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Masahiko Arai - One of the best experts on this subject based on the ideXlab platform.

  • the leaching and re deposition of metal species from and onto conventional supported Palladium catalysts in the heck reaction of iodobenzene and methyl acrylate in n methylpyrrolidone
    Journal of Molecular Catalysis A-chemical, 2002
    Co-Authors: Fengyu Zhao, Masayuki Shirai, Yutaka Ikushima, Masahiko Arai

    When supported Palladium catalysts are used for Heck vinylation of iodobenzene with methyl acrylate in N-methylpyrrolidone (NMP) in the presence of triethylamine and sodium carbonate bases, the reaction proceeds homogeneously with dissolved active Palladium species that are formed through coordination of NMP and triethylamine with Palladium. These active species easily react with iodobenzene (oxidative addition), beginning the catalytic cycle of Heck coupling. The last step of catalyst regeneration takes place with the action of sodium carbonate. The active Palladium species are not stable and deposit the metal to support when they cannot find iodobenzene to react in the reaction mixture after this substrate is completely consumed. The re-deposition of Palladium occurs on the surfaces of bare support and/or Palladium particles remaining on it, depending on the nature of support surface and the number and size of residual metal particles. The growth of Palladium particles has been observed after the reuse of catalyst in some case. However, the supported catalysts are recyclable without loss of activity.

  • Palladium catalyzed homogeneous and heterogeneous heck reactions in nmp and water mixed solvents using organic inorganic and mixed bases
    Journal of Molecular Catalysis A-chemical, 2000
    Co-Authors: Fengyu Zhao, Masayuki Shirai, Masahiko Arai

    The use of mixed solvent of N-methylpyrrolidone (NMP) and water and of mixed base of triethylamine and sodium carbonate was effective for promoting the rate of homogeneous Heck reaction of iodobenzene and methyl acrylate with Palladium acetate as catalyst in the absence of any ligands. It was observed that Palladium precipitated and formed colloidal particles during the reaction and this was promoted by the presence of water. However, the reaction proceeded with Palladium species dissolved in the solvent, which were more active than those in pure NMP solvent. When the same Heck reaction was conducted with an activated carbon-supported Palladium catalyst, the presence of water was also effective for promoting the rate of reaction. It was observed that Palladium was leached out from the support and the dissolved Palladium species were also active species for this heterogeneous reaction. The presence of water seemed to suppress the leaching of Palladium into the solvent and/or enhance the formation of colloidal Palladium particles. The concentration of active Palladium species was smaller in the mixed solvent than in pure NMP solvent. However, these were more active, resulting in the larger rate of reaction in the presence of water.

Benno Bildstein - One of the best experts on this subject based on the ideXlab platform.

  • efficient telomerization of 1 3 butadiene with alcohols in the presence of in situ generated Palladium 0 carbene complexes
    Journal of Molecular Catalysis A-chemical, 2002
    Co-Authors: Ralf Jackstell, Matthias Beller, Anja C Frisch, Dirk Rottger, Michael Malaun, Benno Bildstein

    The Palladium-catalyzed telomerization of 1,3-butadiene with alcohols has been studied in presence of Palladium and imidazolium salts, which form in situ carbene ligands. Among the different imidazolium salts tested 1,3-dimesitylimidazolium chloride (7), 1,3-bis(ferrocenylmethyl)benzimidazolium tetraphenylborate (12) and 1,3-bis(2-ferrocenylethyl)benzimidazolium bromide (13) gave the best yields of the desired octadienyl ethers. Significantly improved regioselectivities for the linear octadienylethers are obtained in the reaction of 1,3-butadiene and methanol compared to the previously optimized Palladium/triphenylphosphine catalyst. Using n-butanol and iso-propanol the Palladium carbene catalysts lead to a considerable increase in the corresponding telomerization products compared to standard Palladium/triphenylphosphine catalysts.

Klaus Kohler - One of the best experts on this subject based on the ideXlab platform.

  • Palladium leaching dependent on reaction parameters in suzuki miyaura coupling reactions catalyzed by Palladium supported on alumina under mild reaction conditions
    Journal of Catalysis, 2010
    Co-Authors: Saeeda S Soomro, Farzana Latif Ansari, Konstantinos Chatziapostolou, Klaus Kohler

    Abstract Suzuki couplings of aryl bromides as well as activated aryl chlorides can be efficiently performed by Palladium supported on alumina under mild reaction conditions ( T  = 65 °C). The catalyst prepared by controlled precipitation of Palladium hydroxide shows a high activity and represents a perfect and robust alternative to other complex and expensive preparation procedures. Palladium leaching in the Suzuki couplings by supported Palladium catalysts has been investigated in detail for the first time for such mild reaction conditions. Catalytic activity is due to Palladium species dissolved from the solid particles. The active Palladium species in solution are re-deposited onto the support with the completion of the reaction. Catalytic activity correlates with the amount of Palladium dissolved. Palladium leaching is a prerequisite for highly efficient catalysis in particular for more demanding substrates. Palladium leaching depends on a variety of parameters (temperature, solvent, base, substrates, and additives). Transmission electron microscopic investigations show that – depending on the parameters – the efficient Palladium re-deposition process can leave the catalyst with an unchanged dispersion and particle size distribution after the reaction.

