Palladium tetrazole-supported complex as an efficient catalyst for the Heck reaction

Article abstract of DOI:10.1007/s11243-010-9442-2

A tetrazole-supported polymeric ligand has been synthesized. The palladium complex derived from the polymeric material has been evaluated as a catalyst for the Heck reaction of aryl iodides and bromides with styrene to provide the corresponding products in high yields. The reaction proceeded smoothly in the presence of 1 mol% with respect to Pd of catalyst in DMF at 125 or 140 °C within 1-3 h. Recycling studies showed that the catalyst can be readily recovered and reused for several times without significant loss of catalytic activity. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11243-010-9442-2

Transition Met Chem (2011) 36:113–117
DOI 10.1007/s11243-010-9442-2
Palladium tetrazole–supported complex as an efficient catalyst
for the Heck reaction
•
Ying He Chun Cai
Received: 9 October 2010 / Accepted: 3 November 2010 / Published online: 18 November 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A tetrazole-supported polymeric ligand has
been synthesized. The palladium complex derived from the
polymeric material has been evaluated as a catalyst for the
Heck reaction of aryl iodides and bromides with styrene
to provide the corresponding products in high yields.
The reaction proceeded smoothly in the presence of 1 mol%
with respect to Pd of catalyst in DMF at 125 or 140 °C
within 1–3 h. Recycling studies showed that the catalyst can
be readily recovered and reused for several times without
significant loss of catalytic activity.
chemistry because of their toxicity, high price and air-
sensitivity.
In this regard, stable, reusable and active catalysts
anchored to solid supports are of current interest. Many
organic and inorganic supports such as activated carbon,
zeolites, metal oxides, clays, organic polymers, mesopor-
ous silica and capsules have been used in the preparation
of novel catalytic systems [9–17]. Among these, organic
polymers offer different ways of metal attachment to the
polymer matrix via covalent or non-covalent bonding,
through hydrogen bonding, as well as through ionic,
hydrophobic or fluorous interactions [18]. Correspond-
ingly, palladium complexes anchored on polymers with
Schiff bases [19], N-heterocyclic carbene groups [20, 21]
and dendrimers [22] have been described recently.
Introduction
Palladium-catalyzed cross-coupling reactions have been
recognized as powerful synthetic tools and a major area of
interest in multiple organic transformations for academic
and industrial processes [1]. The cross-coupling reaction of
aryl halides with styrene, i.e., the Heck reaction, is among
the most commonly used methods for C–C bond formation
and widely applied to organic and fine chemical synthesis
[2, 3]. In the past few years, various ligands such as
phosphorus ligands [4], N-heterocyclic carbenes [5], P,O-
based ligands [6], thiosemicarbazones [7] and bis(thiourea)
ligands [8] have been used to prepare catalytic palladium
species. However, homogeneous processes suffer from the
problems of separation from the reaction mixture, reuse of
expensive palladium catalysts and palladium contamina-
tion in the products. In addition, most of the available
ligands for Heck reactions are undesirable for industrial
In recent years, N-donor ligands, such as imidazole or
tetrazole derivatives, have attracted the interest of synthetic
organic chemists. As examples, Welton and co-workers
have reported on the Suzuki coupling reaction catalyzed by
(CH3CN)2PdCl2-imidazole and its derivatives [23]. Gupta
and co-workers [24] have reported on the homogeneous
Heck reaction catalyzed by a tetrazole palladium ligand. In
continuation of our work on the development of hetero-
geneous palladium catalysts for C–C bond formation
reactions, we have investigated whether a tetrazole func-
tionalized polymer-supported palladium complex could
catalyze the coupling reaction efficiently. We were able to
use this catalyst for room temperature Suzuki cross-cou-
pling reactions in our previous study [25]. In this paper, we
report on the use of this catalyst for the Heck reaction
(Scheme 1). As we expected, the catalyst exhibited excel-
lent catalytic activity and stability in the Heck reaction.
Furthermore, the catalyst can be readily recovered by
simple filtration and reused several times with only slight
decrease in its activity.
Y. He Á C. Cai (&)
Chemical Engineering College, Nanjing University of Science &
Technology, 210094 Nanjing, People’s Republic of China
e-mail: c.cai@mail.njust.edu.cn
123

