A Copper-Oxo Intermediate from O₂?

Recent research from the groups of Professors William B. Tolman and Christopher J. Cramer.

Understanding the properties of synthetic copper-oxygen intermediates is important for evaluating mechanisms of oxidation by enzymes and other catalysts. A growing class of reactive copper-oxygen complexes have been isolated or identified spectroscopically to date, including well-studied examples with [Cu₂(O₂)]²⁺, [Cu₂(m-O)₂]²⁺, or [CuO₂]⁺ cores. While mononuclear copper-oxo species ([Cu^II-O^-· « Cu^III=O^2-]⁺) have been proposed as possible reactive intermediates in catalysis by copper enzymes, they have only been observed in the gas phase and are less well understood. In contrast, several routes have been developed to access the closely related [Fe^IV=O]²⁺ moieties, of which one (found in enzymes and model systems) involves the O₂-induced decarboxylation of an a-ketocarboxylate ligand. Drawing an analogy to this pathway, graduate student Sungjun Hong working in the laboratory of Professor William Tolman prepared Cu^I-a-ketocarboxylate complexes with bidentate N,N-donor ligand sets containing an arene substituent and showed that upon reaction with O₂, decarboxylation and arene hydroxylation occurred (Figure). These results have been rationalized through theoretical calculations performed by postdoctoral associate Dr. Stefan M. Huber, Professor Christopher Cramer, and Professor Laura Gagliardi (University of Geneva), the results of which support two different hydroxylation pathways that involve novel [Cu^I-OOC(O)R] and [Cu^II-O^-· « Cu^III=O^2-]⁺ species (the latter is shown in the Figure). These findings illustrate a new pathway for the generation of novel copper-oxygen intermediates relevant to oxidation catalysis.

Figure. The X-ray structure of a Cu^I-a-ketocarboxylate complex, its reaction with O₂ resulting in hydroxylation of the appended arene ring, and the calculated structure of a [Cu^II-O^-· « Cu^III=O^2-]⁺ intermediate for one of two proposed reaction pathways.

These results have appeared online: “Copper(I)-a-Ketocarboxylate Complexes: Characterization and O₂ Reactions That Yield Copper-Oxygen Intermediates Capable of Hydroxylating Arenes” Hong, S.; Huber, S. M.; Gagliardi, L.; Cramer, C. C.; Tolman, W. B. J. Am. Chem. Soc. 2007, 46, DOI: 10.1021/ja0760426.