Reusable Pd nanoparticles immobilized on functional ionic liquid co-polymerized with styrene for Suzuki reactions in water-ethanol solution

Article abstract of DOI:10.1016/j.tetlet.2011.01.092

Co-polymer of 3-(2,3-dihydroxypropyl)-1-vinylimidazolium chloride and styrene was synthesized and used as a support of Pd nanoparticles. The Pd@poly-Sty-co-diOH-Cl catalyst can efficiently catalyze Suzuki reactions for a wide range of aryl iodides and bromides with 0.05 mol % Pd at 70 °C in water-ethanol solution under air, and the catalyst can be recycled and reused for several times without significant loss of activity.

Full text of DOI:10.1016/j.tetlet.2011.01.092

Tetrahedron Letters 52 (2011) 1477–1480
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reusable Pd nanoparticles immobilized on functional ionic liquid
co-polymerized with styrene for Suzuki reactions in water–ethanol solution
⇑
Jiayi Wang, Gonghua Song , Yanqing Peng
Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Co-polymer of 3-(2,3-dihydroxypropyl)-1-vinylimidazolium chloride and styrene was synthesized and
used as a support of Pd nanoparticles. The Pd@poly-Sty-co-diOH-Cl catalyst can efficiently catalyze
Suzuki reactions for a wide range of aryl iodides and bromides with 0.05 mol % Pd at 70 °C in water–eth-
anol solution under air, and the catalyst can be recycled and reused for several times without significant
loss of activity.
Received 3 December 2010
Revised 13 January 2011
Accepted 21 January 2011
Available online 27 January 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Suzuki reaction
Ionic liquid
Co-polymer
Palladium
Palladium catalyzed Suzuki reactions between organoboronic
acid and halides play an important role in organic synthesis. Usu-
To realize that di-OH group is a good stabilizer of Pd11 and proba-
bly more compatible with water-containing solvents, and to seek a
new functional ionic liquid co-polymer supported Pd catalyst, we
report herein the Suzuki reactions catalyzed by Pd nanoparticles
1
ally, these cross-coupling reactions are catalyzed by palladium
complexes with phosphine ligands in organic solvents. A major
drawback of such a process is the difficulty in recovering and reus-
ing the expensive catalysts, which is often composed of expensive,
usually toxic as well, ligands and noble metals. To overcome this
shortcoming, much work has been devoted to the development
of reusable Pd catalysts, including immobilizing Pd on polymers
2
3
or biopolymers, magnetically recoverable palladium catalysts,
supported Pd-NHC catalysts,4 and so on.5 Although the immobili-
zation of nanoparticles on polymers or porous frameworks can
facilitate the separation process, decreasing in catalytic activity
was often observed probably due to the leaching of active species
and aggregation of nanoparticles.2
,3,6
The design and preparation
of an effective and stable supported palladium catalyst are highly
desirable.
Recently, the application of polymerized/co-polymerized ionic
liquids in noble-metals catalyzed reactions has attracted more
and more attention. Kou et al. reported the soluble ionic liquid
co-polymers of imidazolium and PVP as good stabilizers of Rh or
7
Pt nanoparticle catalysts. (3-Butyl-1-vinylimidazolium)-styrene
8
co-polymers, NH
2
functional imidazolium-divinylbenzene co-
9
10
polymer and poly(p-phenylene) grafted guanidinium (PPPIL)
were also synthesized to immobilize Pd nanoparticles. The
0
PPPILÁPd catalyst showed high catalytic activity in the Suzuki
1
0
reactions, but the reactions need to be conducted under argon.
⇑
Corresponding author. Tel.: +86 21 6425 3140; fax: +86 21 6425 2603.
E-mail address: ghsong@ecust.edu.cn (G. Song).
Figure 1. Synthesis of Pd@poly-Sty-co-diOH-Cl.