114
Transition Met Chem (2011) 36:113–117
Preparation of 5-(p-tolylene)-1H-tetrazole
X
Cat. (1 mol% Pd )
K CO , DMF
R
R
A mixture of 4-tolunitrile (4.68 g, 0.04 mol), sodium azide
(2.86 g, 0.044 mol) and ammonium chloride (2.35 g,
0.044 mol) in DMF (20 ml) was stirred and heated at
100 °C for 18 h. After being allowed to cool to room
temperature, the mixture was poured into ice water and
acidified with aq HCl (6 N) to pH 2–3. The precipitate was
filtered off, washed several times with H2O and further
purified by crystallization to afford the desired product.
2
3
13 examples
X = I, Br
yields 60%-98%
Cat.
N
N
N
O
N
N
PS
Pd(OAc)
2
Preparation of the polymer-supported tetrazole ligand
Pre-washed chloroacetylated styrene–divinylbenzene
copolymer beads (0.5 g, 0.9 mmol of Cl) were swollen in
10 ml of DMF for about 12 h. 5-(p-tolylene)-1H-tetrazole
(0.16 g, 1.0 mmol) was added and the mixture was stirred
at 50 °C for 24 h in the presence of NaH (0.029 g,
1.2 mmol). The color of the beads changed from brown
to yellow. The beads were then filtered off, washed with
distilled water and methanol, respectively, and dried at
80 °C under vacuum overnight. The tetrazole unit content
of the beads was 1.46 mmol g-1 according to the results of
elemental analyses.
Scheme 1 Palladium tetrazole–supported complex for the Heck
reaction
Experimental
All of the reagents and solvents were commercial products.
Chloroacetylated polystyrene (7% divinylbenzene,
1.8 mmol/g of Cl, grain size range: 240–280 lm, surface
area: 32 m2 g-1, pore diameter: 80–100 A, total pore
˚
volume: 0.11 cm3 g-1
) was obtained from Nanjing
Microspheres Co., Ltd. (Nanjing, China). IR spectra were
recorded as KBr disks with a Shimadzu IRPrestige-21 FT-
IR spectrometer. Elemental analyses were performed on
a Vario EL III recorder. Scanning electron microscopy
(SEM) analyses were performed on a JEOL JSM-6380LV
instrument. 1H NMR spectra were measured with a Bruker
Avance III 500 analyzer. GC–MS analyses were performed
on a Saturn 2000 GC/MS instrument. Palladium content of
the catalyst was measured by inductively coupled plasma
(ICP) on a Perkin-Elmer 5300DV analyzer.
Preparation of the catalyst
The functional beads (0.2 g) were swollen in toluene
(5 ml) for about 2 h. The catalyst was then prepared in
toluene using 1.2 equiv. of Pd(OAc)2 per tetrazole unit.
The mixture was stirred at 30 °C for about 24 h, and the
resulting beads were filtered off, washed with methanol and
dried at 100 °C under vacuum overnight. The content of
palladium was 0.76 mmol/g as determined by ICP.
Preparation of the palladium tetrazole–supported
complex
General procedure for the Heck reaction
The palladium tetrazole–supported complex was prepared
in three steps from chloroacetylated polystyrene resin as
reported in the literature [25] (Scheme 2).
In a typical reaction, a mixture of aryl halide (1.0 mmol),
styrene (1.5 mmol), K2CO3 (2.0 mmol), DMF (6 ml) and
catalyst (1 mol% with respect to Pd) was stirred at
O
N
N
N
N
O
Cl
H
N
PS
NH Cl
4
(a) NaN
N
N
3
PS
CN
+
(b) H
N
N
N
N
N
O
Pd(OAc)
Toluene
2
PS
Pd(OAc)2
Scheme 2 Preparation of palladium tetrazole–supported complex
123

