Insight into 2,3,6-Trimethylphenol oxidation by comparing the difference between cupric acetate and cupric chloride catalysis

The oxidation of 2,3,6-trimethylphenol with molecular oxygen produced 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) by using CuCl2 as the catalyst, however, it led to coupling when the catalyst was replaced by Cu(OAc)2. To gain deeper insight into the difference of the catalytic mechanism, the in-depth studies were carried out by controlled experiments such as adding CuCl2 at different times into the system initially catalyzed by Cu(OAc)2 and by using the electron paramagnetic resonance (EPR). In controlled experiments, the oxygenation reaction is dominant when adding CuCl2. In EPR, the phenoxyl radical was detected in the Cu(OAc)2 catalysis system, but not found in the CuCl2 system. All these results suggested that the rate of oxygenation catalyzed by CuCl2 was much faster than that of coupling, which was reinforced by the kinetics experiments. The attacking of activated molecular oxygen by CuCl2 on the phenoxyl radical is proposed as the major route to obtain TMBQ. In the process, the interaction existed between Cl and H-atom at para-position to help eliminating proton by DFT calculation. Moreover, the CuCl2 was inactivated by multidentates ligands suggested the coordination sites on the catalyst should play key roles in oxygenation.