N-Heterocyclic Carbenes as C1 and C–N Synthons: A Mini Review

N-heterocyclic carbenes (NHCs) have traditionally been employed as organocatalysts or metal ligands. Recently, a paradigm shift has emerged in which NHCs serve as reactive building blocks rather than mere auxiliaries. This minireview highlights the use of imidazolium-derived NHCs as C1 and C–N synthons for the construction of valuable heterocycles and other complex molecules. Three major reaction manifolds are discussed: (i) single-carbon atom doping (SCAD) of α,β-unsaturated amides to γ-lactams, (ii) NHC-enabled synthesis of quinazolin-4-ones and isoxazoles from nitrobenzenes and nitroalkenes, and (iii) the use of NHCs as C1 sources for the synthesis of bis(indolyl)methanes. Mechanistic commonalities involving nucleophilic addition, spirocyclic intermediates, and C–N bond cleavage are emphasized.