Design Concepts for N-Heterocyclic Carbene Ligands

The design and development of new N-heterocyclic carbene (NHC) ligands are crucial elements for the advancement of homogeneous metal catalysis. Design concepts such bulky-yet-flexible, ring-expanded, and supramolecular NHC ligands, as well as NHC bearing pendant labile ligands, have already made an impact in this field. NHC ligands are nowadays easily accessible and efficiently tuned to meet the necessary reaction requirements, rendering them highly diverse with a wide range of applicability. Parallel to recent design breakthroughs, advancements in NHC understanding and in improved quantification of their properties have widely contributed to their recent success. N-heterocyclic carbenes (NHCs) are a widely used class of ligands that have enabled an exciting variety of organometallic chemistry and catalysis. As researchers design and customize these ligands for specific applications, various design concepts have emerged. This review covers several of these design concepts and their impact on and promise for organometallic chemistry and catalysis. These concepts are bulky ligands that remain flexible for substrate binding and reactivity, ring-expanded NHCs with different steric and electronic properties, NHCs appended to supramolecular assemblies that control the microenvironment around the metal center; and NHCs with pendant labile functional groups. N-heterocyclic carbenes (NHCs) are a widely used class of ligands that have enabled an exciting variety of organometallic chemistry and catalysis. As researchers design and customize these ligands for specific applications, various design concepts have emerged. This review covers several of these design concepts and their impact on and promise for organometallic chemistry and catalysis. These concepts are bulky ligands that remain flexible for substrate binding and reactivity, ring-expanded NHCs with different steric and electronic properties, NHCs appended to supramolecular assemblies that control the microenvironment around the metal center; and NHCs with pendant labile functional groups. interaction of a coordinatively unsaturated transition metal with a C−H bond. reactions of transition-metal complexes in which an organic ligand undergoes intramolecular metalation with formation of a metal-carbon σ bond. pyridine-enhanced precatalyst preparation stabilization and initiation. percent buried volume; a measurement of the bulk of a ligand coordinated to a transition metal.