35 research outputs found

    N-Heterocyclic carbene-stabilized gold nanoparticles and their assembly into 3D superlattices

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    The facile one-phase synthesis of N-heterocyclic carbene-stabilized gold nanoparticles (NHC-AuNP) by reduction of NHC-gold(I) complexes and their self-assembly into 3D super-lattices is presented

    Approches de complexes carbéniques hétérosubstitués originaux

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    TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

    Azolium hydrogen carbonates and azolium carboxylates as organic pre-catalysts for N-heterocyclic carbene-catalysed group transfer and ring-opening polymerisations

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    Various imidazolium-2-carboxylates and (benz) imidazolium hydrogen carbonates, denoted as NHC-CO2 adducts and [NHC(H)][HCO3], respectively, were employed as masked N-heterocyclic carbenes (NHCs) to bring about both the ring opening polymerisation (ROP) of D, L-lactide (LA) and the group transfer polymerisation (GTP) of methyl methacrylate (MMA). Polymerisation reactions could be performed at room temperature by generation of related free NHCs by a simple solvation effect. Catalytic efficiencies of imidazolium-2-carboxylates were found to be approximately three times higher than that of their hydrogen carbonate counterparts for the ROP of D, L-LA, except in the particular case of 1,3-dicyclohexylbenzimidazolium hydrogen carbonate that exhibited a similar catalytic performance to that of NHC-CO2 adducts. The catalytic efficiencies of free NHCs and NHC precursors were thus in the following order: [NHC(H)][HCO3] < NHC-CO2 adducts << free NHCs. Only NHC-CO2 adducts allowed the catalysis of the GTP of MMA in bulk in a controlled manner, [NHC(H)][HCO3] precursor salts being poorly soluble in the monomer substrate, causing a loss of control of GTP under solvent-less conditions

    Stable Noncyclic Singlet Carbenes

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    International audienc

    Poly(N-heterocyclic-carbene)s and their CO(2) Adducts as Recyclable Polymer-Supported Organocatalysts for Benzoin Condensation and Transesterification Reactions

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    The synthesis of poly(N-heterocyclic carbene)s, denoted poly-(NHC)s, and of their poly(NHC-CO(2)) adducts for a use in organocatalysis is described. Poly(NHC)s were readily obtained in a three-step sequence of reactions, involving i) the free-radical polymerization of ionic liquid monomers, that is, 1-vinyl-3-alkylimidazolium-type monomers with bromide (Br(-)) as counteranion, followed by ii) anion exchange of Br(-) for bis(trifluoromethanesulfonyl)imide ((-)NTf2), of the poly(1-vinyl-3-alkylimidazolium bromide) precursors, affording poly(1-vinyl-3-alkylimidazolium bis(trifluorornethanesulfonyl)imide) derivatives, and iii) deprotonation of the latter polymeric ionic liquids with a strong base. Carbon dioxide (CO(2)) was found to reversibly react with poly(NHC)s forming relatively air-stable and thermolabile poly(NHC-CO(2)) adducts. Both poly(NHC)s and their poly(NHC-CO(2)) adducts were used as polymer-supported organic catalysts and precatalysts, respectively, in transesterification and benzoin condensation reactions under homogeneous conditions. Both types of polymer-supported NHCs were recycled and used several times, but the manipulation of poly(NHC)s like their molecular NHC analogues-was more complicated owing to their air and moisture sensitivity. In this regard, zwitterionic poly(NHC-CO(2)) adducts like their molecular NHC-CO(2) analogues could be easier manipulated than their bare poly(NHC) counterparts, providing good to excellent yields even after several organocatalytic cycles, in particular toward the transesterification reaction

    N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

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    The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(alpha-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis

    Imidazolium-Based Poly(Ionic Liquid)s Featuring Acetate Counter Anions: Thermally Latent and Recyclable Precursors of Polymer-Supported N-Heterocyclic Carbenes for Organocatalysis

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    Statistical copoly(ionic liquid)s (coPILs), namely, poly(styrene)-co-poly(4-vinylbenzylethylimidazolium acetate) are synthesized by free-radical copolymerization in methanolic solution. These coPILs serve to in situ generate polymer-supported N-heterocyclic carbenes (NHCs), referred to as polyNHCs, due to the noninnocent role of the weakly basic acetate counteranion interacting with the proton in C2-position of pendant imidazolium rings. Formation of polyNHCs is first evidenced through the quantitative formation of NHC-CS2 units by chemical postmodification of acetate-containing coPILs, in the presence of CS 2 as electrophilic reagent (= stoichiometric functionalization of polyNHCs). The same coPILs are also employed as masked precursors of polyNHCs in organocatalyzed reactions, including a one-pot two-step sequential reaction involving benzoin condensation followed by addition of methyl acrylate, cyanosilylation, and transesterification reactions. The catalytic activity can be switched on and off successively upon thermal activation, thanks to the deprotonation/ reprotonation equilibrium in C2-position. NHC species are thus in situ released upon heating at 80 degrees C (deprotonation), while regeneration of the coPIL precursor occurs at room temperature (reprotonation), triggering its precipitation in tetrahydrofuran. This also allows recycling the coPIL precatalyst by simple filtration, and reusing it for further catalytic cycles. The different organocatalyzed reactions tested can thus be performed with excellent yields after several cycles

    Update and challenges in organo-mediated polymerization reactions

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    Organocatalysis has become a very powerful tool for precision macromolecular chemistry, as judged by the number of articles published in this field in the past decade. A variety of small organic molecules, including Bronsted/Lewis bases and acids, based on amines, phosphines or carbenes, but also on bi-component systems, have been employed as a means to catalyze the polymerization of miscellaneous monomers. Not only can organocatalysts be employed to promote the ring-opening polymerization of various heterocyclics (e.g. lactones, lactide, cyclic carbonates, epoxides, lactams, cyclocarbosiloxanes), but some of them also allow activating vinylic monomers such as (meth)acrylics, or triggering the step growth polymerization of monomers such as diisocyanates and diols for polyurethane synthesis. The reduced toxicity of organocatalysts in comparison to their metallic counterparts is also driving their development in some sensitive applications, such as biomedical or microelectronics. Overall, organocatalysts display specific monomer activation modes, thereby providing a unique opportunity to control the polymerization of various functional monomers, under mild conditions. This review article focuses on advances of the past 4 years (>150 publications) in polymerization reactions utilizing small organic molecules either as direct initiators or as true catalysts, with a special emphasis on monomer activation modes, as well as polymerization mechanism aspects.POLYMERISATIONS ORGANOCATALYSEES PAR LES CARBENES : VERS UNE PLATEFORME CATALYTIQUE MULTI-TACHEEuropean Joint Doctorate in Organocatalysis and Sustainable Polymer
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