Proline-functionalized chitosan-palladium(ii) complex, a novel nanocatalyst for C-C bond formation in water

Article abstract of DOI:10.1039/c5ra01187f

An environmentally friendly palladium-based catalyst supported on proline-functionalized chitosan was successfully prepared and evaluated as a heterogeneous nanocatalyst in the Suzuki cross-coupling reaction of various aryl halides with phenylboronic acid. The catalyst was characterized by FT-IR, FE-SEM, TEM, XRD, SEM-EDX, ICP and TGA techniques and exhibited reasonable catalytic activity in the reaction system, producing substituted biaryls in good to excellent yields. In addition, the catalyst could be recovered in a facile manner from the reaction mixture and recycled several times.

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ARTICLE
Proline-functionalized chitosan-palladium (II)
complex, a novel nanocatalyst for C-C bond
formation in water
Cite this: DOI: 10.1039/x0xx00000x
a,b*
a
a
A.R. Hajipour,
E. Boostani and F. Mohammadsaleh
Received 00th January 2012,
Accepted 00th January 2012
An environmentally friendly palladium-based catalyst supported on proline-functionalized
chitosan was successfully prepared and evaluated as heterogeneous nanocatalyst in Suzuki cross-
coupling reaction of various aryl halides and phenylboronic acid. The resultant catalyst was
characterized by FT-IR, FE-SEM, TEM, XRD, SEM-EDX, ICP and TGA techniques and
exhibited reasonable catalytic activity in the reaction system and the substituted biaryls were
produced in good to excellent yields. In addition, the catalyst could be recovered in a facile
manner from the reaction mixture and recycled for the several times.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
preparation of a heterogeneous nanocatalyst. All of these
procedures result in faster times and lower temperatures and
In recent years, many efforts have been focused on
development of green chemistry in the synthesis routes which
minimize contamination and pollution in chemical synthesis.
The goal is following usage of green reagents in producing
materials. To reach these goals, greatly active immobilized
greener media, in comparison with conventional investigations,
22,23
and definitely cause to form a brilliant nanostructure
. In
continuation of our recent investigations on the synthesis and
24
applications of palladium-based catalysts , we present a novel
synthetic method for the preparation of CS–proline-Pd (II)
complexes in the state of a recyclable solid catalyst. (Scheme 1)
1
–3
metal catalysts
were used to meaningfully facilitate these
reactions. The heterogeneous catalyst is often readily removed
by filtration and can be successfully recycled several times.
Polysaccharides have been widely applied as a suitable
architecture for solid catalysts. Chitosan (CS), the N-
deacetylated derivative of chitin, is a sample of such
polysaccharides that is widely spread in living organisms. Due
to the presence of a free-amino group, CS and its Schiff-base
derivatives are considered to be proper solid supports for the
4
immobilization of a metal catalyst . A limiting issue in the
consuming of CS is poor chemical resistance and mechanical
strength, which significantly reduces the recycle life of this
biopolymer. Physical and chemical modifications have been
used to modify fresh CS properties. In order to improve pore
size, mechanical strength, chemical stability, hydrophilicity and
biocompatibility, modifications using cross-linking agents such
as glutaraldehyde, epichlorohydrin, diisocyanates or ethylene
5
–13
glycol glycidyl ether have been reported . Amino acids,
pyridine, alkaloids, biguanidine and piperidine are one of the
Scheme 1 The nanocatalyst preparation
The NH functional group in CS was reacted with esterified
proline and therefore proline-functionalized chitosan is formed
via an amid-bond formation. Then, the resultant proline-
2
1
4-19
most supported on solid substrates nanocatalysts
a green and applicable amino acid in the catalytic systems
. Proline is
2
0
especially for modification of various supports. These days,
there are widespread trend to the use of modern heterogeneous
nanocatalysts based on using amino acids such as proline for
modifications and also using CS as a green support for
functionalized chitosan was treated with Pd(OAc) to form the
complex shown in scheme 1.
