Catalytic specificity of linear polystyrene-stabilized Pd nanoparticles during Ullmann coupling reaction in water and the associated mechanism

Article abstract of DOI:10.1016/j.jorganchem.2017.11.016

In the present work, heterogeneous Pd nanoparticle catalysts supported on linear polystyrene (PS-PdNPs) were used to promote Ullmann coupling reactions in water. The coupling reaction of 4-bromotoluene proceeded readily in the presence of methanol as the external reductant to give 4,4'-dimethylbiphenyl in 85% yield. In contrast, only a trace of the coupling product was obtained when using ethanol or 2-propanol. Higher yields were also observed for aryl bromides than for aryl iodides. The difference in the yields obtained from 4-iodotoluene and 4-bromotoluene is attributed to variations in the rate of reduction to reform Pd(0) species. The leaching of Pd species into the reaction medium has a negative effect on the Ullmann coupling reaction. The unique reactivity of PS-PdNPs and mechanistic pathway are discussed herein.

Full text of DOI:10.1016/j.jorganchem.2017.11.016

Accepted Manuscript
Catalytic specificity of linear polystyrene-stabilized Pd nanoparticles during Ullmann
coupling reaction in water and the associated mechanism
Atsushi Ohtaka, Akira Sakon, Asuka Yasui, Taira Kawaguchi, Go Hamasaka,
Yasuhiro Uozumi, Tsutomu Shinagawa, Osamu Shimomura, Ryôki Nomura
PII:
S0022-328X(17)30665-4
10.1016/j.jorganchem.2017.11.016
JOM 20181
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 26 September 2017
Revised Date: 11 November 2017
Accepted Date: 21 November 2017
Please cite this article as: A. Ohtaka, A. Sakon, A. Yasui, T. Kawaguchi, G. Hamasaka, Y. Uozumi,
T. Shinagawa, O. Shimomura, Ryô. Nomura, Catalytic specificity of linear polystyrene-stabilized Pd
nanoparticles during Ullmann coupling reaction in water and the associated mechanism, Journal of
Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.11.016.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
Catalytic Specificity of Linear Polystyrene-Stabilized Pd Nanoparticles during
Ullmann Coupling Reaction in Water and the Associated Mechanism
a,*
a
a
a
b
b
Atsushi Ohtaka, Akira Sakon, Asuka Yasui, Taira, Kawaguchi, Go Hamasaka, Yasuhiro Uozumi,
c
a
a,d
Tsutomu Shinagawa, Osamu Shimomura and Ryôki Nomura
a
Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1
b
Ohmiya, Asahi, Osaka 535-8585, Japan. Institute for Molecular Science (IMS), Higashiyama 5-1,
c
Myodaiji, Okazaki 444-8787, Japan. Osaka Municipal Technical Research Institute, 1-6-50
d
Morinomiya, Joto, Osaka, 536-8553, Japan. Nanomaterials and Microdevices Research Center, Osaka
Institute of Technology, 5-16-1 Ohmiya, Asahi, Osaka 535-8585, Japan.
*
Corresponding Author
To whom correspondence should be addressed;
Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology
5
-16-1 Ohmiya, Asahi, Osaka 535-8585, Japan.
Tel. +81-6-6954-4729
Fax. +81-6-6957-2135
E-mail: atsushi.otaka@oit.ac.jp
1

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1
. Introduction
Biaryls are one of the most important classes of organic compounds, and are found in dyes
1-3], natural products [4], pharmaceutical compounds [5], advanced materials [6] and the backbones
[
of some ligands [7,8] for metal catalysts. Over the past several decades, much effort has been devoted
to the synthesis of biaryls via the coupling of aromatic compounds. The Pd-catalyzed Ullmann
coupling reaction [9-11] is a versatile technique for the synthesis of biaryls, and many studies have
focused on the Ullmann coupling reaction in water in conjunction with heterogeneous Pd catalysts
[
12,13]. Aryl iodides were initially used in such studies because the C-I bond is readily activated due to
low dissociation energy of this bond compared with those of the C-Br and C-Cl bonds [14-18].
Metal nanoparticles immobilized by solid support generally exhibit improved catalytic activity
compared to their bulk metal counterparts because they have high surface-to-volume ratios along with
greater concentrations of available active catalytic sites per unit area. In many cases, however, the
leaching of Pd species into the reaction medium occur during the reaction [19-21], and consequently
metal nanoparticles tend to lose their catalytic activity during use because of decreasing the amount of
catalyst or Ostwald ripening [22,23]. Our own group successfully developed linear
polystyrene-stabilized metal nanoparticles with applications to several reactions in water [24-26] and a
release-and-catch system for soluble Pd species in water using linear polystyrene as an efficient
reservoir [27,28]. On the other hand, it has been thought that the metal nanoparticles catalyzed reaction
2

