Internal Catalytic Effect of Bulky NHC Ligands in Suzuki-Miyaura Cross-Coupling Reaction

The Suzuki-Miyaura reaction is one of the most interesting coupling schemes and is widely used in industrial applications. The catalytic cycle of Suzuki-Miyaura coupling has been investigated using N-heterocyclic carbene (NHC) ligands. Detailed analysis of the effect of the NHC ligand points out that bulky groups can promote the reaction in two different ways. First, the bulkiness of the NHC groups can cause steric repulsion in the precursor Pd complexes, which triggers the reaction toward the active Pd(0) compound. Second, in the oxidative addition step, favorable intramolecular pi-pi and C-H/pi interactions between the aryl-chloride reagent and the bulky groups of NHC can decrease the rate determining activation barrier, which can explain previous surprising experimental results. On the basis of our results, we show how appropriate substituents can be designed to promote the key step of catalytic reactions.