Highly efficient recyclable polymer anchored palladium catalyst for CC and CN coupling reactions

Article abstract of DOI:10.1016/j.molcata.2012.10.011

An inexpensive, air-moisture stable and reusable PS-Pd(II)-furfural complex was synthesized by reacting polymeric amine with furfural to get polymer anchored ligand which was then reacted with Pd(OAc)₂ to get polymer anchored complex. This complex was characterized by different spectroscopic and elemental analyses. The activities of the Pd-complex were tested for the CC and CN cross-coupling reactions under various reaction conditions. The catalyst exhibits high catalytic activities for the coupling of various aryl halides with organoboronic acid, alkene, alkyne and amine providing excellent yields of desired product. Further, the catalyst can be easily recovered by simple filtration and reused up to five times without significant loss of its catalytic activity.

Full text of DOI:10.1016/j.molcata.2012.10.011

Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Highly efficient recyclable polymer anchored palladium catalyst for C C and C N
coupling reactions
Sk. Manirul Islam∗, Noor Salam, Paramita Mondal, Anupam Singha Roy
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235 W.B., India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2012
Received in revised form 12 October 2012
Accepted 15 October 2012
Available online 22 October 2012
An inexpensive, air-moisture stable and reusable PS–Pd(II)-furfural complex was synthesized by reacting
polymeric amine with furfural to get polymer anchored ligand which was then reacted with Pd(OAc)2 to
get polymer anchored complex. This complex was characterized by different spectroscopic and elemental
analyses. The activities of the Pd-complex were tested for the C C and C N cross-coupling reactions
under various reaction conditions. The catalyst exhibits high catalytic activities for the coupling of various
aryl halides with organoboronic acid, alkene, alkyne and amine providing excellent yields of desired
product. Further, the catalyst can be easily recovered by simple filtration and reused up to five times
without significant loss of its catalytic activity.
Keywords:
Polymer anchored
Pd(II) catalyst
© 2012 Elsevier B.V. All rights reserved.
C
C coupling
Amination
Aryl halides
1. Introduction
selectivity offer several significant practical advantages in synthetic
and industrial chemistry; among those, the ease of separation of the
Palladium-catalyzed reactions have become part of the standard
repertoire of the synthetic organic chemist [1,2]. Palladium
metal is a very well-known catalyst for a variety of organic
reactions, especially hydrogenation [3–6], coupling [7–10], cya-
nation [11–13], carbonylation reactions [14–18] and amination.
Palladium-catalyzed cross-coupling reaction for the formation of
carbon–carbon bonds has recently emerged as a powerful method
in an organic synthesis [19]. The palladium-catalyzed coupling of
an aryl halide with aryl boronic acid (Suzuki coupling) [20] or ter-
minal alkyne (Sonogashira coupling) [21] is recognized as the most
successful method for forming a C(sp2) C(sp2) and C(sp) C(sp2)
bond, respectively. These reactions have been widely employed in
the synthesis of natural products, biologically active molecules and
materials science [22]. Despite the remarkable utility of Pd catalysts
in organic synthesis, they suffer from one significant drawback in
that they often remain as contaminants in the organic products
at the end of the reaction. Another problem with homogeneous
palladium catalysts is related to catalyst recovery and/or reuse,
affecting the overall economics of the process [23,24]. In the past
few decades, heterogenization of metal complexes on solid sup-
ports has attracted great interest as a suitable method for solving
many practical problems including catalyst recovery and recycling.
Solid-phase organopalladium complexes having high activity and
catalyst from the desired reaction products and the ease of recov-
ery and reuse of the catalysts are most important. The C C coupling
reaction generally proceeds in the presence of homogeneous pal-
ladium catalysts which possess high efficiency and are suitable
for the study of the reaction mechanism but their susceptibility
to drastic reaction conditions and the difficulties associated with
their isolation from the product mixture restrict their reusability.
This is a great disadvantage in C C couplings using homogeneous
catalysts. Recycling of catalysts is a task of great economic and
environmental importance in the chemical and pharmaceutical
industry, especially when expensive or toxic heavy metal com-
plexes are employed [25]. A way to overcome these difficulties
would be the use of heterogeneous palladium catalysts. Nowadays,
efficient and recyclable heterogeneous catalysts have gained much
attention [26–29]. The most readily available form of supported
catalyst is palladium on charcoal, which is widely used for het-
erogeneous hydrogenation and also has a growing importance in
carbon–carbon bond-forming reactions [30–32]. There are many
examples of heterogeneous catalysts for C C coupling reactions,
and they are prepared by different approaches like encapsulation
or immobilization of a catalytically active metal complex on solid
supports such as zeolites [33–35], covalent grafting of such active
complexes onto reactive polymer surfaces [36–38] or inorganic
porous matrices [39,40], and ordered mesoporous silica such as
MCM-41 [41–43].
In this present work, we report the synthesis and characteri-
zation of a new furfural functionalized polymer grafted with Pd(II)
∗
Corresponding author. Tel.: +91 33 2582 8750; fax: +91 33 2582 8282.
E-mail address: manir65@rediffmail.com (Sk.M. Islam).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.10.011