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proceed either in solution by leached species or on the surface of nanoparticles [19-21,29,30]. In the
course of our research on the Hiyama coupling reaction catalyzed by linear polystyrene-stabilized PdO
nanoparticles (PS-PdONPs), we determined that both reactions in solution and on the surface of
nanoparticles were involved in the catalytic cycle and that the reaction sequence was different from the
conventional mechanism [31]. Our continuing interest in the reaction mechanism of the catalytic
reaction in water with linear polystyrene-stabilized metal nanoparticles as a catalyst led us to examine
the Ullmann coupling reaction using PS-PdNPs in the present work. This study demonstrated that
PS-PdNPs exhibit unique reactivity for Ullmann coupling reaction in aqueous solutions.
2
. Experimental
2
.1 Preparation of PS-PdNPs by reduction with 4-methylphenylboronic acid (1a)
To a screw-capped vial with a stirring bar were added 13 mg of polystyrene (0.13 mmol of styrene unit),
-1
2
Pd(OAc) 8.4 mg (37 µmol), 4-methylphenylboronic acid 0.012 g (88 µmol), and 1.5 mol·L aqueous
KOH solution (3 mL). After stirring at 90 °C for 5 h, the aqueous solution was decanted. Subsequently,
the polystyrene stabilized Pd nanoparticles were washed with water (5 × 3.0 mL), MeOH (1 × 3.0 mL),
2
and Et O (5 × 3.0 mL).
2
4
.2 Preparation of PS-PdNPs by reduction with NaBH (1b)
3

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To a screw-capped vial with a stirring bar were added 13 mg of polystyrene (0.13 mmol of styrene unit),
-1
2 4
Pd(OAc) 8.4 mg (37 µmol), and water (2 mL). After adding methanol solution of NaBH (0.37 mol·L ,
1
mL) dropwise, the mixture was stirring at 25 °C for 1 h, and then the aqueous solution was decanted.
Subsequently, the polystyrene stabilized Pd nanoparticles were washed with water (5 × 3.0 mL), MeOH
1 × 3.0 mL), and Et O (5 × 3.0 mL).
(
2
2
.3 Preparation of PS-PdNPs by reduction with benzyl alcohol (1c)
To a screw-capped vial with a stirring bar were added 13 mg of polystyrene (0.13 mmol of styrene unit),
-1
2
Pd(OAc) 8.4 mg (37 µmol), benzylalcohol 12 mg (0.11 mmol), and 1.5 mol·L aqueous KOH solution
(
3 mL). After stirring at 90 °C for 24 h, the aqueous solution was decanted. Subsequently, the
polystyrene stabilized Pd nanoparticles were washed with water (5 × 3.0 mL), MeOH (1 × 3.0 mL), and
2
Et O (5 × 3.0 mL).
2
.4 Determination of loading of the palladium
PS-PdNPs (1a-1c; 2.9 mg) was placed in a screw-capped vial and then added 13 M nitric acid (5
mL). The mixture was heated at 80 °C to dissolve completely. After cooled to room temperature, the
solution was adjusted to 50 mL by water and then measured the amount of Pd metal by ICP-AES analysis
(
1a: 15.3 ppm, 1b: 15.0 ppm, 1c: 15.1 ppm).
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2
.5 Typical procedures for Ullmann coupling reaction
To a screw-capped vial with a stirring bar were added 4-bromotoluene (86.4 mg, 0.5 mmol),
-1
PS-PdNPs (2.9 mg, 1.5 mol% of Pd), and 1.5 mol·L aqueous NaOH solution (1 mL). After stirring at
0 °C for 3 h, the reaction mixture was cooled to room temperature by immediately immersing the vial
in water (~20 °C) for about 10 min. After separating the catalyst and the aqueous phase by
8
2
centrifugation, the aqueous phase was decanted. Recovered catalyst was washed with H O (5 × 3.0
mL) and diethyl ether (5 × 3.0 mL), which were then added to the aqueous phase. The aqueous phase
was extracted eight times with diethyl ether. The combined organic extracts were dried over MgSO
4
1
and concentrated under reduced pressure. The product was analyzed by H NMR.
3
. Results and discussion
Linear polystyrene-stabilized Pd nanoparticles (PS-PdNPs, 1a-1c) [32] were prepared using
-methylphenylboronic acid, NaBH , and benzyl alcohol as reductants. An X-ray photoelectron
4
4
spectroscopy (XPS) analysis showed binding energy at 335.0 and 340.2 eV, which assigned to Pd 3d5/2
and Pd3d3/2 for palladium, respectively (Figure 1). Transmission electron microscopy (TEM) images
showed Pd nanoparticles were dispersed on linear polystyrene with average particle size of 2.7 ± 0.3
nm for 1a and 4.1 ± 0.7 nm for 1b, respectively. In the case of 1c, aggregates with 5 nm or more were
5

