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Christophe Copéret - One of the best experts on this subject based on the ideXlab platform.
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C-H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)-Oxo Complex
Journal of the American Chemical Society, 2018Co-Authors: Ka Wing Chan, Erwin Lam, Florian Allouche, Carine Michel, Olga Safonova, Philippe Sautet, Vincenza D’anna, Christophe CopéretAbstract: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.
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C–H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)–Oxo Complex
Journal of the American Chemical Society, 2018Co-Authors: Ka Wing Chan, Erwin Lam, Vincenza D’anna, Florian Allouche, Carine Michel, Olga Safonova, Philippe Sautet, Christophe CopéretAbstract: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.
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Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents
ACS central science, 2016Co-Authors: Victor Mougel, Ka Wing Chan, Olga Safonova, Georges Siddiqi, Kento Kawakita, Haruki Nagae, Hayato Tsurugi, Kazushi Mashima, Christophe CopéretAbstract: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...
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Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS.
Angewandte Chemie International Edition, 2016Co-Authors: Ta-chung Ong, David Gajan, Anne Lesage, Lyndon Emsley, Victor Mougel, Wei-chih Liao, Christophe CopéretAbstract:Obtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction-based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy (SENS) towards surface species in heterogeneous Alkene Metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles, which are the cycloaddition intermediates in the Alkene Metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous Metathesis catalysts.
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Cationic Silica-Supported N-Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis.
Angewandte Chemie (International ed. in English), 2016Co-Authors: Margherita Pucino, Victor Mougel, Roman Schowner, Alexey Fedorov, Michael R. Buchmeiser, Christophe CopéretAbstract:Designing supported Alkene Metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ-donating N-heterocyclic carbene ligands with weak σ-donating surface silanolates and cationic tungsten sites leading to highly active and stable Alkene Metathesis catalysts. These well-defined silica-supported catalysts, [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe2 Ph)(IMes)(+) ][B(Ar(F) )4 (-) ] [IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, B(Ar(F) )4 =B(3,5-(CF3 )2 C6 H3 )4 ] catalyze Alkene Metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.
Steven T Diver - One of the best experts on this subject based on the ideXlab platform.
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A Macrocyclic Ruthenium Carbene for Size-Selective Alkene Metathesis
Journal of the American Chemical Society, 2020Co-Authors: Yutong Zhang, Steven T DiverAbstract: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.
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Alkene Metathesis approach to β-unsubstituted anti-allylic alcohols and their use in ene-yne Metathesis.
Journal of Organic Chemistry, 2012Co-Authors: Joseph R. Clark, Jonathan M. French, Steven T DiverAbstract: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.
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Catalyst takes control to heart
Nature, 2008Co-Authors: Steven T DiverAbstract: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.
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Organometallic chemistry: Catalyst takes control to heart
Nature, 2008Co-Authors: Steven T DiverAbstract: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. 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.
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Tandem cyclopropanation/ring-closing Metathesis of dienynes.
Journal of the American Chemical Society, 2004Co-Authors: Brian P Peppers, Steven T DiverAbstract:Certain dienynes give cyclorearrangement by tandem cyclopropanation/ring-closing Alkene Metathesis, triggered by either a ruthenium carbene or noncarbene ruthenium(II) precatalyst. The process represents a variation of enyne Metathesis where presumed cyclopropyl carbene intermediates undergo a consecutive ring-closing Metathesis. A mechanistic proposal is offered, and sequential use of catalysts provided a tandem ring-closing enyne/Alkene Metathesis product.
Odile Eisenstein - One of the best experts on this subject based on the ideXlab platform.
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Metallacyclobutanes from Schrock-Type d0 Metal Alkylidene Catalysts: Structural Preferences and Consequences in Alkene Metathesis
Organometallics, 2015Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile EisensteinAbstract: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...
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Metallacyclobutanes from Schrock-Type d0 Metal Alkylidene Catalysts: Structural Preferences and Consequences in Alkene Metathesis
Organometallics, 2015Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile EisensteinAbstract: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.
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Oxo vs Imido Alkylidene d0-Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis?
Organometallics, 2012Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile EisensteinAbstract: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...
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Oxo vs Imido Alkylidene d0-Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis?
Organometallics, 2012Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile EisensteinAbstract: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 with a slightly larger energy difference between the lowest energy intermediate (SP or separated reactants) and the highest energy transition state (cycloreversion) than the imido complex, which accounts for a slightly lower activity. Changing the imido into an oxo ligand disfavors strongly the deactivation pathway by raising considerably the energy barrier of the β-H transfer at the SP metallacycle that begins the entry into the channel for deactivation and byproduct formation as well as that of the subsequent ethene insertion. This makes the oxo catalysts more selective and stable toward deactivation than the corresponding imido catalysts, when dimerization can be avoided.
