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Personne :
Rochette, Étienne

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Rochette

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Étienne

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Département de chimie, Faculté des sciences et de génie, Université Laval

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ncf11914405

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Voici les éléments 1 - 10 sur 12
  • PublicationAccès libre
    Ambiphilic molecules : from organometallic curiosity to metal-free catalysts
    (ACS Publications, 2018-01-08) Fontaine, Frédéric-Georges; Rochette, Étienne
    Ambiphilic molecules were first used as functional ligands for transition elements, which could enable intriguing organometallic transformations. In the past decade, these intramolecular Lewis pairs, first considered organometallic curiosities, have become staples in organometallic chemistry and catalysis, acting as Z ligands, activating inert molecules using the concept of frustrated Lewis pair (FLP) chemistry, and acting as metal-free catalysts. In this Account, we detail our contribution to this blossoming field of research, focusing on the use of ambiphilic molecules as metal-free catalysts for CO2 reduction and C-H borylation reactions. A major emphasis is put on the mechanistic investigations we carried out using reactivity studies and theoretical tools, which helped us steer our research from stoichiometric transformations to highly active catalytic processes. We first report the interaction of aluminum-phosphine ambiphilic molecules with carbon dioxide. Although these Lewis pairs can bind CO2, a study of the deactivation process in the presence of CO2 and hydroboranes led us to discover that simple phosphinoborane molecules could act as active precatalysts for the hydroboration of carbon dioxide into methanol precursors. In these systems, the Lewis basic sites interact with the reducing agents rather than with the electrophilic carbon of CO2, increasing the nucleophilicity of hydroboranes. Simultaneously, the weak Lewis acids stabilize the oxygen of the gas molecule in the transition state, leading to high reaction rates. Replacing the phosphine by an amine leads to a system enabling CO2 hydrogenation, albeit only in stoichiometric transformations. Investigation of the protodeborylation deactivation of aminoboranes led us to develop metal-free catalysts for the C-H borylation of heteroarenes. By protecting the Lewis acid sites of these catalysts using fluoride, we were able to synthesize practical, air-stable precatalysts allowing the convenient synthesis of heteroarylboronic esters on a multigram scale. Contrary to general perception of FLP chemistry, we also demonstrated that a significant increase in activity could be obtained by reducing the steric bulk around the active site. These smaller systems exist as stable dimers and are more energetically costly to dissociate into active FLPs, but the approach of the substrate and the C-H activation step are significantly favored compared to the bulkier analogues. An in-depth study of the stability and reactivity of these aminoborane molecules also allowed us to develop a metal-free catalytic S-H bond borylation system, and to report stoichiometric and spontaneous B-B bond formation and Csp3-H bond activation processes, highlighting the importance of H2 release as a thermodynamic driving force in these FLP transformations.
  • PublicationAccès libre
    Metal-free borylation of heteroarenes using ambiphilic aminoboranes : on the importance of sterics in frustrated lewis pair C-H bond activation
    (American Chemical Society, 2017-10-18) Légaré-Lavergne, Julien; Fontaine, Frédéric-Georges; Misal Castro, Luis Carlos; Rochette, Étienne; Jayaraman, Arumugam
