Investigations into Ruthenium Metathesis Catalysts with Six-Membered Chelating NHC Ligands: Relationship between Catalyst Structure and Stereoselectivity

A series of ruthenium catalysts bearing five-membered chelating NHC architectures that exhibit very high <i>Z</i>-selectivity in a variety of metathesis reactions have recently been reported. It was envisioned that catalysts possessing six-membered chelates could similarly exhibit high <i>Z</i>-selectivity and address limitations of this methodology. We thus prepared a number of new catalysts and systematically investigated the impact of the NHC and anionic ligand on their stereoselectivity. In standard metathesis assays, only catalysts containing six-membered chelated NHC structures and η²-bound anionic ligands favored the <i>Z</i>-olefin products. In addition, substitution with bulkier <i>N</i>-aryl groups led to improved <i>Z</i>-selectivity. The effect of ligand structure on stereoselectivity discovered in this study will be useful in the future design of highly active and <i>Z</i>-selective ruthenium catalysts