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Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
catalyst and used it in the Suzuki, Heck, Sonogashira, and amination
of aryl halides in the presence of inorganic or organic bases. The
effects of the various reaction parameters on the catalytic activ-
ity were studied. The catalyst shows a high catalytic activity in the
coupling reactions of various aryl halides. This polymer anchored
catalyst remains in a separate solid phase in the reaction mixture,
as a result of which further easy catalyst recovery and excellent
recycling efficiency of the catalyst make it an ideal system for
coupling reactions without any precaution of inert atmospheric
conditions.
having an integrating sphere attachment for solid samples. The
thermal stability of the immobilized catalyst was determined using
a Mettler Toledo, Switzerland, TGA/DTA 851e instrument. The
reaction products were analyzed using a Varian, USA, 3400 gas
chromatograph equipped with a 30 m CP-SIL8CB capillary column
and a flame ionization detector. All reaction products were identi-
fied by using an Agilent, USA, GC–MS (QP-5050) equipped with a
30 m HP-5ms capillary column.
2.2. Synthesis of catalyst
The synthetic procedure of polymer anchored palladium com-
plex is illustrated in Scheme 1. Catalyst was prepared in two steps.
Firstly, p-amino-polystyrene was prepared earlier [44]. The suspen-
sion of macroporous amino polystyrene (2 g) in toluene (20 ml) was
taken in a round bottom flask. Furfural (0.01 mol) was added drop
wise to the stirring suspension of amino-polystyrene. The reaction
mixture was refluxed for 24 h. After cooling to room temperature,
the polymer anchored ligands were filtered, washed thoroughly
with toluene followed by methanol and finally dried under vacuum.
Finally, this polymer anchored furfural ligand (500 mg) subse-
quently reacted with Pd(OAc)2 (25 mg) in acetic acid (10 ml) at 80◦ C
for 10 h to generate the corresponding polymer anchored Pd(II)
complex.
2. Experimental
2.1. Materials and instruments
All the reagents were analytical grade and used as such without
further purification. Solvents were purified and dried according to
standard procedures. Polystyrene was purchased from Aldrich and
Pd(OAc)2 was procured from Arora Matthey. Other reagents were
purchased from Merck.
The palladium content was determined by Varian, USA, AA240
atomic absorption spectrophotometer (AAS). A Perkin Elmer, USA,
2400C elemental analyzer was used to collect microanalytical data
(C, H and N). Surface morphology of functionalized polystyrene
ligand and metal complex were analyzed using a scanning electron
microscope (ZEISS EVO40, England) equipped with EDX facility.
Fourier transform infrared (FT-IR) spectra for the catalyst and its
precursors were recorded on a Perkin Elmer, USA, FT-IR 783 spec-
trophotometer using KBr pellets. UV–vis spectrum was taken using
a Shimadzu, Japan, UV-2401PC double beam spectrophotometer
2.3. General experimental procedure for Suzuki coupling reaction
A mixture of aryl halide (1.0 mmol), arylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), DMF–H2O (3–3 ml), and n-dodecane (15–20 mg)
as an internal GC standard and 0.5 mol% of catalyst was stirred at
Scheme 1. Synthesis of polymer anchored Pd(II) catalyst.