    Two novel frustrated Lewis pair (FLP) aminoboranes, (1-Pip-2-BH2-C6H4)2 (2; Pip = piperidyl) and (1-NEt2-2-BH2-C6H4)2 (3; NEt2 = diethylamino), were synthesized, and their structural features were elucidated both in solution and in the solid state. The reactivity of these species for the borylation of heteroarenes was investigated and compared to previously reported (1-TMP-2-BH2-C6H4)2 (1; TMP = tetramethylpiperidyl) and (1-NMe2-2-BH2-C6H4)2 (4; NMe2 = dimethylamino). It was shown that 2 and 3 are more active catalysts for the borylation of heteroarenes than the bulkier analogue 1. Kinetic studies and density functional theory calculations were performed with 1 and 2 to ascertain the influence of the amino group of this FLP-catalyzed transformation. The C–H activation step was found to be more facile with smaller amines at the expense of a more difficult dissociation of the dimeric species. The bench-stable fluoroborate salts of all catalysts (1F–4F) have been synthesized and tested for the borylation reaction. The new precatalysts 2F and 3F are showing higher reaction rates and yields for multigram-scale syntheses.
  • PublicationAccès libre
    Frustrated Lewis pair mediated Csp3-H activation
    (Wiley, 2017-01-29) Fontaine, Frédéric-Georges; Courtemanche, Marc-André; Rochette, Étienne
    The cleavage of a Csp3−H bond by an N/B frustrated Lewis pair (FLP) is reported. Upon mild heating, the ambiphilic molecule (2‐NMe2‐C6H4)2BH activates the C−H bond of a methyl group in α position of a nitrogen atom to generate an unprecedented N–B heterocycle. Upon further heating, the novel species rearranges through a hydride abstraction/1,2‐aryl shift sequence. The mechanistic details of these transformations are investigated by quantum mechanical calculations.
  • PublicationAccès libre
    Spontaneous reduction of a hydroborane to generate a B-B single bond using a Lewis pair
    (Wiley, 2016-09-14) Légaré-Lavergne, Julien; Fontaine, Frédéric-Georges; Rochette, Étienne; Bouchard, Nicolas; Matta, Chérif F.
    The ansa‐aminohydroborane 1‐NMe2‐2‐(BH2)C6H4 crystallizes in an unprecedented type of dimer containing a B−H bond activated by one FLP moiety. Upon mild heating and without the use of any catalyst, this molecule liberates one equivalent of hydrogen to generate a diborane molecule. The synthesis and structural characterization of these new compounds, as well as the kinetic monitoring of the reaction and the DFT investigation of its mechanism, are reported.
  • PublicationAccès libre
    Design principles in frustrated lewis pair catalysis for the functionalization of carbon dioxide and heterocycles
    (Elsevier Pub. Co., 2016-05-26) Fontaine, Frédéric-Georges; Légaré, Marc-André; Courtemanche, Marc-André; Rochette, Étienne
    This account describes our work on the use of ambiphilic molecules as catalysts for the reduction of carbon dioxide. Starting with the discovery that aluminum ambiphilic species (Me2PCH2AlMe2)2 (1) and Al(C6H4-PPh2)3 (5) could coordinate CO2 but showed significant instability, we found that phosphinoborane Ph2P-2-Bcat-C6H4 (7) was an exceptional catalyst for the hydroboration of CO2 to methoxyboranes, molecules that can be readily hydrolyzed to methanol. A mechanistic investigation allowed us to outline the similarities between the catalytic activity of frustrated Lewis pairs (FLPs) and that of transition metal complexes. We were able to extrapolate four important guidelines, described in this account, for the conception of efficient metal-free catalysts. We demonstrate that using these concepts, it was possible to rationally design FLP catalysts for the C-H borylation of heteroarenes.
  • PublicationAccès libre
    Phosphazenes : efficient organocatalysts for the catalytic hydrosilylation of carbon dioxide
    (Chemical Society, 2015-03-13) Fontaine, Frédéric-Georges; Légaré, Marc-André; Courtemanche, Marc-André; Rochette, Étienne
    Phosphazene superbases are efficient organocatalysts for the metal-free catalytic hydrosilylation of carbon dioxide. They react with CO2 to form the respective phosphine oxides, but in the presence of hydrosilanes, CO2 can be selectively reduced to silyl formates, which can in turn be reduced to methoxysilanes by addition of an extra loading of silanes. Activities reach a TOF of 32 h−1 with a TON of 759. It is also shown that unexpectedly, N,N-dimethylformamide can reduce CO2 to a mixture of silyl formates, acetals and methoxides in the absence of any catalyst.
  • PublicationAccès libre
    Frustrated Lewis pair catalyzed S-H bond borylation
