Heterogeneous Heck reaction catalyzed by Pd/C in ionic liquid

Article abstract of DOI:10.1016/S0040-4039(01)00748-1

Pd/C-catalyzed Heck reactions of aryl halides with olefins proceeded in moderate to satisfactory yields in ionic liquid [bmim]PF₆. The ionic liquid involving a catalyst was easily recycled.

Full text of DOI:10.1016/S0040-4039(01)00748-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4349–4351
Heterogeneous Heck reaction catalyzed by Pd/C in ionic liquid
Hisahiro Hagiwara,a,* Yumiko Shimizu,b Takashi Hoshi,b Toshio Suzuki,b Masayoshi Ando,b
Keisuke Ohkuboc and Chiaki Yokoyamac
aGraduate School of Science and Technology, Niigata University, 8050, 2-nocho, Ikarashi, Niigata 950-2181, Japan
bFaculty of Engineering, Niigata University, 8050, 2-nocho, Ikarashi, Niigata 950-2181, Japan
cInstitute for Chemical Reaction Science, Tohoku University, Katahira 2-chome, Aoba-ku, Sendai 980-8577, Japan
Received 11 April 2001; revised 1 May 2001; accepted 2 May 2001
Abstract—Pd/C-catalyzed Heck reactions of aryl halides with olefins proceeded in moderate to satisfactory yields in ionic liquid
[bmim]PF6. The ionic liquid involving a catalyst was easily recycled. © 2001 Elsevier Science Ltd. All rights reserved.
Palladium-catalyzed carbonꢀcarbon bond forming reac-
tions have contributed remarkably to synthetic organic
chemistry.1 Among these reactions, Heck and its related
reactions have been extensively utilized in preparing a
wide variety of olefinic compounds.2 One major trans-
formation in the Heck reaction is the synthesis of
cinnamic acid and its derivatives, which are very ver-
satile compounds as flavour substances or UV
absorbents and so on. Especially in the case where
preparation of some substituted aryl aldehydes was
difficult, the Heck reaction has great synthetic value. In
view of the economy of the reaction, the recovery as
well as recycling of the expensive Pd catalyst is
required. However, a homogeneous Pd catalyst usually
precipitated from the solution and recycling or recovery
of an active Pd catalyst was difficult. Heterogeneous
catalysts would be much better from such a viewpoint.
Reetz and Lohmer developed colloidal Pd clusters gen-
erated electrochemically.3 Recently, Ying et al. reported
Pd-grafted molecular sieves that effectively catalyze
heterogeneous Heck reactions.4 Pd/C is a recoverable
heterogeneous catalyst and is stable and relatively inex-
pensive, although its catalytic activity was undeveloped5
except in catalytic hydrogenation. Scattered precedents
of Pd/C-catalyzed carbonꢀcarbon bond forming
reactions6 exemplify its catalytic reactivity. Attention
was then focused on the Heck reaction in an ionic
liquid, which has attracted growing interest as an envi-
ronmentally benign re-usable solvent.7–10 Especially, the
highly polar nature of ionic liquid is expected to work
as an activating and stabilizing solvent for Pd/C-cata-
lyzed Heck reactions.
Recent results on the Heck reaction in an ionic liquid,10
as well as our current interests in the selectivity in
Pd-catalyzed arylation leading to aromatized ionone
natural products,11 prompted us to report an expedient
Pd/C-catalyzed Heck reaction in an ionic liquid
(Scheme 1).
The reaction was carried out simply by heating a
solution of aryl substrate, olefin and base in ionic liquid
with 3 mol% of 10% Pd/C. As the ionic liquid,
12
[bmim]PF6 was employed because of its air and mois-
ture stability, easy availability and higher utility in
several organic reactions.7–10 Pd/C dispersed well in
[bmim]PF6. After the reaction, the product was
O
O
I
Pd/C, Et3N
ionic liquid
OEt
OEt
+
R
R
1
2
3
Scheme 1.
Keywords: alkenes; aryl halides; catalysts; Heck reactions; palladium and compounds.
* Corresponding author. Fax: +81-25-262-7368; e-mail: hagiwara@gs.niigata-u.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00748-1

