Copper(II)-catalyzed oxidative coupling of 2,6-dimethylphenol

Abstract

The oxidative coupling reaction of 2,6-dimethylphenol catalyzed by the copper (II) nitrate-acetonitrile complex has been studied. CC coupling product, 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone, was selectively yielded with the present catalytic system. The influence of certain reaction variables, copper (II) nitrate concentration, solvent composition, and H 2O addition, on the CC coupling yield was investigated. The kinetics of the oxidative coupling reaction were first order in copper (II) catalyst concentration and in 2,6-dimethylphenol concentration, respectively. These observations were in accord with the hypothesis that the rate-determining step was between active copper (II) catalyst and 2,6-dimethylphenol. The intermediate copper (II)-phenoxo complex formed must then proceed to yield the CC coupling product and Cu(I) complex which would be reoxidized rapidly with oxygen. The CC coupling activity of the copper (II) catalyst was specific in the case of pyridine addition. The variation of CC coupling product yield with piperidine or diethylamine addition was found to show the similar variation as pyridine was added to the copper (II) complex. On the addition of increasing amounts of piperidine or diethylamine to the catalytic system CO coupling product, poly (2,6-dimethylphenylene oxide), was obtained, in contrast to pyridine addition by which showed no CO coupling formation. Thus it was indicated that some amine addition in the copper (II)-acetonitrile system affected the coupling mode of 2,6-dimethylphenol. Mechanistic aspects are proposed to explain the course of CC coupling of 2,6-dimethylphenol with copper (II) nitrate-acetonitrile catalytic system.