Showing papers on "Palladium published in 2006"
TL;DR: A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years as mentioned in this paper, and there are now many suggestions in the literature that narrow the scope of types of precatalyst that may be considered true catalysts in these coupling reactions.
Abstract: A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true “catalysts” in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid-supported metal ligand complexes, supported palladium nano- and macroparticles, soluble palladium nanoparticles, soluble ligand-free palladium, and palladium-exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems.
1,737 citations
967 citations
TL;DR: Experimental and computational evidence indicates that the pivalate anion is a key component in the palladium-pivalic acid cocatalyst system, that it lowers the energy of C-H bond cleavage and acts as a catalytic proton shuttle from benzene to the stoichiometric carbonate base.
Abstract: A palladium−pivalic acid cocatalyst system has been developed that exhibits unprecedented reactivity in direct arylation This reactivity is illustrated with the first examples of high yielding direct metalation−arylation reactions of a completely unactivated arene, benzene Experimental and computational evidence indicates that the pivalate anion is a key component in the palladation/C−H bond breaking event, that it lowers the energy of C−H bond cleavage and acts as a catalytic proton shuttle from benzene to the stoichiometric carbonate base Eight examples of substituted aryl bromides are included which undergo direct arylation with benzene in 55−85% yield
839 citations
TL;DR: Improvements in Catalytic Systems and Superior Reactivity of N2 as the Nucleofuge 4637 and Related Matsuda−Heck Reactions 4637.
Abstract: 2.1.4. Effect of Bases and Other Additives 4628 2.1.5. Improved Catalytic Systems 4629 2.1.6. Applications in Synthesis 4630 2.1.7. Mechanistic Studies 4632 2.1.8. Related Matsuda−Heck Reactions 4633 2.2. Suzuki−Miyaura Reaction 4634 2.2.1. Early Studies 4634 2.2.2. Modification of the Boronic Counterpart 4635 2.2.3. Improved Catalytic Systems 4637 2.2.4. Superior Reactivity of N2 as the Nucleofuge 4637
668 citations
DSM1
TL;DR: The Heck reaction takes place by attack of the arylating agent on the palladium atoms in the outer rim of the nanoparticles, which leads to formation of monomeric or dimeric anionic palladium complexes that undergo the usual steps of the Heck mechanism as described by Amatore and Jutand as mentioned in this paper.
Abstract: The Heck reaction has been the subject of intense investigation in the past decade. Many new types of catalysts have been developed in addition to the existing palladium/phosphine complexes. Prominent among these are palladacycles, pincers, several types of heterogeneous palladium catalysts, colloids and ligand-free palladium, usually in the form of Pd(OAc)2. Most of the newer types function only at higher temperatures, typically between 120 and 160 °C. It has been shown that irrespective of the catalyst precursor, none of these catalysts are stable at these high temperatures. They all have a tendency to form soluble palladium(0) colloids or nanoparticles, certainly with less reactive substrates such as aryl bromides or chlorides. The Heck reaction takes place by attack of the arylating agent on the palladium atoms in the outer rim of the nanoparticles. This leads to formation of monomeric or dimeric anionic palladium complexes that undergo the usual steps of the Heck mechanism as described by Amatore and Jutand.
558 citations
TL;DR: This communication describes the development of a new Pd-catalyzed method for the fluorination of carbon-hydrogen bonds through palladium-mediated carbon-fluorine coupling, successfully achieved under oxidative conditions using electrophilic N- fluoropyridinium reagents.
Abstract: This communication describes the development of a new Pd-catalyzed method for the fluorination of carbon−hydrogen bonds. A key step of these transformations involves palladium-mediated carbon−fluorine couplinga much sought after, but previously unprecedented, transformation. These reactions were successfully achieved under oxidative conditions using electrophilic N-fluoropyridinium reagents. Microwave irradiation in the presence of catalytic palladium acetate served as optimal conditions for the fluorination of C−H bonds in a variety of substituted 2-arylpyridine and 8-methylquinoline derivatives.
524 citations
TL;DR: The effect of various changes to the palladium catalyst during its stabilization in the course of the selective hydrogenation of ethyne-ethene mixtures (formation of hydride and carbide phases and of carbonaceous deposits) was reviewed in this paper.
Abstract: The effect of various changes to the palladium catalyst during its stabilization in the course of the selective hydrogenation of ethyne‐ethene mixtures (formation of hydride and carbide phases and of carbonaceous deposits) was reviewed. The deposits in the form of a carbonaceous overlayer on the palladium surface create sites at which selective ethyne hydrogenation to ethene can occur. The carbonaceous deposit on the support increases the selectivity to ethane formation by increasing the rate of ethene hydrogenation on support sites (spillover of hydrogen from metal to the support surface) and by decreasing the effective diffusivity of ethyne in the pores.