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Shutting Down Secondary Reaction Pathways: The Essential Role of the Pyrrolyl Ligand in Improving Silica Supported d0-ML4 Alkene Metathesis Catalysts from DFT Calculations
Journal of the American Chemical Society, 2010Co-Authors: Xavier Solans-monfort, Christophe Copéret, Odile EisensteinAbstract:The efficiency of silica supported d0 ML4 Alkene Metathesis catalysts [(≡SiO)M(NR1)(═CHR2)(X)] (M = Mo, W; R1 = aryl and alkyl) is influenced by the nature of the X ancillary ligand. Replacing the alkyl ligand by a pyrrolyl ligand dramatically increases the performance of the catalyst. DFT calculations on the Metathesis, the deactivation, and the byproduct formation pathways for the imido Mo and W and the alkylidyne Re complexes give a rational for the role of pyrrolyl ligand. Dissymmetry at the metal center leads to more efficient catalyst even when the difference in σ-donating ability between X and OSi is not large. β-H transfer at the square based pyramid metallacyclobutane is the key step for catalyst deactivation and byproduct formation. Overall, the greatest benefit of substituting the ancillary alkyl by a pyrrolyl ligand, [(≡SiO)M(ER1)(═CHR2)(pyrrolyl)], is in fact not to improve the efficiency of the catalytic cycle of Alkene Metathesis, but to shut down deactivation and byproduct formation pathwa...
Pierre H. Dixneuf - One of the best experts on this subject based on the ideXlab platform.
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Alkene Metathesis and Renewable Materials: Selective Transformations of Plant Oils
Green Metathesis Chemistry, 2010Co-Authors: Raluca Malacea, Pierre H. DixneufAbstract:The olefin Metathesis of natural oils and fats and their derivatives is the basis of clean catalytic reactions relevant to green chemistry processes and the production of generate useful chemicals from renewable raw materials. Three variants of Alkene Metathesis: self-Metathesis, ethenolysis and cross-Metathesis applied to plant oil derivatives will show new routes to fine chemicals, bifunctional products, polymer precursours and industry intermediates.
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Simple Ruthenium Precatalyst for the Synthesis of Stilbene Derivatives and Ring‐Closing Metathesis in the Presence of Styrene Initiators
Advanced Synthesis & Catalysis, 2007Co-Authors: Renan Cariou, Cédric Fischmeister, Pierre H. DixneufAbstract:The ruthenium complex RuCl 2 (P-cymene)(IMes) was found to be an efficient precatalyst, with styrene as an initiating species, for the Alkene Metathesis of various styrenes into symmetrical and unsymmetrical stilbene derivatives and for the ring-closing Metathesis reaction of a sterically hindered olefin leading to a tetrasubstituted cycloolefin.
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Recent Applications Of Alkene Metathesis For Fine Chemical And Supramolecular System Synthesis
Metathesis Chemistry, 2007Co-Authors: Franc Požgan, Pierre H. DixneufAbstract:Alkene Metathesis has become a synthetic method that has strongly modified approaches in the building of complex molecules. The review will concentrate on the recent applications of ring-closing Metathesis (RCM) and cross- Metathesis (CM) reactions for the design of useful molecules and architectures: new functional macrocycles (RCM) and multifunctional linear Alkenes (CM), but also cyclic ligands, rotaxanes, catenanes, and supramolecular systems.
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Allenylidene-ruthenium complexes as versatile precatalysts for Alkene Metathesis reactions
Journal of Molecular Catalysis A: Chemical, 2004Co-Authors: Ricardo Castarlenas, Christian Bruneau, Cédric Fischmeister, Pierre H. DixneufAbstract:Abstract Recent developments of ruthenium–allenylidene complexes as precatalysts for Alkene Metathesis reactions are described. Since the first evidence, reported in 1998 by the Dixneuf and Furstner’s groups, a variety of ionic 18-electron and neutral 16-electron ruthenium–allenylidene complexes mainly of the type [RuCl(CCCR 2 )(arene)(PR 3 )][X] and RuCl 2 (CCCR 2 )(L′)(L″) have been described. The parallel formation and catalytic activity of related complexes in which the allenylidene moiety is formally transformed into an indenylidene ligand, that may take place via intramolecular rearrangement, is discussed. Applications are described for the production of both fine chemicals and polymers. Attempts for catalyst recycling in ionic liquids, or via heterogenization are also presented.
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The Alkene Metathesis Ruthenium Catalyst Saga
Novel Metathesis Chemistry: Well-Defined Initiator Systems for Specialty Chemical Synthesis Tailored Polymers and Advanced Material Applications, 2003Co-Authors: David Sémeril, Pierre H. DixneufAbstract:The history of the Alkene Metathesis ruthenium catalyst discovery is presented and illustrated with catalyst precursor preparation methods and with references to applications in fine chemistry and polymerisation. These precursors involve well-defined and in situ prepared rutheniumalkylidenes, -vinylidenes and -allenylidenes containing bulky electronrich phosphine or imidazolylidene and imidazolinylidene ligands.
