Environmentally benign synthesis of polyhydroquinolines by Co3O4-CNT as an efficient heterogeneous catalyst

Article abstract of DOI:10.1016/j.catcom.2014.10.023

A novel and eco-friendly synthesis of polyhydroquinolines is efficiently catalyzed by Co₃O₄-CNT nanocomposites. This recyclable catalytic system provides a simple strategy to generate a variety of polyhydroquinolines under mild conditions. Utilization of easy reaction condition, recyclable nanocatalyst, reduced environmental impacts and simple work-up make this methodology an interesting option for the eco-friendly synthesis of polyhydroquinolines.

Full text of DOI:10.1016/j.catcom.2014.10.023

Environmentally benign synthesis of polyhydroquinolines by Co 3O4-CNT as
an efficient heterogeneous catalyst
Zohre Zarnegar, Javad Safari, Zahra Mansouri-Kafroudi
PII:
DOI:
Reference:
S1566-7367(14)00442-7
doi: 10.1016/j.catcom.2014.10.023
CATCOM 4111
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
19 September 2014
20 October 2014
21 October 2014
Please cite this article as: Zohre Zarnegar, Javad Safari, Zahra Mansouri-Kafroudi, Envi-
ronmentally benign synthesis of polyhydroquinolines by Co3O4-CNT as an efficient het-
erogeneous catalyst, Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.10.023
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ACCEPTED MANUSCRIPT
Environmentally benign synthesis of polyhydroquinolines by Co3O4-
CNT as an efficient heterogeneous catalyst
Zohre Zarnegar, Javad Safari*, Zahra Mansouri-Kafroudi
Laboratory of Organic Compound Research, Department of Organic Chemistry, College of Chemistry,
University of kashan, P.O. Box: 87317-51167, kashan, I.R.Iran.
 Corresponding author. Tel.: +98 361 591 2320; fax: +98 361 591 2935;
e-mail: Safari@kashanu.ac.ir
Abstract
A novel and eco-friendly synthesis of polyhydroquinolines is efficiently catalyzed by Co3O4-
CNTs nanocomposites. This recyclable catalytic system provides a simple strategy to generate a
variety of polyhydroquinolines under mild conditions. Utilization of easy reaction condition,
recyclable nanocatalyst, reduced environmental impacts and simple work-up make this
methodology as an interesting option for the eco-friendly synthesis of polyhydroquinolines.
Keywords: Co3O4-CNTs, Polyhydroquinoline, Carbon nanotubes, Supported catalyst,
Heterogeneous catalyst.
1. Introduction
Polyhydroquinolines have attracted considerable interest because they possess various important
pharmacological properties [1]. Polyhydroquinolines have found commercial utility as an
important class of Ca+2‏channel blockers as exemplified by therapeutic agents such as Nifedine,
Nitrendipine and Nimodipine [2]. These examples clearly demonstrate the remarkable potential
of polyhydroquinolines as a source of valuable drugs. In view of the importance of
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polyhydroquinoline compounds, many classical methods reported in literature for the synthesis
of polyhydroquinolines such as conventional heating [3] microwave technology [4], ultrasound
irradiation [5] light induced procedure [6], various types of catalysts [7-12], grinding [13] and by
refluxing in water [14]. Each of the above methods has its own merits, while some of the
methods are plagued by limitations of poor yields, toxicity of solvents, critical product isolation
procedures, restrictions for large scale applications, expensive catalysts, difficulty in recovery of
high boiling solvents, excessive amounts of catalysts and generation of large amounts of toxic
wastes in scaling up for industrial applications leading to environmental issues. Therefore, the
discovery of novel and efficient catalysts is of prime importance to synthesize of
polyhydroquinoline compounds for further improvement toward milder reaction conditions and
higher yields.
