Dioxygen Activation by Copper Complexes. Mechanistic Insights into Copper Monooxygenases and Copper Oxidases

Abstract

Abstract Reactions of copper(I) complexes with molecular oxygen have been examined using a series of N-alkyl-bis[2-(2-pyridyl)ethyl]amine tridentate ligands (R′Py2R) and N,N-dialkyl-2-(2-pyridyl)ethylamine didentate ligands (R′Py1R1,R2) at low temperature. The tridentate ligands predominantly provide (μ-η2 : η2-peroxo)dicopper(II) complexes (side-on type peroxo complex), while the didentate ligands enhance O–O bond homolysis of the peroxo species to produce bis(μ-oxo)dicopper(III) complexes. With the (μ-η2 : η2-peroxo)dicopper(II) complexes supported by the tridentate ligand, efficient oxygenation of phenolates to the corresponding catechols has been accomplished to provide a good model reaction of tyrosinase. The bis(μ-oxo)dicopper(III) complexes, on the other hand, undergo aliphatic ligand hydroxylation as well as oxygen atom transfer to sulfides to give the corresponding sulfoxides. In the reaction of bis(μ-oxo)dicopper(III) complex with 10-methyl-9,10-dihydroacridine (AcrH2) and 1,4-cyclohexadiene (CHD), a new active oxygen intermediate such as a (μ-oxo)(μ-oxyl radical)dicopper(III) or a tetranuclear copper-oxygen complex has been suggested to be involved as the real active oxygen species for the C–H bond activation of the external substrates. A mixed valence bis(μ3-oxo) trinuclear copper(II,II,III) complex has also been assessed using the didentate ligand with the smallest N-alkyl substituent (methyl). Mechanistic details of the above reactions as well as ligand effects on the copper(I)-dioxygen reactivity are discussed systematically.