Search results for "Catalysis"
showing 10 items of 5944 documents
Ultrasmall Rigid Particles as Multimodal Probes for Medical Applications
2011
International audience; Ultrasmall but multifunctional: Rigid imaging particles that are smaller than 5 nm in size can be obtained in a top-down process starting from a core–shell structure (core=gadolinium oxide; shell=polysiloxane). They represent the first multifunctional silica-based particles that are sufficiently small to escape hepatic clearance and enable animal imaging by four complementary techniques.
The catalytic effects of a thiazolium salt in the oxa-Diels–Alder reaction between benzaldehyde and Danishefsky's diene: a molecular electron density…
2021
The oxa-Diels–Alder (ODA) reaction of benzaldehyde with Danishefsky's diene in the presence of a [thiazolium][Cl] salt, as a model of an ionic liquid, has been studied within Molecular Electron Density Theory (MEDT) at the M06-2X/6-311G(d,p) computational level. The formation of two hydrogen bonds (HBs) between the thiazolium cation and the carbonyl oxygen of benzaldehyde modifies neither the electrophilic character of benzaldehyde nor its electronic structure substantially but accelerates the reaction considerably. This ODA reaction presents an activation energy of 4.5 kcal mol−1; the formation of the only observed dihydropyranone is strongly exothermic by −28.8 kcal mol−1. The presence of…
New reactions and step economy: the total synthesis of (±)-salsolene oxide based on the type II transition metal-catalyzed intramolecular [4+4] cyclo…
2006
Studies on the viability of the type II nickel-catalyzed intramolecular [4+4] cycloaddition of bis-dienes show that it is influenced by both diene substitution and geometry. Both E- and Z-isomers of 19 and 20 react, albeit at markedly different rates, to afford cycloadducts, whereas only the Z-isomer of 9 (and not the E-isomer) reacts to give 8 and 25. Chemoselective elaboration of 8 to (±)-salsolene oxide (7) was used to confirm the cycloadduct structure while establishing a step economical route to the natural product.
Selective, homogeneous hydrogenation of cycloheptatriene to cycloheptene catalyzed by (η4-cycloocta-1,5-diene)(η6-cyclohepta-1,3,5-triene)ruthenium(0)
1980
Abstract In the presence of small amounts of [Ru(η 4 -COD)](η 6 -C 8 H 10 )] (1), cycloheptatriene is hydrogenated to cycloheptene, under one atmosphere of hydrogen at room temperature in homogeneous phase. The formation of a small amount of cyclooctene and the existence of an induction period, which do not occur when [Ru(η 4 -COD)(η 6 -C 7 H 8 )] (2) is used as the catalyst, suggest that 2 is the real catalyst. The selectivity of this hydrogenation is 100% in n-hexane as solvent, 99.5% in THF, and low in ethanol. Conversion is quantitative in THF and ethanol, but not more than 65% in n-hexane. In the presence of 1 or 2, cycloheptene is rapidly hydrogenated to cycloheptane in THF and ethano…
Selective homogeneous hydrogenation of cycloocta-1,5, and cycloocta-1,3-diene to cyclooctene in the presence of (η4-cycloocta-1,5-diene)(η6-cycloocta…
1983
Cyclooctadiene isomers (1,3- and 1,5-COD) are selectively, homogenously hydrogenated to cyclooctene (C8H14) in tetrahydrofuran (THF) solution, under mild conditions of temperature (20 °C) and hydrogen pressure (1 atm), in the presence of catalytic amounts of (cycloocta-1,5-diene)(cycloocta-1,3,5,-triene)ruthenium(0), [Ru(η4-COD)(η6-C8H10)] (1). The rate of hydrogenation is higher when 1,3-COD is the substrate. Evidence for the isomerization 1,5-COD → 1,3-COD is reported. Similar studies carried out in several alcoholic solutions, although showing a decreased selectivity in the hydrogenation, confirm the isomerization 1,5-COD → 1,3-COD, and allow a discrimination between the isomerization an…
Stereoselective Synthesis of Chiral Piperidine Derivatives Employing Arabinopyranosylamine as the Carbohydrate Auxiliary.
