ZnO nanorods as an efficient catalyst for the synthesis of imidazo[1,2-a]azines and diazines

Article abstract of DOI:10.1007/s00706-011-0655-7

ZnO nanorods were synthesized using a novel and solvent-free procedure and their catalytic activity in the preparation of imidazo[1,2-a]azines and diazines was studied. The effects of the amount of catalyst and its reusability were investigated to optimiz

Full text of DOI:10.1007/s00706-011-0655-7

Monatsh Chem (2012) 143:653–656
DOI 10.1007/s00706-011-0655-7
ORIGINAL PAPER
ZnO nanorods as an efficient catalyst for the synthesis
of imidazo[1,2-a]azines and diazines
Samahe Sadjadi Mehdi Eskandari
•
Received: 31 August 2010 / Accepted: 4 September 2011 / Published online: 22 October 2011
Ó Springer-Verlag 2011
Abstract ZnO nanorods were synthesized using a novel
and solvent-free procedure and their catalytic activity in the
preparation of imidazo[1,2-a]azines and diazines was
studied. The effects of the amount of catalyst and its
reusability were investigated to optimize the reaction
conditions. The ZnO nanorods exhibited higher catalytic
activity than that of bulk ZnO.
of ZnO nanostructures. Considering the importance of ZnO
nanostructures, development of new procedures for their
synthesis is of great importance.
Several biological and pharmaceutical properties have
been discovered for imidazo[1,2-a]azine and diazine
derivatives [10]. Classically these compounds can be
obtained from the two-component reaction of a-haloke-
tones and 2-aminoazines [11]. Although this procedure
gave the desired products in good to excellent yields, the
application of excessive amounts of sodium bicarbonate
and ionic liquid is one of the disadvantages of this procedure
Keywords ZnO Á Nanorods Á Imidazo[1,2-a]azine
Introduction
[12]. In recent years many procedures for the synthesis of
imidazo[1,2-a]azines and diazines have been developed [13].
Various catalysts have been used for the synthesis of
imidazo[1,2-a]azines such as p-toluenesulfonic acid [14] and
ammonium chloride [15]. Imidazo[1,2-a]azine and diazine
derivatives can be synthesized by the three-component
reaction of a 2-aminoazine, aldehydes, and trimethylsilyl-
cyanide (TMSCN) under microwave irradiation using
The synthesis of nanostructure metal oxides attracts much
attention owing to their enhanced sinterability, mechanical
[
(
1], electrical [2], and chemical properties [3]. Zinc oxide
ZnO), a versatile semiconductor material, has been
attracting attention because of its wide range of applica-
tions in optoelectronic devices, solar cells, gas sensors,
optical waveguides, light emitting diodes, and laser diodes
Sc(OTf) as a catalyst [16]. Recently ionic liquids have been
3
[
4, 5]. Developments in material science will doubtless
used in the synthesis of imidazo[1,2-a]azines and diazines
[17].
reveal further applications for ZnO.
Recently various approaches, e.g., microwave-assisted
solution-phase synthesis [6], wet chemical bath deposition
Herein, we report the synthesis of ZnO nanorods
through a solvent-free procedure and their catalytic activ-
ities in the preparation of imidazo[1,2-a]azines and
diazines are investigated (Scheme 1).
(
CBD) [7], galvanostatic electrodeposition [8], and hydro-
thermal methods [9] have been reported for the preparation
S. Sadjadi (&) Á M. Eskandari
Nanotechnology Research Group,
Academic Center for Education,
Culture & Research (ACECR) on TMU,
Tehran, Iran
Results and discussion
The ZnO nanorods were synthesized through a simple and
solvent-free procedure. As is known, the distinctive ZnO
crystal habit exhibits polar basal [0001]/top [0001] planes
and six nonpolar [1010] planes parallel to the c axis. This
polarity not only directly induces the characteristic
e-mail: samahesadjadi@yahoo.com
S. Sadjadi
Payame Noor University,
Lavasan Branch, Lavasan, Tehran, Iran
123

