Oxidation with the “O2−H2O2—vanadium complex—pyrazine-2-carboxylic acid” reagent: 11. Oxidation of styrene, phenylacetylene, and their derivatives with the formation of benzaldehyde and benzoic acid

Selective Oxidation of Simple Aromatics Catalyzed by Nano-Biomimetic Metal Oxide Catalysts: A Mini Review

The process of selective oxy-functionalization of hydrocarbons using peroxide, O3, H2O2, O2, and transition metals can be carried out by the reactive oxygen species such as hydroxyl/hydroperoxyl radical and/or metal oxygenated species generated in the catalytic reaction. Thus, a variety of mechanisms have been proposed for the selective catalytic oxidation of various hydrocarbons including light alkanes, olefins, and simple aromatics by the biological metalloproteins and their biomimetics either in their homogeneous or heterogeneous platforms. Most studies involving these metalloproteins are Fe or Cu monooxygenases. The pathways carried out by these metalloenzymes in the oxidation of C-H bonds invoke either radical reaction mechanisms including Fenton's chemistry and hydrogen atom transfer followed by radical rebound reaction mechanism or electrophilic oxygenation/O-atom transfer by metal-oxygen species. In this review, we discuss the metal oxide nano-catalysts obtained from metal salts/molecular precursors (M = Cu, Fe, and V) that can easily form in situ through the oxidation of substrates using H2O2(aq) in CH3CN, and be facilely separated from the reaction mixtures as well as recycled for several times with comparable catalytic efficiency for the highly selective conversion from hydrocarbons including aromatics to oxygenates. The mechanistic insights revealed from the oxy-functionalization of simple aromatics mediated by the novel biomimetic metal oxide materials can pave the way toward developing facile, cost-effective, and highly efficient nano-catalysts for the selective partial oxidation of simple aromatics.

Metal Complexes Containing Redox-Active Ligands in Oxidation of Hydrocarbons and Alcohols: A Review

Ligands are innocent when they allow oxidation states of the central atoms to be defined. A noninnocent (or redox) ligand is a ligand in a metal complex where the oxidation state is not clear. Dioxygen can be a noninnocent species, since it exists in two oxidation states, i.e., superoxide (O2−) and peroxide (O22−). This review is devoted to oxidations of C–H compounds (saturated and aromatic hydrocarbons) and alcohols with peroxides (hydrogen peroxide, tert-butyl hydroperoxide) catalyzed by complexes of transition and nontransition metals containing innocent and noninnocent ligands. In many cases, the oxidation is induced by hydroxyl radicals. The mechanisms of the formation of hydroxyl radicals from H2O2 under the action of transition (iron, copper, vanadium, rhenium, etc.) and nontransition (aluminum, gallium, bismuth, etc.) metal ions are discussed. It has been demonstrated that the participation of the second hydrogen peroxide molecule leads to the rapture of O–O bond, and, as a result, to the facilitation of hydroxyl radical generation. The oxidation of alkanes induced by hydroxyl radicals leads to the formation of relatively unstable alkyl hydroperoxides. The data on regioselectivity in alkane oxidation allowed us to identify an oxidizing species generated in the decomposition of hydrogen peroxide: (hydroxyl radical or another species). The values of the ratio-of-rate constants of the interaction between an oxidizing species and solvent acetonitrile or alkane gives either the kinetic support for the nature of the oxidizing species or establishes the mechanism of the induction of oxidation catalyzed by a concrete compound. In the case of a bulky catalyst molecule, the ratio of hydroxyl radical attack rates upon the acetonitrile molecule and alkane becomes higher. This can be expanded if we assume that the reactions of hydroxyl radicals occur in a cavity inside a voluminous catalyst molecule, where the ratio of the local concentrations of acetonitrile and alkane is higher than in the whole reaction volume. The works of the authors of this review in this field are described in more detail herein.

