Showing papers on "Palladium published in 1992"

Catalytic chemistry of supported palladium for combustion of methane

Abstract: The high-temperature catalytic chemistry of supported palladium for methane oxidation has been studied. Palladium oxide supported on alumina decomposes in two distinct steps in air at one atmosphere. The first step occurs between 750 and 800 ° C and is believed to be a decomposition of palladium-oxygen species dispersed on bulk palladium metal designated (PdOx/Pd). The second decomposition is between 800 and 850 ° C and behaves like crystalline palladium oxide designated (PdO). To reform the oxide, the temperature must be decreased well below 650 ° C. Thus, there is a significant hysteresis between decomposition to palladium and re-formation of the oxide. Above 500 ° C, methane oxidation occurs readily when the catalyst contains PdO. However, when only palladium metal is present no oxygen adsorption occurs and no methane activity exists. One may conclude that the high temperature (> 500 ° C) activity of a supported palladium containing catalyst is due to the ability of palladium oxide to chemisorb oxygen. Palladium, as a metal, does not chemisorb oxygen above 650 ° C and thus, is completely inactive toward methane oxidation.

Selectivity in palladium catalysed allylic substitution

Abstract: Palladium catalysed allylic substitution has emerged as one of the more useful synthetic methods for the construction of C-C and C-X bonds. The reaction offers the advantages of mild reaction conditions, as well as the ability to accomodate a wide range of nucleophiles and their electrophilic partners. The issues of regiocontrol, diastereocontrol and enantiocontrol have been documented by a number of researchers over the last twenty years. The levels of selectivity in many cases are very high, and current research is driving these selectivities higher still. A greater understanding of the nature of the process is occurring as more detailed mechanistic and structural studies are being undertaken. Many uses have been found for palladium catalysed allylic substitution methodology, since the strong stereocontrol allows for the selective formation of numerous products. A few other metals are also able to catalyse allylic substitution, with modified stereochemical behaviour, although these are currently less well documented than for palladium catalysed process.

Palladium-Catalysed Reactions of Organotin Compounds

Abstract: Palladium-catalysed reactions involving organotin compounds are of rapidly increasing importance in organic synthesis. Carbon-carbon bond formation via cross-coupling reactions is of primary interest in view of the variety of functionalities which can be used. A variety of applications in natural product synthesis have been described, and intramolecular reactions can be used for the formation of small, medium and large rings. Reactions involving additions of tin-element bonds to multiply bonded systems are also of increasing significance. Although «general-purpose» catalysts are available, specific catalysts are often required for certain applications

Metal complexes of buckminsterfullerene (C60)

Abstract: This account describes the author's work on the attachment of metal complexes directly to the exterior of the C[sub 60] framework via solution chemistry forming metal-C[sub 60] bonds. Ruthenium, platinum, palladium, and nickel complexes are discussed.

Palladium(II) catalysts for living alternating copolymerization of olefins and carbon monoxide

Abstract: Several Pd(II) catalyst systems have been reported which effect prefectly alternating copolymerization of olefins with carbon monoxide to yield polyketones [C(O)CH(R)CH{sub 2}]{sub n}. There has been increasing interest in these polymers, particularly the C{sub 2}H{sub 4}/CO copolymer (T{sub m} = 257{degree}C), due to their unusual properties, the low cost of monomers, the present of the carbonyl functionality, and the potential for further functionalization. The catalyst systems used have typically employed Pd(II) salts in methanol or chloroform. We report here the synthesis of well-defined Pd(II) catalysts that operate in aprotic solvents to yield living alternating copolymers of olefins and CO. In situ spectroscopic studies have established mechanistic details including the identity of the catalyst resting state.

Catalytic activity and structural analysis of polymer-protected gold/palladium bimetallic clusters prepared by the successive reduction of hydrogen tetrachloroaurate(III) and palladium dichloride

Abstract: Colloidal dispersions of the poly(N-vinyl-2-pyrrolidone)-protected gold/palladium bimetallic clusters were prepared by successive reduction in order to compare the catalytic activity and the structure of the clusters. The catalytic activity of the colloidal dispersions of the Au/Pd(1/4) clusters for the selective partial hydrogenation depends on the reduction order in the preparation of the clusters. The clusters prepared by successive reduction starting from the reduction of gold ions (Au→Pd) have higher activity than those prepared by the reverse order (Pd→Au). The EXAFS measurements indicate that the Au/Pd(1/4 and 1/) bimetallic clusters prepared by the successive (Pd→Au) reduction have the cluster-in-cluster structure

Spectroscopic study of the interaction of carbon monoxide with cationic and metallic palladium in palladium–alumina catalysts

Abstract: Palladium–alumina catalysts prepared from various precursors and by different preparation methods have been studied after calcination by O2 and reduction by H2, using IR spectroscopy of adsorbed CO, EPR and diffuse reflectance measurements. In addition to Pd2+ ions, the unreduced samples contain Pd3+ and Pd+ species. Pd2+ and Pd– entities are still present on catalysts reduced at 573 K. The influence of the precursor on the metal fraction exposed (MFE) and on the nature of adsorbed CO species is clearly shown. Bridged entities are more sensitive to the MFE value than the linear ones. At 100 Torr [1 Torr ≈(101 325/760) Pa], the bridged entities are compressed, which leads to a CO-(surface Pd) ratio of near unity. The MFE increases when the precursors are taken in the following order nitrate < chloride (impregnated)≈ acetylacetonate < chloride (exchanged). The sintering brought about by increasing the reduction temperature seems to affect the largest metal particles rather than the small ones which strongly interact with the carrier.

