Ring-expanded N-heterocyclic carbene (reNHC) complexes: Applications in transition metal catalysis

In the past two decades, NHCs (NHC = N-heterocyclic carbenes) have emerged as one of the most important classes of ligands in transition metal catalysis. Although historically 5-membered carbenes, such as imidazol-2-ylidenes, have been the dominant class of NHC ligands in transition metal catalysis, recently, ring expanded carbenes on 6-, 7-, 8-, 9- and 10-membered ring systems have been shown to provide significant advantages in numerous catalytic reactions embracing various metals, transformations and mechanisms. In this comprehensive review, we provide a survey of the applications of ring-expanded N-heterocyclic carbenes (reNHCs) in transition metal catalysis. Spanning from the initial reports on ring-expanded NHCs through June 2023, the review centers on the types of ring-expanded NHCs, their organometallic complexes and applications in catalytic processes. By delving into the impact of ring-expanded NHC ligands, the review aims to provide the reader with an understanding of the significance of this ligand class and its pivotal role in transition metal catalysis and organometallic chemistry.