Showing papers on "Palladium published in 2008"
TL;DR: The conceptual basis and utility of the latest, "fourth-generation" palladium catalyst for the coupling of amines and related reagents with aryl halides is described and the effects of electronic properties on C-C and C-N bond-forming reductive elimination are similar.
Abstract: Many active pharmaceuticals, herbicides, conducting polymers, and components of organic light-emitting diodes contain arylamines. For many years, this class of compound was prepared via classical methods, such as nitration, reduction and reductive alkylation, copper-mediated chemistry at high temperatures, addition to benzyne intermediates, or direct nucleophilic substitution on particularly electron-poor aromatic or heteroaromatic halides. However, during the past decade, palladium-catalyzed coupling reactions of amines with aryl halides have largely supplanted these earlier methods. Successive generations of catalysts have gradually improved the scope and efficiency of the palladium-catalyzed reaction. This Account describes the conceptual basis and utility of our latest, "fourth-generation" palladium catalyst for the coupling of amines and related reagents with aryl halides. In the past five years, we have developed these catalysts using the lessons learned from previous generations of catalysts developed in our group and in other laboratories. The ligands on the fourth-generation catalyst combine the chelating properties of the aromatic bisphosphines of the second-generation systems with the steric properties and strong electron donation of the hindered alkylphosphines of the third-generation systems. The currently most reactive catalyst in this class is generated from palladium and a sterically hindered version of the Josiphos family of ligands that possesses a ferrocenyl-1-ethyl backbone, a hindered di-tert-butylphosphino group, and a hindered dicyclohexylphosphino group. This system catalyzes the coupling of aryl chlorides, bromides, and iodides with primary amines, N-H imines, and hydrazones in high yield. The reaction has broad scope, high functional group tolerance, and nearly perfect selectivity for monoarylation. It also requires the lowest levels of palladium that have been used for C-N coupling. In addition, this latest catalyst has dramatically improved the coupling of thiols with haloarenes to form C-S bonds. Using ligands that lacked one or more of the structural elements of the most active catalyst, we examined the effects of individual structural elements of the Josiphos ligand on catalyst activity. This set of studies showed that each one of these elements contributes to the high reactivity and selectivity of the catalyst containing the hindered, bidentate Josiphos ligand. Finally, we examined the effect of electronic properties on the rates of reductive elimination to distinguish between the effect of the properties of the M-N sigma-bond and the nitrogen electron pair. We have found that the effects of electronic properties on C-C and C-N bond-forming reductive elimination are similar. Because the amido ligands contain an electron pair, while the alkyl ligands do not, we have concluded that the major electronic effect is transmitted through the sigma-bond.
1,535 citations
TL;DR: Four main classes of NHC-containing palladium(II) complexes will be presented: palladium dimers with bridging halogens, palladacycles, palladium acetates and acetylacetonates, and finally pi-allyl complexes.
Abstract: Metal-catalyzed cross-coupling reactions, notably those permitting C−C bond formation, have witnessed a meteoritic development and are now routinely employed as a powerful synthetic tool both in academia and in industry. In this context, palladium is arguably the most studied transition metal, and tertiary phosphines occupy a preponderant place as ancillary ligands. Seriously challenging this situation, the use of N-heterocyclic carbenes (NHCs) as alternative ligands in palladium-catalyzed cross-coupling reactions is rapidly gaining in popularity. These two-electron donor ligands combine strong σ-donating properties with a shielding steric pattern that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the number of well-defined NHC-containing palladium(II) complexes is growing, and their use in coupling reactions is witnessing increasing interest. In this Account, we highlight the advantages of this family of palladium complexes and review their synt...
946 citations
TL;DR: In an effort to identify catalysts based on less expensive and more available metals, density functional calculations were performed that identified relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces that verified the facility of known catalysts.
