Palladium supported on silica-chitosan hybrid material (Pd-CS@SiO2) for Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions

Article abstract of DOI:10.1002/aoc.3290

Palladium supported on silica-chitosan hybrid material was prepared and characterized using thermogravimetric and differential thermogravimetric analyses, scanning electron microscopy, and Fourier transform infrared, energy-dispersive X-ray and X-ray photoelectron spectroscopies. The prepared Pd-CS@SiO₂ catalyst (1 mol%) was used for the Suzuki-Miyaura cross-coupling reaction of various aryl halides and arylboronic acids in 95% ethanol at 80 °C and the Mizoroki-Heck reaction in dimethylformamide at 110 °C using K₂CO₃ as a base. The developed catalyst is well suitable for the 3R approach (recoverable, robust, recyclable) for cross-coupling reactions without appreciable loss of its activity.

Full text of DOI:10.1002/aoc.3290

Full Paper
Received: 29 November 2014
Revised: 18 January 2015
Accepted: 20 January 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3290
Palladium supported on silica–chitosan hybrid
material (Pd-CS@SiO2) for Suzuki–Miyaura and
Mizoroki–Heck cross-coupling reactions
Sanjay Jadhava, Arjun Kumbharb and Rajashri Salunkhea*
Palladium supported on silica–chitosan hybrid material was prepared and characterized using thermogravimetric and dif-
ferential thermogravimetric analyses, scanning electron microscopy, and Fourier transform infrared, energy-dispersive
X-ray and X-ray photoelectron spectroscopies. The prepared Pd-CS@SiO2 catalyst (1 mol%) was used for the Suzuki–
Miyaura cross-coupling reaction of various aryl halides and arylboronic acids in 95% ethanol at 80 °C and the Mizoroki–
Heck reaction in dimethylformamide at 110 °C using K2CO3 as a base. The developed catalyst is well suitable for the 3R
approach (recoverable, robust, recyclable) for cross-coupling reactions without appreciable loss of its activity. Copyright
© 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: cross-coupling reactions; heterogeneous catalysis; chitosan; palladium; silica
chitosan (CS) is in a unique position for the immobilization of
metal complexes as well as metal nanoparticles as it contains
Introduction
Substituted biphenyls and acrylates play an important role in me-
dicinal as well as structural chemistry, and are widely used as versa-
tile building blocks for synthesis of many organic intermediates, in
material sciences, agrochemicals and pharmaceuticals.[1] Number
of methods have been reported for their synthesis. Among which
palladium-catalysed Suzuki–Miyaura[2] and Mizoroki–Heck[3] cross-
coupling reactions are some of the most efficient and facile
methods, due to their mild reaction conditions and functional
group tolerance. Significant progress has been achieved in homo-
geneous palladium-catalysed cross-coupling reactions, as they are
a good and straightforward route for the preparation of many im-
portant compounds. However, the major drawbacks associated
with these protocols are tedious workup procedures, loss of catalyst
and contamination of residual metals in the final product. These
drawbacks can be reduced by immobilizing palladium on suitable
solid supports which facilitates the easy separation and reuse of
catalyst.[4]
NH2 and OH functional groups which can be responsible for immo-
bilization and stabilization of Pd metal.[10] It can easily be obtained
from chitin by deacetylation, which is the second most abundant
natural polysaccharide derived from shrimp and crab shells,
consisting of 2-amino-2-deoxy-(1,4)-β-d-glucopyranose residues
(d-glucosamine units) with
a small amount of N-acetyl-d-
glucosamine units, and is characterized by its strong affinity
towards transition metals.[10] Recently, many reports have demon-
strated that chitosan can acts as excellent stabilizer and support
for the immobilization of Pd metal.[11]
In 1994 Jiang and co-workers reported the first example of
the use of Pd supported on silica-chitosan hybrid material for
the asymmetric hydrogenation of ketones.[12] The results showed
that the amino groups present in chitosan act as ligands to sta-
bilize Pd metal. The utilization of this concept for immobilization
of Pd has been of great interest as it offers easy recovery and
reuse of Pd.
