Immobilized palladium nanoparticles on a silica-starch substrate (PNP-SSS): As an efficient heterogeneous catalyst for Heck and copper-free Sonogashira reactions in water

Article abstract of DOI:10.1039/c1gc15360a

This paper reports an efficient heterogeneous catalyst system based on the immobilization of Pd nanoparticles on a silica-starch substrate (PNP-SSS). The PNP-SSS is found as a highly effective recyclable catalyst in Heck and copper-free Sonogashira reactions. The silica-starch substrate (SSS) can stabilize the Pd nanoparticles effectively so that it can provide a platform and prevent the aggregation of nanoparticles and their separation from the substrate surface. It also provides suitable catalytic sites for reactions in aqueous media. The Heck and copper-free Sonogashira reactions are performed in the presence of a small amount of this catalyst in water as a green solvent. The catalyst can be reused more than six times with almost consistent efficiency and can be recovered by simple filtration.

Full text of DOI:10.1039/c1gc15360a

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PAPER
Immobilized palladium nanoparticles on a silica–starch substrate
(PNP–SSS): as an efficient heterogeneous catalyst for Heck and
copper-free Sonogashira reactions in water
Ali Khalafi-Nezhad* and Farhad Panahi*
Received 4th April 2011, Accepted 15th June 2011
DOI: 10.1039/c1gc15360a
This paper reports an efficient heterogeneous catalyst system based on the immobilization of Pd
nanoparticles on a silica–starch substrate (PNP–SSS). The PNP–SSS is found as a highly effective
recyclable catalyst in Heck and copper-free Sonogashira reactions. The silica–starch substrate
(SSS) can stabilize the Pd nanoparticles effectively so that it can provide a platform and prevent
the aggregation of nanoparticles and their separation from the substrate surface. It also provides
suitable catalytic sites for reactions in aqueous media. The Heck and copper-free Sonogashira
reactions are performed in the presence of a small amount of this catalyst in water as a green
solvent. The catalyst can be reused more than six times with almost consistent efficiency and can
be recovered by simple filtration.
powerful and versatile strategies to obtain these compounds.9
Introduction
An impressive variety of methodology modifications for this
reaction have been developed,10 but these modified methods
often suffer from some drawbacks. The most important limita-
tions associated with the application of these modified methods
are, using only reactive substrates, consuming high amounts
of palladium catalyst and ligands as the major cost factors,
using copper(I) iodide as the co-catalyst, which makes the system
more complicated, using toxic solvents and producing alkyne-
homocoupling products.
Hence, preparation of high performance Pd catalysts and the
use of green reaction conditions are two important strategies
for the preparation of new Pd catalyst systems for palladium
catalyzed Heck and Sonogashira reactions. The large fraction
of metal atoms at the surface (high surface-to-volume ratio) and
strong synergistic interaction between noble metal nanoparticles
and the support leads to these materials being considered as
efficient catalysts.11 Furthermore, these systems often possess
the capability for the recovery, which is an important factor
for the development of green catalysts.12 There is great interest
in supported nano catalysts on metal oxides, especially oxides
of Si such as silica.13 Since silica is abundantly available and
has high stability, in many cases it is used as a solid surface
for heterogeneous catalysts. On the other hand, organic groups
can be linked to the silica surface with strong bonding to
generate catalytic sites. The silica surface is highly porous and
can trap and stabilize the Pd nanoparticles, hence it has been
used as a substrate for the immobilization of Pd nanoparticles.14
The main problem with this catalytic system is that the Pd
nanoparticles are easily separated from the substrate surface
Carbon–carbon bond formation using Pd-catalyzed cross-
coupling reactions has received great attention in recent years.1
The Heck reaction has been used as a synthetic tool for C–
C bond formation from aryl halides and alkenes.2 There are
many chemicals and pharmaceutical intermediates with alkene
structural units, which are synthesized using the Heck reaction
in the presence of palladium catalysts. Thus, the synthetic
importance of this reaction is obvious.3 This reaction has
been studied with different homogeneous and heterogeneous
palladium catalytic systems. However, there are widespread
studies to improve the synthetic methods for this reaction and
its applications to prepare new materials.4 High performance
Pd catalysts allow the use of aryl chlorides, which are less
expensive than aryl bromides or iodides, for the Heck reaction.
