Suzuki, Heck, and copper-free Sonogashira reactions catalyzed by 4-amino-5-methyl-3-thio-1,2,4-triazole-functionalized polystyrene resin-supported Pd(II) under aerobic conditions in water

Article abstract of DOI:10.1016/j.jorganchem.2012.11.008

4-amino-5-methyl-3-thio-1,2,4-triazole-functionalized polystyrene resin-supported Pd(II) complex was found to be an efficient catalyst in the palladium-catalyzed Suzuki-Miyaura coupling reactions of aryliodides and bromides, Mizoroki-Heck reactions of aryliodides and bromides, and copper-free Sonogashira reactions of aryliodides and bromides in water. Under appropriate conditions, all of these reactions give the desired products in moderate to excellent yields. The catalyst is air-stable and easily available. The palladium catalyst is easily separated, and can be reused for several times without a significant loss in its catalytic activity.

Full text of DOI:10.1016/j.jorganchem.2012.11.008

Journal of Organometallic Chemistry 724 (2013) 206e212
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Suzuki, Heck, and copper-free Sonogashira reactions catalyzed by 4-amino-5-
methyl-3-thio-1,2,4-triazole-functionalized polystyrene resin-supported Pd(II)
under aerobic conditions in water
*
Mohammad Bakherad , Ali Keivanloo, Bahram Bahramian, Saeideh Jajarmi
School of Chemistry, Shahrood University of Technology, Shahrood, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
4-amino-5-methyl-3-thio-1,2,4-triazole-functionalized polystyrene resin-supported Pd(II) complex was
found to be an efficient catalyst in the palladium-catalyzed SuzukieMiyaura coupling reactions of ary-
liodides and bromides, MizorokieHeck reactions of aryliodides and bromides, and copper-free Sonoga-
shira reactions of aryliodides and bromides in water. Under appropriate conditions, all of these reactions
give the desired products in moderate to excellent yields. The catalyst is air-stable and easily available.
The palladium catalyst is easily separated, and can be reused for several times without a significant loss
in its catalytic activity.
Received 28 July 2012
Received in revised form
8 October 2012
Accepted 6 November 2012
Keywords:
Suzuki reaction
Heck reaction
Ó 2012 Elsevier B.V. All rights reserved.
Sonogashira reaction
Polystyrene-supported catalyst
Aryl halides
1. Introduction
In contrast, heterogeneous catalysis exhibits the advantage of
easy separation of the catalyst from the products, and it can be
Palladium-catalyzed cross-coupling reactions are versatile and
efficient methods for carbonecarbon bond formations [1]. Among
them, the SuzukieMiyaura [2e4], MizorokieHeck [5e7], and
Sonogashira [8e10] coupling reactions play important roles in
modern synthetic chemistry.
The original Heck, Suzuki, and Sonogashira reactions generally
proceed in the presence of a homogeneous palladium catalyst,
which makes its separation and recovery tedious, if not impossible,
and might result in unacceptable palladium contamination of the
products. A way to overcome this difficulty would be the use of
a heterogeneous palladium catalyst.
Development of recoverable and recyclable catalysts for indus-
trial applications has become important from both the environ-
mental and economical viewpoints, and has been well-reviewed in
the literature [11]. To achieve this objective, several catalysts have
been anchored or immobilized onto polymeric and inorganic
supports [12,13]. Despite the advantages and progress, homoge-
neous catalysis has a share of less than 20% in industrial processes,
because of the issues related to recoverability and recyclability of
expensive metal catalysts.
easily adapted to continuous flow processes [14,15].
Palladacycles have recently emerged as one of the most promising
classes of catalysts or catalyst precursors in the Pd-catalyzed CeC
bond formation reactions such as the HeckeMizoroki [16e19],
SuzukieMiyaura [20e23], and Sonogashira reactions [24e26].
Improvement of these reactions greatly relies on reactivity of
the palladium catalyst using increasingly efficient supporting
ligands. To date, many efforts have been made to the search for
more efficient ligands. During the past decades, the most common
ligands used for these coupling reactions have been the phosphine-
based ones [27,28]. Since most of the phosphine-based ligands are
air and/or moisture-sensitive, in recent years, phosphine-free
ligands as N-heterocyclic carbenes (NHCs) have also been
employed [29e31].
