Anionic Palladium Complexes Research Articles

One-pot Sonogashira–Hydroarylation reaction catalyzed by anionic palladium complexes in an aqueous medium

It was found that anionic Pd(II) complexes of type [CA]2[PdCl4] and [CA]2[Pd2Cl6] (CA = imidazolium or pyridinium cation) are effective catalysts for copper-free Sonogashira coupling in an aqueous medium without additional ligands or co-catalysts. Both types of substrates, containing electron donor or electron acceptor groups, formed the corresponding products in good to excellent yield. The most active complex, [bmpy]2[Pd2Cl6] (bmpy = 1-butyl-4-methylpyridinium cation), was employed to the hydroarylation of the Sonogashira product in a one-pot process, using microwave radiation, NaBPh4 as a phenyl source and water as a proton donor. In these reactions the presence of electron donor substituents facilitated a higher conversion of diarylacetylenes to triarylethenes than electron acceptor groups.

Palladium‐Catalyzed Direct C−H Arylation of Arenes Promoted by Quaternary Ammonium Salt

AbstractA new Pd‐based catalytic system for the direct C−H arylation of arenes was developed. The use of a substoichiometric amount of quaternary ammonium salt as an additive with a Pd source achieved the direct arylation of unactivated arenes under milder reaction conditions in moderate to high yields. The ammonium salt generates the anionic palladium complex, thereby facilitating arylation by a concerted metalation‐deprotonation process.

The role of phosphine ligands in the catalytic systems of the Heck reaction with aromatic carboxylic anhydrides

The results of a comparative study of phosphine-containing and phosphine-free catalytic systems of the Heck reaction using aromatic carboxylic anhydrides as arylating agents are presented. It was demonstrated that the patterns of diff erential selectivity of the reaction under competition of two aromatic anhydrides or two alkenes are independent of the presence of a tertiary phosphine additive in the system. It was established that palladium complexes with no phosphine ligands in their coordination sphere are catalytically active at the step of activation of aromatic carboxylic anhydride and alkene. The patterns of diff erential selectivity for regioisomers of arylated products provide the evidence of the participation of phosphine-containing anionic palladium complexes in the regioselectivity-determining step of the catalytic cycle. The data obtained are in agreement with the phosphine involvement in the catalyst transformations proceeding outside the main catalytic cycle.

Oxidative and Reductive Cross-Coupling Reactions Catalyzed by an Anionic “Ligandless” Palladium Complex

The anionic complex [NBu4][Pd(DMSO)Cl3], which can be synthesized on a gram scale in a single step starting from commercially available starting materials, has been shown to be an active catalyst in the Mizoroki–Heck reaction of aryl halides. We present two new catalytic applications of this complex: the base-free oxidative Heck reaction and the reductive homodimerization of aryl halides. This complex outperformed other palladium salts. In the latter reaction, the catalyst loading could be reduced to 0.01 mol %. The scope of the reactions has been explored, demonstrating the potential of the anionic palladium complex in these catalytic transformations.

An efficient Pd/C catalyst design based on sequential ligand exchange method for the direct synthesis of H2O2

A new catalyst preparation method involving the sequential ligand exchange of anionic palladium complexes to cationic complexes via the formation of palladium hydroxide intermediates is proposed. The novel method enables the electrostatic adsorption of an anionic palladium complex on the negatively charged carbon surface, and therefore makes it possible to obtain small and highly dispersed Pd nanoparticles. The H2O2 productivity and H2 selectivity of the ligand-exchanged catalyst were 16 and 3.6 times higher than those of the Pd/C catalyst prepared by using the same anionic palladium precursor. The strikingly different activities mainly attributed to the difference in the nanoparticle size and the oxidation state of Pd, which clearly indicates that the activity of Pd/C catalyst for the direct synthesis of H2O2 can be greatly improved by the sequential ligand exchange method.

Cationic–anionic palladium complexes: effect of hydrogen bond character on their stability and biological activity

The solution state of palladium cationic–anionic complexes (AmH n ) k [PdCl4] prepared for the first time, where Am is morpholine, methylmorpholine, aminoethylmorpholine, 5-aminovaleric acid, L-1-phenyl-2-methylaminopropanol, and m-xylilenediamine, has been studied by electronic absorption spectroscopy, NMR, and pH measurements. The agreement of obtained results for the state of the complexes in water and NaCl solutions with IR and X-ray diffraction data for these complexes has allowed us to substantiate the principle for designing patent formulation (C5H12NO)2[PdCl4], a new type of palladium complexes, palladium(II) cationic–anionic complexes showing high antitumor and antimetastatic activity. Crystallographic data for six obtained complexes have been presented.