In the most recent works, phosphine-free methods, ligand-free
palladium catalysts and non-conventional reaction media are
highly regarded as environmentally friendly methods for the
Heck reaction.5–7
Also, compounds which contain carbon–carbon triple bonds,
especially aryl alkynes and conjugated enynes, are frequently
used in materials science and medicinal chemistry due to
their potential applications in these flieds.8 Sonogashira cross-
coupling reaction of terminal alkynes with aryl halides or
triflates, especially using palladium catalysts, is one of the most
Department of Chemistry, College of Sciences, Shiraz University, Shiraz,
71454, Iran. E-mail: khalafi@chem.susc.ac.ir, panahi@shirazu.ac.ir;
Fax: +98-711-2280926; Tel: +98-711-2282380
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and released into the reaction media. Even when using it for
the first time in a reaction, the performance of this catalyst
can be significantly reduced. Along the line of these studies
and in the course of our investigations into the development
of an applicable new heterogeneous catalyst in green reaction
conditions, herein, we wish to disclose the fact that when starch
is covalently connected with silica, it can provide a platform and
prevent the removal of nanoparticles from the silica surface.
This point was observed in the increased catalyst recovery
times. Also, the silica–starch substrate generates very suitable
catalytic sites for reaction in water media so it provides the
possibility of reaction in an aqueous environment. Considering
the above reports, we decided to check the Heck and copper-
free Sonogashira reactions using this new catalyst system in an
aqueous environment and the details of this work are provided
below.
Fig. 1 A TEM image of the PNP–SSS catalyst that shows the
morphology of Pd nanoparticles on the silica starch support in two
different positions on the surface (a and b). The TEM image of reused
PNP–SSS catalyst after being reused six times (c).
Results and discussion
Preparation and characterization of palladium nanoparticles on
a silica–starch substrate (PNP–SSS)
The TEM image of the PNP–SSS catalyst (Fig. 1) shows
that the Pd nanoparticles with near spherical morphology are
assembled onto the SSS with relatively good monodispersity.
The histogram revealing the size distributions of Pd-
nanoparticles is shown in Fig. 2 and is proposed according to
the data, which is obtained from the TEM image with an average
size of 8 nm.
As you can see in Scheme 1, the PNP–SSS catalyst was
prepared in three steps. First, activated silica15 was converted
to silica chloride using a similar method to that which has
been reported in the literature.16 Second, it was reacted with
starch in chloroform and triethylamine to obtain the silica–
starch substrate. Finally, the palladium acetate was reduced to
Pd nanoparticles using the method of alcohol reduction on SSS.
Fig. 2 A histogram representing the size distribution of Pd nanoparti-
cles on the silica–starch substrate.
The XRD pattern of the PNP–SSS catalyst (Fig. 3) also shows
that we have palladium nanoparticles on the SSS. The strongest
peaks of the XRD pattern correspond to SiO2 and other peaks
are indexed as the (111), (200), (220) and (311) planes of the
palladium nanoparticles.17
Scheme 1 Synthetic routes for the preparation of the PNP–SSS
catalyst.
According to Fig. 3, a sharp peak of palladium is observed at
2q = 40◦, which indicates the crystalline nature of the palladium
species. The size of the palladium nanoparticles was also
determined from X-ray line broading using the Debye–Scherrer
formula18 given as D = 0.9L/bcos(q), where D is the average
crystalline size, L is the X-ray wavelength used, b the angular
line width at half maximum intensity and q is the Bragg’s angle.
