Biplane-like small N-heterocyclic carbenes as effective ligands in challenging Ru-catalyzed metathesis of sterically crowded olefins

Olefin metathesis methods for preparing sterically hindered alkenes remain scarce. They are commonly based on specialized ruthenium catalysts with sterically reduced N-heterocyclic carbene (NHC) ligands, which are able to accommodate large olefinic substrates during the catalytic steps. Yet, these complexes easily undergo intramolecular C–H activation at the ortho position of the N-aryl group of the NHC, which leads to their decomposition. Considering that this deleterious process requires the rotation of one of the NHC’s N-aryl arms, we introduced a second decker of aromatic groups into this ligand. Such steric blockade led to more stable and highly efficient catalysts for challenging metathesis reactions of sterically crowded olefins. The beneficial effect of these upper aromatic “wings” is rationalized through experimental determination of the stereoelectronic properties of the resulting NHC ligands, complemented by density functional theory (DFT) calculations on the nature of the through-space interactions between the “wings” and on the decomposition pathway of these complexes.