Phosphine-free polystyrene-supported palladium(II) complex as an efficient catalyst for the Heck and Suzuki coupling reactions in water

Article abstract of DOI:10.1016/j.crci.2012.06.009

A palladium complex, 1-phenyl-1,2-propanedione-2-oxime thiosemi-carbazone- functionalized polystyrene resin supported Pd(II), is found to be a highly active catalyst for the Heck reaction of methyl acrylate with aryl halides and Suzuki reaction of phenylboronic acid with aryl iodides and bromides, giving excellent yields. The reactions were performed under phosphine-free conditions in an air atmosphere. 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.crci.2012.06.009

C. R. Chimie 15 (2012) 945–949
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Preliminary communication/Communication
Phosphine-free polystyrene-supported palladium(II) complex as an
efficient catalyst for the Heck and Suzuki coupling reactions in water
*
Mohammad Bakherad , Ali Keivanloo, Amir H. Amin, 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:
A
palladium complex, 1-phenyl-1,2-propanedione-2-oxime thiosemi-carbazone-
Received 25 May 2012
functionalized polystyrene resin supported Pd(II), is found to be a highly active catalyst
for the Heck reaction of methyl acrylate with aryl halides and Suzuki reaction of
phenylboronic acid with aryl iodides and bromides, giving excellent yields. The reactions
were performed under phosphine-free conditions in an air atmosphere. The palladium
catalyst is easily separated, and can be reused for several times without a significant loss in
its catalytic activity.
Accepted after revision 21 June 2012
Available online 14 August 2012
Keywords:
Phosphine-free
Supported catalyst
Heck and Suzuki reactions
Aryl halides
´
ß 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
In recent years, phosphine-free ligands include the N-
heterocyclic carbene class of compounds (NHCs) [24];
Transition metal-, especially palladium-catalyzed
cross-coupling reactions have become a very important
protocol since its development in the 1970s with versatile
applications in organic synthesis [1–3]. Among them, the
Suzuki–Miyaura and Mizoroki–Heck coupling reactions
have been used extensively for the synthesis of natural
products, pharmaceutical intermediates, conducting poly-
mers, pesticides, and liquid crystals [4–18].
Improvement of these reactions greatly relies on the
reactivity of the palladium catalyst by using increasingly
efficient supporting ligands. To-date, many efforts have
been made to search for more efficient ligands. During the
past decades, the most common ligands used for these two
coupling reactions are the phosphine-based ones [19–21].
Since most phosphine ligands are air- and moisture-
sensitive, P–C bond degradation sometimes occurs at
elevated temperatures, which poisons the metal leading to
decomposition of the catalyst, and this strongly affects the
conversion and selectivity [22,23].
thioureas [25], tetrazoles [26], phenanthroline [27], bisimi-
dazole [28], bispyridines [29–31], amino acids [32,33],
hydroxyquinolines [34], hydrazones [35,36], N-phenylurea
[37] and Schiff bases [38–40] have also been employed.
In the past few years, the development of ligands has
provided highly active homogeneous palladium catalysts
for the Heck [41–47] and Suzuki [48–51] coupling reactions.
However, homogeneous processes suffer from problems
concerning separation of the expensive palladium catalysts
from reaction mixture, and their reuse. In addition, most of
the ligands used for the Heck and Suzuki reactions are
undesirable in industrial chemistry because of their toxicity,
high price, and air-sensitivity. In principle, the use of
supported palladium catalysts could address some of the
problems mentioned above. Therefore, practical applica-
tions of the Heck [52–57] and Suzuki [58–64] reactions have
greatly driven the need for the development of recyclable
and efficient heterogeneous palladium catalysts.
Furthermore, in most of the catalytic processes, organic
solvents are employed as the reaction media, often
creating safety, health, and environmental problems due
to their flammability, toxicity, and volatility. From an
economic and environmental standpoint, it is desirable to
* Corresponding author.
E-mail address: m.bakherad@yahoo.com (M. Bakherad).
1631-0748/$ – see front matter ß 2012 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2012.06.009