The PNP–SSS catalyst was characterized by the following
techniques: Transmission Electron Microscopy (TEM) (Fig. 1);
Powder X-ray Diffraction (XRD) (Fig. 2); Energy Dispersive
X-ray spectroscopy (EDX) (Fig. 4); and X-Ray photoelectron
spectroscopy (XPS) (Fig. 7). FT-IR spectroscopy was used for
the characterization of SSS (Fig. 5). Thermal behavior of the SSS
was also investigated by a thermogravimetric (TG) analyzer.
◦
˚
For q = 20 , L = 1.06 A, and b = 0.68 mm, the average size of
the Pd nanoparticles on silica starch substrate is estimated to be
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Fig. 4 The EDX spectrum of the PNP–SSS catalyst.
Fig. 3 The XRD spectrum of the PNP–SSS catalyst.
about 15 nm, this value is in good agreement with data obtained
from the TEM image. As clearly observed, according to the
EDX spectrum (Fig. 4), the presence of palladium is indicated
on the SSS in the PNP–SSS catalyst. The EDX spectrum also
shows other elements, including C, O and Si, which are present
in the silica–starch substrate.
Also, to confirm the Pd content, the supported catalyst was
treated with concentrated HCl and HNO3 to digest the Pd
species and then analyzed by ICP analysis. The Pd content was
determined to be 23.8 ppm (23.8 mg L-1), which is equal to 2.38%
w/w. The morphology of the catalyst was studied using FT-IR
spectroscopy. The FT-IR spectra of pure silica, starch and SSS
are shown in Fig. 5. This comparison shows the existence of
starch on silica and confirms that silica–starch substrate was
produced successfully.
in the SSS that are present in both silica and starch. The peaks at
around 2077, 1659, 1451, 1360, 958, 837, 798, 576, 463 cm-1 are
related to the bonds in SSS. The observed shift in the absorptions
of these peaks is related to the chemical bond formed between the
starch and silica because the formation of bond between silica
and starch probably resulted a change in the electron density
around the chemical bonds. The FT-IR transmission spectrum
of the silica starch substrate shows three bands at around 1659,
798 and 463 cm-1, which are presumably due to asymmetric
stretching (nas), symmetric stretching (ns), and bending modes
of Si–O–Si, respectively.15 The weak band at 2077 cm-1 is due to
the stretching vibration of C–H and C–C bonds. Also, the band
at 1360 cm-1 is associated with the stretching vibration of the
C–O bond.19
Fig. 6 depicts a typical TGA curve of the PNP–SSS catalyst
and it shows three main weight losses. The first one occurs at
~120 ◦C and was assigned to adsorbed water. Another one,
Comparison between the FT-IR spectra of pure silica, starch
and the silica–starch substrate, reveals some absorption bands
Fig. 5 A comparison between the FT-IR spectra of pure silica, starch and silica–starch substrate.
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Table 1 PNP–SSS catalyzed Heck reaction between bromobenzene
and ethyl acrylatea
Entry Solvent
Base
T/◦C Time/h Yield (%)b
1
2
3
4
5
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
120
100
100
reflux
reflux 12
24
6
6
6
6
83
93
91
DMF:H2O (5 : 1)
DMF:H2O (1 : 5)
H2O
90
H2O
75c
78c
80c
81d
83e
92f
63
Fig. 6 A typical TGA curve of the PNP–SSS catalyst.
48
6
7
8
9
10
11
12
13
14
H2O
H2O
H2O
H2O
H2O
H2O
H2O
toluene
CH3CN
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
Et3N
reflux 48
reflux 48
reflux 12
80
r.t.
reflux 12
reflux 12
above ~240 ◦C (and continuing to ~580 ◦C) is related to the
decomposition of starch from the silica substrate. This part of the
thermogram reveals the amounts of grafted starch on silica sup-
port. This was estimated to ~22% (w/w). Finally, the decrease
in the weight percentage of the catalyst at temperatures up to
~850 ◦C is related to the decomposition of the silica support. So,
the elevated temperature for starch removal indicates the high
thermal stability for the silica–starch substrate, because starch
is covalently bonded to silica.
