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.

/pdf/aerobic-copper-catalyzed-organic-reactions-3ctljpvivz.pdf

Aerobic Copper-Catalyzed Organic Reactions

Pyrrole is widely known as a biologically active scaffold which possesses a diverse nature of activities. The combination of different pharmacophores in a pyrrole ring system has led to the formation of more active compounds. Pyrrole containing analogs are considered as a potential source of biologically active compounds that contains a significant set of advantageous properties and can be found in many natural products. The marketed drugs containing a pyrrole ring system are known to have many biological properties such as antipsychotic, β-adrenergic antagonist, anxiolytic, anticancer (leukemia, lymphoma and myelofibrosis etc.), antibacterial, antifungal, antiprotozoal, antimalarial and many more. Due to the diversity of these analogs in the therapeutic response profile, many researchers have been working to explore this skeleton to its maximum potential against several diseases or disorders. In this review, attempts have been made to disclose various tactical approaches to synthesize pyrrole and pyrrole containing analogs. The structure–activity relationship studies have been discussed along with their therapeutic applications which have been reported during last decade. Some molecules as the main components of the market and clinical trials have also been discussed.

/pdf/pyrrole-a-resourceful-small-molecule-in-key-medicinal-hetero-c6dc7ae6wv.pdf

Pyrrole: a resourceful small molecule in key medicinal hetero-aromatics

Transition metal-mediated formation of C-N bonds is an essential synthetic methodology. The discovery of the Chan-Lam amination provided a C-N bond forming process that was mild, convenient, and inexpensive, offering an alternative to complementary methods using other transition metals (TMs). Over the past 20 years, this reaction has seen considerable development in its scope of application, uptake into industry, and understanding of its mechanism. This review provides an account of the development of the Chan-Lam amination, highlighting progress and notable examples of application since 2011. Focus is given to evolution in mechanistic understanding and selected applications of the methodology within medicinal and process chemistry.

/pdf/mechanistic-development-and-recent-applications-of-the-chan-13paelgcwf.pdf

Mechanistic Development and Recent Applications of the Chan-Lam Amination.

Transmetalation of Unsaturated Carbon Nucleophiles from Boron-Containing Species to the Mid to Late d-Block Metals of Relevance to Catalytic C−X Coupling Reactions (X = C, F, N, O, Pb, S, Se, Te)

N-Arylated aliphatic and aromatic amines are important substituents in many biologically active compounds. In the last few years, transition-metal-mediated N-aryl bond formation has become a standard procedure for the introduction of amines into aromatic systems. While N-arylation of simple aromatic halides by simple amines works with many of the described methods in high yield, the reactions may require detailed optimization if applied to the synthesis of complex molecules with additional functional groups, such as natural products or drugs. We discuss and compare in this review the three main N-arylation methods in their application to the synthesis of biologically active compounds: Palladium-catalysed Buchwald–Hartwig-type reactions, copper-mediated Ullmann-type and Chan–Lam-type N-arylation reactions. The discussed examples show that palladium-catalysed reactions are favoured for large-scale applications and tolerate sterically demanding substituents on the coupling partners better than Chan–Lam reactions. Chan–Lam N-arylations are particularly mild and do not require additional ligands, which facilitates the work-up. However, reaction times can be very long. Ullmann- and Buchwald–Hartwig-type methods have been used in intramolecular reactions, giving access to complex ring structures. All three N-arylation methods have specific advantages and disadvantages that should be considered when selecting the reaction conditions for a desired C–N bond formation in the course of a total synthesis or drug synthesis.

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-7-10.pdf

Palladium- and copper-mediated N-aryl bond formation reactions for the synthesis of biological active compounds

Copper-Catalyzed Cross-Coupling Reactions of Nucleobases with Arylboronic Acids: An Efficient Access to N-Arylnucleobases

An efficient and mild protocol for the preparation of 4,5-disubstituted oxazoles through an improved one-pot van Leusen oxazole synthesis in ionic liquids is described. The oxazole products were prepared from tosylmethyl isocyanide (TosMIC), aliphatic halides and various aldehydes in high yields. The recovered ionic liquids could be reused as solvent for six runs without significant loss of yields.

One-Pot Van Leusen Synthesisof 4,5-Disubstituted Oxazoles in Ionic Liquids

The coupling of arylboroxines with a variety of amines, amides, imides and sulfonamides catalyzed by a copper salt/EtOH system has been developed In the absence of a base or additive the corresponding N-arylation products were obtained in moderate to excellent yields

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-4-40.pdf

N-Arylation of amines, amides, imides and sulfonamides with arylboroxines catalyzed by simple copper salt/EtOH system.

A new and efficient method of synthesizing primary amines from their corresponding nitriles with KBH4 in dry ethanol catalyzed by Raney Ni has been discovered. It is proved that this route is so far the most facile, efficient and easy-to-handle method of reducing aliphatic and aromatic nitriles directly to primary amines. Under mild conditions, up to 93% isolated yields were achieved for aromatic or aliphatic nitriles. This approach opens up a new way for the utilization of potassium borohydride and the classical catalyst Raney Ni.

http://www.arkat-usa.org/get-file/23388/%22

Raney Ni/KBH4: An efficient and mild system for the reduction of nitriles to amines

A synthetic route to novel chiral PNA monomer-cyclen conjugates was described for the first time, the targeted products were obtained in high yields under mild reaction conditions. The preliminary results demonstrated that the uracil-PNA monomer-cyclen conjugates can rapidly bind Zn2+ in aqueous solution, and the structure of the Zn(II) complex was confirmed facilely by HRMS spectra, H-1 NMR spectra and elemental analysis. Copyright (c) 2005 European Peptide Society and John Wiley & Sons, Ltd.

The first synthesis of chiral PNA monomer-cyclen conjugates.

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