A variety of solid matrices including functional organic polymers,
silicas, zeolites, carbonaceous materials, dendrimers and nanoparti-
cles have been used as palladium catalyst immobilizers.[5] But the
synthesis of these material requires tedious preparation
procedures.[6] Among the various adsorbent materials, silica gel, es-
pecially immobilized with various organic ligands having metal
chelating ability, has been widely used in many cross-coupling
reactions[7] due to its highly porous surface area, easy availability
and good stability.[8]
From this point of view, the aim of the present work is to use
a material which itself acts as ligand as well as support to stabi-
lize palladium. Recently, we have efficiently used Pd supported
on cellulose-alumina hybrid material for synthesis of biaryls in
aqueous DMF.[13] We found that, when chitosan is covalently
connected with silica, it can provide a platform to Pd and pre-
vent the removal of palladium from the silica surface. Consider-
ing the above facts, we prepared Pd-CS@SiO2 and
characterized it using thermogravimetric analysis (TGA),
Nowadays, researchers are directed to develop new green
methodologies to reduce and solve environmental issues. In this
regard, biomolecules are attractive and promising options as sta-
bilizers and support for metal catalysts.[9] They have high surface
area, large abundance in nature containing various functional
groups, which can be used for immobilization and stabilization
of Pd metal.[9] Among the various available biopolymers,
*
Correspondence to: Rajashri Salunkhe, Department of Chemistry, Shivaji
University, Kolhapur, 416004, MS, India. E-mail: rss234@rediffmail.com
a Department of Chemistry, Shivaji University, Kolhapur, 416004, MS, India
b PDVP College Tasgaon, 416312, MS, India
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

S. Jadhav et al.
differential thermogravimetric analysis (DTA), scanning electron
microscopy (SEM), and fourier transform infrared (FT-IR), energy-
dispersive X-ray (EDX) and X-ray photoelectron (XPS) spectros-
copies. At the same time we studied its synthetic utility for
Suzuki–Miyaura and Mizoroki–Heck cross-coupling reactions.
Results and Discussion
Preparation of Pd-CS@SiO2 Catalyst
Preparation of Pd-CS@SiO2 catalyst was carried out using a two-
step process reported in the literature,[14] and is described in
Scheme 1. CS (1) was dispersed on silica by treating its 1.5% aque-
ous acetic acid solution with activated silica at room temperature.
The reaction mixture was stirred and the pH was adjusted to 13
using 4 N aqueous sodium hydroxide solution which deposit chito-
san on the silica surface. The product was filtered, washed with wa-
ter until the filtrate became neutral and dried to obtain CS@SiO2
composite (2). The palladium was loaded on the resultant compos-
ite material by slow addition of Pd(OAc)2 in dry acetone and stirring
the resultant suspension for 48 h at room temperature. The slightly
grey-coloured Pd-CS@SiO2 catalyst was then filtered, washed with
distilled water, acetone and dried under vacuum. The content of
palladium in Pd-CS@SiO2 catalyst as determined using inductively
coupled plasma atomic emission spectroscopy was found to be
Figure 1. FT-IR spectra of (a) SiO2, (b) CS, (c) CS@SiO2 and (d) Pd-CS@SiO2.
0.077 mmol gÀ1
.
Characterization of Pd-CS@SiO2 Catalyst
Our initial effort was focused on characterization of the resultant
catalyst, which would be used as a heterogeneous catalyst in
Suzuki–Miyaura and Mizoroki–Heck cross-coupling reactions.