To evaluate the chemical oxidation state of the Pd in PNP–SSS
catalyst, it was analyzed using XPS. The XPS spectrum (Fig. 7)
reveals that we have only Pd(0) on the silica–starch substrate.20 It
seems that the silica–starch substrate efficiently stabilized the Pd
nanoparticles, probably with its abundant hydroxyl groups and
prevents from their oxidation during the preparation process.
12
24
NR
71
54
51
Cs2CO3 reflux 24
K2CO3 reflux 24
37
a Reaction conditions: bromobenzene (1 mmol), ethyl acrylate
(1.2 mmol), base (2.0 mmol), amount of catalyst (0.05 g, 1.2 mol%)
and solvent (5 mL). b Isolated yield. c Amount of catalyst used: 0.025 g
(0.6 mol%) d Amount of catalyst used: 0.033 g (0.8 mol%) e Amount of
catalyst used: 0.042 g (1.0 mol%) f Amount of catalyst used: 0.075 g (1.8
mol%)
entry 3) and regarding this point, aqueous solvent was selected
for this reaction as a green medium (Table 1, entry 4). As you
can see, with increasing the amount of catalyst (1.8 mol %),
the yield of desired product does not change much (Table 1,
entry 6), but reducing quantity of the catalyst (0.6, 0.8 and 1.0
mol %) decreased the yield of product (Table 1, entries 5, 6, 7
and 8). With increasing the time of the reaction at low loading
of the catalyst, the yield of the product was not significantly
improved, even after 48 h. Therefore the low catalyst loading
with prolonged reaction time is not a suitable strategy for this
palladium catalyst. As the type of base was changed the results
revealed that in the presence of K2CO3 a promising yield was
obtained (Table 1, entries 4, 11 and 12). During our optimization
studies, various temperatures were examined and it was found
that the temperature plays a significant role in terms of reaction
rate and isolated yield. As you can see in Table 1, by decreasing
the temperature, the yield of reaction significantly decreases
(entry 9). At the end of the optimization studies, we can say
that the Heck reaction can be done under aerobic conditions
in refluxing water in the presence of the PNP–SSS catalyst (1.2
mol %), using K2CO3 as the base, without the addition of free
ligands or any promoting additives.
Fig. 7 XPS spectra of PNP–SSS catalyst. a) fresh catalyst b) after 6
time of reusability.
Heck reaction catalyzed by PNP–SSS catalyst
The PNP–SSS catalyzed Heck reaction between bromobenzene
and ethyl acrylate was chosen as a model reaction to evaluate the
effects of type of solvent, type of base, temperature and quantity
of catalyst. Optimization conditions are shown in Table 1.
In our initial selection we used dimethylformamide (DMF)
as solvent and K2CO3 as base for the reaction between bro-
mobenzene and ethyl acrylate in the presence of the PNP–SSS
catalyst at 120 ◦C, where ethyl cinnamate was obtained in 83%
isolated yield after 6 h. It was observed that the yield of product
was enhanced to 93% when the ratio of DMF : H2O (5 : 1) was
used as solvent. Is noteworthy that the increased ratio of water
to the DMF (1 : 5) did not reduce the product yield (Table 1,
Heck reaction of various substrates catalyzed by the PNP–SSS
catalyst
After optimizing the reaction conditions using bromobenzene
and ethyl acrylate and in view of the fact that PNP–SSS was an
efficient and reusable catalyst for the Heck reaction in aqueous
media, the catalyst was used with other reactants.