Abstract: The removal of trace acetylene from ethylene is performed industrially by palladium hydrogenation catalysts (often modified with silver) that avoid the hydrogenation of ethylene to ethane. In an effort to identify catalysts based on less expensive and more available metals, density functional calculations were performed that identified relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. This analysis not only verified the facility of known catalysts but identified nickel-zinc alloys as alternatives. Experimental studies demonstrated that these alloys dispersed on an oxide support were selective for acetylene hydrogenation at low pressures.
928 citations
TL;DR: The population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface, and unselective hydrogenation proceeds on hydrogen-saturated β-hydride whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events.
Abstract: Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites. In situ x-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Additional studies of hydrogen content using in situ prompt gamma activation analysis, which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-saturated β-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface.
748 citations
TL;DR: In this article, an extremely simple green approach that generates bulk quantities of nanocrystals of noble metals such as silver (Ag) and palladium (Pd) using coffee and tea extract at room temperature is described.
608 citations
TL;DR: Reactions can now be carried out with aryl chlorides and tosylates bearing a range of substituents affecting both the electronic and steric properties of the substrate.
531 citations
TL;DR: A very high catalytic activity in the cyanosilylation reaction was observed for MIL-101, a chromium based metal-organic framework, which is a remarkably stable support for palladium in hydrogenation reactions, with significantly higher activity than e.g. palladium on activated carbon.
471 citations
TL;DR: This Account reports the methodology, based on the cooperative action of Pd and norbornene, that achieves selective aryl-aryl coupling through C-halide and C-H activation, and obtained the key metallacycle able to selectively direct the reactions by replacing nor Bornene with an aryL-bonded aminocarbonyl group.
Abstract: Catalytic methods are important tools for the synthesis of C−C bonds under mild and ambient conditions. Palladium chemistry predominates in this area because it offers the opportunity to form several different types of bonds in one pot. Palladium can also tolerate a variety of functional groups. Among the many investigations of catalytic aryl−aryl couplings, the most successful technique has been the Suzuki reaction, which uses an arylboronic acid to attack an aryl−Pd bond. This Account reports our methodology, based on the cooperative action of Pd and norbornene, that achieves selective aryl−aryl coupling through C−halide and C−H activation. We are primarily interested in Pd-catalyzed sequential reactions. These reactions combine palladium as an inorganic catalyst and a strained olefin such as norbornene as an organic catalyst and can lead to biphenyl derivatives. While the palladium facilitates C−C bond formation through C−halide and C−H activation, the norbornene contributes to the construction of a pa...
430 citations
TL;DR: An efficient synthesis of functionalized indoles from commercially available anilines by palladiumcatalyzed, intramolecular oxidative coupling is reported, with the ability to vary the aniline moiety so broadly is a distinct advantage of this new indole synthesis.
Abstract: The indole unit is one of the most abundant and relevant heterocycles in natural products and pharmaceuticals. Despite the existence of numerous methods for the synthesis and derivatization of indoles, the development of new, more efficient methods is of great importance. In this context, direct oxidative C C coupling by the selective activation of two C H bonds is a promising synthetic strategy. In contrast to established cross-coupling methods, such as the Suzuki– Miyaura coupling, prefunctionalization of the reaction centers is not required. For example, electron-rich aniline substrates can be activated and functionalized by electrophilic aromatic palladation under acidic conditions to give indolequinones or carbazoles. However, the limited scope of these reactions, the frequent requirement of a stoichiometric amount of a palladium complex, and the low yields often observed limit the usefulness of these methods. Furthermore, simple non-annulated indoles could not be prepared under these acidic conditions. Herein, we report an efficient synthesis of functionalized indoles from commercially available anilines by palladiumcatalyzed, intramolecular