FT-IR spectra of silica, CS, CS@SiO2 composite and Pd-CS@SiO2
catalyst clearly indicate the grafting of CS on the surface of silica
(Fig. 1). The CS@SiO2 composite shows bands at around 1089, 798
and 477 cmÀ1, which are probably due to the asymmetric
stretching (νas), symmetric stretching (νss) and bending modes of
Si–O–Si bonds, respectively.[15] A strong and broad absorption
band at 3000–3750 cmÀ1 is related to the OH and NH group
stretching vibrations and two narrow peaks at 2925 and
2874 cmÀ1 are assigned to CH and CH2 fragments. In addition, the
peak at 1257 cmÀ1 is assigned to stretching vibration of C–O
bonds.[16]
The TGA curves (Fig. 2) show that the catalyst is stable up to
300 °C. The first weight loss (25 to 150 °C) up to 5% is due to the
evaporation of loosely bonded water. Then the composite
begins to lose weight at 200–300 °C due to the oxidative
Figure 2. TGA and DTA curves of (a) SiO2, (b) CS and (c) Pd-CS@SiO2.
decomposition of chitosan, which is evidenced by exothermic
peaks in the DTA curves.
The SEM image of CS (Fig. 3(b)) clearly shows a sheet-like fibrous
morphology, while Pd-CS@SiO2 catalyst is ‘fluffy’ with porous na-
ture (Fig. 3(d)). As has been reported,[17] compared with powder
and sheet-like catalysts, an open and porous structure usually offers
Scheme 1. Preparation of Pd-CS@SiO2 catalyst.
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Palladium supported on silica–chitosan hybrid catalyst (Pd-CS@SiO2)
Figure 4. EDX spectra of (a) CS@SiO2 and (b) Pd-CS@SiO2.
catalyst (Fig. 5) shows a characteristic binding energy peak at
337.7eV for 3d5/2, which clearly indicates that Pd species are pres-
ent in zero-valent and divalent states and both forms are
abundant.[18]
Pd-CS@SiO2 catalyst for the Suzuki-Miyaura cross-coupling
reaction
To verify the practicability of the prepared catalyst (Pd-CS@SiO2),
our study commenced with its application for the Suzuki–Miyaura
Figure 3. SEM images of (a) SiO2, (b) CS, (c) CS@SiO2 and (d) Pd-CS@SiO2.
better mass transfer. Hence porous support increases catalytic
activity and also avoids metal leaching.
The EDX spectra of CS@SiO2 and Pd-CS@SiO2 were obtained
from random points on the surface for qualitative analysis of ele-
ments. The comparison of both spectra clearly indicates the depo-
sition of palladium on the composite material (Fig. 4).
To evaluate the chemical oxidation state of palladium in the Pd-
CS@SiO2 catalyst, XPS was performed. The XPS analysis of the
Figure 5. XPS analysis of Pd-CS@SiO2.
Appl. Organometal. Chem. (2015)
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S. Jadhav et al.
cross-coupling reaction of various aryl halides and boronic acids in
ethanol, with K2CO3 as a base, at 80 °C.
toluene, CH3CN and THF shows moderate conversions (Table 3, en-
tries 1, 3, 6 and 7). Among polar solvents, 95% ethanol is the choice
of solvent for this catalytic system, which makes this protocol quite
practical and amenable for large-scale synthesis (Table 3, entry 9).
When the reaction is carried out in H2O–EtOH mixture, the time re-
quired for reaction is increased with decreased product yield
(Table 3, entries 10–12).
During the optimization studies, the effect of temperature was
also examined. It is observed that, with increasing the temperature,
the yield of the reaction significantly increased with decreased
reaction time (Table 4).
In order to evaluate the effect of base, catalyst loading, sol-
vent and temperature, our initial investigation started with the
cross-coupling reaction of 4-bromobenzophenone (3) and
phenylboronic acid (4), using 1 mol% of catalyst (Scheme 2).
Initially, the influence the bases[19] was investigated by per-
forming the model reaction with different bases in 95% ethanol
at 80 °C. It is observed that inorganic bases are more effective
(Table 1, entries 1–3) than organic base Et3N (Table 1, entry 4).
