DOI: 10.1002/chem.201404166
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Cross-Metathesis
Cross-Metathesis of Terminal Alkynes
[a]
Rudy Lhermet and Alois Fꢀrstner*
Abstract: Terminal acetylenes are amongst the most prob-
lematic substrates for alkyne metathesis because they tend
to undergo rapid polymerization on contact with a metal al-
kylidyne. The molybdenum complex 3 endowed with triphe-
nylsilanolate ligands, however, is capable of inducing surpris-
ingly effective cross-metathesis reactions of terminal alkyl
acetylenes with propynyl(trimethyl)silane to give products of
conventional tactics of deprotonation/silylation and excels
as an orthogonal way of alkyne protecting group chemistry
for substrates bearing base-sensitive functionalities. More-
over, it is shown that even terminal aryl acetylenes can be
cross-metathesized with internal alkyne partners. These un-
precedented transformations are compatible with various
functional groups. The need to suppress acetylene forma-
tion, which seems to be a particularly effective catalyst
poison, is also discussed.
1
type R ꢀCꢁCSiMe . This unconventional way of introducing
a silyl substituent onto an alkyne terminus complements the
Introduction
ticularly favorable in the presence of external donor ligands L.
Although X-ray data show that the ligand in the resulting de-
3
[4–8]
The great strides in alkyne metathesis during the last decade
were powered by a new generation of catalysts that combine
protiometallacycle C is h -bound in the solid state,
a haptici-
ty change suffices to reveal an alkynyl–alkylidene motif (D),
[1,2]
[9,10]
high activity with an excellent chemoselectivity profile.
Ade-
which can trigger rapid substrate polymerization.
quate post-metathetic transformations allow the resulting
functionalized alkyne products to be converted into a host of
The productive channel II is not free of serious complica-
tions either. Cycloreversion of E forms the desired product
RCꢁCR and leads to the metal methylidyne F, which then
reacts with a terminal alkyne substrate RCꢁCH via metallacycle
H to regenerate the loaded complex A, as necessary for sus-
tained metathesis; at the same time, however, acetylene is pro-
duced which is well soluble in most organic media, despite its
low boiling point. For its small size and good donor properties,
HCꢁCH starts to outcompete the actual substrate and hence
to deplete the loaded catalyst A. An increasing concentration
of the methylidyne F, favors the dimerization of this fairly un-
[
1–3]
structural motifs.
With regard to the substrate scope, how-
ever, the reaction remains largely confined to the use of inter-
nal acetylene derivatives, whereas applications to terminal al-
kynes are exceedingly rare. As such substrates are often more
readily accessible, however, attempts at changing this situation
are deemed relevant from the synthetic vantage point.
The initial forays to do so go back to the pioneering studies
of Schrock and co-workers on the chemical behavior and
metathesis activity of well-defined high-valent metal alkyli-
[
4,5]
[11]
dynes of tungsten and molybdenum.
There authors clearly
stable species with formation of dimetallatetrahedranes G
[12,13]
delineated the many pitfalls for the productive conversion of
terminal alkynes. As in every metathesis of unsymmetrical sub-
strates, the two possible orientations of the reactants in the
first [2+2] cycloaddition open two competing pathways I and
II which are interconnected by virtue of the fact that this initial
step is reversible (Scheme 1). In case of a terminal alkyne, how-
ever, channel I is not just degenerate but potentially destruc-
tive. Specifically, it was shown that a metallacyclobutadiene of
type B is prone to transannular CꢀH activation with concomi-
tant loss of one of the ancillary ligands X that picks up the
proton and hence formally acts as a base; this pathway is par-
(Scheme 1).
Such m-bridging acetylene complexes, in turn,
are able to first insert additional HCꢁCH and thence progres-
sively will add further substrate, thus opening yet another
polymerization channel. In any case, the available evidence
suggests that accumulating acetylene is a veritable catalyst
[14]
poison.
Early preparative attempts supported the notion that pro-
ductive metathesis of terminal alkynes is difficult, if not impos-
sible. Thus, Schrock and co-workers had already noticed that
phenylacetylene is polymerized on contact with (tBuO) WꢁCR
[4,10]
(1a, R = Ph).
Likewise, the Mortreux group found that
treatment of various terminal alkynes with complex 1b (R =
tBu) resulted only in a short initial phase of metathesis, which
[
a] Dr. R. Lhermet, Prof. Dr. A. Fꢀrstner
Max-Planck-Institut fꢀr Kohlenforschung
[15]
was then quickly superseded by polymerization. In a coura-
geous endeavor, these authors tried to empirically optimize
4
5470 Mꢀlheim/Ruhr (Germany)
Fax: (+49) 208 306 2994
E-mail: fuerstner@kofo.mpg.de
[16]
the system such that metathesis would endure. Using quinu-
clidine as an additive, they actually managed to accomplish
a first successful self-metathesis reaction, by which 1-heptyne
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2014, 20, 1 – 7
1
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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