The first catalytic method for Heck alkynylation of unactivated aryl bromides (copper-free Sonogashira) in an ionic liquid: 1 mol-% palladium/triphenylphosphane/pyrrolidine in [BMIM][BF4] as a simple, inexpensive and recyclable system

Article abstract of DOI:10.1002/ejoc.200600976

Herein we report the studies of Heck alkynylation (copper-free Sonogashira) with aryl halides (I, Br, Cl) employing various metallic precursors, tertiary phosphanes and bases in [BMIM][BF₄] as the solvent. As a result, we provide the first method that allows the coupling of a large array of substrates, either activated or deactivated bromides in an ionic liquid. Furthermore, the system of highest efficiency is unexpectedly the simplest and cheaper combination that employs [Pd(η³-C₃H ₅)Cl]₂/PPh₃ at only a 1 mol-% loading with pyrrolidine as the base and in the absence of a copper salt. The coupling of sterically and electronically deactivated bromides bearing different functional groups to aryl- and alkyl acetylenes, as well as the possibility of recycling, make these results of high interest to the future development of Heck-and Sonogashira-type reactions in ionic liquids. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200600976

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200600976
The First Catalytic Method for Heck Alkynylation of Unactivated Aryl
Bromides (Copper-Free Sonogashira) in an Ionic Liquid: 1 mol-% Palladium/
Triphenylphosphane/Pyrrolidine in [BMIM][BF4] as a Simple, Inexpensive and
Recyclable System
Jean-Cyrille Hierso,*[a] Julien Boudon,[a] Michel Picquet,*[a] and Philippe Meunier[a]
Keywords: Alkynylation / Aryl bromides / Ionic liquids / Heck reaction / Copper-free Sonogashira / Cross-coupling
Herein we report the studies of Heck alkynylation (copper-
free Sonogashira) with aryl halides (I, Br, Cl) employing vari-
ous metallic precursors, tertiary phosphanes and bases in
[BMIM][BF4] as the solvent. As a result, we provide the first
method that allows the coupling of a large array of sub-
strates, either activated or deactivated bromides in an ionic
liquid. Furthermore, the system of highest efficiency is unex-
pectedly the simplest and cheaper combination that employs
[Pd(η3-C3H5)Cl]2/PPh3 at only a 1 mol-% loading with pyr-
rolidine as the base and in the absence of a copper salt. The
coupling of sterically and electronically deactivated bro-
mides bearing different functional groups to aryl- and alkyl
acetylenes, as well as the possibility of recycling, make these
results of high interest to the future development of Heck-
and Sonogashira-type reactions in ionic liquids.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
The development of efficient methods for the incorpora- accomplish, but can be conducted by employing some very
tion of alkynes into organic molecules is a crucial objective. efficient catalytic systems.[5] Aside from their wide func-
Indeed, alkynes are recurring building blocks in a wide tional group tolerance,[6] the important features of these
range of useful industrial intermediaries and a great cross-coupling methods include the use of a reduced
number of bioactive natural products, pharmaceuticals, ag- amount of metal when compared, for instance, to other alk-
rochemicals and conjugated molecules (molecular materials ynylation routes that require the stoichiometric addition of
for optics or electronics) are built around an aryl alkyne metallic reagents.
backbone.[1] The commonly used aryl- or vinyl alkynylation
In the course of our studies aimed at the development of
process which typically employs a palladium precursor/li- ubiquitous multidentate auxiliary ligands,[7,8] which could
gand/base system in the presence of a few mol-% of a cop- allow efficient cross-coupling reactions at low catalyst load-
per cocatalyst (Sonogashira reaction)[2] is recognized as ex- ings ranging from 10–2 to 10–4 mol-% of palladium and
tremely powerful.[3] The even more desirable copper-free phosphane auxiliary,[9–11] we reported the catalytic effi-
alkynylation (Heck alkynylation)[4] is often more difficult to ciency of new ferrocenyl triphosphane Fc(P)2tBu(PiPr), 1,
Scheme 1. Aryl alkynylation system employing Pd/1 at 10–1 to 10–4 mol-% in DMF.
