Tandem Epoxidation‐Alcoholysis or Epoxidation‐Hydrolysis of Glycals Catalyzed by Titanium(IV) Isopropoxide or Venturello’s Phosphotungstate Complex

Venturello’s phosphotungstate complex and titanium(IV) isopropoxide [Ti(O‐i‐Pr)₄] were successfully used as catalysts for the epoxidation‐alcoholysis of glycals using hydrogen peroxide [H₂O₂]. Reaction substrates included a range of variously protected glycals and different alcohols were used as solvents. Ti(O‐i‐Pr)₄ was only effective in methanol as solvent, but gave methyl glycosides in high yields and high selectivities. The Venturello complex proved to be a very versatile and efficient catalyst. Apart from epoxidation‐alcoholysis in alcoholic solvents it also showed activity in biphasic conditions to allow for glycosylation of long‐chain alcohols and was very effective in the stereoselective dihydroxylation of benzylated glucal.     

Silicon Nitride/Molybdenum Disilicide Composite with Superior Long-Term Oxidation Resistance at 1500°C

The long-term high-temperature cyclic oxidation (100 cycles, 10⁴ h, 1500°C) of a Si₃N₄ material and a Si₃N₄/MoSi₂ composite, both fabricated with Y₂O₃ as a sintering additive, was studied. Both materials exhibited similar oxidation rates because of surface SiO₂ formation described by an almost parabolic law and a total weight gain of 3–4 mg/cm² after 10⁴ h. As a consequence of oxidation processes in the bulk, microstructural damage was found in the Si₃N₄ material. These effects were not observed in the composite. The remarkable microstructural stability observed offers the high potential of Si₃N₄/MoSi₂ composites for long-term structural applications at elevated temperatures up to 1500°C.     

Selective Hydrogenation of Amides using Ruthenium/ Molybdenum Catalysts

Recyclable, heterogeneous bimetallic ruthenium/molybdenum catalysts, formed in situ from triruthenium dodecacarbonyl [Ru₃(CO)₁₂] and molybdenum hexacarbonyl [Mo(CO)₆], are effective for the selective liquid phase hydrogenation of cyclohexylcarboxamide (CyCONH₂) to cyclohexanemethylamine (CyCH₂NH₂), with no secondary or tertiary amine by‐product formation. Variation of Mo:Ru composition reveals both synergistic and poisoning effects, with the optimum combination of conversion and selectivity at ca. 0.5, and total inhibition of catalysis evident at ≥1. Good amide conversions are noted within the reaction condition regimes 20–100 bar hydrogen and 145–160 °C. The order of reactivity of these catalysts towards reduction of different amide functional groups is primary>tertiary≫secondary. In situ HP‐FT‐IR spectroscopy confirms that catalyst genesis occurs during an induction period associated with decomposition of the organometallic precursors. Ex situ characterisation, using XRD, XPS and EDX‐STEM, for active Mo:Ru compositions, has provided evidence for intimately mixed ca. 2.5–4 nm particles that contain metallic ruthenium, and molybdenum (in several oxidation states, including zero).     

Efficient Osmium/Rhenium‐Catalyzed Dihydroxylation of Olefins with Hydrogen Peroxide under Acidic Conditions

Simple addition of citric acid confers great stability to the catalytically active osmium and rhenium species involved in a triple catalytic system utilizing aqueous hydrogen peroxide as the terminal oxidant. The resulting system is capable of dihydroxylating traditionally resistant olefins in high yields.     

Efficient Stereoselective Synthesis of Nitrocyclopropanes by the Oxidative Cyclization of Michael Adducts of Nitroolefins with Activated Methylene Compounds

An efficient oxidative cyclopropanation of the Michael adducts of nitroolefins with activated methylene compounds by the combination of iodobenzene diacetate and tetrabutylammonium iodide is reported. Highly functionalized nitrocyclopropanes are synthesized in moderate to good yields via the Michael addition and cyclopropanation with high diastereoselectivity and enantioselectivity under mild conditions.     