  • supported Palladium catalysts for suzuki reactions structure property relationships optimized reaction protocol and control of Palladium leaching
    Advanced Synthesis & Catalysis, 2008
    Co-Authors: Klaus Kohler, Roland G Heidenreich, Saeeda S Soomro, Sandra S Prockl

    : Palladium on metal oxides and on activated carbon with particular properties (high Palladium dispersion, low degree of reduction, water content) are shown to be highly active (tunrover number, TON =20,000; turnover frequency, TOF= 16,600), selective and robust catalysts for Suzuki cross-couplings of aryl bromides and activated aryl chlorides. Catalysts and reaction protocol offer combined advantages of high catalytic efficiency under ambient conditions (air and moisture), easy separation and reuse and quantitative recovery of Palladium. The Palladium concentration in solution during the reaction correlates clearly with the progress of the reaction and indicates that dissolved molecular Palladium is in fact the catalytically active species. Dissolved Palladium is redeposited onto the support at the end of the reaction. Additional minimization of the Palladium content in solution (down to 0.1 ppm) could be achieved by simple procedures which meet the requirements of pharmaceutical industry.

Kilian Muñiz - One of the best experts on this subject based on the ideXlab platform.

  • high oxidation state Palladium catalysis new reactivity for organic synthesis
    Angewandte Chemie, 2009
    Co-Authors: Kilian Muñiz

    Recent years have seen the rapid development of a new field of Palladium catalysis in organic synthesis. This chemistry takes place outside the usually encountered Pd 0 /Pd II cycles. It is characterized by the presence of strong oxidants, which prevent further Palladium(II)-promoted reactions at a given point of the catalytic cycle by selective metal oxidation. The resulting higher-oxidation-state Palladium complexes have been used to develop a series of new synthetic transformations that cannnot be realized within conventional Palladium catalysis. This type of catalysis by Palladium in a higher oxidation state is of significant synthetic potential.

  • oxidative diamination of alkenes with ureas as nitrogen sources mechanistic pathways in the presence of a high oxidation state Palladium catalyst
    Journal of the American Chemical Society, 2008
    Co-Authors: Kilian Muñiz, Claas H Hovelmann, Jan Streuff

    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...

Christopher W Jones - One of the best experts on this subject based on the ideXlab platform.

  • strong evidence of solution phase catalysis associated with Palladium leaching from immobilized thiols during heck and suzuki coupling of aryl iodides bromides and chlorides
    Journal of Catalysis, 2007
    Co-Authors: John M Richardson, Christopher W Jones

    A 3-mercaptopropyl-functionalized silica, SH-SBA-15, is used both as a support for Palladium acetate, Pd-SH-SBA-15, and as a selective poison of soluble Palladium complexes. Pd-SH-SBA-15 is used as a precatalyst for Heck and Suzuki couplings of an aryl iodide, bromide, and chloride under a range of reaction conditions. In all reactions in which metal-free SH-SBA-15 is added, the catalysis associated with either homogeneous Palladium acetate or from Pd-SH-SBA-15 ceased. This strongly suggests that (i) SH-SBA-15 can be used as an effective scavenger and poison of soluble Palladium and (ii) catalysis with Pd-SH-SBA-15 is solely associated with leached metal. Supporting evidence is given from hot filtration tests, self-quenching behavior of partially metalated Pd-SH-SBA-15, and poisoning by other solid-phase poisons such as poly(4-vinylpyridine), Quadrapure TU, and 3-mercaptopropyl-functionalized silica gel. It is postulated that the SH-SBA-15 poisons by over coordination of soluble monomeric or dimeric Palladium, which rules out catalysis by Palladium nanoparticle surfaces. SH-SBA-15 is an effective and selective poison of homogeneous Palladium species and can be used as a test for heterogeneity of Palladium catalysts developed for Heck and Suzuki reactions.

  • on the nature of the active species in Palladium catalyzed mizoroki heck and suzuki miyaura couplings homogeneous or heterogeneous catalysis a critical review
    Advanced Synthesis & Catalysis, 2006
    Co-Authors: Nam T S Phan, Matthew Van Der Sluys, Christopher W Jones

    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.