It is important to develop metal oxides nanoparticles as heterogeneous catalysts in organic
reactions, because of their interesting structure, high catalytic activities and improved selectivity
[15, 16]. In general, heterogeneous nanocatalysts offer higher surface area and lower
coordinating sites, which are responsible for the higher catalytic activity. On the other hand,
among all metal oxides, the tricobalt tetraoxide (Co3O4) can be considered as a one of the best
example to understand the catalytic activity based on the variable particle size [17-19]. However,
the specific surface area of the pure Co3O4 is not high enough for the practical applications. In
addition, Co3O4 cannot be recycled easily, and the leaching cobalt cations in the reacting liquid-
phase can lead to secondary pollution [20].
Stability of the nanoparticles is an important issue and supported metal nanoparticles as catalytic
systems have potential to show greater efficiency. Recently, interest in carbon nanotubes (CNTs)
as nanocatalyst supports has been increasing because of their unique morphologies and various
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potential applications [21-23]. CNTs as excellent supports possess a high surface area for high
dispersion of nanocatalysts, well-defined porosity of structure for maximum reactant contact,
excellent crystallinity or low electrical resistance to facilitate electron transport during chemical
transformations and good interaction between the nanosized catalysts and the carbon support
[23]. Co3O4 supported on CNTs are attracting significant attention owing to their wide
applications as Schottky-junction diode [24], electrochemical capacitors [25], Li-ion battery
anode applications [26], and nanocatalyst [27]. Combination of Co3O4 and CNTs with excellent
electron transfer rate and large specific surface area is expected to provide a chance for
improvement of the durability and performance of pure Co3O4 particles. We devised an effective
and novel Co3O4-CNTs catalyzed synthesis of polyhydroquinolines in an efficient reaction
(Scheme 1). However, there are no reports on the use of Co3O4-CNTs nanocomposites for the
synthesis of polyhydroquinolines under mild condition.
<scheme 1>
2. Experimental
2.1. Materials and apparatus
Chemical reagents were obtained from the Merck Chemical Company. MWNTs with surface
area of 136 m2/g and 10–20 nm in diameter were supplied from Neutrino Company, Iran.
Melting points (°C) were determined on an Electro thermal MK3 apparatus using an open-glass
capillary and are uncorrected. FTIR spectra were recorded as KBr pellets on a Perkin–Elmer 781
spectrophotometer and on an Impact 400 Nicolet FTIR spectrophotometer.1H NMR (400 MHz)
13
and C NMR (100 MHz) spectra were measured on a Bruker DPX-400 spectrometer. X-ray
diffraction analysis was carried out on a Holland Philips Xpert X-ray diffraction (XRD)
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diffractometer (CuK, radiation, λ = 1.5406A˚), at a scanning speed of 2˚ min-1 from 10˚ to 100˚
(2θ). The TEM images were recorded using a Philips CM10 transmission electron microscope
operated at a 100 kV accelerating voltage.
2.2. Preparation of Co3O4-CNT nanocomposites
Oxidized CNTs were prepared by purification in a ratio of 3:1 70% nitric acid and 98 sulfuric
acid at 50 ºC under ultrasonication for 6 h. Composite material of Co3O4-CNT was prepared
according to reported procedures in the literature [25]. In short, 0.5 g Co(NO3)2·6H2O was
dissolved into 40 mL n-hexanol to form a red solution. 70 mg of oxidized CNTs were dispersed
◦
in the red solution by sonication for 2 h. Then, this solution was refluxed at 140 C for 10 h.
After cooled to ambient temperature, the products were washed with cyclohexane repeatedly to
remove any impurities and dried. The black powder was filtered and washed with ethanol and
dried at 100 °C to obtain the Co3O4-CNT nanocomposite.
2.3. General procedure for the Synthesis of 2-Ketomethylquinolines
Benzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate
(1.5 mmol) and Co3O4-CNT (0.03 g) were successively charged into a 50 mL round-bottomed
flask and the contents were refluxed in EtOH (5 mL) for an appropriate period while the product
formation was monitored by TLC (petroleum ether–EtOAc; v/v = 7 : 3). Upon completion, the
nanocatalyst was recovered by simple filtration. Then, the ethanol was evaporated under reduced
pressure to yield the crude product, which was then purified by recrystallization from hot ethanol
and water to afford pure polyhydroquinolines.
