Heterogeneous Suzuki and copper-free Sonogashira cross-coupling reactions catalyzed by a reusable palladium(II) complex in water medium

Article abstract of DOI:10.1016/j.tetlet.2010.02.079

A new polystyrene anchored Pd(II) azo complex has been synthesized and characterized. The present Pd(II) azo complex behaves as a very efficient heterogeneous catalyst in the Suzuki coupling and Sonogashira coupling reaction in water medium. Aryl halides, coupled with phenylboronic acids (Suzuki-Miyaura reaction) or terminal alkyne (Sonogashira reaction), smoothly afford the corresponding cross-coupling products in excellent yields (83-100% yield for Suzuki reaction and 68-96% yield for Sonogashira reaction of aryl halides) under phosphine-free reaction conditions in the presence of polystyrene anchored Pd(II) azo complex catalyst in water medium. Furthermore, the catalyst has shown good thermal stability and recyclability. This polymer-supported Pd(II) catalyst could be easily recovered by simple filtration of the reaction mixture and reused for more than six consecutive trials without a significant loss of its catalytic activity.

Full text of DOI:10.1016/j.tetlet.2010.02.079

Tetrahedron Letters 51 (2010) 2067–2070
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Heterogeneous Suzuki and copper-free Sonogashira cross-coupling
reactions catalyzed by a reusable palladium(II) complex in water medium
*
S. M. Islam , Paramita Mondal, Anupam Singha Roy, Sanchita Mondal, Dilder Hossain
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235 WB, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new polystyrene anchored Pd(II) azo complex has been synthesized and characterized. The present
Pd(II) azo complex behaves as a very efficient heterogeneous catalyst in the Suzuki coupling and Sono-
gashira coupling reaction in water medium. Aryl halides, coupled with phenylboronic acids (Suzuki–
Miyaura reaction) or terminal alkyne (Sonogashira reaction), smoothly afford the corresponding cross-
coupling products in excellent yields (83–100% yield for Suzuki reaction and 68–96% yield for Sonogash-
ira reaction of aryl halides) under phosphine-free reaction conditions in the presence of polystyrene
anchored Pd(II) azo complex catalyst in water medium. Furthermore, the catalyst has shown good ther-
mal stability and recyclability. This polymer-supported Pd(II) catalyst could be easily recovered by simple
filtration of the reaction mixture and reused for more than six consecutive trials without a significant loss
of its catalytic activity.
Received 26 November 2009
Revised 11 February 2010
Accepted 15 February 2010
Available online 18 February 2010
Keywords:
Suzuki reaction
Sonogashira reaction
Water medium
Polystyrene anchored Pd(II) azo complex
Ó 2010 Elsevier Ltd. All rights reserved.
Palladium-catalyzed cross-coupling reaction for the formation
of carbon–carbon bonds has recently emerged as a powerful meth-
polymer-supported palladium catalysts13 for the Suzuki and Sono-
gashira cross-coupling reactions have been reported.
1
od in an organic synthesis. The palladium-catalyzed coupling of
Herein we report the synthesis and characterization of a new
polystyrene anchored Pd(II) azo complex catalysts and illustrate
its application for the Suzuki cross-coupling and Sonogashira
cross-coupling reactions in water medium under aerobic condi-
tion. The catalyst shows a high catalytic activity in the coupling
reactions of various aryl halides. Further easy catalyst recovery
and excellent recycling efficiency of the catalyst make it an ideal
system for coupling reactions in aqueous phase.
2
an aryl halide with aryl boronic acid (Suzuki coupling) or terminal
3
alkyne (Sonogashira coupling) is recognized as the most success-
2
2
2
ful method for forming a C(sp )–C(sp ) and C(sp)–C(sp ) carbon–
carbon bond, respectively. These reactions have been widely
employed in the synthesis of natural products, biologically active
molecules and materials science.4
5
Although homogeneous catalysts have many advantages, cata-
lyst immobilization is a well-known methodology to allow effi-
The preparation procedure followed to obtain the catalyst is
indicated in Scheme 1. Polymeric amine was prepared according
to the literature method. The suspension of polymeric amine
6
cient catalyst separation and to obtain metal-free products.
Several types of solid materials such as zeolites, alumina, silicas,
7
8
9
14
1
0
and polymers have been used to support palladium species. From
the standpoint of environmentally benign organic synthesis, the
development of highly active and easily reusable immobilized
catalysts and the use of water instead of organic compounds as sol-
vent are of great interest to chemists. Solid-phase organopalladium
metal complexes having high activity and selectivity offer several
significant practical advantages in synthetic and industrial chemis-
try; among those, the ease of separation of the catalyst from the
desired reaction products and the ease of recovery and reuse of
the catalyst are most important. In addition, the use of water in
palladium-catalyzed coupling reaction has become popular be-
cause water base synthetic processes are inherently safer as well
(3 g) treated with 1-nitroso-2-naphthol (1.0 g) in methanol
(10 ml) to produce the corresponding polymer anchored azo ligand
(3) which was then reacted with Pd(II) acetate (50 mg) in glacial
acetic acid (10 ml) to yield the catalyst as light brown solid (4).
Due to insolubility of the different functionalized polystyrene
and the corresponding polystyrene anchored Pd(II) complex in all
common solvents, their structural investigations were limited only
to their physico-chemical properties, SEM, TGA-DTA, IR, and UV–
vis spectral data. Atomic absorption spectroscopy suggested 5.45%
of Pd in the complex. The polystyrene anchored azo ligand shows a
À1
sharp peak at 1452 cm due to the N@N bond which on complexa-
À1
tion with Pd disappears and appears as a band at 1440 cm . The
1
1
as inexpensive. Moreover, products can be isolated easily by
lowering in frequency of N@N peak indicates an effective coordina-
tion interaction between the ligand and Pd center. The polystyrene
anchored Pd(II) azo complex exhibited important IR peaks at
1
2
extraction. Recently, a variety of aqueous catalytic systems and
À1
À1
À1
15
À1
1
585 cm , 1430 cm
(m
COO bridged), 520 cm
(m Pd–C),
*
Corresponding author. Tel.: +91 33 2582 8750; fax: +91 33 2582 8282.
16
À1
17
E-mail address: manir65@rediffmail.com (S.M. Islam).
722 cm (orthometallation), and 455 cm
(m Pd–N).
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.02.079

