The chemistry of copper is extremely rich because it can easily access Cu0, CuI, CuII, and CuIII oxidation states allowing it to act through one-electron or two-electron processes. As a result, both radical pathways and powerful two-electron bond forming pathways via organmetallic intermediates, similar to those of palladium, can occur. In addition, the different oxidation states of copper associate well with a large number of different functional groups via Lewis acid interactions or π-coordination. In total, these feature confer a remarkably broad range of activities allowing copper to catalyze the oxidation and oxidative union of many substrates.

Oxygen is a highly atom economical, environmentally benign, and abundant oxidant, which makes it ideal in many ways.1 The high activation energies in the reactions of oxygen require that catalysts be employed.2 In combination with molecular oxygen, the chemistry of copper catalysis increases exponentially since oxygen can act as either a sink for electrons (oxidase activity) and/or as a source of oxygen atoms that are incorporated into the product (oxygenase activity). The oxidation of copper with oxygen is a facile process allowing catalytic turnover in net oxidative processes and ready access to the higher CuIII oxidation state, which enables a range of powerful transformations including two-electron reductive elimination to CuI. Molecular oxygen is also not hampered by toxic byproducts, being either reduced to water, occasionally via H2O2 (oxidase activity) or incorporated into the target structure with high atom economy (oxygenase activity). Such oxidations using oxygen or air (21% oxygen) have been employed safely in numerous commodity chemical continuous and batch processes.3

However, batch reactors employing volatile hydrocarbon solvents require that oxygen concentrations be kept low in the head space (typically <5–11%) to avoid flammable mixtures, which can limit the oxygen concentration in the reaction mixture.4,5,6 A number of alternate approaches have been developed allowing oxidation chemistry to be used safely across a broader array of conditions. For example, use of carbon dioxide instead of nitrogen as a diluent leads to reduced flammability.5 Alternately, water can be added to moderate the flammability allowing even pure oxygen to be employed.6 New reactor designs also allow pure oxygen to be used instead of diluted oxygen by maintaining gas bubbles in the solvent, which greatly improves reaction rates and prevents the build up of higher concentrations of oxygen in the head space.4a,7 Supercritical carbon dioxide has been found to be advantageous as a solvent due its chemical inertness towards oxidizing agents and its complete miscibility with oxygen or air over a wide range of temperatures.8 An number of flow technologies9 including flow reactors,10 capillary flow reactors,11 microchannel/microstructure structure reactors,12 and membrane reactors13 limit the amount of or afford separation of hydrocarbon/oxygen vapor phase thereby reducing the potential for explosions.

Enzymatic oxidizing systems based upon copper that exploit the many advantages and unique aspects of copper as a catalyst and oxygen as an oxidant as described in the preceding paragraphs are well known. They represent a powerful set of catalysts able to direct beautiful redox chemistry in a highly site-selective and stereoselective manner on simple as well as highly functionalized molecules. This ability has inspired organic chemists to discover small molecule catalysts that can emulate such processes. In addition, copper has been recognized as a powerful catalyst in several industrial processes (e.g. phenol polymerization, Glaser-Hay alkyne coupling) stimulating the study of the fundamental reaction steps and the organometallic copper intermediates. These studies have inspiried the development of nonenzymatic copper catalysts. For these reasons, the study of copper catalysis using molecular oxygen has undergone explosive growth, from 30 citations per year in the 1980s to over 300 citations per year in the 2000s.

A number of elegant reviews on the subject of catalytic copper oxidation chemistry have appeared. Most recently, reviews provide selected coverage of copper catalysts14 or a discussion of their use in the aerobic functionalization of C–H bonds.15 Other recent reviews cover copper and other metal catalysts with a range of oxidants, including oxygen, but several reaction types are not covered.16 Several other works provide a valuable overview of earlier efforts in the field.17 This review comprehensively covers copper catalyzed oxidation chemistry using oxygen as the oxidant up through 2011. Stoichiometric reactions with copper are discussed, as necessary, to put the development of the catalytic processes in context. Mixed metal systems utilizing copper, such as palladium catalyzed Wacker processes, are not included here. Decomposition reactions involving copper/oxygen and model systems of copper enzymes are not discussed exhaustively. To facilitate analysis of the reactions under discussion, the current mechanistic hypothesis is provided for each reaction. As our understanding of the basic chemical steps involving copper improve, it is expected that many of these mechanisms will evolve accordingly.

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Aerobic Copper-Catalyzed Organic Reactions

Recent advances in copper-catalyzed oxidation of organic compounds

TMQ is an important precursor in industrial vitamin E synthesis. We report a "green chemistry approach" with respect to the highly selective and environmentally friendly oxidation of 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (TMQ) with molecular oxygen as oxidant and a copper catalyst immobilized in a molten salt. n-Butanol as co-solvent has a positive effect on the activity and selectivity. The structurally characterized catalyst, a 1-n-butyl-3-methylimidazolium oxotetracuprat, is formed in situ via hydrolysis of CuCl2 in the presence of imidazolium chloride. We propose a mechanism of oxidative phenolate activation at a [Cu4(mu4-O)]6+ core as electronically coupled electron acceptor, formation of a copper-bound phenolate radical anion, spin delocalization into the aromatic ring, and attack by triplet oxygen at the para position. Attack of Cu(I) as reduction equivalent at the peroxy radical, proton-mediated elimination of a copper(II)-hydroxo species, will either substitute a copper(I) site in the reduced oxo cluster or take up an electron from the reduced mixed valent cluster [Cu4(mu4-O)]6+ to regenerate the oxidized cluster as the active electron acceptor.

