Heck reactions in a non-aqueous ionic liquid using silica supported palladium complex catalysts

Article abstract of DOI:10.1016/S0040-4039(02)01320-5

The Heck reaction of iodobenzene and 4-methyl-iodobenzene with olefins using silica-supported palladium complex catalysts was conducted in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF₆), exhibiting higher activities than in DMF in addition to easy product isolation and catalyst recycling.

Full text of DOI:10.1016/S0040-4039(02)01320-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 7115–7118
Heck reactions in a non-aqueous ionic liquid using silica
supported palladium complex catalysts
Keisuke Okubo,a Masayuki Shiraib and Chiaki Yokoyamaa,*
aInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai 980-8577, Japan
bSupercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, Sendai 983-8551, Japan
Received 18 March 2002; revised 20 June 2002; accepted 28 June 2002
Abstract—The Heck reaction of iodobenzene and 4-methyl-iodobenzene with olefins using silica-supported palladium complex
catalysts was conducted in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), exhibiting higher activities than in
DMF in addition to easy product isolation and catalyst recycling. © 2002 Elsevier Science Ltd. All rights reserved.
Ionic liquids are attractive alternatives to conventional
organic solvents because their non-volatile nature can
reduce the emission of toxic organic compounds and
facilitate the separation of products and/or catalysts
from the reaction solvents.1 Especially, ionic liquids
consisting of dialkylimidazolium cations have several
useful properties, such as keeping the liquid condition
under a wide range of temperature and having good
solubility for many substrates and molecular catalysts.2
The range of applications of ionic liquids has been
expanded into solvents for catalytic reactions over the
past few years, because ionic liquids are capable of
separation of products and reuse of the solvents with
catalysts is possible.3
plex catalysts could be recycled six times without loss of
activity. Recently, Hagiwara et al.7 reported that Heck
reactions proceeded to give satisfactory yields with a
charcoal-supported palladium catalyst (Pd/C) and with-
out phosphine ligands in [bmim]PF6 and under the
system the ionic liquid could be easily recycled involv-
ing a heterogeneous palladium catalyst as well. How-
ever, the reaction conditions were time-consuming (>12
h) and required high temperature (>373 K) because of
the high viscosity and low solubility of the ionic liquid
[bmim]PF6.
In this paper we report the highly active and re-usable
supported palladium complex catalyst in [bmim]PF6 for
the Heck reaction with triethylamine and without
phosphine.
Palladium-catalysed carbon–carbon bond forming
Heck reactions have contributed remarkably to syn-
thetic organic chemistry.4,5 Ionic liquids are a candidate
for use as solvents for Heck reactions because of the
facile separation of the products and recycling of the
ionic liquids containing the palladium catalysts. Seddon
et al.6 first reported the Heck reaction of aryl halides
and benzoic aldehydes with alkenes using homogeneous
palladium complex catalysts and phosphine ligands in
The ionic liquid [bmim]PF6 was prepared according to
the procedures reported.8 A silica-supported palladium
complex catalyst (PdII/SiO2) was prepared by an ion-
exchange method.9 Palladium was loaded by adding a
weighed quantity of silica gel powder (Aldrich,
Davisil™ 646) to the desired volume of tetraammine
palladium dichloride (Pd(NH3)4Cl2) solution of 20
mmol/l. The pH was adjusted to 11.5–12.0 by adding
ammonium solution, and the support was kept
immersed in this solution at room temperature for 3
days. Under the conditions used, almost all of the
palladium precursors present in the solution were
adsorbed by the support. After filtration, the powder
was washed with distilled water and vacuum-dried at
393 K for 6 h. The quantity of palladium loaded was
fixed at 5 wt%. The actual amounts of palladium
loaded were examined by measuring the residual
1-butyl-3-methylimidazolium
hexafluorophosphate
([bmim]PF6). The hydrophobic ionic liquid [bmim]PF6
could be separated from the products and salt byprod-
ucts by extraction through the simple decantation of
the organic solvent and water, respectively. They also
reported that the [bmim]PF6 containing palladium com-
Keywords: Heck reaction; ionic liquid; supported palladium catalyst.
