S-Benzylisothiourea complex of palladium on magnetic nanoparticles: A highly efficient and reusable nanocatalyst for synthesis of polyhydroquinolines and Suzuki reaction

Article abstract of DOI:10.1002/aoc.3665

S-Benzylisothiourea complex of palladium supported on modified Fe₃O₄ magnetic nanoparticles (Pd-SBTU@Fe₃O₄) is reported for carbon–carbon coupling through the Suzuki coupling reaction. Also, the synthesis of pol

Full text of DOI:10.1002/aoc.3665

Received: 1 September 2016
DOI 10.1002/aoc.3665
Revised: 22 September 2016
Accepted: 25 September 2016
F U L L PA P E R
S‐Benzylisothiourea complex of palladium on magnetic
nanoparticles: A highly efficient and reusable nanocatalyst for
synthesis of polyhydroquinolines and Suzuki reaction
Arash Ghorbani‐Choghamarani | Bahman Tahmasbi | Zahra Moradi
Department of Chemistry, Faculty of Science, Ilam
S‐Benzylisothiourea complex of palladium supported on modified Fe O magnetic
University, Ilam, Iran
3 4
nanoparticles (Pd‐SBTU@Fe O ) is reported for carbon–carbon coupling through
the Suzuki coupling reaction. Also, the synthesis of polyhydroquinoline derivatives
Correspondence
3 4
Arash. Ghorbani‐Choghamarani, Department of
Chemistry, Ilam University, PO Box 69315516,
Ilam, Iran.
is reported in the presence of Pd‐SBTU@Fe O as nanocatalyst. The prepared
3
4
nanoparticles were characterized using Fourier transform infrared spectroscopy,
thermogravimetric analysis, scanning electron microscopy, vibrating sample
magnetometry and inductively coupled plasma atomic emission spectroscopy. The
nanocatalyst was easily recovered using an external magnet and reused several times
without significant loss of its catalytic efficiency. The heterogeneity of Pd‐
SBTU@Fe O was studied using hot filtration.
Email: arashghch58@yahoo.com
3
4
KEYWORDS
magnetic nanoparticle, palladium, polyhydroquinoline, Suzuki reaction
1
| INTRODUCTION
(Pd‐SBTU@Fe O ) was synthesized and characterized as a
3
4
new organometallic catalyst for the synthesis of
polyhydroquinoline derivatives and carbon–carbon coupling
reactions. Carbon–carbon coupling reactions are used as an
interesting method in modern synthesis of natural products,
agrochemicals, pharmaceuticals, herbicides, biologically
In green catalytic reactions, the recovery and reuse of
catalysts is an important factor for ecological and economic
[
1]
demands. Nanoparticles have recently emerged as recover-
able and efficient supports for the immobilization of homoge-
[
2]
[8–10]
neous catalysts. However, particles with diameters of less
than 100 nm are difficult to separate by filtration. In such
cases, expensive ultracentrifugation is often the only way to
active compounds, polymers and advanced materials.
Also, polyhydroquinoline derivatives have a wide range of
biological properties and pharmaceutical activities such
as hepatoprotective, vasodilator, anti‐atherosclerotic, anti‐
tumour, anti‐diabetic, geroprotective and bronchodilator
activities, and also they have the ability to modulate calcium
[3]
separate product and catalyst. This drawback can be over-
come by using magnetic nanoparticles, which can be rapidly
separated from a reaction mixture using an external mag-
[1]
[11–15]
net. More importantly, magnetic separation is more effec-
channels.
[4]
tive and easier than filtration or centrifugation. Fe O
3
4
magnetic nanoparticles are efficient, non‐toxic, readily avail-
able, low cost, simply synthesized and of high surface area
resulting in high catalyst loading capacity for immobilization
2
2
| EXPERIMENTAL
[5]
of homogeneous catalysts. Therefore, Fe O nanoparticles
3
4
.1 | Preparation of catalyst
are considered as ideal supports for the heterogenization of
[6,7]
homogeneous catalysts.
The Fe O4 nanoparticles modified with 3‐choloropro-
3
Therefore in the work reported here, a moisture‐ and air‐
stable catalyst of S‐benzylisothiourea complex of palladium
supported on modified Fe O magnetic nanoparticles
pyltriethoxysilane (CPr‐Si@Fe O ) were prepared according
3
4
[
16]
to our recently reported procedure.
Then CPr‐Si@Fe O4
3
3
4
(1 g) was dispersed in 25 ml of ethanol, and
Appl Organometal Chem 2016; 1–6
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
GHORBANI‐CHOGHAMARANI ET AL.