Cross-Coupling Catalysis by an Anionic Palladium Complex

Recent studies have shown that anionic palladium complexes are viable catalysts for a range of catalytic cross-coupling reactions. We present a one-step synthesis of the anionic “ligandless” palladium complex [NBu4][Pd(DMSO)Cl3] together with its crystal structure. This compound has been shown to be an active precatalyst in the Mizoroki–Heck reaction. Under Jeffery conditions, activated aryl chlorides can be coupled in yields of up to 94% without the need of an additional ligand. The presence of a small amount of water was necessary for product formation. An Amatore–Jutand-type catalytic cycle is consistent with the results presented herein. For comparison with the known mixed complex [(pym-Im-Me)2PdCl][Pd(DMSO)Cl3], the cationic complex [(pym-Im-Me)2PdCl]PF6 (pym = pyrimidyl, Im = imidazolin-2-ylidene) has been synthesized and characterized using standard techniques.

Effect of imidazolinium salts bearing hydroxy substituents on palladium-catalysed Suzuki–Miyaura and Heck coupling reactions

Palladium anionic complexes with imidazolinium cations containing hydroxyl substituents were used as catalyst precursors in the Suzuki–Miyaura and the Heck cross-coupling. High activity of anionic complexes was noted in the Suzuki–Miyaura reaction of 2-bromotoluene with phenylboronic acid at 40 oC in 2-propanol and a 2-propanol/water mixture using conventional heating or microwaves. Similarly, bromonaphtalene and iodonaphtalene reacted efficiently with naphtylboronic acid and 4-methylnaphtylboronic acid. A remarkably lower activity was noted when anionic palladium complexes were employed in the Heck coupling of 2,3-dihydrofuran with iodobenzene. An increase in conversion in the Heck reaction was achieved when Pd(OAc)2 was used as a catalyst precursor together with imidazolinium salts as co-catalysts.

Synthesis, crystal structure, and π–π stacking magnetism of the mononuclear radical complex [N-hydrogenpyridinium][Pd(mnt)2

The mononuclear radical anionic complex [N-hydrogenpyridinium][Pd(mnt)2] (mnt = malonitrile-2,3-dithiolate) with a new countercation has been prepared and its crystal structure determined by X-ray crystallography. In the mononuclear radical anionic complex the palladium ion assumes a slightly distorted square-planar geometry and is bridged by four S atoms of two mnt anionic ligands. There are two kinds of slipped π–π stacking interactions between adjacent mononuclear radical anionic complexes in the crystal (i.e., PS-1 and PS-2). The ESR spectra, the variable-temperature susceptibilities, and the theoretical calculations reveal a very strong antiferromagnetic interaction between the adjacent mononuclear radical anionic complexes, leading to diamagnetism of the present complex. Further, the theoretical calculations indicate that the strong antiferromagnetic strength should be attributed to the PS-1 slipped π–π stacking interaction; the large spin densities on the associated short contact atoms should be a major factor leading to the strong magnetic coupling strength. This study is the first to reveal the mechanism of the strong antiferromagnetic interaction of a mononuclear radical anionic palladium complex with mnt as ligand.

Oxygen-promoted coupling of arylboronic acids with olefins catalyzed by [CA]2[PdX4] complexes without a base

An efficient method was developed for the oxidative Heck reaction between arylboronic acids and olefins catalyzed by anionic palladium complexes of the type [CA]x[PdCl4] (x=1,2) and [CA]2[Pd2Cl6] (CA=imidazolium or pyridinium cation). Molecular oxygen was employed as an environmentally benign oxidant to regenerate Pd(II) species during the reaction. The elaborated protocol could be applied to the coupling of different arylboronic acids with both electron-rich and electron-poor olefins. The catalyst system was further employed for the one-pot oxidative Heck reaction followed by the Heck coupling to build conjugated compounds in good yield.Mass spectrometry (ESI–MS) and UV–vis spectroscopy was used to identify palladium-containing complexes in the reactions. A plausible reaction mechanism of the oxidative Heck reaction was proposed.

