Mikhail M. Vinogradov

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

Isomeric cage-like tetracopper(II) silsesquioxane complexes [(PhSiO1.5)12(CuO)4(NaO0.5)4] (1a), [(PhSiO1.5)6(CuO)4(NaO0.5)4(PhSiO1.5)6] (1b) and binuclear complex [(PhSiO1.5)10(CuO)2(NaO0.5)2] (2) have been studied by various methods. These compounds can be considered as models of some multinuclear copper-containing enzymes. Compounds 1a and 2 are good pre-catalysts for the alkane oxygenation with hydrogen peroxide in air in an acetonitrile solution. Thus, the 1a-catalyzed reaction with cyclohexane at 60 °C gave mainly cyclohexyl hydroperoxide in 17% yield (turnover number, TON, was 190 after 230 min and initial turnover frequency, TOF, was 100 h−1). The alkyl hydroperoxide partly decomposes in the course of the reaction to afford the corresponding ketone and alcohol. The effective activation energy for the cyclohexane oxygenation catalyzed by compounds 1a and 2 is 16 ± 2 and 17 ± 2 kcal mol−1, respectively. Selectivity parameters measured in the oxidation of linear and branched alkanes and the kinetic analysis revealed that the oxidizing species in the reaction is the hydroxyl radical. The analysis of the dependence of the initial reaction rate on the initial concentration of cyclohexane led to a conclusion that hydroxyl radicals attack the cyclohexane molecules in proximity to the copper reaction centers. The oxidations of saturated hydrocarbons with tert-butylhydroperoxide (TBHP) catalyzed by complexes 1a and 2 exhibit unusual selectivity parameters which are due to the steric hindrance created by bulky silsesquioxane ligands surrounding copper reactive centers. Thus, the methylene groups in n-octane have different reactivities: the regioselectivity parameter for the oxidation with TBHP catalyzed by 1a is 1 : 10.5 : 8 : 7. Furthermore, in the oxidation of methylcyclohexane the position 2 relative to the methyl group of this substrate is noticeably less reactive than the corresponding positions 3 and 4. Finally, the oxidation of trans-1,2-dimethylcyclohexane with TBHP catalyzed by complexes 1a and 2 proceeds stereoselectively with the inversion of configuration. The 1a-catalyzed reaction of cyclohexane with H216O2 in an atmosphere of 18O2 gives cyclohexyl hydroperoxide containing up to 50% of 18O. The small amount of cyclohexanone, produced along with cyclohexyl hydroperoxide, is 18O-free and is generated apparently via a mechanism which does not include hydroxyl radicals and incorporation of molecular oxygen from the atmosphere.

Oxidation of hydrocarbons with H2O2/O2 catalyzed by osmium complexes containing p-cymene ligands in acetonitrile

The soluble osmium complexes containing p-cymene (π-p-cym) ligands, [(η6-p-cym)OsCl2]2 (1), [(η6-p-cym)Os(bipy)Cl]PF6 (2), and [(η6-p-cym)2Os2(μ-H)3]PF6 (3), are efficient catalysts for the oxidation of alkanes (cyclohexane, n-heptane, methylcyclohexane, isooctane, and cis- and trans-1,2-dimethylcyclohexane) with hydrogen peroxide in air to the corresponding alkyl hydroperoxides in acetonitrile solution if a small amount of pyridine is present in the solution. The binuclear complex 1 is the most active precatalyst in the oxidation whereas compound 2 containing the bipyridine ligand is much less efficient. The oxidation of cyclohexane at 60 °C and low concentration [1]0 = 10−7 M gave a turnover number (TON) of 200 200 after 24 h. A study of the selectivity parameters in the oxidation of linear and branched alkanes and the kinetic peculiarities of the cyclohexane oxidation led to the conclusion that the main reaction mechanism includes the formation of hydroxyl radicals. The effective activation energy Ea for the cyclohexane oxidation catalyzed by complex 1 was 10 ± 2 kcal mol−1. A kinetic analysis verified also that monomerization of complex 1 occurs before the oxidizing species is involved in the catalytic cycle. The 1-catalyzed reaction of cyclohexane, c-C6H12, with H216O2 in an atmosphere of 18O2 gave labeled cyclohexyl hydroperoxide, c-C6H11–18O–18OH. In addition, a small amount of “light” cyclohexanone, c-C6H1016O, is produced apparently via a mechanism which includes neither hydroxyl radicals nor incorporation of molecular oxygen from the atmosphere. The oxidation of benzene with H216O2 under 18O2 gave phenol which did not contain the 18O isotope. The reactions with cyclohexane and benzene were shown to proceed also via an alternative minor mechanism with oxo derivatives of high-valent osmium “OsO” as key oxidizing species.