To date, development of green chemistry through organic
reactions conducted in water has become one of the most exciting
research endeavors in organic synthesis [32e34]. Nevertheless,
only a few examples of SuzukieMiyaura, MizorokieHeck, and
Sonogashira reactions by using water as the solvent have been
reported in the literature [35,36].
Very recently, Chungu Xia et al. have described cross-linked
polymer supported Palladium-catalyzed carbonylative Sonoga-
shira coupling reactions in water [37]. Vasundhara Singh et al.
* Corresponding author. Tel./fax: þ982733395441.
E-mail address: m.bakherad@yahoo.com (M. Bakherad).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.11.008

M. Bakherad et al. / Journal of Organometallic Chemistry 724 (2013) 206e212
207
have reported synthesis and characterization of recyclable and
2. Results and discussion
recoverable MMT-clay exchanged ammonium tagged carbapalla-
dacycle catalyst for MizorokieHeck and Sonogashira reactions in
ionic liquid media [38]. Furthermore, Nasser Iranpoor et al. have
illustrated palladium nano-particles supported on agarose as an
efficient catalyst and a bioorganic ligand for CeC bond formation
via the solventless MizorokieHeck reaction and Sonogashira reac-
tion in polyethylene glycol (PEG 400) [39]. Haihong Wu et al. have
exemplified ionic liquid functionalized phosphine-ligated palla-
dium complex for the Sonogashira reactions under aerobic and
copper-free conditions [40].
First polystyrene-bound triazole (PS-tazo) 2 was prepared by
treating chloromethylated polystyrene (cross-linked with 2%
divinylbenzene, 4e5% Cl content, 1.14e1.40 mmol/g Cl) with an
appropriate quantity of 4-amino-3-methyl-5-mercaptotriazole 1 in
DMF for 20 h at 100 ꢀC. PS-tazo prepared was characterized by
elemental analysis. The nitrogen content of this resin was 5.13%.
According to this value, the degree of triazole introduced into the
polymer was 0.916 mmol/g of the support. This shows that only
65e80% of the total chlorine content have been substituted by
triazole. Then triazole-functionalized polystyrene resin-supported
Polystyrene is one of the most popular polymeric supports used
in synthetic organic chemistry because of its in expense, ready
availability, mechanical robustness, chemical inertness, and facile
functionalization. S. M. Islam et al. have described synthesis and
characterization of reusable polystyrene anchored Pd(II) azo
complex catalyst for Suzuki and Sonogashira coupling reaction in
water medium [41]. Moreover, Ying He et al. have developed
successful copper-free Sonogashira coupling reaction catalyzed by
a reusable polystyrene-supported macrocyclic Schiff base palla-
dium complex in water [42]. Recently, our research team have re-
ported synthesis of the polystyrene-supported bidentate
phosphine palladium(0) complex and the polystyrene-supported
palladium(II) 1-phenyl-1,2-propanedione-2-oxime thiosemie
carbazone complex, and found that these complexes are highly
active and recyclable catalysts for Sonogashira reaction of aryl
iodides [43,44] or benzoyl chlorides [45,46] with terminal alkynes.
Although all these methods provide good yields, some of the
reactions are sluggish, requiring high temperature for completion,
and the reactions must be performed in the presence of a relatively
large amount of palladium.
Pd(II) [PS-tazo-Pd(II)]complex
3
was prepared by stirring
a suspension of PS-tazo in a solution of PdCl2(PhCN)2 in refluxing
EtOH for 15 h (Scheme 1). The catalyst prepared, PS-tazo-Pd(II), was
characterized by FT-IR, SEM, and ICP. The amount of palladium
incorporated into the polymer was also determined by inductively
coupled plasma (ICP), which showed a value of 0.28 mmol/g of the
heterogenized catalyst.
Presence of the triazole ligand on the polystyrene was
confirmed by FT-IR spectra. The sharp CeCl peak (due to eCH2Cl
groups) at 1264 cmꢁ1 in the starting polymer was practically
omitted after introduction of the triazole ligand on the polymer.
The IR spectra of chloromethylated polystyrene-supported triazole
ligand show sharp bands around 3347 cmꢁ1 due to the NeH
vibrations (Fig. 1).
Scanning electron micrograph (SEM) was recorded to under-
stand the morphological changes occurring on the surface of the
polymer. The images showed that polystyrene microspheres have
been broken upon attachment of the triazole and palladium
complex (Fig. 2).