oxidative coupling. As this cyclization does not proceed through electrophilic aromatic palladation, a large variety of anilines can be used in this reaction. Our investigation commenced with the cyclization of methyl (Z)-3-(phenylamino)but-2-enoate (1a) to give the corresponding indole 2a. In experiments to optimize the reaction, the best results were obtained with a catalytic amount of Pd(OAc)2, Cu(OAc)2 as the oxidant, and K2CO3 as the base in DMF (Table 1, entry 1). Under these conditions, conversion was complete within 3 h at 80 8C (72% yield of the isolated product), or within less than 15 min at 140 8C, even when only 5 mol% of Pd(OAc)2 was used (not shown). Variation of the oxidant (Table 1, entries 3–6), the base (Table 1, entries 7 and 8), or the solvent (Table 1, entries 9–11) led to a decrease in the yield. The use of acetic acid as the solvent resulted in the rapid decomposition of the substrate and therefore no product formation (Table 1, entry 11). The results with Pd(TFA)2 (TFA= trifluoroacetate) were similar to those observed under the optimal conditions (Table 1, entry 12), whereas the addition of PPh3 resulted in the formation of a less active catalyst (Table 1, entry 13). Interestingly, chloride anions do not influence the reaction at all (Table 1, entry 14). A great variety of substituted anilines can be transformed into the corresponding indoles under the optimized reaction conditions (Table 2). In some cases, an increased reaction temperature (and, consequently, a shorter reaction time) led to higher yields. Substrates with a variety of electron-donating (Table 2, entries 2–8) and electron-withdrawing substituents (Table 2, entries 9–19) were converted directly into the indole products, which are versatile building blocks for subsequent synthetic modification, for example, through modern crosscoupling reactions (Table 2, entries 12–14). The ability to vary the aniline moiety so broadly is a distinct advantage of this new indole synthesis. In the case of meta-substituted substrates 1, two regioisomeric indole products 2 can be formed. Intriguingly, exclusive Table 1: Optimization of the reaction conditions.
385 citations
385 citations
TL;DR: Mild conditions permit a broad set of functionalities both in the indole and in the aryl iodide units such as free alcohols, phenols, aldehydes, bromides, or nitriles, thus allowing the synthesis of a variety of novel compounds in excellent yields.
Abstract: We report the first room-temperature direct C-2 arylation of indoles with iodoarenes by using a highly electrophilic palladium catalyst generated in situ from Pd(OAc)2 and a silver carboxylate. These mild conditions permit a broad set of functionalities both in the indole and in the aryl iodide units such as free alcohols, phenols, aldehydes, bromides, or nitriles, thus allowing the synthesis of a variety of novel compounds in excellent yields.
TL;DR: In this paper, a review of the last three decades of kinetics of selective hydrogenation of ethyne in ethene-rich streams on palladium catalysts is presented.
Abstract: Developments in the last three decades of kinetics of selective hydrogenation of ethyne in ethene‐rich streams on palladium catalysts are reviewed. Most of the studies can be described comprehensively by a model that assumes carbonaceous deposits (i) create irreversibly on the palladium surface small A types of active site (selective to ethene) and large E types of active site (selective to ethane), and (ii) are involved in hydrogenation of ethene on E s sites on the support. The relative importance of these sites, with varying (i) reaction conditions, (ii) palladium dispersion, (iii) process modifiers, and (iv) promoters, is discussed.
TL;DR: A new class of one-component Pd precatalysts bearing biarylphosphine ligands that are easily activated under normal reaction conditions at or below room temperature, and ensure the formation of the highly active monoligated Pd(0) complex necessary for oxidative addition is described.
Abstract: A new class of one-component Pd precatalysts bearing biarylphosphine ligands is described. These precatalysts are air- and thermally stable, are easily activated under normal reaction conditions at or below room temperature, and ensure the formation of the highly active monoligated Pd(0) complex necessary for oxidative addition. The use of these precatalysts as a convenient source of LPd(0) in C−N cross-coupling reactions is explored. The reactivity that is demonstrated in this study is unprecedented in palladium chemistry.
TL;DR: The size dependencies of the hydrogen-storage properties in polymer-coated Pd nanoparticles with diameters of 2.6 ± 0.4 and 7.0 ± 1.9 nm were investigated by a measurement of hydrogen pressure-composition isotherms.
