N-Heterocyclic Carbene-Based Catalysis Enabling Cross-Coupling Reactions

N-Heterocyclic carbene (NHC) organocatalysis to promote so-called umpolung reactions has emerged as a powerful tool for modern organic synthesis. NHC-bound nucleophiles known as Breslow intermediates can be obtained by reacting an NHC catalyst with an aldehyde and will add to carbon–heteroatom or carbon–carbon double bonds. On the other hand, the substitution-type cross-couplings of carbon electrophiles (that is, R–X species) with NHC-bound nucleophiles to generate carbon–carbon bonds have also been demonstrated even though such processes are difficult and less mature at present. This Perspective briefly summarizes the NHC-based catalysis enabling cross-coupling reactions. Three types of systems are examined: anionic, hybrid, and radical catalysis.