Palladium Complexes of N-Heterocyclic Carbenes as Catalysts for Cross-Coupling Reactions-A Synthetic Chemist's Perspective

Palladium‐catalyzed CC and CN bond‐forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N‐heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd‐mediated cross‐coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong σ‐electron‐donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong PdNHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user‐friendly, and powerful NHC–Pd precatalysts, the goal of a universal cross‐coupling catalyst is within reach. This Review discusses the basics of Pd–NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in CC and CN cross‐coupling as well as carbopalladation reactions. Universal cross‐coupling catalysts: Pd0 complexes of bulky N‐heterocyclic carbene (NHC) ligands show unprecedented substrate scope and high activity. The σ‐electron donating NHCs renders the oxidative addition, even into highly functionalized and deactivated substrates, facile whereas the steric bulk expedites reductive elimination. At the same time, the strong Pd–NHC bonds ensure excellent catalyst longevity.