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
323
80◦ C in air. Progress of the reaction was monitored by withdrawing
the reaction mixtures periodically and analyzed by GC/GC–MS. GC
yields were based on the amount of aryl halide employed. At the
end of the reaction, the catalyst was separated by simple filtra-
tion. Filtrate was dried over Na2SO4, filtered, concentrated and the
residue was purified by flash column chromatography on silica gel.
The product was analyzed by GC–MS. All the prepared compounds
are known and compared with authentic samples.
chromatography on silica gel to afford the desired product, which
was confirmed by GC–MS. All the prepared compounds are known
and compared with authentic samples.
3. Results and discussion
3.1. Characterization of the PS–Pd(II)-furfural complex
Due to insolubility of the polymer anchored Pd(II) complex in
all common organic solvents, its structural investigations were
limited only to its physicochemical properties, SEM–EDX, TGA, IR
and UV–vis spectral data. The complete incorporation of the organic
substructure in the material was confirmed by elemental analysis
(Table S1).
2.4. General experimental procedure for Sonogashira reaction
A
round-bottomed flask was charged with aryl iodide
(1.0 mmol), phenylacetylene (1.5 mmol), catalyst (1.0 mol% Pd),
triethylamine (2.0 mmol) and DMF (6 ml), and n-dodecane
(15–20 mg) as an internal GC standard. The resulting mixture was
stirred at the appropriate temperature (90 ◦C). The reaction mix-
tures were withdrawn periodically and analyzed by GC/GC–MS. GC
yields are based on the amount of aryl halide employed. Upon com-
pletion of the reaction, the reaction mixture was cooled and filtered
to remove the catalyst which could be used for further reaction. The
filtrate obtained was purified by flash column chromatography on
silica gel to afford the desired product, which was confirmed by
GC–MS. All the prepared compounds are known and compared with
authentic samples.
3.1.1. Scanning electron micrographs (SEM) and energy
dispersive X-ray analyses (EDX)
Scanning electron micrographs of the polymer supported Schiff
base ligand and the prepared catalyst were recorded to understand
the morphological changes occur on the surface of the polymer
matrix. In Fig. 1 the SEM images of the polymer anchored Schiff
base ligand (A) and the immobilized palladium complex on modi-
fied polystyrene (B) are shown. SEM pictures show a morphological
difference between the free Schiff base ligand and the catalyst. EDX
data also supports the metal attachment on the surface of polymer
matrix (Fig. 2).
2.5. General experimental procedure for Heck reaction
3.1.2. TGA studies
A mixture of aryl halide (1.0 mmol), styrene (2.0 mmol), K2 CO3
(2.0 mmol), DMF (6 ml), n-dodecane (15–20 mg) and 0.5 mol% of
catalyst was stirred at 90 ◦C under air. To study the progress of
the reaction, the reaction mixtures were collected at different time
interval and quantified by GC analysis. At the end of the reaction,
the catalyst was separated by simple filtration. Filtrate was dried
over Na2SO4, filtered, concentrated and the residue was purified
by flash column chromatography on silica gel. The product was
analyzed by GC–MS. All the prepared compounds are known and
compared with authentic samples.
Thermal stability of the polymer supported Schiff base ligand
and Pd complex (Fig. 3) were investigated at a heating rate of
10 ◦C/min in air over a temperature range of 30–600 ◦C. It is shown
that thermal stability of the metal complex is slightly greater than
its precursor Schiff base ligand. The complex is stable up to 350 ◦C
and above this temperature it decomposes. Thermogravimetric
study suggests that the polymer supported Pd(II) complex degrades
at considerably high temperature.
3.1.3. IR spectroscopy
The polymer anchored Schiff base exhibits an absorption at
1658 cm−1 assigned to C N stretching. On complexation with Pd,
the frequency of C N (azomethine) is reduced in intensity and
present at 1635 cm−1 (␯C N, coordinated) indicating the formation
of a metal–ligand bond. Two new peaks at 1629 cm−1 (␯C O, free)
and 1313 cm−1 (␯C–O, coordinated) [45], a weak peak at 450 cm−1
(␯Pd–N azomethine) [46] were observed in polymer anchored pal-
ladium complex (Fig. 4).
2.6. General experimental procedure for amination reaction
A mixture of amine (2.0 mmol), aryl halides (1.0 mmol), catalyst
(1.0 mol% Pd), and DMSO (6 ml) was stirred for 16 h under nitrogen
atmosphere at 120 ◦C. The progress of reaction was monitored by
gas chromatography. After completion, the reaction mixture was
cooled and filtered to remove the catalyst which could be used for
further reaction. The filtrate obtained was purified by flash column
Fig. 1. FE SEM images of polymer anchored Schiff base ligand (A) and polymer anchored Pd(II) catalyst (B).

324
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
Fig. 2. EDX images of polymer anchored Schiff base ligand (A) and polymer anchored Pd(II) catalyst (B).
3.1.4. UV–vis spectra
of the ligand and supports the coordination of the ligand to metal-
lic ion. From the microanalytical and spectroscopic data, a probable
structure of the polymer anchored palladium(II) complex may be
proposed according to Scheme 1.
The electronic spectrum (Fig. 5) of the polymer anchored Pd(II)
complex was recorded in diffuse reflectance mode as a BaSO4 disk.
The absorption around 270 nm may be attributed to ␲→␲* transi-
tions of the furfural and phenyl moieties in the polymer anchored
Schiff base ligand. The absorption at 320–380 nm is due to ␲→␲*
and n→␲* transition of imines. After complexation, these absorp-
tions are slightly changed but they are present in complex. This shift
in the spectra of the complex attributed the complexation behavior
Fig. 3. Thermogravimetric weight loss plots for polymer anchored Schiff base ligand
Fig. 4. FT-IR spectra of polymer anchored Schiff base ligand (a) and Pd(II) catalyst
and polymer anchored Pd(II) catalyst.
(b).

