LETTER
Palladium-Catalysed Alkynylation of Aryl Bromides in an Ionic Liquid
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R
4 hours: from bromobenzene (82%, entry 6, row 2), from
4-bromonitrobenzene (58%, entry 8) and from 3,5-bis(tri-
fluoromethyl)bromobenzene (92%, entry 9). Therefore,
and contrary to the case of deactivated substrates, an ac-
curate control of the reaction time is required to increase
the selectivity towards the desired enyne.
R
H
[Pd]/L/pyrrolidinium
[BMIM][BF4]
R'
R'
H
+
R
H
R'
H
(E)
(Z)
Scheme 2 Hydrogenation side products obtained from enynes.
The coupling reaction of phenylacetylene to 4-bromoni-
trobenzene appeared as a particular case, for which a sig-
nificant concurrent reaction was the arylation of the
pyrrolidine base. Thus, substantial amounts of pyrrolidi-
nyl-4-nitrobenzene (17–34%, entry 8) were formed what-
ever the reaction time (2–20 h). Eventually, this product
spontaneously crystallised from the crude ethereal extrac-
tion mixture upon standing few hours at low temperature:
the X-ray crystal structure determined is presented in
Figure 2.15
In summary, we have disclosed the first methodology
allowing the alkynylation of a variety of functionalised
aryle bromides in an ionic liquid medium. The copper-
free system combining 0.5 mol% of [Pd(allyl)Cl]2, 3
mol% PPh3 and 1.2 equiv pyrrolidine is economic, simple
and readily accessible. In [BMIM][BF4] the coupling of
demanding bromides is more selective comparatively to
the coupling of activated aryl bromides. This fact is prob-
ably due to the inertness of demanding substrates (elec-
tronic or steric in origin) as supported by the various side
reactions obtained from activated bromides. Finally, the
presumed palladium-catalysed hydrogenation of aryl/
alkyne subsequent to Heck or Sonogashira reaction has
never been documented before, and could lead, after de-
velopment and optimisation, to very elegant cascade-type
catalysis.
Figure 2 POV-Ray representation (ORTEP) of the molecular struc-
ture of the amine arylation product pyrrolidinyl-4-nitrobenzene.
Acknowledgment
Thanks are due to S. Royer and G. Delmas for their technical help.
We sincerely thank the ‘Conseil Régional de Bourgogne’ for finan-
cial support, especially concerning GC apparatus.
We checked the possibility, using our catalytic system, to
employ aliphatic acetylenes for Heck alkynylation in
[BMIM][BF4]. For this purpose, we chose the long-chain
aliphatic terminal alkyne 1-decyne. Without optimisation,
good to excellent conversions were obtained using either
activated or deactivated bromides (Table 1, entries 10–
13); the side products obtained were mostly the result of
triple-bond isomerisation, with some hydrogenation prod-
ucts also detected.
References and Notes
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The enyne hydrogenation observed (Scheme 2) suggests
that after completion of the coupling reaction, the palladi-
um catalyst then acts as a hydrogen-transfer catalyst. Al-
though at this point the formation of a palladium-hydride
species through insertion of the metal into the imidazoli-
um C2–H bond cannot be totally ruled out,16 it seems more
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Synlett 2006, No. 18, 3005–3008 © Thieme Stuttgart · New York