Magnetic carbon nanotube supported Cu (CoFe2O4/CNT-Cu) catalyst: A sustainable catalyst for the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines

Article abstract of DOI:10.1016/j.catcom.2016.02.004

A magnetic carbon nanotube supported Cu catalyst (CoFe₂O₄/CNT-Cu) was prepared, characterized and evaluated as a recoverable catalyst for synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines via one-pot three-component reactions of 2-a

Full text of DOI:10.1016/j.catcom.2016.02.004

Magnetic carbon nanotube supported Cu (CoFe2O4/CNT-Cu) catalyst: A
sustainable catalyst for the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines
Mo Zhang, Jun Lu, Jia-Nan Zhang, Zhan-Hui Zhang
PII:
DOI:
Reference:
S1566-7367(16)30041-3
doi: 10.1016/j.catcom.2016.02.004
CATCOM 4585
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
13 December 2015
31 January 2016
2 February 2016
Please cite this article as: Mo Zhang, Jun Lu, Jia-Nan Zhang, Zhan-Hui Zhang, Magnetic
carbon nanotube supported Cu (CoFe2O4/CNT-Cu) catalyst: A sustainable catalyst for
the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines, Catalysis Communications (2016),
doi: 10.1016/j.catcom.2016.02.004
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ACCEPTED MANUSCRIPT
Magnetic carbon nanotube supported Cu (CoFe2O4/CNT-Cu) catalyst: A
sustainable catalyst for the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines
Mo Zhang, Jun Lu, Jia-Nan Zhang, Zhan-Hui Zhang*
College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, China
ABSTRACT
A magnetic carbon nanotube supported Cu catalyst (CoFe2O4/CNT-Cu) was prepared, characterized
and evaluated as a recoverable catalyst for synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines via one-pot
three-component reactions of 2-aminopyridines, aldehydes and nitromethane. The reactions proceeded in
PEG 400 under aerobic conditions and the products were obtained in good to excellent yields. The catalyst
can be successfully recycled for eight runs without appreciable loss of its activity.
Keywords:
Magnetic nanoparticles; Carbon nanotubes; Heterogeneous catalyst; Multi-component reaction;
3-Nitro-2-arylimidazo[1,2-a]pyridines
___________________________________________________________________________
*Corresponding author. Fax: +86 31180787431. E-mail address: zhanhui@126.com (Z.-H. Zhang)
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1. Introduction
In view of the principles of green chemistry, the designing of new, efficient reaction protocols, media
and catalysts is still important topics of research in organic synthesis. An efficient and recoverable
heterogeneous catalyst system constitutes an important aspect of green organic synthesis. In this context,
carbon catalysts have been paid much attention as they are cheap and easily obtained. Carbon materials
such as activated carbon [1], graphene oxide [2-3], carbon nanofibers [4-5] and carbon nanotubes (CNTs)
have been widely used as heterogeneous catalysts. Particularly, CNTs have become one of the most
commonly used catalyst supports due to their excellent chemical, thermal, and mechanical stability,
inertness, high surface area, and chemically tunable topography. Most importantly, CNTs have rich surface
functional groups (mainly hydroxyl and carboxyl), which made CNTs as a suitable support material for
metal and metal oxide nanoparticles [6-7]. These catalysts showed high catalytic activity and may be reused
for several cycles, but they were difficult to separate from the reaction mixtures, and filtration or high speed
centrifugation was usually required. Accordingly, magnetic nanoparticles (NPs) have been emerged as
easily reusable catalysts in organic reactions owing to their unique properties such as the remarkable
catalytic activity, easy preparation, noncorrosive and nonhazardous nature, eco-friendliness, low cost, as
well as facile separation by magnetic force. Magnetic separation is an intriguing alternative to filtration or
centrifugation because it reduces loss of catalyst and makes the recovery and reusability of catalyst easier
after completion of the reaction [8-10]. Furthermore, replacement of conventional hazardous organic
solvents by safe and environmentally benign media is also of key relevance, since organic solvents account
for more than 80% of the waste production of a process [11]. Besides, multicomponent reactions (MCRs)
are attractive synthetic strategy for the rapid and efficient construction of structurally diverse heterocyclic
compounds in a safe, step-economy, convergence, and energy-efficient manner [12-18]. With these aspects
of green synthesis in mind, developing new strategies combining magnetic nanocatalysts with carbon
nanotubes, environmentally benign media and multicomponent reactions might be one exciting and
desirable direction.
