Synthesis of efficient and reusable palladium catalyst supported on pH-responsive colloid and its application to Suzuki and Heck reactions in water

Article abstract of DOI:10.1016/j.jcat.2007.06.030

An efficient and reusable pH-responsive colloid supported palladium catalyst was synthesized by loading 3-nm Pd nanoparticles into the pH-responsive colloid of core-shell microspheres of poly[styrene-co-2-(acetoacetoxy)ethyl methacrylate-co-methyl acrylic acid] (PS-co-PAEMA-co-PMAA). The colloidal scaffold of the core-shell microspheres, which contained a pH-responsive shell of PMAA segment and a coordinative core of PAEMA and PS segments, was synthesized by one-stage soap-free emulsion polymerization in water. The pH-responsive colloid supported palladium catalyst was dispersed in basic aqueous medium like a homogeneous catalyst, and also could be simply separated and recovered like a heterogeneous one just by adjusting pH of the aqueous medium. The pH-responsive colloid-supported palladium catalyst proved to be efficient and reusable for the Suzuki and Heck reactions performed in water.

Full text of DOI:10.1016/j.jcat.2007.06.030

Journal of Catalysis 250 (2007) 324–330
www.elsevier.com/locate/jcat
Synthesis of efficient and reusable palladium catalyst supported
on pH-responsive colloid and its application to Suzuki and Heck reactions
in water
∗
Peiwen Zheng, Wangqing Zhang
Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China
Received 17 April 2007; revised 19 June 2007; accepted 21 June 2007
Abstract
An efficient and reusable pH-responsive colloid supported palladium catalyst was synthesized by loading 3-nm Pd nanoparticles into the
pH-responsive colloid of core–shell microspheres of poly[styrene-co-2-(acetoacetoxy)ethyl methacrylate-co-methyl acrylic acid] (PS-co-PAEMA-
co-PMAA). The colloidal scaffold of the core–shell microspheres, which contained a pH-responsive shell of PMAA segment and a coordinative
core of PAEMA and PS segments, was synthesized by one-stage soap-free emulsion polymerization in water. The pH-responsive colloid supported
palladium catalyst was dispersed in basic aqueous medium like a homogeneous catalyst, and also could be simply separated and recovered like
a heterogeneous one just by adjusting pH of the aqueous medium. The pH-responsive colloid-supported palladium catalyst proved to be efficient
and reusable for the Suzuki and Heck reactions performed in water.
©
2007 Elsevier Inc. All rights reserved.
Keywords: Colloid; Heck reaction; Palladium catalyst; pH-responsive; Suzuki reaction
1
. Introduction
brane. All of these accomplishments suggest that the polymer-
supported catalyst is a promising alternative. However, com-
pared with its homogeneous counterparts, the catalyst immobi-
lized in insoluble polymeric scaffold often shows lower activity,
due in part to the heterogeneous polymeric scaffold and in part
to leaching of catalyst from the polymeric scaffold.
The Suzuki reaction (the Pd-catalyzed cross-coupling of
aryl boronic acids with aryl halides) is recognized as the
most promising and versatile method for construction of no-
ble biaryls, which are widely used in the synthesis of fine
chemicals, agrochemicals, and active pharmaceutical inter-
mediates [11–15]. The Heck reaction is a well-established
method for the coupling of aryl halides with olefins [16–18].
The general conditions for performing both the Suzuki and
Heck reactions include a catalyst of palladium nanoparticles
From the standpoint of environmentally benign organic syn-
thesis, the development of highly active and easily reusable
immobilized catalysts and the use of water instead of organic
compounds as solvent are of great interest to chemists [1–3].
Ideally, the catalytic synthesis is efficiently performed in wa-
ter, and the catalyst can be easily separated from the reac-
tion mixture by simple filtration. Cross-linked silica gels and
polystyrene resins have been widely used as insoluble supports
for the immobilization of catalysts [3–5]. To date, numerous
polymer-supported catalysts have been reported, including the
assembled titanium and palladium catalysts bounded to non-
cross-linked amphiphilic polymers [6,7], an Rh-based catalyst
immobilized in the polymeric matrix of polyvinyl alcohol [8],
and an oxime carbapalladacycle catalyst anchored on a sol-
uble polyethyleneglycol scaffold [9]. Abrol et al. [10] even
successfully immobilized the lipase catalyst in polymeric mem-
(
usually generated in situ from the corresponding palladium
salt [19–22]) and a base such as K2CO3, K3PO4, or tri-
ethylamine. In this work, we used the pH-responsive colloid
of core–shell microspheres containing a core-forming seg-
ment of poly[2-(acetoacetoxy)ethyl methacrylate] (PAEMA)
and a shell-forming segment of poly(methyl acrylic acid)
*
Corresponding author. Fax: +86 22 23503510.
