Unhindered Biplane-Shaped N-Heterocyclic Carbenes as Powerful Ligands in Challenging Ru-Catalysed Olefin Metathesis of Sterically-Crowded Substrates

Despite notable progress, olefin metathesis methods for preparing sterically crowded C-C double bonds remain scarce. They are commonly based on specialised ruthenium catalysts with sterically reduced N-heterocyclic carbene (NHC) ligands, able to accommodate more crowded olefinic substrates during the catalytic steps. Yet, although being highly active, these complexes are rather unstable. In the case of Ru complexes containing NHC ligands with N-aryl groups, the catalyst deactivation is mainly due to intramolecular C-H activation at the ortho position of the N-aryl group of the NHC ligand. Considering that the deleterious C-H activation process requires the rotation of the N-aryl arm of the NHC ligand, we introduced a second decker of aromatic groups in benzimidazolylidene-based N-phenyl NHC ligands, which led to robust and highly efficient ruthenium metathesis catalysts in challenging metathesis reactions of tri-and tetra-substituted olefins. The beneficial effect of these upper aromatic “wings” on the stability and activity of the Ru-complexes is rationalised through the experimental determination of the stereoelectronic properties of the NHC ligands, complemented by DFT calculations on the nature of the through-space interactions between the aromatics and on the decomposition pathway of these second-generation Hoveyda precursors.