Palladium Trifluoroacetate Research Articles

Synthesis of Ultrahigh Molecular Weight Poly(methyl Methacrylate) via the Polymerization of MMA Initiated by the Combination of Palladium Carboxylates with Thiols.

A novel synthesis of ultrahigh molecular weight poly(methyl methacrylate) (PMMA) using organosulfur compounds combined with a catalytical amount of transition metal carboxylates as an initiator has been developed. The combination of 1-octanethiol with palladium trifluoroacetate (Pd(CF3COO)2) was found to be a very efficient initiator for the polymerization of methyl methacrylate (MMA). An ultrahigh molecular weight PMMA with a number-average molecular weight of 1.68 × 106 Da and a weight-average molecular weight of 5.38 × 106 Da has been synthesized at the optimal formulation of [MMA]:[Pd(CF3COO)2]:[1-octanethiol] = 94,300:8:23 at 70 °C. A kinetic study showed that the reaction orders with respect to Pd(CF3COO)2, 1-octanethiol, and MMA are 0.64, 1.26, and 1.46, respectively. A variety of techniques such as proton nuclear magnetic resonance spectroscopy (1H NMR), electrospray ionization mass spectroscopy (ESI-MS), size exclusion chromatography (SEC), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electron paramagnetic resonance spectroscopy (EPR) were employed to characterize the produced PMMA and palladium nanoparticles (Pd NPs). The results revealed that Pd(CF3COO)2 was firstly reduced by the excess of 1-octanethiol to form Pd NPs at the early stage of the polymerization, followed by the adsorption of 1-octanethiol on the surface of nanoparticles and subsequent generation of corresponding thiyl radicals to initiate the polymerization of MMA.

Why does the synthesis of N-phenylbenzamide from benzenesulfinate and phenylisocyanate via the palladium-mediated Extrusion–Insertion pathway not work? A mechanistic exploration

The gas-phase extrusion–insertion (ExIn) reactions of the palladium complexes [(phen)nPd(O2SC6H5)]+ (phen = 1,10-phenanthroline, n = 1 or 2), were investigated in the gas phase by multistage mass spectrometry (MSn) experiments consisting of electrospray ionisation and a linear ion trap combined with density functional theory (DFT) calculations. Desulfination of palladium sulfinate cations under collision-induced dissociation (CID) generates the organopalladium intermediates [(phen)nPd(C6H5)]+. Of these two organometallic cations, only [(phen)Pd(C6H5)]+ reacts with phenyl isocyanate via insertion to yield [(phen)Pd(NPhC(O)C6H5)]+. The formation of a coordinated amidate anion is supported by DFT calculations. In exploring this reactivity in the solution phase, we found that heating a mixture of benzenesulfinic acid, phenylisocyanate and palladium trifluoroacetate under a range of different conditions (ligand free versus with ligand, different solvents, addition of acid or base) failed to lead to the formation N-phenyl-benzamide in all cases. Instead, biphenyl was formed and could be isolated in a yield of 46%. DFT calculations using a solvent continuum reveal that the barrier associated with the insertion reaction lies above the competing sequential reactions of desulfination of a second phenyl sulfinate followed by reductive elimination of biphenyl.

Ligand Rearrangement Leads to Tetrahydrothiophene-Functionalized N,S-Heterocyclic Carbene Palladium(II) Complexes.

Tetrahydrothiophene-functionalized N,S-heterocyclic carbene palladium(II) complexes are synthesized through an unexpected rearrangement that proceeds with palladium(II) trifluoroacetate and not with palladium(II) acetate, palladium(II) bromide, or palladium(II) chloride. A series of these complexes were isolated and characterized by X-ray crystallography. The mechanism of formation of these [3.2.1]-palladabicycles was explored, and the catalytic capabilities of these complexes were demonstrated in representative C-C coupling reactions.

Studies of hydrogen isotope scrambling during the dehalogenation of aromatic chloro-compounds with deuterium gas over palladium catalysts.

Catalytic dehalogenation of aromatic halides using isotopic hydrogen gas is an important strategy for labelling pharmaceuticals, biochemicals, environmental agents and so forth. To extend, improve and further understand this process, studies have been carried out on the scrambling of deuterium isotope with protium during the catalytic deuterodehalogenation of model aryl chlorides using deuterium gas and a palladium on carbon catalyst in tetrahydrofuran solution. The degree of scrambling was greatest with electron-rich chloroarene rings. The tetrahydrofuran solvent and the triethylamine base were not the source of the undesired protium; instead, it arose, substantially, from the water content of the catalyst, though other sources of protium may also be present on the catalyst. Replacement of the Pd/C catalyst with one prepared in situ by reduction of palladium trifluoroacetate with deuterium gas and dispersed upon micronised polytetrafluoroethylene led to much reduced scrambling (typically 0-6% compared with up to 40% for palladium on carbon) and to high atom% abundance, regiospecific labelling. The improved catalytic system now enables efficient polydeuteration via the dehalogenation of polyhalogenated precursors, making the procedure viable for the preparation of MS internal standards and, potentially, for high specific activity tritium labelling.