S‐benzylisothiourea (2 mmol) and K CO (2 mmol) were
2
3
added to the reaction mixture and stirred for 24 h under
reflux. The resulting nanoparticles (SBTU@Fe O ) were
3
4
separated by magnetic decantation, washed with ethanol
and dried at room temperature. Then, SBTU@Fe O (0.5 g)
3
4
was dispersed in ethanol, and Pd(OAc) (0.25 g) was added
2
and stirred for 20 h under reflux. Finally, NaBH (0.5 mmol)
4
was added to the reaction mixture and was allowed to run for
another 2 h. The final nanoparticles (Pd‐SBTU@Fe O ) were
3
4
separated by magnetic decantation, washed with ethanol and
dried at room temperature.
2.2 | General procedure for suzuki reaction
A mixture of aryl halide (1 mmol), phenylboronic acid
(
1 mmol), K CO (3 mmol) and Pd‐SBTU@Fe O4
2
3
3
(
0.005 g) was added to a reaction vessel. The resulting
3 4
SCHEME 1 Synthesis of Pd‐SBTU@Fe O
mixture was stirred in poly(ethylene glycol) (PEG) at 60 °C.
After completion of the reaction (monitored by TLC), the
catalyst was separated using an external magnet and washed
with ethyl acetate. The reaction mixture was extracted with
(VSM) and inductively coupled plasma atomic emission
spectroscopy (ICP‐OES).
water and ethyl acetate and dried over anhydrous Na SO4
2
(
1.5 g). Then the solvent was evaporated and pure biphenyl
3
.2 | Catalyst characterization
derivatives were obtained in good to excellent yields.
The size and morphology of Pd‐SBTU@Fe O were studied
3
4
using SEM. The SEM image of Pd‐SBTU@Fe O shows that
the catalyst is formed of nanometre‐sized particles, quasi‐
3
4
2.3 | General procedure for synthesis of
polyhydroquinoline derivatives
spherical, with an average diameter of about 20 ± 5 nm
(
Figure 1).
Also, we determined the exact amount of Pd on the
magnetic nanoparticles using ICP‐OES. The amount of Pd
A mixture of aldehyde (1 mmol), dimedon (1 mmol),
ethylacetoacetate (1 mmol), ammonium acetate (1.2 mmol)
and Pd‐SBTU@Fe O (0.01 g) was stirred in PEG at 80 °C
and the progress of the reaction was monitored by TLC. After
completion of the reaction, the catalyst was separated using
an external magnet and washed with ethanol. Then, the
solvent was evaporated and all products were recrystallized
from ethanol. The pure polyhydroquinoline derivatives were
obtained in good to excellent yields.
3
4
−3
−1
in Pd‐SBTU@Fe O is found to be 1.87 × 10 mol g .
3
4
In order to investigate the presence of Pd complex
on Fe O4 magnetic nanoparticles, TGA/differential
thermal analysis (DTA) was performed. The TGA curve of
3
3
3
| RESULTS AND DISCUSSION
.1 | Catalyst preparation
In continuation of our studies of the preparation of supported
[17,18]
magnetic nanocatalysts,
herein we describe a new palla-
dium complex immobilized on Fe O as a reusable catalyst.
3
4
Initially, CPr‐Si@Fe O was prepared according to a
3
16]
4
[
reported procedure
and, for the the preparation of
SBTU@Fe O , S‐benzylisothiourea was grafted on CPr‐
3
4
Si@Fe O via substitution reaction of NH with terminal Cl
3
4
groups. Finally, the catalyst was synthesized by reaction of
SBTU@Fe O with Pd(OAc) (Scheme 1). This catalyst
3
4
2
was characterized using Fourier transform infrared (FT‐IR)
spectroscopy, thermogravimetric analysis (TGA), scanning
electron microscopy (SEM), vibrating sample magnetometry
3 4
FIGURE 1 SEM image of Pd‐SBTU@Fe O

GHORBANI‐CHOGHAMARANI ET AL.
3
1
Pd‐SBTU@Fe O shows a mass loss of the organic func-
tional groups as it decomposes upon heating (Figure 2).
443 cm− which result from stretching vibration of Fe─O
3
4
[
11]
bonds in Fe O nanoparticles.
In the FT‐IR spectrum of
3
4
The TGA curve shows a small amount of weight loss below
CPr‐Si@Fe O4 (Figure 3b), the anchored 3‐choloro-
3
2
00 °C that is related to removal of adsorbed solvents. Also,
propyltriethoxysilane is confirmed by C─H stretching vibra-
−1
a weight loss of about 9% from 200 to 500 °C is due to the
decomposition of organic species on Fe O surface. On the
tions at 2926 cm and also O─Si stretching vibration modes
−
1 [17]
at 1044 cm .
