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 Z-selectivity
in a variety of metathesis reactions have recently been reported.
It was envisioned that catalysts possessing six-membered chelates
could similarly exhibit high Z-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 Z-olefin products. In addition, substitution with bulkier N-aryl groups led to improved Z-selectivity.
The effect of ligand structure on stereoselectivity discovered in
this study will be useful in the future design of highly active and Z-selective ruthenium catalysts.