N-Heterocyclic carbenes/imidazolium salts as substrates in catalysis: the catalytic 2-substitution and annulation of heterocyclic compounds

N-Heterocyclic carbenes (NHCs) are commonly regarded as strong donor ligands that are valuable in coordination chemistry and catalysis. Many reports describing this aspect of their chemistry have been published. The alternative view of NHCs as reaction intermediates has been little explored, and yet excellent examples exist and will be reviewed in this perspective. Group 10 hydrocarbyl complexes of NHCs [(R)(NHC)ML(2) where R = H, alkyl, aryl, acyl; M = Ni, Pd, Pt] undergo a facile reductive elimination to generate M(0) and R-substituted-azolium salt as products. On the other hand, 2-Hazolium salts will oxidatively add to M(0) complexes to afford reactive NHC-M-H compounds, suitable as catalysts for selected reactions. Combining the oxidative addition and reductive elimination steps into a single cycle, in the presence of an alkene, provides of a novel and potentially exciting, atom efficient catalytic C-C bond forming process for the substitution, and annulation of heterocyclic rings.