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

Abstract : 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.
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Submitted on : Friday, March 9, 2012 - 4:07:22 PM
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Julien Pinaud, Joan Vignolle, Yves Gnanou, Daniel Taton. Poly(N-heterocyclic-carbene)s and their CO(2) Adducts as Recyclable Polymer-Supported Organocatalysts for Benzoin Condensation and Transesterification Reactions. Macromolecules, American Chemical Society, 2011, 44 (7), pp.1900-1908. ⟨10.1021/ma1024285⟩. ⟨hal-00677776⟩

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