2006
Stereoselective synthesis of 2-substituted dehydropiperidinones and their further transformation to variously disubstituted piperidine derivatives was achieved employing D-arabinopyranosylamine as the stereodifferentiating carbohydrate auxiliary. A domino MannichMichael reaction of 1-methoxy-3-(trimethylsiloxy)butadiene (Danishefsky's diene) with O-pivaloylated arbinosylaldimines furnished N-arabinosyl dehydropiperidinones in high diastereoselectivity. Subsequent conjugate cuprate addition gave 2,6-cis-substituted piperidinones, while enolate alkylation furnished 2,3-trans-substituted dehydropiperidinones. Electrophilic substitution at the enamine structure afforded 5-nitro- and 5-halogen…
Assessment of Catalysis by Arene‐Ruthenium Complexes Containing Phosphane or NHC Groups bearing Pendant Conjugated Diene Systems
2015
Two p-cymene-ruthenium complexes 1 and 2 were isolated in high yields by treating the [RuCl2(p-cymene)]2 dimer with new hybrid phosphane- or NHC-linked diene ligands. Both complexes were fully characterized by NMR spectroscopy, and the molecular structure of the ruthenium–p-cymene complex 1, containing the phosphane–diene ligand system, was determined by X-ray diffraction analysis. The catalytic activities of both compounds were probed in atom-transfer radical addition (ATRA) and polymerization (ATRP), in the cyclopropanation of olefins, in the ring-opening metathesis polymerization (ROMP) of norbornene, and in the synthesis of enol esters from hex-1-yne and 4-acetoxybenzoic acid.
Atom Transfer Radical Addition Catalyzed by Ruthenium–Arene Complexes Bearing a Hybrid Phosphine–Diene Ligand
2018
International audience; The synthesis and characterization of a series of arene ruthenium complexes bearing either (3,5-cycloheptadienyl)diphenylphosphine or (cycloheptyl)-diphenylphosphine are reported. Upon irradiation or heating, all these complexes lose their arene ligand but then exhibit a different behavior depending on the nature of the phosphine ligand. (Cycloheptadienyl)phosphine complexes 1 and 3 give a cationic dinuclear Ru complex 5 for which the two Ru atoms are bridged by three chlorido ligands and flanked by two tridendate (cycloheptadienyl)phosphines. (Cycloheptyl)-diphenylphosphine complexes 2 and 4 undergo arene exchange when toluene is used as solvent or degrade in dithlo…
Transition Metal Compound Oxide Catalysts for Lowering the Light Off-Temperature of Particles from Diesel Exhaust
1993
Publisher Summary It has been realized that particle emissions from diesel engines are a severe environmental problem, which is difficult to solve by engine modifications alone. Even if the combustion occurs in a high excess of air, appreciable amounts of soot particles are found. Cracking processes, dehydrogenation, and polymerization play an important role in the formation of soot. To reduce soot fraction, several methods have been developed in the past, but most of them have fundamental disadvantages. Thus, a new procedure based on the catalytic burning of the soot in the exhaust gas is becoming important. This chapter describes the effect of several compound oxides. Further investigatio…
Microwave assisted syntheses of solvent-based colloidal sols of tailored ceria nanoparticles
2009
AbstractMicrowave assisted syntheses of stable solvent based colloidal sols of tailored ceria nanoparticles Olivier Poncelet*, Julien Jouhannaud*, Denis Chaumont** *CEA Liten DTNM/L2T , 17 rue des Martyrs F-38054 Grenoble Cedex9, France **Institut Carnot Bourgogne (ICB), UMR5209 CNRS, Facult� des Sciences Mirande, Universit� de Bourgogne, 9 Av. Alain Savary, BP 47870, F-21078 Dijon Cedex The pressure of environmental laws in many advanced countries becoming more restricting year after year, it is asked to automobile companies to strongly control the carburant consumption of cars that they put on the market and also to eliminate toxic chemicals coming from exhaust emission gases. This is par…