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Table 2 The Heck reaction between different aryl halides and acrylate
of the product did not change much and decreasing the amount
of catalyst (0.6 mol%; 68% isolated yield) leads to a decrease
in the yield of product. The copper-free Sonogashira reaction
is not accomplished at room temperature using the PNP–SSS
catalyst in water. It seems that temperature plays an important
role in the progress of the Sonogashira reaction under optimized
conditions, because the yield of product is reduced when the
temperature is decreased (for example: at 80 ◦C, 73% isolated
yield was obtained). In this way, we have selected optimum
conditions for the copper-free Sonogashira reaction as follows:
PNP–SSS catalyst (0.05 g, 1.2 mol%), K2CO3 as base (2.0 mmol
per 1 mmol of aryl halide) and refluxing water.
or styrene in the presence of PNP–SSS catalyst
Entry
R
X
Y
Time/h
Yield (%)a
1
2
3
4
5
6
7
8
H
H
H
H
H
H
Br
Cl
OMe
SO2Me
NO2
H
Cl
Br
I
Cl
Br
I
CO2Et
CO2Et
CO2Et
Ph
Ph
Ph
12
6
2
8
5
1.5
2
4
12
6
2
61
90
92
64
92
95
91
88
55
77
87
89
86
I
Ph
Ph
Copper-free Sonogashira reaction of various substrates catalyzed
by the PNP–SSS catalyst
Br
Cl
Br
Cl
Cl
Br
9
CO(CH2)3CH3
4-Br–Ph
CO2Et
Ph
10
11
12
13
According to optimum condition, PNP–SSS is also best
candidate catalyst for copper-free Sonogashira reaction. To
determine the scope of the this reaction for preparation of ethyne
derivatives, a number of commercially available terminal alkynes
have coupled with some aryl halides under optimized reaction
conditions, and the results are depicted in Table 3.
12
12
OMe
Ph
a Isolated yield.
Table 2 clearly demonstrates that the PNP–SSS catalyst is
effective for the Heck reaction. As clearly shown in Table
2, the catalytic activity of PNP–SSS depended on the type
of halide and all aryl iodides were rapidly converted to the
corresponding products with excellent yields (Table 2, entries
3, 6 and 7). Also, the electron-withdrawing groups on the aryl
ring increased the reaction rate (Table 2, entries 10, 11 and 12).
The Heck reaction of both electron-rich and electron-deficient
aryl bromides also proceeded smoothly to furnish the desired
products with excellent yields (Table 2, entries 10 and 13). As
you can see in Table 2, moderate yields of products were achieved
using aryl chlorides in the presence of the PNP–SSS catalyst.
Considering Table 3, it is clear that the PNP–SSS catalyst
is also effective for the copper-free Sonogashira reaction.
Although the catalytic activity of PNP–SSS is dependent on
the type of halide, all aryl halides (chloride, bromide and iodide)
were converted to the corresponding products with excellent
yields. Electron-withdrawing groups on the aryl ring increased
the reaction rate and also isolated yield, for example 1-chloro-4-
nitrobenzene after 8 h give 85% isolated yield of desired product.
The Sonogashira reactions of both aliphatic and aromatic
alkynes were carried out with excellent yields in the presence of
the PNP–SSS catalyst (Table 3, for example: entries 3, 6 and 9).
Moderate yields of products were achieved using 2-methylbut-
3-yn-2-ol as a hydroxy-functionalized alkyne in the presence of
the PNP–SSS catalyst (entries 4, 5 and 6).
Copper-free Sonogashira reaction catalyzed by the PNP–SSS
catalyst
Now in this part, we report another important application of
PNP–SSS as a catalyst for the efficient Sonogashira coupling
reaction between terminal alkynes and aryl halides in neat water
under copper-free conditions. Catalytic efficiency of the PNP–
SSS for coupling between bromobenzene and phenylacetylene as
a simple model substrate was first evaluated. According to data
obtained from the Heck reaction, water was used as reaction
medium in this protocol as well, potassium carbonate as the
base and PNP–SSS (1.2 mol%) as the catalyst.