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.092

1
478
J. Wang et al. / Tetrahedron Letters 52 (2011) 1477–1480
supported and stabilized by a di-OH functional ionic liquid–sty-
rene co-polymer (Fig. 1). The immobilized Pd nanoparticles can
efficiently catalyze the Suzuki reactions with 0.05 mol % Pd at
1
2
7
0 °C under air and the catalyst can be recycled and reused several
times without a significant loss of activity.
As shown in Figure 1, the functional ionic liquid monomer was
synthesized by the reaction of 1-methylimidazole and 3-chloro-
1
2
propane-1,2-diol. The co-polymerization of the monomers with
13
styrene afforded poly-Sty-co-diOH-Cl. Finally, the co-polymer
was mixed with H PdCl under vigorous stirring, then filtered
and washed. After the reduction with NaBH the desired
2
4
3
4
,
14
Pd@poly-Sty-co-diOH-Cl was obtained. The loading amount of
Pd was 0.96 wt % based upon the weight of the poly-Sty-co-
diOH-Cl according to ICP–AES analysis. The catalyst was also char-
acterized by TEM and FTIR. The Pd nanoparticles were well dis-
persed in the polymer as shown in Figure 2. The FTIR spectrum
of poly-Sty-co-diOH-Cl before Pd loading showed a broad band
1
: poly-Sty-co-diOH-Cl;
2: fresh Pd@poly-Sty-co-diOH-Cl;
3: Pd@poly-Sty-co-diOH-Cl after reused for 3 times.
1000
1500
2000
2500
3000
3500
-
1
Wavenumbers (cm )
À1
centered at around 3380 cm (Fig. 3, curve 1), which was attrib-
uted to the O–H stretch vibrations from di-OH groups in the poly-
mer. It is clear that the broad band nearly disappeared after Pd
nanoparticles were supported on the polymer (Fig. 3, curve 2), indi-
cating the binding of Pd nanoparticles to the polymer through the
Figure 3. FTIR spectra of the copolymer and catalysts.
Table 1
Optimization of the reaction conditions
9
di-OH groups. In the solubility tests, the co-polymer and catalyst
a
were found to be insoluble in water and commonly used organic
solvents, such as methanol, ethanol, toluene, acetonitrile, diclo-
romethane, acetone, and ethyl acetate. Therefore, the catalyst can
be easily recycled by simple filtration after the reaction.
The synthesized catalyst was then applied in the Suzuki cou-
pling reactions. The catalytic activities of Pd@poly-Sty-co-diOH-Cl
under different conditions were evaluated in the model reaction
of 4-iodomethylbenzene with phenylboronic acid. It was found
that Pd@poly-Sty-co-diOH-Cl could catalyze the mode reaction
much better in ethanol–water (2:1, v/v) solution than in pure
water (Table 1, entries 1 and 2). Relatively weak inorganic base
Yieldb (%)
Entry
Pd (mol %)
Solvent
Base
T (°C)
1
h
2 h
3 h
1
2
3
4
5
6
7
8
9
0.5
0.5
0.5
0.5
0.2
0.1
0.05
0.05
0.05
0.05
H
2
O
K
K
2
CO
CO
3
3
70
70
70
70
70
70
70
60
50
30
—c
98.7
75
89
98
99
99
92
75
46
—c
99
77
93
99
99
99
96.7
85
58
39
99
80
97.5
99
99
99
98
91
68
EtOH/H
EtOH/H
EtOH/H
2
2
2
O
O
O
2
KOH
NEt
K CO
K
2
CO
3
overmatched both strong inorganic base KOH and organic
(Table 1, entries 2–4). Based upon the amount of 4-
3
base NEt
3
2
EtOH/H O
2
3
3
3
3
3
3
iodomethylbenzene used, even 0.05 mol % Pd was enough to afford
the coupling product in almost quantitative yield (Table 1, entries
EtOH/H
EtOH/H
EtOH/H
EtOH/H
EtOH/H
2
O
2
O
2
O
2
O
2
O
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
2
and 5–7). Lowering reaction temperature remarkably slowed
down the reaction (entries 7–10). The optimized conditions for Su-
zuki coupling reaction were thus established as follows:
10
a
For each reaction, 1 mmol 4-iodo-1-methylbenzene, 1.5 mmol phenylboronic
acid, 2 mmol base and 3 ml H O or EtOH/H O (2:1, v/v) were used. The amount of
Pd was based on 4-iodo-1-methylbenzene.
