Palladium nanoparticles supported on a poly(N-vinyl-2-pyrrolidone)-modified mesoporous carbon nanocage as a novel heterogeneous catalyst for the Heck reaction in water

Article abstract of DOI:10.1016/j.tetlet.2012.05.046

Palladium nanoparticles supported on poly(N-vinyl-2-pyrrolidone)/CKT-3 (a mesoporous carbon nanocage) were employed effectively as novel heterogeneous catalyst for the Heck coupling reaction. The results showed that aryl halides (-Cl, -Br, -I) underwent coupling with a variety of alkenes at 60 °C in water under aerobic conditions. The very stable nanocomposite catalyst was easily recovered, showed negligible Pd leaching, and retained good activity for at least ten successive runs without any additional activation.

Full text of DOI:10.1016/j.tetlet.2012.05.046

Tetrahedron Letters 53 (2012) 3763–3766
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Tetrahedron Letters
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Palladium nanoparticles supported on a poly(N-vinyl-2-pyrrolidone)-modified
mesoporous carbon nanocage as a novel heterogeneous catalyst for the
Heck reaction in water
Roozbeh Javad Kalbasi a, , Neda Mosaddegh a, Alireza Abbaspourrad b
⇑
a Department of Chemistry, Shahreza Branch, Islamic Azad University, 311-86145 Shahreza, Isfahan, Iran
b School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium nanoparticles supported on poly(N-vinyl-2-pyrrolidone)/CKT-3 (a mesoporous carbon nano-
cage) were employed effectively as novel heterogeneous catalyst for the Heck coupling reaction. The
results showed that aryl halides (–Cl, –Br, –I) underwent coupling with a variety of alkenes at 60 °C in
water under aerobic conditions. The very stable nanocomposite catalyst was easily recovered, showed
negligible Pd leaching, and retained good activity for at least ten successive runs without any additional
activation.
Received 8 December 2011
Revised 23 April 2012
Accepted 10 May 2012
Available online 16 May 2012
Keywords:
Heck reaction
Ó 2012 Elsevier Ltd. All rights reserved.
Mesoporous carbon nanocage
Pd nanoparticles
Water
Palladium-catalyzed cross-coupling reactions are a versatile
tool for the generation of C–C bonds.1–5 Among various methods,
the coupling of aryl halides with olefinic compounds (Heck reac-
tion) is an excellent procedure for the synthesis of useful organic
compounds.6,7 Many Pd complexes have been used in these reac-
tions,8–15 and although some have the potential to be recycled,11,12
most suffer from problems such as deactivation and difficulties in
separating them from solution due to the aggregation of Pd nano-
particles formed in situ during the Heck reaction. Thus, it is neces-
sary to develop supported catalysts which can easily be recovered
from the reaction system and reused.
Porous materials such as alumina, silica, and zeolites have many
advantages as supports due to their high surface areas and easy
separation from reaction mixtures. After the successful preparation
of mesoporous silica crystals, the synthesis of mesoporous carbon
crystals was developed by Ryoo and co-workers using silica meso-
porous crystals as a hard template.16 This new group of materials
designated as ordered mesoporous carbons (OMCs) possess several
advantages compared to mesoporous silica molecular sieves, for
example high thermal stability up to 1600 °C under an N2 atmo-
sphere, high stability in strong acids and bases, high mechanical
stability, electrical conductivity, etc. These interesting properties
result from their high surface areas, narrow pore size distributions
and regular frameworks.17 Therefore, mesoporous carbons are of
interest for adsorption, catalysis, catalyst supports, electronic
applications, and host-guest chemistry.18–20 Carbon-based cata-
lysts have several advantages as solid catalysts because of their
high surface area and large porosity as well as hydrophobic surface
nature. As a result of these advantages, research has increased
using carbon-based catalysts.21–25
Our present paper reports the synthesis and application of Pd
nanoparticles supported on poly(N-vinyl-2-pyrrolidone)/mesopor-
ous carbon nanocage (CKT-3) using three dimensional large cage-
type face-centered cubic Fm3m mesoporous silica materials (KIT-
5)26 as inorganic templates. This novel heterogeneous nanocom-
posite was used effectively for the Heck reaction in water under
aerobic conditions.
Pd-poly(N-vinyl-2-pyrrolidone)/CKT-3 (Pd-PVP/CKT-3) nano-
composite was prepared via a procedure similar to that reported
in our previous work27 using Pd(OAc)2 as the palladium source
and hydrazine hydrate (N2H4ÁH2O) as the reducing agent in the
presence of PVP/CKT-3 as the stabilizer.
The surface morphologies of the obtained CKT-3 and Pd-PVP/
CKT-3 were observed by SEM, as shown in Figure 1. By comparing
the SEM images of CKT-3 and Pd-PVP/CKT-3, we can see that there
are many agglomerate particles on the outer surface of Pd-PVP/
CKT-3 which are related to the Pd nanoparticles distributed on
the outer surface of the support.
The TEM micrograph of Pd-PVP/CKT-3 is depicted in Figure 2.
The regions with darker contrast are assigned to the presence of
Pd particles with different dispersions (5–40 nm). The small dark
⇑
Corresponding author. Tel.: +98 321 3213211; fax: +98 321 3213103.
E-mail address: rkalbasi@iaush.ac.ir (R.J. Kalbasi).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.046