Our approach was guided by three imperatives: (i) the support
should be easily accessible; (ii) developing an efficient synthetic
process for the facile conversion of the Heck, Suzuki, and Sonoga-
shira coupling reactions; and (iii) the ligand anchored on the
support should have thermal stability and be air-stable at room
temperature, which should allow its storage in normal bottles with
unlimited shelf-life.
In the continuation of our interest in the development of new
ligands for Pd-catalyzed CeC bond formation reactions, we report
synthesis of the PS-tazo-Pd(II) catalyst and its application to cross-
couplings such as the SuzukieMiyaura, MizorokieHeck, and
copper-free Sonogashira reactions in water under aerobic conditions.
The ease of preparation of the complex, its long shelf-life, its stability
toward air, and its compatibility with a wide variety of aryl halides,
and alkynes make it ideal for the above-mentioned reactions.
Efficiency of the triazole-functionalized polystyrene resin-
supported Pd(II) complex 3 was tested in the Suzuki, Heck, and
Sonogashira reactions.
Initial studies wereperformedupontheSuzukieMiyauracoupling
reaction of phenyliodide 4a with phenylboronic acid 5 as a model
reaction using PS-tazo-Pd(II) complex (0.1 mol%) as the catalyst in
water at 70 ꢀC for 10 h (Table 1). As itcan be seen inthistable, from the
bases screened, K2CO3 shows the best result, and the corresponding
coupling product 6a was obtained in 99% yield (Table 1, entry 6).
Whentheamountofthecatalystwasdecreasedfrom0.1to0.05mmol
%, the yield of 6a was decreased to 75% (entry 7). However using
0.1 mmol% of PS-tazo-Pd(II), biphenyl was obtained with 99% in 10-h
(entry 6). Almost similar yield was achieved when escalating the
catalyst amount from 0.1 to 0.15 mmol% (entry 8). Effect of tempera-
ture on the activity of PS-tazo-Pd(II) complex was also studied. As the
N
N
SH
N
Me
N
DMF, 100 oC, 20 h
+
Cl
N
N
S
H2N
Me
NH2
1
2
[PdCl2(PhCN)2]
EtOH, reflux, 15 h
N
Me
N
N
S
H2N
Pd
Cl
Cl
3
Scheme 1. Preparation of the heterogeneous catalyst PS-tazo-Pd(II) 3.

208
M. Bakherad et al. / Journal of Organometallic Chemistry 724 (2013) 206e212
Fig. 1. FT-IR spectra of (a) chloromethylated polystyrene, and (b) supported Pd-tazo complex.
temperature decreased from 70 to 25 ꢀC, the yield of product 6a
decreased from 99% to 70% (entry 9).
Using the optimized reaction conditions, we explored the
general applicability of PS-tazo-Pd(II) complex with phenylboronic
acid and aryl iodides containing electron withdrawing or donating
substituents, and the results were tabulated in Table 2. We were
pleased to find out that all reactions afforded the desired coupling
products 6 in excellent yields within 10 h, and the substituent,

M. Bakherad et al. / Journal of Organometallic Chemistry 724 (2013) 206e212
209
Table 2
Suzuki reaction of aryl halides with phenylboronic acid using PS-tazo-Pd(II) com-
plexa.
OH
OH
PS-tazo-Pd(II)
X
Ph B
+
Ph
K2CO3, H2O, 70 oC
R
R
4
5
6
Entry
R
X
Product
Yieldb (%)
TON
1
2
3
4
5
6
7
8
H
I
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Br
6a
6b
6c
6d
6e
6f
6g
6a
6b
6c
6h
6i
99
99
99
99
97
96
96
98
99
99
98
98
97
95
990
990
990
990
970
960
960
980
990
990
980
980
970
950
3-NO2
4-NO2
MeCO
Cl
Br
MeO
H
3-NO2
4-NO2
4-CN
4-CHO
Cl
9
10
11
12
13
14
6e
6g
MeO
a
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol),
K2CO3 (2.0 mmol), PS-tazo-Pd(II) (0.001 mmol), H2O (3 ml), 10 h at 70 ꢀC.
b
GC yield.
electron-rich aryl bromides with phenylboronic acid proceeded
smoothly to give the corresponding coupling products in high
yields (entries 9e14).
