Heck reaction catalyzed by mesoporous SBA-15-supported ionic liquid-Pd(OAc)2

Article abstract of DOI:10.1055/s-0028-1087488

Palladium acetate was immobilized into the pores of modified SBA-15 with absorbed ionic liquid. The immobilized catalyst system was studied for Heck coupling. The catalyst exhibited high catalytic activity in the coupling of aryl iodides and activated aryl bromides with acrylates. Moreover, it could be recycled without significant loss of activity. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087488

LETTER
39
Heck Reaction Catalyzed by Mesoporous SBA-15-Supported Ionic
Liquid–Pd(OAc)2
Heck
R
eac
i
tionC
-
Y
bySBA-15-Suppo
o
rte
d
Ionic
L
uiquid–Pd(OA 2
n
c) g Jung, Abu Taher, Hyun-Jin Kim, Wha-Seung Ahn, Myung-Jong Jin*
School of Chemical Science and Engineering, Inha University, Incheon 402-751, Korea
Fax +82(32)8720959; E-mail: mjjin@inha.ac.kr
Received 19 September 2008
involve ionic liquids as solvents suffer from severe prob-
lems related with high cost as well as high viscosity of
ionic liquids. It would be desirable to minimize the
amount of ionic liquid in a potential process. Search for
Abstract: Palladium acetate was immobilized into the pores of
modified SBA-15 with absorbed ionic liquid. The immobilized cat-
alyst system was studied for Heck coupling. The catalyst exhibited
high catalytic activity in the coupling of aryl iodides and activated
aryl bromides with acrylates. Moreover, it could be recycled with- recyclable, convenient and practical catalytic systems is
out significant loss of activity.
still a major challenge. In this regard, a new concept of
supported ionic liquid phase has been adopted for immo-
bilization of catalysts.5 Our interest in this area led us to
explore the immobilization of palladium acetate with ion-
ic liquid in the channels of mesoporous silica SBA-15.
SBA-15 silica has been served as a potential solid support
for the immobilization of homogeneous catalysts because
of its well-defined, uniform mesopores and facile surface
modification.6 Our strategy is to immobilize an ionic liq-
uid on the pore surface of SBA-15 through the interaction
with covalently anchored ionic liquid fragments. In this
context, we report the application of SBA-15-supported
ionic liquid–palladium acetate matrix for Heck coupling
reaction.
Key words: Heck reaction, palladium, immobilization, ionic
liquid, SBA-15
Heck coupling reaction is one of the most powerful meth-
ods for carbon–carbon bond formation in organic synthe-
sis.1 Most of the Heck reaction proceeds in the presence of
palladium catalyst. Considerable efforts have been made
for this reaction in order to achieve a high degree of effi-
ciency under mild condition.2 While palladium catalysts
offer widespread utility in the coupling reaction, they are
expensive, air-sensitive, and can not be reused continu-
ously in many cases. This limit is of environmental and
economic concern in large-scale synthesis. Homogeneous
catalysts have been immobilized onto a solid support in
order to overcome the limit.3 Recently, ionic liquids have
received much attention as promising media to reuse
homogeneous catalysts.4 However, the reactions which
First, the surface of SBA-15 was modified with a co-
valently anchored ionic liquid fragment in order to en-
hance the interaction between the free ionic liquid and the
surface. As shown in Scheme 1, N-methylimidazole was
reacted with (3-chloropropyl)triethoxysilane to give
Me
Cl–
Me
Cl–
Me
Cl–
Me
Cl–
N
N
N
N
+
+
+
+
Me
Me
N
N
N
N
N
N
a
c, d
b
Cl–
+
N
N
Si
Si
EtO
Si
EtO
EtO
Si
EtO
O
O
O
O
O
O
O
O
Si(OEt)3
1
SBA-15 support
SBA-15 support
3
2
Pd(OAc)2
=
Bu
N
+
=
–
PF6
N
Me
Scheme 1 Reagents and conditions: (a) (3-chloropropyl)triethoxysilane, 100 °C, 24 h; (b) SBA-15, toluene, reflux; (c) 1-butyl-3-methylimi-
dazolium hexafluorophosphate, Pd(OAc)2, THF, r.t., 1 h; (d) removal of THF.
SYNLETT 2009, No. 1, pp 0039–0042
x
x.
x
x.
2
0
0
8
Advanced online publication: 12.12.2008
DOI: 10.1055/s-0028-1087488; Art ID: U10008ST
© Georg Thieme Verlag Stuttgart · New York