2
068
S. M. Islam et al. / Tetrahedron Letters 51 (2010) 2067–2070
Ac O / AcOH
AcOH
2
NaOH
MeOH
P
NO2
P
NH Cl
3
P
P
NH
2
HNO3
SnCl 2 / HCl
(
1)
NO
(2)
Reflux
MeOH
OH
P
HO
N
N
N
P
OAc
OAc
N
Pd
Pd
OH
Pd(OAc) 2 / AcOH
Reflux
N
N
(
3)
OH
P
(
4)
Scheme 1. Preparation of polystyrene anchored Pd(II) azo complex.
The palladium-catalyzed Suzuki cross-coupling of aryl halides
converted efficiently to the desired products in good to excellent
yields within 6–8 h (entries 1–12). The less reactive chlorobenzene
showed a moderate conversion. However, the activated aryl chlo-
ride, 4-acetylchlorobenzene gave the corresponding product in
good yield using the present catalyst (entry 14). The sensitive het-
eroaryl halides bearing pyridyl moiety produced the corresponding
aryl-heteroaryl coupled product in good yields (entry 12). Steri-
cally hindered 2-nitrobromobenzene was found to react efficiently
under the present reaction condition. However, the ortho-substi-
tuted aryl bromide required longer reaction times than para-
substituted aryl bromide.
with arylboronic acids has become a general and convenient syn-
thetic route in organic chemistry toward biaryls, which has been
employed in many areas of organic synthesis, including natural
product syntheses. To explore the catalytic activity, the Suzuki
cross-coupling reaction of aryl halides with phenylboronic acid
was conducted with the present catalyst. For optimization of the
reaction conditions, we choose the cross-coupling of 4-iodotoluene
with phenylboronic acid in water medium under aerobic condition
as the model reaction, and the effects of the temperature and base
on the reaction were examined. The results are presented in Table
1
. At room temperature, low amount of coupled product was ob-
Another important Pd catalyzed coupling reaction is the alkany-
lation of aryl halides which is known as Sonogashira reaction.
Application of this reaction is to be found in the synthesis of
numerous natural products including enediyne antibiotics. In our
present study, the Sonogashira coupling reactions of iodo and bro-
mo arenes with phenyl acetylene in water medium was carried out
under the copper-free and phosphine-free condition. Since no cop-
per salt used, the undesired formation of oxidative homocoupling
product, a diyne, was also avoided. The reaction was performed
tained. Increasing the temperature from 30 to 70 °C improved
the conversion (Table 1, entries 1–3), while further increase in
temperature to 80 °C, no such improvement in the conversion
was obtained. We found that the reaction was highly dependent
on the amount of base as well as its nature. The base-substrate ra-
tio of 2:1 is the best for this catalytic system. Various bases such as
Na
the reaction. The results revealed that the inorganic bases used
were more effective than organic bases like NaO Bu and Et
t
2
CO
3
2
, K CO
3
, KOH, Et
3
N, NaO Bu, and Na
PO
3 4
were examined for
t
3
N (Ta-
CO
with Et
rate of the reaction by transferring haloarene to the aqueous phase
3
N as a base and TBAB as an additive. TBAB enhances the
ble 1, entries 8 and 9) and hence the economically cheaper K
was chosen as a base for these coupling reactions.
2
3
1
8
and hence reaction with terminal alkyne is faster. We employed
the coupling reaction of iodo benzene with phenylacetylene as a
model reaction to study the effect of base on the reaction
(Table 3). Among various bases, triethylamine was found to be
best. The reaction works well when organic bases are used.
Next, to demonstrate versatility of the catalytic system for the
Suzuki coupling reaction, aryl halides substituted with various
electrons-withdrawing and electrons-donating groups were cho-
sen. The results are summarized in Table 2. Both iodobenzene
and bromobenzene showed excellent conversion in Suzuki reac-
tion. The various substituted aryl iodides and bromides with deac-
tivated (electron rich) and activated (electron poor) were
2 3 2 3 3 4
Inorganic bases, such as K CO , Na CO , K PO , and NaOH, were
less effective to organic ones.
Next, several substituted iodo and bromoarenes were examined
in Sonogashira reaction using polystyrene anchored Pd(II) azo cat-
alyst to explore the catalytic activity of the present catalytic proto-
col. The results are summarized in Table 4. The aryl halides with
electron-withdrawing substituents, such as nitro, (Table 4, entries
2 and 7) and acetyl, (Table 4, entries 3 and 8) gave excellent yields,
while aryl halides with electron-donating groups, such as methyl
and methoxy (Table 4, entries 4, 5, 9, and 10) gave the coupling
products in slightly lower yield. The coupling reaction of aryl bro-
mides required higher temperature and extended reaction time
than aryl iodides. The heteroaromatic compound, 2-bromopyri-
dine, also reacted with phenyl acetylene to give the desired prod-
uct in good yield (entry 11). However, chloroarenes basically do
not react under present reaction conditions, and yields are less
than 6%.
Table 1
Effect of base on the Suzuki coupling reaction of 4-iodotoluene with phenylboronic
acid in water medium
Entry
Base
Temp (°C)
Yielda (%)
1
2
3
4
5
6
7
8
9
K
K
K
K
2
2
2
2
CO
CO
CO
CO
3
3
3
30
50
70
80
70
70
70
70
70
35
87
96
96
84
57
48
45
32
3
Na
2
CO
3
KOH
Na PO
Et
NaO Bu
3
4
3
N
t
Reaction conditions: 4-iodo toluene (1.0 mmol), phenylboronic acid (1.5 mmol),
catalyst (0.5 mol % Pd), base (2.0 mmol), water (6 ml).
In order to find out the effect of anchoring, the activity of the
present polystyrene anchored Pd(II) azo complex catalyst was
a
Yield determined by GC and GCMS analysis.