[a] Laboratoire de Synthèse et Electrosynthèse Organométalliques
UMR-CNRS 5188, Université de Bourgogne
9, avenue Alain Savary 21078 Dijon, France
Fax: +33-3-80-39-3682
for the alkynylation of aryl bromides and aryl chlorides
(Scheme 1).[10]
These interesting results prompted us to extend the study
of the reactivity of our Pd/ferrocenyltriphosphane system
for alkynylation into room temperature ionic liquids (IL),
especially into imidazolium salts. Because of their negligible
E-mail: jean-cyrille.hierso@u-bourgogne.fr
michel.picquet@u-bourgogne.fr
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 583–587
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J.-C. Hierso, J. Boudon, M. Picquet, P. Meunier
SHORT COMMUNICATION
vapour pressure, ease of handling and potential for recycl- tions summarised in Table 1. On the basis of previous re-
ing, these novel solvents have shown great promise. As a ports,[13] we anticipated that a significant advance in this
reaction medium, their high compatibility with transition methodology would be the efficient coupling of terminal
metals and limited miscibility with common solvents enable alkynes with aryl chlorides or bromides. The relative reac-
easy separation of organic products and possibly retention tivity of the aryl halides is well-established as Ar–I ϾϾ Ar–
of the catalyst at the resting-state in the ionic phase.[12] Br Ͼ Ar–Cl,[3b,15] and chloride and bromide substrates are
Apart from the pioneering work recently reported by Ryu more attractive starting materials with regard to sustainable
and coworkers,[13a] and by Alper and Park,[13b] which were chemistry and economic concerns. Therefore, 4-bromoace-
the first groups to disclose catalytic systems for the coupling tophenone (electron-poor activated substrate) and 4-bromo-
of aryl iodides in 1-n-butyl-3-methylimidazolium hexafluo- anisole (electron-rich deactivated substrate) were chosen as
rophosphate ([BMIM][PF6]), aryl alkynylation in ionic li- benchmarks for the preliminary screening. With regard to
quids is still underdeveloped.[12,13] Herein we report our terminal alkyne candidates, phenylacetylene and alkynyl-
studies on the Heck alkynylation (copper-free Sonogashira) silanes are among the most studied in organic solvents; the
reaction with aryl halides (I, Br, Cl) by employing various former, which we chose as a benchmark, appeared generally
metallic precursors, tertiary phosphanes and bases in as a more demanding substrate.[1b,16] As the inorganic base
[BMIM][BF4] as the solvent[14] (Scheme 2). As a result, we K2CO3 that we originally used[10] was not suitable in this
provide the first method that allows the coupling of a large reaction because of solubility troubles, we chose to examine
array of substrates, either activated or deactivated bromides. the effect of pyrrolidine as an organic amine base.[17] Pre-
Furthermore, the system of highest efficiency is unexpec- liminary experiments indicated that the coupling of aryl
tedly the simplest and cheaper combination that employs iodides to phenylacetylene is easy and gives satisfactory re-
[Pd(η3-C3H5)Cl]2/PPh3 at only a 1 mol-% loading with pyr- sults in [BMIM][BF4], which is similar to reactions con-
rolidine as the base and in the absence of a copper salt.
ducted in [BMIM][PF6].[13a,13b] Then, we were delighted to
discover and show for the first time that the coupling of aryl
bromides is also possible in the absence of a copper cocata-
lyst, and within only a few hours, by employing the catalytic
systems in Table 1. Whereas the alkynylation of 4-bromo-
acetophenone was not effective in the absence of a palla-
dium/ligand system, when 0.5 mol-% of [Pd(allyl)Cl]2
(1 mol-% Pd, no auxiliary ligand) was employed as the cata-
lyst, less than 10% of coupling product was obtained
(Table 1, Entries 1 and 2). The alkynylation reactions with
the use of catalytic system Pd/1, which is remarkably active
in DMF,[10] were not as promising as we expected: only
52% yield of 4-(2-phenylethynyl)acetophenone and 31% of
Scheme 2.