* Corresponding author. Tel./fax: +81-22-217-5646; e-mail: chiaki@
tagen.tohoku.ac.jp
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01320-5

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K. Okubo et al. / Tetrahedron Letters 43 (2002) 7115–7118
is a major advantage of the present study. The palla-
dium concentration was not proportional to the activity
in our results. The PdII/SiO2 catalyst showed higher
yields than the mix-PdII-SiO2 catalyst, though the
amount of palladium dissolved from the PdII/SiO2 cata-
lyst into [bmim]PF6 was lower than that from the
mix-PdII-SiO2 catalyst. It is probable that the number of
the active palladium sites in the [bmim]PF6 liquid with
the PdII/SiO2 catalyst was higher than that with the
mix-PdII-SiO2 catalyst.
amounts of the metal remaining in the solution by
atomic absorption spectroscopy. The actual weights
loaded were in good agreement with the nominal value.
The PdII/SiO2 catalyst was reduced at 573 K for 3 h
under flowing hydrogen to prepare a palladium metal
catalyst (Pd0/SiO2). A mixture of Pd(NH3)4Cl2 and silica
(mix-PdII-SiO2) was also prepared. The palladium load-
ing of the catalyst was 5 wt%. The reaction was carried
out in a test tube. Typical experimental procedure was
as follows. To a stirred slurry of a 5 wt% silica-sup-
ported palladium catalyst (21.6 mg, 0.010 mmol) in
bmimPF6 (1.5 ml) was added iodobenzene (204 mg, 1.0
mmol), ethyl acrylate (163 ml, 1.5 mmol) and triethyl-
amine (140 ml, 1.0 mmol). The resulting slurry was
heated at 353 K under a nitrogen atmosphere. After
cooling to room temperature, the product was extracted
three times with t-butyl methyl ether by vigorous stir-
ring, followed by decantation of the organic layer. The
components in the organic solution were analyzed by a
gas chromatograph with a FID detector (Shimazu GC-
14A, ULBON HR-52 capillary column 25 m×0.32 mm).
The Heck reaction in organic solvents normally
requires more than 1 equiv. of a base to remove halogen
atoms and to reproduce the active metal palladium
Fig. 1 shows the conversion profiles for the Heck
reactions of iodobenzene with ethylacrylate (Scheme 1)
catalyzed by supported palladium catalysts. (E)-Ethyl-
cinnamate was the only product, and the reaction pro-
ceeded without phosphine ligands on all the catalysts
used. The reaction proceeded with time on palladium
metal catalysts (Pd/C and Pd0/SiO2). The induction
periods were observed in the system with unreduced
catalysts (PdII/SiO2 and mix-PdII-SiO2). In the case of
the PdII/SiO2 catalyst, the color of the catalyst gradually
changed from pale yellow to black, and then the Heck
reaction proceeded. The induction period would be the
reductive process from the divalent to an active metal
palladium (Pd0) in the ionic liquid. The final conversion
with the PdII/SiO2 catalyst was higher than that of the
mix-PdII-SiO2 catalyst (Pd(NH3)4Cl2 complex). Seddon
et al.6 reported that the Heck reaction of iodobenzene
and ethyl acrylate proceeded in [bmim]PF6 with
Pd(OAc)2 and triphenylphosphine at 373 K. Our results
indicate that the supported complex catalyst proceeds
the Heck reaction without phosphine ligands at 353 K,
and the catalytic activity of the palladium complex
supported on silica was higher than that of the palla-
dium complex itself. Fig. 2 shows the palladium concen-
tration profiles in [bmim]PF6 solvent during the Heck
reactions. The palladium species was dissolved from the
silica support during the reactions. Half amounts of the
palladium supported were eluted from the supports into
[bmim]PF6 after 1 h. The palladium concentrations in
the ionic liquid were higher than those in organic
solvent (Tables 1 and 2). Biffs et al.10 proposed that the
palladium species dissolved from the supported Pd
catalysts into the solvents were the active sites in the
heterogeneous Heck reaction. The Heck reaction of
iodobenzene with ethylacrylate was conducted again
with the ionic liquid [bmim]PF6 recycled, from which
the catalyst powder was separated after the reaction.