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observed (Figure 2).
Figure 1 XPS spectrum of PS-PdNPs.
6

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Figure 2 TEM micrographs and size distributions of 1a-1c (scale bar = 20 nm).
The effect of the Pd nanoparticle size was first examined by performing the Ullmann coupling
-1
reaction of 4-bromotoluene in a 1.5 mol·L aqueous NaOH solution at 80 ºC for 3 h, using methanol as
the external reductant. The desired coupling product was obtained efficiently only in the case of 1a,
indicating that the size of nanoparticles has a significant effect on the catalytic activity (Table 1, entries
1
-3). In addition, the leaching of Pd into the reaction medium appears to be a challenge in this method,
because the amount of leaching increased with increasing particle size [33,34]. Running the reaction
with ethanol or 2-propanol as the external reductant lowered the yield of the desired product (entries 4
and 5), even though Zhang et al. has reported that 2-propanol is an excellent solvent and reductant [10].
2
To better understand the different performances of the reductants, the reduction of PdBr by various
-
1
alcohols in a 1.5 mol·L aqueous NaOH solution at 80 °C was monitored by UV-vis spectroscopy.
2
After heating a mixture of PdBr and methanol (10 equiv), the disappearance of the peak at 368 nm
(
II)
(II)
(0)
corresponding to Pd was observed within 2 h (Figure 3a), indicating the reduction of Pd to Pd . In
7

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contrast, no reduction was observed when using ethanol or 2-propanol under the same conditions
(
Figures 3b and 3c).
Table 1. Effect of size of Pd nanoparticles and external reductant.
PS-PdNPs
1a: 1.5 mol% of Pd)
Br
(
NaOH (3 equiv), additive,
H O, 80 ℃, 3 h
2
0.5 mmol
yield (%)a
entry
additive
1
MeOH (3 equiv)
85
<1
2b
3c
4
MeOH (3 equiv)
MeOH (3 equiv)
EtOH (3 equiv)
i-PrOH (3 equiv)
<1
trace
trace
5
a
NMR yield. b PS-PdNPs (1b) prepared by NaBH4
was used as a catalyst. c PS-PdNPs (1c) prepared
by BnOH was used as a catalyst.
8

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Figure 3. UV-vis spectra of PdBr in a 1.5 mol/L aqueous NaOH solution at 80 °C: (a) in the presence
2
of MeOH, (b) in the presence of EtOH, (c) in the presence of isopropanol.
The scope of the reaction was subsequently studied, employing various aryl halides (Table 2).
Both electron-rich and electron-deficient aryl bromides were found to be applicable, affording the
desired coupling products in good yields (entries 1-4). When the reaction was performed at 50 °C,
similar yields were obtained with either 4-bromobenzotrifluoride or bromobenzene. In contrast, the
yield was decreased dramatically when using 4-bromotoluene or 4-bromoanisole, suggesting that the
oxidative addition of the aryl halide is the rate-determining step. The Ullmann coupling reaction of
9