1-n-Butyl-3-methylimidazolium
tetrafluoroborate
([BMIM][BF4]).
Because our original goal was to extend the previously
reported methodology by employing palladium combined
with tridentate ferrocenyl polyphosphane 1 to valuable alk-
ynylations in ionic liquids, we first explored different condi-
Table 1. Screening of catalytic systems.
Entry
Aryl halide
Catalytic system, [Cat.][a]
[Cat.] ratio [mol-%]
Yield [%][b]
1
2
3
4
5
6
7
8
4-Bromoacetophenone
None
/
1
1
1
1
0
Ͻ10
52
100 (88)
96
86
47
91
0
0
31
61 (55)
15
51
[Pd(allyl)Cl]2, no ligand
[Pd(allyl)Cl]2/1
[Pd(allyl)Cl]2/3 PPh3
[Pd(allyl)Cl]2/dppe
[Pd(allyl)Cl]2/dppf
[Pd(allyl)Cl]2/1
[Pd(allyl)Cl]2/3 PPh3
Ni(NO3)2(PPh3)2/PPh3
Ni(SCN)2(PPh3)2/PPh3
[Pd(allyl)Cl]2/1
[Pd(allyl)Cl]2/3 PPh3
[Pd(allyl)Cl]2/dppe
[Pd(allyl)Cl]2/dppf
1
0.1
0.1
5
5
1
1
1
1
9
10
11
12
13
14
4-Bromoanisole
[a] Reaction conditions: 1.0 equiv. aryl bromide, 1.2 equiv. phenylacetylene, 1.2 equiv. pyrrolidine, 3 mL [BMIM][BF4], 130 °C; catalyst:
0.5 mol-% [Pd(allyl)Cl]2 and 1 mol-% 1, or dppe (1,2-bis[diphenylphosphanyl]ethane), or dppf (1,1Ј-bis[diphenylphosphanyl]ferrocene),
or 3 mol-% PPh3. [b] Yields determined after 2 h for 4-bromoacetophenone and 4 h for 4-bromoanisole (GC), isolated yields are in
brackets, which is the average of several consistent runs.
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Heck Alkynylation of Unactivated Aryl Bromides in an Ionic Liquid
SHORT COMMUNICATION
(4-methoxyphenyl)phenylacetylene were obtained in short- (Entry 13), 51% with Pd/dppf (Entry 14) and 31% with Pd/
time reactions. In an effort to comparatively evaluate the 1 (Entry 11). We noticed as well that the coupling of 4-
properties of cheap and robust classical phosphane ligands bromoacetophenone is efficiently achieved with only one-
(typically PPh3, dppe and dppf), we eventually identified tenth the catalyst concentration that was originally em-
the superior effectiveness of the catalytic system that com- ployed (Entries 7 and 8). Finally, the ionic nickel complexes
bines 1 mol-% Pd with 3 mol-% PPh3; by using this system, bearing triphenylphosphane proved to be completely inac-
4-bromoacetophenone (Entry 4) was selectively and quanti- tive in this reaction.[18]
tatively alkynylated in two hours. Under similar conditions,
From these screening results we selected the simple cata-
with the employment of dppe or dppf as auxiliary ligands, lytic system combining 1 mol-% palladium/3 mol-% tri-
96% and 86% of coupling product were obtained, respec- phenylphosphane in [BMIM][BF4] for the detailed studies
tively, with some secondary products being formed.[18]
presented in Table 2.[18] In addition to the quantitative con-
Consistently, the alkynylation of 4-bromoanisole is ef- version of 4-bromoacetophenone (Entry 1), good to excel-
fected in [BMIM][BF4] at 130 °C in 4 h with a 61% conver- lent conversions were obtained in 2 to 4 h for the coupling
sion with the use of Pd/PPh3 (Entry 12), 15% with Pd/dppe of phenylacetylene with the electronically activated 4-bro-
Table 2. Scope of the copper-free alkynylation of aryl bromides with 1 mol-% of Pd/PPh3 in [BMIM][BF4].[a]
[a] Reaction conditions: 1.0 equiv. of aryl bromide, 1.2 equiv. of phenylacetylene, 1.2 equiv. of pyrrolidine, 3 mL [BMIM][BF4], 130 °C;
catalyst: 0.5 mol-% [Pd(allyl)Cl]2, 3 mol-% PPh3. [b] Yields determined by GC (external standard), isolated yields obtained after column
chromatography reach 90–95% of these values. [c] Secondary products identified by GC–MS. [d] 1.6 equiv. of phenylacetylene and
pyrrolidine. [e] 3 equiv. of phenylacetylene and pyrrolidine. Ar-X = unconverted halide.