The ionic liquid recycled showed the same conversion as
that of the first run, indicating that the reaction pro-
ceeded by the palladium species dissolved in the ionic
liquid. This re-usability without loss of catalytic activity
Figure 1. The conversion profiles for Heck reactions of
iodobenzene with ethylacrylate with several catalysts (ꢀ:
PdII/SiO2, ꢁ: Pd0/SiO2, ꢂ: mix-PdII-SiO2, ꢃ: Pd/C) and
Et3N 1 mmol (1 equiv.) in [bmim]PF6. The reaction tempera-
ture was 353 K.
Scheme 1.
Figure 2. The palladium concentration in [bmim]PF6 during
the Heck reactions of iodobenzene and ethylacrylate (ꢀ:
PdII/SiO2, ꢁ: Pd0/SiO2, ꢂ: mix-PdII-SiO2, ꢃ: Pd/C). The
broken line is the total amount of the palladium in the system.
The reaction temperature was 353 K. One equivalent of Et3N
(1 mmol) was added.

K. Okubo et al. / Tetrahedron Letters 43 (2002) 7115–7118
7117
Table 1. Heck reaction of iodobenzene with ethylacrylate using supported palladium catalysts
Entrya
Catalyst
Solvent
Et3N (mmol)
Conversion (%)c
Pd concentration (ppm)d
1
PdII/SiO2
PdII/SiO2
PdII/SiO2
Pd0/SiO2
Pd0/SiO2
Pd/C
bmimPF6
bmimPF6
bmimPF6
bmimPF6
bmimPF6
bmimPF6
bmimPF6
bmimPF6
bmimPF6
DMF
1
1
2
1
2
1
2
1
2
1
2
1
2
66
71
99
57
59
66
66
49
82
85
88
81
83
382
395
311
261
246
555
638
871
743
145
72
2b
3
4
5
6
7
8
9
10
11
12
13
Pd/C
Pd(NH3)4Cl2
Pd(NH3)4Cl2
PdII/SiO2
PdII/SiO2
Pd0/SiO2
Pd0/SiO2
DMF
DMF
DMF
57
65
a All reactions were carried out at 353 K for 1 h except for entry 2.
b Reaction was carried out at 353 K for 24 h.
c Yields were determined by GLC analysis. Only E-isomers were obtained.
d The Pd concentrations in solvents were analyzed by an atomic absorption spectroscopy.
Table 2. Heck reactions with PdII/SiO2 catalysts in several solvents
Entrya
Aryl iodide (R=)
Olefin
Et3N (mmol)
Solvent
Time (h)
Conversion (%)b Pd concentration (ppm)c
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Bu
Bu
Bu
Bu
Et
Et
Et
Et
Et
1
2
1
2
1
2
1
2
1
2
2
1
2
2
1
2
2
bmimPF6
bmimPF6
DMF
1
1
1
1
1
1
1
1
2
2
2
6
6
6
6
6
6
52
98
90
88
57
70
72
67
64
99
78
16
79
85
1
545
468
122
85
497
413
127
156
514
268
61
442
421
30
517
496
30
DMF
bmimPF6
bmimPF6
DMF
DMF
9
4-Me
4-Me
4-Me
4-OMe
4-OMe
4-OMe
4-COMe
4-COMe
4-COMe
bmimPF6
bmimPF6
DMF
bmimPF6
bmimPF6
DMF
bmimPF6
bmimPF6
DMF
10
11
12
13
14
15
16
17
Et
Et
Et
Et
33
99
a All reactions were carried out at 353 K.
b Yields were determined by GLC analysis.
c The Pd concentrations in solvents were analyzed by an atomic absorption spectroscopy.