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
325
Table 1
1.5
1.0
0.5
0.0
The effect of organic solvent on the polymer-anchored Pd(II) Schiff base-catalyzed
Suzuki cross-coupling reaction of 4-bromoanisole with phenylboronic acid.a
Entry
Solvent
Temperature (◦C)
Atmosphere
Yields (%)b
1
2
3
4
5
6
7
8
9
DMF
80
110
80
100
70
100
80
80
Air
Air
Air
Air
Air
Air
Air
Argon
Air
Air
Air
80
20
75
30
18
12
96
96
70
12
37
Toluene
Methanol
Acetonitrile
Acetone
␳-Xylene
DMF–H2O
DMF–H2O
H2O
(b)
80
30
50
10
11
DMF–H2O
DMF–H2O
(a)
200
300
400
500
600
700
800
a
Reaction conditions: 4-bromoanisole (1.0 mmol), phenylboronic acid
(1.2 mmol), catalyst (0.5 mol% Pd), K2CO3 (1.5 mmol), and time (6 h).
Wavelength (nm)
b
Yields were determined by GC and GC–MS analysis.
Fig. 5. DRS–UV–vis absorption spectra of polymer anchored Schiff base ligand (a)
and polymer anchored Pd(II) catalyst.
Table 2
The effect of base and base–substrate ratio on the polymer-anchored Pd(II) Schiff
base catalyzed Suzuki cross-coupling reaction of 4-bromoanisole with phenyl-
boronic acid.a
3.2. Catalytic activities
Entry
Base
Substrate:base
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
K2CO3
K2CO3
K2CO3
Na2CO3
NaHCO3
Cs2CO3
K3PO4
Na3PO4
NaOH
KF
1:1
1:1.5
1:2
50
76
96
88
70
84
53
45
30
–
3.2.1. Suzuki cross-coupling reaction
Suzuki cross-coupling reaction represents a powerful method
for the C C bond formation. Construction of biaryl compounds
via the palladium-catalyzed Suzuki reaction is an interesting area
in organic synthesis. The importance of biaryl units as molecular
components in pharmaceuticals, herbicides and natural products,
as well as in engineering materials such as conducting polymers,
molecular wires and liquid crystals, has attracted enormous inter-
est. The reaction represents an attractive alternative over other
methods using organometallics because organoboranes are air
and moisture-stable with relatively low toxicity. We started our
investigation for Suzuki reaction (Scheme 2) with the coupling of
4-bromoanisole and phenylboronic acid as model reaction. The per-
formance of a palladium-catalyzed Suzuki reaction is known to be
governed by the number of factors such as the bases, solvents, and
reaction temperatures.
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
NaOtBu
Et3N
–
24
41
NaOAc
a
Reaction conditions: 4-bromoanisole (1.0 mmol), phenylboronic acid
(1.2 mmol), catalyst (0.5 mol% Pd), DMF–H2O (1:1, 6 ml), temperature (80 ◦C),
and time (6 h).
b
Yields were determined by GC and GC–MS analysis.
To verify the solvent effect in Suzuki cross-coupling reactions,
a series of reactions were investigated by taking the model reac-
tion in different solvents. Quantitative yields were observed with
polar solvents such as DMF and methanol (Table 1). On the con-
trary, unsatisfactory yields were obtained with nonpolar solvents
like toluene or p-xylene. The catalytic activity was also observed in
water and the desired product was obtained in 70% yield (Table 1,
entry 9). It was found that the reactions performed in a mixture
of water and DMF (1:1) medium with K2CO3 as a base at 80 ◦C
showed the best performance (Table 1, entry 8). Since the starting
4-bromoanisole and the biphenyl product had poor solubility in
water, the addition of DMF should be helpful in solving the solubil-
ity problems and accelerate the reaction. This coupling reaction was
found to be highly sensitive to the reaction temperature. At lower
temperatures (30–50 ◦C) only low to moderate yield was obtained.
A reaction temperature of 80 ◦C was found to be optimal for the
model reaction.
The influence of base on the catalytic performance of this system
was investigated by employing various bases (Table 2). Compari-
son of inorganic bases utilized showed that carbonate bases were
more stable than the other ones. Within a short time the reaction
proceeded in high yield in presence of Na2CO3 or K2CO3. We found
that using K2CO3 as base in DMF–H2O (1:1) at 80 ◦C gave the cou-
pled product in 96% yields. The other inorganic bases such as NaOH,
K3PO4, NaOAc and organic bases like Et3N were not as effective as
K2CO3, only affords moderate to low yields of coupling products. No
conversion was observed with either NaOtBu or KF. The quantity of
K2CO3 was also found to be important. The base–substrate molar
ratio of 2:1 was found to be ideal for present catalytic system.
Under optimum reaction conditions, the Suzuki cross-coupling
reactions were carried out with arylboronic acids (1.2 mmol) and
aryl halides (1.0 mmol), which has different steric and electronic
properties, using K2CO3 as base in DMF–H2O solvent and the
polymer anchored Pd(II) catalyst. The results are summarized in
PS-Pd(II)-furfural
X
R1
(HO)2B
R2
R2
R1
K2CO3, DMF
80 0C
X = Br, I, Cl
R1 = H,CH3,OMe,COMe, NO2
R2 = H, CH3, OMe
Scheme 2. Suzuki cross-coupling reaction.

