Oxidative coupling reaction of 2,6-di-alkylphenols catalyzed with copper(II) complexes

Abstract

The oxidative coupling reactions of 2,6-di-tert-butyl and 2,6-dimethyl-phenols catalyzed by copper(II) nitrate—acetonitrile system, with or without some amines, are described. It was found that the rate of the oxidative coupling reaction of 2,6-di-tert-butylphenol catalyzed by the copper(II) nitrate—acetonitrile system was first order with respect to both the concentrations of copper(II) nitrate and the phenol, and was independent of the partial pressure of oxygen. When an amine was added to the copper(II) catalytic system, the oxidation activities of 2,6-substituted phenols varied with the amount of amine added. The basicity and steric hindrance of amines markedly affected the rates of oxidation of both phenol derivatives; thus with the additions of small amounts of piperidine, morpholine, or diethylamine, the rate of oxidation decreased markedly, but the further addition of these amines resulted in a re-increase of the oxidation activity to yield the coupling products almost quantitatively. Under the latter reaction conditions, 2,6-dimethylphenol was selectively oxidized to a corresponding poly-(2,6-dimethylphenylene oxide), C—O coupling product, in contrast to the former catalytic system by which a corresponding diphenoquinone, C—C coupling product, was selectively obtained. On the other hand, addition of pyridine, triethylamine, or bidentate amine such as ethylenediamine resulted in a monotonous decrease of C—C coupling product and no C—O coupling product was obtained in the 2,6-dimethylphenol oxidation under further addition of these amines. The heterogeneous part resulting from amine addition to a homogeneous copper(II)—acetonitrile catalytic system was found not to have catalytic activity for the oxidative coupling reaction. The ESR and electronic absorption spectra were recorded to investigate the variation of these spectra with the addition of amines. The copper(II) catalysts that had tetragonal symmetry from addition of amine, from the observation of these ESR spectra, favored C—O coupling for 2,6-dimethylphenol oxidation.