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The synthesis of poly(N-heterocyclic carbene)s, denoted poly(NHC)s, and of their poly(NHC−CO2) 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(trifluoromethanesulfonyl)imide) derivatives, and iii) deprotonation of the latter polymeric ionic liquids with a strong base. Carbon dioxide (CO2) was found to reversibly react with poly(NHC)s forming relatively air-stable and thermolabile poly(NHC−CO2) adducts. Both poly(NHC)s and their poly(NHC−CO2) 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−CO2) adducts like their molecular NHC−CO2 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.
Pinaud et al. (Wed,) studied this question.