Kinetic isotope effects for oxidation reaction of olefins by p-quinones in water-acetonitrile solutions of cationic and chloride anionic palladium complexes

Kinetic isotope effects for oxidation reactions of ethylene and cyclohexene in solutions of cationic palladium(ii) complexes in MeCN-H2O(D2O) systems, were measured. It was established that the ratio of the initial reaction rates \({{R_0^{H_2 O} } \mathord{\left/ {\vphantom {{R_0^{H_2 O} } {R_0^{D_2 O} }}} \right. \kern-\nulldelimiterspace} {R_0^{D_2 O} }} \) is equal to 1 for both reactions with the use of cationic complexes of the type Pd(MeCN) x (H2O)4−x 2+, which differs from oxidation reactions catalyzed by chloride palladium complexes in the same solutions, where the ratio \({{R_0^{H_2 O} } \mathord{\left/ {\vphantom {{R_0^{H_2 O} } {R_0^{D_2 O} }}} \right. \kern-\nulldelimiterspace} {R_0^{D_2 O} }} \) = 5.0±0.16 and 4.73±0.14 at H+ molar fraction of 0.48 and 0.16, respectively (H+ molar fraction was calculated based on the sum of [H+] and [D+]).

Solvent Effect on Palladium‐Catalyzed Cross‐Coupling Reactions and Implications on the Active Catalytic Species

Suzuki coupling of the bifunctional substrate 1 using [Pd(2)(dba)(3)]/PtBu(3) gives selectivity for C-Cl in nonpolar solvents but for C-OTf in polar solvents. The results of computational and experimental studies suggest that the catalytically active species in polar solvents under conditions employing coordinating additives is inconsistent with monoligated [Pd(PtBu(3))]. Instead, the data are consistent with an anionic palladium complex as the active species.

A unifying mechanism for all high-temperature Heck reactions. The role of palladium colloids and anionic species

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 degrees 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.

Oxidative Addition of Aryl Sulfonates to Palladium(0) Complexes of Mono- and Bidentate Phosphines. Mild Addition of Aryl Tosylates and the Effects of Anions on Rate and Mechanism

Oxidative additions of aryl halides to palladium complexes have been studied extensively in recent years, but studies on the oxidative addition of aryl sulfonates to palladium complexes are less common. Here, oxidative additions of aryl triflates to palladium(0) complexes of hindered mono- and bis(phosphines) and the rare oxidative addition of aryl tosylates to palladium(0) complexes of hindered bis(phosphines) are reported. Pd[P(o-tolyl)3]2 reacted with PhOTf in the presence of added halide salts to produce the corresponding {Pd[P(o-tolyl)3](Ar)(μ-X)}2 complexes (X = halide). The rate of oxidative addition was accelerated by the addition of coordinating anions but was not affected by the addition of weakly coordinating anions. This result suggests that the oxidative addition of aryl triflates occurs to an anionic palladium complex ligated by halide. The addition of phenyl triflate to Pd(PPF-t-Bu)[P(o-tolyl)3] in the presence of added bromide produced [(PPF-t-Bu)Pd(Ph)(Br)]. The rate of this oxidative addition was accelerated by added bromide but was more greatly accelerated by the addition of PF6-, a typically noncoordinating anion. This result indicates that a medium effect, not coordination of bromide prior to oxidative addition, accounts for the acceleration of the rate of addition of aryl triflates to Pd(PPF-t-Bu)[P(o-tolyl)3]. The oxidative addition of phenyl tosylate to Pd(PPF-t-Bu)[P(o-tolyl)3] and to Pd(CyPF-t-Bu)[P(o-tolyl)3] also occurred at room temperature to produce the corresponding phenylpalladium tosylate complexes. In the presence of added bromide, arylpalladium bromide complexes were formed. Like the addition of phenyl triflate to Pd(PPF-t-Bu)[P(o-tolyl)3], the rate of oxidative addition was accelerated by addition of either strongly or weakly coordinating anions. The additions of aryl tosylates were also faster in more polar solvents. Thus, the effect of halide on the rate of addition of aryl tosylates to Pd(PPF-t-Bu)[P(o-tolyl)3] is also more likely to result from a medium effect, rather than from coordination of anion to palladium prior to oxidative addition.

Reactivity of tetra-n-butylammonium di-µ-chloro-bis[bis(pentafluorophenyl)palladate(II)]. A general method for the synthesis of cis isomers of neutral and anionic palladium(II) complexes

The reaction of [NBun4]2[Pd2(µ-Cl)2(C6F5)4] with L (L = N, P, As, or Sb donor ligand) leads to the synthesis of novel anionic palladium complexes of the type [NBun4][PdCl(C6F5)2L], which react with L to give the neutral complexes [Pd(C6F5)2L2]. Both families of compounds are the cis derivatives. Moreover, replacement of Cl by SCN or CN, and of L (SbPh3 or AsPh3) by PPh3, takes place without a change in configuration, giving again cis isomers.