6
54
S. Sadjadi, M. Eskandari
NH
2
NH
2
R2
X
N
Catalyst
N
1
R1
R CHO
+
+
TMSCN
N
X
R2
X = N, CH
Scheme 1
piezoelectricity and spontaneous polarization, but also
plays a key role in the crystal growth. The growth veloc-
ities of the ZnO crystals in different directions are as
follow:
[0001] [ [0111] [ [0110] [ [0111] [ [0001].
Normally, ZnO is apt to grow along the [0001] direction
owing to its faster growth rate. Therefore, the ZnO crystals
were grown along the [0001] direction and nanorods were
synthesized by heat treatment in a furnace.
Fig. 2 XRD pattern of ZnO nanorods
Scanning electron microscopy (SEM) images of the
synthesized ZnO nanorods grown at 500 °C are shown in
Fig. 1. The average diameter and length of the nanorods
are about 50 nm and 2 lm, respectively.
was carried out in the presence of various amounts of
catalyst (0.1, 0.3, 0.5, 0.7, 1 mg) under refluxing condi-
tions. As shown in Table 2, the optimum amount of
catalyst was 0.5 mg.
Figure 2 shows the X-ray diffraction (XRD) pattern of the
ZnO nanorods. All the diffraction peaks were indexed as a
˚
ZnO wurtzite structure (JCPDS36-1451, a = 3.24982 A,
˚
c = 5.20661 A). The crystal size of the ZnO nanorods was
It is presumed that the ZnO nanorods can catalyze the
reaction through coordination to the carbonyl group of
benzaldehyde. The catalytic activity of these nanorods for
the synthesis of imidazo[1,2-a]azines and diazines was
compared with that of bulk ZnO. As shown in Table 3, the
catalytic activity of the ZnO nanorods is much better than
that of bulk ZnO. The higher activity of the ZnO nanorods
is due to their higher surface area.
measured using the Debye–Scherrer relation (D = 0.94k/
Bcosh). The average crystal size is 42 nm.
The synthesis of imidazo[1,2-a]azines and diazines by
the reaction of benzaldehydes, 2-aminoazine, and TMSCN
was then performed at reflux in the presence of a catalytic
amount of ZnO nanorods. As shown in Table 1, benzal-
dehydes containing electron-withdrawing groups led to
products with slightly higher yields than those obtained
with benzaldehydes containing electron-donating groups.
To optimize the reaction conditions, the synthesis of
In conclusion, ZnO nanorods were synthesized using a
solvent-free procedure and were successfully used in the
synthesis of imidazo[1,2-a]azines and diazines. The rea-
sonable reaction times, very good yields, simple workup
procedure, and reusability of the catalyst are the main
merits of this method. The ZnO nanorods exhibit much
better catalytic activity than that of bulk ZnO.
2
-(4-nitrophenyl)imidazo[1,2-a]pyrimidine-3-amine was
selected as a model reaction. To study the effect of the
amount of catalyst on the reaction yield, the model reaction
Fig. 1 SEM image of ZnO nanorods
1
23

ZnO nanorods as an efficient catalyst
655
Table 1 Synthesis of
1
2
a
Entry
R
R
X
Time/min
Yield /%
M.p./°C
Lit. m.p./°C [10]
imidazo[1,2-a]azines and
diazines using 0.5 mg ZnO
nanorods under reflux
1
2
3
4
5
6
4-NO
2
-C
6
H
4
H
N
30
40
30
30
20
40
81
70
82
85
85
85
213–215
211–215
231
215–218
212–214
230
4-MeO-C
6
H
4
H
N
6
C H
5
Br
Me
Me
Me
CH
CH
CH
CH
3-NO
4-Cl-C
4-Me-C
-C
2 6
H
4
221–224
250
220–223
248–250
230
6 4
H
a
Yields refer to isolated
products
6
H
4
230–232
Table 2 Results of using various amounts of ZnO nanorods on the
yields of 2-(4-nitrophenyl)imidazo[1,2-a]pyrimidine-3-amine under
reflux
Table 4 Reusability of ZnO nanorods in the synthesis of imi-
dazo[1,2-a]azines and diazines derivatives under reflux after three
runs
a
1
2
R
a
Yield/% /run
Entry
Catalyst amount/mg
Yield/%
Entry
R
X
First
Second
Third
1
2
3
4
5
a
0.1
0.3
0.5
0.7
1
69
75
81
80
80
1
2
3
4
5
6
a
4-NO
2
-C
6
H
4
H
N
81
71
85
88
86
85
80
69
82
88
86
85
78
68
80
85
85
83
4-MeO-C
6
H
4
H
N
C
H
6 5
Br
Me
Me
Me
CH
CH
CH
CH
3-NO
4-Cl-C
4-Me-C
2
-C
H
6 4
Yields refer to isolated products
6 4
H
6 4
H
Yields refer to isolated products
Table 3 Comparison of efficiency of ZnO nanorods and bulk ZnO in
the synthesis of 2-(4-nitrophenyl)imidazo[1,2-a]pyrimidine-3-amine
under reflux
comparing their physical and spectral data with those of
authentic samples [10].
a
Entry
Catalyst (0.5 mg)
Yield/%
1
2
a
ZnO nanorods
Bulk ZnO
81
66
Reuse of the catalyst
Yields refer to isolated products
At the end of the reaction, the catalyst could be recovered
by centrifugation. The recycled catalyst was washed with
diethyl ether and subjected to further reaction runs. The
efficiency of this catalyst in the synthesis of imidazo[1,2-a]
azine and diazine derivatives after three runs is shown in
Table 4, which indicates that the reduction in the yields
using reused catalyst is slight.
Experimental
Synthesis of ZnO nanorods
Zinc acetate dihydrate [Zn(CH COO) Á2H O] was used as
3
2
2
the precursor for the synthesis of the ZnO nanorods.
Zn(CH COO) Á2H O was heated in a furnace at 500 °C for
Acknowledgments The authors are thankful to the Nanotechnology
Research Group, Academic Center for Education, Culture & Research
3
2
2
3
h. Samples were characterized using SEM (Philips) and
˚
(
ACECR) on TMU for the partial financial support.
XRD with Cu Ka radiation, k = 1.54178 A (XPERT
model 95).
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1,2-a]azines and diazines
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