The results reveal that, in the presence of K2CO3, a promising yield
is obtained in minimum time (Table 1, entry 5).
To explore the scope and generality of the catalyst, the
optimized protocol (aryl bromide (1 equiv.), arylboronic acid
(1.1 equiv.), K2CO3 (2 equiv.), catalyst (1 mol%), 95% ethanol (5 ml),
80 °C, under air) was applied for reactions of various substituted
aryl halides and arylboronic acids (Table 5). It is observed that a
time of 1–8 h were required for complete conversion of aryl bro-
mides with good to excellent yield (68–94%), but a much longer
reaction time (10 h) is required for less reactive aryl chlorides with
only 60% yield of the desired product (Table 5, entry 22). The ste-
rically hindered 2,4,6-trimethylbromobenzene affords acceptable
yield (73%) in 7 h (Table 5, entry 14). The reactions of dibromo-
arenes with arylboronic acids give desired products in quantitative
Among the various loadings, 1 mol% of catalyst proves to be the
best, affording the desired product in 2.5h with 94% yield using
95% ethanol at 80 °C (Table 2, entry 4). It is observed that on
increasing the amount of catalyst from 1 to 2 mol%, the yield of
desired product does not change much (Table 2, entries 4 and 5).
However, on reducing quantity of catalyst from 1 to 0.1 mol%, the
yield of product decreases with increasing reaction time (Table 2,
entries 1–3).
Solvent has a crucial effect on the reaction rate and yield of
desired product of Suzuki–Miyaura cross-coupling reaction. Screen-
ing of solvents[20] indicated that non-polar solvents such as xylene,
Table 3. Effect of solvent on Suzuki–Miyaura cross-coupling reactiona
Entry
Solvent
Time (h)
Yield (%)b
1
Xylene
8
2.5
5
55
92
70
91
40
45
65
89
94
80
60
10
Scheme 2. Pd-CS@SiO2-catalysed Suzuki–Miyaura cross-coupling reaction.
2
EtOH
3
CH3CN
4
DMF
2
Table 1. Screening of various bases for Suzuki–Miyaura cross-coupling
reactiona
5
CH2Cl2
9
6
THF
7
Entry
Base
Time (h)
Yield (%)b
7
Toluene
Methanol
95% EtOH
EtOH–H2O (80:20)
EtOH–H2O (50:50)
H2O
10
2
1
2
3
4
5
K3PO4
NaOAc
Na2CO3
Et3N
4
95
80
92
79
94
8
6
9
2.5
3.5
4
3
10
11
12
5
K2CO3
2.5
12
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenylboronic
acid (1.1 mmol), catalyst (1 mol%), base (2.0 mmol), 95% ethanol
(5 ml), at 80 °C in an open vessel.
bIsolated yield.
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenylboronic
acid (1.1 mmol), catalyst (1 mol%), K2CO3 (2.0 mmol), solvent
(5 ml), at reflux temperature in an open vessel.
bIsolated yield.
Table 2. Optimization of catalyst loading for Suzuki–Miyaura cross-
coupling reactiona
Table 4. Effect of temperature on Suzuki–Miyaura cross-coupling
reactiona
Entry
Catalyst loading (mol%)
Time (h)
Yield (%)b
Entry
Temp. (°C)
Time (h)
Yield (%)b
1
2
3
4
5
0.1
0.25
0.50
1
6
45
86
90
94
93
4
1
2
3
4
Room temp.
12
7
20
40
85
94
3
40
60
80
2.5
2.5
5
2
2.5
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenylboronic
acid (1.1 mmol), catalyst (0.1–2 mol%), K2CO3 (2.0 mmol), 95%
ethanol (5 ml), at 80 °C in an open vessel.
bIsolated yield.
