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

Abstract

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