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 ability of copper proteins to process dioxygen at ambient conditions has inspired numerous research groups to study their structural, spectroscopic and catalytic properties. Catechol oxidase is a type-3 copper enzyme usually encountered in plant tissues and in some insects and crustaceans. It catalyzes the conversion of a large number of catechols into the respective o-benzoquinones, which subsequently auto-polymerize, resulting in the formation of melanin, a dark pigment thought to protect a damaged tissue from pathogens. After the report of the X-ray crystal structure of catechol oxidase a few years earlier, a large number of publications devoted to the biomimetic modeling of its active site appeared in the literature. This critical review (citing 114 references) extensively discusses the synthetic models of this enzyme, with a particular emphasis on the different approaches used in the literature to study the mechanism of the catalytic oxidation of the substrate (catechol) by these compounds. These are the studies on the substrate binding to the model complexes, the structure–activity relationship, the kinetic studies of the catalytic oxidation of the substrate and finally the substrate interaction with (per)oxo-dicopper adducts. The general overview of the recognized types of copper proteins and the detailed description of the crystal structure of catechol oxidase, as well as the proposed mechanisms of the enzymatic cycle are also presented.

Synthetic models of the active site of catechol oxidase: mechanistic studies

The critical review describes the known dicopper systems mediating the aromatic hydroxylation of monophenolic substrates. Such systems are of interest as structural and functional models of the type 3 copper enzyme tyrosinase, which catalyzes the ortho-hydroxylation of tyrosine to DOPA and the subsequent two-electron oxidation to dopaquinone. Small-molecule systems involving μ-η²:η² peroxo, bis-μ-oxo and trans-μ-1,2 peroxo dicopper cores are considered separately. These tyrosinase models are contrasted to copper–dioxygen systems inducing radical reactions, and the different mechanistic pathways are discussed. In addition to considering the stoichiometric conversion of phenolic substrates, the available catalytic systems are described. The second part of the review deals with tyrosinase. After an introduction on the occurrence and function of tyrosinases, several aspects of the chemical reactivity of this class of enzymes are described. The analogies between the small-molecule and the enzymatic system are considered, and the implications for the reaction pathway of tyrosinase are discussed (140 references).

Copper–O2 reactivity of tyrosinase models towards external monophenolic substrates: molecular mechanism and comparison with the enzyme

1.2.2. Davis Oxaziridines 4305 1.2.3. Metal Complexes in Enantioselective Oxidation 4305 1.3. New Applications 4307 1.3.1. Chiral Sulfinate Method 4307 1.3.2. Oxidation with Chiral Oxaziridines 4309 1.3.3. Oxidation Using Metal Complexes 4310 1.4. New Systems 4313 1.4.1. C-S Bond Formation 4314 1.4.2. Organic Oxidants 4315 1.4.3. Oxidations Catalyzed by Metal Complexes 4316 1.5. Diastereoselective Oxidations 4330 1.6. Heterogenized Systems 4332 1.7. Summary 4335 2. Biological Oxidations 4336 2.

Enantioselective synthesis of sulfoxides: 2000-2009.

The catecholase activity of a series of dicopper(II) complexes containing different numbers of phenol groups coordinated to the metal centers was studied to identify functional as well as structural models for the type III copper enzymes tyrosinase and catechol oxidase. The syntheses and characterization of complexes [Cu(2)(H(2)bbppnol)(mu-OAc)(H(2)O)(2)]Cl(2).2H(2)O (1) and [Cu(2)(Hbtppnol)(mu-OAc)](ClO(4))(2) (2) were previously reported by us (Inorg. Chim. Acta 1998, 281, 111-115; Inorg. Chem. Commun. 1999, 2, 334-337), and complex [Cu(2)(P1-O(-))(OAc(-))](ClO(4))(2) (3) was previously reported by Karlin et al. (J. Am. Chem. Soc. 1997, 119, 2156-2162). The catalytic activity of the complexes 1-3 on the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butyl-o-benzoquinone characteristic absorption band at about 400 nm over time in methanol saturated with O(2)/aqueous buffer pH 8 solutions at 25 degrees C. The complexes were able to oxidize 3,5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity. A kinetic treatment of the data based on the Michaelis-Menten approach was applied. The [Cu(2)(H(2)bbppnol)(mu-OAc)(H(2)O)(2)]Cl(2) small middle dot2H(2)O complex showed the highest catalytic activity of the three complexes as a result of a high turnover rate (k(cat) = 28 h(-1)) combined with a moderate substrate-catalyst binding constant (K(ass) = 1.3 x 10(3) M(-1)). A mechanism for the oxidation reaction is proposed, and reactivity differences, k(cat)/K(M) of the complexes, were found to be dependent on (DeltaE)(1,2), the difference in the driving force for the reduction reactions Cu(II)(2)/Cu(II)Cu(I) and Cu(II)Cu(I)/Cu(I)(2).

Catecholase activity of a series of dicopper(II) complexes with variable Cu-OH(phenol) moieties.

