Syntheses and Applications of Indol-2-ylidene-Ligated Ruthenium-Based Olefin Metathesis Catalysts

Ru-based
catalysts bearing ambiphilic carbenes exhibit exceptionally
high turnover numbers (TONs) in olefin metathesis reactions. However,
the syntheses of these catalysts are still challenging because of
the strong ambiphilicities of the carbenes. Herein, we prepared a
family of indol-2-ylidene (IdY)-ligated Ru-based (Ru–IdY) olefin metathesis catalysts and
obtained two X-ray crystal structures of Ru–IdY complexes showing a distinct conformation of the ruthenium
centers. An exceptional stability of the complexes was demonstrated
by a decomposition test conducted in deuterated benzene at 40 °C,
indicating that approximately 98% of the most stable Ru–IdY catalyst was retained even after 3 weeks.
The catalytic activities of Ru–IdY catalysts were investigated for ring-closing metathesis, ring-opening
metathesis polymerization (ROMP), and ethenolysis. The most sterically
accessible Ru–IdY catalyst, 2e, exhibited the best catalytic activity for ethenolysis
(TON 61200 and selectivity for methyl oleate 95%; TON 53200 and selectivity
for cis-cyclooctene 70%), whereas the sterically
demanding catalyst 2f demonstrated the most efficient
catalytic activity for ROMP in comparison to the other Ru–IdY catalysts, achieving a complete conversion
in 2 min.