Transition Met Chem (2011) 36:113–117
115
125–140 °C for a certain time as the reaction was moni-
tored by GC. After the completion of the reaction, the
mixture was cooled to room temperature. The solid catalyst
was separated by filtration, washed with water to remove
base and salt and finally with dichloromethane to remove
adsorbed organic substrate and dried under vacuum for the
next cycle. The filtrate was diluted with water followed by
extraction with dichloromethane. The combined organic
phase was washed with brine and dried over anhydrous
Na2SO4. The solvent was removed and the crude products
were purified by flash chromatography with n-hexane/
EtOAc as eluent to afford the corresponding products. All
the products were known compounds and were identified
by comparison with their physical and spectroscopic data
with those of authentic samples.
Table 1 Optimization of the Heck reaction conditions
I
Cat.
base, solvent
Entry Solvent Base
T (°C) Time (h) All yield (%)a E/Zb
1
DMF
DMF
DMF
DMF
DMF
DMF
K2CO3 125
NaOH 125
K3PO4 125
DIPEA 125
1
1
1
3
3
5
3
3
5
5
5
5
98
87:13
86:14
84:16
82:18
71:29
88:12
83:17
78:22
81:19
–
2
96
3
97
4
72
5
NEt3 125
NaOAc 125
81
6
46
7
DMAc K2CO3 125
DMSO K2CO3 125
73
8
90
9
CH3CN K2CO3
81
35
10
11
12
Toluene K2CO3 110
Trace
92
DMF
DMF
K2CO3 100
K2CO3 80
80:20
84:16
73
Results and discussion
Reaction conditions: iodobenzene (1.0 mmol), styrene (1.5 mmol),
K2CO3 (2.0 mmol); catalyst (1 mol%) in 6 ml solvent
Optimization of the reaction conditions
a
Isolated yield
b
The ratio of E/Z was determined by GC of the crude reaction
Initially, the Heck reaction of iodobenzene and styrene was
chosen as a model reaction. Various parameters including
bases, solvents and loadings of the catalyst were screened
to optimize the reaction conditions (Table 1). Base plays a
significant role in both the rate and product distribution of
the Heck reaction. Hence, K2CO3, NaOAc, NaOH, K3PO4,
NEt3 and diisopropylethylamine (DIPEA) were all inves-
tigated as bases. K2CO3 was found to be the most effective
base, as shown in Table 1, entries 1–6. Slightly lower
yields and selectivities were obtained when K3PO4 and
NaOH were used as base (Table 1, entries 2, 3). When the
organic bases NEt3 and DIPEA were used, moderate yields
were obtained but longer reaction times were required
(Table 1, entries 4, 5). However, NaOAc gave lower yields
than other bases, even when 5 h reaction times were used
(Table 1, entry 6).
mixture
Table 2 Heck reaction of aryl halides and styrene
X
Cat. (1 mol% Pd )
R
R
K2CO3, DMF
Entry
X
R
T (°C) Time (h) All yield (%)a E/Zb
1
I
H
125
125
125
125
125
140
140
140
1
1
1
1
1
3
3
3
3
3
5
5
5
8
8
8
98
97
98
96
95
90
87
89
95
93
60
67
70
14
51
3
87:13
85:15
83:17
86:14
85:15
90:10
93:7
2
I
p-NO2
p-Me
m-Me
o-Me
H
3
I
4
I
5
I
6
Br
7
Br p-NO2
Br p-CHO
8
77:23
92:8
Next, the effects of different solvents were studied.
According to publications from Hartwig and co-workers
¨
[26], Zapf and Beller [27], and Bohm and Herrmann [28],
9
Br p-COMe 140
10
11
12
13
14
15
16
Br p-Cl
140
140
140
140
140
140
140
90:10
78:22
86:14
90:10
84:16
80:20
–
Br p-Me
Br p-OMe
Br o-Me
polar, aprotic solvents tend to give the best results for Heck
coupling. Among the evaluated polar and non-polar sol-
vents, DMF was the most productive (Table 1, entry 1).
Lower catalyst activities were found in other solvents such
as DMAc, DMSO, CH3CN and toluene (Table 1, entries
7–10). We further evaluated the effects of temperature on
the reaction. As illustrated in Table 1, entries 11, 12,
reducing the amount of the catalyst led to a decrease in the
yields. Thus, we selected K2CO3 as the base, DMF as
solvent and 1 mol% with respect to Pd of catalyst as the
best conditions for the Heck reaction.
Cl
H
Cl p-NO2
Cl p-CH3
Reaction conditions: aryl halides (1.0 mmol), styrene (1.5 mmol),
K2CO3 (2.0 mmol); catalyst (1 mol%) in 6 ml DMF
a
Isolated yield
b
The ratio of E/Z was determined by GC of the crude reaction
mixture
123