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4
-bromonitrobenzene proceeded with formation of the desired product in good yield only in the
presence of TBAC (entry 5), likely due to the high melting point of 4-bromonitrobenzene (ca. 125 °C).
In addition, steric hindrance associated with the substrate clearly retarded the Ullmann coupling
reaction, such that 2-bromotoluene gave a low yield, while no product was obtained with either
1
-bromo-m-xylene or 1-bromonaphthalene (entries 6-8). When 4-chlorotoluene was used as the
substrate, the yield of the coupling product was low, as predicted, due to the difficulty of cleavage of
C-Cl bond (entry 9). However, the yield obtained from the reaction of 4-iodotoluene was unexpectedly
lower than that from 4-bromotoluene (entry 10) [35] in contrast to previous works by other researchers
[
9-13]. Given the ease of the cleavage of C-X bond, the difference in the yields obtained from
(
0)
4
-iodotoluene and 4-bromotoluene is attributed to variations in the rate of reduction to re-form Pd
species. Indeed, a quantitative yield was achieved when the Ullmann coupling reaction of
-iodotoluene was performed in the absence of methanol using a large amount of PS-PdNPs (50 mol%
4
2
of Pd). No coupling product was obtained from the reaction of 4-bromoanisole using PdI as the
catalyst, although the coupling proceeded, giving 4,4’-dimethoxybiphenyl in 20% yield, in the presence
-1
2 2
of PdBr (entries 11 and 12). In addition, the reduction of PdBr by methanol in a 1.5 mol·L aqueous
NaOH solution at 80 °C was confirmed by UV-vis spectroscopy, whereas little reduction occurred in
2
the case of PdI (Figure 3a and Figure 4).
1
0

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Table 2. Ullmann coupling reaction of aryl halide in water.
PS-PdNPs
(
1a: 1.5 mol% of Pd)
Ar
X
Ar Ar
NaOH (3 equiv),
MeOH (3 equiv),
0
.5 mmol
H O, 80 ℃, 3 h
2
yield (%)a
entry
Ar
X
1
2
3
4
5
6
7
8
9
phenyl
Br
Br
Br
Br
Br
Br
Br
71 (66)b
85 (8)b
63 (7)b
75 (76)b
44 (73)c
14
4-methylphenyl
4-methoxyphenyl
4-trifluoromethylphenyl
4-nitrophenyl
2-methylphenyl
2,6-dimethylphenyl
0
1-naphtyl
Br
Cl
I
0
4-methylphenyl
4-methylphenyl
4-methoxyphenyl
4-methoxyphenyl
30
9 (99)d
1
1
1
0
1e
2f
Br
Br
20
0
a
NMR yield. b 50 ℃, 3 h. c In the presence of TBAC. d 50 mol% of Pd
was used. e PdBr was used as the catalyst. PdI was used as the
f
2
2
catalyst.
11

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Figure 4. UV-vis spectra of PdI in a 1.5 mol/L aqueous NaOH solution at 80 °C in the presence of
2
MeOH.
When the reusability of the catalyst was examined, decreasing in yield was observed (Figure
5
). Catalytic deactivation would be caused by decreasing of the catalyst amount because leaching of Pd
34] and slightly smaller Pd nanoparticle were confirmed by ICP-AES and TEM, respectively (Figure
). Decrease in catalytic activity was also observed when the reaction time for the recycling trials was
[
6
extended to 5 h. Although no palladium species in the reaction medium after the reaction was observed
by ICP-AES analysis, TEM observation suggested Ostwald ripening occurred (Figure 7).
Figure 5. A recyclability test for PS-PdNPs in Ullmann coupling reaction of 4-bromotoluene in a 1.5
-1
mol·L aqueous NaOH solution at 80 °C in the presence of MeOH.
1
2

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th
Figure 6 TEM micrographs and size distributions of the recovered catalyst after 4 run (reaction time
=
3 h).
th
Figure 7 TEM micrographs and size distributions of the recovered catalyst after 5 run (reaction time
=
5 h).
Deuterium labeling experiments were also conducted to obtain further mechanistic details
Scheme 1). The control reaction produced a 66% yield of coupling product, whereas reactions with
(
3 3 3
CH OD or CD OD furnished the biphenyl in 16% or 8% yields, respectively. Because both the CH OD
1
3