Correction. Isomerization of (π-Allyl)Palladium Complexes via Nucleophilic Displacement by Palladium(0). A Common Mechanism in Palladium(0)-Catalyzed Allylic Substitution

Abstract: Treatment of (π-allyl)palladium complexes such as 6 and 9 with Pd(PPh 3 ) 4 leads to rapid isomerization at -15 o C in tetrahydrofuran and other solvents. At 0 o C and in the presence of more than 2 equiv of triphenylphosphine per palladium, the phosphine attacks the π-allyl group to give allylic phosphonium salts 7 with concomitant formation of a palladium(0)-phosphine complex, and isomerization of 6 is observed

Identification of two types of oxidized palladium on γ-alumina by X-ray photoelectron spectroscopy

Abstract: It was demonstrated by X-ray photoelectron spectroscopy (XPS) that oxidized palladium supported on γ-alumina can exist in two chemically different entities, distinguishable by their observed electron binding energies and by selective reduction. Larger oxidized particles are characterized by a binding energy attributed to PdO. This oxide is reduced to metallic palladium at room temperature in hydrogen, as would be expected for bulk PdO. On the other hand, at low palladium loadings of 0.5 wt.-% and less, the majority of the palladium is present at a binding energy shifted approximately 1.6 eV higher than that observed for PdO. The shift appears to be associated with a metal-support interaction at concentrations where the palladium is highly dispersed. Contrary to that observed at loadings greater than 0.5 wt.-%, this palladium remains mostly oxidized when exposed to hydrogen at room temperature. The results are compared with literature data reported for platinum/γ-alumina.

Palladium sulfate solution for the selective seeding of the metal interconnections on polyimide dielectrics for electroless metal deposition

Abstract: A method of selectively seeding or activating metal interconnections patterned on polyimide dielectric surfaces using an aqueous solution of palladium sulfate, palladium perchlorate, palladium trifluoromethane sulfonate, palladium nitrate or other palladium salts having poorly coordinating counter ions. This strongly selective seeding and the corresponding ability to reliably remove all traces of the seeding material from the polyimide surface eliminates shorting, bridging and reduction of breakdown voltage during electroless plating of a thin layer of nickel or cobalt.

Enantioselective Synthesis of Optically Active Pyridine Derivatives and C2-Symmetric 2,2′-Bipyridines

Abstract: Optically active pyridine derivatives 2, 15, 18, 19, 21, 26, and 27 are obtained by enantioselective reduction of the corresponding ketones 5, 7, 11–13, 24, and 25 using the chiral borane reagent chlorodiisopinocampheylborane [(Ipc)2BCl]. Nickel(0)-mediated coupling of bromopyridines 2, 15, and 31 gives C2-symmetric 2,2′-bipyridines (R,R)-32, (R,R′)-33, and (S,S)-38, respectively, which form metal complexes with CoII, PdII, CuI, and AgI. Aryl-substituted pyridines 26, and 39–41 are synthesized by palladium(0)-catalyzed cross couplings of 2 and 15 with boronic acids 42–44.

Synthesis of palladium and platinum nanoclusters within microphase-separated diblock copolymers

Abstract: Small palladium and platinum clusters have been synthetized within a purely hydrocarbon polymer matrix. The procedure consists of ring-opening metathesis polymerisation of norbornene-derived organometallic complexes, Pd(Cp N )(PA) and Pt(Cp N )Me 3 (Cp N = endo-2-(cyclopentadienylmethyl)norbon-5-ene and PA = η 3 -1-phenylallyl), using W or Mo alkylidene initiators, followed by static casting of films and subsequent reduction of the organometallic complexes under mild conditions using molecular hydrogen

Catalytic asymmetric synthesis of quarternary carbon centers. Palladium-catalyzed formation of either enantiomer of spirooxindoles and related spirocyclics using a single enantiomer of a chiral diphosphine ligand

Abstract: The asymmetric synthesis of eight spirooxindoles and related spirocyclics from the corresponding aryl iodide by palladium catalyzed cyclizations in the presence of the diphosphine ligand (R)-(+)-BINAP is reported

Preparation of enantiomerically pure protected 4-oxo .alpha.-amino acids and 3-aryl .alpha.-amino acids from serine

