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Alkene Metathesis

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Christophe Copéret – One of the best experts on this subject based on the ideXlab platform.

  • C-H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)-Oxo Complex
    Journal of the American Chemical Society, 2018
    Co-Authors: Ka Wing Chan, Erwin Lam, Florian Allouche, Carine Michel, Olga Safonova, Philippe Sautet, Vincenza D’anna, Christophe Copéret

    Abstract:

    In Alkene Metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species-generating alkylidenes-is still an open question. Here, we report the syntheses of tungsten(IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)2py3 (R = CMe(CF3)2 (2a), R = Si(O tBu)3 (2b), and R = C(CF3)3 (2c); py = pyridine), and show that upon activation with B(C6F5)3 they display Alkene Metathesis activities comparable to W(VI)-oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the Alkene reactant is crucial for initiating Alkene Metathesis. Deuterium labeling of the allylic C-H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C-H bond and show that formation of the metallacyclobutane from the allyl “hydride” involves a proton transfer facilitated by the coordination of a Lewis acid (B(C6F5)3) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the Metathesis active species.

  • C–H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)–Oxo Complex
    Journal of the American Chemical Society, 2018
    Co-Authors: Ka Wing Chan, Erwin Lam, Vincenza D’anna, Florian Allouche, Carine Michel, Olga Safonova, Philippe Sautet, Christophe Copéret

    Abstract:

    In Alkene Metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species—generating alkylidenes—is still an open question. Here, we report the syntheses of tungsten(IV)–oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)2py3 (R = CMe(CF3)2 (2a), R = Si(OtBu)3 (2b), and R = C(CF3)3 (2c); py = pyridine), and show that upon activation with B(C6F5)3 they display Alkene Metathesis activities comparable to W(VI)–oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the Alkene reactant is crucial for initiating Alkene Metathesis. Deuterium labeling of the allylic C–H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C–H bond and show that formation of the metallacyclobutane from the allyl “hydride” involves a proton transfer facilitated by the coordination of a Lewis acid (B(C6F5)3) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the Metathesis active species.

  • Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents
    ACS central science, 2016
    Co-Authors: Victor Mougel, Ka Wing Chan, Olga Safonova, Georges Siddiqi, Kento Kawakita, Haruki Nagae, Hayato Tsurugi, Kazushi Mashima, Christophe Copéret

    Abstract:

    Alkene Metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular Alkene Metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C…

Steven T Diver – One of the best experts on this subject based on the ideXlab platform.

  • A Macrocyclic Ruthenium Carbene for Size-Selective Alkene Metathesis
    Journal of the American Chemical Society, 2020
    Co-Authors: Yutong Zhang, Steven T Diver

    Abstract:

    The synthesis of a macrocyclic Ru carbene catalyst for selective cross Alkene Metathesis is reported. The new catalyst showed different reactivity for various type 1 Alkenes in homodimerization which correlated with the aggregrate size of the allylic substituent. The altered reactivity profile allowed for selective product formation in competition cross Alkene Metathesis between two different type 1 Alkenes and tert-butyl acrylate. Selectivity in these reactions is attributed to the ability of the macrocyclic catalyst to differentiate Alkenes based on their size. Two preparative examples of cross Metathesis with the macrocyclic catalyst are also provided.

  • Alkene Metathesis approach to β-unsubstituted anti-allylic alcohols and their use in ene-yne Metathesis.
    Journal of Organic Chemistry, 2012
    Co-Authors: Joseph R. Clark, Jonathan M. French, Steven T Diver

    Abstract:

    The synthesis of β-unsubstituted, anti-allylic alcohols using a catalytic Evans aldol reaction conjoined with a relay-type ring-closing Alkene Metathesis is reported. The Metathesis step serves to remove a β-alkenyl group, which facilitated the aldol step. The β-substituted enals serve as acrolein surrogates. The products were employed in ene–yne cross Metathesis.

  • Catalyst takes control to heart
    Nature, 2008
    Co-Authors: Steven T Diver

    Abstract:

    Alkene Metathesis (also called olefin Metathesis or transalkylidenation) is an organic reaction widely used to synthesize products including medicines, polymers and fuels. Its importance was recognized in 2005, when Yves Chauvin, Robert Grubbs and Richard Schrock shared the Nobel prize for work on Metathesis. A new class of molybdenum-based chiral catalyst, capable of initiating Alkene Metathesis with exceptional efficiency and enantioselectivity, is reported in this issue. The new catalysts bear a stereogenic metal centre and carry only monodentate ligands. Their effectiveness was demonstrated in an enantioselective synthesis of the Aspidosperma alkaloid, quebrachamine, via a Metathesis reaction that cannot be promoted by any of the previously reported catalysts. Some transition-metal catalysts control organic reactions so that, given a choice of two mirror-image products, only one forms. The metal atom in these catalysts has been ignored as a source of control — until now.