Abstract: The size dependencies of the hydrogen-storage properties in polymer-coated Pd nanoparticles with diameters of 2.6 ± 0.4 and 7.0 ± 0.9 nm were investigated by a measurement of hydrogen pressure-composition isotherms. Their storage capacities per constituent Pd atom in the particles decreased with decreasing particle size, whereas the hydrogen concentrations in the two kinds of nanoparticles were almost the same and 1.2 times as much, respectively, as that in bulk palladium after counting zero hydrogen occupancy on the atoms in the first surface layer of the particles. Furthermore, apparent changes in hydrogen absorption behavior with decreasing particle size were observed, that is, a narrowing of the two-phase regions of solid-solution and hydride phases, the lowering of the equilibrium hydrogen pressure, and a decrease in the critical temperature of the two-phase state. By analyzing the isotherms, we quantitatively determined the heat of formation (ΔHα→β) and the entropy change (ΔSα→β) in the hydride form...
TL;DR: The use of pivalic acid as the reaction solvent, instead of acetic acid, results in greater reproducibility, higher yields, and broader scope, and the use of electron-rich diarylamines as illustrated in the synthesis of three naturally occurring carbazole products.
Abstract: New reaction conditions for intramolecular palladium(II)-catalyzed oxidative carbon-carbon bond formation under air are described. The use of pivalic acid as the reaction solvent, instead of acetic acid, results in greater reproducibility, higher yields, and broader scope. This includes the use of electron-rich diarylamines as illustrated in the synthesis of three naturally occurring carbazole products: Murrayafoline A, Mukonine, and Clausenine. A variety of side products have also been isolated, casting light on competing reaction pathways and revealing new reactivity with palladium(II) catalysis.
TL;DR: Recent advances in gold acting as a catalyst for a broad range of redox reactions are explored.
TL;DR: This novel CO2-fixation reaction is thought to proceed through the catalytic generation of sigma-allyl palladium species via hydropalladation of allenes, followed by its regioselective nucleophilic addition to CO2 in the presence of an appropriate reducing agent.
Abstract: Tridentate PSiP pincer-type palladium complex-catalyzed hydrocarboxylation of allenes under carbon dioxide to give synthetically useful β,γ-unsaturated carboxylic acids was developed. This novel CO2-fixation reaction is thought to proceed through the catalytic generation of σ-allyl palladium species via hydropalladation of allenes, followed by its regioselective nucleophilic addition to CO2 in the presence of an appropriate reducing agent. The reaction is successfully applied to various allenes bearing functional groups such as ester, carbamate, ketone, and alkene, showing high synthetic utility of this protocol.
TL;DR: Recent progress in the direct synthesis of hydrogen peroxide using supported palladium and gold-palladium alloy catalysts is described and some of the problems that have to be overcome are considered.
Abstract: Today hydrogen peroxide is produced by an indirect process in which an alkyl anthraquinone is sequentially hydrogenated and oxidized. In this way hydrogen and oxygen are kept separate during the manufacturing process. A process where molecular oxygen is directly hydrogenated could be preferred if control of the sequential hydrogenation can be achieved, particularly if high rates can be attained under intrinsically safe, non-explosive conditions. Herein we describe recent progress in the direct synthesis of hydrogen peroxide using supported palladium and gold-palladium alloy catalysts and consider some of the problems that have to be overcome.
TL;DR: Two high-valent arylpalladiumfluoride complexes that afford carbon-fluorine bond formation upon thermolysis are observed.
Abstract: We have observed two high-valent arylpalladiumfluoride complexes that afford carbon−fluorine bond formation upon thermolysis.
TL;DR: In this paper, the performance of PdGa and Pd3Ga7 was investigated under different acetylene hydrogenation reaction conditions, in absence and in excess of ethylene, in temperature-programmed and isothermal long-term experiments.
TL;DR: In this paper, the fabrication of bimodal nanoporous palladium with pore sizes of ∼50 and 5 nm by electrochemically dealloying a ternary Pd30Ni50P20 metallic glass was reported.
