Development of six-membered saturated cyclic diaminocarbene in main-group chemistry

Abstract

In recent years, there has been a remarkable surge in the utilization of saturated N-heterocyclic carbenes (NHCs) as ligands in main group chemistry. While the field has predominantly focused on five-membered NHCs and cyclic alkyl(amino)carbene (CAAC) ligands, the investigation of six-membered NHCs (6-NHC) has only just begun. Despite possessing higher nucleophilicity than their five-membered counterparts, 6-NHCs have been less explored due to their lack of structural rigidity. This feature article aims to highlight recent developments of 6-NHCs in only main group chemistry. Exciting new studies have demonstrated the clean activation of B-H bonds in HBpin, ring expansion from a six-membered to a seven-membered ring at ambient conditions, and the stabilization of transient units "H-B=O" and "(OH)B=O," among other intriguing phenomena. A major focus of this review is on synthetic approaches to 6-NHC-stabilized main-group compounds, and their unusual properties, as revealed through spectroscopic and crystallographic data. While the chemistry of 6-NHC is still in its nascent stage, the findings presented in this feature article underscore the need for its exploration and further investigation. Furthermore, this review provides valuable insights into meaningful synthetic methods for creating new 6-NHC·main group (B, Al, Si, P, Zn, etc.) complexes, along with mechanistic explanations for some of these reactions. These advances hold great promise for the future development of this exciting and rapidly evolving field of N-heterocyclic chemistry.