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R. J. Kalbasi et al. / Tetrahedron Letters 53 (2012) 3763–3766
Figure 1. Scanning electron microscopy (SEM) photographs of (a) CKT-3 and (b) Pd-PVP/CKT-3.
spots in the image are ascribed to Pd nanoparticles with average
diameters of ꢀ5 nm, probably located in the support channels.
The larger dark areas over the channels most likely correspond to
Pd nanoparticle agglomerates on the external surface with average
diameters of ꢀ12–40 nm (Fig. 2).
The XPS spectrum of the Pd nanoparticles, dispersed on PVP/
CKT-3 for the Pd 3d region is presented in Figure 3. The results
show that for the binding energies of Pd 3d5/2 and Pd 3d3/2 two
peaks are observed at about 334.5 and 339.6 eV, respectively. This
indicates that the Pd nanoparticles are stable in the metallic state
on PVP/CKT-3 [the XPS spectrum of Pd(II) ions revealed two peaks
at about 337 and 342 eV which refer to Pd 3d5/2 and 3d3/2].28,29
Heck reactions are generally performed at relatively high tem-
peratures, thus we conducted our reactions at 60 °C. In addition,
the solvent plays a crucial role in the rate and the product distribu-
tion of Heck coupling reactions. Since water is known to increase
the activity of the Heck catalyst, two different solvents were used:
H2O and MeOH/H2O (3:1 v/v). The reaction was catalyzed using
0.12 g of Pd-PVP/CKT-3 (containing about 11.6 wt % palladium) in
the presence of 5 equiv of base and with an iodobenzene/styrene
ratio of 1–2 (Table 1). The time required for completion of the reac-
tion was shorter when using H2O as the solvent.
Careful selection of the base in terms of its solubility and basi-
city in the reaction mixture was crucial. The role of the base in the
mechanistic cycle is to re-activate the Pd species in solution, mak-
Figure 2. Transmission electron microscopy (TEM) of Pd-PVP/CKT-3
nanocomposite.
Figure 3. XPS spectrum of Pd 3d region of Pd-PVP/CKT-3.