Yields were determined by GC (internal standard: dodecane) and products were
identified by GC–MS.
0
.05 mol %Pd in Pd@poly-Sty-co-diOH-Cl as catalyst, ethanol–
2
2
water (2:1, v/v) as solvent, K CO as base, and reaction at 70 °C.
2
3
b
Suzuki coupling reactions of various substituted aryl halides
with phenylboronic or 4-methyl phenylboronic acid were then
investigated under optimized conditions.15,16 Both the electron
c
Not determined.
Figure 2. TEM images of freshly prepared Pd@poly-Sty-co-diOH-Cl (a) and after being reused three times (b).

J. Wang et al. / Tetrahedron Letters 52 (2011) 1477–1480
1479
Table 2
(
curve 2). On the other hand, the small difference between the FTIR
spectra of freshly prepared (curve 2) and recycled catalysts (curve
) implied that no great conformational change had happened with
a
Pd@poly-Sty-co-diOH-Cl catalyzed Suzuki coupling reactions
3
catalyst recycling.
In conclusion, a co-polymer of di-OH functional ionic liquid
with styrene was synthesized and applied in the immobilization
of Pd nanoparticles. The grafted Pd@poly-Sty-co-diOH-Cl showed
efficient catalytic activity in Suzuki coupling reactions for a wide
range of aryl iodides and bromides with 0.05 mol % Pd at 70 °C in
water–ethanol solution under air. The catalyst could be recycled
and reused at least five times without significant decline in the cat-
alytic activity. Further investigations on metal nanoparticle cata-
lysts grafted on functional ionic liquid polymers are being carried
out in our lab.
Entry
R
1
X
R
2
Time
Yieldb (%)
1
2
3
4
5
6
7
8
9
H
I
I
I
I
I
I
I
I
H
H
H
H
p-Me
p-Me
p-Me
p-Me
H
p-Me
H
p-Me
H
40 min
1 h
1 h
40 min
1 h
1 h
1 h
1 h
2 h
2 h
97
99
97
98
96
98
97
98
84
81
92
90
96
94
p-Me
p-OMe
p-Cl
H
p-Me
p-OMe
p-Cl
o-CF
o-CF
H
Acknowledgments
3
I
I
10
11
12
13
14
15
3
Br
Br
Br
Br
Cl
6 h
6 h
6 h
6 h
Financial support for this work from the NSFC (Grant
H
2
0676033), National Basic Research Program of China (973 Pro-
m-NO
m-NO
H
2
2
gram) (Grant 2010CB126101) and Shanghai Leading Academic Dis-
cipline Project (Project Number: B507) is gratefully acknowledged.
p-Me
H
17,c,d 24c,e
12 h
a
For each reaction, 1 mmol aryl iodide or bromide, 1.5 mmol arylboronic acid,
mmol K CO , 3 ml EtOH/H O (2:1, v/v) and 0.05 mol % Pd based on aryl iodide or
Supplementary data
2
2
3
2
bromide were used.
b
Isolated yields.
GC yield.
Reaction at 80 °C.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.01.092.
c
d
e
0
.5 mol % Pd was used as catalyst.
References and notes
rich and electron deficient aryl iodides delivered the products with
good to excellent yields within 1 h (Table 2, entries 1–8). The ste-
rically demanding aryl iodide also gave good yields (Table 2, en-
tries 9 and 10). Prolonged reaction time was necessary to fulfill
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2
the reaction when less active phenyl bromide or 3-NO phenyl bro-
mide was used (Table 2, entries 11–14). The yield for the coupling
of phenyl chloride with phenylboronic acid was quite poor even
with strengthened reaction conditions (Table 2, entry 15).
