N-Heterocyclic Carbenes as Efficient Organocatalysts for CO2 Fixation Reactions

摘要

During the last decade, N-heterocyclic carbenes (NHCs) have attracted attention because of the intensive development of general synthetic methods and wide applications in the field of molecular catalysis. The unique properties of the unsaturated NHC carbon atom, stabilized by the electron-donating heteroatoms on either side, are highlighted by their utilization as versatile ligands for transition metals and organocatalysts as they exhibit strong basicity. Most NHC-mediated organocatalysis includes the formation of covalent, active intermediates by their addition to C=O bonds as the key step, leading to nucleophilic incorporation of the carbonyl functional group. Such an apparent σ-donor character of NHCs has also been applied to CO2 capture, and the resulting imidazolium-2-carboxylates are identified as the typical NHC-CO2 adducts (Figure 1). Whereas the reverse reaction has been proven to be decisive for delivering a number of NHC complexes, in principle, the zwitterionic CO2 adduct could be utilized as a convenient CO2 carrier to accomplish CO2 fixation through the nucleophilic incorporation of the O=C=O unit as proposed for various Lewis base catalyst systems.