Persistent carbene

A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the 'R's are various functional groups. The groups can be bridged so that the carbon with unfilled orbitals is part of a heterocycle, such as imidazole, thiazole or triazole.

Quotes

 * In conclusion, it is obvious that N-heterocyclic carbenes and their unique, but concurrently versatile reactivity have already proven wide applicability. The offered catalytic (and enantioselective) access to important reactive intermediates suchas homoenolates or enolates as well as their additional catalytic properties will smooth the way for mild and sustainable synthesis of multifunctionalized compounds.
 * K. Zeitler, "N-Heterocyclic Carbenes: Organocatalysts Displaying Diverse Modes of Action", in Organocatalysis (2008) edited by M.T. Reetz, B. List, S. Jaroch, H. Weinmann