The synthesis of CS-supported Pd (II) has already been
reported in the literature as well as CS-imine derivatives used in
2
2
1
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DOI: 10.1039/C5RA01187F
Journal Name
2
5,26
C–C coupling reactions
. A study on the structure of CS Microscopy) images of catalyst. TEM micrographs show the
2
7
supported ligand-free Pd (II) has already been reported and, as distribution of particles and confirm that the nano-sized
far as we searched, only a few reports refer to their application particles have been well distributed throughout the polymer
in coupling reactions especially by employing amino acids such matrix. The black spots and translucent parts show
2
8
as proline .
nanoparticles and polymer, respectively.
The palladium-catalyzed Suzuki cross-coupling reaction is a The palladium amount of the catalyst was 2 wt%, which was
considerable method for Carbon–Carbon bond formation, determined by ICP technique. We proposed that the complex is
especially for the synthesis of biaryl derivatives compounds, formed randomly on the support surface between CS-proline
which are used as the building blocks for most of the natural and pd (II) ions and this help the system to make a better
2
9
products, pharmaceuticals and advanced materials . Phosphine nanostructure.
3
0,31
ligands are so toxic and sensitive to air and moistur
phosphine free catalyst are so valuable.
, so
Result and Discussion
In Fig. 1a, the XRD pattern shows that palladium (II)
complexes spread on the amorphous modified-chitosan support.
By the SEM-EDX analysis the presence of palladium in the
catalyst was approved (Fig 1b).
Fig.2 FE-SEM image of CS-proline-Pd (II) complex
The TGA thermograms of weight loss as a function of
temperature for CS-proline-Pd (II) nanocatalyst is shown in fig.
4
. The TGA analysis results illustrate that the catalyst is stable
over 200 ˚C and a total of 52.7% weight loss is due to the
decomposition of polysaccharide chain.
Fig.1 XRD pattern (a) and SEM-EDX spectrum (b) of the CS-proline-
Pd (II) complex
Fig.4 TGA thermogram
Field emission scanning electron microscopy (FE-SEM) images
in Fig. 2 demonstrate the phase morphology of CS-proline-Pd
To explore the activity of the nanocatalyst, we use it in Suzuki
cross-coupling reaction (Scheme 2). The reusability of
(
II). Fig.
3
shows the TEM (Transmission Electron
2
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DOI: 10.1039/C5RA01187F
X
B(OH)
2
nanocatalyst was investigated, too. A model reaction with 4-
bromonitrobenzene with phenylboronic acid was used to
optimize reaction conditions. The catalytic activity of the
catalyst with different amounts of catalyst was studied and
comparable yields were obtained (Table 1).
Nanocatalyst , K CO
2 3
R
+
5
0-50% H O/EtOH, 70ºC
R
2
OH
NH
OH
OH
OH
NH
OH
O
O
O
HO
n
O
O
HO
HO
HO
HO
AcO
AcO
NH
2
H N
Pd
HN
O
AcO
AcO
H N
2
AcO
AcO
Pd
HN
Pd
HN
O
O
Nanocatalyst
Scheme 2 Suzuki cross-coupling rection by CS-proline-Pd (II)
Fig.3 The TEM image of CS-proline-Pd (II) complex
Table 1 Optimization of main parameters in Suzuki cross-coupling
4.
a
H
2
O
70
K
2
2
CO
3
3
(1eq)
(1eq)
0.8
0.8
45
50
99
88
reaction by CS-proline-Pd (II) nanocatalyst
5
6
.
.
EtOH/H
2:1)
EtOH/H
1:2)
EtOH/H
1:1)
EtOH/H
2
2
2
2
O
O
O
O
7
7
7
0
0
0
K
CO
(
Temperature
ºC)
Catalyst
mol %
Time
(min)
b
Entry
Solvent
Base
Yield
(
K
2
CO
3
(1eq)
0.8
56
73
1
2
.
.
(
PEG-200
70
70
K
2
2
CO
3
(1eq)
0.8
0.8
40
30
97
80
CH
EtOH/H
1:1)
3
OH
K
CO
3
(1eq)
(1eq)
7.
8.
