The state‐of‐the‐art in olefin metathesis is application of N‐heterocyclic carbene (NHC)‐containing ruthenium alkylidenes for the formation of internal C=C bonds and of cyclic alkyl amino carbene (CAAC)‐containing ruthenium benzylidenes in the production of terminal olefins. A straightforward synthesis of bis(CAAC)Ru indenylidene complexes, which are highly effective in the formation of both terminal and internal C=C bonds at loadings as low as 1 ppm, is now reported.
The
active species generated from ruthenium complexes bearing N-heterocyclic
carbene (NHC) ligands exhibit limited stability under certain conditions
(high dilution, high temperature) and in the presence of nitrile functionality.
As a consequence, industrial implementation of metathesis for the
production of important materials such as macrocyclic musks and polyamide
11 is uneconomical. Over the past decade, hundreds of ruthenium complexes
bearing NHC ligands have been obtained. However, to this date, they
have brought rather limited improvements in efficiency. In this paper,
we report on cyclic alkyl amino carbene (CAAC) ruthenium complexes
that promote highly challenging macrocyclization and cross metathesis
(CM) with acrylonitrile reactions at loadings as low as 10–20
ppm.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.