Fabrication of Nanoreactors Based on End-Functionalized Polymethacrylate and Their Catalysis Application

Polymer-based nanoreactor, which can be achieved by self-assembling of amphiphilic polymers in water, has attracted more and more attention in recent years. In this study, a kind of Pd-nanoreactor was fabricated by self-assembling of chiral palladium complex-containing amphiphilic polymers in water. Firstly, chain transfer reagent (CTA) CEPA-PyOx was obtained by condensation reaction of (S,S)-4-hydroxymethyl-5-phenyl-2-(2′-pyridinyl)-1,2-oxazoline (PyOx) and 4-cyano-4-[((ethylthio)carbonothioyl)thio] pentanoic acid (CEPA) in the presence of EDC·HCl. Next, polymethacrylate with a terminal chiral oxazoline ligand (CEPA-POEGMA36-PyOx) was synthesized through Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization. After removal of the thioester bond and coordination with palladium trifluoroacetate, the polymer self-assembled in water affording Pd-nanoreactor, which possess core-shell structure and with chiral palladium complex in the core. At last, the nanoreactor was successfully applied in the asymmetric catalytic synthesis of flavanone in aqueous phase, and the catalyst loading was greatly reduced compared to unsupported palladium complex. In the presence of low palladium loading (0.5 mol%), flavanone was successfully synthesized with excellent yield (98%) and high enantioselectivity (82%).

From Pincer to Paddlewheel: C-H and C-S Bond Activation at Bis(2-pyridylthio)methane by Palladium(II).

The bis(2-pyridylthio)methanidopalladium(II) pincer complex (1), containing a Pd-C bond, was obtained from the reaction of bis(2-pyridylthio)methane (H2L) with palladium(II) acetate in toluene under reflux. When palladium(II) trifluoroacetate was used, H2L reacted to generate the tetrakis(pyridine-2-thiol)palladium(II) complex (2). Complex 2 was converted to a heterobimetallic palladium(II)-iron(II) paddlewheel complex (3) upon treatment with iron(II) triflate in the presence of a base in acetonitrile at room temperature.

Palladium‐Catalyzed Regioselective Synthesis of 1‐Benzoazepine Carbonitriles from o‐Alkynylanilines via 7‐endo‐dig Annulation and Cyanation

AbstractWe herein report a three‐component, one‐pot cascade reaction involving an imination/annulation/cyanation sequence for the synthesis of 1‐benzoazepine carbonitrile derivatives using readily available o‐alkynylanilines, cyclic ketones and trimethylsilyl cyanide. This regio‐ and stereoselective reaction was achieved by combining palladium(II) trifluoroacetate and copper(II) acetate in dimethyl sulfoxide. The important features of this method include a broad substrate scope, the use of trimethylsilyl cyanide as a cyanating agent, the formation of two C−C bonds and one C−N bond, mild reaction conditions and good product yields.magnified image

Palladium-Catalyzed Cross-Coupling of 1,3,4-Oxadiazoles and Styrenes: An Efficient Method to Synthesize 2-Alkenyl-1,3,4- Oxadiazoles§

2-Alkenyl-1,3,4-oxadiazole derivatives have been prepared by cross-coupling of 1,3,4-oxadiazoles with styrenes by using palladium(II) trifluoroacetate [Pd(TFA)2] as a catalyst, 1,10-phenanthroline as a ligand, silver trifluoroacetate as an oxidant, and toluene as a solvent. The products were formed in high yields and no byproducts were detected.

Enantioselective, Palladium-Catalyzed Conjugate Additions of Arylboronic Acids to Form Bis-benzylic Quaternary Stereocenters.

We report enantioselective, palladium-catalyzed conjugate additions of arylboronic acids to β-aryl, β,β-disubstituted enones to generate ketones containing bis-benzylic quaternary stereocenters. A catalyst generated from palladium trifluoroacetate and (S)-4-tert-butyl-2-(2-pyridyl)oxazoline ligand ((S)-t-BuPyOx) promotes conjugate additions of a wide range of arylboronic acids to a variety of β-aryl, β,β-disubstituted enones. Iterative addition of the arylboronic acid to minimize undesired protodeboronation pathways leads to efficient formation of the corresponding ketones containing bis-benzylic quaternary stereocenters in up to 92% yield and up to 93% enantioselectivity.

