Kinetics and selectivity of the copper-catalysed oxidative coupling of 4-(2 ',6 '-dimethylphenoxyl-2,6-dimethylphenol

The kinetics of the copper/N-methylimidazole catalysed oxidative coupling reaction with the C-O coupled dimer of 2,6-dimethylphenol (DMP or monomer), viz. 4-(2',6'-dimethylphenoxy)-2,6-dimethylphenol (dimer), as the substrate have been studied. The reaction was found to obey Michaelis-Menten kinetics. The dimer is more easily oxidised than the monomer, but the formation of a copper-substrate complex is more difficult. The reaction rates are higher than in the case of the monomer, and the amounts of diphenoquinone (DPQ) formed are much lower. With the dimer as the substrate, the order of the reaction in copper is 2, confirming that the formation of a dinuclear copper complex is an important step in the reaction mechanism. The amount of DPQ formed is proportional to the initial amount of the dimer. A slight, but clear preference for the dimer over the monomer as the substrate has been observed from experiments with mixtures of monomer and dimer. The amount of DPQ formed decreases exponentially with an increase in the fraction of dimer in the mixture, which can be ascribed mainly to a statistical effect. (C) 1999 Elsevier Science B.V. All rights reserved.