Heterocycle Syntheses with Anionic N‐Heterocyclic Carbenes: Ring Transformations of Sydnone Imine Anions

Abstract

Sydnone imines were deprotonated at the C4 position to give lithium‐stabilized anions, which proved to be stable for several months in solution. These anions are elements of the intersection of the substance classes of N‐heterocyclic carbenes and mesomeric betaines. They can be formulated as anionic N‐heterocyclic carbenes. The negative charge translates into high‐field shifts of the 13C NMR resonance frequencies of C4 in comparison to other NHCs. Similar to the chemistry of N‐heterocyclic carbenes, reactions with isocyanates and isothiocyanates gave 4‐(N‐carbamoyl)‐benzoylsydnone imines in trapping reactions. As a result of ring transformations involving the characteristic π‐architecture of the mesoionic framework, imidazolidine‐2,4‐dithiones, imidazolin‐2,4‐diones and 1,2,4‐triazol‐5‐ones were formed depending on the substitution pattern of the starting materials and the reaction conditions.