116
Transition Met Chem (2011) 36:113–117
Heck reaction of aryl halides with styrene catalyzed
by the supported catalyst
Reusability of the catalyst
The reusability of the catalyst is a very important theme,
especially for commercial applications. Therefore, the
recovery and reusability of the catalyst were investigated
using the reaction of iodobenzene with styrene as a model
system. After the reaction was completed, the mixture was
cooled to room temperature. The solid catalyst could be
separated readily from the reaction mixture by filtration.
The recycled catalyst was then used in the model reaction
under the same conditions, giving 94, 89, 81 and 72%
yields in successive experiments (Fig. 1). ICP analysis
showed that leaching of palladium from the polymer was
3–5% after five cycles. TEM analyses of the recovered
catalyst were also performed. The TEM analysis of the
structure of the catalyst was used to gain information on the
Pd of the catalyst after the reaction, in terms of the nano-
particle size and shape. We found that the size of the
palladium nanoparticles inside the beads had increased
from 2 to 5 nm for the first run to 15–20 nm for the fifth
run (Fig. 2). Therefore, the slight decrease in catalytic
activity in subsequent runs may be due to passivation of the
nanocluster surface [29].
Encouraged by the efficiency of the reaction protocol
described earlier, we investigated the substrate scope. As
can be seen in Table 2, a range of aryl iodides and bro-
mides were found to give the desired products in high
yields. Aryl iodides with electron-withdrawing or electron-
donating groups underwent efficient couplings with styrene
for reaction times of 1 h at 125 °C (Table 2, entries 1–5).
As for aryl bromides, satisfactory yields were obtained
when the reaction was carried out for 3 h at 140 °C. Aryl
bromides with electron-withdrawing groups in the para
positions reacted smoothly, while aryl bromides with
electron-donating groups in the para and ortho positions
were less reactive (Table 2, entries 7–13). We also exam-
ined whether aryl chlorides were active for the Heck
reaction. However, unsatisfactory yields were obtained
even after prolonged reaction times at 140 °C (Table 2,
entries 14–16).
Hot filtration experiments
100
80
Although extensive studies have been carried out to elu-
cidate the true catalytic species in the heterogeneous Pd-
catalyzed Heck reaction, it is still unclear in many cases
whether the reaction takes place on the surfaces of the solid
Pd catalyst [30] or whether the active catalysts are Pd
species leached out from the support, which simply acts as
a reservoir of Pd [31]. Thus, a hot filtration test was carried
out, using the reaction of bromobenzene with styrene as a
model reaction. After 20 min of reaction time, the solid
catalyst was separated by hot filtration and the hot filtrate
was further reacted with fresh K2CO3 at 140 °C for an
60
98
94
89
81
40
20
0
72
1
2
3
4
5
Run
Fig. 1 Recycling experiment
Fig. 2 TEM images of
a recovered catalyst after first
run for the Heck reaction;
b recovered catalyst after fifth
run for the Heck reaction
123

Transition Met Chem (2011) 36:113–117
117
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Conclusion
In conclusion, we have successfully applied a palladium
tetrazole–supported complex in the Heck reaction. The
catalyst exhibits high activity, affording a diverse range of
products in good to excellent yields. Furthermore, the
catalyst is stable to the reaction conditions and can be
recycled, albeit with a slow progressive decrease in activ-
ity. The easy separation and availability make such sup-
ported palladium catalysts an interesting alternative to
homogeneous catalysts.
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