In the FT‐IR spectrum of SBTU@Fe O4
3
4
3
basis of this result, the good grafting of organic groups
including the palladium complex on the Fe O is verified.
(Figure 3c), the presence of the immobilized SBTU groups
−1
is indicated by C═N vibrations (1621 cm ). This band is
3
4
−
1
Figure 3 shows the FT‐IR spectra of Fe O nanoparticles,
shifted to lower frequency (1616 cm ) in the spectrum of
3
4
CPr‐Si@Fe O , SBTU@Fe O and Pd‐SBTU@Fe O . The
Pd‐SBTU@Fe O (Figure 3d), which indicates the formation
3
4
3
4
3
4
3 4
FT‐IR spectrum of Fe O (Figure 3a) shows strong bands at
of the palladium complex on surface of functionalized nano‐
3
4
−
1
[17]
[6]
3
393 cm , which correspond to surface O─H bonds.
In
boehmite.
all the FT‐IR spectra, several peaks appear at 582 and
The magnetic property of Pd‐SBTU@Fe O was studied
3
4
using VSM. The room temperature magnetization analysis of
the catalyst is shown in Figure 4. The magnetic measurement
shows that Pd‐SBTU@Fe O has a saturation magnetization
3
−
4
1
(
M ) value of 49.88 emu g . The M value of the catalyst is
s
s
lower than that of Fe O nanoparticles, which is due to the
3
4
organic layer and Pd complex on the surface of the Fe O
nanoparticles (39.1 emu g ).
3
4
−1 [11]
3
.3 | Catalytic activity of Pd‐SBTU@Fe O in C─C
3
4
coupling reactions
After synthesis and characterization of the catalyst, it was
applied in carbon–carbon bond formation of the Suzuki reac-
tion using the coupling of various aryl halides with
phenylboronic acid (Scheme 2).
3 4
FIGURE 2 TGA/DTA curves of Pd‐SBTU@Fe O
In order to optimize the reaction conditions, the coupling
of 4‐bromotoluene with phenylboronic acid in the presence
of various amounts of Pd‐SBTU@Fe O was selected as
3
4
a model reaction (Table 1, entries 1–6). The reaction does
FIGURE
4
Magnetization curve for Pd‐SBTU@Fe
3
O
4
at room
temperature
FIGURE
3
FT‐IR spectra of (a) Fe
and (d) Pd‐SBTU@Fe
3
O
4
,
3 4
(b) CPr‐Si@Fe O , (c)
SBTU@Fe
3
O
4
3
O
4
3 4
SCHEME 2 Pd‐SBTU@Fe O ‐catalysed Suzuki reaction

4
GHORBANI‐CHOGHAMARANI ET AL.
TABLE 1 Optimization of reaction conditions for the C─C coupling reac-
aryl halides including chloride, bromide and iodide and the
results are summarized in Table 2. The experimental method
is simple and has the ability to tolerate a variety of electron‐
donating and electron‐withdrawing functional groups such
as CHO, NO , CN, OCH and CH . Therefore, the results
tion of 4‐bromotoluene with PhB(OH)
2
3 4
in presence of Pd‐SBTU@Fe O at
6
0 °C
Catalyst
(mg)
Amount of
base (mmol)
Time
(min)
Yield
(%)
a
Entry
Solvent
Base
2
3
3
b
1
2
3
4
5
6
7
8
9
—
3
PEG
PEG
PEG
PEG
PEG
PEG
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
3
3
100
70
45
40
30
15
45
45
45
45
45
45
45
—
reveal that this procedure is effective for a wide range of aryl
halides.
3
3
3
3
3
3
3
3
80
93
96
95
96
26
37
69
90
69
46
64
5
3
To extend the scope of our work, we next investigated the
catalytic activity of Pd‐SBTU@Fe O for the synthesis of
7
3
3
4
10
14
5
3
polyhydroquinoline derivatives (Scheme 3). In order to
optimize the reaction conditions, the reaction of 4‐
chlorobenzaldehyde, dimedon, ethylacetoacetate and ammo-
3
H
2
O
3
5
DMF
PEG
PEG
PEG
PEG
PEG
3
nium acetate (NH OAc) was selected as a model reaction.
4
5
NaOEt
Na CO
Et
KOH
CO
3
The effect of solvents and amount of Pd‐SBTU@Fe O were
3
4
10
11
12
13
5
2
3
3
studied in the model reaction (Table 3). As evident from
5
3
N
3
Table 3, 0.01 g of Pd‐SBTU@Fe O in PEG at 80 °C are
3
4
5
3
found to be optimized reaction conditions for the outcome
of the model reaction. While increasing the amount of cata-
lyst (from 10 to 12 mg) has no significant effect on the model
reaction.