New Oxidovanadium(IV) Complexes with 2,2′-bipyridine and 1,10-phenathroline Ligands: Synthesis, Structure and High Catalytic Activity in Oxidations of Alkanes and Alcohols with Peroxides

Reactions of [VCl3(thf)3] or VBr3 with 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) in a 1:1 molar ratio in air under solventothermal conditions has afforded polymeric oxidovanadium(IV) four complexes 1–4 of a general formula [VO(L)X2]n (L = bpy, phen and X = Cl, Br). Monomeric complex [VO(DMF)(phen)Br2] (4a) has been obtained by the treatment of compound 4 with DMF. The complexes were characterized by IR spectroscopy and elemental analysis. The crystal structures of 3 and 4a were determined by an X-ray diffraction (XRD) analysis. The {VOBr2(bpy)} fragments in 3 form infinite chains due to the V = O…V interactions. The vanadium atom has a distorted octahedral coordination environment. Complexes 1–4 have been tested as catalysts in the homogeneous oxidation of alkanes (to produce corresponding alkyl hydroperoxides which can be easily reduced to alcohols by PPh3) and alcohols (to corresponding ketones) with H2O2 or tert-butyl hydroperoxide in MeCN. Compound 1 exhibited the highest activity. The mechanism of alkane oxidation was established using experimental selectivity and kinetic data and theoretical DFT calculations. The mechanism is of the Fenton type involving the generation of HO• radicals.

Transformations of alkynes to carboxylic acids and their derivatives via C[triple bond, length as m-dash]C bond cleavage

Alkynes are building blocks of high synthetic value and their usefulness as precursors to many chemical and biological systems is widely established. Amongst several transformations of alkynes, cleavage of the C[triple bond, length as m-dash]C bond for obtaining diverse compounds is considered to be important. This review, in particular, comprehensively assimilates the transformations of alkynes to carboxylic acids including esters, amides and nitriles resulting from the cleavage of the C[triple bond, length as m-dash]C bond either under the influence of a metal catalyst or via a metal-free approach.

New oxidovanadium(iv) complex with a BIAN ligand: synthesis, structure, redox properties and catalytic activity

The combination of a new oxidovanadium(iv) complex1with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the efficient oxidation of saturated hydrocarbons.

Cage-like copper(II) silsesquioxanes: transmetalation reactions and structural, quantum chemical, and catalytic studies

The transmetalation of bimetallic copper-sodium silsesquioxane cages, namely, [(PhSiO1.5 )10 (CuO)2 (NaO0.5 )2 ] ("Cooling Tower"; 1), [(PhSiO1.5 )12 (CuO)4 (NaO0.5 )4 ] ("Globule"; 2), and [(PhSiO1.5 )6 (CuO)4 (NaO0.5 )4 (PhSiO1.5 )6 ] ("Sandwich"; 3), resulted in the generation of three types of hexanuclear cylinder-like copper silsesqui- oxanes, [(PhSiO1.5 )12 (CuO)6 (C4 H9 OH)2 (C2 H5 OH)6 ] (4), [(PhSiO1.5 )12 (CuO)6 (C4 H8 O2 )4 (PhCN)2 (MeOH)4 ] (5), and [(PhSiO1.5 )12 (CuO)6 (NaCl)(C4 H8 O2 )12 (H2 O)2 ] (6). The products show a prominent "solvating system-structure" dependency, as determined by X-ray diffraction. Topological analysis of cages 1-6 was also performed. In addition, DFT theory was used to examine the structures of the Cooling Tower and Cylinder compounds, as well as the spin density distributions. Compounds 1, 2, and 5 were applied as catalysts for the direct oxidation of alcohols and amines into the corresponding amides. Compound 6 is an excellent catalyst in the oxidation reactions of benzene and alcohols.

Alkane oxidation with peroxides catalyzed by cage-like copper(ii) silsesquioxanes

Copper(ii) silsesquioxanes [(PhSiO_1.5)₁₂(CuO)₄(NaO_0.5)₄] or [(PhSiO_1.5)₁₀(CuO)₂(NaO_0.5)₂] are catalysts for alkane oxidation with H₂O₂ort-BuOOH.

