Georgiy B. Shul'pin

Oxidations by the system “hydrogen peroxide - manganese(IV) complex - acetic acid” — Part II. Hydroperoxidation and hydroxylation of alkanes in acetonitrile

Abstract Higher alkanes (cyclohexane, n -pentane, n -heptane, methylbutane, 2- and 3-methylpentanes, 3-methylhexane, cis - and trans -decalins) are oxidized at 20 °C by H 2 O 2 in air in acetonitrile (or nitromethane) solution in the presence of the manganese(IV) salt [L 2 Mn 2 O 3 ](PF 6 ) 2 (L = 1,4,7-trimethyl-1,4-7-triazacyclononane) as the catalyst. An obligatory component of the reaction mixture is acetic acid. Turnover numbers attain 3300 after 2 h, the yield of oxygenated products is 46% based on the alkane. The oxidation affords initially the corresponding alkyl hydroperoxide as the predominant product, however later these compounds decompose to produce the corresponding ketones and alcohols. Regio- and bond selectivities of the reaction are high: C(1) : C(2) : C(3) : C(4) ≈ 1 : 40 : 35 : 35 and 1° : 2° : 3° is 1 : (15–40) : (180–300). The reaction with both isomers of decalin gives (after treatment with PPh 3 ) alcohols hydroxylated in the tertiary positions with the cis/trans ratio of ∼ 2 in the case of cis -decalin, and of ∼ 30 in the case of trans -decalin (i.e. in the latter case the reaction is stereospecific). Light alkanes (methane, ethane, propane, normal butane and isobutane) can be also easily oxidized by the same reagent in acetonitrile solution, the conditions being very mild: low pressure (1–7 bar of the alkane) and low temperature (−22 to +27 °C). Catalyst turnover numbers attain 3100, the yield of oxygenated products is 22% based on the alkane. The yields of oxygenates are higher at low temperatures. The ratio of products formed (hydroperoxide: ketone: alcohol) depends very strongly on the conditions of the reaction and especially on the catalyst concentration (at higher catalyst concentration the ketone is predominantly produced).

Extremely Efficient Alkane Oxidation by a New Catalytic Reagent H2O2/Os3(CO)12/Pyridine

Triosmium dodecacarbonyl catalyzes a very efficient oxidation of alkanes by H(2)O(2) in MeCN to afford alkyl hydroperoxides (primary products) as well as alcohols and ketones (aldehydes) at 60 degrees C if pyridine is added in a low concentration. Turnover numbers attain 60,000, and turnover frequencies are up to 24,000 h(-1).

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.

Oxidations by the reagent O2 - H2O2 - vanadium complex - pyrazine-2-carboxylic acid Part 7. Hydroperoxidation of higher alkanes

Abstract Alkanes ( n -heptane, 2- and 3-methylhexane, cis - and trans -decalin) are readily oxidized under air in acetonitrile by the O 2 - H 2 O 2 - PCA - VO 3 − reagent at room temperature to produce alkyl hydroperoxides as main products as well as minor amounts of the corresponding alcohols and carbonyl compounds. The site selectivities of the reactions are very similar to those observed with hydroxylation of the alkanes with hydrogen peroxide under UV irradiation. The proposed mechanism involves the catalytic formation of hydroxyl radicals from hydrogen peroxide which abstract hydrogen atoms from the alkanes. The alkyl radicals react rapidly with molecular oxygen to produce peroxyl radicals which are transformed mainly into the hydroperoxides.

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 ‘H2O2–vanadium complex–pyrazine-2-carboxylic acid’. Part 4. Oxidation of alkanes, benzene and alcohols by an adduct of H2O2 with urea

In the presence of catalytic amounts of a vanadium complex (Bu4nVO3) and pyrazine-2-carboxylic acid, the adduct H2O2·urea, in MeCN solution at 22–60 °C, oxidizes cyclohexane to yield mainly cyclohexyl hydroperoxide which may be easily converted into cyclohexanol by the action of triphenylphosphine (at room temperature after the reduction the ratio cyclohexanol : cyclohexanone is 25 : 1). Linear and branched alkanes are also oxidized by this reagent to give the corresponding alcohols, the regioselectivity being low; e.g., for oxidation of hexane C(1) : C(2) : C(3)= 1.0 : 6.8 : 6.0. The effective activation energy of cyclohexane oxidation is ∼ 70 kJ mol–1. Dependencies of initial rate of the reaction on concentration of substrate and the components of the reagent have been obtained. The postulated mechanism includes, as the key stage, the abstraction of a hydrogen atom from alkane, RH, to produce radical R˙ which then rapidly reacts with O2 to afford ROO˙. The hydrogen atom is most probably abstracted by hydroxyl radical formed in the interaction of H2O2 with the vanadium complex. Benzene is oxidized by the reagent under consideration to yield phenol, and alcohols are transformed into aldehydes or ketones.

Alkane oxygenation catalysed by gold complexes

Abstract Gold(III) and gold(I) complexes, NaAuCl 4 and ClAuPPh 3 , efficiently catalyse the oxidation of alkanes by H 2 O 2 in acetonitrile solution at 75°C. Turnover numbers (TONs) attain 520 after 144 h. Alkyl hydroperoxides are the main products, whereas ketones (aldehydes) and alcohols are formed in smaller concentrations. It is suggested on the basis of the bond selectivity study that at least one of the pathways in Au-catalysed alkane hydroperoxidation does not involve the participation of free hydroxyl radicals. Possibly, the oxidation begins from the alkane hydrogen atom abstraction by a gold oxo species. The oxidation of cyclooctane by air at room temperature catalysed by NaAuCl 4 in the presence of Zn/CH 3 COOH as a reducing agent and methylviologen as an electron-transfer agent gave cyclooctanol (TON=10).

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.