0
10
Pd@poly-Sty-co-diOH-Cl was superior over the PPPILÁPd catalyst
with respect to lower reaction time for aryl iodides and less
amount of catalyst for aryl bromides under air.
2085–2088; (b) Amali, A. J.; Rana, R. K. Green Chem. 2009, 11, 1781–1786; (c)
The reaction of 4-iodomethylbenzene and phenylboronic acid
was used to test the reusability of Pd@poly-Sty-co-diOH-Cl. After
each cycle, the catalyst was recovered by filtration, followed by
washing with EtOH. After drying, the recovered catalyst
Pd@poly-Sty-co-diOH-Cl was used for the next run in the same
conditions. The results were presented in Table 3. Limited decline
in the catalytic activity of Pd@poly-Sty-co-diOH-Cl was observed
after five cycles, which implied that the catalyst was highly active
and stable. The content of Pd in the catalyst after being used for
three times was 0.87 wt % as determined by ICP-AES, and no Pd
leaching was detected in the aqueous phase. The TEM images of
Pd@poly-Sty-co-diOH-Cl before and after being reused three times
Polshettiwar, V.; Baruwati, B.; Varma, R. S. Green Chem. 2009, 11, 127–131; (d)
Polshettiwar, V.; Varma, R. S. Org. Biomol. Chem. 2009, 7, 37–40.
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Jun, B. H.; Byun, J. W.; Lee, Y. S. Tetrahedron Lett. 2004, 45, 5827–5831; (c)
Michrowska, A.; Mennecke, K.; Kunz, U.; Kirschning, A.; Grela, K. J. Am. Chem.
Soc. 2006, 128, 13261–13267; (d) Lee, D. H.; Kim, J. H.; Jun, B. H.; Homan, K.;
Park, J.; Lee, Y. S. Org. Lett. 2008, 10, 1609–1612; (e) Taher, A.; Kim, J. B.; Jung, J.
Y.; Ahn, W. S.; Jin, M. J. Synlett 2009, 2477–2482.
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.
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Wrzyszcz, J. J. Catal. 2008, 254, 121–130.
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Green Chem. 2010, 12, 228–233.
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9, 2470–2474.
8
9
.
(
Fig. 2) clearly showed that the Pd nanoparticles with an average
.
Liu, G.; Hou, M.; Song, J.; Jiang, T.; Fan, H.; Zhang, Z.; Han, B. Green Chem. 2010,
diameter of 4 nm were well dispersed in the co-polymer, and there
1
2, 65–69.
was no obvious aggregation of Pd in the reused catalyst. As can be
1
0. Li, S.; Wang, J.; Kou, Y.; Zhang, S. Chem. Eur. J. 2010, 16, 1812–1818.
À1
seen in Figure 3, the broad band centered at around 3380 cm
,
11. (a) Namboodiri, V. V.; Varma, R. S. Green Chem. 2001, 3, 146–148; (b) Kwong, F.
Y.; Klapars, A.; Buchwald, S. L. Org. Lett. 2002, 4, 581–584; (c) Cassez, A.;
Ponchel, A.; Hapiot, F. Dr.; Monflier, E. Org. Lett. 2006, 8, 4823–4826; (d) Han,
W.; Liu, C.; Jin, Z. L. Org. Lett. 2007, 9, 4005–4007; (e) Cai, Y.; Lu, Y.; Liu, Y.; He,
M.; Wan, Q. Catal. Commun. 2008, 9, 1209–1213; (f) Cai, Y.; Liu, Y. Catal.