ACCEPTED MANUSCRIPT
and CD
3
OD gave lower yields, it appears that both cleavage of C-H bond of methanol by β-H
(
II)
elimination and O-H bond such as oxidative addition to Pd species [36] or deprotonation from
methanol on the metal surface [37] include in the catalytic cycle.
PS-PdNPs
Br
(
1a: 1.5 mol% of Pd)
NaOH (3 equiv),
CH OH (4.5 equiv),
3
0
.5 mmol
66%
H O, 50 ℃, 3 h
2
PS-PdNPs
(
1a: 1.5 mol% of Pd)
NaOH (3 equiv),
CH OD (4.5 equiv),
3
1
6%
D O, 50 ℃, 3 h
2
PS-PdNPs
(
1a: 1.5 mol% of Pd)
NaOH (3 equiv),
CD OD (4.5 equiv),
3
8
%
D O, 50 ℃, 3 h
2
Scheme 1. Deuterium labelling experiments.
Based on the above results, we propose a mechanism for the Ullmann coupling reaction in the
presence of PS-PdNPs (Scheme 2). Following the successive oxidative addition of two aryl halides on
the surface of the nanoparticles (as opposed to the solution phase), reductive elimination proceeds to
(
0)
form the coupling product [38]. And then the reduction by methanol occurs to regenerate Pd . The
result obtained using 4-iodotoluene (Table 2, entry 10) is consistent with the proposed process, in
1
4

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which the successive oxidative addition of two aryl halides occurs prior to the reduction by methanol.
(
II)
There is only minimal reduction of dissolved Pd species by methanol at 50 °C (Figure 8), the
Ullmann coupling reaction of 4-bromobenzotrifluoride took place smoothly at 50 °C (entry 4 in Table
2
), and only 4% of the corresponding product was obtained from the reaction of
-bromobenzotrifluoride using a water-soluble palladium species, which was prepared by dissolving
4
-1
2
PdBr in 1.5 mol·L NaOH aqueous solution (eq 1). These results indicate that the leaching of Pd
species into the reaction medium has a negative effect on the Ullmann coupling reaction.
Ar
X
HX
Pd
H
X
Ar
X
Pd
Pd
Ar
X
HCHO
CH O
X
X
X
3
Ar Ar
Pd
Pd
HX
CH OH
3
Scheme 2. A plausible reaction mechanism for Ullmann coupling reaction.
1
5

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Figure 8. UV-vis spectra of PdBr in a 1.5 mol/L aqueous NaOH solution at 50 °C in the presence of
2
MeOH.
PdBr in NaOH
2
aq
(1.5 mol% of Pd)
CF3
Br
(
3 equiv of NaOH)
(1)
MeOH (4.5 equiv),
F3C
0
5
0 ℃, 3 h
F3C
.50 mmol
4%
4
. Conclusion
In summary, the Ullmann coupling reaction as catalyzed by polystyrene-stabilized Pd
nanoparticles was investigated. This catalyst showed high catalytic activity and the catalytic activity
was also found to be dependent on the size of nanoparticles. The reactivity order of aryl halides was
Ar-Br > Ar-Cl > Ar-I, as a result of variations in the ease with which the C-X bond could be cleaved
(
II)
(0)
and with which Pd could be reduced to Pd by methanol. The reaction evidently starts with the
1
6

