Hydroxylation of phenolic compounds by a peroxodicopper(II) complex : Further insight into the mechanism of tyrosinase

The dicopper(I) complex [Cu2(MeL66)]2+ (where MeL66 is the hexadentate ligand 3,5-bis-{bis-[2-(1-methyl-1H-benzimidazol-2-yl)-ethyl]-amino}-meth ylbenzene) reacts reversibly with dioxygen at low temperature to form a μ-peroxo adduct. Kinetic studies of O2 binding carried out in acetone in the temperature range from −80 to −55 °C yielded the activation parameters ΔH1≠ = 40.4 ± 2.2 kJ mol-1, ΔS1≠ = −41.4 ± 10.8 J K-1 mol-1 and ΔH-1≠ = 72.5 ± 2.4 kJ mol-1, ΔS-1≠ = 46.7 ± 11.1 J K-1 mol-1 for the forward and reverse reaction, respectively, and the binding parameters of O2 ΔH° = −32.2 ± 2.2 kJ mol-1 and ΔS° = −88.1 ± 10.7 J K-1 mol-1. The hydroxylation of a series of p-substituted phenolate salts by [Cu2(MeL66)O2]2+ studied in acetone at −55 °C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant ρ = −1.84. The temperature dependence of the phenol hydroxylation was studied between −84 and −70 °C for a range of sodium p-cyanophenolate concentrations. ...