5
K
2
3
1.5
a
Isolated yield.
b
No reaction.
Then, we extended the reaction to a variety of aldehydes
with dimedon, ethylacetoacetate and ammonium acetate
to confirm the generality of the present method.
not proceed in the absence of Pd‐SBTU@Fe O (Table 1,
entry 1). Also the model reaction was examined in various
3
4
solvents such as water, PEG and dimethylformamide (DMF)
(
Table 1, entries 6–8) and various bases such as K CO ,
2
3
NaOEt, KOH, Et N and Na CO (Table 1, entries 9–12). As
3
2
3
evident from Table 1, the best results are obtained using
K CO (3 mmol) in PEG‐400 and in the presence 0.005 g of
2
3
catalyst at 60 °C. Decreasing the amount of K CO from 3
2
3
to 1.5 mmol leads to a decrease in yield of product from 93
to 69% (Table 1, entry 13).
After optimization of the reaction conditions, we exam-
ined the catalytic activity of Pd‐SBTU@Fe O for various
3 4
SCHEME 3 Pd‐SBTU@Fe O ‐catalysed one‐pot synthesis of polyhydro‐
quinoline derivatives
3
4
TABLE 2 Catalytic C─C coupling reaction of aryl halides with C
6
H
5
B(OH)
2
3 4
in presence of catalytic amounts of Pd‐SBTU@Fe O
Entry
Aryl halide
Iodobenzene
Time (min)
Yield (%)a
Melting point (°C)
1
2
3
4
5
6
7
8
9
420
120
300
45
92
90
89
93
98
86
67–68
66–68
42–43
42–43
53–54
Oil
Bromobenzene
4‐Iodotoluene
4‐Bromotoluene
4‐Bromobenzaldehyde
3‐Bromobenzaldehyde
4‐Iodoanisole
150
120
120
180
240
70
b
85
83–85
82–84
Oil
b
4‐Bromoanisole
90
b
3‐Bromoanisole
80
10
11
12
13
4‐Bromochlorobenzene
4‐Chlorobenzonitrile
4‐Bromonitrobenzene
4‐Chloronitrobenzene
94
96
99
50
69–71
82–84
113–115
108–111
120
120
1440
a
Isolated yield.
b
Reaction conditions: aryl halide (1 mmol), Pd‐SBTU@Fe
3
O
4
(8 mg), C
6
H
5
B(OH)
2
(1 mmol) and K
2
CO
3
(3 mmol) in PEG at 80 °C.

GHORBANI‐CHOGHAMARANI ET AL.
5
3 4
TABLE 3 Optimization of conditions for synthesis of polyhydroquinolines in presence of Pd‐SBTU@Fe O
a
Entry
Solvent
PEG
Catalyst (mg)
Temperature (°C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
10
12
10
10
10
10
80
80
80
80
80
80
60
140
140
120
155
140
140
140
69
95
95
91
55
64
32
PEG
PEG
Ethanol
Ethyl acetate
2
H O
PEG
a
Isolated yield.
3 4
TABLE 4 Synthesis of polyhydroquinolines catalyzed by Pd‐SBTU@Fe O in PEG
a
Entry
Aldehyde
Time (min)
Yield (%)
Melting point (°C)
1
2
3
4
5
6
7
8
9
4‐Methylbenzaldehyde
Benzaldehyde
75
160
300
320
300
150
150
420
240
140
480
90
92
89
90
89
94
90
90
87
95
79
254–256
216–218
249–250
176–178
247–248
184–186
230–232
228–230
200–202
235–237
173–175
4‐Methoxybenzaldehyde
4‐Ethoxybenzaldehyde
4‐Bromobenzaldehyde
4‐Fluorobenzaldehyde
4‐Hydroxybenzaldehyde
3‐Hydroxybenzaldehyde
3,4‐Dimethoxybenzaldehyde
4‐Chlorobenzaldehyde
3‐Nitrobenzaldehyde
10
1
1
a
Isolated yield.
Polyhydroquinoline derivatives are obtained in high yields.
The results of this study are summarized in Table 4. As is
evident, a variety of benzaldehydes bearing electron‐donating
and electron‐withdrawing substituents are successfully
employed to prepare corresponding polyhydroquinoline
derivatives in excellent yields. Therefore, the results reveal
that this methodology is effective for a wide range of
aldehydes.
presence of this catalyst with the results obtained using
previously reported catalysts. As evident from Table 5,
Pd‐SBTU@Fe O affords a better reaction time than the
other catalysts. This new catalyst is comparable to or may
be better in terms of non‐toxicity, stability and ease of
separation.
