolecular decay to produce O2. The oxygen combines with the allyl radical of cyclohexene to produce the oxidized product at allylic reactive center. In contrast, VCl3 catalyst promoted the generation of t-BuOO• radical, which reacted with C=C bond of cyclohexene to yield epoxide directly.