Abstract: We report the fabrication of bimodal nanoporous palladium with pore sizes of ∼50 and 5 nm by electrochemically dealloying a ternary Pd30Ni50P20 metallic glass. The nanostructured palladium is expected to have superior performances in functional applications because of its nanoporous channels for fast mass transport and chemical reactions. This study paves a new way for fabricating novel nanoporous metals by electrochemically dealloying multicomponent metallic glasses.
TL;DR: Using propiolic acid as a difunctional alkyne and using the consecutive reactions of the Sonogashira reaction and the decarboxylative coupling, unsymmetrically substituted diaryl alkynes were obtained in moderate to good yield.
TL;DR: This method features the use of a novel catalytic system consisting of 10 mol % of Pd(II), 50 mol% of Cu(I), and 2 equiv of Bu4NBr that produced variously substituted benzothiazoles in high yields with good functional group tolerance.
TL;DR: A first palladium-catalyzed intramolecular diamination of unfunctionalized terminal alkenes is reported and it is proved that the nucelophile for this step stems from the immediate coordination sphere of the palladium(IV) precursor.
Abstract: A first palladium-catalyzed intramolecular diamination of unfunctionalized terminal alkenes has recently been reported. This study investigates the details of its mechanistic course based on NMR titration, kinetic measurements competition experiments, and deuterium labeling. It concludes a two-step procedure consisting of syn-aminopalladation with an unligated palladium(II) catalyst state followed by oxidation to palladium(IV) and subsequent C−N bond formation to give the final products as cyclic diamines. Related reactions employing sulfamides give rise to aminoalkoxy-functionalization of alkenes. This process was investigated employing deuterated alkenes and found to follow an identical mechanism where stereochemistry is concerned. It exemplifies the importance of cationic palladium(IV) intermediates prior to the final reductive elimination from palladium and proves that the nucelophile for this step stems from the immediate coordination sphere of the palladium(IV) precursor. These results have importan...
TL;DR: In this paper, a series of catalysts with Au:Pd ratios varying from 9.5:0.5 to 2:8 were prepared and characterized by TEM, HRTEM, EDX, and X-ray mapping techniques to obtain morphological information, particle size distributions, crystalline structure and distribution of the two metals.
Abstract: Au nanoparticles are known to be a good catalyst or an effective promoter for a wide range of catalytic reactions. Bimetallic Au−Pd nanoparticles supported on activated carbon were synthesized following a two-step procedure: immobilization of Au sol onto activated carbon followed by immobilization of Pd(0). The catalysts showed superior activities compared to monometallic Pd or Au nanoparticles on the same support. A series of catalysts with Au:Pd ratios varying from 9.5:0.5 to 2:8 were prepared. These catalysts were characterized by TEM, HRTEM, EDX, and X-ray mapping techniques to obtain morphological information, particle size distributions, crystalline structure, and distribution of the two metals. Correlating with the result from catalytic tests of selective oxidation of glycerol to glyceric acid, we found that the surface configuration of Pd monomers isolated by Au atoms has a substantial effect on activity and stability. The Au:Pd ratio on the surface of the particles is the key parameter and can be...
TL;DR: Palladium-catalyzed methods for intermolecular aerobic oxidative amination of unactivated olefins, known as azaWacker reactions, have been recently reported to yield enamide derivatives, and these reactions are proposed to proceed by Markovnikov aminopalla-.
Abstract: The rich variety of nitrogen-containing molecules that occur as natural and synthetic products has inspired considerable interest in the development of new methods for their syntheses. Among the strategies involving the direct amination of olefins by C N bond formation, the synthesis of amine derivatives by intermolecular dioxygen-coupled oxidative amination of olefins is particularly attractive. Palladium-catalyzed methods for intermolecular aerobic oxidative amination of unactivated olefins, known as azaWacker reactions, have been recently reported to yield enamide derivatives (Scheme 1, left arrow). 6] These reactions are proposed to proceed by Markovnikov aminopalla-
TL;DR: Microwave heating greatly accelerates Pd-catalyzed decarboxylative coupling of aromatic acids and aryl iodides, and allows the coupling of benzoic acids with unactivated arenes.