Aerobic oxidation of 2,3,6-trimethylphenol to trimethyl-1,4-benzoquinone with copper(II) chloride as catalyst in ionic liquid and structure of the active species.

Iron(III), chromium(III) and bismuth(III) complexes of amidate ligand 1,2-bis(2-hydroxybenzamido)ethane (H2hybe) encapsulated in the super cages of zeolite-Y have been isolated and characterized by various physico-chemical measurements. 3D model structure generated for [Fe(hybe)·2H2O]+ suggests that zeolite-Y can accommodate these complexes in its super cages without any strain. These encapsulated complexes catalyze hydroxylation of phenol using H2O2 as oxidant to give catechol as a major and hydroquinone as a minor product. A suitable reaction condition has been optimized for [Fe(hybe)·2H2O]Cl–Y by considering the effect of various parameters such as concentration of substrate, amount of oxidant and catalysts, reaction time, temperature, and volume of solvent for the maximum hydroxylation of phenol. The results obtained over these new encapsulated catalysts showed that selectivity of the catechol formation is ca. 85%, though transformation of phenol varied in the order [Fe(hybe)·2H2O]Cl–Y (43.5%)>[Cr(hybe)·2H2O]Cl–Y (32.8%)>[Bi(hybe)·2H2O]Cl–Y (30.6%). Catalytic performance of these catalysts has been compared with different metal complexes encapsulated in zeolite-Y.

Catalytic activity of chromium(III), iron(III) and bismuth(III) complexes of 1,2-bis(2-hydroxybenzamido)ethane (H2hybe) encapsulated in zeolite-Y for liquid phase hydroxylation of phenol

2-(2-pyridyl)benzimidazole (2-pybmz) and 2-(3-pyridyl)benzimidazole (3-pybmz) have been covalently anchored to chloromethylated polystyrene crossed-linked with 5% divinylbenzene in DMF in presence of triethylamine in ethylacetate. These chelating resins readily react with non-isolable metal peroxo species, prepared in situ by stirring V2O5 (in presence of aqueous KOH), MoO3 or WO3 · H2O with an excess of 30% H2O2, to give the corresponding oxodiperoxo complexes, PS-K[VO(O2)2(L)] (L = 2-pybmz: 1, L = 3-pybmz: 4), PS-[MoO(O2)2(L)] (L = 2-pybmz: 2, L = 3-pybmz: 5) and PS-[WO(O2)2(L)] (L = 2-pybmz: 3, L = 3-pybmz: 6). PS-2-pybmz and PS-3-pybmz represent polymer-anchored ligands. Structures of these complexes have been established on the basis of spectroscopic (IR and electronic), thermogravimetric studies and elemental analyses. Oxidation of phenol or styrene with selected catalysts was tested using H2O2 as an oxidant. Oxidation of phenol with ca. 35% conversion gave a mixture of catechol and p-hydroquinone where selectivity towards catechol is ca. 62%. Oxidation of styrene gave five products, styrene epoxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-phenylethane-1,2-diol. Various parameters such as amount of oxidant and catalyst, and temperature of the reaction mixture have been taken into consideration for the maximum conversion of substrates. These catalysts are recyclable without considerable loss in their catalytic activities. IR spectral data of both freshly prepared and recovered catalysts are also identical, which indicate that the metal complex moiety is intact during the catalytic reaction.

Polymer-anchored oxoperoxo complexes of vanadium(V), molybdenum(VI) and tungsten(VI) as catalyst for the oxidation of phenol and styrene using hydrogen peroxide as oxidant

Oxidation of 2,6-disubstituted 4-methylphenols with dioxygen by using a CuCl2-poly(4-methyl-4′-vinyl-2,2′-bipyridine) catalyst gave the corresponding 4-hydroxybenzaldehydes in high yields. The activity of the catalyst and the selectivity of the products significantly depended on the reaction conditions and the composition of the catalyst. When the molar ratio of the bipyridine unit of the polymer ligand to Cu was unity, i.e., N/Cu = 2, the best results were obtained. Moreover, the reaction is likely to be promoted by coordination of the products to the catalyst. Similarly, 2,3,6-trimethylphenol and related compounds were converted to p-benzoquinones selectively with a CuCl2-poly(4-vinylpyridine) catalyst. These polymer-supported catalysts were readily recovered and are reusable without noticeable decrease of their activity.

Yohei Shimasaki

Papers

Selective Oxidation of Phenols to Hydroxybenzaldehydes and Benzoquinones with Dioxygen Catalyzed by Polymer-Supported Copper

Selective Oxidation of Phenols to Hydroxybenzaldehydes and Benzoquinones with Dioxygen Catalyzed by Polymer‐Supported Copper.