Hetarenium salts

This review tries to shed light on the early history of different methods for the generation of N-heterocyclic carbenes (NHCs), i.e., the extrusion of heterocumulenes (decarboxylations) from suitable mesomeric betaines, deprotonations of hetarenium salts, a-eliminations, tautomerizations of mesomeric betaines, and reductive desulfurizations of cyclic thioureas. Selected examples of acyclic and three- to eight-membered NHCs are presented, as well the generation of selected five- and six-membered anionic NHCs. 

Scheme 3 depicts preparative routes to NHC I from cyclic precursors. The extrusion of heterocumulenes, such as carbon dioxide from suitable precursors II, the deprotonation of hetarenium salts HI, the a-ehmination... 

Deprotonations of hetarenium salts as a source of NHCs can be traced back to at least the first half of the 20th century. In 1943, thiazoHum salts were found as catalysts for benzoin condensations (1943JPS(JP)269), and in 1957, Breslow reported on the rapid deuterium exchange of thiazolium salts 42 via the anion at C-2 of thiazolium salts (i.e., the NHC 43 which also plays a role in vitamin Bl) (Scheme 11), and correspondingly, of 1,3-dimethylbenzimidazoHum iodide (1958JA3719). These results also inspired... 

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