Bicyclic (alkyl)(amino)carbenes (BICAACs): synthesis, characteristics, and applications

Carbenes in general and isolable NHCs (N-heterocyclic carbenes) in particular have been useful ligands in recent years. The emergence of CAACs [cyclic(alkyl)(amino)carbenes], BICAACs [bicyclic(alkyl)(amino)carbenes], and many other carbenes has marked revolutionary milestones in this field. These carbenes possess an intriguing blend of highly electrophilic and nucleophilic characteristics, owing to their remarkably narrow HOMO–LUMO energy gap. The isolation and characterization of these carbenes hold significance not only due to their fascinating electronic properties but have demonstrated their prowess across various domains, including isolation of transition metal complexes, medicinal applications, catalysis, and radical stabilization. While the chemistry of 5-membered NHCs and CAACs has been extensively explored, the investigation of BICAACs has just begun. This review covers the synthesis, characterization, and reactivity of BICAACs and outlines the diverse applications of BICAACs in organometallic chemistry, metal-free catalysis, and main-group chemistry.