Letter
Ruthenium Carbene-Mediated Construction of Strained Allenes via
the Enyne Cross-Metathesis/Cyclopropanation of 1,6-Enynes
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ABSTRACT: Herein, we report on the unprecedented dimeriza-
tion of 1,6-enynes using a commercially available ruthenium
complex RuCl2(PPh3)3, which results in a series of bicyclo[3.1.0]-
hexyl allene derivatives in moderate to excellent yields. Mechanistic
investigation indicates that the in-situ-generated ruthenium vinyl-
idene undergoes a site-selective metathesis process to provide
allenyl ruthenium carbene, which can be intramolecularly trapped
by the pendent C=C bond of enyne through a [2 + 2] cycloaddition/metal elimination process.
nyne metathesis, as an efficient strategy for C−C bond-
produces cyclopropane-containing products.10 Several interest-
ing examples were presented by Iwasa et al., who used the
ruthenium phenyloxazoline complex as a catalyst in the
coupling reaction of diazoester and alkene/allene to produce
cyclopropyl ester/ketone.11 In the typical examples reported by
Dixneuf, an intramolecular alkene side chain of enynes was
utilized as a capture partner to successfully realize a
cyclopropanation process to access alkenyl bicyclo[3.1.0]-
hexane (Scheme 1A2).12 In another report made by the Trost
group, a similar strategy was performed to efficiently capture
ruthenium carbene intermediates to create bicyclo[3.1.0]-
hexane.13
Inspired by the above in-situ-generated allenic ruthenium
carbene, together with the extensively studied intramolecular
capture of ruthenium carbene, a ruthenium vinylidene-directed
metathesis/cyclopropanation reaction pathway was designed
for constructing strained allene, as shown in Scheme 1B. As we
envisioned, the site-selective metathesis of in-situ-generated
ruthenium vinylidene complex (A) and 1,6-enynes generates
the allenyl ruthenium carbene intermediate (B), which can be
intramolecularly captured by alkene motif, thereby realizing the
construction of unprecedented bicyclo[3.1.0]hexyl allenes.
This study not only represents a new methodology for the
allenes synthesis to enrich allene chemistry, but also provides
an example for the “yne-then-ene” mechanism of the
metathesis process between ruthenium vinylidene and enyne.
In the proof-of-concept experiments, we have focused on
cross-coupling reactions to test the feasibility of the ruthenium
carbene-directed construction of allenes, as shown in Scheme
2. The stoichiometric tert-butyl ruthenium vinylidene
E
forming reactions, has the tremendous ability to construct
complex molecules containing the 1,3-diene motif.1 Among
those extensively studied metathesis processes, the intermo-
lecular enyne cross-metathesis are especially attractive, since
this simplest cross-coupling or dimerization of two enyne
molecules is able to produce many interesting molecules2 with
inherent selectivity and an atom-economical manner through
those in-situ-generated metal carbenes. However, its applica-
tion for the creation of allene structures has not been
recognized yet, mainly because of the high activity of free
unsaturated carbenes that often lead to complicated by-
products arising from uncontrollable polymerization induced
by consecutive metathesis3 or cyclopropanation processes.4
Through investigations of the mechanism of enyne metathesis,
we find that the key point for the formation of allenes mainly
lies in the formation of in-situ-generated ruthenium vinylidene
and its site-selective metathesis process of ruthenium vinyl-
idene with the alkyne group of second enynes via the so-called
“yne-then-ene” mechanism (Scheme 1B), rather than the “ene-
then-yne” mechanism (Scheme 1A3).5
A brief survey of the published literature found that
homobimetallic ruthenium vinylidene can be activated by the
addition of phenyl acetylene in the ring-closing metathesis
reactions of 1,6-dienes, where Delaude proposed an
intermediate transformation from ruthenium vinylidene to
allenyl ruthenium carbene via a common [2 + 2] cycloaddition
process (Scheme 1A1).6 Their speculation offered valuable
information for the construction of allenes through the efficient
capture of this allenyl ruthenium carbene intermediate. From a
literature review of the reported methods used to capture
ruthenium carbenes, it was found that there are numerous
functional groups, for instance, amine,7 aldehyde,8 acetal,9 that
have the ability to trap ruthenium carbene. In particular, the
trapping of ruthenium carbene with an alkene chain via a [2 +
2] cycloaddition/metal reductive elimination process generally
Received: December 28, 2019
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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