Furthermore, under these optimized conditions, we investi-
gated the usefulness of PS-tazo-Pd(II) complex in the Heck reaction
involving cross-coupling of an aryl halide with methyl acrylate.
Table 3 illustrates that the reaction is effective in the presence of
a wide variety of functional groups on the aryl iodides and aryl
bromides, giving good to excellent conversions to the corre-
sponding products.
Fig. 2. Scanning electron micrograph of (a) chloromethylated polystyrene (b) sup-
ported Pd-tazo complex.
Among the various substituted aryl iodides, both activated
(electron-poor) and deactivated (electron-rich) examples were
converted efficiently to the desired products in excellent to good
yields (entries 2e5). As expected, aryl iodides were more reactive
than aryl bromides, and the substituent effects in the aryl iodides
appear to be less significant than in the aryl bromides. As shown in
Table 3, activated aryl bromides such as p-nitrobromobenzene
underwent the Heck reaction with methyl acrylate under similar
conditions to afford the corresponding product in 98% yield
(entry 7) whereas, unactivated aryl bromides such as p-bromoa-
nisole gave 90% yield (entry 8).
We next investigated the PS-tazo-Pd(II) complex catalytic
system for the copper-free Sonogashira cross-coupling reaction.
The catalytic activity of the PS-tazo-Pd(II) complex 3 (0.1 mol%) was
studied at room temperature under aerobic conditions in a copper-
free Sonogashira reaction using phenylacetylene and phenyliodide
in the presence of various bases in water. Our optimization data is
shown in Table 4.
either an electron-donating group such as methoxy group (entry 7)
or an electron withdrawing group such as NO2, COMe, or Cl (entries
2e5) on the phenyl ring of 4 had almost no significant effect on
these reactions. To extend the scope of our work, we next investi-
gated the coupling reactions of various aryl bromides with phe-
nylboronic acid. The Suzuki reactions of electron withdrawing and
Table 1
Optimization of the conditions for Suzuki reaction of phenyliodide with phenyl-
boronic acida.
HO
PS-tazo-Pd(II)
base, H2O, 70 oC
I
B
+
HO
4a
5
6a
Entry
Base
Time (h)
Catalyst (mol%)
Yeildb (%)
1
2
3
4
5
6
7
8
9c
DIEA
Et3N
Piperidine
Pyrrolidin
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
10
10
10
10
10
10
10
7
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.15
0.1
64
85
90
74
97
99
75
99
70
When the reaction of phenylacetylene with iodobenzene was
performed with K2CO3 as the base, an excellent 99% yield of the
product was obtained (entry 5).
After the optimized conditions were found, we explored the
general applicability of the PS-tazo-Pd(II) complex 3 as a catalyst
for copper-free coupling of different alkynes 9 with aryl iodides and
bromides 4 containing electron withdrawing or donating substit-
uents. The results are shown in Table 5. The coupling of phenyl-
acetylene with phenyliodide took place smoothly at room
temperature in the presence of K2CO3 (2.0 mmol) and 0.1 mol%
palladium of the PS-tazo-Pd(II) complex 3 to give an excellent yield
10
a
Conditions: phenyliodide (1.0 mmol), phenylboronic acid (1.2 mmol), base
(2.0 mmol), H2O (3 ml), 10 h at 70 ꢀC.
b
GC yield.
Reaction at 25 ꢀC.
c

210
M. Bakherad et al. / Journal of Organometallic Chemistry 724 (2013) 206e212
Table 3
Heck reactions of aryl halides with methyl acrylate using PS-tazo-Pd(II) complexa.
CO2Me
CO2Me
PS-tazo-Pd(II)
R
X
+
R
K2CO3, H2O, 70 oC
4
7
8
Entry
R
X
Product
Yieldb (%)
TON
1
2
3
4
5
6
7
8
H
NO2
Cl
Br
MeO
H
I
I
I
I
8a
8b
8c
8d
8e
8a
8b
8e
97
99
98
97
92
95
98
90
970
990
980
970
920
950
980
900
I
Br
Br
Br
NO2
MeO
a
Conditions: aryl halide (1.0 mmol), methyl acrylate (1.5 mmol), PS-tazo-Pd(II) (0.001 mmol), K2CO3 (2.0 mmol), H2O (3 ml), 10 h at 70 ꢀC.