KOH (1eq)
0.8
0.8
35
55
81
87
(
3
.
2
O
7
0
K
2
CO
3
0.8
47
99
70
2 3
Na CO
(
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Journal Name
and used in the next catalytic cycle according to the optimized
conditions. However, no significant decrease of activity was
observed until the fifth reuse.
(
1:1)
EtOH/H
1:1)
0. EtOH/H
1:1)
1. EtOH/H
1:1)
2. EtOH/H
1:1)
3. EtOH/H
1:1)
4. EtOH/H
1:1)
5. EtOH/H
1:1)
(1eq)
CO
(0.5eq)
9
1
1
1
1
1
1
.
2
2
2
2
2
2
2
O
O
O
O
O
O
O
K
2
3
7
7
0
0
0.8
0.8
0.8
0.8
0.4
0.2
0.4
60
50
50
60
47
47
47
45
88
(
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
(2eq)
(1eq)
(1eq)
(1eq)
(1eq)
(2eq)
(
rt
40
(
4
7
7
7
0
0
0
0
80
(
100
91
(
Fig.5 Reusability of CS-proline-Pd (II) catalyst
The supported-Pd nanocatalyst exhibited moderate activity in
the Suzuki cross-coupling reactions in aqueous ethanol solvent.
This catalyst displayed good activity at low Pd loading and in
aqueous media. The chitosan modified by amino acid
environment plays an essential stabilizing role for the supported
catalysts in the coupling reactions. The heterogeneous catalyst
could be easily separated and recovered from the reaction
mixture (by filtration) and reused several times. The
combination of advantages displayed by the supported catalysts
such as: ease of preparation, reasonable catalytic activity,
stability, reusability and versatility prove that this catalyst
should be considered as a viable alternative in cross-coupling
reactions on efficiency, environmental stewardship and
economical grounds.
(
97
(
a
Reaction condition: 4-bromonitrobenzene(1mmmol), phenylboronic acid
1.2mmol), CS-proline-Pd (II) catalyst, solvent (4 ml).
(
b
GC yield
We investigated efficiency of the different bases in Suzuki
reaction (Table 1, entries 6-8). Among the selected bases,
K CO was found to be the most effective base. Several
different solvents were examined, among the tested solvents 1:1
mixture of H O/EtOH gave the best result We also optimized
the catalyst concentration, employing various amounts of
catalyst for this cross coupling reaction. The best result was
obtained when the Suzuki coupling reaction was carried out
with 0.4 mol % of catalyst (Table 1, entry 13). Temperature
also was optimized for this catalytic system and 70 ºC was
selected for showing the best result in which the reaction time
of the model reaction, was significantly reduced in comparison
2
3
.
2
Only as a catalyst reservoir, the use of chitosan beads allowed
loading of palladium because of the ability of metal ion
sorption capacity of chitosan.
Experimental
All chemical reagents were purchased from Merck and were
used without further purification. Chitosan was purchased from
Acros Organics and its molecular weight was 100000-300000.
3
2
with other reports .
The optimized parameters were applied to the reaction of
various aryl halides with phenylboronic acid. the result is on
table 2. The electronic effects on yields and conversion times of
the reactions were examined. The nanocatalyst was compatible
with a wide range of functional groups.
1
H-NMR spectra were recorded on a Bruker 400 spectrometer
using tetramethylsilane as an internal standard in CDCl and
3
FT-IR spectra were obtained as KBr pellets on a JASCO 680-
Plus spectrophotometer. Also we used gas chromatography
Aryl halides with electron donating group in comparison with
aryl halides with electron withdrawing group have shown better
conversions in shorter reaction time. As shown in Table 2, aryl
halides with the leaving group of iodide react faster than aryl
bromides (Table 2. entries 1, 2). Aryl bromides required longer
reaction times and less yields were obtained.
(
GC) (BEIFIN 3420 gas chromatograph equipped with a Varian
CP SIL 5CB column: 30 m, 0.32 mm, 0.25 mm), field emission
scanning electron microscopy [FE-SEM, HITACHI (S-4160)],
Inductive coupled plasma Perkin Elmer Optima 7300 DV, for
examination of reactions conversions.