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Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
Table 3
Suzuki cross coupling reaction of aryl halides with arylboronic acids with polymer anchored Pd(II) Schiff base complex catalyst.a
Entry
1
Aryl halide
Arylboronic acid
Productb
Time (h)
5
Yieldc (%)
I
B(OH)2
100
3a
3a
3a
Br
Cl
B(OH)2
B(OH)2
B(OH)2
2
3
4
5
8
6
98
70
96
N2O
N2O
NO2
NO2
Br
3b
B(OH)2
B(OH)2
5
6
8
93
87
Br
3c
NO2
NO2
Br
12
3d
MeO
MeOC
H3C
OHC
NC
MeO
MeOC
H3C
Br
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
B(OH)2
7
8
6
5
8
6
6
8
8
96
96
93
95
94
88
90
3e
3f
3g
Br
Br
9
OHC
NC
Br
10
11
12
13
3h
3i
Br
Br
HO
HO
3j
3k
HOOC
N
Br
HOOC
Br
14
15
10
6
85
94
N
3l
MeO
NO2
I
MeO
NO2
B(OH)2
B(OH)2
3e
16
6
95
I
3c
H3C
MeOC
NC
I
B(OH)2
B(OH)2
B(OH)2
H3C
17
18
19
8
5
5
93
98
95
3g
3f
I
MeOC
NC
I
3i

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
327
Table 3 (Continued )
Entry
Aryl halide
Arylboronic acid
Productb
Time (h)
8
Yieldc (%)
HO
I
B(OH)2
HO
20
90
79
3j
Ph
I
B(OH)2
B(OH)2
21
10
3m
MeOC
Cl
MeOC
22
23
24
12
5
81
94
92
3f
CH3
3g
B(OH)2
B(OH)2
Br
H3C
CH3
OHC
OHC
OHC
OHC
Br
H3C
6
3n
COCH
3
B(OH)2
H3COC
Br
25
6
90
3o
a
Reaction conditions: ArX (1.0 mmol), ArB (OH)2 (1.2 mmol), K2CO3 (2.0 mmol), DMF:H2O (6 ml), catalyst (0.5 mol% of Pd) and temperature (80 ◦C).
Products were identified by comparison of their IR, GC–MS data those reported in the literature.
Yields refer to those of purified products characterized by spectroscopic data.
b
c
100
80
60
40
20
0
Table 3. Clearly, aryl halides have a great influence on the coupling
reaction. Among the non-substituted aryl halides, iodobenzene
shows the highest conversion (100%). Aryl bromides and iodides
afforded cross-coupling products in 85–99% yield. Trace amounts
of homo-coupled product were detected. Aryl chlorides gave mod-
erate yields of coupling products (Table 3, entries 3 and 22).
Variations of substituents on the aryl halides were also tested.
Generally, the substitution pattern dependence on the reactivity of
mono-substituted aryl halides is in the order para > meta > ortho.
Para-bromonitrobenzene gave higher conversion than the meta
and the sterically hindered ortho isomers under the same reac-
tion conditions (Table 3, entries 4–6). Electron-donating and
electron-withdrawing substituents in the para position were well
tolerated by the catalytic system and the coupling products were
obtained in good-to-excellent yields. However, para-substituted
aryl halides with electron-withdrawing groups reacted faster than
with electron-donating groups. We subsequently investigated the
application of the catalytic system for the reaction of heteroaryl-
bromide and phenylboronic acid and good yields of coupled
product were obtained. Sterically hindered iodonaphthalene also
reacted efficiently, giving 79% yield of the desired product (Table 3,
entry 21). The activity of the catalyst was evaluated for sev-
eral arylboronic acids. Thus, better activity with the catalyst was
observed with electron-rich relative to electron-poor arylboronic
acids (Table 3, entries 23–25).
Present catalyst
Pd/C
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 6. Suzuki cross-coupling reaction between 4-bromo toluene and phenylboronic
acid catalyzed by polymer anchored Pd(II) furfural catalyst and Pd/C respectively.
parameters for the Sonogashira reaction, we chose to examine
K2CO3, NaOH, Cs2CO3, Et3N and piperidine as base and DMF, DMSO,
toluene and DCM as the solvent (Table S2). It was found that the
reaction performed in DMF with Et3N as base at 90 ◦C showed the
best performances (Scheme 3).
Under optimized reaction conditions, the efficiency of this cat-
alyst was studied for Sonogashira coupling reaction of various
aryl halides and the results are summarized in Table 4. From the
results it is seen that the reaction was effective in the presence
of a wide variety of functional groups on the aryl iodides, bro-
mides and chlorides, giving good to excellent conversions to the
corresponding products. Electron-poor aryl halides were coupled
We carried out a comparative study for the Suzuki cross cou-
pling reaction between 4-bromo toluene and phenylboronic acid
under polymer anchored Schiff base catalyst and Pd/C respectively
in DMF–H2O medium. The experimental progress is closely moni-
tored by GC analysis using dodecane as internal standard and both
reactions undergo same reaction condition. Fig. 6 shows that the
reactions catalyzed by the present polymer anchored Pd(II) catalyst
proceed faster rate than that catalyzed by Pd/C.
3.2.2. Sonogashira cross-coupling reaction
PS-Pd(II)-furfural
X
R
R
The present palladium catalyst was then tested on another
most important C C bond forming process in synthetic organic
chemistry, known as Sonogashira coupling reaction, the palladium-
catalyzed coupling of aryl halides with terminal alkynes. The
reaction condition was optimized by using 4-bromoanisole and
phenylacetylene as a model reaction. To survey the reaction
Et3N, DMF
90 0
C
X = Br, I, Cl
R = H,CH3, COMe, NO2
Scheme 3. Sonogashira cross-coupling reaction.