S. M. Islam et al. / Tetrahedron Letters 51 (2010) 2067–2070
2069
Table 2
Suzuki coupling reaction of various aryl halides with phenylboronic acid catalyzed by polystyrene anchored Pd(II) azo complex
Entry
Arylhalide
Temp (°C)
Time (h)
Product
Yielda (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
Iodobenzene
70
70
70
70
70
70
70
70
70
70
70
70
85
85
85
6
6
6
6
6
7
6
6
8
8
10
8
12
12
12
Biphenyl
100(98)
99(94)
99(93)
96(87)
98(94)
99(96)
98
99
94
96
92
4-Nitroiodobenzene
4-Iodoacetophenone
4-Iodotoluene
4-Iodoanisole
Bromobenzene
4-Nitrobromobenzene
4-Bromoacetophenone
4-Bromotoluene
4-Bromoanisole
2-Nitrobromobenzene
2-Bromopyridine
Chlorobenzene
4-Nitrobiphenyl
4-Acetylbiphenyl
4-Methylbiphenyl
4-Methoxybiphenyl
Biphenyl
4-Nitrobiphenyl
4-Acetylbiphenyl
4-Methylbiphenyl
4-Methoxybiphenyl
2-Nitrobiphenyl
2-Phenylpyridine
Biphenyl
1
1
1
1
1
1
90(82)
85
90(84)
83
4-Chloroacetophenone
4-Chlorotoluene
4-Acetylbiphenyl
4-Methylbiphenyl
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.5 mmol), catalyst (0.5 mol % Pd), K
2
CO
3 2
(2.0 mmol), H O (6 ml).
a
Yield determined by GC and GCMS analysis, based on aryl halides, average of two runs. Isolated yield are given in parentheses.
Table 3
applications of coupling reactions. To determine whether the cata-
lyst is actually functioning in a heterogeneous manner, a hot-filtra-
tion test was performed in the Suzuki coupling reaction of 4-iodo
Effect of base on the Sonogashira coupling reaction of iodobenzene with phenylbo-
ronic acid in water medium
21
Entry
Base
CO
Yielda (%)
toluene and phenylboronic acid. During catalytic reaction, the solid
catalyst was separated from the reaction mixture by filtration after
3 h and the determined conversion was 52%. The reaction was
carried out for a further 2 h. The gas chromatographic analysis
showed no increment in the conversion. Atomic absorption
spectrometric analysis of the liquid phase of the reaction mixtures
collected by filtration confirms that Pd is absent in the reaction
mixture. These results suggested that no leaching of palladium
takes place during the reaction and that the catalyst is purely
heterogeneous in nature.
1
2
3
4
5
6
K
2
3
49
42
37
21
94
79
Na CO
2 3
K
3
PO
KOH
Et
Piperidine
4
3
N
Reaction conditions: iodobenzene (1.0 mmol), phenylacetylene (1.5 mmol), catalyst
(
0.5 mol % Pd), TBAB (1.0 mmol), base (2.0 mmol), water (6 ml), 70 °C.
Yield determined by GC and GCMS analysis.
a
The reusability of this heterogeneous palladium catalyst was
also examined for the Suzuki coupling reaction of 4-iodo toluene
with phenylboronic acid using K CO as the base in water medium.
2 3
compared to that of the corresponding homogeneous analogue in
the Suzuki coupling reaction. From the results it is seen that the
present heterogeneous catalyst is more reactive than the corre-
sponding homogeneous catalyst. The reaction using homogeneous
After the first run, the catalyst was separated by simple filtration,
washed thoroughly with water, methanol, and acetone, and dried
