Synthetic Routes to N-Heterocyclic Carbene Precursors

N-Heterocyclic carbenes (NHC) are defined as singlet carbenes, in which the divalent carbenic center is connected directly to at least one nitrogen atom within the heterocycle. N-heterocyclic carbenes are electron-rich nucleophilic species in which the carbene center benefits from the stabilization associated with both the σ-electron-withdrawing and π-electron-donating character of the nitrogen centers. Introduction of the C1 precarbenic unit in the final ringclosing stage is still the most widely used strategy to NHC precursors, since it is generally high-yielding and easy to perform and tolerates various substituents. This synthetic route now remains the most popular standard protocol to access common imidazolinium salts, since it is applicable to a wide variety of primary amines. The results show the possibility to modulate the size and the number of heteroatoms of the heterocycle and to introduce independently various substituents.