Imidazo[1,2-a]pyridines represent an important class of N-heterocycles and have attactive significant
attention due to their various biological and pharmacological activities. Substituted imidazo[1,2-a]pyridines
are a characteristic strucral unit present in some naturally occurring products and pharmacologically
important molecules. In addition, they have been utilized as dyes [19], electroluminescent [20], and
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intermediates in chemical synthesis [21]. The wide demand for substituted imidazo[1,2-a]pyridines has
spurred a vigorous research for the development of new synthetic methods [22-25]. Particularly,
3-nitroimidazo[1,2-a]pyridine can be readily converted to various imidazopyridine derivatives [26]. From a
synthetic point of view, methods for obtained 3-nitro-2-arylimidazo[1,2-a]pyridines are still limited. In
2012, Yan and co-workers proposed Cu(I)-catalyzed synthesis of 3-nitroimidazo[1,2-a]pyridines by mild
oxidative double C–N coupling of nitroolefins with aminopyridines using air as oxidative agent [27]. The
group of Jagadhane reported the use of sodium dichloriodide for the efficient synthesis of
3-nitroimidazo[1,2-a]pyridines by the reaction of 2-aminopyridines and nitrostyrenes [28]. More recently,
an alternative method to the above coupling reactions has been reported by the group of Itoh employing an
iodine source combined with hydrogen peroxide [29]. Despite their effectiveness, some of these methods
encounter limitations, such as the use of harmful organic solvents, unsatisfactory yields, long reaction times,
narrow substrate scope, or the requirement for advance preparation of nitrostyrenes. Thus, the development
of a more efficient catalytic system for accessing these heterocuyclic compounds under environmentally
benign conditions is still highly desirable.
Considering the above reports and as a continuation of our research towards the design of magnetic
nanocatalysts [30-33] and development of environmentally benign methods for the construction of
heterocyclic compounds [34], herein, we describe synthesis and characterization of magnetic
recyclable CoFe2O4/CNT-Cu NPs and their application as efficient catalysts for one-pot synthesis of
3-nitro-2-arylimidazo[1,2-a]pyridines by three-component reaction of 2-aminopyridines, aldehydes, and
nitromethane in PEG 400 under aerobic conditions (Scheme 1).
Scheme 1. Synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines catalyzed by CoFe2O4/CNT-Cu in PEG 400.
2. Experimental
2.1. General
See in supporting information.
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2.2 Preparation of magnetic carbon nanotube supported Cu catalyst (CoFe2O4/CNT-Cu)
The purchased MWCNTs were dispersed in nitric acid solution (40%) and then placed in an ultrasonic
o
bath for treatment at 80 C for 2 h. The obtained acid-treated MWCNTs were separated by centrifuge,
washed with water and ethanol until the pH reached 7.0, and dried under vacuum at 60 oC for 8 h.
CoFe2O4-CNT NPs were prepared by a chemical co-precipitation technique using FeCl3·6H2O,
CoCl2·6H2O and acid-treated carbon nanotubes as precursors. The obtained acid-treated MWCNTs (0.16 g)
was dispersed in distilled water (40 ml), and sonicated for 30 min at room temperature. The mixture of
FeCl3·6H2O (2.70 g), CoCl2·6H2O (1.19 g) and 10 ml distilled water was added to the MWCNTs solution.
Then, 25 ml of 3 mol/L NaOH solution and oleic were added slowly into the above solution under vigorous
o
stirring. The mixture was stirred for 1 h at 100 C. The obtained CoFe2O4-CNT NPs were separated
magnetically, washed with water and ethanol until the pH reached 7.0, and dried under vacuum at 60 °C for
24 h.
