Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole and Indazole

The chemistry of the N-heterocyclic carbenes pyrazol-3-ylidene and indazol-3-ylidene, and of the remote N-heterocyclic carbene pyrazol-4-ylidene, is reviewed. Heterocumulene adducts of pyrazol-and indazol-3-ylidene are pseudo-cross-conjugated heterocyclic mesomeric betaines, from which the corresponding N-heterocyclic carbenes can be generated by thermal cleavage of the carbene-heterocumulene bond. Cross-conjugated heterocyclic mesomeric betaines such as pyrazolium-4-carboxylates are formal 1:1 adducts of the heterocumulene carbon dioxide with the remote N-heterocyclic carbene pyrazol-4-ylidene. These relationships give rise to an interesting interplay between the two compound classes, N-heterocyclic carbenes and heterocyclic mesomeric betaines. 1 Introduction 2 Classifications 2.1 Mesomeric Betaines of Pyrazole and Indazole 2.2 N-Heterocyclic Carbenes and Relatives of Pyrazole and Indazole 3 Conjugated Mesomeric Betaines of Pyrazole and Indazole 4 Ylides of Pyrazole 5 Cross-Conjugated Mesomeric Betaines and Remote N-Heterocyclic Carbenes of Pyrazole (Pyrazol-4-ylidenes) 6 Pseudo-Cross-Conjugated Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole (Pyrazol-3-ylidenes) 7 Pseudo-Cross-Conjugated Mesomeric Betaines and N-Heterocyclic Carbenes of Indazole (Indazol-3-ylidenes) 8 Conclusions