Catalytic decomposition of H<Subscript>2</Subscript>O<Subscript>2</Subscript> in hydrophilic solvents: The kinetic aspect

Decomposition of hydrogen peroxide in organic hydrophilic solvents, catalyzed by cobalt(II) palmitate [Co(palm)₂], was studied by the method of inhibitors.     

Effect of Quinones on the Cobalt(II) Acetate–Catalyzed Mild Oxidation of Cyclohexane with Hydrogen Peroxide in Acetic Acid

The effects of free-radical reaction inhibitors (InH), hydroquinone (HQ) and quinone (Q), on the oxidation of cyclohexane catalyzed by cobalt(II) acetate Co(OAc)₂ · 4H₂O and on the decomposition of hydrogen peroxide in acetic acid (HOAc) at 303 K were studied. It was found that an increase in the concentration of HQ in the starting reaction mixture containing cyclohexane, the catalyst, and H₂O₂ dissolved in HOAc resulted in an exponential decrease in the yields of the target products of oxidation: cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide. In the presence of Q, the dependence of the yield of the target products on the initial inhibitor concentration exhibited a maximum at (1.8–2.5) × 10^–2 M Q. At (2.2–2.4) × 10^–2 M Q concentrations, the yield of the target products was 55–60% of that in an uninhibited process. Based on kinetic, spectrometric, and quantum-chemical data, the effect found was explained by the fact that under the experimental conditions highly active hydroxyl derivatives of radicals rather than a hydroxy quinolide hydroperoxide (the homolysis of which can produce species with a free valence, which are capable of initiating free-radical reactions) were largely formed from Q.     

Mild Oxidation of Cyclohexane Catalyzed by Cobalt(II) Acetate and Initiated by H2O2 in the Acetic Acid Medium

The homogeneous catalytic oxidation of cyclohexane by molecular oxygen and hydrogen peroxide in a solution of acetic acid (HOAc) in the presence of cobalt(II) acetate Co(OAc)₂ is studied. The high yields of cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide (0.10–0.15 mol/l) and the high rate of the process (w = 10^–5–10^–4 mol l^–1 s^–1) are explained by (1) mild conditions of oxidation in the medium of the HOAc solvent and (2) efficient initiation of the process due to the fast kinetics-controlled dissociation of H₂O₂ into radicals in the studied reaction medium under the action of cobalt cations. Quantitative relationships are found for the cyclohexane oxidation rate, the yield of target products, and the ratio of reactants participating in the process. The effect of hydrogen hydroperoxide additives on the concentrations of reduced and oxidized forms of the catalyst is studied by spectrophotometry in model mixtures. Quantum chemistry is employed to calculate the probabilities of some key elementary reactions. Calculated data agree well with the experiment.     

Quantitative accounting for the medium effect at the catalytic decomposition of H<Subscript>2</Subscript>O<Subscript>2</Subscript> in hydrophilic solvents

The experimental data on the effect of physicochemical properties of solvent on the rate of hydrogen peroxide decomposition catalyzed by cobalt ions were obtained using a method of adding inhibitors and were generalized quantitatively by the application of multiparametric linear equations.     

Cyclohexane oxidation catalyzed by variable-valence metal compounds in the presence of propionic aldehyde

The effect of propionic aldehyde additives on the kinetics and mechanism of cyclohexane oxidation by molecular oxygen catalyzed by variable-valence metal salts is studied. The effect of the catalyst metal (M) on the rate of oxygen consumption, yield, and ratio of reaction products (cyclohexanol, cyclohexanone, cyclohexyl hydroperoxide (CHHP), and propionic acid and peracid) is studied. The catalytic functions of the variable-valence metal salts are determined by their ability to influence the rate of the homolysis of the O–O bonds of peroxide compounds, which correlates with the redox potential of the metal ion for most of the catalysts studied. The fact that other salts of variable-valence metals do not fit this correlation is due to the multifunctional action of catalysts involved in chain initiation, termination, and degenerate branching. The main role of aldehyde in the process under consideration is to promote oxidation. According to the quantum-chemical studies, the catalyst cation largely determines both the structure of the [Mⁿ⁺-CHHP] transition complex and the rates of competitive homolysis and heterolysis of cyclohexyl hydroperoxide.     

New data on the mechanism of hydrogen peroxide decomposition in acetic acid in the presence of vanadyl and cobalt (II) acetylacetonates

The process of catalytic hydrogen peroxide decomposition in acetic acid in the presence of vanadyl and cobalt (II) acetylacetonates was studied using modern spectroscopic and kinetic techniques. The formation of intermediates during the catalytic decomposition of hydrogen peroxide in the presence of VO(acac)₂ was observed using UV—Vis and ESR spectroscopy. The decomposition of H₂O₂ occurs both catalytically and via the radical route.     

Benign homogeneous catalytic oxidation of cyclohexane promoted with glyoxal

Evidence of the promoted effect of dialdehyde (glyoxal) in VO(acac)₂- and Co(acac)₂-catalyzed oxidations of cyclohexane by H₂O₂ under ambient conditions is reported. The V-process leads to a mixture of cyclohexanone, cyclohexanol, and cyclohexyl hydroperoxide and TON up to 4400. The Co-process is much less active but leads selectively to cyclohexyl hydroperoxide. Glyoxal significantly accelerates the process rate and enhances the yield of desired products. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 1, pp. 32–37. This article was submitted by the authors in English.     

Kinetics and mechanism of cyclohexane oxidation with molecular oxygen in the presence of propionic aldehyde

The kinetics and mechanism of the liquid-phase oxidation of cyclohexane with molecular oxygen in the presence of the additives of propionic aldehyde are studied at 303.0, 322.5, and 341.5 K by measuring the rates of oxygen and propionic aldehyde consumption and the yields of the main reaction products (cyclohexanol (COL), cyclohexanone (CON), cyclohexyl hydroperoxide, and propionic acid and peracid). A kinetic scheme is proposed and rate constants of elementary reactions are estimated based on the analysis of their rates and the yields of the main cyclohexane products. The key reactions of the main steps (including chain initiation, propagation, and termination) are determined. An increase in the rate of cyclohexane oxidation and the yield of the target products (cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide) in the presence of propionic aldehyde suggests that highly active acylperoxy radicals participate in chain propagation. The [CON]/[COL] ratio indicates that these products are mainly formed in chain propagation. The strong effect of the Baeyer-Villiger rearrangement on both the rate of oxygen consumption and the yield of the target products at the initial stages of the process and at high propionic aldehyde concentrations is explained.     

Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage

An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.