40
J.-Y. Jung et al.
LETTER
Table 1 Textural Properties Determined from Nitrogen Adsorption
Experiments at 77 K
Specific BET Total pore
Mesopore
diameter
(nm)
Loading
(mmol/g)
surface area
volume
(m2/g)
(cm3/g)
SBA-15 750
0.83
0.61
0.38
8.33
7.35
7.09
–
2
3
334
215
1.05
0.21
Table 2 Heck Reaction of Iodobenzene with Acrylates in the
Presence of Catalyst 3a
3, base
I
+
acrylate
dodecane, 3.5 h
CO2R
Entry
Acrylateb
MA
EA
Base
Temp (°C) Yield (%)c
1
2
Bu3N
130
130
130
120
120
120
110
120
120
120
100 (98)
100 (99)
100 (96)
98
Bu3N
3
BA
Bu3N
4
MA
EA
Bu3N
Figure 1 TEM image of 3 and XRD pattern of 2 and 3
5
Bu3N
99
6
BA
Bu3N
98
N-1-(3-triethoxysilylpropyl)-3-methylimidazolium chlo-
ride (1).7 SBA-15-supported ionic liquid 2 was prepared
by treatment of SBA-15 with an excess amount of N-1-(3-
triethoxysilylpropyl)-3-methylimidazolium chloride in
refluxing toluene.8 Both nitrogen analysis and weight gain
confirm the incorporation of ionic liquid 1, giving modi-
fied SBA-15 2 with loading ratio of 1.05 mmol/g. Next,
immobilization of palladium acetate–ionic liquid on the
surface of the pore walls was performed by immersing the
modified SBA-15 2 in THF solution of palladium acetate
and ionic liquid 1-butyl-3-methylimidazolium hexafluoro-
phosphate, followed by removing THF from the mixture.9
In addition, palladium acetate and the ionic liquids are
very miscible in THF and less soluble in nonpolar sol-
vents and water. A weight composition of 0.06 Pd(OAc)2/
0.1 ionic liquid/1.0 modified SBA-15 silica 2 was used.
The Pd content of 0.21 mmol/g was measured by induc-
tively coupled plasma (ICP) analysis. The absorbed ionic
liquid spreads as multiple layers of free ionic liquid in the
pores of SBA-15 support. Palladium acetate in the ionic
liquid layer would act like a homogeneous catalyst.
Therefore, this system may possess the advantages of both
homogeneous and heterogeneous catalysts.
7
BA
Bu3N
84 (82)
80
8
MA
EA
(i-Pr)2EtN
(i-Pr)2EtN
Et3N
9
82
10
MA
71
a Reaction conditions: iodobenzene (1.0 mmol), acrylate (1.3 mmol),
base (1.5 mmol), 3 (1.0 mol%), dodecane (1.3 mL).
b MA = methyl acrylate, EA = ethyl acrylate, BA = butyl acrylate.
c Determined by GC analysis. Isolated yield is given in parenthesis.
tal pore volume and pore diameter of SBA-15 materials
are listed in Table 1. Mesoporous SBA-15 used as a sup-
port has high surface area of 750 m2/g and mesopore di-
ameter of 8.33 nm. As expected, the surface area, total
pore volume and pore diameter of the functionalized
SBA-15 decreased dramatically from those of parent
SBA-15 because of the grafting of organic species on the
surface of the mesopores. Mesoporous SBA-15 may be a
suitable support in the development of supported ionic
liquid catalysis.
The catalytic activity of the immobilized Pd(OAc)2 was
investigated in Heck coupling reaction.10 We tested the
coupling of iodobenzene with acrylate in dodecane as a
model reaction (Table 2). In our initial experiments, the
coupling of iodobenzene with methyl acrylate was carried
out in the presence of 1.0 mol% of 3 and Bu3N as a base.
The reaction was complete at 130 °C in 3.5 hours. Quan-
titative conversions were also observed in the couplings
with ethyl acrylate and butyl acrylate (entries 2 and 3).
The HRTEM image of modified SBA-15 3 is very similar
to symmetry of the pore arrays (Figure 1). It reveals that
the hexagonal symmetry of the pore arrays is still con-
served after the immobilization of ionic liquid–palladium
acetate matrix onto the modified SBA-15. This mesopo-
rous character is also confirmed by XRD (Figure 1). Ap-
parently, there is less change of the lattice parameters
upon the immobilizing process. The BET surface area, to-
Synlett 2009, No. 1, 39–42 © Thieme Stuttgart · New York