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(
II)
successive oxidative addition of two aryl halides, with the reduction of Pd species by methanol
occurring after the formation of the coupling product. It was further determined that the reaction takes
place on the surface of the Pd nanoparticles and that leaching of Pd into reaction medium impedes the
reaction.
Acknowledgement
The authors are grateful to the Nanomaterials and Microdevices Research Center (NMRC) of OIT for
financial and instrumental supports. This work was supported by the Joint Studies Program (2016) of
the Institute for Molecular Science.
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23] C.C. Cassol, A.P. Umpierre, G. Machado, S.I. Wolke, J. Dupont, The Role of Pd Nanoparticles in
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Ionic Liquid in the Heck Reaction, J. Am. Chem. Soc. 127 (2005) 3298-3299.
[
24] A. Ohtaka, Recyclable Polymer-Supported Nanometal Catalysts in Water, Chem. Rec. 13 (2013)
74-285.
25] A. Ohtaka, T. Yamaguchi, R. Nishikiori, O. Shimomura, R. Nomura, One-pot Synthesis of
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Dibenzyls and 3-Arylpropionic Acids Catalyzed by Linear Polystyrene-Stabilized Pallasium Oxide
Nanoparticles in Water, Asian J. Org. Chem. 2 (2013) 399-402.
[
26] A. Ohtaka, M. Kozono, K. Takahashi, G. Hamasaka, Y. Uozumi, T. Shinagawa, O. Shimomura, R.
Nomura, Linear Polystyrene-Stabilized Pt Nanoparticles Catalyzed Indole Synthesis in Water via
Aerobic Alcohol Oxidation, Chem. Lett. 45 (2016) 758-760.
[
27] A. Ohtaka, E. Sakaguchi, T. Yamaguchi, G. Hamasaka, Y. Uozumi, O. Shimomura, R. Nomura, A
Rycyclable “Boomerang” Linear Polystyrene-Stabilized Pd Nanoparticles for the Suzuki Coupling
Reaction of Aryl Chlorides in Water, ChemCatChem 5 (2013) 2167-2169.
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28] A. Ohtaka, T. Okagaki, G. Hamasaka, Y. Uozumi, T. Shinagawa, O. Shimomura, R. Nomura,
Application of “Boomerang” Linear Polystyrene-Stabilized Pd Nanoparticles to a Series of C-C
Coupling Reactions in Water, Catalysts 5 (2015) 106-118.
[
29] P.J. Ellis, I.J.S. Fairlamb, S.F.J. Hackett, K. Wilson, A.F. Lee, Evidence for the Surface-Catalyzed
Suzuki-Miyaura Reaction over Palladium Nanoparticles: An Operando XAS Study, Angew. Chem. Int.
Ed. 49 (2010) 1820-1824.
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[
30] D.B. Eremin, V.P. Ananikov, Understanding Active Species in Catalytic Transformations: From
Molecular Catalysis to Nanoparticles, Leaching, “Cocktails” of Catalysts and Dynamic Systems, Cood.
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31] A. Sakon, R. Ii, G. Hamasaka, Y. Uozumi, T. Shinagawa, O. Shimomura, R. Nomura, A. Ohtaka,
Detailed Mechanism for Hiyama Coupling Reaction in Water Catalyzed by Linear
Polystyrene-Stabilized PdO Nanoparticles, Organometallics 36 (2017) 1618-1622.
[
32] XPS analysis and FT-IR measurement were performed to reveal the interaction between
polystyrene and Pd nanoparticles. Unfortunately, however, we could not confirm any obvious
differences between polystyrene and PS-PdNPs.
[
33] Metal leaching will work against the Ullmann coupling reaction. See: R.N. Dhital, M. Ehara, H.
Sakurai, Gold/Palladium Bimetallic Nanoclusters for C-X Bond Activation: A Unique Effect of Gold,
Yuki Gosei Kagaku Kyokaishi 73 (2015) 1130-1140.
[
34] 1.3%, 8.4% of 17.1% of palladium leaching was confirmed by ICP-AES analysis when the
reaction medium was checked after the Ullmann coupling reaction of 4-bromotoluene using 1a, 1b, or
c, respectively.
35] A similar tendency was observed in the hydrodehalogenation of aryl halide. See: X. Ma, S. Liu, Y.
1
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Liu, G. Gu, C. Xia, Comparative Study on Catalytic Hydrodehalogenation of Halogenated Aromatic
Compounds over Pd/C and Raney Ni Catalysts, Sci. Report (2016) 25068.
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[
36] P.K. Monaghan, R.J. Puddephatt, Oxidation of Dimethylplatinum(II) Complexes with Alcohols:
Synthesis and Characterization of Alkoxoplatinum(IV) Complexes, Organometallics 3 (1984) 444-449.
37] Z. Ahmadi, J.S. Mclndoe, A Mechanistic Investigation of Hydrodehalogenation Using ESI-MS,
Chem. Commun. 49 (2013) 11488-11490.
38] Indeed, the formation of biaryl product was confirmed from the reaction of aryl halide with aryl
group on the surface of Pd(II) nanoparticles: See ref 31.
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Highlights
Catalytic Specificity of Linear Polystyrene-Stabilized Pd Nanoparticles
during Ullmann Coupling Reaction in Water and the Associated
Mechanism
*
Atsushi Ohtaka, Akira Sakon, Asuka Yasui, Taira, Kawaguchi, Go Hamasaka, Yasuhiro
Uozumi, Tsutomu Shinagawa, Osamu Shimomura and Ryôki Nomura
ꢀ
Polystyrene-stabilized Pd nanoparticles showed high catalytic activity for Ullmann
coupling reaction of aryl bromides in water.
ꢀ
ꢀ
ꢀ
The reactivity order of aryl halides was Ar-Br > Ar-Cl > Ar-I.
The size of nanoparticles has a significant effect on the catalytic activity.
The reaction takes place on the surface of the Pd nanoparticles and that leaching of
Pd into reaction medium impedes the reaction.