Linear polystyrene-stabilized PdO nanoparticle-catalyzed mizoroki-heck reactions in water

Article abstract of DOI:10.3390/molecules16119067

Linear polystyrene-stabilized PdO nanoparticles (PS-PdONPs) were prepared by thermal decomposition of Pd(OAc)2 in the presence of polystyrene. X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated the production of PdO nanoparticles. The loading of palladium was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). PS-PdONPs exhibited high catalytic activity for Mizoroki-Heck reactions under air in water and could be recycled without loss of activity.

Full text of DOI:10.3390/molecules16119067

Molecules 2011, 16, 9067-9076; doi:10.3390/molecules16119067
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Linear Polystyrene-Stabilized PdO Nanoparticle-Catalyzed
Mizoroki-Heck Reactions in Water
1
,
1
1
1
Atsushi Ohtaka *, Tomohiro Yamaguchi , Takuto Teratani , Osamu Shimomura and
1
,2,
Ryôki Nomura
*
1
Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology,
-16-1 Ohmiya, Asahi, Osaka 535-8585, Japan
5
2
Nanomaterials and Microdevices Research Center, Osaka Institute of Technology, 5-16-1 Ohmiya,
Asahi, Osaka 535-8585, Japan
*
Authors to whom correspondence should be addressed; E-Mails: otaka@chem.oit.ac.jp (A.O.);
nomura@chem.oit.ac.jp (R.N.); Tel.: +81-6-6954-4729; Fax: +81-6-6957-2135.
Received: 19 September 2011; in revised form: 22 October 2011 / Accepted: 26 October 2011 /
Published: 27 October 2011
Abstract: Linear polystyrene-stabilized PdO nanoparticles (PS-PdONPs) were prepared by
thermal decomposition of Pd(OAc)
2
in the presence of polystyrene. X-ray diffraction
(XRD) and transmission electron microscopy (TEM) indicated the production of PdO
nanoparticles. The loading of palladium was determined by inductively coupled plasma-
atomic emission spectroscopy (ICP-AES). PS-PdONPs exhibited high catalytic activity for
Mizoroki-Heck reactions under air in water and could be recycled without loss of activity.
Keywords: polystyrene; PdO nanoparticles; Mizoroki-Heck reaction; water
1
. Introduction
The coupling reaction of vinyl or aryl halides with various alkenes in the presence of palladium
catalyst is known as the Mizoroki-Heck reaction and represents a powerful tool for building up a new
carbon-carbon bonds [1,2]. Recently, attention has focused on the use of palladium nanoparticles
(
PdNPs) as catalysts in organic synthesis [3-5]. PdNPs supported by a variety of substrates, including
poly(N-vinylpyrrolidone)- or several organic moieties-grafted silica [6,7], alumina-based oxides [8],
mesoporous silica or NiFe [9,10], and chitosan [11], have been shown to exhibit high catalytic
activity for the Mizoroki-Heck reaction in aprotic, polar solvents such as N,N-dimethylformamide and
2
O
4