E-mail address: wqzhang@nankai.edu.cn (W. Zhang).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.06.030

P. Zheng, W. Zhang / Journal of Catalysis 250 (2007) 324–330
325
(
PMAA) as a scaffold to immobilize the palladium catalyst,
2.2. Synthesis of PS-co-PAEMA-co-PMAA core–shell
microspheres
and then studied the Suzuki and Heck reactions via the re-
sultant pH-responsive colloid-supported palladium catalyst.
We chose the core–shell microspheres of poly[styrene-co-2-
To a 500-mL flask were added 3.0 g of MAA, 3.6 g of
styrene, and 5.5 g of AEMA (at a molar ratio of 4:4:3). Sub-
sequently, 390 mL of double-distilled water was added. The
mixture was vigorously stirred for 30 min at room tempera-
ture. Then 0.47 g of K S O was added, and the mixture was
(
acetoacetoxy)ethyl methacrylate-co-methyl acrylic acid] (PS-
co-PAEMA-co-PMAA) as the scaffold for three reasons. First,
because PAEMA is a typical coordination polymer contain-
ing tethered ligands, the present colloid is able to coordinate
2
2
8
with the ions of Pd2 , and thus no additional ligand is needed
+
degassed under nitrogen purge. Finally, polymerization was
◦
[
23–25]. Second, the pH-responsive colloid and the colloid-
performed at 80 C for 24 h under vigorously stirring. The re-
supported palladium catalyst can be dispersed in basic aqueous
solution. This suggests that the present palladium catalyst has
a potential to bridge the gap between the easy recoverable het-
erogeneous catalyst and the highly efficient homogeneous one.
Third, the pH-responsive colloid-supported palladium catalyst
can be easily separated from the reaction mixture and recovered
by first acidifying and then filtrating the dispersion of the cat-
alyst due to the pH-responsive PMAA segment. Furthermore,
the PS segment with a relatively high glass transition tempera-
sultant core–shell microspheres of PS-co-PAEMA-co-PMAA
were first precipitated in 0.1 mol/L HCl aqueous solution, fil-
trated, washed with water, and finally dried under vacuum.
2.3. Synthesis of pH-responsive colloid-supported palladium
catalyst (pH-responsive catalyst)
First, a given amount of the core–shell microspheres of PS-
co-PAEMA-co-PMAA was dispersed in aqueous solution at
pH 8.0 to form 1.50 mg/mL of pH-responsive colloidal dis-
persion. Subsequently, a given volume of 10.0 mmol/L PdCl2
aqueous solution was added to keep the molar ratio of AEMA to
◦
ture (∼103 C) is also introduced into the core of the core–shell
microspheres to increase the thermal stability of the colloids,
because the glass transition temperature of the core-forming
◦
2+
segment of PAEMA is as low as 3 C [24].
Pd equal to 4:1. The mixture was kept overnight at room tem-
Compared with heterogeneous palladium catalysts immobi-
lized in cross-linked silica gels or insoluble polystyrene resins,
the present pH-responsive colloid-supported palladium catalyst
has four merits. First, it has the obvious advantage of high
degree of dispersion, and thus its high efficiency is expected.
Second, it can be easily separated from the reaction mixture
and recovered simply by adjusting pH of the aqueous medium.
Third, it can work efficiently in water. Finally, its preparation is
simple. The pH-responsive colloid of core–shell microspheres
can be synthesized by one-stage, soap-free emulsion polymer-
ization in water.
perature, after which a 20-fold excess of 0.40 mol/L NaBH4
aqueous solution was added under vigorous stirring. The mix-
ture immediately turned a deep-brown color and was main-
tained at room temperature under vigorous stirring for 4 h.
The resultant pH-responsive colloidal dispersion was dialyzed
against water at room temperature for 4 days.
