Copper(I)-alpha-ketocarboxylate complexes: characterization and O2 reactions that yield copper-oxygen intermediates capable of hydroxylating arenes.

Understanding the properties of synthetic copper-oxygen intermediates is important for evaluating mechanisms of oxidations by enzymes and other catalysts.1,2 A growing class of reactive copper-oxygen complexes have been isolated or identified spectroscopically to date, including well-studied examples with [Cu2(O2)], [Cu2(μ-O)2], or [CuO2] cores.3 Mononuclear copperoxo species ([CuII-O-• T CuIIIdO2-]+) are less well understood. Such species have been considered as a possible reactive intermediate in catalysis by copper enzymes (cf. peptidylglycine R-hydroxylating monooxygenase,4,5 particulate methane monooxygenase6) and in some synthetic reactions,7 but to our knowledge, they have only been observed in the gas phase.8 Theory suggests that they should be powerful oxidants,4,8,9 perhaps even more reactive than the related [FeIVdO]2+ unit that has been extensively studied.10 Among the various routes by which [FeIVdO]2+ moieties may be accessed is that used by R-ketoglutarate-dependent non-heme iron enzymes (Scheme 1).11 Experimental and theoretical studies12 support a mechanism that involves reaction of O2 with a bidentate R-ketocarboxylate complex of FeII to yield an O2 adduct (A) that attacks the R-keto position to induce loss of CO2 and generate B. Formally a FeII-OOC(O)R species, B, then undergoes O-O bond heterolysis to yield the [FeIVdO]2+ moiety (C) that is generally deemed to be responsible for attacking the C-H bond of the substrate(s). Drawing an analogy to this chemistry, we envisioned that reaction of O2 with a CuI-R-ketocarboxylate could similarly generate novel [CuI-OOC(O)R] and/or derived [CuII-O-• T CuIIIdO2-]+ species. In view of their potentially high reactivity that could make these species difficult to observe directly, we postulated that they might be trapped by using a supporting ligand with an arene substituent appropriately positioned to be susceptible to intramolecular attack.13,14 Herein we report the synthesis and structural characterization of new CuI-R-ketocarboxylate complexes with such ligands, the discovery of decarboxylation and arene hydroxylation upon reaction of these complexes with O2, and theoretical calculations that provide provocative mechanistic insights into the reactions. These findings serve to illustrate a new pathway for the generation of novel copper-oxygen intermediates relevant to oxidation catalysis. Using modifications of known methods,15 we prepared bidentate LH,16 LMe, and Lm-OMe (Scheme 2), which were chosen because they would favor low coordination numbers in their complexes, provide a single appended arene in a position suited for intramolecular oxidation,7d,13,14,17 and contain a sterically bulky 2,6diisopropylphenyl group to inhibit formation of bis-ligand complexes (e.g., L2Cu) and dicopper-oxygen intermediates.3 Copper(I)R-ketocarboxylate complexes 1-4 were synthesized by treatment of [Cu(Mes)]4 with benzoyl or mesitoyl formic acid, followed by addition of the N-donor ligand. For comparative O2 reactivity studies (see below), we also prepared 5 by treating LMe with CuCl and then AgO3SCF3. The compounds 1-5 were isolated as red-brown (1-4) or purple (5) solids in good yields (∼75-85%) and were fully characterized by NMR and UV-vis spectroscopy, CHN analysis, and X-ray crystallography.15 The structures of 1 and 2 are shown in Figure 1, while those of 3-5 appear in the Supporting Information (Figures S1-S3). The R-ketocarboxylate coordinates in chelating bidentate fashion via the carboxylate and the ketone O atoms in 1, but in 2-4, it binds solely through a carboxylate O atom with the keto group oriented approximately orthogonal to the carboxylate plane (torsional angle O1-C26-C27-O3 ) -103.5(3)°). In 5, the triflate anion binds as a monodentate ligand to yield a three-coordinate complex. All structures display metal-ligand bond distances typical for CuI complexes. For 1-4, similar 1H and 13C{1H} NMR spectra are † University of Minnesota. § University of Geneva. Scheme 1. General Mechanism for Reactivity of FeII-R-Ketocarboxylate Sites in Enzymes and Model Complexes