In order to prepare the supported Cu catalyst, CoFe2O4/CNT NPs (0.5 g) was dispersed in water (5 ml)
by sonication. CuCl (0.2 g) dissolved in water (5 ml) was added to the above solution. After stirring
overnight at ambient temperature, the solid was collected using a permanent magnet, followed by washing
three times with H2O. Finally, the obtained CoFe2O4/CNT-Cu was dried under vacuum at 60 °C for 24 h.
2.3. General procedure for synthesis of imidazo[1,2-a]pyridines in PEG 400
A mixture of 2-aminopyridine (1.0 mmol), benzaldehyde (1.0 mmol), nitromethane (1.1 mmol), and
CoFe2O4/CNT-Cu (0.05 mmol) in PEG 400 (5 ml) was stirred at 80 °C with a reflux condenser under an
open atmosphere. After completion of the reaction as monitored by TLC, the reaction mixture was brought
to room temperature and diluted with EtOAc (2 ml). The catalyst was separated by a magnetic bar. The
organic layer was washed with water and dried with anhydrous sodium sulfate and concentrated under
reduced pressure. The crude product was purified through a silica gel column chromatography eluting with
an ethyl acetate and petroleum ether mixture to afford the pure product.
3. Results and discussion
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Scheme 2. Synthesis of CoFe2O4/CNT–Cu.
As shown in scheme 2, the catalyst was prepared in a three-step process. At first, raw CNTs were purified
o
and oxidized by nitric acid in an ultrasonic bath at 80 C for 2 h to increase the concentration of
oxygen-containing functional groups, finally filtered, thoroughly washed and dried for further use. It has
been confirmed that acid treatment could improve the purity of raw CNTs, and introduce hydrophilic
groups (such as COOH and OH) to the CNT surface to help adsorb the active metals. Magnetic carbon
nanotubes (CoFe2O4/CNT) were achieved by a chemical co-precipitation technique using FeCl3, CoCl2 and
acid-treated carbon nanotube as precursors. CuCl was added to a suspension of CoFe2O4/CNT in H2O,
while being dispersed by stirring. The magnetic carbon nanotube supported Cu (CoFe2O4/CNT-Cu)
nanoparticles were isolated by magnetic decantation and washed with H2O and dried. The copper metal
amount of the immobilized catalyst was found to be 51.7 % based on inductively coupled plasma mass
spectrometry (ICP-MS) analysis.
As shown in Fig. 1 (A), the energy dispersive spectrum (EDS) indicated the presence of Fe, Co, O, C, Cl
and Cu in the nanocatalyst. Fig. 1 (B) shows the XRD patterns of the prepared CoFe2O4/CNT-Cu
nanoparticles. The position and relative intensities of observed diffraction peaks appearting at 2θ = 36.5o,
42.3o, 50.3o, 53.8o and 61.4o corresponding to the diffractions of (1 2 1), (0 1 4), (4 0 1), (0 1 5) and (3 3
1), well mathed with literature data (JCPDS 22-1066), indicating retention of the crystalline cubic spinel
structure during functionalization of CoFe2O4. The XRD data (JCPDS 001-0793) clearly demonstrates the
presence of copper with the characteristic (1 1 1) and (2 2 0) Bragg’s peaks located at 2θ = 32.4o and 47.9o.
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Diffraction peaks at around 39.8o and 73.5o corresponding to the diffractions of (2 0 5) and (3 3 6) are
readily recognized from the XRD pattern, The observed diffraction peaks agree well with that of carbon
nanotubes (JCPDS 01-0258).
The magnetic properties of the samples CoFe2O4-CNT-Cu were measured by a vibrating sample
magnetometery (VSM) at room temperature. The prepared CoFe2O4/CNT-Cu is superparamagnetic and the
saturation magnetization value is 21.8 emug-1 (Fig. 1C). The magnetic behavior of the synthesized nano
catalyst is critical for their application, which prevents aggregation and enables them to redisperse rapidly
when the magnetic field is removed.
