Thermally Stable, Latent Olefin Metathesis Catalysts

Highly thermally stable N-aryl, N-alkyl N-heterocyclic
carbene
(NHC) ruthenium catalysts were designed and synthesized for latent
olefin metathesis. These catalysts showed excellent latent behavior
toward metathesis reactions, whereby the complexes were inactive at
ambient temperature and initiated at elevated temperatures, a challenging
property to achieve with second-generation catalysts. A sterically
hindered N-tert-butyl substituent
on the NHC ligand of the ruthenium complex was found to induce latent
behavior toward cross-metathesis reactions, and exchange of the chloride
ligands for iodide ligands was necessary to attain latent behavior
during ring-opening metathesis polymerization (ROMP). Iodide-based
catalysts showed no reactivity toward ROMP of norbornene-derived monomers
at 25 °C and upon heating to 85 °C gave complete conversion
of monomer to polymer in less than 2 h. All of the complexes were
very stable to air, moisture, and elevated temperatures up to at least
90 °C and exhibited a long catalyst lifetime in solution at elevated
temperatures.