GC yield.
b
Table 4
of diphenylacetylene (entry 1). The Sonogashira coupling of phe-
nylacetylene with p-iodoanisol (electron-rich) gave the corre-
sponding biarylacetylenes 10d in 97% yield, (entry 4).
p-bromoiodobenzene and p-chloroiodobenzene also underwent
the Sonogashira coupling reaction with phenylacetylene under
similar conditions to afford the corresponding biarylacetylenes 10b
and 10c in excellent yields (99%) (entries 2 and 3).
Copper-free Sonogashira reaction of phenylacetylene with aryl halides in the pres-
ence of different basesa.
PS-tazo-Pd(II)
I
+
Base, H2O, r.t.
4a
9a
10a
When the less reactive acetylene, 1-hexyne, was used, the
coupling product was produced efficiently. Coupling of p-substituted
iodobenzene having bromo, chloro, and methoxy groups took place
with 1-hexyne to give the corresponding products 10hej in excellent
yields (entries 10e12).
We next investigated the coupling of various aryl bromides with
terminal alkynes. As shown in Table 5, high catalytic activity was
observed in the coupling of aryl bromides such as nitro-
bromobenzenes(entries6and14)andp-bromoanisole(entries8and
16) as well as the iodobenzenes. Moreover, p-nitrobromobenzene
and p-bromobenzonitrile having electron-deficient aromatic rings
also underwent Sonogashira coupling reactions with terminal
Entry
Base
Yeildb (%)
1
2
3
4
5
6
DIEA
Et3N
Piperidine
Pyrrolidin
K2CO3
70
95
85
95
99
96
Na2CO3
a
Reaction conditions: phenylacetylene (1.2 mmol), iodobenzene (1.0 mmol),
base (2.0 mmol), PS-tazo-Pd(II) (0.001 mmol), H2O (3 ml), 3 h, room temperature,
aerobic conditions.
b
GC yield.
Table 5
Copper-free Sonogashira reactions of terminal alkynes with aryl halidesa.
PS-tazo-Pd(II)
Y
R
Y
X
R
+
K2CO3, H2O, r.t.
4
9
10
Entry
R
X
Y
Product
Yieldb (%)
TON
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
n-C4H9
n-C4H9
n-C4H9
n-C4H9
n-C4H9
n-C4H9
n-C4H9
n-C4H9
I
I
I
I
Br
Br
Br
Br
I
I
I
I
Br
Br
Br
Br
H
Br
Cl
MeO
H
NO2
CN
MeO
H
Br
Cl
MeO
H
10a
10b
10c
10d
10a
10e
10f
10d
10g
10h
10i
99
99
99
97
91
96
95
95
99
99
98
98
98
98
99
98
990
990
990
970
910
960
950
950
990
990
980
980
980
980
990
980
9
10
11
12
13
14
15
16
10j
10g
10k
10l
NO2
CN
MeO
10j
a
Reaction conditions: aryl halide (1.0 mmol), terminal alkyne (1.2 mmol), PS-tazo-Pd(II) (0.001 mmol), K2CO3 (2.0 mmol), H2O (3 ml), 3 h, room temperature, aerobic
conditions.
b
GC yield.

M. Bakherad et al. / Journal of Organometallic Chemistry 724 (2013) 206e212
211
Table 6
thoroughly with DMF, and dried in vacuo for 12 h. The triazole-
functionalized polymer 2 (1.5 g) was treated with ethanol (50 ml)
for 30 min. An ethanolic solution of PdCl2(PhCN)2 (1.5 g) was added,
and the resulting mixture was heated to 80 ꢀC for 15 h. The
resulting bright yellow colored polymer, impregnated with the
metal complex, was filtered and washed with ethanol to obtain
PS-tazo-Pd(II) 3 (Scheme 1).
The Suzuki, Heck and Sonogashira reactions catalyzed by the recycled catalyst.a
Entry
Cycle
Sonogashira yeildb (%)
Suzuki yeildb (%)
Heck yeildb (%)
1
2
3
4
5
1
2
3
4
5
99
99
95
90
90
99
97
90
85
85
99
98
95
90
87
a
Reaction conditions: phenyliodide (1.0 mmol), phenylacetylene (Sonogashira
reaction) (1.2 mmol), phenylboronic acid (Suzuki reaction) (1.2 mmol), methyl
acrylate (Heck reaction) (1.5 mmol), PS-tazo-Pd(II) (0.001 mmol), K2CO3 (2.0 mmol),
H2O (3 ml), room temperature (for Sonogashira reaction), 70 ꢀC (for Suzuki and Heck
reactions).
