Homogeneous oxidative coupling catalysts. Mechanism of conversion of 2,6-dimethylphenol [DMP] to 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone [DPQ] by [(Pip)nCuX]4O2 (n = 1 or 2, Pip = piperidine, X = Cl, Br or I) in aprotic media

This paper represents the mechanism of the second half of the catalytic cycle, Scheme 1, which represents the conversion of 2,6-dimethylphenol [DMP] to 3,3′,5,5′-tetramethyl, 4,4′-diphenoquinone [DPQ] by homogenous oxidative coupling catalysts [(Pip)nCuX]4O2 in aprotic media. The mechanism can be represented as a pre-equilibrium, K, between the catalyst and 2,6-dimethylphenol to form a complex intermediate which is converted into the activated complex through the rate determining step, k2, to form the final products. The observed pseudo first-order rate constant is given by kobs = K k2[DMP]y/(1 + K[DMP]y). When the coordination number around copper(II) is equal to five as in [(Pip)CuX]4O2, the system suffers from kinetic saturation due to strong complex formation between catalyst and [DMP] and therefore K[DMP]y > 10 and kobs = k2. Kinetic saturation has been avoided by using six coordinate copper(II) as in [(Pip)2CuX]4O2. The influence of the coordination saturation of copper(II) in [(Pip)2CuX]4O2 helps to evaluate both thermodynamic and kinetic parameters for the system as well as for the structure of the activated complex, (y = 2), which consists of one [(Pip)2CuX]4O2 and two [DMP]. Reduction of copper(II) to copper(I) has been suggested as a rate determining step due to halogen, X, and solvent effects.