3. Results and discussion
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In this research, Co3O4-CNTs were prepared according to the schematic 2. The positive cobalt
ions in the hydrophobic n-hexanol solution are adsorbed to the surface of the acid treated CNTs
through electrostatic attraction and then in situ decomposes into Co3O4 according to the
following process [25]:
air, n-hexanol
3Co(NO3)2
Co3O4 + 6NO2 + O2
140-180 ◦C
<scheme 2>
The XRD patterns of CNTs and Co3O4-CNTs composites are presented in Fig. 1. The diffraction
peaks at 18.63, 35.17, 43.55, 54.22, 60.08 and 65.19 correspond to (1 1 1), (3 1 1), (4 0 0), (3 3
1), (4 2 2)and (5 1 1) reflections, respectively, which they are correlated with Co3O4 in spinel
structure (JCPDS card No. 74-1656). This confirms the crystalline nature and phase purity of
Co3O4 nanoparticles. Also, a peak is clearly observed near 26.22, which is typical for CNTs [28].
The XRD result clearly indicates that Co3O4 nanoparticles were decorated on the surface of
CNTs. The average crystal size of the Co3O4-CNTs was calculated by the Debye–Scherrer
formula to be 20 nm.
<fig 1>
The physical morphologies of the acid-treated CNTs and Co3O4-supported CNTs were examined
using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 2
shows SEM results about the interaction of CNTs and the deposited Co3O4nanoparticles. From
the TEM micrograph, the Co3O4 nanoparticles were dispersed completely homogeneous on the
outer wall of CNTs.
<fig 2>
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After the characterization of the catalyst, the reaction conditions were optimized with the model
substrates of benzaldehyde (1), dimedone (2), ethyl acetoacetate (3) and ammonium acetate (4)
in the presence of Co3O4-CNTs nanocomposites in EtOH. At first, we examined the reaction
parameters such as amount and type of catalyst, effect of temperature, various solvents, and also
reusability of supported catalyst. Initially, the various catalysts were studied for their activities in
the model reaction. As shown in Table 1, Co3O4-CNTs were found to be highly active in term of
reaction yield under refluxing EtOH. The studies reported that CNTs not only provided a large
surface area support for the nanosized metal oxide catalyst, but also stabilized the charge
separation by trapping the electrons transferred from the metal to hinder charge recombination
[29].
< Table 1>
Next, we performed the model reaction with different amounts of Co3O4-CNTs to evaluate the
effect of the catalyst in the synthesis of polyhydroquinolines. As shown in Table 2, it was found
that the highest yield was obtained using 0.03 g catalyst in the model reaction. Moreover, in the
absence of nanocatalyst, the product was afforded only in trace amounts even after extended time
(720 min). In the presence of Co3O4-CNTs, the reaction yield improves gradually in a shorter
time (40 min) with an increase in the amount of nanocatalyst (0.03 g). A higher amount of
catalyst (0.04 g) leads to decreasing the yield to 92% in the same period. With increasing the
amount of nanocatalyst, an increase in the number of acidic active sites occurs, which causes a
decrease in the concentration of reactants at the active sites and finally a decline in yield.
Therefore, optimum amount of catalyst was found to be 0.03 g to prepare polyhydroquinolines
[23].
< Table 2>
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To select the appropriate solvent, the reaction with test substrates was carried out in several
solvents and the solvent-free system (Table 3). The polar solvents such as EtOH and MeOH were
found to be better solvents than the non-polar solvents like toluene and cyclohexane. The non-
occurrence of reaction in non-polar solvents may be due to the poor solubility of the products in
these solvents. Moreover, the results could be explained with the much better solubility of the
reagents and the much better dispersion of the catalyst in the polar solvents. Therefore, EtOH
was used as the solvent of choice, with its fast conversion, high yield and low toxicity. The effect
of temperature was also studied by carrying out the model reaction at different temperatures
(Table 3, entries 8-10). It was observed that the product 5a was obtained in excellent yield if the
reaction was carried out in EtOH under refluxing (Table 3, entry 8).
