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

Hydrogen bonds are like human beings in the sense that they exhibit typical grouplike behavior. As an individual they are feeble, easy to break, and sometimes hard to detect. However, when acting together they become much stronger and lean on each other. This phenomenon, which in scientific terms is called cooperativity, is based on the fact that 1+1 is more than 2. By using this principle, chemists have developed a wide variety of chemically stable structures that are based on the reversible formation of multiple hydrogen bonds. More than 20 years of fundamental studies on these phenomena have gradually developed into a new discipline within the field of organic synthesis, and is nowadays called noncovalent synthesis. This review describes noncovalent synthesis based on the reversible formation of multiple hydrogen bonds. Starting with a thorough description of what the hydrogen bond really is, it guides the reader through a variety of bimolecular and higher order assemblies and exemplifies the general principles that determine their stability. Special focus is given to reversible capsules based on hydrogen-bonding interactions that exhibit interesting encapsulation phenomena. Furthermore, the role of hydrogen-bond formation in self-replicating processes is actively discussed, and finally the review briefly summarizes the development of novel materials (nanotubes, liquid crystals, polymers, etc.) and principles (dynamic libraries) that recently have emanated from this intriguing field of research.

Noncovalent synthesis using hydrogen bonding

Supramolecular Polymers: Historical Development, Preparation, Characterization, and Functions.

The supramolecular crosslinking of polymer chains in water by specific, directional and dynamic non-covalent interactions has led to the development of novel supramolecular polymeric hydrogels. These aqueous polymeric networks constitute an interesting class of soft materials exhibiting attractive properties such as stimuli-responsiveness and self-healing arising from their dynamic behaviour and that are crucial for a wide variety of emerging applications. We present here a critical review summarising the formation of dynamic polymeric networks through specific non-covalent interactions, with a particular emphasis on those systems based on host–guest complex formation, as well as the characterisation of their physical characteristics. Aqueous supramolecular chemistry has unlocked a versatile toolbox for the design and fine-tuning of the material properties of these hydrogels (264 references).

Supramolecular polymeric hydrogels

Levulinic acid (LA) can be produced cost effectively and in high yield from renewable feedstocks in a new industrial process. The technology is being demonstrated on a 1 ton/day scale at a facility in South Glens Falls, New York. Low cost LA can be used as a platform chemical for the production of a wide range of value-added products. This research has demonstrated that LA can be converted to methyltetrahydrofuran (MTHF), a solvent and fuel extender. MTHF is produced in >80% molar yield via a single stage catalytic hydrogenation process. A new preparation of δ-aminolevulinic acid (DALA), a broad spectrum herbicide, from LA has also been developed. Each step in this new process proceeds in high (>80%) yield and affords DALA (as the hydrochloride salt) in >90% purity, giving a process that could be commercially viable. LA is also being investigated as a starting material for the production of diphenolic acid (DPA), a direct replacement for bisphenol A.

Production of levulinic acid and use as a platform chemical for derived products

Telechelic oligo- and poly(dimethylsiloxanes) 1 and 2, with two ureidopyrimidone (UPy) functional groups, have been prepared via a hydrosilylation reaction. The compounds have been characterized in solution by 1H NMR and viscometry and in the solid state by 1H NMR and 13C NMR, FTIR, and rheology measurements. The measurements show that the UPy groups of 1 and 2 are associated via quadruple hydrogen bonds in a donor−donor−acceptor−acceptor (DDAA) array. In many aspects, the materials behave like entangled, high molecular weight polymers. Compound 2 has a Tg at −119 °C and shows melting of microcrystalline domains of associated UPy units at −25 °C. Compound 1 has a crystalline form (Tm = 112 °C) and an amorphous modification with a Tg of 25 °C. Solid-state NMR was used to investigate the mobility of these phases. WISE spectra show a higher mobility of the UPy groups in the amorphous phase than in the crystals of 1. Amorphous 1 and 2 behave like entangled polymers. Their mechanical behavior is characterized ...

