Developing Mononuclear Copper–Active-Oxygen Complexes Relevant to Reactive Intermediates of Biological Oxidation Reactions

Shinobu Itoh *

Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

Acc. Chem. Res., Article ASAP

DOI: 10.1021/acs.accounts.5b00140

Publication Date (Web): June 18, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.accounts.5b00140

*E-mail: shinobu@mls.eng.osaka-u.ac.jp.

 Special Issue

Published as part of the Accounts of Chemical Research special issue “Synthesis in Biological Inorganic Chemistry”.

Biography

Shinobu Itoh received his doctor’s degree in engineering from Osaka University in 1986. He joined the Department of Applied Chemistry of Osaka University as an Assistant Professor (1986–1994) and an Associate Professor (1994–1999). During that period, he spent a year (1987–1988) as a postdoctoral fellow in Professor Teddy G. Traylor’s group at the University of California, San Diego. In 1999, he moved to Osaka City University as a full Professor, and came back to Osaka University in 2008. His current research interest is chemical modeling and application of redox active sites in biological systems and development of artificial metalloenzymes.

Abstract

Conspectus

Active-oxygen species generated on a copper complex play vital roles in several biological and chemical oxidation reactions. Recent attention has been focused on the reactive intermediates generated at the mononuclear copper active sites of copper monooxygenases such as dopamine β-monooxygenase (DβM), tyramine β-monooxygenase (TβM), peptidylglycine-α-hydroxylating monooxygenase (PHM), and polysaccharide monooxygenases (PMO). In a simple model system, reaction of O2 and a reduced copper(I) complex affords a mononuclear copper(II)–superoxide complex or a copper(III)–peroxide complex, and subsequent H• or e–/H+transfer, which gives a copper(II)–hydroperoxide complex. A more reactive species such as a copper(II)–oxyl radical type species could be generated via O–O bond cleavage of the peroxide complex. However, little had been explored about the chemical properties and reactivity of the mononuclear copper–active-oxygen complexes due to the lack of appropriate model compounds. Thus, a great deal of effort has recently been made to develop efficient ligands that can stabilize such reactive active-oxygen complexes in synthetic modeling studies.

In this Account, I describe our recent achievements of the development of a mononuclear copper(II)–(end-on)superoxide complex using a simple tridentate ligand consisting of an eight-membered cyclic diamine with a pyridylethyl donor group. The superoxide complex exhibits a similar structure (four-coordinate tetrahedral geometry) and reactivity (aliphatic hydroxylation) to those of a proposed reactive intermediate of copper monooxygenases. Systematic studies based on the crystal structures of copper(I) and copper(II) complexes of the related tridentate supporting ligands have indicated that the rigid eight-membered cyclic diamine framework is crucial for controlling the geometry and the redox potential, which are prerequisites for the generation of such a unique mononuclear copper(II)–(end-on)superoxide complex.

Reactivity of a mononuclear copper(II)–alkylperoxide complex has also been examined to get insights into the intrinsic reactivity of copper(II)–peroxide species, which is usually considered as a sluggish oxidant or just a precursor of copper–oxyl radical type reactive species. However, our studies have unambiguously demonstrated that copper(II)–alkylperoxide complex can be a direct oxidant for C–H bond activation of organic substrates, when the C–H bond activation is coupled with O–O bond cleavage (concerted mechanism). The reactivity studies of these mononuclear copper(II) active-oxygen species (superoxide and alkylperoxide) will provide significantly important insights into the catalytic mechanism of copper monooxygenases as well as copper-catalyzed oxidation reactions in synthetic organic chemistry.