946
M. Bakherad et al. / C. R. Chimie 15 (2012) 945–949
Table 1
Ph
N
Me
Optimization of the conditions for Heck reaction of iodobenzene with
methyl acrylate.a
Entry
Base
Cat (mol %)
Yield (%)b
HNSCHN
N
OH
1
2
3
4
5
6
7
8c
9
Et3N
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
95
90
86
94
90
93
97
40
82
Pd
DIEA
Pyridine
Piperidine
Pyrrolidine
KOH
Cl
Cl
Fig. 1. Heterogeneous catalyst Ps-ppdot-Pd (II).
K2CO3
K2CO3
avoid the use of hazardous and expensive organic solvents.
The use of water or aqueous solutions represents
economically and environmentally viable alternatives to
organic solvents for metal-catalyzed reactions [65–69].
Several examples of Pd-catalyzed Heck [70–73] and Suzuki
[74–77] reactions in aqueous media have been reported.
As part of our continuing interest in heterogeneous
palladium-catalyzed carbon–carbon cross-coupling reac-
tions [78–83], we have recently reported a mild protocol
for the copper- and solvent-free Sonogashira coupling
reactions catalyzed by 1-phenyl-1,2-propanedione-2-ox-
ime thiosemi-carbazone-functionalized polystyrene resin
supported Pd(II) [PS-ppdot-Pd(II)] complex under aerobic
conditions [84].
K2CO3
a
Conditions: iodobenzene (1.0 mmol), methyl acrylate (1.5 mmol),
base (2.0 mmol), H2O (5 mL), 70 8C.
b
GC yield.
c
Reaction at 25 8C.
thiosemi-carbazone ligand on the polymer. The N-content
of the resin was obtained to be 4.2% (0.72 mmol/g), which
indicates thatonly57% ofthe totalchlorines was substituted
by ligand. The amount of palladium incorporated into the
PS-ppdot-Pd(II) complex was also determined by induc-
tively coupled plasma (ICP), which showed a value of
0.30 mmol/g for the heterogenized catalyst.
In order to show the merit of the application of
heterogeneous catalysts in organic synthesis, we applied
the PS-ppdot-Pd(II) complex as the catalysts in the Heck
and Suzuki reactions.
In this article, we wish to report the development of a
mild protocol for the Suzuki–Miyaura coupling reactions of
aryl halides, including iodides and bromides, and Mizor-
oki–Heck reactions of aryl iodides and bromides catalyzed
by PS-ppdot-Pd(II) as catalyst (Fig. 1).
In our initial investigations, the catalytic activities of
PS-ppdot-Pd(II) complex (1.0 mol%) were studied in the
Heck reaction using iodobenzene and methyl acrylate as
substrates, in the presence of K2CO3 as base, in water at
70 8C for 10 h (Table 1). As it can be seen in Table 1, K2CO3
showed the best result, and the corresponding coupling
product was obtained in 97% yield (Table 1, entry 7). The
effect of temperature on the activity of PS-ppdot-Pd(II)
complex was also studied. As the temperature decreased
from 70 to 25 8C, the yield of product decreased from 98%
to 40% (entry 8). A low palladium concentration gave a
decreased yield (entry 9).
2. Results and discussion
The PS-ppdot-Pd(II) complex was synthesized accord-
ing to the procedure previously reported [85]. Successful
synthesis of PS-ppdot-Pd(II) complex was characterized by
FT-IR, SEM, and ICP.
The IR spectrum of PS-ppdot-Pd(II) shows a band around
3420 cmÀ1 due to the N–H vibrations. The sharp C–Cl peak
(due to –CH2Cl groups) at 1264 cmÀ1 in the starting polymer
was practically omitted or was seen as a weak band after
introduction of the 1-phenyl-1,2-propanedione-2-oxime
Table 2
Heck reactions of aryl halides with methyl acrylate using PS-ppdot-Pd(II) complex.a
CO2Me
CO2Me
PS-ppdot-Pd(II)
X
R
R
H2C
+
K2CO3, H2O, 70 °C
1
2
3
Entry
R
X
Product
Yield(%)b
1
2
3
4
5
6
7
H
I
3a
3b
3c
3d
3a
3c
3d
97
98
98
95
72
85
60
Cl
I
NO2
MeO
H
I
I
Br
Br
Br
NO2
MeO
a
Conditions: aryl halide (1.0 mmol), methyl acrylate (1.5 mmol), PS-ppdot-Pd(II) (1.0 mol %), K2CO3 (2.0 mmol), H2O (5 mL), 70 8C.
GC yield.
b