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Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
Table 4
Sonogashira cross-coupling reaction of aryl halides and phenylacetylene using PS–Pd(II)-furfural.a
Entry
ArX
Productb
Time (h)
8
Yieldc (%)
100
I
1
Br
2
12
99
I
MeOC
MeOC
H3C
3
4
5
6
8
8
97
94
93
83
I
H3C
H3C
MeO
Br
H3C
12
12
Br
MeO
NC
Br
NC
7
8
9
12
12
12
85
90
95
O2N
MeOC
Br
Br
O2N
MeOC
CH3
CH3
Br
10
11
24
12
74
76
Br
Br
N
N
12
13
15
80
Cl
24
24
40
70
Cl
MeOC
MeOC
14
a
Reaction conditions: ArX (1.0 mmol), phenylacetylene (1.5 mmol), Et3N (2.0 mmol), DMF (6 ml), catalyst (1.0 mol% of Pd), and temperature (90 ◦C).
Products were identified by comparison of their IR, GC–MS and 1HNMR data those reported in the literature.
Yields refer to those of purified products characterized by spectroscopic data.
b
c
with phenylacetylene efficiently in the presence of catalyst. The
3.2.3. Heck cross-coupling reaction
electron-rich aryl halides also affords moderate to good conver-
sions. The electronically neutral iodo and bromobenzene produced
high amount of desired product when coupled with phenylacety-
lene. The ortho-substituted aryl halides, and 2-bromotoluene,
gave good yield of coupled product but requires longer reaction
times which might be due to steric hindrance. Under the same
reaction conditions, heteroaryl bromide such as 2-bromopyridine
furnished the desired coupled product in good yields. In additions,
1-bromonaphthalene was successfully coupled with phenylacety-
lene affords the corresponding coupled product in 80% yields. The
less reactive chlorobenzene showed low conversion. The activated
aryl chloride, and 4-chloroacetophenone, gave the corresponding
coupled product in moderate yields. However, in both reactions
high temperature and longer reaction time were required.
The Heck cross-coupling reaction, the palladium-catalyzed cou-
pling of aryl halides with alkenes, is an extremely valuable method
for carbon–carbon bond formation. It is now widely used in the
special chemical and pharmaceutical manufacturing industries,
because it is simple, versatile, and relatively mild. Our catalyst was
next tested for the Heck coupling reaction (Scheme 4). Initial cat-
alytic studies with 0.5 mol% of catalyst were performed on the Heck
cross-coupling of bromobenzene with styrene as a model reaction
in different temperature, solvents and bases under air atmosphere
(Table S3). The reaction was conveniently carried out at 90 ◦C. At
lower temperature, incomplete conversion of bromobenzene was
obtained. Several organic and inorganic bases were screened and it
was observed that the potassium carbonate was the most efficient
base for present catalytic reaction. Other bases were significantly