4.2. General procedure for the Suzuki coupling reaction
A mixture of aryl halide (1.0 mmol), phenylboronic acid
(1.2 mmol), PS-tazo-Pd(II) (0.001 mmol), K2CO3 (2.0 mmol), and
water (3 ml) was stirred at 70 ꢀC for 10 h. After completion of the
reaction, the mixture was filtered to recover the catalyst. The
polymer was washed with water and acetonitrile, vacuum dried,
and stored for a new run. After GC analysis, the solvent was
removed under vacuum, and the crude product was subjected to
silica gel column chromatography using CHCl3eCH3OH (98:2) as
eluent to afford the pure product.
b
GC yield.
alkynes under similar conditions to afford the corresponding prod-
ucts in excellent yields (entries 6, 7,14 and 15).
The reusability of the catalyst is a very important theme, espe-
cially for commercial applications. Therefore, the recovery and
reusability of the catalyst were investigated using the reaction of
iodobenzene with phenylboronic acid (Suzuki reaction), iodo-
benzene with methyl acrylate (Heck reaction), and iodobenzene
with phenylacetylene (Sonogashira reaction) as the representative
reactants, and in the presence of 0.1 mol% of PS-tazo-Pd(II) in order
to study the recyclability of this heterogeneous catalyst. Similarly,
the reactions for the repeated runs were conducted after separation
of the organic compounds from the reaction mixture by extraction,
and the recovered solid catalyst was recycled for another run. The
recycling process was repeated for five cycles with some decrease
in the catalytic activity of the catalyst (Table 6).
To determine the degree of leaching of the metal from the
heterogeneous catalyst, the catalyst was removed by filtration after
the reaction (for Suzuki reaction) was completed and the palladium
content of the filtrate was determined by ICP. It was shown that less
than 0.2% of the total amount of the original palladium species was
lost into solution during the course of a reaction. This leaching level
was negligible which was also confirmed by the excellent recov-
erability and reusability of this heterogeneous catalyst.
4.3. General procedure for the Heck reaction
A mixture of aryl halide (1.0 mmol), methyl acrylate (1.5 mmol),
PS-tazo-Pd(II) (0.001 mmol), K2CO3 (2.0 mmol), and water (3 ml)
was stirred at 70 ꢀC for 10 h. After completion of the reaction, the
mixture was filtered to recover the catalyst. The polymer was
washed with water and acetonitrile, vacuum dried, and stored for
a new run. After GC analysis, the solvent was removed under
vacuum, and the crude product was subjected to silica gel column
chromatography using CHCl3eCH3OH (98:2) as eluent to afford the
pure product.
4.4. General procedure for the Sonogashira coupling reaction
An aryl halide (1.0 mmol) and a terminal alkyne (1.2 mmol) were
added to
a mixture of PS-tazo-Pd(II) (0.001 mmol), K2CO3
(2.0 mmol), and water (3 ml) in a glass flask under vigorous stirring.
The mixture was stirred at room temperature for 3 h under aerobic
conditions. After completion of the reaction, the mixture was
filtered to recover the catalyst. The polymer was washed with water
and acetonitrile, vacuum dried, and stored for a new run. After GC
analysis, the solvent was removed under vacuum, and the crude
product was subjected to silica gel column chromatography using
CHCl3eCH3OH (98:2) as eluent to afford the pure product.
3. Conclusion
In conclusion, we developed a clean and safe protocol for the
Suzuki, Heck, and copper-free Sonogashira reactions catalyzed by
the PS-tazo-Pd(II) complex. The catalyst used is easily separated,
and can be reused for several times without a noticeable change in
its activity. The ease of preparation of the complex, indefinite shelf-
life, and stability toward air make it an ideal complex for the above
transformations. Further work is in progress to broaden the scope
of this catalytic system for aryl chlorides and other organic
transformations.
Acknowledgements
We gratefully acknowledge the financial support of the Research
Council of Shahrood University of Technology.
References
4. Experimental
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Wiley, New York, NY, 1995.
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