Search results for " Catalysis"
showing 10 items of 585 documents
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.
Palladium-Catalysed Intermolecular Direct C–H Bond Arylation of Heteroarenes with Reagents Alternative to Aryl Halides: Current State of the Art
2022
Abstract: This unprecedented review with 322 references provides a critical up-to-date picture of the Pd-catalysed intermolecular direct C–H bond arylation of heteroarenes with arylating reagents alternative to aryl halides that include aryl sulfonates (aryl triflates, tosylates, mesylates, and imidazole-1-sulfonates), diaryliodonium salts, [(diacetoxy)iodo]arenes, arenediazonium salts, 1-aryltriazenes, arylhydrazines and N’-arylhydrazides, arenesulfonyl chlorides, sodium arenesulfinates, arenesulfinic acids, and arenesulfonohydrazides. Particular attention has been paid to summarise the preparation of the various arylating reagents and to highlight the practicality, versatility, and limit…
A study of the influence of ionic liquids properties on the Kemp elimination reaction
2009
The morpholino-induced elimination of 5-nitrobenzisoxazole into the relevant 2-cyano-4-nitrophenolate has been used as a sample reaction in order to investigate molecular properties of some room temperature ionic liquids. The kinetic study was carried out at 298 K by means of spectrophotometric measurements. Ionic liquids, which differ in both their cation and anion properties, were used as solvent systems. In particular, aliphatic (pyrrolidinium, piperidinium, and ammonium) and aromatic (imidazolium and pyridinium) ionic liquids were used. For aromatic cations, imidazolium ions having different hydrogen-bond donor ability or a different alkyl-chain length were taken into account. The anion…
Water in Organocatalytic Reactions
2013
Water is a green solvent: it is cheap, safe and environmentally friendly compared with organic solvents. Pfizer Corp. has defined a solvent guide that encourages the use of preferred solvents, for example, water or ethyl acetate, over usable solvents, for example, dimethyl sulfoxide (DMSO), and undesirable solvents, for example, dimethylformamide (DMF) [1]. However, chemical reactions carried out in or on water are not necessarily green reactions. Indeed, if we consider a reaction involving two species (A and B) and a product (C), the requirement for an ideal situation is when the product C has zero solubility and is quantitatively recovered by filtration, leaving no starting compounds A an…
Organocatalytic Enantioselective Alkylation of Pyrazol-3-ones with Isatin-Derived Ketimines: Stereocontrolled Construction of Vicinal Tetrasubstitute…
2016
A quinine-derived thiourea catalysed the enantioselective addition of 4-substituted pyrazolones to isatin-derived ketimines, providing a variety of aminooxindole-pyrazolone adducts containing congested vicinal tetrasubstituted stereocentres with excellent outcomes (up to 98% yield, >20:1 dr and 98% ee).
Room temperature ionic liquids structure and its effect on the mononuclear rearrangement of heterocycles: an approach using thermodynamic parameters.
2006
The kinetics of the rearrangement of the Z-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (1) into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole (2) induced by amines have been studied in five room-temperature ionic liquids (RTILs) at different temperatures. The kinetic data collected show that both cationic and anionic parts of RTILs significantly influence the reactivity of the title reaction. The calculated activation parameters allow us to advance hypotheses about the weak interactions operating in RTIL solutions.
On the Rearrangement in Dioxane/Water of (Z)-Arylhydrazones of 5-Amino-3-benzoyl-1,2,4-oxadiazole into (2-Aryl-5-phenyl-2H-1,2,3-triazol-4-yl)ureas: …
2006
We have recently evidenced an interesting differential behavior in the reactivity in dioxane/water between the (Z)-2,4-dinitrophenylhydrazone (1a) and the (Z)-phenylhydrazone (1b) of 5-amino-3-benzoyl-1,2,4- oxadiazole. The former rearranges into the relevant triazole 2a only at pS+ > 4.5 while undergoing hydrolysis at high proton concentration (pS+ < 3.5); on the contrary, the latter rearranges into 2b in the whole pS+ range examined (0.1 e pS+ e 14.9). Thus, for a deeper understanding of these differences we have now collected kinetic data on the rearrangement in dioxane/water of a series of 3- or 4-substituted (Z)-phenylhydrazones (1c-l) of 5-amino-3-benzoyl-1,2,4-oxadiazole in a wide ra…
Oxygen isotopic exchange: A useful tool for characterizing oxygen conducting oxides
2005
International audience; The 18O/16O isotope exchange depth profile technique (IEDP) followed by SIMS characterizations was applied to dense membranes of pure ionic conductors and mixed ionic/electronic conducting materials. It is a very useful tool since we obtain in both cases data concerning the oxygen diffusion in the bulk and the oxygen exchange at the surface of the materials. Comparisons were done, including results concerning the role of the surface with regards to the oxygen reduction reaction. Detailed experimental and analytical processes are given.
Oxidative degradation properties of Co-based catalysts in the presence of ozone
2007
Four series of cobalt-based catalysts, such as bare Co3O4 and CoO, CoOx-CeO2 mixed oxides, CoOx supported over alumina and alumina-baria and CoMgAl and CoNiAl hydrotalcites have been synthesized and investigated for the oxidative degradation of phenol in the presence of ozone. Characterizations were obtained by several techniques in order to investigate the nature of cobalt species and their morphological properties, depending on the system. Analyses by XRD, BET, TPR, UV-visible diffuse reflectance spectroscopy and TG/DT were performed. The CoNiAl hydrotalcite exhibits, after 4 h of reaction, the highest phenol ozonation activity followed by Co(3 wt%)/Al2O3-BaO and CoMgAl. The samples Co(1 …
Few-layer Black Phosphorous Catalyzes Radical Additions to Alkenes Faster than Low-valence Metals
2020
Abstract The substitution of catalytic metals by p‐block main elements has a tremendous impact not only in the fundamentals but also in the economic and ecological fingerprint of organic reactions. Here we show that few‐layer black phosphorous (FL‐BP), a recently discovered and now readily available 2D material, catalyzes different radical additions to alkenes with an initial turnover frequency (TOF0) up to two orders of magnitude higher than representative state‐of‐the‐art metal complex catalysts at room temperature. The corresponding electron‐rich BP intercalation compound (BPIC) KP6 shows a nearly twice TOF0 increase with respect to FL‐BP. This increase in catalytic activity respect to t…