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
329
PS-Pd(II)-furfural
R
X
R
K2CO3, DMF
90 0C
X = Br, I, Cl
R = H,CH3, COMe, NO2
Scheme 4. Heck cross-coupling reaction.
methanol yields were comparative low. DMSO is found to be the
best solvent for the amination of aniline. This arylation is also
found to be highly sensitive to reaction temperature and time. At
lower temperatures (30 and 60 ◦C), the reaction totally fails and
with lower reaction time (6 and 12 h) only low to moderate yield
was obtained. It was found that the reactions performed in DMSO
medium with KOH at 120 ◦C for 18 h showed the best performance.
On the basis of the above optimized reaction conditions, the cou-
pling reactions between a variety of aryl halides and amines were
investigated. Representative results are summarized in Table 6. It
was observed that the reaction of iodobenzene and aniline pro-
ceeded at 120 ◦C resulting in corresponding coupling product in
high yield (Table 6, entry 1). Iodoarenes with electron-withdrawing
group (Table 6, entries 2, 3) reacted at a faster rate than iodoarenes
with electron donating group (Table 6, entry 4). Substituted anilines
coupled with iodobenzene to give the desired product in moderate
yield (Table 6, entry 5). Bromobenzene remained inactive in ami-
nation reaction under present reaction conditions (Table 6, entry
6).
I
NH2
H
N
PS-Pd(II)-furfural
KOH, DMSO
120 0C
Scheme 5. Amination reaction.
less effective and gave moderate yield. The influence of solvents
on Heck coupling reaction was also examined in different polar
and non-polar solvents. Among different polar solvents, such as
DMF, DMSO, and NMP, high catalytic activity was observed in DMF
medium. Non-polar solvents were not favorable for the Heck cou-
pling reaction under present reaction condition. In the absence of
catalyst, the reaction did not proceed.
With these optimized reaction conditions, the reaction of vari-
ous substituted and non-substituted aryl halides with styrene were
examined to explore the scope and generality of this catalyst for
the synthesis of various aryl-alkenes and the results are summa-
rized in Table 5. As shown in Table 5, the substituent groups on
the aromatic ring of the aryl halides had a significant effect on the
yields. It was observed that aryl halides with electron-withdrawing
functionality undergo Heck coupling reaction efficiently to afford
the corresponding products in excellent yields. Aryl halides with
electron donating functionality gave moderate to good yields of
coupled product. Iodobenzene gave excellent yield of coupled prod-
uct (Table 5, entry 1). Compared with substituted aryl bromides, the
reaction of bromobenzene with styrene gave slight lower product
yield (Table 5, entry 6). Sterically hindered 1-iodo-naphthalene also
coupled with styrene to give desired product in good yield (Table 5,
entry 5). The entire coupled products showed E-stereochemistry.
These results showed that the scope of the reaction is quite broad
and the conditions are tolerant of various functional groups of
substituted on aryl bromides and iodides with an extremely good
selectivity to the trans-isomer. However no coupling product was
obtained in the reaction of less reactive chlorobenzene (Table 5,
entry 10).
Many efficient heterogeneous catalysts have been reported
for the Suzuki, Sonogashira, Heck coupling and amination reac-
tions. Table 7 provides a comparison of the results obtained for
our present catalytic system with those reported in the literature
[50–53]. The present catalyst demonstrates a comparable efficiency
in all the reactions.
3.3. Heterogeneity test
3.3.1. Hot-filtration test
Hot-filtration test was performed in the Suzuki cross-coupling
reaction of 4-bromoanisole with phenylboronic acid to investigate
whether the reaction proceeded in a heterogeneous or a homo-
geneous fashion. After continuing the reaction for 3 h, the catalyst
was removed by filtration and the determined conversion was 40%.
The resulting filtrate was subjected to heating for further 6 h. From
GC result it reveals that after separation of the catalyst no conver-
sion takes place in the filtrate part. This confirms that the reaction
did not proceed upon the removal of the solid catalyst. Further, no
evidence for leaching of palladium or decomposition of the com-
plex catalyst was observed during the catalytic reaction and no
palladium could be detected by atomic absorption spectroscopic
measurement of the filtrate after removal of catalyst. These studies
clearly demonstrated that palladium was intact to a considerable
extent with the heterogeneous support, and there is no significant
amount of leaching occurred during reaction.
3.2.4. Amination reaction
C
N bond formation by palladium-catalyzed cross-coupling
reaction is one of the powerful techniques in synthetic organic
chemistry. This method has been widely used in biological, phar-
maceutical, and material sciences for various applications [47–49].
To test the further applicability of the PS–Pd(II) furfural catalyst,
we examined the amination reaction (Scheme 5) between aniline
with various aryl halides. In the absence of catalyst the reaction
did not proceed. Initially we screened typical reaction parameters
including solvents and bases and we varied the temperature and
time taking iodobenzene and aniline as a model reaction (Table S4).
Bases such as K2CO3, Cs2CO3, Et3N, K3PO4, NaOMe are found to
facilitate this coupling reaction and among them KOH was the best.
In order to determine the best reaction medium, we tested differ-
ent solvents such as methanol, toluene, DMF and DMSO for the
amination reaction of aniline with aryl halides. With DMF, toluene,
3.3.2. Solid phase poisoning test
Further the heterogeneous nature of the catalyst was checked
by the solid phase poisoning test. This test was performed for the
Suzuki coupling reaction of 4-bromoanisole. We used commercially
available 3-mercaptopropyl-functionalized silica as effective pal-
ladium scavenger which selectively coordinates and deactivates
the leached out palladium. For Suzuki reaction, a mixture of 4-
bromoanisole (1.0 mmol), phenylboronic acid (1.2 mmol), K2 CO3
(2.0 mmol), DMF (6 ml), 0.005 g 3-mercaptopropyl-functionalized

