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 C-13 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 p-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.