Commun. 2009, 10, 1390–1393; (g) Yan, N.; Xue, Y.; Fei, Z.; Li, Y.; Kou, Y.;
Dyson, P. J. Organometallics 2009, 28, 937–939.
which could be clearly seen in the FTIR spectrum of poly-Sty-co-
diOH-Cl (curve 1), disappeared after loading Pd nanoparticles
Table 3
a
Recycle and reuse of Pd@poly-Sty-co-diOH-Cl
1
2. Synthesis of 3-(2,3-dihydroxypropyl)-1-vinylimidazolium chloride. A mixture of 1-
methylimidazole (9.4 g, 0.1 mol) and 3-chloropropane-1, 2-diol (11 g, 0.1 mol)
in the absence of solvent was heated with stirring at 75 °C for 48 h. Then, the
product was washed first with diethyl ether (50 ml  4), then EtOAc
Cycle
0
99
1
97
2
98
3
96
4c
94
5c
95
Yieldb (%)
a
b
c
Reaction conditions were the same as that of entry 2 in Table 2.
Isolated yields.
Prolonged to 1.5 h.
(100 ml  2). After filtration and drying, the product was obtained as
a
1
yellow solid powder (15.2 g, 74.5%): H NMR (D
2
O, 400 MHz): d 9.00 (s, 1H),
= 4.4 Hz, 1H), 5.74 (dd,
7.73 (s, 1H), 7.53 (s, 1H), 7.08 (dd, J = 7.6 Hz, J
1
2

1
1
480
J. Wang et al. / Tetrahedron Letters 52 (2011) 1477–1480
J
1
(
1
= 6.8 Hz, J
H), 4.15–4.20 (m, 1H), 3.98–4.08 (m, 1H), 3.56 (d, J = 2.4 Hz, 2H); C NMR
O, 100 MHz) d 128.14, 123.47, 119.37, 109.59, 95.11, 69.69, 62.36, 52.06;
2
= 1.2 Hz, 1H), 5.36 (dd, J
1
= 4 Hz, J
2
= 4.4 Hz, 1H), 4.34–4.38 (m,
iodomethylbenzene (218 mg, 1 mmol), phenylboronic acid (183 mg,
1.5 mmol), K CO (276 mg, 2 mmol), and ethanol/H O (3 ml, 2:1, v/v) were
13
2
3
2
D
2
added into a 25 ml round bottomed flask, which was then heated to 70 °C for
1 h as monitored by TLC. After the reaction was finished, the catalyst was
filtered and washed three times with ethyl acetate (3 ml). Then, 10 ml water
was added to the filtrate, and 10 ml EtOAc was added to extract the organic
compound for three times. The combined extract was evaporated to leave the
crude products which were purified by column chromatography over silica gel
to furnish the pure 4-methylbiphenyl as a white solid: MS (GC–MS): m/z 168
+
+
ESI-MS: 169.1 [MÀCl] , 373.2 [2MÀCl] .
3. Synthesis of poly-Sty-co-diOH-Cl. A mixture of styrene (5.2 g, 0.05 mol), 3-(2, 3-
dihydroxypropyl)-1-vinylimidazolium chloride (1.02 g, 0.005 mol), methanol
(
50 ml) was heated to 80 °C. Then, AIBN (82 mg, 1 mol %) was added into the
reaction solution. The reaction was continued for 72 h under nitrogen, and a lot
of solid appeared. On completion, the upper methanol was poured out, and
methanol (80 ml) was added into the flask for further stirring for 6 h at room
temperature. After filtration, the obtained solid was washed by methanol
+
(M ), 152, 91.
