Imidazol-2-and-4-ylidene by decarboxylation. Studies on the cross-conjugated mesomeric betaine-alkaloid norzooanemonine and its pseudo-cross-conjugated isomer.

1,3-Dimethylimidazolium-2-carboxylate and -4-carboxylate (norzooanemonine), which belong to two distinct classes of heterocyclic mesomeric betaines, undergo thermal decarboxylations to the N-heterocyclic carbenes imidazol-2-ylidene and imidazol-4-ylidene, respectively. These carbenes can be detected by ESI mass spectrometry and can be trapped by isocyanates to imidazolium-amidates, the structure of which was proved by independent syntheses. We performed calculations to characterize the different types of conjugation in the imidazolium-carboxylates.