Integrating Activity with Accessibility in Olefin Metathesis: An Unprecedentedly Reactive Ruthenium-Indenylidene Catalyst

Access to leading olefin metathesis
catalysts, including the Grubbs,
Hoveyda, and Grela catalysts, ultimately rests on the nonscaleable
transfer of a benzylidene ligand from an unstable, impure aryldiazomethane.
The indenylidene ligand can be reliably installed, but to date yields
much less reactive catalysts. A fast-initiating, dimeric indenylidene
complex (Ru-1) is reported, which reconciles high activity
with scaleable synthesis. Each Ru center in Ru-1 is stabilized
by a state-of-the-art cyclic alkyl amino carbene (CAAC, C1) and a bridging chloride donor: the lability of the latter elevates
the reactivity of Ru-1 to a level previously attainable
only with benzylidene derivatives. Evaluation of initiation rate constants
reveals that Ru-1 initiates >250× faster than
indenylidene
catalyst M2 (RuCl2(H2IMes)­(PCy3)­(Ind)), and 65× faster than UC (RuCl2(C1)2(Ind)). The slow initiation previously
regarded as characteristic of indenylidene catalysts is hence due
to low ligand lability, not inherently slow cycloaddition at the Ru=CRR′
site. In macrocyclization and “ethenolysis” of methyl
oleate (i.e., transformation into α-olefins via cross-metathesis
with C2H4), Ru-1 is comparable
or superior to the corresponding, breakthrough CAAC-benzylidene catalyst.
In ethenolysis, Ru-1 is 5× more robust to standard-grade
(99.9%) C2H4 than the top-performing catalyst,
probably reflecting steric protection at the quaternary CAAC carbon.