2.4. Typical procedures for Suzuki reaction
To a screw-capped vial with a side tube 2.0 mmol of aryl
halide, 6.0 mmol of K CO , 3.0 mmol of benzenboronic acid
2
3
(
50% excess), and 5.0 mL of dispersion of the pH-responsive
catalyst were added. The mixture was degassed under nitrogen
purge for 10 min at room temperature, after which the vial was
put in a preheated oil bath at a given temperature and mag-
netically stirred under nitrogen. After the reaction was com-
pleted, the mixture was cooled to room temperature instantly,
and then acidified with 0.1 mol/L HCl aqueous solution. The
pH-responsive catalyst precipitated and was separated by sim-
ple filtration. Subsequently, a 4-fold excess volume of CH2Cl2
was added into the filtrate to extract organic compounds for
3 times. The organic phase was collected and washed with
10 mL of water 3 times. Subsequently, the organic phase was
concentrated and the resultant product was dried under vacuum
2
. Experimental
2
.1. Materials
Styrene (St, >98%), methylacrylic acid (MAA, >99%) and
methyl acrylate (MA, >99%) were purchased from Tianjin
Chemical Company and distilled under vacuum before be-
ing used. The monomer of 2-(acetoacetoxy)ethyl methacrylate
(
AEMA, >95%; Aldrich) and the reagents such as benzen-
boronic acid (>99%; Beijing Wisdom Chemicals Company),
p-chloroacetophenone (>99%, Merck-Schuchardt), p-bro-
mophenol (>99%, Tianjin Guangfu Fine Chemical Re-
search Institute), p-bromoacetophenone (>99%, Alfa Aesar),
iodobenzene (>98%, Alfa Aesar), p-bromobenzoic acid
◦
1
at 40 C, weighted, and analyzed by H NMR.
2.5. Typical procedures for Heck reaction
(
(
(
>99%, Beijing Henye Fine Chemical Company), K S O
2 2 8
>99.5%, Tianjin Chemical Company), PdCl2·2H2O
To a screw-capped vial with a side tube containing 2.0 mmol
of aryl halide, 6.0 mmol of K CO , 3.0 mmol of methyl acrylate
>99.99%, Tianjin Chemical Company), NaBH4 (>98.9%,
2
3
Tianjin Chemical Company), p-bromoanisole (>99%, Tian-
jin Chemical Company) and p-chlorophenol (>99.5%, Tianjin
Chemical Company) were used as received.
(50% excess), and 5.0 mL of dispersion of the pH-responsive
catalyst were added. The mixture was degassed under nitrogen
purge for 10 min at room temperature, after which the vial was

326
P. Zheng, W. Zhang / Journal of Catalysis 250 (2007) 324–330
immersed in a preheated oil bath and magnetically stirred un-
der nitrogen. After the reaction was completed, the mixture was
cooled to room temperature instantly and then acidified with
0
.1 mol/L HCl aqueous solution. Subsequently, a 4-fold excess
volume of diethyl ether was added into the filtrate to extract the
organic compounds. The collected organic phase was washed
with water and then concentrated. Finally, the resultant product
◦
was dried under vacuum at 40 C, weighted, and analyzed by
H NMR.
1
2
.6. Recovery of the palladium catalyst
Two kinds of catalyst recycling were performed. Follow-
ing the first recycling method, the reaction mixture was first
acidified with 0.1 mol/L HCl aqueous solution and then the
precipitate of the pH-responsive catalyst was collected. The
collected catalyst was reused in another run of Suzuki reac-
tion of p-bromoacetophenone, with the proportional amounts
of reactants and solvent being added to keep the same solvent-
to-catalyst ratios and the same concentrations described above.
Following the second method, the pH-responsive catalyst
was not acidified and separated from the reaction mixture.
The aqueous layer containing the catalyst was extracted with
Scheme 1. Schematic structure of the core–shell microspheres of PS-co-
PAEMA-co-PMAA.
and the total conversion of the three monomers is >96%. The
monomer of MAA is hydrophilic, and the monomers of styrene
and AEMA are hydrophobic in water; therefore, the resultant
microspheres are expected to contain a shell rich in PMAA seg-
ment and a complex core rich in PS and PAEMA segments,
as shown in Scheme 1. Fig. 1A shows the TEM image of the
core–shell microspheres. The core–shell structure of the mi-
crospheres can be clearly distinguished. The average diameter,
the core thickness, and the shell thickness of the core–shell mi-
crospheres are about 208, 70, and 34 nm, respectively. Fig. 1B
shows the FTIR spectra of the core–shell microspheres of PS-
co-PAEMA-co-PMAA. Along with the characteristic absorp-
4
-fold excess volume of CH2Cl2, after which the substrates
of p-bromoacetophenone (2.0 mmol), benzenboronic acid
3.0 mmol), and K2CO3 (3.0 mmol) were directly charged into
(
the screw-capped vial to perform the Suzuki reaction in another
cycle. As explained previously, the organic extracts were con-
1
centrated, dried, and analyzed by H NMR.
