Exploring the Myth of Nascent Hydrogen and its Implications for Biomass Conversions

Iron (and to a lesser extent manganese) in the wall of a 316 stainless steel (SS) reactor is responsible for the hydrogenation of cyclohexanone to cyclohexanol when using an aqueous formic acid solution under high temperature and pressure water (HTPW) conditions. However, not only dilute formic acid but also aqueous solutions of several other organic and mineral acids in the presence of iron are active in this reaction covering a range of aldehydes and ketones, even under ambient conditions. The stoichiometry, kinetics, and the possible mechanisms of both dihydrogen production as well as of the hydrogenation of the model compound cyclohexanone were examined. The reduction is essentially stoichiometric with respect to metallic iron, and the conversions are highly dependent on the speed of stirring as well as temperature and reactant concentrations. Importantly, it is established unequivocally that water participates in dihydrogen gas formation (hydrogen atoms originate from both the acid and water molecules) and facilitates substrate reduction.     

The mechanism of the formation and hydrolysis of cyclohexanone oxime in aqueous solutions

The rate of formation of cyclohexanone oxime from cyclohexanone and hydroxylamine and the rate of the reverse reaction, the hydrolysis of cyclohexanone oxime, were studied in the pH range 1–7. The relative position and the bell shape of the two experimental reaction-rate curves can be fully explained by supplementing the reaction mechanism proposed by Jencks for the oximation only with the reverse reaction, applying the principle of microscopic reversibility. An essential part of this mechanism is the intermediate formation of a 1:1 adduct of cyclohexanone and hydroxylamine. It is concluded that the concentration of this adduct is negligibly small under all the conditions used here.     

Photo-catalytic oxidation of cyclohexane over TiO2: a novel interpretation of temperature dependent performance

The rate of cyclohexane photo-catalytic oxidation to cyclohexanone over anatase TiO(2) was studied at temperatures between 23 and 60 °C by in situ ATR-FTIR spectroscopy, and the kinetic parameters were estimated using a microkinetic model. At low temperatures, surface cyclohexanone formation is limited by cyclohexane adsorption due to unfavorable desorption of H(2)O, rather than previously proposed slow desorption of the product cyclohexanone. Up to 50 °C, the activation energy for photocatalytic cyclohexanone formation is zero, while carboxylates are formed with an activation energy of 18.4 ± 3.3 kJ mol(-1). Above 50 °C, significant (thermal) oxidation of cyclohexanone contributes to carboxylate formation. The irreversibly adsorbed carboxylates lead to deactivation of the catalyst, and are most likely the predominant cause of the non-Arrhenius behavior at relatively high reaction temperatures, rather than cyclohexane adsorption limitations. The results imply that elevating the reaction temperature of photocatalytic cyclohexane oxidation reduces selectivity, and is not a means to suppress catalyst deactivation.     

Oxidation of cyclohexylamine by air to its oxime

The oxidation of cyclohexylamine was studied over tungsten, molybdenum and vanadium oxides containing catalysts supported on silica, γ-alumina or hydrotalcite in the temperature range 170°C–230°C. Depending on the catalyst and reaction conditions, the main products of the reaction are cyclohexanone oxime, cyclohexylidenecyclohexylamine or cyclohexanone. About 70% selectivity of cyclohexanone oxime formation at about 20% conversion of cyclohexylamine is obtained over tungsten catalysts. The activity of the tested catalysts usually passes through a maximum and, then, gradually decreases with time on stream. The deactivation of the catalyst is caused by the formation of tar products on the catalyst surface. The mechanism of oxidation involves formation of oxygen species on the catalyst surface which oxidizes cyclohexylamine.     

Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its derivatives

Pd/TiN nanocomposite catalysts were fabricated for one-step selective hydrogenation of phenol to cyclohexanone successfully. High conversion of phenol (99%) and selectivity of cyclohexanone (98%) were obtained at 30℃ and 0.2 MPa H₂ for 12 h in the mixed solvents of H₂O and CH₂Cl₂. The Pd nanoparticles were stable in the reaction, and no aggregation was detected after four successive runs. The catalytic activity and selectivity depended on slightly the Pd particle sizes. The generality of the catalysts for this reaction was demonstrated by the selective hydrogenation of phenol derivatives, which showed that the catalyst was selective for the formation of cyclohexanone.     

Dy2O3 as a catalyst for a Meerwein-Ponndorf-Verley reaction

Dy₂O₃ activated at high temperature is reported as a catalyst for the liquid phase reduction of cyclohexanone. The catalytic activity of Dy₂O₃ activated at 300, 500 and 800°C and its mixed oxides with alumina for the reduction of cyclohexanone with 2-propanol has been reported. The data have been correlated with the electron donating properties of the catalysts which were reported from the adsorption of electron acceptors [EA] of various electron affinity on the surface of these oxides.     

Regioselective reduction of keto groups in Michael adducts of levoglucosenone and cyclohexanone

Conditions for the highly regioselective reduction of the specified keto groups in the Michael adduct of levoglucosenone and cyclohexanone have been developed. Selective reduction of the keto group in the cyclohexanone moiety was performed under the action of lithium metal in NH₃/THF system. Reduction of the keto group in the carbohydrate residue was accomplished microbiologically or by refluxing with NaBH(OAc)₃ in benzene.     

Palladium activity in 3-thiolene-1,1-dioxide hydrogenation in the presence of alkalis

Surface acidity/Basicity of mixed oxides of La and Zn activated at three different temperatures were determined. The data have been correlated with the catalytic activity for liquid phase reduction of cyclohexanone in isopropanol.     

Kinetic Studies of Picolinic Acid Catalyzed Chromic Acid Oxidation of Cyclohexanone

The chromic acid oxidation of cyclohexanone catalyzed by picolinic acid in water undergoes a change from first-to zero-order dependence in both cyclohexanone and acidity. The mechanism proposed indicates the formation of an intermediate C₁ by picolinic acid and chromic acid. Then C₁ would react with enol form of cyclohexanone to give another intermediate C₂. C₂ finally cleaves into products.     

Kinetic isotope effects in the thermal gas phase reactions of 1, 2-epoxycyclohexane-3, 3, 6, 6-d4

First-order rate constants for formation of cyclohexanone and 2-cyclohexen-1-ol from 1,2-epoxycyclohexane and 1,2-epoxycyclohexane-3,3,6,6-d₄ have been determined over the temperature range of 677–746°K. The observed kinetic isotope effects are used in an attempt to determine the mechanism for formation of products. A distinction between a biradical and a concerted mechanism for the alcohol formation could not be made. However, if a common biradical is the precursor of both cyclohexanone and 2-cyclohexenl-ol then the rate of ring closure of this biradical must be much faster than the rates of hydrogen transfer to give the ketone and the alcohol.     

Synthesis,Structure, and Hydrophosphorylation of π-Complexes of Hexacarbonyltungsten(0) with Cyclohexanone,Cyclohexanethione, and <Emphasis Type="Italic">N</Emphasis>-Cyclohexylideneaniline

New carbonyl π-complexes of tungsten(0) with cyclohexanone, cyclohexanethione, and N-cyclo-hexylideneaniline were synthesized. Geometric and electronic parameters of the ligands, as well as energy parameters of the complex formation process, were determined by quantum-chemical calculations. Hydrophosphorylation with diethyl phosphonate changed the reactivity of coordinated N-cyclohexylideneaniline, while no analogous effect was observed for cyclohexanone and cyclohexanethione.     