The dicopper(II) complex with the ligand N,N,N‘,N‘,N‘‘-pentakis[(1-methyl-2-benzimidazolyl)methyl]dipropylenetriamine (LB5) has been synthesized and structurally characterized. The small size and the quality of the single crystal required that data be collected using synchrotron radiation at 276 K. [Cu2(LB5)(H2O)2][ClO4]4:  platelet shaped, P1, a = 11.028 A, b = 17.915 A, c = 20.745 A, α = 107.44°, β = 101.56°, γ = 104.89°, V = 3603.7 A3, Z = 2; number of unique data, I ≥ 2σ(I) = 3447; number of refined parameters = 428; R = 0.12. The ligand binds the two coppers nonsymmetrically; Cu1 is coordinated through five N donors and Cu2 through the remaining three N donors, while two water molecules complete the coordination sphere. Cu1 has distorted TBP geometry, while Cu2 has distorted SP geometry. Voltammetric experiments show quasireversible reductions at the two copper centers, with redox potential higher for the CuN3 center (0.40 V) and lower for the CuN5 center (0.17 V). The complex binds azide in the ter...

Tyrosinase Models. Synthesis, Structure, Catechol Oxidase Activity, and Phenol Monooxygenase Activity of a Dinuclear Copper Complex Derived from a Triamino Pentabenzimidazole Ligand.

The role of myoglobin (Mb) is not yet completely understood and recent evidence indicates it is involved in a variety of pseudo-enzymatic functions. Mb is also extensively used as a tool to redesign novel active-site features and introduce new activities into a protein. This review summarizes our approach to enhance the peroxidase-like activity of Mb toward phenolic compounds by combining two different strategies of protein modification, i.e. active-site engineering and cofactor replacement. The main objective of active-site engineering is to increase the rate of hydrogen peroxide activation upon reaction at the iron(III) center, while cofactor replacement facilitates substrate interaction at the protein active site, thereby increasing the efficiency of the catalytic process and stereoselective recognition of the substrate. The thermodynamic stability of the modified Mb derivatives has been evaluated through guanidinium chloride induced and thermal unfolding experiments. As expected, both protein mutation and cofactor replacement decrease the stability of the protein, the latter effect being somewhat more pronounced for the Mb derivatives studied here. For evaluating the catalytic efficiency of the protein in non-natural reactions, the chemical stability of the protein during catalysis is also important and therefore a new protocol was developed to evaluate the competitive degradation undergone by the protein upon substrate oxidation. It was found that autodegradation can occur through two different pathways involving oxidation of protein residues by the Mb active species or by a substrate-derived phenoxy radical. The peroxidase-like nitrating activity and the sulfoxidation reactions promoted by Mb, both involving the nitrite ion as a cosubstrate, have also been briefly described. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Engineering and Prosthetic-Group Modification of Myoglobin: Peroxidase Activity, Chemical Stability and Unfolding Properties

The deuterohemin complex carrying a deca-L-alanine peptide residue covalently linked to one of the propionic acid side chain of the porphyrin has been synthesized and its catalytic behaviour in biomimetic oxidations has been investigated by kinetic measurements. In the catalytic oxidation of a series of para-substituted thioanisoles by hydrogen peroxide a correlation has been found between the reaction rates and the Hammettσp parameter suggesting that a direct oxygen transfer from an iron-oxo species to the substrate sulfur atom takes place. This trend is similar to that found for chloroperoxidase whereas for normal peroxidases the rate of oxidation of the same substrates correlates with Hammett σ+ values. The catalytic oxidation of L- and D-tyrosine by hydrogen peroxide exhibit substrate saturation kinetics mimicking the enzymic reactions catalyzed by peroxidases. The reaction shows stereoselective effects, the oxidation of D-tyrosine being favoured with respect to that of the L isomer.

Biomimetic oxidation catalysis by iron (III) deuteroporpbyrin carrying a deca-L-alanine peptide chain

Engineering and Prosthetic-Group Modification of Myoglobin: Peroxidase Activity, Chemical Stability and Unfolding Properties.

Heme-peptide complexes can be used as model systems for peroxidases. The build-up of these complexes depends on which of the enzyme features have to be introduced into the model; i.e. the reactivity vs. peroxide or some substrate selectivity. Fourdifferent approaches are described: (a) linking a peptide residue to hemin or deuterohemin, with the aim of introducing substrate selectivity or stereoselectivity in the catalytic activity; (b) coordinating an axial histidine to the heme iron, to increase the reactivity; the efficiency of the models then depends on the

Enrico Monzani

Papers

Tyrosinase Models. Synthesis, Structure, Catechol Oxidase Activity, and Phenol Monooxygenase Activity of a Dinuclear Copper Complex Derived from a Triamino Pentabenzimidazole Ligand.

Engineering and Prosthetic-Group Modification of Myoglobin: Peroxidase Activity, Chemical Stability and Unfolding Properties

Biomimetic oxidation catalysis by iron (III) deuteroporpbyrin carrying a deca-L-alanine peptide chain

Engineering and Prosthetic-Group Modification of Myoglobin: Peroxidase Activity, Chemical Stability and Unfolding Properties.

Account / Revue Heme-peptide complexes as peroxidase models