M. Bakherad et al. / C. R. Chimie 15 (2012) 945–949
947
Table 3
Suzuki reaction of aryl halides with phenyl boronic acid using PS-ppdot-Pd(II) complex.a
OH
PS-ppdot-Pd(II)
Ph
X
Ph
B
+
K2CO3, H2O, 70 oC
R
R
OH
4
Entry
R
X
Product
Yield(%)b
1
H
I
4a
4b
4c
4d
4e
4f
98
97
98
97
100
100
96
68
80
98
97
90
80
2
4-Cl
I
3
4-Br
I
4
4-MeCO
4-NO2
3-NO2
MeO
I
5
I
6
I
7
I
4g
4a
4b
4h
4e
4f
8
H
Br
Br
Br
Br
Br
Br
9
4-Cl
10
11
12
13
4-CN
4-NO2
3-NO2
MeO
4g
a
Reaction conditions: aryl halide (1.0 mmol), phenyl boronic acid (1.2 mmol), K2CO3 (2.0 mmol), PS-ppdot-Pd(II) (1.0 mol%), H2O (5 ml), 70 8C.
GC yield.
b
Using the optimized reaction conditions, we explored
corresponding product in excellent yield (entries 10 and
11).
the general applicability of PS-ppdot-Pd(II) complex with
methyl acrylate and aryl halides containing electron
withdrawing or donating substituents, and the results
are tabulated in Table 2. Among the various substituted
aryl iodides, both deactivated (electron-rich) and activated
(electron-poor) examples were converted efficiently to the
desired products in good to excellent yields (entries 1–4).
As expected, aryl iodides were more reactive than aryl
bromides, and the substituent effects in the aryl iodides
appeared to be less significant than in the aryl bromides. As
shown in Table 2, activated aryl bromides such as 4-
nitrobromobenzene underwent the Heck reaction with
methyl acrylate under similar conditions to afford the
corresponding product in 85% yield (entry 6) whereas,
unactivated aryl bromides such as 4-methoxybromoben-
zene gave 60% yield (entry 7).
One of the purposes for designing this heterogeneous
catalyst is to enable recycling of the catalyst for use in
subsequent reactions. For the recycle experiment, we used
iodobenzene with methyl acrylate (Heck reaction) and
iodobenzene with phenyl boronic acid (Suzuki reaction) as
the representative reactants and in the presence of 1.0
mol% of PS-ppdot-Pd(II) to study the recyclability of this
heterogeneous catalyst. After the completion of the
reaction, the mixture from the first-run reaction was
centrifuged, and the solid obtained was washed alternately
with ethanol and CH3CN. After drying, the recovered
catalyst was then reused in the same reaction under
identical conditions. We found that the product yield
decreased slightly over four recycling runs (Table 4).
Furthermore, under these optimized conditions, we
investigated the usefulness of PS-ppdot-Pd(II) complex in
the Suzuki reaction involving cross-coupling of an aryl
halide with phenylboronic acid. Table 3 illustrates that the
reaction was 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 corresponding
products.
The Suzuki reactions of electron-rich aryl iodides with
phenyl boronic acid proceeded smoothly to give the
corresponding coupling products in high yields (entry 7).
Unsurprisingly, 4-nitroiodobenzene and 3-nitroiodoben-
zene were found to be the most reactive among the aryl
iodides studied (entries 5 and 6).
3. Conclusion
In conclusion, we showed that the PS-ppdot-Pd(II)
complex efficiently catalyzes the Heck reaction of methyl
Table 4
The Heck and Suzuki reactions catalyzed by the recycled catalyst.a
Entry
Cycle
Heck
Suzuki
Yield (%)b
Yield (%)
1
2
3
4
1
2
3
4
97
95
90
85
98
96
93
90
To extend the scope of our work, we next investigated
the coupling reaction of various aryl bromides with phenyl
boronic acid. The less reactive bromobenzene showed low
yield (entry 8). However, the activated aryl bromides, 4-
bromobenzonitrile and 4-nitrobromobenzene, gave the
a
Reaction conditions: methyl acrylate (1.5 mmol), or phenyl boronic
acid (1.2 mmol), iodobenzene (1.0 mmol), Catalyst (1.0 mol%), K2CO3
(2.0 mmol), H2O (5 ml), 70 8C
b
GC yield.

948
M. Bakherad et al. / C. R. Chimie 15 (2012) 945–949
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A round-bottomed flask was charged with aryl halide
(1.0 mmol),
phenylboronic
acid
(1.2 mmol), K2CO3
(2.0 mmol), PS-ppdot-Pd(II) complex (1.0 mol %), and
H2O (5 mL). The reaction mixture was heated at 70 8C
for 8 h.
Both reactions were monitored by gas chromatography.
Upon completion of the reaction, the reaction mixture was
dissolved in acetonitrile (10 mL). The palladium catalyst
was separated from the mixture by filtration, washed with
water (10 mL) and acetonitrile (10 mL), and reused in the
next run. The solution was concentrated in vacuo, and the
crude product was subjected to silica gel column
chromatography using CHCl3–CH3OH (98:2) as eluent to
afford the pure product.
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