Poly(N-heterocyclic-carbene)s and their CO2 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-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 (-NTf 2), 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-CO 2) 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. © 2011 American Chemical Society.