General procedure for Suzuki cross-coupling reaction by CS-
The achievability of recycling the catalyst also examined. After
the reaction the catalyst was simply recovered by filtration and proline-Pd (II) nanocatalyst
washed with water and acetone, then dried at room temperature
4
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The aryl halides (1 mmol) and the phenylboronic acid (1.1 The proline-functionalized chitosan (0.026 g) and Pd(OAc)2
mmol, 0.13 g) were dissolved in H O/EtOH 50:50 (4 ml). CS– (0.02 g) in 10 ml acetone in a round-bottom flask, stirred for 3
2
proline-Pd (II) nanocatalyst and K CO (1 mmol, 0.14 g) were days. Then, the powder was filtered and washed with acetone.
2
3
added. The mixture was stirred at 70 ºC (see Table 1) by The CS-proline-Pd (II) catalyst was obtained as a brown
conventional heating. The mixture was stirred continuously powder.
during the reaction and monitored by both TLC and GC. After
the reaction was completed, the mixture was cooled to room
temperature, 30 ml water was added and the product was Conclusions
extracted with n-hexane (3*10 ml). The organic phase was
In this work we have reported the use of a chitosan-Proline-
dried over CaCl , concentrated under vacuum and purified by
2
Pd(II) complex nanocatalyst for the Suzuki coupling carried out
column chromatography on silica gel (n-hexane:EtOAc, 9:1).
in aqueous media. Excellent yields have been achieved in
The products were characterized by comparing their melting
1
relatively short reaction times on a wide array of iodo- and
bromo-arenes bearing both electrondonating and electron-
withdrawing groups. The gradation of the aryl bromide
point, FT-IR spectra, H-NMR spectra with those reported in
3
3
the literature .
General Procedure for the synthesis of chitosan modified by conversion slightly depended on the electronic effect of
methyl prolinate (CS-proline)
substituent in para position to the bromine. Remarkably
advantageous condition like the low catalyst loading, the use of
eco-friendly support (chitosan) and green solvent provide to the
method great benefits in terms of safety, economy and
sustainability. In addition, the use of water as an additive
proved to be particularly advantageous for its ability to dissolve
boron side-products coating the catalyst surface. Consequently,
we believe that it can compete with the most efficient known
protocols, and due to its simple operating procedure we can
anticipate that it will find wide applicability. Further
application of this new catalytic system is under investigation
in our laboratory.
Methyl prolinate (5 mmol, 0.64 g) and chitosan (0.5 g) in DMF
(
20 ml) in a round-bottom flask refluxed for 3 days under N2
atmosphere. The reaction mixture was cooled down to room
temperature followed by filtration to produce the proline-
functionalized chitosan. After that, the product was collected,
washed with methanol several times to remove the impurities
and dried.
Genereal procedure for synthesis of CS-proline-Pd nanocatalyst
Table 2. Suzuki cross-coupling reaction of various aryl halides with phenylboronic acid by Cs-proline-Pd (II) nanocatalyst
b
Entry
1
Aryl halide
Product
Time (min)
Yield
8
100
2
3
3
0
6
100
99
3
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4
60
81
5
6
1
3
0
0
100
100
7
39
100
8
9
3
h
89
40
99
1
1
1
0
1
2
60
60
53
89
93
95
6
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1
3
6
9
68
87
1
1
4
5
6
0
9
6
85
a
Reaction condition: aryl halide (1mmmol), phenylboronic acid (1.2mmol), K
2
CO
3
(1mmmol), 1:1 EtOH/H
2
O (4 ml), CS-proline-Pd (II)
catalyst 0.4 mol%, 70 °C.
b
GC yield
1
0
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We gratefully acknowledge the funding support received for
this project from the Isfahan University of Technology (IUT),
IR of Iran, and Isfahan Science and Technology Town (ISTT),
IR of Iran. Further financial support from the Center of
Excellence in Sensor and Green Chemistry Research (IUT) is
gratefully acknowledged.
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Electronic Supplementary Information (ESI) available:
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