(A)
(B)
(C)
Fig. 1. (A) EDS spectrum of CoFe2O4/CNT–Cu, (B) XRD pattern of CoFe2O4/CNT–Cu and (C)
magnetization curve of (a) CoFe2O4/CNT and (b) CoFe2O4/CNT–Cu.
Fig. 2 shows fourier transform infrared (FT-IR) spectra of carbon nanotubes, acid-treated carbon
nanotubes, magnetic CoFe2O4/CNT nanoparticles and CoFe2O4/CNT-Cu. The presence of OH and C=O
bonds in the acid-treated carbon nano tubes is confirmed by the characteristic peaks appeared at 3457 and
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1758 cm-1. The presence of Co-O and Fe-O bonds in the magnetic particles is confirmed by the
characteristic peak appeared at 600 cm-1. The absorption at 472 cm-1 is assigned to the Cu-O bridge.
Fig. 2. IR spectra of carbon nanotubes (a), acid-treated carbon nanotubes (b), CoFe2O4/CNT (c) and
CoFe2O4/CNT–Cu (d).
The shape, size and morphology of the synthesized CoFe2O4/CNT-Cu NPs were analyzed by using TEM
(Fig. 3) and SEM (Fig. 4). The average nanopartical diameter of CoFe2O4/CNT-Cu was estimated to be
50-52 nm based on the TEM image, which is also in accordance with the result calculated by the
Debye–Scherrer’s equation. The specific surface area of CoFe2O4/CNT-Cu obtained by the N2 adsorption
isotherms was 26.7 m2g-1.
Fig. 3. TEM images of CNT (left), CoFe2O4/CNT (middle) and CoFe2O4/CNT–Cu (right).
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Fig. 4. SEM image of CoFe2O4/CNT–Cu: fresh (left), after eight reaction cycles (right).
The activity of the prepared catalyst was tested for one-pot reaction of 2-aminopyridine, benzaldehyde
and nitromethane under an air atmosphere and the key results are summarized in Table 1. The absence of
catalyst failed to give desired product ever after heating the reaction mixture for 6 h in PEG 400 (Table 1,
entry 1). Further investigation showed that the reaction occurred smoothly in the presence of
CoFe2O4/CNT-Cu, affording 3-nitro-2-phenylimidazo[1,2-a]pyridine (4a) in 95% yield (entry 9). The use
of CuCl2 as a homogeneous catalyst, showed a drop in reaction rate and gave 65% yield in 6 h (entry 6). In
contrast, magnetic nano-Fe3O4, -Fe2O3, CoFe2O4 and NiFe2O4 were ineffective for the formation of the
expected product. The observed results inferred that copper plays a main catalytic role. We also performed
the model reaction in the presence of nano CuFe2O4, the desired product was obtained in 50% yield. We
recently reported that nano CuFeO2 could be used as an efficient catalyst for promoting three-component
reaction of 2-aminopyridines, aldehydes and alkynes to give imidazo[1,2-a]pyridines in high yield [34],
81% yield of product was obtained for above model reaction (entry 8).
Next, we screened the effect of the solvent, catalyst loading and temperature. Among the different
solvents screened, PEG 400 was found to be the best solvent of choice and afforded the product in high
yield. The formatting product 4a was not observed when the mixture of 2-aminopyridine, benzaldehyde
and nitromethane was stirred under reflux temperature or 80 °C in CH2Cl2, MeOH, and EtOH in the
presence of 5 mol% of CoFe2O4-CNT-Cu (entries 10-12). When the reaction was conducted in H2O,
EtOH/H2O (1:1), CH3CN, DMF and toluene the desired product was obtained in 20%-65% yield (entries
13-17). Based on the fact that PEG plays the role of a ligand in Cu-mediated reaction [35], PEG appeared
to play the pivotal role as a solvent and also as a ligand in the present reaction. Furthermore, the relation
between the yields of the model reaction and catalyst loading was also studied. The result showed that 5
mol % of catalyst was sufficient to complete the reaction, and further increasing the amount of catalyst to
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more than 5 mol% showed no significant beneficial effect on the reaction. However, reducing the amount
of catalyst led to the declined yield of product (entry 21). Accordingly, the effect of the reaction
o
temperature was investigated. It was found that heating at 80 C gave the best yield while the reaction did
not occur at room temperature. Taken together, the optimal reaction conditions included using 5 mol% of
CoFe2O4-CNT-Cu in PEG 400 at 80 oC.
