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.

Looking for the Noncyclic(amino)(alkyl)carbene Ruthenium Catalyst for Ethenolysis of Ethyl Oleate: Selectivity Is on Target.

A wide set of 65 diverse Ru metathesis catalysts was investigated in the ethenolysis reaction of biosourced ethyl oleate to allow the comparison between the catalyst structure and its activity and selectivity. Handling of the oleic substrate, weighing of the catalysts, and charging the reactor were done in air, with exclusion of a glovebox or Schlenk techniques. A catalyst bearing the unsymmetrical N-heterocyclic ligand featuring a thiophene fragment (Ru-63) was selected to offer the best combination between high selectivity and sufficient activity under conditions mimicking oil industry practice. A proof-of-concept large-scale ethenolysis experiment was also done with the selected catalyst to prove its high selectivity at the 1 L scale reaction with a 90% pure non-distilled substrate.