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

摘要

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)(n)CuX](4)O-2 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, k(2), to form the final products. The observed pseudo first-order rate constant is given by k(obs) = Kk(2)[DMP](y)/(l + K[DMP](y)). When the coordination number around copper(II) is equal to five as in [(Pip)CuX](4)O-2, the system suffers from kinetic saturation due to strong complex formation between catalyst and [DMP] and therefore K[DMP](y) > 10 and k(obs) = k(2). Kinetic saturation has been avoided by using six coordinate copper(II) as in [(PiP)(2)CuX](4)O-2. The influence of the coordination saturation of copper(II) in [(Pip)(2)CuX](4)O-2 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)(2)CuX](4)O-2 and two [DMP]. Reduction of copper(II) to copper(l) has been suggested as a rate determining step due to halogen, X, and solvent effects. (c) 2006 Elsevier B.V. All rights reserved.