Table 3 The copper-free Sonogashira reaction between different aryl
halides and alkynes in the presence of the PNP–SSS catalyst
Entry
G
X
Y
Time/h
Yield (%)a
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
Cl
Br
I
Cl
Br
I
Cl
Br
I
Cl
Br
Cl
Br
Ph
Ph
Ph
12
8
3
12
8
4
12
8
3
8
8
81
91
95
84
90
92
91
93
97
91
85
85
90
According to Scheme 2, under the mentioned conditions the
PNP–SSS catalyst is effective in the environmentally benign
system and 1,2-diphenylethyne compound was obtained with
satisfactory 91% isolated yield after 8 h. We also checked the
effect of amount of catalyst and we found that with increasing
the amount of catalyst (1.8 mol%; 92% isolated yield), the yield
C(CH3)2OH
C(CH3)2OH
C(CH3)2OH
CH3CH2CH2CH2
CH3CH2CH2CH2
CH3CH2CH2CH2
Ph
Ph
Ph
Ph
9
H
10
11
12
13
4-CN
4-OMe
4-NO2
4-Me
8
8
a Isolated yield.
Scheme 2
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Table 5 Reusability of the PNP–SSS catalyst in the copper-free Sono-
gashira reactiona
Along the line of this study and in the course of our
investigations on the PNP–SSS catalyst and its application in
the Heck and copper-free Sonogashira reactions, we observed
that the experimental conditions (K2CO3, refluxing water) do
not have any destructive influence on behavior of the grafted
starch on the silica during the Heck and copper-free Sonogashira
reactions. Although phenomena (such as dehydration) may
happen for the silica starch substrate during the reaction, these
changes do not have any adverse effect on the progress of
the Heck and copper-free Sonogashira reactions. It should be
noted that the PNP–SSS catalyst system in some cases is more
efficient for Heck21 and copper-free Sonogashira22 reactions in
comparison with other heterogeneous catalysts, especially for
aryl chlorides.
Entry
Yield of product (%)
Recovery of PNP–SSS (%)
fresh
91
91
91
89
89
87
99
99
98
98
97
97
1
2
3
4
5
a Reaction conditions: PNP–SSS (0.1 g), K2CO3 (4.0 mmol), H2O (4 mL),
bromobenzene (2.0 mmol) and phenylacetylene (2.2 mmol).
Table 6 The ICP analysis results of fresh and reused catalyst after 5
time of reusability
Entry
PNP–SSS catalyst condition
ppm of Pd
% (w/w)
Recycling of the PNP–SSS catalyst for the Heck and
copper-free Sonogashira reactions
1
2
3
fresh
23.8
22.4
2.38
2.24
2.21
reused from Heck reactiona
For practical applications of heterogeneous catalysts, the level
of reusability is a very important factor. The possibility of
recycling the catalyst for the Heck reaction was examined
using the reaction of bromobenzene with ethyl acrylate under
optimized reaction conditions. When the reaction was complete,
the reaction mixture was filtered and the solid was washed with
water (2 ¥ 10 mL) and dichloromethane (2 ¥ 10 mL), and the
recycled catalyst was dried in an oven (at 120 ◦C for 2 h) to use
in the next operation. The recycled catalyst could be reused five
times and no observation of any appreciable loss in the catalytic
activity of the PNP–SSS catalyst was observed (Table 4).
The catalytic reusability of the PNP–SSS catalyst for the
copper-free Sonogashira reaction was tested using the reaction
between bromobenzene and phenylacetylene under optimized
conditions. The reaction mixture was filtered (after finishing the
reaction) and then the filtered sediment was washed with water
(2 ¥ 10 mL) and dichloromethane (2 ¥ 10 mL). The recovered
catalyst was heated in an oven at 120 ◦C for 2 h and then the dried
catalyst was applied for the next operation. The recycled catalyst
could be reused five times without any treatment (Table 5).
