Non-classical N-Heterocyclic Carbene Complexes

University College Dublin, School of Chemistry and Chemical Biology, Belfield, Dublin 4, Ireland 

The transition metal chemistry of N-heterocyclic carbenes (NHCs) is dominated by imidazol-2-ylidenes and their derivatives including oxazoles, thiazoles, 1,2,4-triazoles, and pyrimidines, as well as related saturated systems/ These carbenes have found widespread application as strongly donating ligands in transition metal-catalysed reactions and also as organocatalysts in their own right/ They have become highly popular because the free carbenes are stabilised by two adjacent heteroatoms, which renders the carbene often sufficiently stable to be manipulated and isolated under ambient conditions/ 

In contrast to these classical types of NHCs, isomers with less extensive heteroatom stabilisation, termed here non-classical carbenes , initially received much less attention/ Non-classical carbenes can contain just one, or even no heteroatom in positions a to the carbene carbon atom. Due to the lower number of heteroatoms, n stabilising effects are substantially diminished when compared to classical carbenes, which in turn reduces the stability of the free carbene. On the other hand, the electron-withdrawing inductive influence of the heteroatoms is significantly decreased. As a consequence, these non-classical carbenes are surmised to be stronger donors than their classical analogues. Lately, they have gained interest especially as ligands in transition metal complexes because of their distinct electronic properties, which open up new synthetic and catalytic opportunities. 

N-Heterocyclic Carbenes From Laboratory Curiosities to Efficient Synthetic Tools 

Despite these formal ambiguities, non-classical variations of NHCs constitute an attractive class of ligands with great opportunities for catalysis and synthesis in general. This Chapter aims to overview the most recent advances in using non-classical NHC ligands in transition metal-mediated catalysis and to illustrate the attractive prospects that emerge from these achievements. 

---

Non-classical N-Heterocyclic Carbene Complexes C-2 bound carbene... 

---