LETTER
Heck Reaction Catalyzed by SBA-15-Supported Ionic Liquid–Pd(OAc)2
41
Table 3 Heck Reaction of Aryl Halides with Acrylates in the
Presence of Catalyst 3a
Table 4 Reusability of Catalyst 3 over Six Runs
3, Bu3N
I
X
+ acrylate
3, Bu3N
dodecane, 120 °C
CO2R'
R
R
+
dodecane, 120 °C, 3.5 h
CO2Me
Entry
Aryl halide
Acrylateb
Time (h) Yield (%)c
CO2Me
Recycle
1st
2nd
100
3rd
98
4th
98
5th
99
6th
99
I
1
MA
5
83 (81)
Yield (%)
100
OMe
2
3
MA
3
3
98 (95)
94
MeO
MeO
I
I
ene and 4-iodophenol (entries 1–6) as well as activated
1-iodo-4-nitrobenzene and 1-iodo-3-nitrobenzene (entries
7–11). Deactivated aryl iodides possessing electron-
donating groups showed a slight drop in reactivity com-
pared to those possessing electron-withdrawing groups. A
little longer reaction time was required to reach a satisfac-
tory conversion. High reactivity was still observed in the
coupling of activated 1-bromo-4-nitrobenzene (entry 12).
However, an attempt to couple 1-chloro-4-nitrobenzene
was not successful (entry 13).
BA
I
4
MA
MA
5
84 (80)
5
6
5
3
87 (83)
94
I
MA
HO
I
The recycling of catalyst is an important issue in the het-
erogeneous reaction. Thus, we turned our attention to the
reusability of our catalyst 3. The catalyst was recovered
by a simple filtration and reused in the reaction of iodo-
benzene with methyl acrylate. The reusability of 3 was
demonstrated by conducting six runs without consider-
able loss of catalytic activity (Table 4). Furthermore, ICP
analysis of the solution indicated that less Pd metal (< 0.1
ppm) leached into the reaction solution. This excellent re-
usability and high stability of the catalyst would be ex-
plained by strong binding of Pd(OAc)2 to SBA-15-
supported ionic liquid and site isolation, that is, the ab-
sence of interactions between catalytic sites, which causes
aggregation of the Pd complex and formation of less ac-
tive Pd catalyst. The successful recycling allows for a
more economic and environmentally friendly process.
I
7
8
9
MA
EA
BA
3
3
3
100 (99)
99
O2N
O2N
I
I
97
O2N
O2N
10
11
12
13
MA
2
2
100 (99)
100
I
BA
O2N
O2N
O2N
I
MA
5
97 (94)
21
In summary, palladium acetate with ionic liquid was suc-
cessfully confined onto modified SBA-15 silica. We have
shown that the immobilized palladium acetate can be effi-
ciently used as a recyclable catalyst for Heck reaction.
Moreover, SBA-15 was proven to be an excellent support
for the supported ionic liquid catalysis. Further studies of
other reactions are currently in progress.
Br
BA
12
Cl
a Reaction conditions: aryl halide (1.0 mmol), acrylate (1.3 mmol),
Bu3N (1.5 mmol), 3 (1.0 mol%), dodecane (1.3 mL).
b MA = methyl acrylate, EA = ethyl acrylate, BA = butyl acrylate.
c Determined by GC analysis. Isolated yield is given in parenthesis.
Acknowledgment
The catalytic activity was not significantly decreased at
120 °C (entries 4–6). The turnover frequency (TOF) was
as high as 29 h–1 for the coupling. In contrast, the reaction
was somewhat slow at 110 °C (entry 7). Lower yields
were obtained when i-Pr2EtN or Et3N was used as a base
(entries 8–10).
This work was supported by the Carbon Dioxide Reduction &
Sequestration R&D Center, one of the 21st Century Frontier R&D
Programs in Korea.
References and Notes
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In order to extend the scope of our catalytic system, we
next performed Heck reaction of substituted iodobenzene
with acrylates. As shown in Table 3, catalyst 3 showed
outstanding activities in the coupling of deactivated aryl
iodides such as 2-iodoanisole, 4-iodoanisole, 2-iodotolu-
Synlett 2009, No. 1, 39–42 © Thieme Stuttgart · New York

42
J.-Y. Jung et al.
LETTER
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(2.0 g). The mixture was stirred at 105 °C for 12 h. After
cooling, the reaction mixture was filtered and washed
completely with CH2Cl2, and dried at 90 °C under vacuum to
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dodecane (1.5 mL) was stirred at 120 °C. The reaction was
periodically monitored by GC analysis. The mixture was
diluted with Et2O (15 mL) and H2O (3 mL). Catalyst 3 was
separated from the mixture and washed with H2O and Et2O.
The ethereal layer was dried over MgSO4 and then evap-
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column chromatography on silica gel.
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Synlett 2009, No. 1, 39–42 © Thieme Stuttgart · New York

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