Eight-membered NHCs

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. 

Needless to say, we are, therefore, neither able to provide a comprehensive overview of each aspect of the chemistry of NHCs here, nor to give a broad picture of the character of this class of compounds. We try to shed some light on the history of NHCs (1998CUZ6) and show lines that lead from the early history to the synthetic methods for the generation of NHCs that are widely appHed nowadays. In addition, we present a very short survey of selected structural variations ofacychc to eight-membered NHCs. Not unexpectedly, we came soon across the works of Alan R. Katritzky during our hterature research. 

Synthetic routes to NHCs are very flexible, and a vast nimiber of structures are possible. NHCs based on imidazole and imidazolidine cores are the most common, although 1,2,3- and 1,2,4-triazole and oxazole, thiazole, and benzimidazole cores have also been used (among others). Several researchers, most notably Cavell and coworkers, have also explored the chemistry of so-called ring-expanded NHCs, where a six-, seven-, or eight-membered ring forms the core of the NHC [50]. By suitable modification of the synthetic route, it is possible to introduce a variety of substituents on the backbone of the core, or at the N-sub-stituents. This enormous flexibility has contributed to the rapid growth of this area of chemistry. 

NHCs featuring an aliphatic seven- or eight-membered membered ring scaffold of type or 147c were also obtained by deprotonation... 

Figure 1.12 Stable NHCs with six-, seven- and eight-membered heterocycles.

Figure 10.5 [C N N]-pincer, six-, seven- and eight-membered ring, frares-diisocy-anato, T-shape [(NHC)Ni] complexes.

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