Table 1
Optimization of the reaction conditions for the synthesis of 3-nitro-2-phenylimidazo[1,2-a]pyridine (4a).a
Entry
1
Catalyst
Solvent
Time (h)
Yield (%)
NR
NR
NR
NR
NR
65
no
PEG 400
PEG 400
PEG 400
PEG 400
PEG 400
PEG 400
PEG 400
PEG 400
PEG 400
CH2Cl2
6
6
6
6
6
6
6
6
3
46
6
6
6
6
6
6
6
6
3
3
3
3
2
nano Fe3O4
3
nano -Fe2O3
4
nano CoFe2O4
nano NiFe2O4
CuCl2
5
6
7
nano CuFe2O4
nano CuFeO2
50
8
81
9
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
CoFe2O4/CNT-Cu
95
10
11
12
13
14
15
16
17
18b
19c
20d
21e
22f
NR
NR
NR
20
MeOH
EtOH
DMF
CH3CN
60
H2O
50
EtOH/H2O (1:1)
Toluene
PEG 400
PEG 400
PEG 400
PEG 400
PEG 400
65
40
NR
86
94
82
95
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a
Experimental conditions: 2-aminopyridine (1.0 mmol), benzaldehyde (1.0 mmol), nitromethane (1.1
mmol), catalyst (0.05 mmol) unless otherwise specified in the table, solvent (2 ml), 80 oC, open flask to air.
b room temperature.
c 60 oC.
d 90 oC.
e Catalyst (0.02 mmol).
f Catalyst (0.10 mmol).
To examine the substrate scope and functional group tolerance of this three-component coupling reaction,
we first investigated structurally diverse aldehydes under the optimal conditions. As evidenced in Table 2,
various aromatic aldehydes, regardless of the presence of electron-donating or electron-withdrawing
functional groups attached to benzene rings could react with 2-aminopyridine and nitromethane smoothly
to provide the desired products in high to excellent yields. However, compared to 3- and 4-positions, the
substituents at 2-position have slight impact on the result, which might be due to steric hindrance. For
example, 2-chlorobenzaldehyde required prolonged reaction time and afforded the desired product 4l in a
relatively lower yield (entry12). Strikingly, this procedure could tolerate a wide range of synthetically
useful functional groups such as thioether, C-F bond, C-Cl bond and C-Br bond, which could be used for
further transformation at the substituted positions. Also, the present catalytic system showed excellent
promise for the transformation of aromatic heterocyclic aldehydes such as thiophene-2-carbaldehyde (entry
17). When the aromatic ring was replaced by a hindered naphthyl group, the desired product 4r was
obtained in 80% yield (entry 18). Subsequently, pyridin-2-amines bearing different functional groups were
evaluated. Satisfactorily, 2-aminopyridine having either electron-donating or electron-withdrawing group
worked well and delivered the expected products in high to excellent yields.
Table 2.
Scope of reaction of 2-aminopyridines with aldehydes and nitromethane in CoFe2O4-CNT-Cu/PEG 400
catalytic system.