330
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
Table 5
Heck cross-coupling reaction of aryl halides and styrene using PS–Pd(II)-furfural.a
Entry
1
ArX
Productb
Time
10
Yieldc (%)
94 (>99)
I
Me
I
2
3
4
10
10
10
85 (97)
96 (98)
96 (95)
Me
I
NO2
MeOC
NO2
I
MeOC
I
5
12
90 (98)
Br
6
7
16
16
85 (>99)
86 (98)
NO2
Br
NO2
8
9
MeOC
16
16
89 (96)
76 (93)
Br
MeOC
Br
N
N
10
24
No reaction
Cl
a
Reaction conditions: ArX (1.0 mmol), styrene (2.0 mmol), K2CO3 (2.0 mmol), DMF (6 ml), catalyst (0.5 mol% of Pd) and temperature (90 ◦C).
Products were identified by comparison of their IR, GC–MS and 1HNMR data those reported in the literature.
Yields refer to those of purified products characterized by spectroscopic data. Selectivity of the E-products is given in parenthesis.
b
c
silica and 0.5 mol% of Pd catalyst was stirred at 80 ◦C for 8 h. In this
case no change in conversion was observed which suggests that
the catalyst is heterogeneous in nature since 3-mercaptopropyl-
functionalized silica form a complex with the leached palladium
and there by deactivate the catalyst. From this result we may
conclude that no leaching of palladium is occurred during Suzuki
coupling reaction and that the catalyst is purely heterogeneous in
nature.
3.4. Catalyst reusability
Recovery and catalyst reuse are important issues in the cou-
pling reactions. Easy catalyst separation and recycling in successive
batch operations can greatly increase the efficiency of the reaction.
We studied the reusability of the present heterogeneous palla-
dium catalyst in the Suzuki coupling reaction taking the reaction
Fig. 7. Recycling efficiency for the Suzuki cross-coupling reaction of 4-bromoanisole
with phenylboronic acid catalyzed by PS–Pd(II)-furfural.

Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
331
Table 6
Amination reaction of aryl halides using PS–Pd(II)-furfural.a
Entry
ArX
Productb
Time
18
Yieldc (%)
85
NH
I
1
NH
NO2
I
I
2
18
88
NO 2
NH
MeOC
CH3
3
4
5
18
18
76
72
70
MeOC
CH3
NH
I
NH
I
18
18
CH3
NH
Br
6
No reaction
a
Reaction conditions: ArX (1.0 mmol), amine (2.0 mmol), KOH (2.0 mmol), DMSO (6 ml), catalyst (1.0 mol% of Pd) and temperature (120 ◦C). All the reactions were carried
out in nitrogen atmosphere.
b
Products were identified by comparison of their IR, GC–MS and 1HNMR data those reported in the literature.
Yields refer to those of purified products characterized by spectroscopic data.
c
Table 7
Comparison of catalytic activity of the present catalyst in the cross-coupling and cyanation reactions with other related reported systems.
Reaction
Catalyst
Reaction conditions
Yield (%)
Reference
PS–Pd(II)-furfural
NHC–Pd complex
PS–Pd(II)-furfural
Pd–PMMA
PS–Pd(II)-furfural
Pd–LHMS-3
K2CO3, DMF–H2O, 0.5 mol% catalyst, 80 ◦C, 5 h
Na2CO3, DMF–H2O, 50 ◦C, 6 h
98
95
85
84
99
80
85
82
Present
[50]
Present
[51]
Present
[52]
Present
[53]
Suzuki reaction
(bromobenzene + phenylboronic acid)
Heck reaction (bromobenzene + styrene)
K2CO3, DMF, 0.5 mol% catalyst, 90 ◦C, 16 h
Et3N, DMF, 1 wt% of Pd, 100 ◦C, 8 h
Et3N, DMF, 1.0 mol% catalyst, 90 ◦C, 12 h
Hexamine, DMF, 120 ◦C, 14 h, N2 atm
KOH, DMSO, 1.0 mol% catalyst, 120 ◦C, 18 h
NaOMe, toluene, 0.05 mmol Pd(OAc)2, 110 ◦C, 24 h
Sonogashira reaction
(bromobenzene + phenylacetylene)
Amination reaction (iodobenzene + aniline)
PS–Pd(II)-furfural
Pd (DPPE)
of 4-bromoanisole and phenylboronic acid (Fig. 7). After comple-
tion of the reaction, the catalyst was recovered by simple filtration
and washed with ethyl acetate followed by acetone then dried in
reduced pressure at 40◦ C. The recovered catalyst was employed
in the next run with further addition of substrates in appropriate
amount under optimum reaction conditions. The catalyst shows
unchanged catalytic activity up to five reaction cycles. No cata-
lyst deterioration was observed, confirming the high stability of
the heterogeneous catalyst under the reaction conditions.
cycles with consistent catalytic activity. Further work is in progress
to broaden the scope of this catalytic system for other organic trans-
formation.
Acknowledgements
We thank the Indian Association for the Cultivation of Science,
Kolkata for providing the instrumental support. SMI acknowl-
edges Department of Science and Technology (DST), Council of
Scientific and Industrial Research (CSIR) and University Grant Com-
mission (UGC), New Delhi, India for funding. NS acknowledges
CSIR-UGC, New Delhi, for providing his Junior research fellow-
ship. PM acknowledge CSIR-New Delhi for her senior research
fellowship.
4. Conclusions
In conclusion, we have reported the simple preparation of
polystyrene supported Pd(II) complex and its successful applica-
tion for the coupling and amination reactions of aryl halides under
air atmosphere using. The present system is highly air and moisture
stable and the catalyst can be synthesized readily from inexpen-
sive and commercially available starting materials. The system
works equally well for wide variety of aryl halides, alkenes, alkynes,
amines and organoboronic acid and tolerates a variety of func-
tional groups. Moreover the catalyst was reused for five consecutive
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.10.011.

332
Sk.M. Islam et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 321–332
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