The ICP analysis of the aqueous phases of reaction mixture
after five cycle of reusability (for both Heck and sonogashira
reactions) showed that only a very small amount of the Pd metal
is removed from the silica starch substrate (Table 6).
reused from Sonogashira reactionb 22.1
a The Heck reaction was accomplished between bromobenzene and ethyl
acrylate. b The Sonogashira reaction was done between bromobenzene
and phenylacetylene.
was complete and the hot filtration had been accomplished, the
obtained aqueous solution from the filtrate was applied for the
next reaction between bromobenzene with ethyl acrylate under
optimized condition. After a similar purification processes, the
products from the Heck reaction were obtained in less than 5%
isolated yield. The ICP analysis of the aqueous solution that was
obtained from filtration is in good agreement with these results,
due to only 1.1 ppm of Pd being observed in this solution. It may
be caused by the redeposit ion process23 for leached Pd metal on
the support surface. To resolve this issue, the recovered catalysts
were characterized using TEM images and ICP analysis to
investigate any changes in Pd particle morphology/size and Pd
content on the catalyst surface in comparison with fresh catalyst.
The TEM image of the catalyst showed that the morphology
and size of the catalyst after recycling five times does not change
significantly (Fig. 1c).
Also, the obtained ICP data from the reused catalyst (after
recycling six times) show that only a low amount of Pd
nanoparticles (1.4 ppm) are lost from the silica–starch substrate
during reaction process. So, these results confirmed that the
PNP–SSS catalyst provides the high catalytic activity, and not
any leached palladium.
To confirm this point that the activity originated from
the supported palladium nanoparticles on the silica starch–
substrate and not from leached Pd, we did the following test.
When the reaction between bromobenzene with ethyl acrylate
Conclusions
Table 4 Reusability of the PNP–SSS Catalyst in the Heck Reactiona
In conclusion, we report herein the preparation, characterization
and utilization of a new and high performance catalyst system
for the Heck and copper-free Sonogashira reactions. The silica–
starch substrate was prepared from abundant, commercially
available, and relatively cheap starting materials. The palladium
nanoparticles with near spherical morphology and average size
of 8 nm were immobilized on the silica–starch substrate using
the alcohol reduction method. By use of this catalyst system, we
synthesized some of the alkene and alkyne derivatives with high
yield in a green environment. Reusability and easy workup were
Entry
Yield of product (%)
Recovery of PNP–SSS (%)
fresh
90
90
89
89
87
88
> 99
99
98
98
98
1
2
3
4
5
97
a Reaction conditions: PNP–SSS (0.1 g), K2CO3 (4.0 mmol), H2O (4 mL),
bromobenzene (2.0 mmol) and ethyl acrylate (2.4 mmol)
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two other advantages of this catalyst system. Also, this catalyst
provides great promise toward further useful applications in
other palladium transformations in the future.
groups are eliminated. After drying in the oven at 120 ◦C for
2h, the silica–starch substrate was obtained as a white powder
(6.35 g).
Pd nanoparticles on the silica–starch substrate (PNP–SSS)
Experimental section
To a mixture of the silica–starch substrate (5 g) in absolute
ethanol (30 mL), palladium acetate (0.3 g, 1.3 mmol) was added
and stirred 24 h at room temperature. Then, the mixture was
filtered and washed with ethanol (3 ¥ 15 mL) and diethyl ether
(2 ¥ 15 mL). After drying in a vacuum oven at 100 ◦C overnight,
the PNP–SSS catalyst was obtained as a dark solid.
