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S. Sadjadi, M. Eskandari / Ultrasonics Sonochemistry 20 (2013) 640–643
Table 3
The comparison of efficiency of ZnO nanorods and bulk ZnO on the yields of 2-(4-
nitro-phenyl)-imidazo[1,2-a]pyrimidine-3-ylamine under ultrasonic irradiation.
Entry
Catalyst (0.5 mg)
Yield (%)a
1
2
ZnO nanorods
Bulk ZnO
90
76
a
Yields refer to isolated products.
Table 4
Synthesis of imidazo[1,2-a]azine using 0.5 mg ZnO nanorods under stirring, refluxing
and ultrasonic condition.
Entry
R
R’
X
Time (min) Yield %a
mp.(found) mp.(r.f.)
[1]
Sb Rc Ud Sb Rc Ud
1
2
4-NO2Ph
4-OMe
Ph
H
H
N
N
30 20
7
77 70 90 213–215
215–218
212–214
40 35 12 70 65 83 211–215
3
4
5
6
Ph
Br CH 30 20 10 70 66 85 231
230
3-NO2Ph Me CH 30 20
4-ClPh Me CH 20 20
4-Me Ph Me CH 40 35 12 76 71 85 230–232
8
7
80 76 88 221–224
80 75 90 250
220–223
248–250
230
Fig. 2. XRD pattern of ZnO nanorods.
a
b
c
Table 1
Yields refer to isolated products.
S: Stirring condition.
R: Refluxing condition.
U: Ultrasonic condition.
Synthesis of imidazo[1,2-a]azine using 0.5 mg ZnO nanorods under ultrasonic
irradiation.
d
Entry
R
R’
X
Time (min) Yield %a mp.(found) mp.(r.f.) [10]
1
2
3
4
5
6
4-NO2Ph
4-OMe Ph
Ph
3-NO2Ph Me CH
4-ClPh
H
H
N
N
7
12
90
83
85
88
90
85
213–215
211–215
231
221–224
250
215–218
212–214
230
220–223
248–250
230
As seen with other metal oxides, once they are made into nano-
Br CH 10
particles, their reactivity is greatly enhanced. This is thought to be
due to the morphological differences; whereas larger crystallites
have only a small percentage of the reactive sites on the surface,
smaller crystallites will possess a much higher surface concentra-
tion of such sites [19].
8
7
Me CH
Me CH 12
4-Me Ph
230–232
a
Yields refer to isolated products.
To investigate the role of ultrasonic irradiation in this method,
the reactions were carried out in the presence of the same amount
of ZnO nanorods under various conditions, including stirring and
refluxing condition in EtOH at room temperature and the results
were compared with sonication. The results are summarized in Ta-
ble 4. It is clear that, under the same reaction conditions, reactions
under ultrasonic irradiation led to relatively higher yields and
shorter reaction times than both stirring and refluxing condition.
It is presumed that the efficiency using ultrasound irradiation is
due to the cavitation phenomena. An ultrasonic wave is a pressure
wave with alternate compressions and rarefactions which are able
to break the intermolecular forces maintaining the cohesion of the
liquid and produces a cavity in the rarefaction section of the wave.
The chemical and physical effects of ultrasound derive primarily
from acoustic cavitation which includes formation, growth and col-
lapse of the cavity [17–22]. Bubble collapse in liquids results in an
enormous concentration of energy from the conversion of kinetic
energy of liquid motion into heating of the contents of the bubble.
The high local temperatures and pressures produced by cavitation
lead to a diverse set of applications of ultrasound such as acceler-
ating the rate of the reaction, changing the reaction pathway,
enhancing chemical reactivity and important uses in synthetic or-
ganic compounds [23].
In Table 1, the results of synthesis of imidazo[1,2-a]azine in the
presence of ZnO nanorods under ultrasonic irradiation have been
summarized. As is shown in this table, the benzaldehyde with elec-
tron withdrawing groups led to products with higher yields and
benzaldehydes with electron donating groups gave the corre-
sponding products with slightly lower yields.
It is presumed that ZnO nanorods can catalyze the reaction
through coordination to the carbonyl group of benzaldehyde.
To optimize the reaction conditions, the synthesis of 2-(4-nitro-
phenyl)-imidazo[1,2-a]pyrimidine-3-ylamine was selected as a
model reaction.
To study the effect of catalyst amounts on the yields of reac-
tions, the model reaction has been carried out in the presence of
various amounts of catalysts (0.1, 0.3, 0.5, 0.7, 0.1 mg) under ultra-
sonic condition. The results have been shown in Table 2. The re-
sults show that the optimum amount of catalyst was 0.5 mg.
The catalytic activity of synthesized ZnO nanorods for the syn-
thesis of imidazo[1,2-a]azines has been compared with bulk ZnO.
The results are summarized in Table 3.
As shown in this table, the catalytic activity of ZnO nanorods is
much better than bulk ZnO.
Table 2
The results of using various amounts of ZnO nanorods on the yields of 2-(4-nitro-
phenyl)-imidazo[1,2-a]pyrimidine-3-ylamine under ultrasonic irradiation.
3.1. Recycling of the catalyst
Entry
Catalyst amount (mg)
Yield (%)a
After completing the model reaction, the solid product was
solved in CH3CN and catalyst was separated by simple centrifuga-
tion then washed three times with 10 ml portions of dichlorometh-
ane and dried at 150 °C overnight and subjected to a second run of
the reaction process with the same substrate.
1
2
3
4
5
0.1
0.3
0.5
0.7
1
76
84
90
90
90
The comparison of efficiency of this catalyst in the synthesis of
the model reaction after three times is shown in Table 5. As shown
a
Yields refer to isolated products.