Palladium-Catalyzed C–H Activation: Mass Spectrometric Approach to Reaction Kinetics in Solution

We report a new method for determination of rate constants of processes in solution using electrospray ionization mass spectrometry (ESI-MS). The investigated reaction is C–H activation of acetanilides by palladium(II)trifluoroacetate leading to stable organopalladium complexes. The rate constants can be determined from an experiment with a couple of differently substituted acetanilides being in competition for being activated by the palladium salt. The formed organopalladium complexes can be detected by ESI-MS. The time dependence is achieved by adding one of the acetanilides to the reaction mixture with a time delay. The kinetics can be then evaluated from the evolution of the ratio of the ESI-MS signals of differently substituted complexes as a function of the time delay. The Hammett analysis of the rate constants obtained for a series of meta- and para-substituted acetanilides provides a ρ value of −1.5, which is in agreement with values reported for similar C–H activations. We have also investigated ...

Palladium-catalyzed conjugate addition of arylboronic acids to 2-substituted chromones in aqueous media

Palladium-catalyzed conjugate additions of arylboronic acids to 2-alkylchromones are reported. The conjugate additions occur in aqueous media in the presence of a catalyst generated from palladium trifluoroacetate and 1,10-phenanthroline, and the flavanone derivatives containing a fully-substituted carbon center are formed in up to 90% yield.

ChemInform Abstract: Palladium Catalyzed C‐Arylation of Amino Acid Derived Hydantoins.

AbstractVarious aryl iodides are suitable reactants under conditions A) to produce the desired 5,5‐disubstituted hydantoin structures.

Kinetic Studies on the Palladium(II)-Catalyzed Oxidative Cross-Coupling of Thiophenes with Arylboron Compounds and Their Mechanistic Implications.

Reaction orders for the key components in the palladium(II)-catalyzed oxidative cross-coupling between phenylboronic acid and ethyl thiophen-3-yl acetate were obtained by the method of initial rates. It turned out that the reaction rate not only depended on the concentration of palladium trifluoroacetate (reaction order: 0.97) and phenylboronic acid (reaction order: 1.26), but also on the concentration of the thiophene (reaction order: 0.55) and silver oxide (reaction order: -1.27). NMR spectroscopy titration studies established the existence of 1:1 complexes between the silver salt and both phenylboronic acid and ethyl thiophen-3-yl acetate. A low inverse kinetic isotope effect (kH /kD =0.93) was determined upon employing the 4-deuterated isotopomer of ethyl thiophen-3-yl acetate and monitoring its reaction to the 4-phenyl-substituted product. A Hammett analysis performed with para-substituted 2-phenylthiophenes gave a negative ρ value for oxidative cross-coupling with phenylboronic acid. Based on the kinetic data and additional evidence, a mechanism is suggested that invokes transfer of the phenyl group from phenylboronic acid to a 1:1 complex of palladium trifluoroacetate and thiophene as the rate-determining step. Proposals for the structure of relevant intermediates are made and discussed.

Palladium Catalyzed C-Arylation of Amino Acid Derived Hydantoins.

Palladium(II) trifluoroacetate (5 mol %) catalyzes the C-arylation of N,N-disubstituted hydantoins by aryl iodides in good yield. The reaction proceeds through base-promoted enolization of the amino acid derived hydantoins, and the resulting 5,5-disubstituted hydantoins may be deprotected at one or both N atoms to yield biologically active structures or alternatively hydrolyzed to the parent α-aryl α-amino acids. The reaction is successful with a variety of parent amino acids and a range of electron-rich and electron-poor aryl iodides.

Surface engineering and palladium dispersion in MCM-41 for methane oxidation

Highly dispersed palladium oxide nanoparticles (Ø∼2nm) were obtained in mesostructured silica of MCM-41 type from divalent palladium complexes tethered from grafted propylamine ligands themselves highly dispersed using a surface engineering approach based on a molecular stencil patterning technique (MSP). A partial surface coverage obtained by incomplete extraction of the surfactant produces a self-assembled and patterned molecular mask that is fixed by silanol capping. After removal of the masking surfactant, the unreacted silanol groups are reacted with 3-(aminopropyl) trimethoxysilane ligands, which are further metallated using an acetonitrile solution of palladium trifluoroacetate Pd(O2CCF3)2. Then, calcination yields PdO nanoparticles of ca. 2 or 6nm depending of the masking percentage of the surface. In some case, calcination was accompagnied by a partial collapse that is undergone by the structure. The structural and textural characteristics of these materials were studied by means of different physico-chemical techniques such as XRD, N2 sorption isotherms, TGA, elemental analysis (EA) by ICP, 29Si-NMR, UV–visible, FT-IR, and Raman spectroscopies. Light-off reactivity curves of methane combustion reveals that these catalysts possess no external PdO phase and follow the gold rule of higher activity for higher active phase dispersion. Eventually, intermediate structural collapse that decreases the molecular diffusion limitation may invert this tendency.

