Galina V. Nizova

Hydroperoxidation of methane and other alkanes with H2O2 catalyzed by a dinuclear iron complex and an amino acid

Abstract The compound [Fe2(HPTB)(μ-OH)(NO3)2](NO3)2·CH3OH·2H2O ( 1 ) containing a dinuclear iron(III) complex in which HPTB=N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane catalyzes the oxidation of alkanes with hydrogen peroxide in acetonitrile solution at room temperature only if certain amino acids (pyrazine-2-carboxylic, pyrazine-2,3-dicarboxylic or picolinic acid) are added to the reaction mixture. Alkyl hydroperoxides are formed as main reaction products. The turnover numbers attain 140 for cyclohexane, 21 for ethane and four for methane oxidation. The oxidation proceeds non-stereoselectively and bond selectivity parameters are low which testifies the participation of hydroxyl radicals in alkane functionalization.

Oxidations by the “hydrogen peroxide–manganese(IV) complex–carboxylic acid” system.

Whereas the dinuclear manganese(IV) complex [LMn(O)3MnL](PF6)2 (1a, L = 1,4,7-trimethyl-1,4,7-triazacyclononane) does not react with H2O2 in acetonitrile solution containing cyclohexane, acetic acid added to this mixture in small amounts induces the catalytic decomposition of hydrogen peroxide to O2 and H2O (catalase activity) and the transformation of cyclohexane to cyclohexyl hydroperoxide (oxygenase activity). Addition to the acetic acid containing solution only 2 equivalents (relative to the Mn catalyst) of a base enhances the catalase activity and suppresses the oxygenase activity. The proposed mechanism includes the formation of dinuclear dihydroperoxy derivatives of manganese, which can be transformed under the action of acetic acid to OMn(V)–Mn(IV)–OOH species. The latter can abstract a hydrogen atom from an alkane. The interaction of the so-formed R˙ radical with Mn(IV)–OOH can give the alkyl hydroperoxide, ROOH, which is the main primary product of the oxidation process.

Hydrocarbon Oxidations with Hydrogen Peroxide Catalyzed by a Soluble Polymer-Bound Manganese(IV)Complex with 1,4,7-Triazacyclononane

Soluble manganese(IV) complexes with polymer-bound 1,4,7-triazacyclononanes as ligands (compound 2) catalyze the oxidation of alkanes by hydrogen peroxide in acetonitrile at room and lower temperatures. The corresponding alkyl hydroperoxides are the main products. The presence of a relatively small amount of acetic acid is obligatory for this reaction. The oxidation of alkanes and olefins exhibits some features (kinetic isotope effect, bond selectivities) that distinguish this system from an analogous one based on the dinuclear Mn(IV) complex 1.

Oxidations by the reagent «O2 - H2O2 - vanadium complex - pyrazine-2-carboxylic acidå-8: Efficient oxygenation of methane and other lower alkanes in acetonitrile☆

Methane, ethane, propane, n-butane and isobutane can be readily oxidized in acetonitrile solution by air and H2O2 at 20–75 °C using the catalytic system [n-Bu4N]VO3/pyrazine-2-carboxylic acid. Apart from alkyl hydroperoxides which are the primary oxidation products, more stable derivatives (alcohols, aldehydes or ketones and carboxylic acids) are obtained with high total turnover numbers (e.g., at 75 °C after 4 h: 420 for methane and 2130 for ethane). It was shown in the case of ethane and cyclohexane that alkanes do not yield oxygenated products in the absence of air. The cyclohexane oxidation under an 18O2 atmosphere showed a high degree of 18O incorporation into the oxygenated products. Thus in the oxidation reaction described here H2O2 is only the promoter while O2 is the “true” oxidant.

Oxidations by the reagent `O2–H2O2 – vanadate anion – pyrazine-2-carboxylic acid'.: Part 10. Oxygenation of methane in acetonitrile and water

Abstract The oxidation of methane by a combination of air and hydrogen peroxide is effectively catalyzed in solution by a system composed of vanadate and pyrazine-2-carboxylic acid (PCA). In acetonitrile solution, containing the vanadate anion as tetrabutylammonium salt, the reaction gives, over a temperature range of 25 to 50°C, methanol, carbon monoxide, formaldehyde, formic acid and carbon dioxide, the latter three compounds, however, being partially due to the oxidation of the acetonitrile used as the solvent, especially at higher temperatures. In aqueous solution, containing the vanadate anion in the form of the sodium salt, the reaction affords, over a temperature range of 40 to 70°C, selectively methyl hydroperoxide within 4 h. The yield of CH3OOH attains 24%, based on H2O2, after 24 h at 50°C, the catalytic turnover number being 480. The process seems to involve hydroxyl radicals, generated by the catalyst from H2O2 even at low temperatures. At 120°C, methane is oxidized by O2 and H2O2 to give formaldehyde and formic acid, even in the absence of the catalyst, presumably due to the formation of HO· radicals from H2O2 in the presence of very low concentrations of metal ions from the autoclave under high temperature conditions.

