Showing papers on "Cyclohexene published in 1995"

Palladium-catalyzed allylic acetoxylation of olefins using hydrogen peroxide as oxidant

Abstract: A new and efficient system for the allylic acetoxylation of olefins has been developed, which consists of a palladium (II) catalyst and hydrogen peroxide as oxidant in acetic acid. The acetoxylation reaction competes with the epoxidation by the peracetic acid generated in situ in this system. Increase of the concentration of palladium catalyst and addition of benzoquinone (BQ), enhance the acetoxylation considerably. For the Pd(OAc)2BQH2O2 system, more than 1000 turnover number could be achieved in the acetoxylation of cyclohexene. Most internal and cyclic olefins tested gave the corresponding allylic acetates in fair to high yields. 1,3-Cyclohexadiene gave 1,4-diacetoxy-cyclohex-2-ene while 1,5-hexadiene afforded 3-acetoxy-methylenecyclopentane; 1,7-octadiene produced 1-octen-7-one and octan-2,7-dione. Terminal olefins gave methyl ketones in good yield with high turnover number both in the presence and absence of BQ. Possible mechanisms and involvement of palladium peroxidic species in both the catalytic acetoxylation and ketonization of olefins are discussed.

Cationic iroporphyrins as catalyst in comparative oxidation of hydrocarbons: homogeneous and supported on inorganic matrices systems

Abstract: Cationic ironporphyrins in solution and supported on imidazole propyl gel (IPG) and silica gel (SG) as catalyst in cyclohexane hydroxylation using iodosylarene as oxygen donor were studied. FeTM4PyP 5+ , 1 as homogeneous catalyst for cyclohexane hydroxylation, in acetonitrile (CH 3 CN) and ultrasound stirring, gives cyclohexanol (C-ol) yields of 20%. The FeTM2PyP 5+ , 2 as catalyst for cyclohexene oxidation in CH 3 CN and ultrasound stirring is a very efficient system, giving 93% of total yield, with an epoxide:alcohol:ketone selectivity of 77:12:11. The supported systems, 1 -IPG and 1 -SG are particularly efficient in dichloromethane (CH 2 Cl 2 ), giving C-ol yields of 37% and 53%, respectively. These systems consist of polar hemin in isolated sites, which selectively catalyze cyclohexane oxidation in apolar solvent. They represent good cytochrome P-450 model systems. In the same way, the active site of P-450 consists of a polar protohemin in a hydrophobic pocket, promoting selective cyclohexane oxidation. These rigid cationic 1 -IPG, 1 -SG, 2 -IPG, 2 -SG, 3 -IPG and 3 -SG systems can catalyze with very small amounts of ironporphyrins, which correspond to about 30–80 times fewer numbers than the hemin in P-450 catalytic site.

Catalytic enantioselective allylic oxidation

Abstract: Several chiral Cu(II)-complexes of cyclic amino acids catalyse the enantioselective allylic oxidation of cyclohexene to cyclohexenyl esters. Cyclohexenyl propionate was obtained in 86% yield with e.e.'s up to 61%.

Semiconductor Photocatalysis. ZnS-Nanocrystallite-Catalyzed Photooxidation of Organic Compounds

Abstract: Freshly prepared ZnS (nano-ZnS) suspensions catalyze photooxidation of organic substrates under band-gap irradiation with water as a good electron acceptor, while H2 evolves concomitantly The organic substrates with hetero atoms or carbon–carbon double bonds (π-bonds), such as triethylamine (TEA), diethylamine (DEA), methanol, ethanol, cyclopentene, cyclohexene, 2-methylfuran, toluene, and ethylbenzene, undergo effective one-hole oxidation This leads to efficient carbon–carbon bond forming reactions between cumulatively formed radicals at the α-carbon adjacent to the hetero atom or the π-bond The photooxidation in the presence of a larger quantity of water results in successive oxidation of the intermediary α-carbon radicals, giving the two-hole oxidation products, eg, DEA and acetaldehyde from TEA, and formaldehyde from methanol The formation of the intermediary α-carbon radical has been clarified by ESR analysis using 2-propanol as an organic substrate Semi-empirical molecular orbital calculation

Effect of tetrahedral Ti in titania–silica mixed oxides on epoxidation activity and Lewis acidity

Abstract: The states of Ti in titania–silica mixed oxides have been studied by varying the Ti content. EXAFS analyses indicated that the amount of tetrahedral Ti species increased with a decrease in the Ti content reaching a maximum at 10 to 20 mol% of Ti while octahedrally coordinated Ti predominated in the high Ti content region (Ti 50 mol%). Tetrahedral Ti species catalyse the eposidation of oct-1-ene and cyclohexene using tert-butyl hydroperoxide as an oxidant. Lewis acid sites of the titania–silica also originated from the tetrahedral Ti species. Both epoxidation activity and Lewis acidity of the titania–silica were well explained by the coordinative unsaturation of the tetrahedral Ti site.