Selective Autooxidation of Ethanol over Titania‐Supported Molybdenum Oxide Catalysts: Structure and Reactivity

We study the selective catalytic oxidation of ethanol with air as a sustainable alternative route to acetaldehyde. The reaction is catalysed by molybdenum oxide supported on titania, in a flow reactor under ambient pressure. High selectivity to acetaldehyde (70%–89%, depending on the Mo loading) is obtained at 150 °C. Subsequently, we investigate the structure/performance relationship for various molybdenum oxide species using a combination of techniques including diffuse reflectance UV‐visible, infrared, X‐ray photoelectron spectroscopies, X‐ray diffraction and temperature programmed reduction. As their surface density increases, the monomeric molybdenum oxide species undergo two‐dimensional and three‐dimensional oligomerisation. This results in polymolybdates and molybdenum oxide crystallites. Importantly, the ethanol oxidation rate depends not only on the overall molybdenum loading and dispersion, but also on the type of molybdenum oxide species prevalent at each surface density and on the domain size. As the molybdenum oxide oligomerisation increases, electron delocalisation becomes easier. This lowers the absorption edge energy and increases the reaction rate.     

Grafting of Molecularly Ordered Mesoporous Phenylene‐Silica with Molybdenum Carbonyl Complexes: Efficient Heterogeneous Catalysts for the Epoxidation of Olefins

Arenetricarbonyl complexes, or the general formula  C₆H₄Mo(CO)₃ , were incorporated into crystal‐like mesoporous phenylene‐silica by liquid‐phase deposition of molybdenum hexacarbonyl [Mo(CO)₆]. By adjusting the reaction conditions, different molybdenum loadings of 1.5 and 5.9 wt% were obtained, which correspond to 3% and 14% of the phenylene contents. The texture properties of the materials as well as the nature of the surface‐fixed complexes were characterized by powder X‐ray diffraction, transmission electron microscopy (TEM), N₂ adsorption, FT‐IR, UV‐vis and MAS (¹³C, ²⁹Si) NMR spectroscopy. The derivatized organosilicas were examined as catalyst precursors for the liquid‐phase epoxidation of cis‐cyclooctene, 1‐octene, trans‐2‐octene and (R)‐(+)‐limonene at 55 °C, using tert‐butyl hydroperoxide as the oxidant. For each olefin the corresponding epoxide was the only product detected. In the case of cyclooctene, the intrinsic reaction rates per surface molybdenum atom were similar for both Mo loadings (TOF∼1150 mol mol_Mo⁻¹ h⁻¹), suggesting that the resultant materials act as single site epoxidation catalysts. Leaching tests and metal analyses of reaction solutions showed that the catalytic activity stemmed from the immobilized species and not from the leaching of active species into solution. The oxidation of limonene gave limonene oxide as the only product in 95% yield at 3 h, which reveals an outstanding regioselectivity to the epoxidation of the endocyclic double bond.     

Regio‐ and Stereochemical Controlled Koenigs–Knorr‐Type Monoglycosylation of Secondary Hydroxy Groups in Carbohydrates Utilizing the High Site Recognition Ability of Organotin Catalysts

The catalytic regio‐ and stereoselective monoglycosylation of carbohydrates using organotin catalysts is demonstrated. The one‐step reaction affords various oligosaccharides linked at the secondary hydroxy group in high chemical yield and good regio‐ and stereoselectivities. The regioselectivity of the glycosylation is shown to depend on the spatial arrangement of the hydroxy groups in the carbohydrates.

     

Nanocrystalline Magnesium Oxide‐Stabilized Molybdenum: An Efficient Heterogeneous Catalyst for the Aerobic Oxidation of Alcohols to Carbonyl Compounds

A nanocrystalline magnesium oxide‐stabilized molybdenum(VI) complex catalyzed the oxidation of primary and secondary alcohols to carbonyl compounds in excellent yields using molecular oxygen as stoichiometric oxidant. The nanomaterials with their three‐dimensional structure and defined size and shape act as suitable supports for metal complexes. The catalyst can be reused for four runs without any significant loss of activity.     

Selective Light Supported Oxidation of Hexoses Using Air as Oxidant – Synthesis of Tetrahydroxyazepanes

Unprotected alkyl glycosides have been oxidized using air as oxidant and copper(I) chloride/TEMPO as the catalytic system. The primary hydroxy group in the 6 position is selectively transformed into an aldehyde equivalent. The process was optimized by irradiation with visible light. Without further purification and with high yields, the resulting lactols have been transformed into tetrahydroxyazepane.     

