Theoretical study of aromatic hydroxylation of the [Cu(H-XYL)O] complex mediated by a side-on peroxo dicopper core and Cu-ligand effects.

In this work, the aromatic hydroxylation mechanism of the [Cu(H-XYL)O] complex mediated by a peroxo dicopper core and Cu-ligand effects are investigated by using hybrid density functional theory (DFT) and the broken symmetry B3LYP method. Based on the calculated free-energy profiles, we proposed two available mechanisms. The first reaction steps of both mechanisms involve concerted O-O bond cleavage and C-O bond formation and the second step involves the Wagner-Meerwein rearrangement of the substrate by a [1,2] H shift (H shift from C to C) or (H shift from C to O) across the phenyl ring to form stable dienone intermediates, and this is followed by the protonation of bridging oxygen atoms to produce the final hydroxylated dicopper(ii) product. The H shift from C to C mechanism is the energetically most favorable, in which the first reaction step is the rate-limiting reaction, with a calculated free-energy barrier of 19.0 kcal mol and a deuterium kinetic isotope effect of 1.0, in agreement with experimental observations. The calculation also shows that the reaction started from the P-type species of [Cu(H-XYL)O] which is capable of mediating the direct hydroxylation of aromatic substrates without the intermediacy of an O-type species. Finally, we designed some new complexes with different Cu-ligands and found the complex that computationally possesses a higher activity in mediating the hydroxylation of the ligand based aromatic substrate; here, Cu loses a pyridyl ligand donor by dissociation, compared to the [Cu(H-XYL)O] complex.