2
.7. Characterization
−
1
tions at 1600, 1493, 1452, 757, and 697 cm ascribed to the
PS segment, a broad absorption at 1660–1780 nm ascribed to
the two kinds of carbonyl groups in the PAEMA segment and
the carboxyl group in the PMAA segment and an absorption at
Transmission electron microscopy (TEM) measurements
were performed using a Philips T20ST electron microscope at
an acceleration of 200 kV, whereby a small drop of the colloidal
dispersion was deposited onto a piece of copper grid and then
dried at room temperature under vacuum. The FTIR measure-
ment was performed on a Bio-Rad FTS-6000 IR spectrometer.
For FTIR measurement, the mixture of the purified powder
of the core–shell microspheres and KBr was first pressed to
form a pellet, after which the spectra were recorded. The pow-
der X-ray diffraction (XRD) measurement was performed on a
Rigaku D/max 2500 X-ray diffractometer. The XRD patterns
were recorded by using CuKα irradiation (λ = 1.54178 Å).
−
1
1
160 cm attributed to the vibration of the C–O in the PAEMA
segment also can be observed, confirming the component of the
PS-co-PAEMA-co-PMAA core–shell microspheres.
As demonstrated above, because the core–shell microspheres
of PS-co-PAEMA-co-PMAA contain an acidic shell-forming
segment of PMAA, the core–shell microspheres can be dis-
persed in aqueous solution at pH >8 to form a colloidal dis-
persion with a copolymer concentration as high as 30 mg/mL.
At pH <3, the colloid becomes unstable, and precipitation oc-
curs. The precipitate can be redispersed when the pH value of
the aqueous solution is further adjusted to >8.
1
The H NMR measurement was carried out on a Varian UNITY
PLUS-400 NMR spectrometer.
3
. Results and discussion
3.2. Synthesis and characterization of the pH-responsive
catalyst
3
.1. Synthesis and characterization of the pH-responsive
colloid of core–shell microspheres
The pH-responsive colloid of the PS-co-PAEMA-co-PMAA
core–shell microspheres is used as a scaffold to load palladium
nanoparticles because the PAEMA segment is a coordinative
polymer able to dissolve metal compounds due to coordination
The core–shell microspheres of PS-co-PAEMA-co-PMAA
were synthesized by one-stage soap-free emulsion polymeriza-
tion in water. The synthesis of the PS-co-PAEMA-co-PMAA
core–shell microspheres is very similar to those of the PS-co-
PMAA core–shell microspheres as described elsewhere [26]
[23–25]. In the present study, the ions of Pd2 first coordinate
with the chelating groups in the PAEMA segment and then are
reduced by NaBH4 to form discrete Pd nanoparticles in situ.
+

P. Zheng, W. Zhang / Journal of Catalysis 250 (2007) 324–330
327
Fig. 2. The TEM image (A) and XRD pattern (B) of the pH-responsive colloid-
supported palladium catalyst.
Fig. 1. The TEM image (A) and FTIR spectra (B) of the core–shell micro-
spheres of PS-co-PAEMA-co-PMAA.
pH-responsive catalyst has an obvious advantage of high dis-
persion and consequently is expected to be more active than
those immobilized in insoluble cross-linked silica gels and
polystyrene resins. When the pH value decreases to 3, precipi-
tation occurs. The precipitate of the pH-responsive catalyst can
be recovered when the pH value is further adjusted to >8. This
suggests that the present catalyst can be easily separated by sim-
ple filtration and further recovered by adjusting the pH value of
the aqueous medium.
Fig. 2A shows a TEM image of the resultant Pd nanoparticles
supported on the core–shell microspheres. Clearly, the size of
the core–shell microspheres is as same as that shown in Fig. 1A,
and the resultant Pd nanoparticles (about 3 nm in size) are im-
mobilized uniformly in the core of the core–shell microspheres.
