Mechanism Of Aerobic Oxidation Of 2,3,6-Trimethvlphenol By Copper Catalysts And Application Of Chelate-based Ionic Liquids In This Reaction

2,3,5-Trimethyl-l,4-benzoquinone(TMBQ),the one-step method oxygenation product of 2,3,6-trimethylphenol(TMP),was of great significance for the industrial synthesis of vitamin E.The general method was that using CuCl2 as the catalyst,organic ammonia-contain chlorides or lithium chloride as co-catalysts under aerobic conditions.However,TMBQ was produced just using CuCl2 as the catalyst,however,only oxidative couplings(including 2,2',3,3',6,6'-hexamethyl-4,4'-biphenol(HBP)and oligomers)were formed when the catalyst was replaced by Cu(OAc)2.Herein,we tried to explain the difference in catalytic aerobic oxidation of TMP by C1UCl2 or Cu(OAc)2 by spectroscopic methods.On this basis,the mechanism of oxidation of TMP by CuClb was further explored.In literatures,some groups proposed that the phenoxyl radicals reacted with hydrogen chloride to form 4-chloro-2,3,6-trimethylphenol(CI-TMP),as the intermediate,which could be oxidized to TMBQ,and some believed that the HBP was the intermediate.In addition,the phenoxyl radicals reaction with activated oxygen to yield TMBQ was also proposed.Hence,combined logical experimental study,kinetics and computational investigation was applied to explore the mechanism of oxidation of TMP in detail.On the basis of EPR results,the mechanism with deeper insight of Oxidation of TMP was discussed.The EPR results shown that the CuCl2 versus Cu(OAc)2 were different,and phenoxyl radicals were not detected in the EPR of CuCl2 system while appearing in the Cu(OAc)2 system.We proposed that radicals were rapidly reacted with copper-oxygen adduct,which created oxygenation rather than formed oxidative coupling.Moreover,the CuCl2 plays a leading role in the reaction.When mixed with other copper salts,the reaction still led to form TMBQ.The logical experiments and computational investigation to suggest the solution and proposed that the attacking of activated molecular oxygen on phenoxyl radical is the major route to obtain TMBQ,and Cl proposed to eliminate para H-atom to form the oxygenation products.Based on above studies,we have designed the chlorine contained chelate-based ionic liquids.The chelate-based ionic liquids was applied to oxidation of TMP.The results showed that reaction rate became slow and the selectivity of TMBQ was decreased to 50%comparing with CuCl2-system.By adding hydroxylamine hydrochloride as a co-catalyst to the reaction system,the reaction selectivity was increased to 95%.If the ionic liquid is directly subjected to solvent-free catalytic oxidation,the selectivity of the reaction to TMBQ was only 37%due to the high viscosity.