518 citations
TL;DR: Overall, the measurements demonstrate the particle size effect on the electronic properties of palladium that yields different catalytic activity in the HCOOH oxidation reaction.
Abstract: We report a combined X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and chronoamperometry (CA) study of formic acid electrooxidation on unsupported palladium nanoparticle catalysts in the particle size range from 9 to 40 nm. The CV and CA measurements show that the most active catalyst is made of the smallest (9 and 11 nm) Pd nanoparticles. Besides the high reactivity, XPS data show that such nanoparticles display the highest core-level binding energy (BE) shift and the highest valence band (VB) center downshift with respect to the Fermi level. We believe therefore that we found a correlation between formic acid oxidation current and BE and VB center shifts, which, in turn, can directly be related to the electronic structure of palladium nanoparticles of different particle sizes. Clearly, such a trend using unsupported catalysts has never been reported. According to the density functional theory of heterogeneous catalysis, and mechanistic considerations, the observed shifts are caused by...
465 citations
TL;DR: The high selectivity of the CuI-beta-diketone catalyst for aliphatic amines represents a useful complement to the palladium-based methods and is highly tolerant of a number of reactive functional groups.
Abstract: Through the use of cyclic beta-diketones as supporting ligands, the copper-catalyzed coupling of aryl iodides with aliphatic amines occurs at room temperature in as little as 1 h. These high reaction rates allow for the coupling of a wide range of aryl and heteroaryl iodides at room temperature. This method is highly tolerant of a number of reactive functional groups, including -Br and aromatic -NH2 as well as phenolic and aliphatic -OH. The high selectivity of the CuI-beta-diketone catalyst for aliphatic amines represents a useful complement to the palladium-based methods.
394 citations
337 citations
TL;DR: In this article, a computational study with the Becke3LYP DFT functional is carried out on the cross-coupling reaction of vinyl bromide H2CCHBr and vinylboronic acid H 2CCHB(OH)2 catalyzed by palladium diphosphine [Pd(PH3)2] in the presence of an excess of base OH-.
TL;DR: The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)(2) in the presence of dioxygen, leading to symmetrical biaryls, has been fully elucidated.
Abstract: The mechanism of the palladium-catalyzed homocoupling of arylboronic acids ArB(OH)2 (Ar = 4-Z-C6H4 with Z = MeO, H, CN) in the presence of dioxygen, leading to symmetrical biaryls, has been fully elucidated. The peroxo complex (η2-O2)PdL2 (L = PPh3), generated in the reaction of dioxygen with the Pd(0) catalyst, was found to play a crucial role. Indeed, it reacts with the arylboronic acid to generate an adduct (coordination of one oxygen atom of the peroxo complex to the oxophilic boron atom of the arylboronic acid) characterized by 31P NMR spectroscopy and ab initio calculations. This adduct reacts with a second molecule of arylboronic acid to generate trans-ArPd(OH)L2 complexes. A transmetalation by the arylboronic acid gives trans-ArPdArL2 complexes. The biaryl is then released in a reductive elimination. This reaction is at the origin of the formation of biaryls as byproducts in palladium-catalyzed Suzuki−Miyaura reactions when they are not conducted under oxygen-free atmosphere.
TL;DR: A mild, palladium-catalyzed coupling of aryl halides with ammonia or lithium amide to form primary arylamines as the major product is described and the first organopalladium complex with a terminal -NH2 ligand has been isolated.
Abstract: A mild, palladium-catalyzed coupling of aryl halides with ammonia or lithium amide to form primary arylamines as the major product is described. These reactions occurred with excellent selectivity for formation of the primary arylamine over formation of the diarylamine (9.5:1 to over 50:1 ratios of arylamine to diarylamine). In addition, the first organopalladium complex with a terminal −NH2 ligand has been isolated. This complex reductively eliminates to form arylamines.
TL;DR: Terminal substitution at the allyl moiety of the palladium complex facilitates its activation at room temperature leading to very active catalytic species enabling the present catalytic transformations to be performed rapidly using very mild reaction conditions.