Stereoselective Alkane Oxidation with meta-Chloroperoxybenzoic Acid (MCPBA) Catalyzed by Organometallic Cobalt Complexes

Cobalt pi-complexes, previously described in the literature and specially synthesized and characterized in this work, were used as catalysts in homogeneous oxidation of organic compounds with peroxides. These complexes contain pi-butadienyl and pi-cyclopentadienyl ligands: [(tetramethylcyclobutadiene)(benzene)cobalt] hexafluorophosphate, [(C4Me4)Co(C6H6)]PF6 (1); diiodo(carbonyl)(pentamethylcyclopentadienyl)cobalt, Cp*Co(CO)I2 (2); diiodo(carbonyl)(cyclopentadienyl)cobalt, CpCo(CO)I2 (3); (tetramethylcyclobutadiene)(dicarbonyl)(iodo)cobalt, (C4Me4)Co(CO)2I (4); [(tetramethylcyclobutadiene)(acetonitrile)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [(C4Me4)Co(bipy)(MeCN)]PF6 (5); bis[dicarbonyl(B-cyclohexylborole)]cobalt, [(C4H4BCy)Co(CO)2]2 (6); [(pentamethylcyclopentadienyl)(iodo)(1,10-phenanthroline)cobalt] hexafluorophosphate, [Cp*Co(phen)I]PF6 (7); diiodo(cyclopentadienyl)cobalt, [CpCoI2]2 (8); [(cyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [CpCo(bipy)I]PF6 (9); and [(pentamethylcyclopentadienyl)(iodo)(2,2′-bipyridyl)cobalt] hexafluorophosphate, [Cp*Co(bipy)I]PF6 (10). Complexes 1 and 2 catalyze very efficient and stereoselective oxygenation of tertiary C–H bonds in isomeric dimethylcyclohexanes with MCBA: cyclohexanols are produced in 39 and 53% yields and with the trans/cis ratio (of isomers with mutual trans- or cis-configuration of two methyl groups) 0.05 and 0.06, respectively. Addition of nitric acid as co-catalyst dramatically enhances both the yield of oxygenates and stereoselectivity parameter. In contrast to compounds 1 and 2, complexes 9 and 10 turned out to be very poor catalysts (the yields of oxygenates in the reaction with cis-1,2-dimethylcyclohexane were only 5%–7% and trans/cis ratio 0.8 indicated that the oxidation is not stereoselective). The chromatograms of the reaction mixture obtained before and after reduction with PPh3 are very similar, which testifies that alkyl hydroperoxides are not formed in this oxidation. It can be thus concluded that the interaction of the alkanes with MCPBA occurs without the formation of free radicals. The complexes catalyze oxidation of alcohols with tert-butylhydroperoxide (TBHP). For example, tert-BuOOH efficiently oxidizes 1-phenylethanol to acetophenone in 98% yield if compound 1 is used as a catalyst.

Photochemical arene exchange in the iron dicarbollide complex [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6H6)]+

Visible light irradiation of the dicarbollide complex [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6H6)]+ (2a) in the presence of the benzene derivatives in CH2Cl2/MeNO2 resulted in cations [(η-9-SMe2-7,8-C2B9H10)Fe(η-C6R6)]+ (2b-g; arene is anisole (b), toluene (c), m-xylene (d), mesitylene (e), durene (f), and hexamethylbenzene (g)) due to the arene exchange. The structures of [2g]PF6 and related tricarbollide complex [(η-1-ButNH-1,7,9-C3B8H10)Fe-(η-C6H6)]PF6 (1) were confirmed by X-ray diffraction analysis. The nature of bonding in cations 1, 2a, and [CpFe(η-C6H6)]+ was analyzed by an energy decomposition analysis.

Oxidative functionalization of C–H compounds induced by the extremely efficient osmium catalysts (a review)

Osmium derivatives are not as popular among catalytic chemists as the compounds of iron (an analog of osmium), copper, or manganese. Although osmium is expensive, poisonous and volatile, it finds applications not only in cis-hydroxylation of olefins but its derivatives have recently been employed in the oxygenation of C–H compounds (saturated and aromatic hydrocarbons and alcohols) by hydrogen peroxide as well as organic peroxides. The turnover in alkane oxidations catalyzed by soluble osmium derivatives has been found to be much higher than analogous reactions that used complexes of other transition metals (e.g., iron, manganese or copper). Using certain additives, the authors of this review have developed osmium-containing systems that are extremely efficient in transforming alkanes into corresponding alkyl hydroperoxides, which can further be reduced to alcohols easily. The new catalytic systems described here are capable of producing ground-breaking results in the field of metal-complex catalysts.

CCDC 1544890: Experimental Crystal Structure Determination

Related Article: Alexander P. Molotkov, Mikhail M. Vinogradov, Alexey P. Moskovets, Olga Chusova, Sergey V. Timofeev, Vasilii A. Fastovskiy, Yulia V. Nelyubina, Alexander A. Pavlov, Denis A. Chusov, Dmitry A. Loginov|2017|Eur.J.Inorg.Chem.||4635|doi:10.1002/ejic.201700498

CCDC 1544888: Experimental Crystal Structure Determination

Related Article: Alexander P. Molotkov, Mikhail M. Vinogradov, Alexey P. Moskovets, Olga Chusova, Sergey V. Timofeev, Vasilii A. Fastovskiy, Yulia V. Nelyubina, Alexander A. Pavlov, Denis A. Chusov, Dmitry A. Loginov|2017|Eur.J.Inorg.Chem.||4635|doi:10.1002/ejic.201700498

CCDC 1544887: Experimental Crystal Structure Determination

Related Article: Alexander P. Molotkov, Mikhail M. Vinogradov, Alexey P. Moskovets, Olga Chusova, Sergey V. Timofeev, Vasilii A. Fastovskiy, Yulia V. Nelyubina, Alexander A. Pavlov, Denis A. Chusov, Dmitry A. Loginov|2017|Eur.J.Inorg.Chem.||4635|doi:10.1002/ejic.201700498