Oxidation of Cyclohexane by Molecular Oxygen Photoassisted by meso-Tetraarylporphyrin Iron(III)-Hydroxo Complexes

The photochemical and photocatalytic properties of iron meso-tetraarylporphyrins bearing an OH(-) axial ligand and different substituents in the beta-positions of the porphyrin ring are reported. Irradiation (lambda = 365 nm) in the absence of dioxygen leads to the reduction of Fe(III) to Fe(II) with the formation of OH(*) radicals. Substituents at the pyrrole beta-positions are found to markedly affect the photoreduction quantum yields. Under aerobic conditions, this photoreaction can induce the subsequent oxidation of cyclohexane to cyclohexanone and cyclohexanol by O(2) itself. The process occurs under mild conditions (22 degrees C; 760 Torr of O(2)) and without the consumption of a reducing agent. The polarity of the solvent and the nature of the porphyrin ring have a remarkable effect on the selectivity of the photooxidation process, likely controlling the cleavage of O-O bonds of possible iron peroxoalkyl intermediates. In particular, in pure cyclohexane, oxidation occurs with the selective formation of cyclohexanone; in contrast, in dichloromethane/cyclohexane mixed solvent, the main oxidation product is cyclohexanol. Phenyl-tert-butylnitrone (pbn) has been found to quench the radical chain autooxidation of the substrate thus increasing the yield of cyclohexanol. This becomes the only oxidation product when iron 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin hydroxide (Fe(III)(TDCPP)(OH)) is used as photocatalyst.     

Kinetics and mechanisms of homogeneous catalytic reactions. Part 4. Hydrogenation of cyclohexanone and 2-cyclohexen-1-one catalysed by the complexes [MH(CO)(NCMe)2(PPh3)2]BF4 (M = Ru,Os)

Kinetic and mechanistic studies of the homogeneous hydrogenation of cyclohexanone were carried out using the cationic complexes [MH(CO)(NCMe)₂(PPh₃)₂]BF₄ (M = Ru, Os) as the catalyst precursors, which were very efficient under mild reaction conditions in 2-methoxyethanol solution. For both complexes, the catalytic hydrogenation of cyclohexanone proceeds according to the rate law r = k[M][H₂]. The activation parameters were also calculated, the activation energy for the osmium catalyst being higher than for the ruthenium(I). All experimental data are consistent with a mechanism involving the oxidative addition of hydrogen as the rate-determining step of the catalytic cycle. Finally, the [MH(CO)(NCMe)₂(PPh₃)₂]BF₄ complexes were efficient precatalysts for the selective reduction of 2-cyclohexen-1-one to cyclohexanone; the reduction of the CO group of cyclohexanone only begins to take place when the ,-unsaturated ketone has been consumed.     

环己酮氨肟化反应中失活TS-1催化剂上沉积物的物化表征

在液相环己酮氨肟化反应中，有机物在钛硅分子筛催化剂（TS-1）上的沉积是造成失活的原因之一。采用傅里叶变换红外光谱、热重-差热、色谱-质谱联用、X-射线衍射、固体核磁共振、N2物理吸附和扫描电镜等分析手段，对失活TS-1的骨架结构及表面沉积物分子的结构和物化性质进行了表征。结果表明，引起催化剂失活的沉积物富集在分子筛的孔道内，主要有环己酮的氧化或还原产物、环己酮的二聚物、环己酮肟深度反应产物、叔丁基环己酮等可溶性沉积物以及它们缩聚而成的不溶性沉积物，其量可占失活催化剂总质量的5.0%。TPO烧炭时靠近Ti中心处的沉积物可以在较低温度下脱除，而孔道内的其他沉积物需要在较高温度下脱除，650℃沉积的炭可完全脱除。失活催化剂经700℃煅烧再生后，催化活性可恢复到新鲜催化剂的水平。     

A practical synthesis of a gamma-secretase inhibitor

A practical and scaleable synthesis of the gamma-secretase inhibitor 1 is reported. The inhibitor consists of a central trisubstituted cyclohexane core with appended propionic acid, 2,5-difluorophenyl, and 4-chlorophenylsulfonyl moieties. Two alternative synthetic strategies, proceeding by way of a common disubstituted cyclohexanone derivative 5, were studied. In the preferred route, conjugate reduction of acrylonitrile derivative 4 with L-Selectride configures the desired relative stereochemistry of the cyclohexane core with >99.9:0.1 dr. A second strategy, based on catalyst-controlled hydrogenation of racemic cyclohexene derivative 2, is more convergent but less diastereoselective (up to 75:25 dr). The common cyclohexanone intermediate 5 was constructed by a regioselective Diels-Alder condensation of a 1,1-disubstituted vinyl sulfone 6 with 2-trimethylsiloxybutadiene.     