Eur. J. Org. Chem. 2007, 583–587
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585

J.-C. Hierso, J. Boudon, M. Picquet, P. Meunier
SHORT COMMUNICATION
mobenzonitrile (80% in 4 h, Entry 2), 3,5-bis(trifluoro-
In summary, herein is provided the first catalytic method
methyl)bromobenzene (92%, 4 h, Entry 3) and with the un- for Heck alkynylation of aryl bromides in an ionic liquid.
activated bromobenzene (82%, 2 h, Entry 4). However, the The system Pd/3 PPh3 (1 mol-%) in [BMIM][BF4] is ef-
most remarkable results were obtained with longer times for ficient for the coupling of a variety of activated and deacti-
the electronically and/or sterically deactivated substrates: in vated aryl bromides: para and/or ortho mono- and disubsti-
20 h, 4-bromanisole was converted in 95% yield (Entry 5), tuted substrates, with the two demanding aromatic (phenyl-
4-bromotoluene in 86% yield (Entry 6) and 2-bromotoluene acetylene) and aliphatic (long chain 1-decyne) terminal
in 99% yield (Entry 7). Whereas cross-coupling reactions acetylenes. The recycling possibility has been shown and
with aryl halides in which one or both the ortho positions ongoing studies are aimed at the optimisation of the reac-
are substituted generally suffer from additional complica- tion. The simplicity and accessibility of the system (low
tions of steric origin,[1b] 2-bromonaphthalene and bromo- price for ligand, organic base and IL)[20] make these results
mesitylene were quantitatively converted into the corre- of high interest to the future development of Heck- and
sponding enyne in 4 and 48 h, respectively (Entries 8 and Sonogashira-type coupling reactions in ionic liquids.
9).[18] Interestingly, we verified that a double alkynylation
was possible starting from a dibromoaryl (98% conversion
in enynes, with 60% monosubstitution and 40% disubstitu-
tion; Entry 10). Finally, the coupling of aryl bromides with
Experimental Section
Representative Catalytic Experiment for the Heck Alkynylation Re-
action in [BMIM][BF4]: The solid mixture of [Pd(allyl)Cl2] (6.3 mg,
a demanding (long chain) aliphatic terminal alkyne, namely
1-decyne, was also explored with success: in only 4 h, 4-
bromoacetophenone was converted in 96% yield (Entry 11),
the electronically and/or sterically deactivated 4-bromotolu-
ene and 2-bromotoluene yielded 76% and 65%, respec-
tively, of the expected alkynylation product. For 4-bromo-
anisole, a 20 h reaction time gave 76% conversion into the
desired enyne. Table 2 also summarises some of the second-
ary products encountered in these reactions. Our attempts
to activate aryl chlorides with the Pd/PPh3 system remained
unsuccessful as even 4-chloroacetophenone was not con-
verted.