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenylboronic
acid (1.1 mmol), catalyst (1 mol%), K2CO3 (2.0 mmol), 95%
ethanol (5 ml), at r. t.-80 °C in an open vessel.
bIsolated yield.
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Palladium supported on silica–chitosan hybrid catalyst (Pd-CS@SiO2)
Table 5. Suzuki–Miyaura cross-coupling reaction of various aryl halides
Table 6. Screening of various solvents and bases for Mizoroki–Heck
and phenylboronic acidsa
cross-coupling reactiona
Entry
Base
Solvent
CH3CN
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
NaOAc
Et3N
5
6
10
92
25
30
10
15
10
50
30
70
DMF
DCM
8
Entry
R1–X
R2
Time (h) Yield (%)b
THF
9
Toluene
H2O
5
1
p-Ph-CO-C6H4-Br
p-CH3CO-C6H4-Br
p-OHC-C6H4-Br
p-OHC-C6H4-Br
p-Ph-CO-C6H4-Br
p-Ph-C6H4-Br
C6H5
C6H5
C6H5
2.5
3.5
2.2
2.5
3
95
93
92
91
90
88
90
90
91
92
90
92
90
73
90
90
68
94
90
88
80
60
12
10
9
2
95% EtOH
DMF
3
4
p-H3C-C6H4
p-F-C6H4
C6H5
DMF
10
8
5
DMF
6
3
7
p-Ph-C6H4-Br
p-H3C-C6H4
C6H5
5
aReaction conditions: 4-nitrobromobenzene (1.0 mmol), methyl
acrylate (1.1 mmol), catalyst (1 mol%), base (2.0 mmol), solvent
(5 ml) at reflux temperature in an open vessel.
bIsolated yield.
8
C6H5-Br
1
9
C6H5-Br
p-H3C-C6H4
C6H5
2
10
11
12
13
14
15
16
17
18
19
20
21
22
p-O2N-C6H4-Br
p-O2N-C6H4-Br
p-Br-C6H4-Br
1.5
2
p-H3C-C6H4
C6H5
3
p-OH-C6H4-Br
Bromomesitylene
p-CH3O-C6H4-Br
p-CH3CO-C6H4-Br
2-Bromofluorene
p-CN-C6H4-Br
p-CN-C6H4-Br
9-Bromoanthracene
C6H5
1.5
7
C6H5
yield. As compared to K2CO3 other bases like K3PO4, NaOAc and
Et3N were not so effective (Table 6, entry 8–10).
1-Naphthyl
1-Naphthyl
C6H5
3.5
4
The scope and limitations of the present protocol were investi-
gated for different aryl bromides possessing electron-withdrawing
and electron-donating groups with methyl acrylate, butyl acrylate
and styrene. In all cases desired (E)-cinnamate derivatives are
obtained in good to excellent yield in 4–8 h. E- and Z-isomers are
confirmed from coupling values (J) of NMR spectra. In the case of
compounds containing electron-donating and electron-
withdrawing groups, 100% E-isomers is formed in the reaction with
methyl acrylate (Table 7, entries 3, 4, 6, 10 and 11), but with bulky
butyl acrylate 84–95% E-isomers formed due to steric impact
(Table 7, entries 2, 5, 7, 9 and 12).
8
C6H5
2
p-H3C-C6H4
C6H5
3
4
9,10-Dibromoanthracene C6H5
C6H5-Cl C6H5
5
10
aReaction conditions: aryl halides (1.0 mmol), arylboronic acids
(1.1 mmol), catalyst (1 mol%), K2CO3 (2.0 mmol), 95% ethanol
(5 ml), at 80 °C in an open vessel.
bIsolated yield.
yields (Table 5, entries 12 and 21). Diphenylanthracene, which is
used as a fluorescent material,[21] is also obtained by coupling
9,10-dibromoanthracene with phenylboronic acid, in 80% yield
in 5 h (Table 5, entry 21).