Abstract: The organozinc reagent 13, prepared from the protected β-iodo alanine derivative 3c using ultrasonic activation, is efficiently acylated using acid chlorides in the presence of bis(triphenylphosphine)palladium dichloride to give enantiomerically pure protected 4-oxo-α-amino acids 17 in 39-90% yield (13 examples). Zinc reagent 13 can also be coupled with aryl iodides in the presence of bis(triphenylphosphine)palladium dichloride to give enantiomerically pure protected phenylalanine analogues 26, 29, and 30 in 10-67% yield (11 examples)

Palladium-catalyzed cross-coupling reactions of chloropyrazines with aromatic heterocycles

Abstract: In the presence of tetrakis(triphenylphosphine) palladium, chloropyrazines were treated with aromatic heterocycles such as furan, thiophene, pyrrole, N-substituted pyrroles, benzo[b]furan, benzo[b]thiophene, oxazole, thiazole, N-methylimidazoles, benz[b]oxazole and benzo[b]thiazole. The corresponding coupling products were obtained in moderate to good yields. The structures of the coupling products were determined

Chiral phosphine ligands modified by crown ethers: an application to palladium-catalyzed asymmetric allylation of .beta.-diketones

Abstract: Chiral ferrocenylphosphine ligands modified by monoaza or diaza crown ethers of varying ring sizes and linker chain lengths (8a-e, 9) were synthesized. The reaction of the phosphine ligand modified by monoaza- 18-crown-6 (8b) and the di-μ-chlorobis(π-allyl)dipalladium(II) complex in CDCl 3 produced the π-allylpalladium(II) complex chelated by the two phosphorus atoms of 8b, leaving the crown ether moiety free

Palladium-catalyzed carbonyl allylation by allylic alcohols with stannous chloride

Abstract: Allylation of carbonyls via activation of allylic alcohols on palladium catalysts is reported SnCl{sub 2} acts as a reducing agent Regioselectivity of the reactions was evaluated in terms of solvent effects Further, reaction intermediates were observed by NMR 46 refs, 14 figs, 5 tabs

.alpha.-Regioselectivity in palladium-catalyzed arylation of acyclic enol ethers

Abstract: Regioselective α-arylation of acyclic enol ethers by aryl trifluoromethanesulfonates, aryl bromide, aryl iodides, and aroyl chlorides is described. The outcome of the reaction proved to be dependent from the relationship between ligand and counterion in the oxidative addition complex

Mechanisms for the reactions between methane and the neutral transition metal atoms from yttrium to palladium

Abstract: Calculations including electron correlations have been performed for the oxidative addition reactions between methane and the whole sequence of second row transition metal atoms from yttrium to palladium. The lowest barrier for the C-H insertion reaction is found for the rhodium atom. Palladium has the lowest methane elimination barrier. The barrier height is governed by two factors. In the reactant channel low repulsion favors a low barrier, and in the product channel strong bond formation is important. The atomic state with lowest repulsion toward methane is the d{sup n+2} state and the strongest bonds are formed to the d{sup n+1} s state. For rhodium both these states are energetically low lying. Only palladium has a bound {eta}{sup 2} precursor state on the ground state potential surface. Another interesting result is that the potential surface for the reaction between methane and the rhodium atom is remarkably similar to the potential surface for the reaction between methane and ClRhL{sub 2}, which has been studied experimentally. 44 refs., 5 figs., 5 tabs.

The aqueous geochemistry of platinum, palladium and gold; recent experimental constraints and a re-evaluation of theoretical predictions

Abstract: ABsrRAcr Re-evaluation of theoretical calculations reaffirms that, at temperatures up to 300"C, significant solubility (>10 ppb) of Pt and Pd as chloride complexes is restricted to highly oxidizing and acidic conditions. This conclusion is supported by recently,reported experimental solubility measurement!. Experiments on the solubility of ft and Pd in sodium hydroxide solutiors at 25'C confirm that a change in speciei from PdIOU)! to p'O(OfDt occurs at a pH of approximately 12, but apparcntly no such change in speciation occurs for Pt. The measured log p2 for R(OH)9 agrees very well with the previously predicted value, whereas- predicted stability-constants for ft(OH)J unO ntOUlo2- stroutO Ue considered only as prwisional- estimates. Hydroxide complexation is unlikely to contribute to the hidrothermal iransport of Pt and Pd in mosi geological environments. Nevertheless, ft, Pd and Au hydroxide complexes may be the dominant inoiganic species of these metals in oxidized surficial waters such as seawater, lake water, and rivei water. Ammonia complexes are predicGd to contnbute substantially to the solubility of Pd and Au, bulProbablY not pr, ar temperarures near 300'C, with a maximum in solubility at the N2-NH3-NHf triple_ point. P:;s"t-l9jtidggf.!' Pd and Au as ammonia complexes decrease very rapidly with decreasing temperatue, owing to the shift of the N2-NHJ-NHf triple point to successively lowei fugacities of o*ygen anA iigner pH. Preliminarry results of an experimental investigation of the solubility of I,t, pd and Au in hydrothermA bisulfide solutioni from 200" to 500'i yield between 2 and 300 ppm Au, benveen 4 and 800 ppb ft, and between 1 and 600 ppb Pd at0.3-2.2mol^|xs,5.91 -N > -O.