Mechanism of Phenol Oxidation by CuCl2

Although copper-catalyzed organic transformations are prevalent, insights into the interactions of phenols with simple copper(II) salts are not well understood. In contrast, inspired by the oxygenase-type modifications of the phenolic substrates, the reactions of substituted phenols with metastable copper–oxygen intermediates are well documented. The present report sheds light on the reactions of substituted phenols with benchtop stable CuCl₂ salt and the role of a common base like triethylamine. Moreover, the reactions of substituted phenols with CuCl₂ in the presence of weakly coordinating tripodal N-nitrosated ligand L^3NO have been illustrated, while a closely related tripodal copper(II) complex L^3HCuCl₂ (2) of the corresponding non-nitrosated ligand L^3H does not react with the phenolic substrates. Phenol reactions with CuCl₂ in the presence of the L^3NO ligand enable in depth mechanistic investigation, thereby illustrating a bimolecular rate law with ΔH^‡ = 15.13 kcal mol⁻¹, ΔS^‡ = −9.6 eu, and kinetic isotope effect k₂(ArOH)/k₂(ArOD) in the range of 1.35–1.43. Thus, these findings suggest that simple copper(II) salts like CuCl₂ are capable of facilitating a proton-coupled electron transfer (PCET) pathway.