Scheme 2.
(0) sites.11 We examined the effect of the quantity of the
base for the Heck reactions in [bmim]PF6. The conver-
sions were increased with increasing the quantity of
triethylamine added from 0 to 1 equiv. for iodobenzene
on all the catalysts used. Adding more than 1 equiv. of
the base in the reaction with such organic solvents as
DMF and acetonitrile did not enhance the conversion.
The conversions were increased with increasing the
amount of the base added to more than 1 equiv. and
became maxima on adding 2 equiv. of triethylamine to
the [bmim]PF6 solvent with non-reduced palladium cat-
alysts, though the palladium concentrations were
decreased (entries 3, 5 and 9 in Table 1).
The base effects for the Heck reaction using the PdII/SiO2
catalyst in [bmim]PF6 were studied with a combination
of other iodobenzene derivatives and olefins (Scheme 2,
Table 2). The enhancement of conversion with the
PdII/SiO2 catalyst was observed for all of the experiments
by adding 2 equiv. of the base to [bmim]PF6.

7118
K. Okubo et al. / Tetrahedron Letters 43 (2002) 7115–7118
The conversion of 66% after 1 h was slightly improved
after 24 h with the PdII/SiO2 catalyst and 1 equiv. of the
base (entry 2 in Table 1); however, the Heck reaction
was almost completed by adding 1 further equiv. (total
2 equiv.) of triethylamine after the reaction of entry 2,
indicating that 2 equiv. of the base were needed to
proceed the Heck reactions in [bmim]PF6. Supported
metal palladium catalysts showed high activities for
Heck reactions in organic solvents;12 however, we
showed that the unreduced PdII/SiO2 catalyst with 2
equiv. of base in [bmim]PF6 were more active than
supported palladium catalysts in DMF in this study.
3. (a) Wasserschied, P.; Keim, W. Angew. Chem., Int. Ed.
Engl. 2000, 19, 1123; (b) Sheldon, R. J. Chem. Soc.,
Chem. Commun. 2001, 2399.
4. (a) Heck, R. F.; Nolley, J. P., Jr. Palladium Reagents in
Organic Synthesis; Academic Press, 1985; (b) Tsuji, J.
Palladium Reagents and Catalysts; John Wiley: New
York, 1996.
5. (a) de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed.
Engl. 1994, 33, 2379; (b) Beletskaya, I. P.; Cheprakov, A.
V. Chem. Rev. 2000, 100, 3009.
6. Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; MaCor-
mac, P. B.; Sedonn, K. R. Org. Lett. 1999, 1, 997.
7. Hagiwara, H.; Shimizu, Y.; Hoshi, T.; Suzuki, T.; Ando,
M.; Okubo, K.; Yokoyama, C. Tetrahedron Lett. 2001,
42, 4349.
We observed that [bmim]PF6 was decomposed by heat-
ing at 403 K; however, the decomposition temperature
was increased above 433 K by adding a small amount
of triethylamine, suggesting that triethylamine
molecules interact with the ionic solvent and more than
1 equiv. of the base was needed to proceed the reaction
in the ionic liquid.
8. (a) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.;
Visser, A. E.; Rogers, R. D. J. Chem. Soc., Chem.
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K. J. Catal. 1996, 161, 704.
10. Biffis, A.; Zecca, M.; Basato, M. J. Mol. Cat. A 2001,
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In summary, high conversions of the Heck reactions
were obtained in ionic liquid [bmim]PF6 with a non-
reduced PdII/SiO2 catalyst and 2 equiv. of triethylamine
and without phosphine ligands at 375 K.
11. (a) Jeffery, T. J. Chem. Soc., Chem. Commun. 1984, 1287;
(b) Shaughnessy, K. H.; Hamann, B. C.; Hartwig, J. F. J.
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Shirai, M.; Arai, M. J. Mol. Cat. A 1999, 142, 383.
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