Mechanistic Understanding towards the Role of Cyclohexene in Enhancing the Efficiency of Manganese Porphyrin-Catalyzed Aerobic Oxidation of Diphenylmethane

The selective aerobic oxidation of nonactivated C-H bonds is a key challenge in both industrial and academic research. The catalytic activity of manganese porphyrins (MnTPPCl) towards the aerobic oxidation of diphenylmethane (DPM) is investigated. Compared with the oxidation-catalyzed MnTPPCl alone, the conversion of DPM is enhanced ca. 6 times (from 7.0% to 42.5%) in the presence of cyclohexene. Some in situ characterizations, such as in situ EPR (electron paramagnetic resonance) spectroscopy and in situ UV/Vis spectroscopy are used to investigate the role of cyclohexene in catalytic oxidation. Mechanistic studies suggest that both high-valent manganese species and cyclohexenyl alkoxyl radicals are favorable for the formation of the carbon radicals. The enhanced activity is related to the easier generation of secondary carbon radicals, triggered by high-valent manganese species and cyclohexenyl alkoxyl radicals.