Abstract: The use of second-generation [(NHC)Pd(R-allyl)Cl] complexes for Suzuki-Miyaura and Buchwald-Hartwig cross-coupling reactions involving heteroaromatic halides at room temperature is reported. The first examples of room temperature Suzuki-Miyaura cross-coupling of deactivated aryl chlorides with alkenyl boronic acids are also disclosed. Terminal substitution at the allyl moiety of the palladium complex facilitates its activation at room temperature leading to very active catalytic species enabling the present catalytic transformations to be performed rapidly using very mild reaction conditions. Catalyst loadings can be as low as 10 ppm for the Buchwald-Hartwig aryl amination and 50 ppm for the Suzuki-Miyaura reaction.
TL;DR: In this paper, the structure sensitivity of 1-pentyne over various palladium catalysts was studied under various conditions, and the effect of carbon dissolution in the crystal lattice near the surface was evidenced by high-resolution transmission electron microscopy.
TL;DR: The results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions.
Abstract: To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH4 in alkaline media, the kinetics of the reaction has been investigated by 11B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH4- concentration in the range 0.05−0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03−0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH4 and its perdeuterated analogue NaBD4 in H2O, in D2O and H2O/D2O mixtures. When the process was carried out in D2O, deuterium incorporation in BH4- afforded BH4-nDn- (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hyd...
TL;DR: Palladium-catalyzed ligand directed C-H activation/halogenation reactions have been extensively explored in this paper, where both the nature of the directing group and the substitution pattern on the arene ring of the substrate lead to different reactivity profiles, and often different and complementary products, in the presence and absence of the catalyst.
TL;DR: TEM coupled with EDX analysis indicate the formation of Pd nanoparticles within the immobilized IL layer and the application of the resulting catalyst in the Heck reaction of a variety of different haloarenes is described.
TL;DR: The cross-coupling reaction of aryl chlorides with aliphatic and aromatic thiols catalyzed by palladium complexes of the strongly binding bisphosphine CyPF-tBu ligand is reported, with excellent yields, broad scope and high tolerance of functional groups.
Abstract: The cross-coupling reaction of aryl chlorides with aliphatic and aromatic thiols catalyzed by palladium complexes of the strongly binding bisphosphine CyPF-tBu ligand (1) is reported. Most of the reactions catalyzed by complexes of ligand 1 occur with turnover numbers that exceed those of previous catalysts by two orders of magnitude. The reactions occur with excellent yields, broad scope and high tolerance of functional groups. Coupling of aryl halides with thiols in the presence of low loadings of catalysts derived from other Josiphos type ligands, as well as ligands of other structural types, are also described.
TL;DR: Spectroscopic studies and accompanying kinetic data demonstrate that an apparent positive order in the concentration of amine during reactions of secondary amines is best attributed to catalyst decomposition, and reactions catalyzed by [Pd(BINAP)2] with added BINAP are found to be first- order in bromoarene and inverse first-order in ligand, in contrast to previous work indicating zero-order kinetics in both.
Abstract: Two previous mechanistic studies of the amination of aryl halides catalyzed by palladium complexes of 1,1'-binaphthalene-2,2'-diylbis(diphenylphosphine) (BINAP) are reexamined by the authors of both studies. This current work includes a detailed study of the identity of the BINAP-ligated palladium complexes present in reactions of amines with aryl halides and rate measurements of these catalytic reactions initiated with pure precatalysts and precatalysts generated in situ from [Pd2(dba)3] and BINAP. This work reveals errors in both previous studies, and we describe our current state of understanding of the mechanism of this synthetically important transformation. 31P NMR spectroscopy shows that several palladium(0) species are present in the catalytic system when the catalyst is generated in situ from [Pd2(dba)3] and BINAP, and that at least two of these complexes generate catalytic intermediates. Further, these spectroscopic studies and accompanying kinetic data demonstrate that an apparent positive order in the concentration of amine during reactions of secondary amines is best attributed to catalyst decomposition. Kinetic studies with isolated precatalysts show that the rates of the catalytic reactions are independent of the identity and the concentration of amine, and studies with catalysts generated in situ show that the rates of these reactions are independent of the concentration of amine. Further, reactions catalyzed by [Pd(BINAP)2] with added BINAP are found to be first-order in bromoarene and inverse first-order in ligand, in contrast to previous work indicating zero-order kinetics in both. These data, as well as a correlation between the decay of bromobenzene in the catalytic reaction and the predicted decay of bromobenzene from rate constants of studies on stoichiometric oxidative addition, are consistent with a catalytic process in which oxidative addition of the bromoarene occurs to [Pd(BINAP)] prior to coordination of amine and in which [Pd(BINAP)2], which generates [Pd(BINAP)] by dissociation of BINAP, lies off the cycle. By this mechanism, the amine and base react with [Pd(BINAP)(Ar)(Br)] to form an arylpalladium amido complex, and reductive elimination from this amido complex forms the arylamine.