in room temperature under vacuum for 8 h. The dried catalyst
was then reused with a fresh reaction mixture without any further
activation. The catalyst could be almost recovered and recycled for
further reactions upto six runs without much loss of efficiency.
In summary, we have reported a polystyrene anchored Pd(II)
azo complex which was efficiently used as a heterogeneous cata-
lyst for Suzuki cross-coupling reaction and copper free Sonogashira
cross-coupling reactions involving various aryl halides in water
medium under aerobic condition. The catalyst shows not only high
catalytic activity, but also offers many practical advantages such as
oxygen insensitivity, thermal stability, and recyclability. The
2
catalyst with 4-iodo toluene and phenylboronic acid in H O med-
ium at 70 °C for 6 h results in 58% conversion, while the analogous
heterogeneous reaction was performed successfully with 96% con-
version by GC analysis. The activity of the catalyst was also supe-
rior with other reported heterogeneous catalysts for the reaction
of aryl halides under aerobic condition.1
1a,19
The catalytic activities
are also comparable for copper-free Sonogashira coupling reaction
in water medium under air atmosphere with other heterogeneous
2
0
catalysts.
An important point concerning the use of heterogeneous cata-
lyst is its lifetime, particularly for industrial and pharmaceutical
Table 4
Sonogashira coupling reaction of various aryl halides with phenylacetylene catalyzed by polystyrene anchored Pd(II) azo complex
Entry
Arylhalide
Temp (°C)
Time (h)
Product
Yielda (%)
1
2
3
4
5
6
7
8
9
Iodobenzene
70
70
70
70
70
80
80
80
80
80
80
80
8
8
8
Diphenylacetylene
94(88)
96(87)
96(90)
87(82)
90(85)
84
91
86
68(63)
75(70)
77(71)
6
4-Nitroiodobenzene
4-Iodoacetophenone
4-Iodotoluene
4-Iodoanisole
Bromobenzene
4-Nitrobromobenzene
4-Bromoacetophenone
4-Bromotoluene
4-Bromoanisole
2-Bromopyridine
Chlorobenzene
1-Nitro-4-(phenylethynyl) benzene
1-Acetyl-4-(phenylethynyl) benzene
4-(Phenylethynyl) toluene
1-Methoxy-4-(phenylethynyl) benzene
Diphenylacetylene
1-Nitro-4-(phenylethynyl) benzene
1-Acetyl-4-(phenylethynyl) benzene
4-(Phenylethynyl) toluene
1-Methoxy-4-(phenylethynyl) benzene
2-Phenylethynyl pyridine
10
10
12
12
12
12
12
12
12
1
1
1
0
1
2
Diphenylacetylene
Reaction conditions: aryl halide (1.0 mmol), phenylacetylene (1.5 mmol), catalyst (0.5 mol % Pd), TBAB (1.0 mmol), Et
3
N (2.0 mmol), H
2
O (6 ml).
a
Yield determined by GC and GCMS analysis, based on aryl halides, average of two runs. Isolated yield are given in parentheses.

2
070
S. M. Islam et al. / Tetrahedron Letters 51 (2010) 2067–2070
catalyst was reused for six consecutive cycles with consistent
activity. The excellent catalytic performance in water medium
and the easy preparation and separation of the catalyst make it a
good heterogeneous system and a useful alternative to other heter-
ogeneous palladium catalysts.
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