N-Heterocyclic carbene-catalyzed dimerization, cyclotetramerization and polymerization of Michael acceptors

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N-Heterocyclic carbenes (NHCs), which were first isolated in the early 1990s, have received a great deal of scientific attention as ligands for transition metal complexes and organocatalysts for more than a decade. Organocatalysis by NHCs primarily involves the reaction of carbonyl compounds, particularly the umpolung of aldehydes, although we and others have been developing the reactions and the polymerizations of Michael acceptors. This review focuses on the NHC-catalyzed transformations of Michael acceptors that were developed by our research group, including (1) tail-to-tail dimerization of a wide variety of substrates, (2) cyclotetramerization of acrylates, (3) tandem oxa-Michael addition and head-to-tail dimerization of methacrolein and (4) oxa-Michael addition polymerization of hydroxyl-functionalized acrylates. For the former two reactions, the NHCs turn the β-carbon of the Michael acceptors into nucleophilic sites (umpolung), thereby generating the deoxy-Breslow intermediate and enabling bond formation between the β-carbon and electrophiles. The latter two reactions are based on the O–C bond formation between alcohols and Michael acceptors, in which the NHC catalysts act as a Lewis base. Thus, the use of NHC catalysts allowed new modes of reactivity of Michael acceptors other than the conventional addition polymerizations. This review describes the N-heterocyclic carbene-catalyzed transformations of Michael acceptors that were developed by our research group, including (1) tail-to-tail dimerization of a wide variety of substrates, such as vinyl pyridines, acrylates, methacrylates and methacrylonitrile, (2) cyclotetramerization of acrylates to form trisubstituted cyclopentenones, (3) tandem oxa-Michael addition and head-to-tail dimerization of methacrolein and (4) oxa-Michael addition polymerization of hydroxyl-functionalized acrylates.