The XRD pattern of the colloid-supported palladium catalyst,
shown in Fig. 2B, displays three diffraction peaks at 2θ of
◦
◦
◦
3
9.6 , 45.1 , and 67.5 , corresponding to the diffraction of the
(
111), (200), and (220) lattice planes of the face-centered cu-
bic crystalline structure of the Pd nanoparticles. Furthermore,
the broad diffraction with 2θ ranging from 8 to 35 ascribed
◦
3.3. Suzuki reaction in DMF–water
to the scaffold of the core–shell microspheres also can be ob-
served clearly. In terms of Scherrer’s equation [27], using the
diffraction peak of (111) lattice plane of the Pd nanoparticles,
the average size of the Pd nanoparticles is calculated as 2.3 nm,
consistent with that observed on TEM.
Similar to the colloid of the PS-co-PAEMA-co-PMAA core–
shell microspheres, the colloid-supported palladium catalyst
also is pH-responsive (see supplementary information). At pH
The Suzuki reaction is usually performed in the DMF–water
solvent mixture, which aids solvation of the organic substrates
and the inorganic base [11–15]. In this work, the Suzuki re-
action was first performed in the DMF–water mixture (1:1
by volume) via the pH-responsive catalyst using K2CO3 as a
base. It was found that the pH-responsive catalyst can be dis-
persed in the reaction solution similar to a homogeneous one.
To cover a wide range of substrates, the Suzuki coupling reac-
>
8, the pH-responsive catalyst is dispersed in water; thus, the

328
P. Zheng, W. Zhang / Journal of Catalysis 250 (2007) 324–330
Table 1
Suzuki coupling reaction of aryl halide and benzenboronic acid via the pH-responsive colloid supported palladium catalyst in the mixture of DMF/water and the
a
sole solvent of water
0
.20 mol% catalyst
R–X + Ph–B(OH) −−−−−−−−−−→ Ph–R
2
K CO , solvent
2
3
c
d
Entry
R–X
Solvent
T
Time
(h)
Product
Yield
(%)
◦
(
C)
b
1
2
3
D/W
80
6
6
6
98
b
D/W
120
120
120
120
150
150
80
99 (100)
99
b
D/W
b
4
5
6
7
8
D/W
10
8
97
b
D/W
98
b
D/W
10
10
6
(3)
b
D/W
51
H O
2
83
9
0
6
6
99
9
H O
2
80
50
90
(100)
H O
2
1
63
57
1
0
1
H O
10
2
1
H O
2
90
1
2
99
99
99
93
3
50
12
H O
2
80
0.5
1
50
1
1
3
H O
90
10
2
e
4
a
H O
2
90
10
(–)
Reaction conditions: aryl halide (2.0 mmol) and benzenboronic acid (3.0 mmol, 1.5 eq.), K CO (6.0 mmol, 3 eq.), 5.0 mL of colloidal dispersion containing
2
3
0
.20 mol% palladium catalyst.
b
D/W represented the mixture of DMF and H O (1:1 by volume).
2
c
d
e
Oil bath temperature.
1
1
Isolated yield and the purity of isolated product was confirmed by H NMR. The conversion of aryl halide shown in parentheses was determined H NMR.
1
No p-hydroxybiphenyl was detected by H NMR.
tions of benzenboronic acid with iodobenzene and brominated
and chlorinated aromatic compounds were tested; the catalysis
results are summarized in Table 1. When iodobenzene was used
as the substrate, the coupling reaction was almost completed
electron-deficient acryl chloride of p-chloroacetophenone (en-
try 7). These results demonstrate that the present pH-responsive
catalyst is suitable for the Suzuki coupling of iodobenzene or
aryl bromides with benzenboronic acid in the DMF–water sol-
vent mixture.
