Catalytic Conversion of Monophenols to Ortho-Quinones in a Tyrosinase-Like Fashion: Towards More Biomimetic and More Efficient Model Systems

A new tyrosinase model based on the mononuclear copper(I) complex CuL(bzm)1 is synthesized and characterized. The ligand L(bzm)1 of this system contains a combination of an imine and a benzimidazole function which renders the system more biomimetic in comparison to the recently published L(py)1 model of tyrosinase (M. Rolff, J. Schottenheim, G. Peters, F. Tuczek, Angew. Chem. Int. Ed. 2010, 122, 6583). As shown by UV/Vis and NMR spectroscopy, the CuL(bzm)1 complex catalytically mediates the conversion of the monophenol DTBP-H to the o-quinone DTBQ with a TON of 31. This reaction was also conducted in a stoichiometric fashion to get information about the involved intermediates and identify possible reasons for the observed increase in catalytic performance with respect to the L(py)1 system. DFT calculations were performed for the mu-catecholato dicopper intermediate, compound 4(bzm). These calculations indicate a mixed valent Cu-I-semiquinone-Cu-II structure, indicating that one-electron transfer from the monohydroxylated substrate to the copper centers has already occurred at this stage.