A plausible mechanism for copper-catalyzed oxidative chlorophenol coupling reactions. Semi-empirical AM1 and ab initio 3-21G molecular orbital studies

Semi-empirical AM1 and ab initio molecular orbital calculations were performed to gain more insight into the mechanism of copper-catalyzed oxidative phenol coupling reactions. Partial charges were determined for chlorophenols, chlorophenolate anions, chlorophenoxy radicals and chlorophenoxonium cations (in both the singlet and triplet states) at the AM1 level. Calculations of these species indicate that the only aromatic carbon bearing a substantial positive charge is the para carbon of the cation in the singlet state. The results support a mechanism for copper-catalyzed oxidative phenol coupling in which dinuclear phenolate-bridged copper(II) species act as intermediates affording chlorophenoxonium cations after a double-bond one-electron transfer.