A density functional theory study of the mechanism of free radical generation in the system vanadate/PCA/H2O2

Experimental studies by Shul'pin and co-workers have shown that vanadate anions in combination with pyrazine-2-carboxylic acid (PCA identical with pcaH) produce an exceptionally active complex that promotes the oxidation of alkanes and other organic molecules. Reaction of this complex with H2O2 releases HOO* free radicals and generates V(IV) species, which are capable of generating HO* radicals by reaction with additional H2O2. The oxidation of alkanes is initiated by reaction with the HO* radicals. The mechanism of hydrocarbon oxidation with vanadate/PCA/H2O2 catalyst has been studied using density functional theory. The proposed model reproduces the major experimental observations. It is found that a vanadium complex with one pca (PCA identical with pcaH) and one H2O2 ligand is the precursor to the species responsible for HOO* generation. It is also found that species containing two pca ligands and an H2O2 molecule do not exist in the solution, in contradiction to previous interpretations of experimental observations. Calculated dependences of the oxidation rate on initial concentrations of PCA and H2O2 have characteristic maxima, the shapes of which are determined by the equilibrium concentration of the active species. Conversion of the precursors requires hydrogen transfer from H2O2 to a vanadyl group. Our calculations show that direct transfer has a higher barrier than pca-assisted indirect transfer. Indirect transfer occurs by migration of hydrogen from coordinated H2O2 to the oxygen of a pca ligand connected to the vanadium atom. The proposed mechanism demonstrates the important role of the cocatalyst in the reaction and explains why H2O2 complexes without pca are less active. Our work shows that the generation of HOO* radicals cannot occur via cleavage of a V-OOH bond in the complex formed directly from the precursors, as proposed before. The activation barrier for this process is too high. Instead, HOO* radicals are formed via a sequence of additional steps involving lower activation barriers. The new mechanism for free radical generation underestimates the observed rate of hexane oxidation by less than an order of magnitude; however, the calculated activation energy (67-81 kJ/mol) agrees well with that determined experimentally (63-80 kJ/mol).

Oxidation of 2-propanol and cyclohexane by the reagent "hydrogen peroxide-vanadate anion-pyrazine-2-carboxylic acid": kinetics and mechanism

The vanadate anion in the presence of pyrazine-2-carboxylic acid (PCA [identical with] pcaH) efficiently catalyzes the oxidation of 2-propanol by hydrogen peroxide to give acetone. UV-vis spectroscopic monitoring of the reaction as well as the kinetics lead to the conclusion that the crucial step of the process is the monomolecular decomposition of a diperoxovanadium(V) complex containing the pca ligand to afford the peroxyl radical, HOO(.-) and a V(IV) derivative. The rate-limiting step in the overall process may not be this (rapid) decomposition itself but (prior to this step) the slow hydrogen transfer from a coordinated H2O2 molecule to the oxygen atom of a pca ligand at the vanadium center: "(pca)(O=)V...O2H2" --> "(pca)(HO-)V-OOH". The V(IV) derivative reacts with a new hydrogen peroxide molecule to generate the hydroxyl radical ("V(IV)" + H2O2 --> "V(V)" + HO(-) + HO(.-)), active in the activation of isopropanol: HO(.-) + Me2CH(OH) --> H2O + Me2C(.-)(OH). The reaction with an alkane, RH, in acetonitrile proceeds analogously, and in this case the hydroxyl radical abstracts a hydrogen atom from the alkane: HO(.-) + RH --> H2O + R(.-). These conclusions are in a good agreement with the results obtained by Bell and co-workers (Khaliullin, R. Z.; Bell, A. T.; Head-Gordon, M. J. Phys. Chem. B 2005, 109, 17984-17992) who recently carried out a density functional theory study of the mechanism of radical generation in the reagent under discussion in acetonitrile.