Table 7. Mizoroki–Heck cross-coupling reaction of various aryl bro-
mides and olefinic compounds over Pd-CS@SiO2 catalysta
Pd-CS@SiO2 Catalyst for Mizoroki-Heck Cross-Coupling Reaction
Entry
R1
R2
Time (h)
Yield (%)b (E:Z, %)c
A satisfactory result of Pd-CS@SiO2 catalyst for the Suzuki–Miyaura
cross-coupling reaction prompted us to extend the same protocol
for the Mizoroki–Heck cross-coupling reaction (Scheme 3).
When 4-nitrobromobenzene (7) is stirred in 95% ethanol with
methyl acrylate (8), using 1 mol% catalyst and K2CO3 as a base at
80 °C for 10h, the reaction is incomplete, with formation of methyl
(E)-cinnamate in only 10% yield (Table 6, entry 7). However the
same reaction is carried out in, DMF gives satisfactory yield (92%)
in 6 h using K2CO3 as a base at 110 °C (Table 6, entry 2). The other
solvents CH3CN, DCM, THF, toluene and water gave only 10–30 %
1
H
H
H
Ph
4
5
6
6
6
7
7
6
5
7
7
8
91 (100:0)
88 (90:10)
90 (100:0)
92 (100:0)
89 (95:5)
2
COOBu
COOMe
COOMe
COOBu
COOMe
COOBu
Ph
3
4
NO2
5
NO2
6
OMe
85 (100:0)
92 (84:16)
90 (100:0)
81 (90:10)
83 (100:0)
85 (100:0)
88 (85:15)
7
OMe
8
COCH3
COCH3
COCH3
COPh
COPh
9
COOBu
COOMe
COOMe
COOBu
10
11
12
aReaction conditions: aryl halides (1.0 mmol), olefins (1.1 mmol),
catalyst (1 mol%), K2CO3 (2.0 mmol), DMF (5 ml), at 110 °C in an
open vessel.
bIsolated yield.
cE:Z ratio determined by 1H NMR spectroscopy.
Scheme 3. Pd-CS@SiO2-catalysed Mizoroki–Heck cross-coupling reaction.
Appl. Organometal. Chem. (2015)
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S. Jadhav et al.
measured with an Alpha-Bruker instrument. A JEOL (Tokyo, Japan)
JSM-5200 scanning electron microscope was used for SEM
observations and EDX analysis. XPS of palladium was carried
out with a UG Multilab 2000 (Thermo Scientific, USA) with Kα ra-
diation. An SDT Q600 V20.9 Build 20 thermogravimetric analyser
was used for DSC/TGA analysis. Mass spectra were recorded
using a Shimadzu QP2010 GCMS instrument with an ion source
temperature of 280 °C. All reagents were purchased from Aldrich
and were used without further purification, unless otherwise
stated.
Preparation of the Silica–Chitosan Composite (CS@SiO2, 2)
The CS@SiO2 composite was prepared as described in the
literature.[14] In a typical reaction, about 1 g of chitosan and 57ml
of 1.5% aqueous acetic acid solution were placed in 250 ml three-
necked flask equipped with mechanical stirrer, thermometer and
dropping funnel. After dissolving the chitosan with stirring at
50°C, 5 g of silica was added. The mixture was stirred, and then
4 N aqueous NaOH solution was added dropwise to deposit chito-
san on the surface of silica by adjusting the pH13. The product
was filtered and washed with water until the filtrate become neu-
tral, and then dried at room temperature to obtain 5.85 g of fine
white particles of CS@SiO2.
Figure 6. Recyclability of Pd-CS@SiO2 for Suzuki–Miyaura and Mizoroki–
Heck cross-coupling reactions.