Odile Eisenstein – One of the best experts on this subject based on the ideXlab platform.

  • Metallacyclobutanes from Schrock-Type d0 Metal Alkylidene Catalysts: Structural Preferences and Consequences in Alkene Metathesis
    Organometallics, 2015
    Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile Eisenstein

    Abstract:

    Metallacyclobutanes have been observed as intermediates in the Alkene Metathesis reaction, with the 5-coordinated metal center, in a trigonal bipyramidal or a square-planar coordination. Previous calculations have shown that only the trigonal bipyramidal form is directly on the reaction pathway of Alkene Metathesis. The square-pyramidal form, which is obtained by a trigonal-bipyramidal (TBP)-square-based pyramidal (SP) interconversion at the metal, is an intermediate that can be responsible for catalyst deactivation. This computational study aimed at establishing the factors that control the properties of the two metallacyclobutanes (structural and 13C NMR features, stability, and TBP-SP interconversion) that can influence the efficiency of the Metathesis reaction. Metallacyclobutanes resulting from the addition of ethene to a large set of methylidene complexes where the metal fragment is M(E)(X)(Y) (M = Mo or W; E = alkyl imido, aryl imido, or oxo), (X and Y) = alkyl, pyrrolyl, alkoxy and fluoroalkoxy, a…

  • Metallacyclobutanes from Schrock-Type d0 Metal Alkylidene Catalysts: Structural Preferences and Consequences in Alkene Metathesis
    Organometallics, 2015
    Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile Eisenstein

    Abstract:

    Metallacyclobutanes have been observed as intermediates in the Alkene Metathesis reaction, with the 5-coordinated metal center, in a trigonal bipyramidal or a square-planar coordination. Previous calculations have shown that only the trigonal bipyramidal form is directly on the reaction pathway of Alkene Metathesis. The square-pyramidal form, which is obtained by a trigonal-bipyramidal (TBP)-square-based pyramidal (SP) interconversion at the metal, is an intermediate that can be responsible for catalyst deactivation. This computational study aimed at establishing the factors that control the properties of the two metallacyclobutanes (structural and 13C NMR features, stability, and TBP-SP interconversion) that can influence the efficiency of the Metathesis reaction. Metallacyclobutanes resulting from the addition of ethene to a large set of methylidene complexes where the metal fragment is M(E)(X)(Y) (M = Mo or W; E = alkyl imido, aryl imido, or oxo), (X and Y) = alkyl, pyrrolyl, alkoxy and fluoroalkoxy, and large monoaryloxy) have been studied by density functional calculations (B3PW91, and M06). From the study of these numerous complexes that include in particular all characterized complexes, properties of the metallacyclobutanes could be derived: Metallacyclobutanes with W are more stable than those with Mo relative to reactants; electron withdrawing ligands stabilize the two isomeric forms of the metallacyclobutane but more the TBP than the SP form, and conversely, electron donating ligands destabilize both forms but more the TBP isomer. The energy barrier for the TBP-SP interconversion is found to be lower than that for the productive ethene Metathesis pathway for W complexes but is generally higher for Mo species. These facts rationalize the experimental evidence and accounts in particular for the high efficiency of the Mo catalysts.

  • Oxo vs Imido Alkylidene d0-Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis?
    Organometallics, 2012
    Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile Eisenstein

    Abstract:

    Density functional calculations have been carried out to analyze the origin of the differences in reactivity, selectivity, and stability toward deactivation in Metathesis of d0 oxo alkylidene complexes vs their isoelectronic imido counterparts. DFT calculations show that the elementary steps and geometries of the extrema are similar for the oxo and imido complexes, but that the energy profiles are different, the greatest difference occurring for the deactivation pathway. For the Alkene Metathesis pathway, replacing the imido by an oxo ligand slightly lowers the energy barrier for Alkene coordination but raises that for the [2+2]-cycloaddition and cycloreversion; it also destabilizes the trigonal bipyramidal (TBP) metallacyclobutane intermediate with respect to the separated reactants. The isomeric square-based pyramid (SP) metallacyclobutane is in general more stable, and its stability relative to the separated reactants is similar for oxo and imido systems. Consequently, the oxo complex is associated wit…