Palladium-Catalyzed Oxidative Heck Coupling of Vinyl Pyridines with Aryl Boronic Acids

An efficient methodology has been developed for the oxidative cross-coupling of vinyl pyridine with various boronic acids catalyzed by palladium. In this reaction, vinyl pyridines reacted with various aryl boronic acids in the presence of 10 mol% palladium(II) trifluoroacetate, 10 mol% 1,10- phenanthroline, and 1 equivalent silver(I) oxide, to give the corresponding aryl vinyl pyridine products as a single stereoisomer, with N,N-dimethylformamide as the solvent and under 1 atmosphere of oxygen gas. The aryl vinyl pyridine products were obtained in moderate to good yields after 24 hours. A mechanism for the reaction is proposed.

Selective Aryl α‐Diimine/Palladium‐Catalyzed Bis‐Alkoxy‐ carbonylation of Olefins for the Synthesis of Substituted Succinic Diesters

AbstractAryl α‐diimine derivatives have been used, for the first time, as efficient new ligands for the palladium‐catalyzed oxidative bis‐alkoxycarbonylation reaction of olefins. The most active catalyst was formed in situ from bis(9‐anthryl)‐2,3‐dimethyl‐1,4‐diazabutadiene and palladium(II) trifluoroacetate [Pd(TFA)2]. This catalytic system was able to selectively convert olefins into succinic diesters in good yields (up to 97%) and low catalyst loading (up to 0.5 mol%) under mild reaction conditions [4 bar of carbon monoxide (CO) at 20 °C in the presence of p‐toluenesulphonic acid as additive and p‐benzoquinone as oxidant]. The optimized conditions could be successfully applied to both aromatic and aliphatic olefins, by using methanol, benzyl alcohol or isopropyl alcohol as nucleophiles.magnified image

Mechanism and Enantioselectivity in Palladium-Catalyzed Conjugate Addition of Arylboronic Acids to β-Substituted Cyclic Enones: Insights from Computation and Experiment

Enantioselective conjugate additions of arylboronic acids to β-substituted cyclic enones have been previously reported from our laboratories. Air- and moisture-tolerant conditions were achieved with a catalyst derived in situ from palladium(II) trifluoroacetate and the chiral ligand (S)-t-BuPyOx. We now report a combined experimental and computational investigation on the mechanism, the nature of the active catalyst, the origins of the enantioselectivity, and the stereoelectronic effects of the ligand and the substrates of this transformation. Enantioselectivity is controlled primarily by steric repulsions between the t-Bu group of the chiral ligand and the α-methylene hydrogens of the enone substrate in the enantiodetermining carbopalladation step. Computations indicate that the reaction occurs via formation of a cationic arylpalladium(II) species, and subsequent carbopalladation of the enone olefin forms the key carbon-carbon bond. Studies of nonlinear effects and stoichiometric and catalytic reactions of isolated (PyOx)Pd(Ph)I complexes show that a monomeric arylpalladium-ligand complex is the active species in the selectivity-determining step. The addition of water and ammonium hexafluorophosphate synergistically increases the rate of the reaction, corroborating the hypothesis that a cationic palladium species is involved in the reaction pathway. These additives also allow the reaction to be performed at 40 °C and facilitate an expanded substrate scope.

ChemInform Abstract: Regiocontrolled Aerobic Oxidative Coupling of Indoles and Benzene Using Pd Catalysts with 4,5‐Diazafluorene Ligands.

AbstractThe oxidative coupling of indoles with benzene in the presence of palladium(II) trifluoroacetate leads to 3‐substituted indoles (IV) as main reaction products, whilst the use of palladium(II) pivalate favors the formation of 2‐substituted products (III), in general.

Pd-Catalyzed Asymmetric Hydrogenation of C=C Bond of α,β-Unsaturated Ketones

Homogenous palladium-catalyzed asymmetric hydrogenation of C=C double bond of alpha,beta-unsaturated ketones has been developed by using palladium(II) trifluoroacetate/(S)-7,7'-bis[ di(4-methoxyphenyl]phosphino)-1,1'-spirobiindane complex [Pd(OCOCF(3))(2)-(S)-An-SDP] as the catalyst under ambient hydrogen pressure and room temperature with up to 89% ee.