3.1. C-M and X-H as Nucleophiles 1806 3.2. C-H and X-M as Nucleophiles 1809 3.2.1. C-Halogen Bond Formations 1809 3.2.2. C-O Bond Formations 1812 3.3. C-H and X-H as Nucleophiles 1812 3.3.1. C-O Bond Formations 1812 3.3.2. C-N Bond Formations 1815 4. Oxidative X-X Bond Formations between Two Nucleophiles 1819 5. Conclusions 1819 Author Information 1819 Biographies 1819 Acknowledgment 1820 References 1820

Bond Formations between Two Nucleophiles: Transition Metal Catalyzed Oxidative Cross-Coupling Reactions

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

Ordered mesoporous and microporous molecular sieves functionalized with transition metal complexes as catalysts for selective organic transformations.

https://pubs.rsc.org/en/content/articlepdf/2013/cc/c3cc41143e

Exploration of the medical periodic table: towards new targets

V2O5 nanowires exhibit an intrinsic catalytic activity towards classical peroxidase substrates such as 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 3,3,5,5,-tetramethylbenzdine (TMB) in the presence of H2O2. These V2O5 nanowires show an optimum reactivity at a pH of 4.0 and the catalytic activity is dependent on the concentration. The Michaelis-Menten kinetics of the ABTS oxidation over these nanowires reveals a behavior similar to that of their natural vanadium-dependent haloperoxidase (V-HPO) counterparts. The V2O5 nanowires mediate the oxidation of ABTS in the presence of H2O2 with a turnover frequency (k(cat)) of 2.5 x 10(3) s(-1). The K-M values of the V2O5 nanowires for ABTS oxidation (0.4 mu M) and for H2O2 (2.9 mu M) at a pH of 4.0 are significantly smaller than those reported for horseradish peroxidases (HRP) and V-HPO indicating a higher affinity of the substrates for the V2O5 nanowire surface. Based on the kinetic parameters and similarity with vanadium-based complexes a mechanism is proposed where an intermediate metastable peroxo complex is formed as the first catalytic step. The nanostructured vanadium-based material can be re-used up to 10 times and retains its catalytic activity in a wide range of organic solvents (up to 90%) making it a promising mimic of peroxidase catalysts.

V2O5 Nanowires with an Intrinsic Peroxidase-Like Activity

We report the synthesis and characterization of several chiral salen- and salan-type ligands and their vanadium complexes, which are derived from salicylaldehyde or salicylaldehyde derivatives and chiral diamines (1R,2R-diaminocyclohexane, 1S,2S-diaminocyclohexane, and 1S,2S-diphenylethylenediamine). The structures of H2sal(R,R-chan)2+·2Cl−·(CH3)2CHOH·H2O (1c; H2sal(R,R-chan) = N,N′-salicyl-R,R-cyclohexanediaminium), Etvan(S,S-chen) (3c; Etvan(S,S-chen) = N,N′-3-ethoxy-salicylidene-S,S-cyclohexanediiminato), and naph(R,R-chen) (6c; naph(R,R-chen) = N,N′-naphthylidene-R,R-cyclohexanediiminato) were determined by single-crystal X-ray diffraction. The corresponding vanadium(IV) complexes and several other new complexes involving different salicylaldehyde-type precursors were prepared and characterized in the solid state and in solution by spectroscopic techniques: UV−vis, circular dichroism, electron paramagnetic resonance, and 51V NMR, which provide information on the coordination geometry. The salan comple...

Synthesis, characterization, and application of vanadium-salan complexes in oxygen transfer reactions.

Development of the coordination chemistry of vanadium through bis(acetylacetonato)oxovanadium(IV): synthesis, reactivity and structural aspects

Vanadium complexes immobilized on solid supports and their use as catalysts for oxidation and functionalization of alkanes and alkenes

Coordination chemistry of schiff base complexes of molybdenum

Reaction between [VO(acac)(2)] and the ONN donor Schiff base Hsal-ambmz (I) (Hsal-ambmz = Schiff base obtained by the condensation of salicylaldehyde and 2-aminomethylbenzimidazole) resulted in the formation of the complexes [V(IV)O(acac)(sal-ambmz)] (1), [V(V)O(2)(acac-ambmz)] (2) (Hacac-ambmz = Schiff base derived from acetylacetone and 2-aminomethylbenzimidazole), and the known complex [V(IV)O(sal-phen)] (3) (H(2)sal-phen = Schiff base derived from salicylaldehyde and o-phenylenediamine). Similarly, [V(IV)O(acac)(sal-aebmz)] (7) has been isolated from the reaction with Hsal-aebmz (II) (Hsal-aebmz derived from salicylaldehyde and 2-aminoethylbenzimidazole). Aerial oxidation of the methanolic solutions/suspensions of 1 and 7 yielded the dioxovanadium(V) complexes [V(V)O(2)(sal-ambmz)] (4) and [V(V)O(2)(sal-aebmz)] (8), respectively. Reaction of VOSO(4) with II gave [{V(IV)O(sal-aebmz)}(2)SO(4)] (9) and [V(IV)O(sal-aebmz)(2)] (10), along with 3 and 8. Under similar reaction conditions, I gave only [{V(IV)O(sal-ambmz)}(2)SO(4)] (5) and 3 as major products. Treatment of 1 and 7 with benzohydroxamic acid (Hbha) yielded the mixed-chelate complexes [V(V)O(bha)(sal-ambmz)] (6) and [V(V)O(bha)(sal-aebmz)] (11). The crystal and molecular structures of 2, 3.1/2DMF, 7.1/4H(2)O, 8, 9.2H(2)O, 10, and 11 have been determined, confirming the ONN binding mode of the ligands. In complex 10, one of the ligands is coordinated through the azomethine nitrogen and phenolate oxygen only, leaving the benzimidazole group free. In the dinuclear complex 9, bridging functions are the phenolate oxygens from both of the ligands and two oxygens of the sulfato group. The unstable oxoperoxovanadium(V) complex [V(V)O(O(2))(sal-aebmz)] (12) has been prepared by treatment of 7 with aqueous H(2)O(2). Acidification of methanolic solutions of 7 and 10 lead to (reversible) protonation of the bemzimidazole, while 8 was converted to an oxo-hydroxo species. Complexes 2, 4, and 8 catalyze the oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide and methyl phenyl sulfone, a reaction mimicking the sulfideperoxidase activity of vanadate-dependent haloperoxidases. These complexes are also catalytically active in the oxidation of styrene to styrene oxide, benzaldehyde, benzoic acid, and 1-phenylethane-1,2-diol.

Mannar R. Maurya

Papers

Synthesis, characterization, and application of vanadium-salan complexes in oxygen transfer reactions.

Development of the coordination chemistry of vanadium through bis(acetylacetonato)oxovanadium(IV): synthesis, reactivity and structural aspects

Vanadium complexes immobilized on solid supports and their use as catalysts for oxidation and functionalization of alkanes and alkenes

Coordination chemistry of schiff base complexes of molybdenum

Synthesis, Characterization, Reactivity, and Catalytic Potential of Model Vanadium(IV, V) Complexes with Benzimidazole-Derived ONN Donor Ligands