TL;DR: To overcome catalyst poisoning associated with some N-oxide substrates, a benefical effect of copper(I) salts was uncovered, and it is demonstrated that the N- oxide functionality can be removed easily after cross-coupling or transformed into a wide range of other functional groups.
Abstract: The use of palladium-catalyzed cross-coupling reactions in biaryl synthesis is linked to, and limited by, the synthetic and commercial availability of organometallic reagents such as aryl boronic acids. Not only are most of these compounds expensive, important classes of aryl organometallic are very challenging prepare and/or use in cross-coupling reactions, including electron-deficient nitrogen heterocycles. The importance of these building blocks in medicinal chemistry and materials sciences has prompted continued methodological efforts, and two very recent reports highlight the importance of this goal. Absent from these and the predominance of reports are some of the most problematic organometallic reagents—azines bearing the metal adjacent to a nitrogen atom. The problem is even more severe with diazines (Scheme 1). These organometallic compounds are unstable, can rarely be isolated, and commonly decompose under crosscoupling reaction conditions. While some are commercially available, the price reflects both their value and the challenge associated with their preparation. Recently, the potential of direct arylation as an efficient alternative to standard cross-couplings is becoming recognized, and we have been studying the use of N-oxides as replacements for unstable/unreactive organometallics. In the context of this strategy, diazine N-oxides are more challenging than simple pyridine N-oxides since they possess a free nitrogen atom that could bind and poison the catalyst. They are also more p-electron-deficient and less nucleophilic than pyridine N-oxides. Herein we describe the establishment of reaction conditions that enable the use of readily available, benchstable diazine N-oxides as cheap, high-yielding reagents in palladium-catalyzed cross-coupling reactions (Scheme 1). To overcome catalyst poisoning associated with some N-oxide substrates, a benefical effect of copper(I) salts was uncovered, and we demonstrate that the N-oxide functionality can be removed easily after cross-coupling or transformed into a wide range of other functional groups. These new reactions can be performed with aryl iodides, bromides, and chlorides and include the first examples of N-oxide arylation with equimolar ratios of the two coupling partners. Furthermore, the relative reactivities and regioselectivities point to C–H acidity as a critical factor in reactivity, encouraging consideration of this property in the design of other novel direct arylation processes. Initial reaction screens with N-oxides 1–3 under previously described conditions led to disappointing results. Noting that the N-oxides were only sparingly soluble in toluene, we reinvestigated the reaction conditions. We found that changing the solvent from toluene to dioxane provides superior conversions with N-oxides 1 and 3, giving the cross-coupled products 4 and 5 in yields of 75% and 72%, respectively (Table 1, entries 1 and 2). These two substrates are actually more reactive than pyridine N-oxide, as demonstrated by a competition experiment between 1 and pyridine N-oxide, which resulted in exclusive arylation of 1 (Table 1, entry 4). In contrast to the excellent results obtained with 1 and 3, the reaction of pyrimidine N-oxide (2) proceeds in very low yield (Table 1, entry 3). Further investigations revealed that the poor outcomes associated with 2 do not result from low reactivity alone. For example, the addition of pyrimidine N-oxide (2) to a reaction with pyrazine N-oxide (1) results in the exclusive formation of 4 but in a significantly lower yield (9%) than when the reaction was performed in the absence of 2 (75% yield; Table 1, entries 5 and 1). The reason for catalyst inhibition with 2 and not with 1 or 3 is a focus of ongoing study. We note that neither pyridine N-oxide nor pyridine poison the reaction of 1 (Table 1, entries 4 and 6), indicating that these deleterious effects are specific to pyrimidine N-oxide. To overcome catalyst inhibition, a variety of additives were investigated including phosphines, halides, and metals. Scheme 1. Direct arylation in aryl diazine synthesis. Pyrazine, pyrimidine, and pyridazine boronic acids are unstable, difficult to prepare, and unsuited for cross-coupling reactions. Their N-oxides are readily prepared, bench-stable replacements for the organometallic reagent in biaryl synthesis.
TL;DR: The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution and it has been shown that the reduction of Pd( II) to metallic Pd has been successfully achieved and that the obtained suspensions have a long shelf life.