Lyndon Emsley - One of the best experts on this subject based on the ideXlab platform.
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Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS.
Angewandte Chemie International Edition, 2016Co-Authors: Ta-chung Ong, David Gajan, Anne Lesage, Lyndon Emsley, Victor Mougel, Wei-chih Liao, Christophe CopéretAbstract:Obtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction-based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy (SENS) towards surface species in heterogeneous Alkene Metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles, which are the cycloaddition intermediates in the Alkene Metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous Metathesis catalysts.
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Evidence for metal-surface interactions and their role in stabilizing well-defined immobilized Ru-NHC Alkene Metathesis catalysts.
Journal of the American Chemical Society, 2013Co-Authors: Manorja K Samantaray, Johan Alauzun, David Gajan, Santosh Kavitake, Ahmad Mehdi, Laurent Veyre, Moreno Lelli, Anne Lesage, Lyndon Emsley, Christophe CopéretAbstract:Secondary interactions are demonstrated to direct the stability of well-defined Ru–NHC-based heterogeneous Alkene Metathesis catalysts. By providing key stabilization of the active sites, higher catalytic performance is achieved. Specifically, they can be described as interactions between the metal center (active site) and the surface functionality of the support, and they have been detected by surface-enhanced 1H–29Si NMR spectroscopy of the ligand and 31P solid-state NMR of the catalyst precursor. They are present only when the metal center is attached to the surface via a flexible linker (a propyl group), which allows the active site to either react with the substrate or relax, reversibly, to the surface, thus providing stability. In contrast, the use of a rigid linker (here mesitylphenyl) leads to a well-defined active site far away from the surface, stabilized only by a phosphine ligand which under reaction conditions leaves probably irreversibly, leading to faster decomposition and deactivation of t...
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Evidence for Metal−Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru−NHC Alkene Metathesis Catalysts
Journal of the American Chemical Society, 2013Co-Authors: Manorja K Samantaray, Johan Alauzun, David Gajan, Santosh Kavitake, Ahmad Mehdi, Laurent Veyre, Moreno Lelli, Anne Lesage, Lyndon Emsley, Christophe CopéretAbstract:Secondary interactions are demonstrated to direct the stability of well-defined Ru-NHC-based heterogeneous Alkene Metathesis catalysts. By providing key stabilization of the active sites, higher catalytic performance is achieved. Specifically, they can be described as interactions between the metal center (active site) and the surface functionality of the support, and they have been detected by surface-enhanced H-1-Si-29 NMR spectroscopy of the ligand and P-31 solid-state NMR of the catalyst precursor. They are present only when the metal center is attached to the surface via a flexible linker (a propyl group), which allows the active site to either react with the substrate or relax, reversibly, to the surface, thus providing stability. In contrast, the use of a rigid linker (here mesitylphenyl) leads to a well-defined active site far away from the surface, stabilized only by a phosphine ligand which under reaction conditions leaves probably irreversibly, leading to faster decomposition and deactivation of the catalysts.
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Synthesis and reactivity of molybdenum imido alkylidene bis-pyrazolide complexes
Dalton transactions (Cambridge England : 2003), 2010Co-Authors: David Gajan, Anne Lesage, Lyndon Emsley, Christophe Copéret, Jean-marie Basset, Nuria Rendón, Keith M. Wampler, Richard R SchrockAbstract:Reaction of Li(3,5-R2-pyrazolide) (R = tBu or Ph, dXpz) with Mo(NAr)(CHCMe2Ph)(OTf)2(DME) yields Mo(NAr)(CHCMe2Ph)(dXpz)2 in good yield. These complexes react with alcohols or the surface silanols of silica, to yield bis-alkoxy and surface mono-siloxy Alkene Metathesis catalysts, respectively.
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Direct observation of reaction intermediates for a well defined heterogeneous Alkene Metathesis catalyst
Proceedings of the National Academy of Sciences of the United States of America, 2008Co-Authors: Frédéric Blanc, Anne Lesage, Lyndon Emsley, Christophe Copéret, Romain Berthoud, Rojendra Singh, Thorsten Kreickmann, Richard R SchrockAbstract:Grafting of [W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)2] on a silica partially dehydroxylated at 700°C (SiO2- (700)) generates the corresponding monosiloxy complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)] as the major species (≈90%) along with [(≡SiO)W(≡NAr)(CH2tBu)(2,5-Me2NC4H2)2], according to mass balance analysis, IR, and NMR studies. This heterogeneous catalyst displays good activity and stability in the Metathesis of propene. Very importantly, solid state NMR spectroscopy allows observation of the propagating alkylidene as well as stable metallacyclobutane intermediates. These species have the same reactivity as the initial surface complex [(≡SiO)W(≡NAr)(=CHtBu)(2,5-Me2NC4H2)], which shows that they are the key intermediates of Alkene Metathesis.
