MCRs Involving Carbenes as Key Components

Carbenes are neutral species containing a carbon atom with only six valence electrons. In the case of carbenes containing electron-rich substituents (-OR, -NR, -SR), the heteroatom lone pair donation into the formally vacant p-orbital of the carbene carbon produces stabilization of the singlet state of the carbene [1], enhancing its nucleophilicity (Fig. 5.1). 

---

S.2.2.3 MCRs Involving Triazol-5-ylidenes as Key Components MCRs involving triazol-5-ylidenes V are more difficult as a result of the diminished nucleophiUcity of these carbenes compared with previously described thia-zol-2-ylidenes II and imidazol-2-yUdenes ID. However, the importance of triazole moieties in relevant organic compounds has encouraged the use of triazole carbenes as key components in MCRs. To this aim, the reaction of triazol-5-ylidene V, DMAD, and different aromatic aldehydes provided fully substituted furanone derivatives 25 [21]... 

S.2.2.2 MCRs Involving Imidazol-2-yUdenes or Imidazolin-2-ylidenes as Key Components Imidazol-2-ylidenes III can also achieve similar reactivity to thiazol-2-ylidenes II. In 2003, Nair and coworkers described the first-ever MCR of this family of NHCs, wherein two related carbenes reacted differently. As shown in Scheme 5.11, reaction of imidazol-2-ylidenes III (R=aryl) with DM AD and aldehydes afforded furanones 15. However, when more nucleophilic imidazolin-2-ylidenes IV (R=aryl) were used, under the same reaction conditions, 2-oxy-maleate derivatives 16 were obtained [16]. 

---