(
(
entry 1). The coupling reactions of the two electron-deficient
and thus reactive) aryl bromides of p-bromoacetophenone and
p-bromobenzoic acid with benzenboronic acid were almost
quantitive (entries 2 and 3). For the electron-rich acryl bromides
of p-bromoanisole and p-bromophenol, the pH-responsive cat-
alyst afforded a reasonable yield of 4-methoxybiphenyl (en-
try 4) and p-hydroxybiphenyl (entry 5). For the electron-rich
aryl chloride of p-chlorophenol, the pH-responsive catalyst was
inefficient, and the conversion of p-chlorophenol was as low
as 3% (entry 6). The coupling was unsatisfactory even for the
3.4. Suzuki reaction in water
Concerning that one of our aims is to synthesize an en-
vironmentally benign catalyst, we further studied the Suzuki
coupling reactions of aryl halides with benzenboronic acid per-
formed in water via the pH-responsive catalyst. We performed
the Suzuki reactions at a lower temperature due to the limit
of the lower boiling temperature of water. Similar to that in

P. Zheng, W. Zhang / Journal of Catalysis 250 (2007) 324–330
329
the DMF–water solvent mixture, the pH-responsive catalyst
Table 2
Recycling of pH-responsive colloid supported palladium catalyst in the Suzuki
was dispersed in the basic aqueous solution. The catalysis re-
sults are also summarized in Table 1. For the active substrate
such as iodobenzene (entry 8) and the electron-deficient aryl
bromide of p-bromoacetophenone (entry 9), catalysis in wa-
ter was almost as efficient as that in the DMF–water solvent
mixture, and the yields of the corresponding biaryls in water
were satisfactory. In contrast, for the electron-rich aryl bro-
mide of p-bromoanisole, the yield of 4-methoxybiphenyl was
much lower (entry 10). Interestingly, the hydrophilic electron-
rich aryl bromide of p-bromophenol was extremely active, and
its coupling with benzenboronic acid in water was much more
efficient than in the DMF–water solvent mixture even at much
lower temperature (entry 11). This possibly suggests that the
pH-responsive colloid acts beyond a scaffold of the palladium
catalyst as well as a reactor or capsule, which affects parti-
tioning of the reactants between the solvent and colloid and
thus affects the chemical equilibrium in the Suzuki coupling
reaction. To confirm this hypothesis, we tested two substrates
of the hydrophilic p-bromobenzoic acid and the hydropho-
bic p-bromoacetophenone, both of which are electron-deficient
aryl bromides with similar activity in the Suzuki coupling re-
actions with benzenboronic acid in the DMF–water mixture.
Just as expected, the hydrophilic p-bromobenzoic acid was
much more active than the hydrophobic p-bromoacetophenone
whether the Suzuki reactions were performed in water as the
coupling of p-bromoacetophenone with benzenboronic acid in DMF/water and
water, respectively
a
0
.20 mol% catalyst
Br–Ph–COCH + Ph–B(OH) −−−−−−−−−−→
3
2
K CO , solvent
2
3
b
c
Run
Conversion (%)
Conversion (%)
Fresh use
100
89
13
8
100
1st reuse
87 (99)
76 (100)
79 (97)
2nd reuse
3rd reuse
a
Reaction conditions in the first cycle: p-bromoacetophenone (2.0 mmol)
and benzenboronic acid (3.0 mmol, 1.5 eq.), K CO (6.0 mmol, 3 eq.), 5.0 mL
2
3
of colloidal dispersion containing 0.20 mol% palladium catalyst.
b
DMF/water (1:1 by volume) was used as solvent and the conversion to
1
biphenyl was determined by H NMR.
c
Water was used as solvent and the conversion to biphenyl was deter-
1
mined by H NMR. The numbers in parentheses represent the conversion of
p-bromoacetophenone following the second catalyst recycling, where the pH-
responsive catalyst was not separated and was directly used.
linking in the core–shell colloid could enhance the durability
of the colloidal scaffold and thus increase the reusability of the
pH-responsive catalyst in the mixture of DMF–water mixture;
we will study this further in future work. The pH-responsive
catalyst was more durable and had much higher conversion
in the fourth cycle in water than in the DMF–water mixture.
Furthermore, it should be pointed out that even through great
care was taken, a little of the pH-responsive catalyst was lost
during filtration of the catalyst precipitate. Thus the decreased
conversion in water can be ascribed mainly to the loss of cat-
alyst during filtration. To further study the reuse of the pH-
responsive catalyst, we performed another catalyst recycling
procedure similar to the method adopted by SanMartin and co-
workers [28]; that is, after extracting the organic products with
CH Cl , we used the resulting aqueous layer containing the
◦
sole solvent at high temperature (80 C) or at low temperature
◦
(
50 C) (entries 9 and 12). For the hydrophilic aryl chloride of
p-chlorophenol or the hydrophobic p-chloroacetophenone, the
Suzuki coupling reaction with benzenboronic acid is very inef-
ficient, and almost no biaryl is synthesized (entries 13 and 14).
These results demonstrate that the present pH-responsive cata-
lyst is a suitable catalyst for the substrate of iodobenzene or aryl
bromides, especially for the hydrophilic aryl bromides when the
Suziki reaction is performed in water, and that the catalysis is
almost as efficient as or more efficient than those in the DMF–
water solvent mixture.