The reaction of PtCl62− with aromatic compounds to afford anionic σ-aryl complexes of platinum(IV): VIII. Kinetics and mechanisms of thermal, photochemical and γ-induced reactions with arenes and arylmercury compounds (electrophilic substitution involving electron transfer)☆

Abstract The kinetics of the thermal reaction of the PtCl62− ion with various aromatic compounds in CF3COOH/H2O or CH3COOH to afford σ-aryl complexes of platmum(IV) have been studied at temperatures of 60–100 °C. The reaction is first order both in PtCl62− and arene. The process of formation of platinated toluene (as well as some other complexes) is accompanied with its para-meta isomerisation. The rate of formation of the σ-aryl complexes decreases with increasing concentration of LiCl added to the reaction mixture. Additions of AgNO3, Na2PtCl4, Hg(OCOCH3)2, NaOCOCH3, BF3 · OEt2 and SeO2 accelerate the reaction. The activation energies of the formation and the para-meta isomerisation are ca. 100 kJ mol−1. The relative rates of the reaction with different aromatic compounds (C6H5X) decrease in the following sequence of X: OH > OCH3 > CH3 > C2H5 > OC6H5 > CH(CH3)2 > H > C6H5 > F > COCH3 > COOH > Cl > NO2. The logarithms of the relative rates correlate with Hammetts σ and Browns σ+ constants (ϱ = −3.0 and ϱ+ = −1.5). The kinetic hydrogen isotope effect of the reaction is small ( ∼ 3 for benzene and ∼ 2.3 for toluene). The following mechanism of the reaction is proposed. The first stage is the dissociation of PtCl62− followed by coordination with arene to form a π-complex. The π-complex then transforms into a Wheland-type complex, which gives a σ-aryl complex of platmum(IV) after proton elimination. The reaction of PtCl62− with arenes may be carried out at room temperature if it is induced by light or γ-irradiation. The relative rates of the photoinduced reaction decrease in the following sequence: OH > OC2H5 > OCH3 > OC6H5 > CH3 (ϱ+ = −1.5). The isotope effect for the reaction with toluene (kH/kD ∼ 2) was determined. No para-meta isomerisation was observed in the photoinduced reaction at room temperature. The ESR spectra of frozen solutions of PtCl62− and arenes irradiated at 77 K exhibited signals due to platinum(III) complexes and organic radicals. The proposed mechanism involves electron transfer from an arene to a platinum(IV) complex to give an intermediate ion-radical pair, [ArH]+ [PtIIICl52− ]. The latter may then be transformed into a Wheland-type complex. Such a mechanism may be termed the SE2e.t. mechanism. The thermal reaction of PtCl62− with Ar2Hg in aqueous acetone to afford σ-aryl complexes of platinum(IV) is first order in the platinum(IV) complex and zero order in the aryl mercurial. The rate of the reaction decreases upon addition of LiCl. The mechanism of the reaction with Ar2Hg appears to involve an electron-transfer stage.

Aerobic oxidation of saturated hydrocarbons into alkyl hydroperoxides induced by visible light and catalysed by a ‘quinone–copper acetate’ system

Irradiation in air of an alkane solution in MeCN in the presence of catalytic amounts of quinone and copper(II) acetate (ratio 5 : 1) affords mainly alkyl hydroperoxide which by the action of triphenylphosphine may be easily converted into the corresponding alcohol and ketone (aldehyde)(in the case of cyclohexane oxidation the ratio cyclohexanol : cyclohexanone was 50:1 after 10 h). The main peculiarities (the rate dependencies on initial concentrations of a substrate and the catalyst as well as of some additives and the effective activation energy) of the reaction are described. The proposed mechanism includes the abstraction of the hydrogen atom from alkane, RH, by photo-excited quinone to produce radicals R˙ and HOC6H4O˙. The former then reacts rapidly with O2 to give RO2˙, and the latter may be reoxidized by copper(II). A copper(I) derivative thus formed may be converted into copper(II) when oxidized either by RO2˙ or O2.

Catalytic functionalization of methane

A mixture of sodium vanadate and pyrazine-2carboxylic acid (pcaH) efficiently catalyses the reaction of methane with molecular oxygen (from air) and hydrogen peroxide to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde. The reaction takes place under mild conditions (25‐75 °C) either in aqueous or in acetonitrile solution. The complexes formed from the catalyst precursor and the co-catalyst (under the reaction conditions) have been isolated and characterized as the derivatives [VO2(pca)2] ˇ (1) and [VO(O2) (pca)2] ˇ (3). The implications of these species in the catalytic process are discussed. Copyright # 2000 John Wiley & Sons, Ltd.

Photoinduced reactions of organic compounds with transition metal complexes: II. Reaction of PtCl62− with acetone to give a σ-acetonyl complex of platinum(IV). Detection of platinum(III) compounds by ESR

Abstract Light irradiation of a PtCl 6 2− solution in acetone affords a σ-acetonyl complex of platinum(IV), [CH 3 COCH 2 PtCl 5 ] 2− . Prolonged irradiation yields CH 3 COCH 2 Cl and PtCl 4 2− . The signals of a platinum(III) complex and CH 3 COCH 2 . are detected in the ESR spectrum of the frozen solution of PtCl 6 2− in acetone irradiated at 77 K. The proposed mechanism of the reaction involves electron transfer from the enol tautomer of acetone to PtCl 6 2− .

Carboxylation of methane with CO or CO2 in aqueous solution catalysed by vanadium complexes

Reaction of methane with CO or CO2 in aqueous solution in the presence of O2 (catalysed by NaVO3) or H2O2 (catalysed by NaVO3–pyrazine-2-carboxylic acid) at 25–100 °C affords acetic acid and in some cases also methanol, methyl hydroperoxide and formaldehyde.