On the mechanism of catalytic alkene oxidation by molecular oxygen and halogenated iron porphyrins

Abstract: The halogenated porphyrin, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyriato-iron(III) chloride, [Fe(TFPPBr8)Cl], catalyzes the oxidation of cyclohexene in the presence of molecular oxygen or iodosobenzene. With PhIO, 77% epoxide is observed, consistent with a mechanism involving a high-valent metal-oxo species. With dioxygen, however, allylic alcohol and ketone are observed, suggesting a different mechanism. The relatively high activity of the [Fe(TFPPBr 8 )Cl] O 2 system suggests that the reaction involves the formation and decomposition of alkyl peroxides.

Process for producing tenside-stabilized colloids of mono- and bimetals of the group VIII and Ib of the periodic system in the form of precursors for catalysts which are isolable and water soluble at high concentration

Abstract: The invention relates to a process for producing tenside-stabilized colloids of mono- and bimetals of the group VIII and Ib of the periodic system which are isolable in the form of powder and which are soluble at a concentration of at least 100 mg atom of metal/l of water, from metal salts in the presence of strongly hydrophilic tensides with hydrotriorganoborates in THF, or with simple chemical reduction agents like hydrogen or alkali formate in water and alcohols, respectively. Furthermore, the subject matter of the invention is the use of the tenside-stabilized colloids which are produced according to this process as precursor for supported catalysts for the selective cis-hydrogenation of C-C triple bonds, for the selective hydrogenation of functional groups at the aromatic nucleus, for the selective hydrogenation of benzene to cyclohexene, for the partial oxidation of the primary alcohol functionality in carbohydrates, as well as for use as a precursor for electrocatalysts in fuel cells.

Rate constants for the gas‐phase reaction of C5 ? C10 alkenes with ozone

Abstract: The gas-phase reaction of ozone with C5C10 alkenes(eight 1-alkenes, four 1,1-disubstituted alkenes, and cyclohexene) has been investigated at atmospheric pressure and ambient temperature (285–293 K). Cyclohexane was added to scavenge the hydroxyl radical, which forms as a product of the ozone-alkene reaction. The reaction rate constants, in units of 10−18 cm3 molecule−1 s−1, are 9.6±1.6 for 1-pentene, 9.7±1.4 for 1-hexene, 9.4±0.4 for 1-heptene, 12.5±0.4 for 1-octene, 8.0±1.4 for 1-decene, 3.8±0.6 for 3-methyl-1-pentene, 7.3±0.7 for 4-methyl-1-pentene, 3.9±0.9 for 3,3-dimethyl-1-butene, 13.3±1.4 for 2-methyl-1-butene, 12.5±1.1 for 2-methyl-1-pentene, 10.0±0.3 for 2,3-dimethyl-1-butene, 13.7±0.9 for 2-ethyl-1-butene, and 84.6±1.0 for cyclohexene. Substituent effects on alkene reactivity are examined. Steric effect appear to be important for all 1,1-disubstituted alkenes as well as for those 1-alkenes that bear s-butyl and t-butyl groups. The results are briefly discussed with respect to the atomospheric persistence of the alkenes studied. © 1995 John Wiley & Sons, Inc.

Distinguishing direct and quasi-direct mechanisms for an Eley–Rideal gas/surface reaction. Stereochemistry of H addition to cyclohexene on Cu(100)

Abstract: Previous studies have demonstrated that when H atoms are impinged onto copper surfaces precovered with partial monolayers of physisorbed alkenes at 110 K, addition reactions occur to generate surface alkyl groups. Based on the absence of these reactions when alkenes are adsorbed on a copper surface precovered with a partial monolayer of H atoms, it was concluded that these reactions occur by Eley–Rideal mechanisms in which the H reacts with the alkenes prior to thermal accommodation with the surface. The stereochemistry of this Eley–Rideal process has been studied with reference to the addition of hydrogen to cyclohexene on Cu(100). The use of this cyclic alkene eliminates free rotation about the C—C bond in the alkyl product. The stereochemistry of the surface cyclohexyl groups generated by this addition reaction was determined by combining isotope labelling with mass spectrometric detection of cyclohexene that is regenerated as a result of a stereoselective β-hydride elimination by the surface cyclohexyl groups. To quantify the product yields with high accuracy, a chemical displacement protocol has been applied to remove unreacted alkenes from the monolayer and different isotopic variants of the reaction have been studied to confirm the product distributions. It has been shown that H addition to the CC double bond (which is oriented parallel to the surface based on IR results) occurs from the top face (vacuum side) of the molecule with >95% selectivity. This stereoselectivity for the top face is particularly interesting since it is thought that Langmuir–Hinshelwood pathways for analogous processes occur by addition to the bottom face of adsorbed alkenes. Furthermore, since one would expect surface-mediated hot-atom pathways (i.e. quasi-direct Eley–Rideal mechanisms) to show the stereochemistry of the Langmuir–Hinshelwood mechanism, we conclude that H addition to cyclohexene on Cu(100) occurs by a truly direct Eley–Rideal mechanism.

