Search results for "Palladium"
showing 10 items of 956 documents
Influence of crystal defects on the electromigration of the hydrogen in palladium
2001
Abstract Electromigration and diffusion of hydrogen in palladium defected through quenching to liquid nitrogen temperature were investigated. It was found that the obtained experimental results cannot be described by means of a single flux of hydrogen which diffuses in a gradient of electric field. In order to effectively explain the obtained results, a theoretical model was elaborated. It assumed the existence of the following two fluxes of hydrogen in the quenched metal: • Ji—of migration, according to the interstitial mechanism, and • Ji–d—of migration, according to the interstitial-defective mechanism. Dependence of effective valences and diffusion coefficients for both mechanisms of tr…
Direct catalytic cross-coupling of alkenyllithium compounds
2015
A catalytic method for the direct cross-coupling of alkenyllithium reagents with aryl and alkenyl halides is described. The use of a catalyst comprising Pd-2(dba)(3)/XPhos allows for the stereoselective preparation of a wide variety of substituted alkenes in high yields under mild conditions. In addition (1-ethoxyvinyl) lithium can be efficiently converted into substituted vinyl ethers which, after hydrolysis, give readily access to the corresponding methyl ketones in a one pot procedure.
Palladium-catalysed direct cross-coupling of secondary alkyllithium reagents
2014
Palladium-catalysed cross-coupling of secondary C(sp(3)) organometallic reagents has been a long-standing challenge in organic synthesis, due to the problems associated with undesired isomerisation or the formation of reduction products. Based on our recently developed catalytic C-C bond formation with organolithium reagents, herein we present a Pd-catalysed cross-coupling of secondary alkyllithium reagents with aryl and alkenyl bromides. The reaction proceeds at room temperature and on short timescales with high selectivity and yields. This methodology is also applicable to hindered aryl bromides, which are a major challenge in the field of metal catalysed cross-coupling reactions.
Unexpected Reaction of the Unsaturated Cluster Host and Catalyst [Pd3(3-CO)(dppm)3]2+ with the Hydroxide Ion: Spectroscopic and Kinetic Evidence of a…
2006
The title cluster, [Pd(3)(mu(3)-CO)(dppm)(3)](2+) (dppm=bis(diphenylphosphino)methane), reacts with one equivalent of hydroxide anions (OH(-)), from tetrabutylammonium hydroxide (Bu(4)NOH), to give the paramagnetic [Pd(3)(mu(3)-CO)(dppm)(3)](+) species. Reaction with another equivalent of OH(-) leads to the zero-valent compound [Pd(3)(mu(3)-CO)(dppm)(3)](0). From electron paramagnetic resonance analysis of the reaction medium using the spin-trap agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), the 2-tetrahydrofuryl or methyl radicals, deriving from the tetrahydrofuran (THF) or dimethyl sulfoxide (DMSO) solvent, respectively, were detected. For both [Pd(3)(mu(3)-CO)(dppm)(3)](2+) and [Pd(3)(mu…
Isotope dilution inductively coupled plasma quadrupole mass spectrometry in connection with a chromatographic separation for ultra trace determinatio…
2000
An isotope dilution inductively coupled plasma quadrupole mass spectrometric (ID-ICP-QMS) method was developed for the simultaneous determination of the platinum group elements Pt, Pd, Ru, and Ir in environmental samples. Spike solutions, enriched with the isotopes 194Pt, 108Pd, 99Ru, and 191Ir, were used for the isotope dilution step. Interfering elements were eliminated by chromatographic separation using an anion-exchange resin. Samples were dissolved with aqua regia in a high pressure asher. Additional dissolution of possible silicate portions by hydrofluoric acid was usually not necessary. Detection limits of 0.15 ng x g(-1), 0.075 ng x g(-1), and 0.015 ng x g(-1) were achieved for Pt,…
Synthesis of Optically Pure Arylamine Derivatives by Using the Bucherer Reaction
2017
The Bucherer reaction is a common pathway for the conversion of 1- and 2-naphthols into the corresponding 1- or 2-naphthylamines, respectively. Mostly, only singular examples for its preparative use are reported since this particular transformation seems to be very sensitive to the reaction conditions. By choosing different phenolic substrates and chiral amines, we were able to prepare a broad scope of optically pure arylamines using this type of reaction. In contrast to alternative methods forming C−N aryl bonds such as Buchwald-Hartwig or Chan-Lam cross-coupling reactions, no palladium or copper catalysts are required. The use of water as solvent and the easily available starting material…
Palladium-Catalyzed N-Arylation Reactions with Aziridine and Azetidine
2007
Studies on the viability of palladium-catalyzed cross-coupling reactions of aryl- or hetaryl bromides with the parent aziridine or azetidine showed that a wide range of N-arylaziridines and N-arylazetidines are accessible by this method. Ring cleavage of the N-arylaziridines or -azetidines thus produced does not occur under the applied reaction conditions. The synthetic utility of the method is illustrated by examples of double N-arylations with either aziridine or azetidine.
Palladium‐Catalyzed Electrophilic C–H‐Bond Fluorination: Mechanistic Overview and Supporting Evidence
2018
International audience; Palladium-catalyzed electrophilic fluorination is a particularly attractive and challenging synthetic issue. Because of the rapid evolution of this topic, a critical point on the mechanistic and experimental advances is provided herein. In the present review, we focused on current mechanistic understanding in electrophilic fluorination (and related halogenations) catalyzed by palladium, mainly with N-directing group. Our discussion is based on the well-characterized or calculated pertinent metal species and intermediates used for analyzing the plausible catalytic cycles. A particular effort has been devoted to gathering supporting data for the putative species involv…
Selective hydrogenation of acetylene in ethylene feedstocks on Pd catalysts
1996
Abstract Pumice supported palladium catalysts were compared with Pd SiO 2 and Pd Al 2 O 3 in the hydrogenation of acetylene using typical industrial ethylene feedstocks: front-end and tail-end cuts. Pd/pumice catalysts exhibit good activity and excellent selectivity and stability in the title reaction. Their activity/selectivity pattern is controlled by the composition of the reaction mixture. The turnover frequency (TOF) increases, and the apparent activation energy (Ea) decreases, with the H 2 C 2 H 2 ratio, but they are not affected by the C 2 H 2 C 2 H 4 ratio. The selectivity to ethane (SE) does not change with acetylene conversion at low H 2 C 2 H 2 ratio (tail-end cut) and increases …
Hydrogenation of acetylene in ethylene rich feedstocks: Comparison between palladium catalysts supported on pumice and alumina
1998
Abstract The activity–selectivity patterns of Pd/pumice catalysts are compared with industrial and home prepared Pd/Al2O3 catalysts in the hydrogenation of acetylene in ethylene rich feedstocks (front-end and tail-end cuts). The iso-kinetic relationship (IKR) approach and a new mathematical model, surface site evolution model (SSEM), are employed in this comparison. Pumice and alumina supported Pd catalysts show different metal redox properties. A similar reaction mechanism is adequate to describe the reaction pathway independently by the catalysts and the gas mixtures considered. This mechanism involves the formation of surface polymers during the catalytic reactions and different catalyti…