3
4
The Pd‐SBTU@Fe O , as a magnetically recoverable and
3
4
recyclable nanocatalyst, can be easily recovered from the
reaction mixture and reused for the Suzuki reaction and syn-
thesis of polyhydroquinoline derivatives. Therefore, recycla-
bility of Pd‐SBTU@Fe O was studied for the synthesis of
3
4
biphenyl (Table 4, entry 1) and ethyl 4‐(4‐chlorophenyl)‐
,7,7‐trimethyl‐5‐oxo‐1,4,5,6,7,8‐hexahydroquinoline‐3‐car-
2
boxylate (Table 4, entry 10) under similar reaction condi-
tions. As shown in Figure 5, the catalyst was recovered
using an external magnet and recycled over five times with-
out considerable decrease in activity.
3.4 | Comparison of catalyst
In order to describe the catalytic activity of Pd‐
SBTU@Fe O , we compared the results of the synthesis of
3
4
3 4
FIGURE 5 Recycling experiment of Pd‐SBTU@Fe O in synthesis of
biphenyl (a) and ethyl 4‐(4‐chlorophenyl)‐2,7,7‐trimethyl‐5‐oxo‐1,4,5,6,7,8‐
hexahydroquinoline‐3‐carboxylate (b)
ethyl
2,7,7‐trimethyl‐5‐oxo‐4‐(p‐tolyl)‐1,4,5,6,7,8‐hexa-
hydroquinoline‐3‐carboxylate (Table 3, entry 1) in the

6
GHORBANI‐CHOGHAMARANI ET AL.
TABLE 5 Comparison of Pd‐SBTU@Fe
3
O
4
for the synthesis of ethyl 2,7,7‐trimethyl‐5‐oxo‐4‐(p‐tolyl)‐1,4,5,6,7,8‐hexahydroquinoline‐3‐carboxylate with
previously reported catalysts
Entry
Substrate
Catalyst
Time (min)
Yield (%)
[
11]
1
2
3
4
5
6
4‐Methylbenzaldehyde
4‐Methylbenzaldehyde
4‐Methylbenzaldehyde
4‐Methylbenzaldehyde
4‐Methylbenzaldehyde
4‐Methylbenzaldehyde
Cu‐SPATB/Fe
3
O
4
240
250
260
120
110
75
91
96
89
92
91
[
15]
Boehmite‐SSA
GSA@MNPs
Triton X‐100
[
19]
20]
21]
[
[
Fe
3 4
O ‐SA‐PPCA
Pd‐SBTU@Fe
3
O
4
90 (this work)
a
Isolated yield.
4
| CONCLUSIONS
[10] S. Pasa, Y. S. Ocak, H. Temel, T. Kilicoglu, Inorg. Chim. Acta 2013, 405,
93.
4
[
11] A. Ghorbani‐Choghamarani, B. Tahmasbi, P. Moradi, N. Havasi, Appl.
Organomet. Chem. 2016, 30, 619.
In summary, a new type of magnetically recoverable
nanocatalyst (Pd‐SBTU@Fe O ) was prepared and further
3
4
[
[
12] P. N. Kalaria, S. P. Satasia, D. K. Raval, Eur. J. Med. Chem. 2014, 78, 207.
applied as an excellent, highly reusable and air‐ and mois-
ture‐stable nanocatalyst for the Suzuki reaction and the
synthesis of polyhydroquinoline derivatives in PEG‐400 as
a green solvent. The catalyst was characterized using SEM,
VSM, ICP‐OES, FT‐IR spectroscopy and TGA. The catalyst
could be reused five times without any significant loss of its
activity.
13] S. M. Vahdat, F. Chekin, M. Hatami, M. Khavarpour, S. Baghery, Z.
Roshan‐Kouhi, Chin. J. Catal. 2013, 34, 758.
[
[
[
[
14] M. Saha, A. Kumar Pal, Tetrahedron Lett. 2011, 52, 4872.
15] A. Ghorbani‐Choghamarani, B. Tahmasbi, New J. Chem. 2016, 40, 1205.
16] A. Ghorbani‐Choghamarani, H. Rabiei, Tetrahedron Lett. 2016, 57, 159.
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Chem. 2016, 30, 422.
[
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ACKNOWLEDGMENTS
[
[
[
19] M. Hajjami, B. Tahmasbi, RSC Adv. 2015, 5, 59194.
This work was supported by the research facilities of Ilam
University, Ilam, Iran.
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