Interaction between ruthenium and molybdenum in RuMo/Al2O3 catalysts

Abstract: RuMo/Al2O3 catalysts with various molybdenum loadings (0.15–0.6 wt.-%) and 0.6wt.-% ruthenium were prepared and characterized by hydrogen and carbon monoxide chemisorption. n-Hexane hydrogenolysis and cyclohexane dehydrogenation were performed over the reduced catalysts. Cyclohexene dehydrogenation was also carried out over the sulfided catalysts. The results suggest an electron transfer from molybdenum to ruthenium sites.

Microporous poly-(N,N-dimethyl-acrylamide)-(1-methacryloyl-ethylene-2-sulphonate)-(N,N'-methylene-bis-acrylamide) resins as hydrophilic supports for metal catalysts

Abstract: The synthesis of poly-(N,N-dimethyl-acrylamide)-(1-methacryloyl-ethylene-2-sulphonate) (M4K) with 4 mol. % of N,N'-methylene-bis-acrylamide (MBAA) as crosslinker, and of poly-(N,N-dimethyl-acrylamide)-(1-methacryloyl-ethylene-2-sulphonic acid) (M4A) with 4 mol. % MBAA is described. Polymerization was performed under gamma rays irradiation of a water solution of monomers (M4K), or of the monomer mixture (M4A), at room temperature. High affinity to protonic solvents was observed for both materials. Palladium catalysts were prepared from M4K through ion-exchange with [Pd(NH3)4]2+ and from M4A through ion-exchange with palladium acetate; the subsequent Pd(II) reduction with sodium borohydride in ethanol yielded the final catalysts with a uniform distribution of metal throughout the resin particles. The average size of metal crystallites obtained by X-ray diffraction was about 4 nm and did not show any dependence on metal content in the polymer in the range from 0.5 to 8.8 ww % of metal. The rate of hydrogenation of p-nitrotoluene and cyclohexene in catalytic tests with 1 and 0.5 ww % Pd catalysts was proportional to the metal content in catalysts, which confirmed a very good accessibility of these supports. Catalysts with low metal content also exhibit a significant deactivation in the course of the hydrogenation of p-nitrotoluene to p-toluidine in methanol under ambient conditions.

Effects of surface modifiers on the liquid-phase hydrogenation of alkenes over silica-supported platinum, palladium and rhodium catalysts I. Quinoline and carbon tetrachloride

Abstract: Hydrogenation of 1-hexene and cyclohexene was studied over silica-supported Pt, Pd and Rh catalysts in the liquid phase, in the presence of quinoline or carbon tetrachloride at room temperature. As the amount of surface modifiers increased in the reactor, the rate of hydrogenation decreased over Pd SiO 2 and Rh SiO 2 . The activity of Pt SiO 2 could also be lowered with quinoline; however, it remained constant for cyclohexene hydrogenation or decreased just slightly for 1-hexene even when excess CCl4 was supplied. The inhibition by CCl4 was substantial for palladium and rhodium; nevertheless, complete deactivation did not occur. Quinoline effectively deactivated the palladium and rhodium catalysts in cyclohexene hydrogenation, but residual activity for 1-hexene hydrogenation was above 50% for each of the catalysts used.

Unsaturated ketones as accelerators for hydrosilylation

Abstract: A hydrosilation process is described wherein a silicon hydride is reacted with an unsaturated reactant in the presence of a platinum catalyst and an unsaturated ketone accelerator. The described accelerators are especially useful for the hydrosilation of unsaturated reactants where the unsaturation is in the internal portion of the reactant's structure, for example, as in cyclopentene and cyclohexene. The unsaturated ketone accelerators are effective in the absence of oxygen activation of the platinum catalyst and are surprisingly synergetic with oxygen activation of platinum catalyst.