4350
H. Hagiwara et al. / Tetrahedron Letters 42 (2001) 4349–4351
recovered completely even at 140°C with or without a
phosphine ligand.
extracted simply by stirring with n-hexane or diethyl
ether several times followed by decantation of the
upper organic layer. Isolation of products from the
ionic liquid was complete according to GLC analysis of
the remaining ionic liquid. No aqueous work-up was
required.10a,c The Pd/C was kept and suspended com-
pletely in the ionic liquid layer after the extraction.
Under conventional reaction conditions, a homo-
geneous Pd catalyst deteriorated and a Pd black
deposited after the reaction. Recovery of the Pd cata-
lyst is usually not realistic, thus eliminating the possibil-
ity of recycling the Pd catalyst. On the other hand,
since Pd/C remained only in the ionic liquid, the ionic
liquid containing Pd/C can be re-used as a catalyst
system itself, as shown in Table 3. Although a certain
decrease in yields was observed after the second re-use,
probably due to accumulation of triethylammonium
iodide (Table 3, entries 3–5), washing the ionic liquid
layer with water10c recovered the catalytic activity to the
same level as in a fresh system (Table 3, entry 6).
Some representative results are compiled in Table 1. As
shown in entries 1 and 2, ethyl cinnamate was obtained
in high yield. Aryl iodides having electron-donating or
-accepting substituents gave the corresponding cinna-
mates in moderate to acceptable yields (Table 1, entries
4–6). Among the bases investigated, triethylamine gave
the best result. An inorganic base such as sodium
hydrogencarbonate11 provided poor yields (entry 3).
Bromobenzene also gave ethyl cinnamates and the
results are listed in Table 2. However, aryl triflates were
The present reaction conditions were applicable to a
combination of other substrates and olefins (Scheme 2,
Table 4 and Fig. 1). Acrylonitrile, styrene (Table 4,
entries 1–5) and methyl methacrylate (Fig. 1) afforded
Table 1. Pd/C-catalyzed Heck reactions with ethyl acrylate
in ionic liquid
I
R'
R'
Entrya
Aryl iodide
Time (h)
Yield (%)c
+
R
1
R=H
R=H
R=H
R=4-Me
R=4-OMe
R=4-COMe
1
12
12
12
12
24
92
95
R
2
1
4
5
3b
4
(19)d
53
Scheme 2.
5
6
69
65
Table 4. Pd/C-catalyzed Heck reactions with olefins in
a See Ref. 13.
ionic liquid
b Sodium hydrogencarbonate was used.
Entrya
Aryl iodide Olefin 4
Time (h)
Yield (%)b
c Only E-isomers were obtained.
d Yield in parentheses was obtained by GLC analysis.
1
1
2
3
4
5
6
7
8
R=H
R%=CN
R%=CN
R%=Ph
R%=Ph
12
12
24
24
24
92c
88d
59
Table 2. Pd/C-catalyzed Heck reactions of aryl bromide
R=4-OMe
R=4-Me
R=4-OMe
Entrya
Aryl bromide
Time (h)
Yield (%)c
59
R=4-COMe R%=Ph
29
1
2
3
4
5
R=H
12
12
12
12
12
40
45
85
52
25
R=H
R=H
R=H
R%=COMe 12
R%=COMe 12
R%=COEt 12
32e
26f
16
R=Hb
R=4-NO2
R=3-Cl
R=4-OMe
a Reaction was carried out in the same manner as in Table 1.
b Only E-isomers were obtained except entries 1 and 2.
c The ratio of E/Z isomer was 4:1.
a All reactions were carried out in the same manner as in Table 1 at
140°C.
d The ratio of E/Z isomer was 3.6:1.
b Triphenylphosphine (0.04 equiv.) was added.
c Only E-isomers were obtained.
e 4-Phenyl-2-pentanone (15%) was accompanied along with recovered
iodobenzene.
f Sodium hydrogencarbonate was used.
Table 3. Recycle of catalyst system
CO2Me
CO2Et
Entrya
Recycle
Yield (%)
1
2
3
4
0
1
2
3
4
5
92
93
84
81
80
95
6 (70%)
7 (49%)
CO2Et
5
6b
7 (49%)
a Reaction was carried out as described in Table 1 for 12 h.
b Catalyst system in entry 6 was washed with water before use.
Figure 1.

H. Hagiwara et al. / Tetrahedron Letters 42 (2001) 4349–4351
4351
the Heck products in acceptable yields. In the reactions
with vinylketones (Table 4, entries 6–8), yields were low
probably due to instability of both olefins and prod-
ucts. Reaction of bromostyrene and ethyl acrylate also
provided the desired product 7 (Fig. 1).
Ali, B.; Vasapollo, G.; Alper, H. J. Org. Chem. 1993, 58,
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In all cases, reactions were clean and the only side
products were arylhalides. Conversion yields could not
be obtained because of the high volatility of the aryl-
halides recovered.
7. (a) Welton, T. Chem. Rev. 1999, 99, 2071–2083; (b)
Wassersheid, P.; Keim, W. Angew. Chem., Int. Ed. Engl.
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8. Recent examples of metal-catalyzed reactions in ionic
liquid picked up arbitrarily: (a) Chauvin, Y.; Mussmann,
L.; Olivier, H. Angew. Chem., Int. Ed. Engl. 1995, 34,
2698–2700; (b) Dullius, J. E. L.; Suarez, P. A.; Einloft, S.;
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Seddon, K. R. J. Chem. Soc., Chem. Commun. 1998,
2097–2098; (b) Earle, M. J.; McCormac, P. B.; Seddon,
K. R. J. Chem. Soc., Chem. Commun. 1998, 2245–2256;
(c) Adams, C. J.; Earle, M. J.; Seddon, K. R. J. Chem.
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lics 2000, 19, 1123–1127.
11. Hagiwara, H.; Eda, Y.; Morohashi, K.; Suzuki, T.;
Ando, M.; Ito, N. Tetrahedron Lett. 1998, 39, 4055–4058.
12. Huddleston, J. G.; Willauer, H. D.; Swatloslki, R. P.;
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Dupont et al.8b and, subsequently, Xiao et al.8d
reported the isolation and identification of a Pd com-
plex formed from Pd(OAc)2 or PdCl2 with ionic liquid.
Dissolution of Pd black over 100°C was proposed by
Earle et al.10c We investigated the presence of the Pd
species in an ionic liquid before and after the present
Heck reaction by ICP emission spectroscopy. After
filtration of the suspending Pd/C, ICP analysis revealed
that the concentration of Pd in the ionic liquid was
negligible. This important observation implies that the
Pd/C catalyzes the Heck reaction on the surface of Pd
held on the carbon.3
Thus, we have offered an expedient protocol for Heck
arylation reactions by using Pd/C as an inexpensive and
stable catalyst in ionic liquid. One major advantage of
the present protocol is that the catalyst system was
easily re-usable without loss of catalytic activity,
thereby multiplying catalyst turn-over. Another advan-
tage is that the reaction proceeded without phosphine
ligands, which are expensive, toxic and contaminants of
products. Green character of the ionic liquid as well as
easy operation make the present Heck reaction
attractive.
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