Reaction of Tetracyanoethylene with 2-Substituted Cyclohexanones

Reactions of 2-methylcyclohexanone and menthone with tetracyanoethylene gave 2-(1,1,2,2-tetracyanoethyl)cyclohexanones which underwent quantitative transformation in the solid phase into 3,4-cyanosubstituted 2-aminopyrans in 2-3 days at room temperature. 2-Methyl-2-(1,1,2,2-tetracyanoethyl)cyclohexanone reacted with hydroiodic acid to afford 8a-hydroxy-2-iodo-4a-methyl-1,4,4a,5,6,7,8,8a-octahydroquinoline-3,4,4-tricarbonitrile. The reaction of 2,2'-methylenedi(cyclohexanone) with tetracyanoethylene resulted in formation of 7-imino-4,5-tetramethylene-2'-oxo-6-oxabicyclo[3.2.1]octane-2-spiro-1'-cyclohexane-1,8,8-tricarbonitrile.     

Electrosynthesis of adipic acid by undivided cell electrolysis

A preparative method for synthesis of adipic acid in 47% yield was developed. The method is based on cyclohexanol oxidation in an undivided cell on the NiOOH electrode in aqueous alkali. A possibility of the step-by-step process was studied: oxidation of cyclohexanol to cyclohexanone (75% yield) and subsequent oxidation of cyclohexanone to adipic acid (52% yield). The electrosynthesis of adipic acid is accompanied by the formation of minor amounts (up to 10%) of glutaric and succinic acids.     

Cholesterol complexes with tri-n-butylphosphate, tri-n-octylamine and cyclohexanone in chlorinated hydrocarbon solvents

The hydrogen bonding interactions of moderately associated cholesterol with tri-n-butylphosphate, tri-n-octylamine and cyclohexanone in dilute solutions of tetrachloromethane, 1,2-dichloroethane and trichloromethane were studied by conventional IR spectroscopy. Information on the stoichiometry of the complexes formed was derived from least squares plots of the linearized expressions of Bjerrum's degree of system formation. The formation constants of the complexes were also determined in this way. The spectra recorded in the OH region from 3100 to 3700 cm-1 were resolved in to the bands of the cholesterol species and the complexes formed. The complexes responsible for the observed bands were recognized from the dependence of their intensity on cholesterol monomer and free base concentration and from their frequency locations. The presence of the complexes B...HO(R) and B...HO(R)...HO(R) with tri-n-butylphosphate and tri-n-octylamine was established in all of the solutions and also for the system cholesterol+cyclohexanone in 1,2-dichloroethane. On the other hand, for cholesterol bonding to cyclohexanone in tetrachloromethane, the model considering complexes with 3:1 and 1:1 stoichiometry seemed the most appropriate.     

Pd-Ce-B/水滑石催化液相苯酚选择性加氢制环己酮

以Al3 /(Al3 Mg2 )摩尔比为0.2的水滑石(HT)为载体,采用还原浸渍法制备了负载型Pd-Ce-B/HT催化剂,并将其应用于液相苯酚选择性加氢制环己酮.与Pd-Ce-B/Al2O3,Pd-Ce-B/MgO和Pd-Ce-B/SiO2相比,Pd-Ce-B/HT催化剂具有高活性和高环己酮选择性.5.8%Pd-Ce-B/HT上苯酚的转化率和环己酮的选择性分别达82.0%和80.3%,显示了其潜在的工业化应用前景.根据多种表征结果,初步讨论了催化剂的构效关系以及添加剂Ce3 和载体酸碱性对催化性能的促进作用.