Recycling of our catalytic system was also carried out
(Table 3). After extraction, the resulting IL phase was kept
under air for several days (Table 3 for Entries 3–4, cycles 2
were run after 78 days standing!) and was reused without
any pretreatment (no washing with water). Because of the
importance of the phosphane ligand, and to limit the nega-
tive effect of some possible leaks of monophosphane within
the extraction process, the recycling tubes were refilled with
3 mol-% triphenylphosphane. The ionic phase was reused
successfully for different substrates with a slight loss in its
activity[19] for activated 4-bromoacetophenone (Table 3, En-
try 1), electronically deactivated 4-bromotoluene (Entry 2),
and electronically and sterically deactivated (ortho-substi-
tuted) 2-bromotoluene (Entry 3). Recycling of strongly de-
activated 4-bromoanisole gave a more disappointing result.
0.03416 mmol
of
Pd),
triphenylphosphane
(26.9 mg,
0.10256 mmol) and 4-bromoacetophenone (680 mg, 3.416 mmol)
was degassed for 15 min in a 20 mL Schlenk tube equipped with a
magnetic stirring bar and a reflux condenser. Under an atmosphere
of argon, [BMIM][BF4] (3 mL) was added. The mixture was then
degassed under reduced pressure for another 10 min. The Schlenk
tube was heated in an oil bath at 110 °C to give an orange solution.
Once out of the oil bath, pyrrolidine (292 mg, 0.35 mL,
4.099 mmol, d = 0.87) and then phenylacetylene (419 mg, 0.45 mL,
4.099 mmol, d = 0.93) were added to the ionic liquid solution. The
resulting mixture was heated at 130 °C for 2 h under an atmosphere
of argon. The product was extracted from the ionic liquid phase
by the addition of diethyl ether (6ϫ5 or 10 mL) and decanting off
the ether from the IL phase (GC yield 88%). After evaporation,
the residue was purified by silica gel chromatography (diethyl ether/
hexane, 1:9) to give 620 mg (isolated yield 82%) of 4-(2-phenylethy-
nyl)acetophenone.
Recycling Experiment for the Heck Alkynylation Reaction in
[BMIM][BF4]: After extraction with diethyl ether, the resulting dark
coloured ionic liquid was kept under air for several days or weeks
with no particular precaution. The recovered ionic liquid was re-
used without any pretreatment (no water washing), but degassed
under reduced pressure for 15 min. The tube was refilled with tri-
phenylphosphane (26.9 mg), the aryl halide was added and the mix-
ture degassed for 15 min. The reaction was carried out and worked
up under the same conditions employed for the first run.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, GC chromatograms, mass spectra, ele-
mental analyses and NMR spectroscopic data for coupling prod-
ucts and identification of secondary products.
Table 3. Efficiency of the recycled ionic liquid phase of the catalytic
system.[a]
Acknowledgments
This work was supported by the “Conseil régional de Bourgogne”
and the CNRS, which are sincerely acknowledged. The authors
wish to thank Dr. I. Tkatchenko for his continuous support and
interest in this research.
Entry
Aryl bromide
Cycle 1
yield
[%]
Cycle 2
yield
[%]
Cycle 3
yield
[%]
1
2
3
4
4-Bromoacetophenone
4-Bromotoluene
2-Bromotoluene
4-Bromoanisole
Ͼ99
86
Ͼ99
95
80
99
72
61
78
73
61
20
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[a] Reaction conditions identical to Table 2 for each cycle.
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Heck Alkynylation of Unactivated Aryl Bromides in an Ionic Liquid
SHORT COMMUNICATION
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were reported with less demanding aryl iodides (see ref.[13]).
The loss of activity reported here is of great importance with
regard to the genuine nature of the catalyst resting state in the
IL (molecular, colloidal); a question that has, so far, not been
tackled in the few alkynylation reports. Our results of activity
suggest a molecular nature for the catalyst. The IL phase would
therefore be (upon recycling) a reservoir of palladium atoms in
a colloidal form. Ongoing research in our groups is aimed at
addressing this question.
[20] By now, [BMIM][BF4] is generally less expensive than
[BMIM][PF6], see for instance: http://www.solvionic.com.
Received: May 23, 2006
Revised Version Received: November 8, 2006
Published Online: December 12, 2006
Eur. J. Org. Chem. 2007, 583–587
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
587