Molecules 2011, 16
9068
dimethylacetamide. However, these solvents are toxic and have high boiling points, which cause
difficulties in isolating the products after the reactions. This problem can be minimized by using ionic
liquids [12] or performing the reaction under solvent-free conditions [13].
On the other hand, the use of water as a reaction medium for organic synthesis has recently received
much attention because water is a readily available, safe, and environmentally benign solvent [14-18].
Several research groups have reported PdNPs-catalyzed Mizoroki-Heck reactions in water [19-25]. For
example, Cacchi et al. have developed fluorous silica gel-immobilized perfluoro-tagged PdNPs that
can be successfully used and recycled in the Mizoroki-Heck reaction of aryl iodides with allylic
alcohols under aerobic phosphine-free conditions [26].
Recently, we found that PdO nanoparticles (PdONPs) are readily stabilized on linear polystyrene,
and the resultant polystyrene-stabilized PdONPs (PS-PdONPs) have high catalytic activities for Suzuki
and copper-free Sonogashira coupling reactions in water [27-29]. Our continuing interest in the
catalytic utility of PS-PdONPs led us to examine herein the Mizoroki-Heck reaction in water.
2
. Results and Discussion
2
.1. Preparation and Characterization of PS-PdONPs
Linear polystyrene-stabilized PdO nanoparticles (PS-PdONPs) were prepared according to our
3
previous paper [27]. A mixture of Pd(OAc)
2
and linear polystyrene (M
n
= 6.0 × 10 ) was added to
−1
1
2 3
.5 mol·L aqueous K CO solution. After the mixture was stirred at 90 °C for 1 h, the color turned
black. An XRD pattern of PS-PdONPs is presented in Figure 1a. In addition to the broad diffraction
with 2θ ranging from 12° to 28° ascribed to the polystyrene, other five diffraction peaks assigned to
PdO (JCPDS #41-1107) are observed clearly. Figure 1c shows a TEM image of PS-PdONPs, where a
fairly uniform particle size of 2.5 ± 0.4 nm is evident. Inductively coupled plasma-atomic emission
−1
spectroscopy (ICP-AES) revealed that PS-PdONPs contained an average of 2.5 mmol·g of Pd.
Figure 1. (a) XRD patterns of PS-PdONPs; (b) JCPDS data (#41-1107) for PdO; (c) TEM
micrograph of PS-PdONPs (scale bar = 20 nm); (d) Size distribution of PS-PdONPs.
(a)
(b)
1
0
20
30
40
50
60
70
80
90
2
 /degree (CuK)

Molecules 2011, 16
9069
Figure 1. Cont.
2
.2. Coupling Conditions
Initially, an optimum base was investigated at 90 °C in the Mizoroki-Heck reaction of iodobenzene
with acrylic acid using PS-PdONPs. As seen in Table 1, the most effective base was potassium
hydroxide. Reactions with potassium carbonate, cesium carbonate, and sodium acetate gave
trans-cinnamic acid in 27%, 19% and 20% yields, respectively, probably due to the low basicity.
3
When 1.8-diazabicyclo[5.4.0]undec-7-ene (DBU) and NEt was used as a base, the yields of coupling
product were 21% and 96%, respectively. However, the catalyst was not recovered in these cases,
suggesting that the strong binding capability of nitrogen was causing palladium leaching. PS-PdNPs,
4
reduced by NaBH , exhibited slightly lower catalytic activity than PS-PdONPs although reduction of
palladium on the surface of the nanoparticles was observed by XRD after treatment of PS-PdONPs
with acrylic acid (Figure 2). However, the precise reason for this is as yet unclear, although it suggests
that the presence of oxygen is important [27,30]. On the contrary, Pd/C exhibited low catalytic activity.
Table 1. Effect of base on the Mizoroki-Heck reaction of iodobenzene with acrylic acid
using PS-PdONPs in water.
PS-PdONPs
(1.0 mol% of Pd)
O
I
O
+
OH
base (3 equiv),
OH
0.75 mmol
H O, 90 °C, 5 h
2
0
.50 mmol
a
Entry
Base
KOH
Yield (%)
b
1c
99 (32) b
2
3
99 (16)
44
d
4
5
6
7
K
2
CO
CO
COONa
DBU
NEt
3
27
19
20
21
Cs
CH
2
3
3
8
3
96
a
b
c
NMR yields; Reaction time = 1 h; PS-PdNPs was used as a catalyst;
Pd/C was used as a catalyst.
d