Fungicides and photochemistry: photodegradation of the dicarboximide fungicide vinclozolin

Abstract: To examine the potential of the fungicide vinclozolin to undergo photochemical reactions on plant surfaces, model experiments lambda > 280 nm) were performed in various organic solvents simulating the plant cuticle environment. On irradiation in 2-propanol and n-propanol vinclozolin was completely degraded within 1 h, but the degradation was substantially lower in benzene, cyclohexane, cyclohexene, ethanol, methanol, and tert-butyl methyl ether (TBME). In 2-propanol, n-propanol, and cyclohexane, the main reaction was photoaddition of the solvent molecules to the vinclozolin vinyl group; the photoproducts in further reactions were dechlorinated. In the presence of ethanol, photodehalogenation of the fungicide competed with photoaddition. On the other hand, in cyclohexene and benzene solutions, substitutions of the chlorines by solvent molecules were mainly observed. Photolysis in methanol or TBME yielded only dehalogenated photoproducts.

Structures of aluminum chloride grafted on silica surface

Abstract: The structures of silica-grafted aluminum chloride catalysts were investigated by solid-state NMR measurements, together with their catalytic properties in the alkylation of benzene with cyclohexene. Catalyst samples prepared by contacting aluminum trichloride vapor with a silica gel surface at temperatures between 140 and 250°C were found to be active for the alkylation, while other samples prepared at higher temperatures were inactive. The grafted aluminum chloride species were classified into two structural types, corresponding to monomeric and dimeric aluminum chloride. The combination of 27 Al and 29 Si NMR experiments elucidates important structural features of these aluminum chloride/silica systems. The active catalysts were found to have a dimeric aluminum chloride structure, with chemical shifts of 85 and 42 ppm in the 27 Al magic-angle spinning NMR spectra. Those peaks correspond to aluminum species coordinated to chloride and/or oxygen with coordination numbers of 4 and 5, respectively. It is speculated that the catalytically active species is the product of reaction of the aluminum trichloride dimer with one hydroxyl on the silica surface.

Low-temperature hydrogenation of cyclohexene by energetic forms of hydrogen on the Ni(100) surface

Abstract: Adsorbed cyclohexene can be hydrogenated at cryogenic temperatures by both incident gas phase atomic hydrogen and desorbing bulk hydrogen in the presence of adsorbed surface hydrogen on a Ni(100) surface. In both cases, no C-C bond activation is observed, and cyclohexane is the only hydrogenated product. Cyclohexene desorbs without significant reaction from the Ni(100) surface in the presence of coadsorbed surface hydrogen. Selective hydrogenation of adsorbed cyclohexene by bulk hydrogen is observed at 178 K in the leading edge of the bulk hydrogen desorption peak. Hydrogenation of adsorbed cyclohexene by gas phase atomic hydrogen is observed below 140 K. Isotope studies of the hydrogenation mechanism suggest that the hydrogenation of adsorbed cyclohexene by gas phase atomic hydrogen is a sequential process with the first hydrogen adding from the gas phase and the second from the surface. The small amount of benzene observed from adsorbed cyclohexene monolayers indicates that gas phase atomic hydrogen also causes some hydrogen abstraction. 35 refs., 4 figs.

Metabolism of cyclohexane carboxylic acid by the photosynthetic bacterium Rhodopseudomonas palustris

Abstract: Cyclohexane carboxylate supported relatively rapid growth (doubling times 7–8 h) ofRhodopseudomonas palustris under oxic or photosynthetic conditions, but did not serve as a substrate for either of the known aromatic CoA ligases. A CoA ligase that thioesterifies cyclohexane carboxylate was partially purified and did not cross react immunologically with the two CoA ligases purified previously from this bacterium. Crude extracts ofR. palustris cells grown with a range of aromatic or alicyclic acids contained a dehydrogenase that reacted with cyclohexane carboxyl-CoA or cyclohex-1-ene carboxyl-CoA, using 2,6-dichlorophenolindophenol or ferricenium ion as electron carrier. This activity was not detected in extracts of adipate-, glutamate-, or succinate-grown cells. No oxidation or reduction of nonesterified cyclohexane carboxylate or cyclohexene carbocylate was detected in extracts of cells grown with aromatic or aliphatic substrates, neither aerobically nor anaerobically. A constitutively expressed thioesterase that hydrolyzed cyclohexane carboxyl-CoA and also some alicyclic and aliphatic CoA derivatives was purified and characterized. The enzyme had little or no activity on benzoyl-CoA or 4-hydroxybenzoyl-CoA. The presence of a thioesterase that effectively hydrolyzes cyclohexane carboxyl-CoA suggests that transient production of cyclohexane carboxylate is a physiological response to temporary excess of reductant during metabolism of aromatic compounds.