Tandem enzyme‐catalysed reduction/ cis‐dihydroxylation of 2,2,2‐trifluoroace‐ tophenone: chemoenzymatic routes to new enantiopure phenol and benzylic alcohol reagents

Factors that control the competition between toluene dioxgenase‐catalysed arene cis‐dihydroxylation and dehydrogenase‐catalysed ketone reduction have been studied, using whole cells of Pseudomonas putida UV and three alkylaryl ketones. The triol metabolite, obtained from 2,2,2‐trifluoroacetophenone, has been used in the synthesis of single enantiomer chiral phenols and benzylic alcohols. Potential applications of the methylether derivatives of the chiral phenols and benzylic alcohols, as resolving agents, have been found. Copyright © 2007 Society of Chemical Industry     

Ultrafine Amorphous Co–Fe–B Catalysts for the Hydrolysis of NaBH4 Solution to Generate Hydrogen for PEMFC

For hydrogen generation from alkaline sodium borohydride (NaBH₄) solution, Co–Fe–B catalysts with different Co/(Co + Fe) molar ratios (χ_Co) were prepared by the chemical reduction of CoCl₂ and FeCl₃ ethanol solution with KBH₄ solution. The X‐ray diffraction (XRD) and scanning electron microscopy (SEM) analyses revealed that the as‐prepared Co–Fe–B catalysts were in amorphous form and ultrafine. The hydrogen generation measurements showed that as‐prepared Co–B and Co–Fe–B catalysts exhibited excellent catalytic activity. Co–Fe–B with the Co/(Co + Fe) molar ratio (χ_Co) of 0.85 was the best. Its maximum hydrogen generation rate at 298 K was 4,310 mL min^–1 g^–1, while the Co–B was 2,773 mL min^–1 g^–1. The enhanced activity could be attributed to large active surface area, electron transfer from B and Fe to active Co sites, and improvement in the dispersion of Co–B with Fe₂O₃. The activation energy of Co–Fe–B catalyst with the molar ratio χ_Co = 0.85 was decreased to 29.09 kJ mol^–1 as compared to 30.85 kJ mol^–1 obtained with Co–B.     

Stereoselective Monoamine Oxidase‐Catalyzed Oxidative Aza‐Friedel–Crafts Reactions of meso‐Pyrrolidines in Aqueous Buffer

We disclose the highly diastereoselective combination of monoamine oxidase‐catalyzed oxidation of meso‐pyrrolidines and aza‐Friedel–Crafts reactions in aqueous buffer to give valuable enantioenriched 2‐substituted pyrrolidines in a formal double C H activation process. A range of secondary as well as tertiary amines were shown to be suitable substrates for the biocatalytic oxidation and subsequent addition of a variety of C‐nucleophiles.

     

Palladium‐Catalyzed Carbonylative Synthesis of Phthalimides from 1,2‐Dibromoarenes with Molybdenum Hexacarbonyl as Carbon Monoxide Source

We describe here a convenient and mild, carbon monoxide gas‐free palladium‐catalyzed procedure to obtain N‐substituted phthalimides with molybdenum hexacarbonyl as carbon monoxide precursor. These conditions tolerate a number of functional groups on the benzene ring as well as a number of amines and give the corresponding phthalimides in good to excellent yields.

     

Dithiolane‐Directed Tandem Oxidation/1,2‐Benzyl Migration of Tetramic Acids under Ambient Conditions

Reactions directly involving triplet oxygen are an important issue in organic and biological chemistry. We now report that exposure of tetramic acids in the open air (molecular oxygen) at room temperature can lead to the formation of succinimide derivatives in high to excellent yields via tandem oxidation/1,2‐benzyl migration. In this reaction, the dithiolane moiety plays a crucial role and succinimides are produced either in the presence or absence of dibutyl sulfide as reductant.     

Copper(I)‐Catalyzed [3+2] Cycloaddition/Ring‐Opening Rearrangement/[4+2] Cycloaddition/Aromatization Cascade: An Unprecedented Chemo‐ and Stereoselective Three Component Coupling of Sulfonyl Azide,Alkyne and N‐Arylidenepyridin‐2‐ amine to Pyrido[1,2‐a]pyrimidin‐4‐imine

A novel synthetic protocol for the one‐pot chemo‐ and stereoselective construction of diversely functionalized pyrido[1,2‐a]pyrimidin‐4‐imines via copper(I)‐catalyzed [3+2] cycloaddition/ring‐opening rearrangement/[4+2] cycloaddition/aromatization cascade of sulfonyl azides, alkynes and N‐arylidenepyridin‐2‐amines under mild reaction conditions is reported. In addition, the catalytic activity of copper(I)‐modified zeolite, a recyclable, heterogeneous catalyst is also investigated, which gives improved yield compared to its homogeneous equivalents.     

