Mesomeric Betaines and N-Heterocyclic Carbenes of Pyrazole and Indazole

Abstract

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 Nheterocyclic carbene pyrazol-4-ylidene. These relationships give rise to an interesting interplay between the two compound classes, N-heterocyclic carbenes and heterocyclic mesomeric betaines.

A DFT and ab initio study of conjugated and semi-conjugated mesoionic rings and their covalent isomers

“…The subtle difference between semi-conjugated and pseudo semi-conjugated HMBs (and also between cross-conjugated and pseudo cross-conjugated HMBs) has been described previously. 1,5 A comprehensive classification system requires this distinction but in practice the pseudo species appear to have very similar properties to the non-pseudo species and for routine discussion, but not always, 6,7 the distinction is probably not necessary.…”

Synthesis of 2-anilinobenzimidates, anthranilamides, and 2,3-dihydroquinazolin-4(1H)-ones from N-heterocyclic carbenes of indazole

“…We therefore envisaged another approach to intermediates A by ring-cleavage of N-heterocyclic carbenes of indazole E. These are readily available from indazolium salts F or indazolium-3-carboxylates, 28 as summarized in a recent review article. 29 Exchanging the alcohols to water in this procedure should also conveniently produce anthranilamides I by tautomerisation of the O-unsubstituted benzimidates II (R´= H).…”

Organic Synthesis with N‐Heterocyclic Carbenes of Indazole: Synthesis of Benzo(thio)imidates, Benzo[d][1,3]thiazines and Quinazoline‐4‐thiones

“…[31] (Scheme 2) To the best of our knowledge, 2-anilinobenzothioimidates have not been described before, as we were not able to find any examples in the literature. In view of the synthetic potential of N-heterocyclic carbenes of pyrazole [32][33][34] and indazole, [34,35] we envisaged the first synthetic approach as outlined in Scheme 3. We used indazolium salts I as protonated precursors of the N-heterocyclic carbenes II (indazol-3-ylidenes) which can be trapped as thiones or rhodium complexes.…”