Abstract: The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations of palladium(II) nitrate in ethylene glycol and poly(vinylpyrrolidone) (PVP). The resulting mixtures were irradiated with ultrasonic 50 kHz waves in a glass vessel for 180 min. The UV-visible absorption spectroscopy and pH measurements revealed that the reduction of Pd(II) to metallic Pd has been successfully achieved and that the obtained suspensions have a long shelf life. The protective effect of PVP was studied using Fourier transform infrared (FT-IR) spectroscopy. It has been found that, in the presence of ethylene glycol, the stabilization of the nanoparticles results from the adsorption of the PVP chain on the palladium particle surface via the coordination of the PVP carbonyl group to the palladium atoms. The effect of the initial Pd(II) concentration on the Pd nanoparticle morphology has been investigated by transmission electron microscopy. It has been shown that the increase of the Pd(II)/PVP molar ratio from 0.13 x 10(-3) to 0.53 x 10(-3) decreases the number of palladium nanoparticles with a slight increase in particle size. For the highest Pd(II)/PVP value, 0.53 x 10(-3), the reduction reaction leads to the unexpected smallest nanoparticles in the form of aggregates.
TL;DR: These palladium complexes catalyze the efficient C-H arylation of SEM-protected azole heteroarenes and thus provide a good method for preparation of a wide range of arylated free (NH)-azoles including pyrroles, indoles, imidazoles, and imidazo[1,2-a]pyridines.
TL;DR: In this paper, the average particle size of Pt and Pd nanoparticles, which are uniformly dispersed on carbon, is 4 and 5 nm, respectively, and they exhibit four diffraction peaks that are indexed to the {1 1/1/1}, {2 0/0, {2 2/2/0}, {3 2/3/1} planes of Pd and Pt, respectively.
TL;DR: It is shown that both a random distribution of palladium nanoparticles supported on a BDD electrode or a palladium plated BDD microelectrode array can each provide a sensing platform for the electrocatalytic detection of hydrazine.
Abstract: We show that both a random distribution of palladium nanoparticles supported on a BDD electrode or a palladium plated BDD microelectrode array can each provide a sensing platform for the electrocatalytic detection of hydrazine. The palladium nanoparticle modified electrode displays a sensitivity and limit of detection of 60 mA mol−1 L and 2.6 µM respectively while the array has a sensitivity of 8 mA mol−1 L with a detection limit of 1.8 µM. The beneficial cost implications of using palladium nano- or micro-particles in sensors compared to a palladium macroelectrode are evident. Interestingly the array of the nanoparticles shows similar sensitivity and limit of detection to the microelectrode array which probably indicates that the random distribution of the former leads to ‘clumps’ of nanoparticles that effectively act as microelectrodes.
TL;DR: In this paper, the enantioselective Pd-catalyzed α-arylation of N-Boc-pyrrolidine was used for the synthesis of functionalized aryl halides.
Abstract: This communication discloses the first instance of the enantioselective Pd-catalyzed α-arylation of N-Boc-pyrrolidine. The methodology relies on Beak's sparteine-mediated, enantioselective deprotonation of N-Boc-pyrrolidine to form the 2-pyrrolidinolithium specices in high enantiomeric ratio (er). Transmetalation of this intermediate with zinc chloride generates the stereochemically rigid, 2-pyrrolidinozinc reagent, which was readily coupled to a variety of functionalized aryl halides at room temperature using a catalyst generated from Pd(OAc)2 and PtBu3−HBF4. A diverse array of 2-aryl-N-Boc-pyrrolidines was synthesized using this methodology, providing adducts consistently in a 96:4 er. A survey of the stoichiometry revealed that as little as 0.3 equiv of zinc could be used in the coupling reaction, and the 2-pyrrolidinozinc reagent was found to exhibit stereochemical stability up to 60 °C. The method allows for the most convergent and reliable preparation of a broad range of functionalized 2-aryl-N-Boc-...
TL;DR: In this article, a general and mild protocol of oxygen-promoted Pd(II) catalysis resulting in the selective cross-couplings of alkenyl- and arylboron compounds with various olefins was developed.
Abstract: We report herein the development of a general and mild protocol of oxygen-promoted Pd(II) catalysis resulting in the selective cross-couplings of alkenyl- and arylboron compounds with various olefins. Unlike most cross-coupling reactions, this new methodology works well even in the absence of bases, consequently averting undesired homo-couplings. Nitrogen-based ligands including dimethyl-phenanathroline enhance reactivities and offer a highly efficient and stereoselective methodology to overcome challenging substrate limitations. For instance, oxidative palladium(II) catalysis is effective with highly substituted alkenes and cyclic alkenes, which are known to be incompatible with other known catalytic conditions. Most examined reactions progressed smoothly to completion at low temperatures and in short times. These interesting results provide mechanistic insights and utilities for a new paradigm of palladium catalytic cycles without bases.