Search results for "hydrogenation"
showing 10 items of 155 documents
Enantioselective Synthesis of (−)-Dihydrocodeine and Formal Synthesis of (−)-Thebaine, (−)-Codeine, and (−)-Morphine from a Deprotonated α-Aminonitri…
2014
The α-benzylation of a deprotonated bicyclic α-aminonitrile, followed by Noyori's asymmetric transfer hydrogenation combined with the Grewe cyclization onto a symmetrical A-ring precursor, are the key steps of a short and high-yielding enantioselective synthesis of the morphinan (-)-dihydrocodeine. This compound can be converted to (-)-thebaine in high yield by known transformations, while (-)-codeine and (-)-morphine are available from an advanced intermediate.
Cyclodextrines confinantes : synthèse, propriétés complexantes et utilisation en catalyse asymétrique
2014
This manuscript is concerned with the design of novel catalytic systems derived from metallocyclodextrins. The first part describes new ways of functionalising the cyclodextrin primary face regioselectively for accessing inherently chiral P,P’ chelators. These heterodentate ligands gavequantitatively cis-chelate complexes with various d8 cations. Their rhodium(I) complexes were assessed in the asymmetric hydrogenation and hydroformylation of prochiral olefins. Thecoordination and catalytic properties of two phosphines derived from a- and b-cyclodextrin are also reported. With their phosphorus lone pair pointing toward the CD core, these confining ligands force the coordinated metal centre t…
1,4-Benzoquinones with Styryl Substituents
2002
2-Styryl-1,4-benzoquinone (1) and compounds 2 and 3 containing 1 as a substructure all proved to be highly reactive towards thermal or photochemical [4π + 2π] cyclodimerization reactions. Chemo-, regio- and stereoselective processes lead to dimers (compounds 1−10), which can undergo secondary reactions consisting of the addition of nucleophiles combined with a twofold keto-enol tautomerism (10 ⇄ 12). An alternative process is dehydrogenation/oxidation followed by an intramolecular [4π + 2π] cycloaddition (10 ⇄ 11). The same selective [4π + 2π] cyclodimerization can be observed in solution upon irradiation (e.g., 1a ⇄ 10a), in contrast to irradiation in the crystalline state which yields a […
Evolution of the optimal catalytic systems for the oxidative dehydrogenation of ethane: The role of adsorption in the catalytic performance
2022
Three samples that correspond to the evolution of optimal catalytic systems for the oxidative dehydrogenation of ethane have been synthesized and compared in terms of catalytic behavior and adsorption properties: (i) vanadium oxide supported on alumina, (ii) Sn-promoted NiO, and (iii) multicomponent MoVTeNbO with the M1 structure. The main difference in catalytic performance lies in the extent of the overoxidation of the ethylene formed, following the order VOx/Al2O3 > NiSnOx > MoVTeNb-M1. Accordingly, the selectivity to ethylene at medium and high ethane conversion follows the order MoVTeNb-M1 > NiSnOx > VOx/Al2O3. These results are confirmed by the relative reaction rates observed for the…
Direct oxidation of isobutane to methacrolein over V-MCM-41 catalysts
2004
High vanadium content mesoporous vanado-silicates with MCM-41-like structure, obtained by the atrane route, catalyse the direct oxidation of isobutane to methacrolein with 30% selectivity, and a total dehydrogenation (olefin plus methacrolein) selectivity up to 74%.
Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehy…
2021
Cationic amidotitanocene complexes [Cp2 Ti(NPhAr)][B(C6 F5 )4 ] (Cp=η5 -C5 H5 ; Ar=phenyl (1 a), p-tolyl (1 b), p-anisyl (1 c)) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA-NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti-N bond in 1 a-c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a-c undergo photolytic Ti-N cleavage to release Ti(III) species and aminyl radicals ⋅NPhAr. Reaction of 1 b with H3 BNHMe2 results in fast homolytic Ti-N cleavage to give [Cp2 Ti(H3 BNHMe2 )…
Homogeneous Metal-Based Catalysis in Supercritical Carbon Dioxide as Reaction Medium
2016
Above 31.1 degrees C and 73.8 bar, carbon dioxide reaches the supercritical state, being transformed into a fluid (scCO(2)) that has attracted interest in the last few decades as reaction medium for several transition-metal-catalyzed organic transformations. The main feature of this fluid rests on its capability to dissolve large amounts of other gases such as hydrogen, carbon monoxide, ethylene, or even methane and light alkanes. In this manner, very high concentrations of these reactants are available for catalysis. In this contribution a review of the main achievements of the use of transition-metal complexes as catalysts in scCO(2) is presented.
Computational Investigation of Alkynols and Alkyndiols Hydrogenation on a Palladium Cluster
2013
The reaction path leading to the partial and total reduction of alkynols and alkyndiols with general formula R–CH2–C≡C–CH(OH)–R′ and R–CH(OH)–C≡C–CH(OH)–R′ (R, R′ = H, CH3) on a D3h symmetry Pd9 cl...
Organic carbonates as alternative solvents for asymmetric hydrogenation
2009
Organic carbonates like propylene carbonate (PC) or butylene carbonate (BC) belong to the class of aprotic, highly dipolar solvents (AHD). Interestingly, their potential as solvents for asymmetric catalysis has been overlooked for a long time. The aim of this work is to evaluate organic carbonates and other organic solvents like THF, CH2Cl2, and acetonitrile as well as members of the AHD-family (DMF, DMSO, etc.) as media for homogeneous asymmetric hydrogenation. For this reason cationic Rh-complexes based on chiral phosphine ligands were tested in the hydrogenation of typical benchmark substrates. In several trials, significant advantages of organic carbonates were found. In contrast to DMS…
The effect of β-mercaptoethanol on lactate dehydrogenase from rat brain
1963
Abstract Catalytic studies on lactate dehydrogenase from rat brain in the presence of β-mercaptoethanol have been performed. It has been found that pyruvate reduction and lactate dehydrogenation were differently affected by the addition of the sulfhydryl compound to the reaction mixture. The effect of β-mercaptoethanol on the catalyzed reaction has been shown to be dependent on the hydrogen ion concentration. When lactate dehydrogenation is measured, β-mercaptoethanol causes a net enhancement of the reaction rate at the more acidic pH values.