Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Automated summary

Since the discovery of persistent carbenes, the design and applications of this class of ligands and their metal complexes have rapidly grown. N-heterocyclic carbenes (NHCs) have become popular among chemists due to their modular synthesis and adjustability. The strength of the metal-carbon bond in transition metal complexes bearing NHCs depends on the energy match between the NHC ligand and the metal ion. NHC ligands have been explored with both transition metals and main group elements. The catalytic applications of NHC ligands linked with a second tether have been extensively studied, with carbon-carbon coupling reactions, transfer hydrogenation, ketone hydrosilylation, and chiral catalysis being among the most popular reactions. Areas such as CO2 reduction and C-H borylation have received less attention. The synthesis and applications of transition metal complexes are also covered.