2
2
palladium catalyst without further treatment in successive runs,
just addition of the substrates and base. Clearly, this catalyst re-
cycling eliminated the loss of Pd catalyst during filtration. We
repeated the Suzuki reaction in water up to 4 times with no obvi-
ous loss of catalytic activity, as demonstrated by yields >97%.
These results suggest that the leaching of the pH-responsive cat-
alyst in water can be ignored.
3
.5. Recovery and reuse of the pH-responsive catalyst
The reuse of the pH-responsive catalyst was tested in the
Suzuki coupling reaction of p-bromoacetophenone with ben-
zenboronic acid in the sole solvent of water and in the mix-
ture of DMF–water (1:1 by volume), respectively. As discussed
above, the pH-responsive catalyst could be easily recovered first
by adjusting the pH to <3, and then separating the resultant
precipitate by simple filtration. As shown in Table 2, the conver-
sion of p-bromoacetophenone was satisfactory in the first two
cycles when the DMF–water mixture was used as the solvent,
but dropped to 13% in the third cycle, possibly due to the rel-
atively rapid breakage of the colloidal scaffold and leaching of
the palladium catalyst in the DMF–water solvent mixture (be-
cause DMF is a good solvent for the PS, PAEMA, and PMAA
segments). TEM measurements confirmed the hypothesis. It
was found that the polymeric scaffold of the core–shell mi-
crospheres was broken and the Pd nanoparticles conglomerated
3.6. Heck reaction in water
We further studied the activity of the pH-responsive cata-
lyst in the Heck coupling of aryl halides with methyl acrylate
in water. The catalysis results are listed in Table 3. For the
highly reactive iodobenzene, the yield of 2a reached 76% af-
◦
ter 6 h at 80 C, and increasing temperature or time led to
even higher yields (entry 1). For the hydrophilic electron-rich
aryl bromide of p-bromophenol, a satisfactory yield of 2b
was achieved (entry 2). For the hydrophobic electron-deficient
p-bromoacetophenone and p-chloroacetophenone (entries 3
and 4), the yields of 2c were as low as 16% and 1%, re-
spectively. As demonstrated in the Suzuki coupling of aryl
halides and benzenboronic acid in water, the hydrophilic p-bro-
mophenol was extremely active; however, the hydrophilic
(
see supplementary information). We suppose that shell-cross-

330
P. Zheng, W. Zhang / Journal of Catalysis 250 (2007) 324–330
Table 3
Acknowledgments
Heck coupling reaction of aryl halide with methyl acrylate in water via the pH-
responsive colloid supported palladium catalyst
a
Financial support was provided by the National Science
Foundation of China (20504016) and the Program for New
Century Excellent Talents in University.
0
.20 mol% catalyst
R–X + CH =CHCO CH −−−−−−−−−−→ RCH=CHCO CH
2
2
3
2
3
K CO ,H O
2
3
2
b
En- R–X
try
T
Time Product
Yield
(%)
◦
( C) (h)
Supplementary material
80
6
76
89
1
90 12
The online version of this article contains additional supple-
mentary material.
(2a)
Please visit DOI: 10.1016/j.jcat.2007.06.030.
90
16
93
62
2
3
90 24
References
(
2b)
2c)
[
[
[
1] N.E. Leadbeater, Chem. Commun. (2005) 2881.
2] C.J. Li, Chem. Rev. 105 (2005) 3095.
3] B.M.L. Dioos, I.F.J. Vankelecom, P.A. Jacobs, Adv. Synth. Catal. 348
(2006) 1413.
90 16
90 24
16
1
(
[
[
4] C.A. McNamara, M.J. Dixon, M. Bradley, Chem. Rev. 102 (2002) 3275.
5] D. Brunel, A.C. Blanc, A. Galarneau, F. Fajula, Catal. Today 73 (2002)
139.
4
a
(2c)
Reaction conditions: aryl halide (2.0 mmol) and methyl acrylate (3.0 mmol,
.5 eq.), potassium carbonate (4.0 mmol, 3 eq.), 5.0 mL of colloidal dispersion
[
[
[
[
6] Y.M.A. Yamada, K. Takeda, H. Takahashi, S. Ikegami, Org. Lett. 4 (2002)
1
3371.
in water containing 0.20 mol% palladium catalyst.
b
7] Y.M.A. Yamada, M. Ichinohe, H. Takahashi, S. Ikegami, Tetrahedron
Lett. 43 (2002) 3431.