< Table 3>
After establishing the optimal conditions, in order to prove the generality of this method, we
evaluated a variety of aromatic and heterocyclic aldehydes. As shown in Table 4, aromatic
aldehydes bearing electron- deficient or electron-rich substituent on the aromatic ring as well as
heterocyclic aldehydes afforded the corresponding products in excellent yields. Thus, the use of
Co3O4-CNTs as an efficient catalyst is an environmentally clean technique for this condensation.
All products were identified by comparing their physical (M.P.) and spectral data (1H NMR, 13C
NMR and IR) with authentic samples.
< Table 4>
We propose a mechanism for the Co3O4-CNTs catalyzed synthesis of polyhydroquinolines
(Scheme 3). In this process, the presence of CNTs leads to good dispersion and small size of
Co3O4 nanoparticles. Co3O4-CNTs is supposed as a Lewis acid catalyst to facilitate the
condensation between aldehyde 1 and 1,3-diketone 2 for the formation of the corresponding
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Knoevenagel product 6a, and the Michael addition between this intermediate and enamines 7a
obtained from the reaction of ethyl acetoacetate 3 and ammonium acetate 4, for the formation of
an open chain intermediates 8a which undergo cyclohydration to furnish the desired
polyhydroquinolines 5 products [3].
<Scheme 3>
Moreover, the recyclability of the supported nanocatalyst is also important for the large scale
operation. It was found from Fig.3 that there was no significant decrease in activity after seven
runs using the Co3O4-CNTs nanocomposites. It is clear from the TEM micrograph in Fig. 2(e)
that the nanoparticles are stable on the surface of the nanotube after reaction.
<fig. 3>
Conclusions
In summary, this investigation deals with a novel and clean approach to synthesize
polyhydroquinolines in the presence of Co3O4-CNTs. This method has several advantages such
as simple experimental procedure, utilization of heterogeneous catalyst, short reaction times,
milder conditions, excellent yields and reusability of the catalyst.
Acknowledgments
The authors would like express their appreciation for the financial support given for this research
by the University of Kashan under research grant (No. 256722/27).
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Table 1: Synthesis of product Ethyl-1,4,7,8-tetrahydro-2,7,7-trimethyl-4-phenyl-5(6H)-oxoquinolin-3-
carboxylate using different catalysts under reflux conditions in EtOH.a
Entry
Catalyst
CNTs
Time (min)
Yield (%)
40
40
40
40
40
40
35
45
65
70
80
95
1
2
3
4
5
6
CNTs-COOH
Co(NO3)2
Co3O4
Co3O4 NP
Co3O4-CNTs
a
Benzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate (1.5 mmol) and
catalyst (0.03 g) were refluxed in EtOH (5 mL).
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Table 2. Effect of amount of Co3O4-CNTs on the synthesis of compound Ethyl-1,4,7,8-tetrahydro-2,7,7-
trimethyl-4-phenyl-5(6H)-oxoquinolin-3-carboxylate under refluxing EtOH.a
Entry
Catalyst (g)
-
Time (min)
Yield (%)
720
60
60
40
40
40
trace
75
1
2
3
4
5
0.01
0.02
85
0.03
95
0.035
0.04
94
92
6
a
Benzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), and ammonium acetate (1.5 mmol) in
the presence of Co3O4-CNTs in EtOH (5 mL).
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Table 3. Influence of the solvent on the synthesis of Ethyl-1,4,7,8-tetrahydro-2,7,7-trimethyl-4-phenyl-
5(6H)-oxoquinolin-3-carboxylate.a
Entry
Solvent
-
Temperature (°C)
Time (min)
100
Yield (%)
40
75
90
40
45
55
30
90
95
80
75
1
2
-
80
100
CH2Cl2
MeCN
PhMe
cyclohexane
MeOH
EtOH
EtOH
EtOH
reflux
reflux
reflux
reflux
reflux
reflux
50
100
3
100
4
100
5
100
6
40
7
40
8
90
9
40
100
10
a
Benzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol), ammonium acetate (1.5 mmol) and
Co3O4-CNTs (0.03 g) in various of solvent (5 mL).