    (American Chemical Society, 2017-06-30) Boutin, Hugo; Fontaine, Frédéric-Georges; Rochette, Étienne
    The frustrated Lewis pair (FLP) [NMe2-C6H4-BH2]2 is shown to catalyze the dehydrogenative borylation of thiols. The scope of the reaction, the experimental and computational investigation of the mechanism, and the application of the system to a one-pot Michael addition on α–β unsaturated carbonyl leading to the β sulfido ketone are reported.
  • PublicationAccès libre
    Bench-stable frustrated Lewis pair chemistry : fluoroborate salts as precatalysts for the C-H borylation of heteroarenes
    (Royal Society of Chemistry, 2016-03-14) Légaré-Lavergne, Julien; Fontaine, Frédéric-Georges; Légaré, Marc-André; Rochette, Étienne; Bouchard, Nicolas
    While the organotrifluoroborate group is commonly used as a leaving group in cross-coupling reactions, we now show that their high stability can be used to protect the Lewis acidic moieties of frustrated Lewis pair catalysts. Indeed, the air and moisture-stable trifluoro- and difluoroborate derivatives of bulky (tetramethylpiperidino)benzene are shown to be conveniently converted to their dihydroborane analogue which is known to activate small molecules. An efficient synthesis route to these stable and convenient precatalysts, their deprotection chemistry and their benchtop use for the dehydrogenative borylation of heteroarenes is presented.
  • PublicationAccès libre
    From phosphinoboranes to mercaptopyridines : a journey into the reactivity of not so frustrated Lewis pairs
    (2019) Rochette, Étienne; Fontaine, Frédéric-Georges
    La catalyse est une des pierres d’assise de la chimie moderne. Elle permet de faire des transformations difficiles d’une manière efficace et sélective, rendant possible des voies de synthèse plus courtes qui permettent ainsi à l’industrie chimique des économies de temps et d’argent. Par conséquent, le développement de la catalyse est d’une grande importance. Dans les dernières décennies, la plupart des efforts ont été orientés vers l’utilisation de métaux de transition de la seconde et troisième rangée, une approche couronnée de succès. Cependant, la maturité de ce sous-domaine et les améliorations des méthodes de caractérisation et de modélisation ont encouragé les chercheurs académiques à explorer le potentiel d’autres éléments du tableau périodique pour la catalyse. Cette thèse explore la catalyse sans métal, ou comme nous aimons l’appeler, la chimie organométallique sans métal. Elle présente des avancées dans le domaine des paires de Lewis frustrées (PLFs), qui utilisent des molécules comportant des fonctions acide de Lewis et base de Lewis pour rendre possible des transformations qui ne le seraient pas en utilisant seulement l’une ou l’autre des fonctions. Le focus particulier du travail est de comprendre et d’exploiter la chimie des PLFs. Par conséquent, nous ne nous sommes pas limités à seulement une sous-classe de PLFs ni à une seule transformation chimique. Les sujets contenus dans la thèse sont diversifiés et incluent la réduction du CO2, la fonctionnalisation de liens C-H, la chimie des liens B-B, la chimie des liens B-S ainsi que des discussions plus fondamentales sur le futur de la catalyse utilisant les PLFs.
  • PublicationAccès libre
    Metal-free catalytic C-H bond activation and borylation of heteroarenes
    (AAAS, 2015-07-31) Fontaine, Frédéric-Georges; Légaré, Marc-André; Courtemanche, Marc-André; Rochette, Étienne
    Transition metal complexes are efficient catalysts for the C-H bond functionalization of heteroarenes to generate useful products for the pharmaceutical and agricultural industries. However, the costly need to remove potentially toxic trace metals from the end products has prompted great interest in developing metal-free catalysts that can mimic metallic systems. We demonstrated that the borane (1-TMP-2-BH2-C6H4)2 (TMP, 2,2,6,6-tetramethylpiperidine) can activate the C-H bonds of heteroarenes and catalyze the borylation of furans, pyrroles, and electron-rich thiophenes. The selectivities complement those observed with most transition metal catalysts reported for this transformation.