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Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn

X-ray photoelectron studies on some oxides and hydroxides of cobalt, nickel, and copper

X-ray photoelectron spectroscopic studies of iron oxides

Surface studies of supported model catalysts

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

A study of pyridinium ternary molybdophosphate complexes

In the cupric chloride-catalyzed oxygenation of isobutyraldehyde enamines and cyclohexanone enamines of various secondary amines, the amino substituent remarkably affects the reactivity and the relative rates correlate well with their 1st ionization potentials.

Amino substituent effect on oxygenation of enamines catalyzed by cupric chloride

Inhibiting action of thallium(I) ion on uranyl photoreduction

The solution-phase ionization potentials of I−, Br−, and SCN− ions in 16 solvents including water have been measured by a photoelectron emission spectroscopic technique. The values obtained as the threshold energy Et for halides are found to be correlated well with the Mayer–Gutmann acceptor number and better with the Riddle–Fowkes modified acceptor number of the solvent indicating that the electronic states of the halides are predominantly stabilized through the Lewis acid–base interaction. The reorganization energies ΔGr of solvent after the photoionization of the halides are obtained from the Et values. The experimental ΔGr values are reproduced by using a simple model which incorporates the dipole–dipole repulsion and the bond formation energies in the first solvation layer. The solvation mechanism of SCN− is somewhat different from those of the halides because its Et depends less on the acceptor number and its ΔGr values in alcohols are about half of those for the halides.

Ionization potentials of iodide, bromide, and thiocyanate anions in solvents : solvent-solvent interactions in the solvation sphere

The sulfur 1s and 2p photoelectron spectra and KL23L23 Auger electron spectra in various sulfur compounds involving solid sulfur were measured by means of irradiation by Al Kα and Cr Kα X-rays. The chemical shifts of the S1s and S2p photoelectron spectra for compounds referred to the data for solid sulfur differ from each other in magnitude, but all are larger than that for the Auger electron spectra in the corresponding compound. The shift of the Auger electron spectra is different in fashion from those of the photoelectron spectra; the difference is largely attributable to the differences in relaxation energies. The extra-atomic relaxation energies due to core-electron emission were estimated by combining the atomic spectral data and the XPS and AES data of compounds; they were found not necessarily to depend upon the atomic charges. From the calculated sulfur Kα X-ray spectra corresponding to the difference in the binding energy between 1s and 2p, the number of electrons on each sulfur atom was obtaine...

Shigero Ikeda

Papers

A study of pyridinium ternary molybdophosphate complexes

Amino substituent effect on oxygenation of enamines catalyzed by cupric chloride

Inhibiting action of thallium(I) ion on uranyl photoreduction

Ionization potentials of iodide, bromide, and thiocyanate anions in solvents : solvent-solvent interactions in the solvation sphere

Chemical Shifts of Sulfur KLL Auger Electron Energies in Its Compounds