Supramolecular polymers from linear telechelic siloxanes with quadruple- hydrogen- bonded units

Redistribution of poly(2,6-dimethyl-1,4-phenylene ether) (PPE) with a phenolic compound yields a depolymerized polymer having the phenolic compound incorporated as the tail end. The molecular weight of the product is adjusted by use of the ratio of phenol over PPE repeating units. PPE oligomers and telechelics with a large variety of end groups, including reactive end groups, can be prepared in this way. Furthermore, block copolymers can be prepared by reacting a phenol terminated polymer in a redistribution with PPE. This is illustrated by the reaction of phenol terminated polystyrene with PPE, leading to polystyrene−PPE diblock copolymers. TMDPQ or copper/amine catalysts can be used as catalyst depending on the reacting phenolic compound, desired reaction time, and product purity. For a fast PPE redistribution with ortho-unsubstituted phenols, without any oxidation of the methyl substituents of the PPE chain, the use of TMDPQ is preferred. For the preparation of pure oligomers and block copolymers, a sl...

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Modified poly(2,6-dimethyl-1,4-phenylene ether)s prepared by redistribution

The properties of star-shaped poly(2,6-dimethyl-1,4-phenylene ether) (PPE) as prepared by the redistribution of PPE and tyrosine-modified poly(propylene imine) dendrimers, are studied in solution and in 50/50 wt% blends with linear polystyrene. Star polymers with constant armlength but increasing number of arms show the same hydrodynamic volume as measured by Size Exclusion Chromatography (SEC), but decreasing hydrodynamic radius as measured by Dynamic Light Scattering (DLS). This is caused by the restricted mobility of the more densely packed chains at high numbers of arms, also leading to a decrease in intrinsic viscosities. These solution properties are also reflected in the miscibility behaviour in polymer blends. Star-shaped polymers with a high number of PPE arms (16,32 or 64 respectively) give inhomogeneous blends with linear polystyrene, in contrast to the miscible combination of linear polystyrene with linear PPE or starshaped polymers with a low number (4 or 8) of PPE arms.

Star-shaped poly(2,6-dimethyl-1,4-phenylene ether)

Low molar mass poly(2,6-dimethyl-1,4-phenylene-ether)s (PPE) have been prepared by the precipitation polymerization of 2,6-dimethylphenol (DMP) under the action of a Cu(I)Cl/amine-catalyst and O2. The polymers prepared possess a rather narrow molecular weight distribution (D = 1.7−2.2), and the molecular weight can easily be controlled by adjusting the solvent mixture of 2-propanol and toluene. Molecular weights were determined using 1H-NMR spectroscopy, GPC, and FD-MS, and range from M n = 2700 to 7500 g/mol. No Mannich-basetype endgroups and no incorporation of DPQ (3,5,3′,5′-tetramethyl-4,4′-diphenoquinone) has been detected.

Controlled molecular weight by the precipitation polymerization of 2,6-dimethylphenol

New multiblock copolymers based on dimethylsiloxanes and phenylene diacrylate derivatives have been prepared by hydrosilation polymerization. Copolymerization was performed using cither low molecular weight monomers or high molecular weight macromers. The organic macromcr was prepared by acyclic diene metathesis (ADMET) polymerization of para-phenylenediacrylic acid dipent-4-enyl ester. Hexamethyltrisiloxane and α,ω-bishydride-terminated polydimethylsiloxane (PDMS) were used as inorganic macromers. The copolymers show microphase separation of the soft siloxane block and the hard semicrystalline organic block, as observed by differential scanning calorimetry (DSC), X-ray scattering experiments (SAXS and WAXS) and transmission electron microscopy (TEM). The crosslinking rate upon UV-irradiation is slightly higher for the microphase-separated copolymer than for the corresponding homopolymer. When films of the polyester-PDMS multiblock copolymers were irradiated with linearly polarized light, optically anisotropic films were obtained with a polarizing efficiency of 67%. whereas the crosslinked films of the homopolymer had a polarizing efficiency of at most 43%.

H.A.M. van Aert

Papers

Supramolecular polymers from linear telechelic siloxanes with quadruple- hydrogen- bonded units

Modified poly(2,6-dimethyl-1,4-phenylene ether)s prepared by redistribution

Star-shaped poly(2,6-dimethyl-1,4-phenylene ether)

Controlled molecular weight by the precipitation polymerization of 2,6-dimethylphenol

Selective photo‐crosslinking of microphase‐separated multiblock copolymers of poly(dimethylsiloxane) and phenylene diacrylate polyesters