16. Characterization of the products. Table 2, entry 5: liquid; 1H NMR (CDCl
,
3
(
100 ml  2) and dried in vacuo to afford the product as a light yellow solid
powder (4.8 g, 77%): Elemental analysis: N 1.06, C 87.34, H 7.37. The ratio of
m:n in the polymer was 23:1 as calculated from the elemental analysis. The M
400 MHz): d 7.82 (d, J = 3.6 Hz, 1H), 7.61 (t, J = 7.2 Hz,1H), 7.40–7.54 (m, 7H);
13
C NMR (CDCl
3
, 100 MHz) d 141.51, 139.92, 132.08, 131.32, 129.01, 128.99
n
(2C), 127.79, 127.66, 127.38, 126.09 (q), 125.61, 122.89; MS (GC–MS): m/z 222
+
1
and M
GPC.
w
of the polymer were 20,600 and 80,100, respectively, as determined by
(M ), 201, 183, 152, 91. Entry 10: liquid; H NMR (CDCl
J = 3.8 Hz, 1H), 7.60 (t, J = 7.4 Hz, 1H), 7.51 (t, J = 7.6 Hz, 1H), 7.40 (d, J = 3.8 Hz,
1H), 7.28–7.32 (m, 4H), 2.48 (s, 3H); 13C NMR (CDCl
, 100 MHz) d 141.59,
3
, 400 MHz): d 7.81 (d,
1
4. Synthesis of Pd@poly-Sty-co-diOH-Cl. PdCl
2
(2.5 mmol) was dissolved in
a
3
solution of HCl (0.1 M, 50 ml) in an ultrasound water bath for 1 h, and the
mixed solution was stirred under megnic for further 2 h to get a clear solution.
137.37, 137.05, 132.22 (2C), 131.29, 128.88, 128.52 (2C), 127.17, 126.07 (q),
+
125.66, 122.94, 21.23; MS (GC–MS): m/z 236 (M ), 215, 201, 167, 91. Entry 13:
A mixture of poly-Sty-co-diOH–Cl (1 g), 0.05 M H
2
PdCl
4
(50 ml) was stirred at
white solid; 1H NMR (CDCl
3
, 400 MHz): d 8.48 (s, 1H), 8.22 (dd, J
1
= 4.0 Hz,
room temperature for 72 h, then filtered, washed with water (20 ml  2), and
J = 0.4 Hz, 1H), 7.94 (d, J = 3.8 Hz, 1H), 7.61–7.66 (m, 3H), 7.53 (t, J = 7.6 Hz,
2
13
methanol (20 ml  3), respectively. After drying, the obtained solid catalysts
2H), 7.46 (t, J = 7.2 Hz, 1H); C NMR (CDCl
3
, 100 MHz) d 148.75, 142.89,
4
were re-dispersed in 20 ml water and reduced by a fresh made NaBH solution
138.67, 133.05, 129.73 (2C), 129.19, 128.57, 127.18 (2C), 122.05, 121.98; MS
+
1
(
(
185 mg, 5 mmol in 10 ml water). Then they were filtered, washed by water
20 ml  2) and methanol (20 ml  3), and dried under vacuum to afford the
(GC–MS): m/z 199 (M ), 152, 76. Entry 14: white solid; H NMR (CDCl
400 MHz): d 8.46 (s, 1H), 8.19 (d, J = 4.0 Hz, 1H), 7.92 (d, J = 3.6 Hz, 1H), 7.61 (t,
J = 8.0 Hz, 1H), 7.55 (d, J = 4.0 Hz, 2H), 7.33 (d, J = 3.8 Hz, 2H), 2.45 (s, 3H); 13
NMR (CDCl , 100 MHz) d 148.75, 142.81, 138.59, 135.77, 132.81, 129.89 (2C),
3
,
desired Pd@poly-Sty-co-diOH-Cl as a dark grey powder. The content of Pd in
the catalyst was 0.96 wt % as determined by ICP-AES.
C
3
+
1
5. General procedure for Suzuki reactions: representable procedure for Suzuki
reaction of 4-iodomethylbenzene and phenylboronic acid (Table 2, entry 2).
The catalyst Pd@poly-Sty-co-diOH-Cl (5.5 mg, Pd: 0.05 mol %), 4-
129.65, 126.98 (2C), 121.73, 121.69, 21.17; MS (GC–MS): m/z 213 (M ), 165,
152.