8] M. Ahlmann, O. Walter, M. Frank, W. Habicht, J. Mol. Catal. A 249 (2006)
1
Isolated yield and the purity of the product was confirmed by H NMR.
80.
p-bromophenol was not as active as iodobenzene in the Heck
coupling with methyl acrylate. This may be due to the great dif-
ference in hydrophilicity between hydrophilic p-bromophenol
and hydrophobic methyl acrylate, which weakens the accelera-
tion of the hydrophilic aryl bromide.
9] A. Corma, H. García, A. Leyva, J. Catal. 240 (2006) 87.
[10] K. Abrol, G.N. Qazi, A.K. Ghosha, J. Biotechnol. 128 (2007) 838.
[11] S.D. Walker, T.E. Barder, J.R. Martinelli, S.L. Buchwald, Angew. Chem.
Int. Ed. 43 (2004) 1871.
[
[
[
12] A. Suzuki, J. Organomet. Chem. 576 (1999) 147.
13] G.C. Fu, A.F. Littke, Angew. Chem. Int. Ed. 41 (2002) 2176.
14] J. Yin, M.P. Rainka, X.-X. Zhang, S.L. Buchwald, J. Am. Chem. Soc. 124
(2002) 1162.
4
. Conclusion
[
[
[
[
15] S. Kotha, K. Lahiri, D. Kashinath, Tetrahedron 58 (2002) 9633.
16] E. Drege, G. Morganta, D. Desmaele, Tetrahedron Lett. 46 (2005) 7263.
17] F. lonso, I.P. Beletskaya, M. Yus, Tetrahedron 61 (2005) 11771.
18] C. Najera, J. Gil-Molto, S. Karlstroem, L.R. Falvello, Org. Lett. 5 (2003)
In conclusion, pH-responsive core–shell microspheres of
PS-co-PAEMA-co-PMAA, in which the pH-responsive PMAA
segment forms the shell and the PS and the coordinative
PAEMA segments form the core, were synthesized by one-
stage, soap-free emulsion polymerization in water. The pH-
responsive core–shell microspheres were used as scaffolds to
load 3-nm palladium nanoparticles synthesized in situ to form
a pH-responsive colloid-supported palladium catalyst. In this
work, the pH-responsive catalyst was dispersed in basic aque-
ous medium like a homogeneous catalyst; it also can be simply
separated and recovered like a heterogeneous catalyst by simply
adjusting the pH of the aqueous medium. The pH-responsive
catalyst was demonstrated to be a good alternative for the
Suzuki and Heck reactions in view of two advantages: (i) It
is dispersed in the reaction mixture, and the reactions proceed
efficiently in both water and the DMF–water solvent mixture,
and (ii) the pH-responsive catalyst can be easily separated from
the reaction mixture and readily recovered.
1451.
[19] Z. Zhang, Z. Zha, C. Gan, C. Pan, Y. Zhou, Z. Wang, M. Zhou, J. Org.
Chem. 71 (2006) 4339.
[
[
20] Z. Wang, B. Shen, Z. Aihua, N. Hea, Chem. Eng. J. 113 (2005) 27.
21] N. Panziera, P. Pertici, L. Barazzone, A.M. Caporusso, G. Vitulli, P. Sal-
vadori, S. Borsacchi, M. Geppi, C.A. Veracini, G. Martra, L. Bertinetti,
J. Catal. 246 (2007) 351.
[22] M.J. Climent, A. Corma, S. Iborra, M. Mifsud, J. Catal. 247 (2007) 223.
[23] T. Kaliyappan, P. Kannan, Prog. Polym. Sci. 25 (2000) 343.
[24] T. Krasia, R. Soula, H.G. Börner, H. Schlaad, Chem. Commun. (2003)
538.
[
[
[
[
25] V. Chandrasekhar, P. Thilagar, B.M. Pandian, Coord. Chem. Rev. 251
(2007) 1045.
26] X. Jiang, Y. Wang, W. Zhang, P. Zheng, L. Shi, Macromol. Rapid Com-
mun. 27 (2006) 1833.
27] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrys-
talline and Amorphous Materials, Wiley, New York, 1974, p. 687.
28] F. Churruca, R. SanMartin, B. Ines, I. Tellitu, E. Dominguez, Adv. Synth.
Catal. 348 (2006) 1836.