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.
A ruthenium metathesis catalyst Ru1 bearing a cyclic (alkyl)(amino)carbene (CAAC) ligand is used in the ethenolysis reaction of biosourced 90% pure undistilled ethyl oleate or technical fatty acid methyl esters mixture with grade 3 ethylene, to yield 1-decene and valuable terminal unsaturated 9-decenoic acid ester. Under optimized conditions (25 ppm of catalyst Ru1, 20 bar, 50°C, 2 h, use of metal scavenger/quenching agent, SnatchCat), the title reaction proceeds with high productivity (up to 70% conversion) and selectivity, and is tested in scale up to 1 L, with the exclusion of a glovebox or Schlenk techniques usage, and the ethenolysis products are isolated by fractional distillation. In addition, a large-scale synthesis of Ru1 is presented. Practical Applications: Decreasing availability of fossil organic feedstock and current policies of the European Union drives scientists to look for alternative sustainable sources of fine chemicals. In this context, olefin metathesis based ethenolysis of transesterified seed oils studied herein shows a significant potential as a sustainable way to obtain valuable products, such as 1-decene and valuable terminal unsaturated 9-decenoic acid ester. Unlike other published studies on ethenolysis, in the present work, the focus is on optimizing this reaction under industrially practical conditions, thus utilizing an air-stable catalyst, technical grade oleic substrates, grade 3 ethylene, and excluding the use of a glovebox or Schlenk techniques.
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.
Ruthenium-based catalysts bearing quaternary ammonium groups in their N-heterocyclic carbene (NHC) fragments and different counter-ions were synthesised and tested in various olefin metathesis transformations.
A set of olefin metathesis catalysts bearing a ruthenium amide moiety was synthesised. In the ruthenium amide form these complexes exhibit very low activity in standard metathesis reactions. However, a dramatic increase of activity was observed upon in situ activation with trimethylsilyl chloride or HCl, allowing successful application of such catalysts in a number of model ring‐closing metathesis, cross‐metathesis and enyne transformations. Moreover, such activated complexes proved to be very effective catalysts for bulk polymerisation of dicyclopentadiene (DCPD). The influence of factors such as temperature and the nature of additives on the properties of poly‐DCPD was examined.
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