Heterogeneity of Pd-CS@SiO2 Catalyst during Suzuki–Miyaura
Cross-Coupling Reaction
To evaluate the heterogeneity of the catalyst,[22] hot filtration
test was performed by coupling 4-bromobenzophenone with
phenylboronic acid in the presence of catalyst. For heterogeneity
test the catalyst was separated from the reaction mixture at
80 °C after 1 h and the filtrate was stirred for 2.5h at 80 °C. The
GC analysis shows high initial rate of the reaction with 66% of
product within 1 h. The model reaction does not proceed after
filtration of catalyst 3 after 1 h. This result suggest that the cata-
lyst is characteristic heterogeneous in nature.
For practical applications of heterogeneous systems, the life-
time of the catalyst and its level of reusability are very important
features. The recyclability of catalyst was checked for both
reactions and the results are depicted in Fig. 6. After the first
cycle, the catalyst was recovered by filtration and extensively
washed with water, DCM, and acetone. The catalyst was then
dried under vacuum before performing the reusability test. The
catalyst can be recovered and reused for five successive cycles
with no appreciable change in product yield.
Preparation of Pd-CS@SiO2 Catalyst
In a small Schlenk tube, CS@SiO2 (1.0g) was mixed with Pd(OAc)2
(22.6 mg, 0.1 mmol) in dry acetone (20 ml). The mixture was stirred
at room temperature for 48 h. The solid product was filtered,
washed with distilled water and acetone successively, and dried
under vacuum at room temperature to give slightly grey colored
Pd-CS@SiO2 catalyst.
General Procedure for Suzuki–Miyaura Cross-Coupling
Reaction
Aryl halide (1.0mmol), arylboronic acid (1.1mmol), K2CO3 (2mmol),
Pd-CS@SiO2 (1mol%) and 5 ml of solvent were stirred in a Schlenk
flask, equipped with a magnetic stir bar and a condenser, at 80 °C.
Upon complete consumption of starting materials as determined
using TLC analysis, the catalyst was separated by simple filtration
and washed with water, ethanol and acetone. The filtrate was
extracted with diethyl ether (3× 10 ml). The combined organic layer
was collected and dried over anhydrous Na2SO4 and concentrated
under vacuum to afford crude product which was purified using
column chromatography.
Conclusions
An active, stable and recyclable Pd-CS@SiO2 catalyst was pre-
pared using a conceptually and practically simple approach from
inexpensive commercially available materials. The prepared cata-
lyst was characterized and applied in the Suzuki–Miyaura cross-
coupling reaction in 95% ethanol at 80 °C and the Mizoroki–Heck
cross-coupling reaction in DMF at 110° using K2CO3 as a base. It
was found that the catalyst can be recovered by simple filtration
and reused at least four times with moderate decrease in
product yield. These advantages make the process highly valu-
able from the synthetic and environmental points of view and
make a good contribution to supported catalyst used for cross-
coupling reactions in 3R (recoverable, robust, recyclable)
approach.
General Procedure for Mizoroki–Heck Cross-Coupling Reaction
In a Schlenk flask, equipped with a magnetic stir bar, septum and
condenser, aryl halide (1.0mmol), olefin compound (1.1mmol),
K2CO3 (2mmol), catalyst 3 (1 mol%) and DMF (5ml) were added.
The flask was immersed in a heating bath and reaction mixture
was stirred at 110 °C. Upon complete consumption of starting
materials as determined using TLC analysis (petroleum ether–
ethyl acetate, 8:2), the catalyst was separated by filtration,
washed with water, ethanol and acetone. The filtrate was
extracted with diethyl ether (3× 10 ml). The combined organic
layer was collected, dried over anhydrous Na2SO4 and concen-
trated to afford crude product which was purified using column
chromatography.
Experimental
General Remarks
1H NMR (300MHz) and 13C NMR (75 MHz) spectra were recorded
with a Bruker AC instrument using CDCl3 as solvent and
tetramethylsilane as an internal standard. FT-IR spectra were
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Acknowledgment
One of the authors, Sanjay Jadhav, is grateful to the University Grants
Commission, New Delhi, India for providing an SAP-BSR fellowship.
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