mp (oC)
Entry R1
R2
Product Time (h) Yield (%)a
Found
Reported
1
2
3
4
5
H
H
H
H
H
Ph
3
4
4
6
6
95
90
88
85
85
176-177 172-174 [27]
146-147 146-148 [36]
171-172 168-170 [27]
146-147
4a
4b
4c
4d
4e
3-MeOC6H4
4-MeOC6H4
3-PhOC6H4
4-CH3(CH2)2OC6H4
135-136
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6
H
4-CH3(CH2)4OC6H4
4-MeC6H4
4-CMe3C6H4
4-MeSC6H4
3-FC6H4
4
4
4
4
3
3
6
4
3
3
4
5
6
4
3
86
92
84
90
90
95
83
90
96
95
90
86
80
91
94
96-97
4f
7
H
201-202 204-206 [27]
4g
4h
4i
8
H
oil
9
H
125-126
10
11
12
13
14
15
16
17
18
19
20
H
213-214
4j
H
4-FC6H4
222-223 224-226 [28]
4k
4l
H
2-ClC6H4
3-ClC6H4
4-ClC6H4
4-BrC6H4
3-CF3C6H4
Thienyl
oil
oil [27]
H
181-182 180-182 [27]
185-186 184-186 [27]
188-190 186-188 [27]
108-110
4m
4n
4o
4p
4q
4r
4s
4t
H
H
H
H
186-187 188-190 [37]
116-118 118-120 [27]
163-164 156-159 [37]
156-157
H
1-Naphthyl
Ph
4-Me
5-F
Ph
a Isolated yield.
The structures of the prepared products were identified by their IR, 1H NMR, 13C NMR spectra and Mass
spectra. The structure of a representative compound 4t (CCDC 1029046) was also explicitly established by
the single-crystal X-ray crystallographic analysis (Fig. S1 in supporting information).
The recyclability is of great importance for applying a catalytic system in industrial processes. The
recyclability of the catalyst and PEG 400 was investigated in a model reaction. After completion of the
reaction, the reaction mixture was cooled to room temperature, and the product was extracted with ethyl
acetate. The catalyst was separated by a magnetic bar. The recovered catalyst was reused in the model
reaction to the next round. The efficiency and stability of the catalytic system were found to be very good,
even after eight cycles (Fig. 5). SEM images of fresh and recovered catalysts indicated that little
morphological changes occurred (Fig. 4). The amount of Cu leaching was also determined by inductively
coupled plasma techniques and found that the Cu content after eighth cycle was 0.60 ppm in ethyl acetate.
In order to test the recyclability of the used PEG, the PEG was collected after usual workup and dried at 50
oC under vacuum. The recovered PEG was reused in the next run for the model reaction. The results show
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that 4a can be obtained in 85% yield when PEG was reused in the fifth runs.
Fig. 5. Reusability of the CoFe2O4/CNT-Cu.
Finally, based on the observations and previous literature report [36], a probable mechanism for
one-pot three-component reactions of 2-aminopyridines, aldehydes and nitromethane has been postulated in
Scheme 3.
Scheme 3. Plausible reaction mechanism.
4. Conclusion
In summary, a new kind of CoFe2O4/CNT-supported Cu catalyst has been firstly prepared by a
relatively simple route. It has been demonstrated to be an efficient catalyst for one-pot synthesis of a wide
variety of 3-nitro-2-arylimidazo[1,2-a]pyridines via one-pot three-component reactions of
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2-aminopyridines, aldehydes and nitromethane in PEG 400 under aerobic conditions. More importantly, the
magnetic CoFe2O4/CNT-Cu catalyst can be quickly and completely recovered by simply applying an
external magnet and the efficiency remains unaltered even after eight recycles. The experimental simplicity,
easily available and economical starting materials, convenient one-pot operation, high yields, wide
substrate scope, benign solvent, ecologically clean procedure, efficient recycling reaction system make this
methodology a very efficient and green process for the synthesis of biologically important
3-nitro-2-arylimidazo[1,2-a]pyridines.
Acknowledgements
This research was supported by generous grants from the National Natural Science Foundation of China
(No. 21272053) and the Nature Science Foundation of Hebei Province (No. B2015205182).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.catcom.XXXX
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Graphical Abstract:
Magnetic carbon nanotube supported Cu (CoFe2O4/CNT-Cu) catalyst: A
sustainable catalyst for the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines
Mo Zhang, Jun Lu, Jia-Nan Zhang, Zhan-Hui Zhang*
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Highlights
 A novel magnetic carbon nanotube supported Cu catalyst was prepared.
 It showed high catalytic activity for the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines.
 The catalyst could be easily isolated by magnetic decantation.
 The catalyst was reused several times without significant loss in catalytic activity.
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