New Aspects of Recently Developed Rhodium(N-Heterocyclic Carbene)-Catalyzed Organic Transformations

N-Heterocyclic carbene (NHC) compounds have attracted significant attention as ligands for transition metal catalysis and organometallic complexes. Over the last two decades, among the various transition metals, ruthenium-NHC-type metathesis catalysts and palladium-NHC-based cross-coupling catalysts have been extensively studied and developed. Recently, however, there has been a resurgence in developing NHC complexes of group 9 metals (Co, Rh, and Ir) for their use in organic transformations. This review specifically focuses on the advances in rhodium-NHC catalysts. From the classical NHC ligands to the most recently developed CAAC (cyclic alkyl amino carbene)-type ligands, the impact of using NHC ligands for increasing activity and enhancing selectivity across multiple reactions is discussed. We address the recent achievements in Rh(NHC) catalysis, which include (i) reactions involving rhodium hydride species, (ii) cross-coupling reactions, (iii) bond cleavage reactions, and (iv) reduction reactions. Mechanistic insights based on the active catalytic species and a discussion about harnessing their unique reactivity for the future development of rhodium catalysis are given. (Figure presented.).