Recent advances in neutral and anionic N-heterocyclic carbene - betaine interconversions. Synthesis, characterization, and applications

Some mesoionic compounds, i.e. five-membered representatives of the class of conjugated mesomeric betaines (CMB), are in equilibrium with their tautomeric normal N-heterocyclic carbenes (nNHC). In addition, anionic N-heterocyclic carbenes, generated by deprotonation of mesoionic compounds, have been described. The first examples of conversions of cross-conjugated mesomeric betaines (CCMB), 6-oxopyrimidinium-4-olates, into normal N-heterocyclic carbenes have been reported as well. CCMB such as imidazolium-4-carboxylate and pyrazolium-4-carboxylate can decarboxylate to form abnormal (aNHC) or remote N-heterocyclic carbenes (rNHC). Most conversions of betaines into N-heterocyclic carbenes start from pseudocross-conjugated mesomeric betaines (PCCMB) which can be regarded as heterocumulene adducts of nNHC. Thus, decarboxylations of imidazolium-2-carboxylates, 1,2,4-triazole-3-carboxylates, pyrazolium-3-carboxylates or indazolium-3-carboxylates yield N-heterocyclic carbenes which have been used in catalysis, complex chemistry, heterocyclic synthesis, and organocatalysis.