Selective oxidation and ammoxidation of propane on a Mo–V–Te–Nb–O mixed metal oxide catalyst: a comparative study

A comparative study on the selective oxidation and the ammoxidation of propane on a Mo–V–Te–Nb–O mixed oxide catalyst is presented. The catalyst has been prepared hydrothermally at 175 °C and heat-treated in N₂ at 600 °C for 2 h. Catalyst characterization results suggest the presence mainly of the orthorhombic Te₂M₂₀O₅₇ (M = Mo, V and Nb) bronze in samples before and after use in oxidation and ammoxidation, although some little modifications have been observed after its use in ammoxidation reaction. Propane has been selectively oxidized to acrylic acid (AA) in the 340–380 °C temperature range while the ammoxidation of propane to acrylonitrile (ACN) has been carried out in the 360–420 °C temperature interval. The steam/propane and the ammonia/propane molar ratios have an important influence on the activity and the selectivity to acrylic acid and acrylonitrile, respectively. The reaction network in both oxidation and ammoxidation reactions as well as the nature of active and selective sites is also discussed. The catalytic results presented here show that the formation of both ACN and AA goes through the intermediate formation of propene.     

Bench-scale demonstration of an integrated deSoot–deNOx system

A catalytic deSoot–deNO_x system, comprising Pt and Ce fuel additives, a Pt-impregnated wall-flow monolith soot filter and a vanadia-type monolithic NH₃-SCR catalyst, was tested with a two-cylinder DI diesel engine. The soot removal efficiency of the filter was 98–99 mass% with a balance temperature (stationary pressure drop) of 315 °C at an engine load of 55%. The NO_x conversion ranged from 40 to 73%, at a NH₃/NO_x molar ratio of 0.9. Both systems were measured at a GHSV of 52 000 l/(l h). The maximum NO_x conversion was obtained at 400 °C. The reason for the moderate deNO_x performance is discussed. No deactivation was observed after 380 h time on stream. The NO_x emission at high engine loads is around 15% lower than that of engines running without fuel additives.     

Chemoselectivity Improvement via Partial Shielding of an Imidazole Active Site in Branched/Dendritic Homogeneous Catalysts of the Baylis–Hillman Reaction

While comparing analogous polystyrene‐supported and homogeneous catalysts for the Baylis–Hillman reaction, we hypothesized that the hydrophobic envelopment of the imidazole catalytic sites of the former is responsible for the significantly better chemoselectivity exhibited by the heterogeneous catalysts compared to their homogeneous counterparts. In order to test this hypothesis, we prepared a series of branched/dendritic homogeneous catalysts, with an imidazole active site near the focal point and flexible tails of various lengths and polarities, capable of providing partial shielding of this site. The design of the catalysts was based on a 5‐hydroxyisophthalate scaffold, and they were prepared through a number of multistep synthetic pathways. The comparison of the catalysts under a variety of conditions in a model Baylis–Hillman reaction demonstrated that long hydrophobic tales enhance the chemoselectivity parameter of the catalysis, while reducing the rate of the consumption of the substrates, and that the chemoselectivity is further improved by the presence of a free phenolic moiety in the vicinity of the catalytic imidazole unit. Moreover, in second‐generation catalysts, the peripheral long tails could be either hydrophobic or polar, since the dendritic inner backbone itself presumably partially provides the necessary isolation of the catalytic site. Thus, experimental results support our hypothesis.

     

Polymerization of pyrrole by isopolymetallates of vanadium/molybdenum and preparation of a polypyrrole–molybdenum blues nanocomposite

Oxidative polymerization of pyrrole (PY) was achieved in high yield (>90 %) by adding the monomer directly to isopolyvanadate (IPV) and isopolymolybdate (IPMo) solutions at selected pH. In the case of the IPV solution instantaneous precipitation of a black mass was obtained while in the IPMo solution in situ precipitation of polypyrrole (PPY) onto molybdenum blues (MB) formed through reduction of IPMo by PY resulting in a PPY‐encapsulated MB nanocomposite (PPY–MB). Formation of PPY was confirmed from Fourier‐transform infrared (FTIR) and thermogravimetric (TG) analyses. Transmission electron microscope (TEM) photographs for the composite revealed the formation of spherical particles of average size ≈75 nm. Scanning electron microscope (SEM) analyses showed the distinctive morphological patterns for PPY and PPY–MB composite. The direct current conductivity values for PPY prepared by IPV, IPMo and PPY–MB were of the order of 10^?2, 10^?4 and 10^?6 S cm^?1, respectively. Copyright © 2004 Society of Chemical Industry