Molecules 2011, 16
9070
Figure 2. (a) XRD patterns of the catalyst after treatment with acrylic acid at 90 °C;
b) JCPDS data (#87-0641) for Pd (H-loaded).
(
2
.3. Substrate Tolerance
Employing the optimized reaction conditions, we next examined a variety of substituted aryl iodides
having either an electron-releasing or an electron-withdrawing group (Table 2). The reaction of
iodobenzene with acrylic acid took place smoothly in water at 90 °C for 5 h to give trans-cinnamic
acid in 99% yield (entry 1). The Mizoroki-Heck reaction of 4-iodotoluene and 4-iodoanisole, bearing
electron donating groups at the para-position, gave the corresponding cinnamic acids in 99% and 96%
yields, respectively (entries 2 and 3). Substrates with electron-deficient aromatic rings, i.e.,
4
-iodoacetophenone and 4-iodobenzotrifluoride, also underwent the Mizoroki-Heck reaction with
acrylic acid under similar conditions to afford 4-acetylcinnamic acid and 4-trifluoromethylcinnnamic
acid, respectively, both in near quantitative yield (entries 4 and 5). Sterically hindered substrates were
also examined. The reaction of 2-iodotoluene and 2-iodophenol with acrylic acid gave the
corresponding cinnamic acids in 99% and 99% yields, respectively (entries 6 and 7).
1
-Iodonaphthalene was also reactive, with the desired product being obtained in 98% yield (entry 8).
It is noteworthy that the formation of 2,6-dimethylcinnamic acid was achieved by the reaction of
2
4
-iodo-m-xylene with acrylic acid in 59% yield (entry 9). The reaction proceeded well with
-bromoacetophenone, although a longer reaction time was needed (entry 4). However, reactions with
styrene and bromobenzene gave low yields (entries 1 and 10).
Table 2. PS-PdONPs-catalyzed Mizoroki-Heck reaction in water.
PS-PdONPs
(
1.0 mol% of Pd)
R
R
Ar X
+
Ar
KOH (3 equiv),
0.50 mmol
0.75 mmol
H O, 90 °C, 5 h
2

Molecules 2011, 16
9071
Table 2. Cont.
a
Entry Aryl iodides Alkenes
Yield (%)
I
O
b,c
1
2
3
99 (13)
OH
OH
I
O
99
96
I
O
O
OH
OH
MeO
O
I
b,c
4
99 (99)
I
O
O
O
5
6
7
92
99
99
F C
OH
OH
OH
3
I
I
OH
I
O
O
8
9
98
OH
OH
I
I
c
59
c
1
0
14
a
b
c
NMR yields; Aryl bromide was used as a substrate; Reaction time = 20 h.
2
.4. Recycling Experiments
Recycling studies were then performed. After the first reaction, which gave a nearly quantitative
yield of the product (Table 2, entry 1), the catalyst was recovered and successively subjected to nine
more runs of the reaction under the same conditions. As shown in Scheme 1, the yields remained
essentially constant for the ten successive runs. After every run, the reaction solutions were analyzed
by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) to determine the amount of
palladium leached during the reaction. The amount of palladium leaching after every run was <1.4%.
Similar sizes of palladium nanoparticles were observed by TEM after the recycling experiments (after
the fifth run, 3.0 ± 0.6 nm; after the tenth run, 3.0 ± 0.4 nm, Figure 3). When the reaction was
interrupted at 16% conversion and continued after removal of the catalyst (hot filtration test), the
residual activity of the reaction mixture was significant (47% after 20 h). This suggests that leached
palladium species are, obviously, participating in the catalytic process. However, the data in Table 1
(entries 1 and 2) and the hot filtration test indicate that the soluble forms of palladium are not the only
catalytically active species.

Molecules 2011, 16
9072
Scheme 1. Recycling experiments.
PS-PdONPs
(1.0 mol% of Pd)
O
I
O
+
OH
KOH (3 equiv),
H O, 90 °C, 5 h
2
OH
0.75 mmol
0
.50 mmol
1
2
3
4
st run: 98%
nd run: 92%
rd run: 90%
th run: 96%
6th run: 99%
7th run: 99%
8th run: 99%
9th run: 98%
5
th run: 95% 10th run: 97%
Figure 3. (a) TEM image of the recovered catalyst after the fifth run; (b) Size distribution
of the recovered catalyst after the fifth run; (c) TEM image of the recovered catalyst after
the tenth run; (d) Size distribution of the recovered catalyst after the tenth run.
3
. Experimental
3
.1. General
1
H-NMR spectra in CDCl
3
were recorded with a 300 MHz NMR spectrometer (UNITY 300,
Varian, Palo Alto, CA, USA) using tetramethylsilane (δ = 0) as an internal standard. Inductively
coupled plasma-atomic emission spectroscopy (ICP-AES) was performed using ICPS-8100 (Shimadzu
Co., Kyoto, Japan). Pd nanoparticles were investigated by transmission electron microscopy (TEM) on

