Chemistry: A European Journal 2015-12-01

Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex.

Claudia Wilfer, Patricia Liebhäuser, Alexander Hoffmann, Hannes Erdmann, Oleg Grossmann, Leander Runtsch, Eva Paffenholz, Rahel Schepper, Regina Dick, Matthias Bauer, Maximilian Dürr, Ivana Ivanović-Burmazović, Sonja Herres-Pawlis

Index: Chemistry 21 , 17639-49, (2015)

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Abstract

Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η(2) :η(2) )-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox , was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.