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

The selective oxidation of C-H bonds and the use of O(2) as a stoichiometric oxidant represent two prominent challenges in organic chemistry. Copper(II) is a versatile oxidant, capable of promoting a wide range of oxidative coupling reactions initiated by single-electron transfer (SET) from electron-rich organic molecules. Many of these reactions can be rendered catalytic in Cu by employing molecular oxygen as a stoichiometric oxidant to regenerate the active copper(II) catalyst. Meanwhile, numerous other recently reported Cu-catalyzed C-H oxidation reactions feature substrates that are electron-deficient or appear unlikely to undergo single-electron transfer to copper(II). In some of these cases, evidence has been obtained for the involvement of organocopper(III) intermediates in the reaction mechanism. Organometallic C-H oxidation reactions of this type represent important new opportunities for the field of Cu-catalyzed aerobic oxidations.

Copper-Catalyzed Aerobic Oxidative C ? H Functionalizations: Trends and Mechanistic Insights

Oxidative polymerization of phenols revisited

Publisher Summary The liquid phase oxidation of hydrocarbons by molecular oxygen forms the basis for a wide variety of petrochemical processes including the manufacture of phenol and acetone from cumene, adipic acid from cyclohexane, terephthalic acid from p -xylene, acetaldehyde and vinyl acetate from ethylene, and propylene oxide from propylene. The majority of these processes employ catalysis by transition metal complexes to attain maximum selectivity and efficiency. This chapter describes the subject of metal-catalyzed oxidations of organic compounds in the liquid phase, largely within a mechanistic framework. A better understanding of the catalytic action of metal complexes is essential from the point of view of increasing selectivity and efficiency. Metal-catalyzed oxidations may be conveniently divided into two types—homolytic and heterolytic. The homolytic catalysis involves soluble transition metal salts (homogeneous), such as the acetates or naphthenates of Co, Mn, and Cu, or the metal oxides (heterogeneous). Heterolytic catalysis involves reactions of organic substrates coordinated to transition metals.

Metal-Catalyzed Oxidations of Organic Compounds in the Liquid Phase: A Mechanistic Approach

Die selektive Oxidation von C-H-Bindungen und der Einsatz von O2 als stochiometrisches Oxidationsmittel stellen zwei wesentliche Herausforderungen in der organischen Chemie dar. Kupfer(II) ist ein vielseitiges Oxidationsmittel fur verschiedene oxidative Kupplungen, die durch Einelektronen-Transfer (SET) von elektronenreichen organischen Molekulen ausgelost werden. Viele dieser Reaktionen konnen mit Kupfer katalytisch durchgefuhrt werden, wenn molekularer Sauerstoff als Oxidationsmittel eingesetzt wird, um den aktiven Kupfer(II)-Katalysator zu regenerieren. Daneben wurden auch zahlreiche neue Cu-katalysierte C-H-Oxidationen mit elektronenarmen Substraten beschrieben, von denen ein SET zum Kupfer(II) unwahrscheinlich scheint. In einigen dieser Falle konnte die Beteiligung von Organokupfer(III)-Intermediaten im Reaktionsmechanismus nachgewiesen werden. Metallorganische C-H-Oxidationen dieser Art eroffnen neue bedeutende Moglichkeiten fur das Gebiet der Cu-katalysierten aeroben Oxidationen.

H. S. Blanchard

Papers

Polymerization by Oxidative Coupling. The Function of Copper in the Oxidation of 2,6-Dimethylphenol