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1
H
R
O
O
O
+
O
O
H
5
R
CH3CH2O
Co3O4-CNTs
OCH2CH3
O
O
EtOH, reflux
N
H
3
2
CH3COONH4
4
Scheme 1. One-step synthesis of polyhydroquinolines using Co3O4-CNTs.
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O
Co3O4
C
O
O
Co(NO3)2
hexanol
Co2+
H2SO4/HNO3
sonication
C
O
OH
C
air
O
OH
O
oC
140-180
O
C
Co2+
C
C
OH
O
OH
Co3O4-CNTs
Scheme 2. Schematic illustration of decoration of the CNTs with Co3O4 nanoparticles.
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Co3O4-CNTs
1
O
O
O
R
O
O
NH4OAc
4
O
O
H
R
OEt
OEt
Me
OH
NH
H
Me
7a
3
2a
6a
Co3O4-CNTs
Co3O4-CNTs
O
2
O
R
O
O
R
O
H
-
H2O
OEt
Me
OEt
O
N
H
Me
HN
O
8a
5
Scheme 3: Proposed mechanism for the Co3O4-CNTs catalyzed synthesis of
polyhydroquinolines
.
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Table 4. Co3O4-CNTs catalyzed synthesis of polyhydroquinolines.
M.P. (°C)
Entry
R
Time (min)
Yield (%)
Obs.
203-204
244-246
210-211
229-232
258-259
245-247
209-210
179-181
204-206
247-248
242-243
148-149
Lit.
1
2
C6H5
4-Cl-C6H4
40
35
45
45
40
30
40
30
50
35
35
40
95
97
93
88
95
98
94
98
93
94
95
92
202-204 [7]
244-246 [30]
209-211 [13]
229-231 [7]
257-259 [7]
245-246 [13]
208-211 [31]
179-181 [32]
204-206 [30]
246-248 [7]
241-244 [13]
147-148 [7]
3
2-Cl-C6H4
4
4-(CH3)2N-C6H4
4-MeO-C6H4
4-O2N-C6H4
2-O2N-C6H4
3-O2N-C6H4
C6H5-CH=CH
2-Furyl
5
6
7
8
9
10
11
12
2-Thienyl
3-Pyridyl
Ethyl-1,4,7,8-tetrahydro-2,7,7-trimethyl-4-phenyl-5(6H )-oxoquinolin-3-carboxylate
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(a)
0 0 2
(b)
3 1 1
4 0 0
3 3 1
5 1 1
4 2 2
1 1 1
Fig. 1. XRD patterns of (a) CNT and (b) Co3O4-CNT nanocomposite.
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(a)
(b)
(c)
(d)
(e)
Fig. 2. SEM images of (a) CNTs, (b) Co3O4-CNT and TEM images of (c) CNTs, (d) Co3O4-CNT before
the reaction, (e) Co3O4-CNT after the reaction.
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.
95
95
94
93
93
92
92
1
2
3
4
5
6
7
Run
Fig. 3. Recyclability of catalyst in one-pot synthesis of Ethyl-1,4,7,8-tetrahydro-2,7,7-trimethyl-4-
phenyl-5(6H)-oxoquinolin-3-carboxylate.
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Graphical Abstract
Environmentally benign synthesis of polyhydroquinolines by Co3O4-CNT nanocomtposites as a
novel heterogeneous catalys
1
H
R
O
O
O
+
O
O
H
5
R
CH3CH2O
3
CNT-Co3O4
OCH2CH3
O
O
EtOH, reflux
N
H
2
CH3COONH4
4
O
C
Co3O4
O
O
Co(NO3)2
hexanol
Co2+
H2SO4/HNO3
sonication
C
O
OH
C
air
O
OH
O
C
oC
140-180
O
Co2+
C
C
OH
O
OH
Co3O4-CNTs
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Highlights
 CNTs as an excellent support for the metal oxide in catalytic systems.
 Introducing Co3O4-CNTs for the synthesis of polyhydroquinolines.
 Generality of the catalyst, easiness of separation, and recycled several times.

22