R. J. Kalbasi et al. / Tetrahedron Letters 53 (2012) 3763–3766
3765
Table 1
Next, the Heck reactions of different aryl halides and styrene
over Pd-PVP/CKT-3 as the catalyst were investigated (Table 2).
The turnover frequency (TOF) value indicates that Pd-PVP/CKT-3
was a good candidate for this reaction. TOF is defined as the mol
product/(mol catalyst/h); and this was calculated from the isolated
yield, the amount of palladium used, and the reaction time. The re-
sults are shown in Table 2. The Heck reaction of iodobenzene with
styrene reached complete conversion within one hour. In addition,
bromobenzene (entry 5) and chlorobenzene (entry 7) gave rela-
tively high yields of the desired products, albeit longer reaction
times were needed. The more easily accessible and cheaper aryl
chlorides have not been employed to any great extent in palla-
dium-catalyzed coupling reactions, primarily because oxidative
addition of the C–Cl bond to the Pd(0) species is usually difficult.
Few heterogeneous Pd catalysts have been found to convert acti-
vated aryl chlorides at high temperature.32–35
Aryl iodides with electron-donating substituents (entries 2 and
3) as well as iodobenzene (entry 1) afforded excellent yields of cou-
pled products at 60 °C. The coupling of 4-bromoacetophenone with
styrene resulted in a moderate yield (63%) at 60 °C and the yield
did not improve at elevated temperature. Thus, the catalyst affor-
ded average to excellent yields of the stilbene products.36,37
Another important issue concerning the application of a heter-
ogeneous catalyst is its reusability and stability under the reaction
conditions employed. To gain insight into this issue, catalyst recy-
cling experiments were carried out using the Heck reaction of
iodobenzene and styrene over Pd-PVP/CKT-3. The results are
Effect of different bases and solvents on the Heck reactiona,b
Base
Time (h)
Yieldc (%)
d
e
K2CO3
K2CO3
1
5
1
1
98
84
65
40
d
Na3PO4
Et3Nd
a
Reaction conditions: Pd-PVP/CKT-3 (0.12 g), iodobenzene (1 mmol), styrene
(2 mmol), base (5 mmol), 60 °C.
b
E/Z stereoselectivity was higher than 99:1 (determined by 1H NMR
spectroscopy).
c
Isolated yield of the E isomer.
Solvent: H2O (5 mL).
Solvent: MeOH/H2O (3:1 v/v) (5 mL).
d
e
ing it available to be recycled.30 The results revealed that the inor-
ganic bases used were more effective than Et3N (Table 1), most
probably due to their complete solubility in water. Hence, cheaper
K2CO3 was chosen as the base for the coupling reactions. From the
data obtained, it can be concluded that the nature of the base
played a significant role in the conversion using Pd nanoparticle-
PVP/CKT-3 as the catalyst. These observations agree with a previ-
ous report in the literature.31
In order to study the effect of the support (PVP/CKT-3), the reac-
tion was run over palladium nanoparticles in water as a reference
reaction using the same reaction conditions, and no activity was
observed. This result showed the important role of the support in
this reaction.
Table 2
Heck reaction of aromatic aryl halides and styrene catalyzed by Pd-PVP/CKT-3a,b
CH
CH2
n
0
Pd
N
R
O
CKT-3
X
+
K2CO3, H2O, 60 °C
R
Entry
Substrate
Product
TOFc (hÀ1
)
Time (h)
Yieldd (%)
Mp (°C)
Ref.
Found
Rep.
I
1
2
7.66
0.63
1
98
97
122–124
121–12336
135–13736
OCH3
OCH3
12
136–138
117–119
I
CH3
CH
3
3
1.48
5
95
116–11836
I
Cl
4
5
6
7
3.75
0.94
0.41
0.62
6
98
98
63
98
38–40
38–3937
I
Cl
Br
10
12
14
122–124
142–144
122–124
121–12336
142–14536
121–12336
Br
O
O
Cl
a
b
c
Reaction conditions: Pd-PVP/CKT-3 (0.12 g), iodobenzene (1 mmol), styrene (2 mmol), K2CO3 (5 mmol), H2O (5 mL), 60 °C.
E/Z stereoselectivity was higher than 99:1 (determined by 1H NMR spectroscopy).
Turn-over frequency.
d
Isolated yield of the E isomer.

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R. J. Kalbasi et al. / Tetrahedron Letters 53 (2012) 3763–3766
Table 3
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