Molecules 2011, 16
9073
a JEM 2100F transmission electron microscope (JEOL Ltd., Tokyo, Japan). The samples were
prepared by placing a drop of the solution on carbon-coated copper grids and allowed to dry in air.
Polystyrene of narrow molecular weight distribution standards was purchased from Tosoh Co., Ltd.
(Tokyo, Japan). Pd(OAc)
2
was obtained from Sigma-Aldrich Co. (St Louis, MI, USA).
3
.2. Preparation of PS-PdONPs
To a screw-capped vial with a stirring bar was added polystyrene (9.0 mg, 85 μmol of styrene
units), Pd(OAc) (5.5 mg, 25 μmol), and 1.5 M aqueous K CO solution (3 mL). After stirring at 90 °C
2
2
3
for 1 h, the reaction mixture was filtered with hot water. Subsequently, the polystyrene stabilized Pd
nanoparticles were washed with hot water (5 × 1.0 mL) and acetone (5 × 1.0 mL).
3
.3. Determination of the Amount of Palladium
PS-PdONPs (2.9 mg) was placed in a screw-capped vial and then 13 M nitric acid (5 mL) was
added. The mixture was heated at 80 °C to dissolve completely. After cooling to room temperature, the
solution was adjusted to 50 g by water and then the amount of Pd metal was measured by ICP-AES
analysis (15.3 ppm). After the catalytic reaction, the aqueous phase was adjusted to 10 g by nitric acid
and then the amount of Pd metal was measured by ICP-AES analysis.
3
.4. Typical Procedures for Mizoroki-Heck Reaction
To a screw-capped vial with a stirring bar were added iodobenzene (0.25 mmol), acrylic acid
(
0.25 mmol), PS-PdONPs (1.0 mol% of Pd), 1.5 M aqueous KOH solution (1 mL). After stirring at
0 °C for 5 h, the reaction mixture was cooled to room temperature by immediately immersing the vial
9
in water (~20 °C). Subsequently, the aqueous phases were removed, and recovered catalyst was
washed with water (5 × 1.5 mL) and diethyl ether (5 × 1.5 mL), which were then added to the aqueous
−1
phase. After 6.0 mol·L HCl aqueous solution (0.22 mL) was added to the aqueous phase, the aqueous
phase was extracted five times with diethyl ether. The combined organic extracts were dried over
1
MgSO
4
, concentrated under reduced pressure. The product was analyzed by H-NMR. The recovered
catalyst was dried in vacuo and successfully reused. Furthermore, the amount of Pd metal in the
aqueous phase determined by ICP-AES analysis was 0.1 ppm.
4
. Conclusions
PS-PdONPs was prepared with a simple procedure and demonstrated to be an efficient and reusable
catalyst for the Mizoroki-Heck reaction in water. ICP-AES analysis confirmed that palladium leached
into the aqueous solution during the reaction. Hot filtration tests indicated the leached palladium
species are participating in the catalytic process. In addition, no obvious change in particle size was
observed by TEM. Currently, further efforts to extend the application of polystyrene-stabilized metal
nanoparticles to other organic reaction in water are under way in our laboratory.

Molecules 2011, 16
9074
Acknowledgments
This work was grateful to the Nanomaterials and Microdevices Research Center (NMRC) of OIT
for financial and instrumental supports. We wish to thank Y. Uozumi and G. Hamasaka of Institute for
Molecular Science for assistance with TEM measurements. We also thank T. Shinagawa and
Y. Kashiwagi of Osaka Municipal Technical Research Institute for ICP-AES and XRD measurements.
Conflict of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds PS-PdONPs and PS-PdNPs are available from the authors.
©
2011 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
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