Chemical, instrumentation and analysis
Chemicals were purchased from Fluka, Merck and Aldrich
1
Chemical Companies. For recorded HNMR and 13C NMR
spectra we used Brucker (500 and 250 MHZ) Avance DRX in
pure deuterated DMSO-d6 and CDCl3 solvents with tetram-
ethylsilane (TMS) as internal standards. Mass spectra were
recorded on a FINNIGAN-MAT 8430 mass spectrometer
operating at 70 eV. FT-IR spectroscopy (Shimadzu FT-IR
8300 spectrophotometer), was employed for characterization
of the compounds and PNP–SSS catalyst. The transmission
electron microscopy (TEM) was obtained using TEM apparatus
(CM-10-philips, 100 kV) for characterization of the PNP–
SSS catalyst. The thermogravimetry analysis (TGA) of the
samples was analyzed using a lab-made TGA instrument.
The X-ray diffraction (XRD, D8, Advance, Bruker, axs) was
employed for characterization of the PNP-SSS catalyst. Melting
points were determined in open capillary tubes in a Barnstead
Electrothermal 9100 BZ circulating oil melting point apparatus.
The reaction monitoring was accomplished by TLC on silica gel
PolyGram SILG/UV254 plates. Column chromatography was
carried out on columns of silica gel 60 (70–230 mesh).
General procedure for the Heck reaction in the presence of the
PNP–SSS catalyst
In a typical experiment, to a mixture of aryl halide (1 mmol),
acrylate or styrene (1.2 mmol), and K2CO3 (2 mmol) in 2 mL wa-
ter, PNP–SSS catalyst◦(0.05 g, 1.2 mol%) was added and heated
in an oil bath at 100 C for the time specified in Table 2. The
reaction was followed by TLC. After completion of the reaction,
the mixture was cooled down to room temperature and filtered
and the remaining solid was washed with dichloromethane (3 ¥
5 mL) in order to separate the catalyst. After the extraction of
dichloromethane from water, the organic extract was dried over
Na2SO4. The products were purified by column chromatography
(hexane/ethyl acetate) to obtain the desired purity.
General procedure for Sonogashira reaction catalyzed by the
PNP–SSS catalyst
Catalyst Preparation
Silica chloride
Into a conical flask (10 mL) a mixture of aryl halide (1 mmol),
terminal alkyne (1.1 mmol), K2CO3 (2 mmol), water (2 mL)
and PNP–SSS catalyst (0.05 g, 1.2%) were stirred at refluxing
temperature of water. The reactions were monitored by TLC.
Stirring was continued until the consumption of the starting
materials based on reaction time in Table 3. After completion
of the reaction, the mixture was filtered and washed with water
(2 ¥ 10 mL) dichloromethane (2 ¥ 10 mL) and the catalyst
was separated by simple filtration. The solvent was removed
under reduced pressure and the product was purified by silica
gel column chromatography employing n-hexane/ethyl acetate
as the eluent, affording the pure corresponding product.
In a three-necked flask (100 mL) that was equipped with a
dropping funnel containing thionyl chloride (SOCl2, 40 mL),
condenser and a gas inlet tube for conducting HCl gas over
an adsorbing solution (10% NaOH), 10 g of silica was charged.
Thionyl chloride was added drop-wise over a period of 30 min at
room temperature. The HCl gas evolved from the reaction vessel
immediately. When the addition was completed, the mixture was
stirred for 24 h at the refluxing temperature of SOCl2. Then,
the untreated thionyl chloride was removed by distillation. The
silica chloride was dried in a vacuum at 90 ◦C and the resulting
grayish powder was stored in a desiccator under vacuum. To
determine the amount of chlorosilyl groups on the silica surface
we prepared a suspension containing 1 g of silica chloride in
100 mL of sodium hydrogen carbonate (0.005 M). This solution
was stirred for 1 h at room temperature. Then the released
chloride ions were titrated with silver nitrate (AgNO3) solution
(0.01 M, 88 mL) using potassium chromate solution as an
indicator and in this way the amount of chlorosilyl groups on
silica surface was obtained ~0.9 mmol g-1.24
Acknowledgements
We acknowledge Shiraz University for partial support of this
work. Also, we are thankful to Dr Reza Yousefi for their helpful
comments.
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