Ultrasound-Assisted Preparation of Copper(I) Oxide Nanocubes: High Catalytic Activity in the Synthesis of Quinazolines

Article abstract of DOI:10.1002/cctc.201601330

A facile and green ultrasound-assisted synthesis of Cu₂O nanocubes at room temperature is presented. The Cu₂O nanocubes were characterized using XRD, SEM, TEM, energy-dispersive X-ray spectroscopy, UV/Vis spectroscopy, differential scanning calorimetry with thermogravimetric analysis, and FTIR spectroscopy. The Cu₂O nanocubes were employed as a heterogeneous nanocatalyst and were found to be highly active in the preparation of quinazolines by the tandem cyclization of 2-bromobenzaldehydes with amidines under ligand-free conditions. Various quinazolines could be prepared in excellent yields within a very short reaction time. Furthermore, the Cu₂O nanocatalyst could be recycled and reused up to four times without any significant loss of catalytic activity.

Full text of DOI:10.1002/cctc.201601330

Accepted Article
Title: Ultrasonic irradiation assisted preparation of Cu2O nanocubes
and their high catalytic activity in synthesis of quinazolines
Authors: Bhalchandra Mahadeo Bhanage, Amol Bhanudas Raut, and
Abhishek Rajesh Kumar Tiwari
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To be cited as: ChemCatChem 10.1002/cctc.201601330
Link to VoR: http://dx.doi.org/10.1002/cctc.201601330
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Ultrasonic irradiation assisted preparation of Cu2O-nanocubes
and their high catalytic activity in synthesis of quinazolines
Amol B. Raut, Abhishek R. Tiwari and Bhalchandra M. Bhanage*[a]
Abstract: In this work, we report a facile, green ultrasonic irradiation
assisted synthesis of Cu2O-nanocubes at room temperature. The
Cu2O-nanocubes were examined using different characterization
techniques such as XRD, FEG-SEM, TEM, EDS, UV-vis, DSC-TGA
and FTIR. The Cu2O nanocubes were employed as a heterogeneous
nanocatalyst and was found to be highly active in preparation of
quinazolines via tandem cyclization of 2-bromobenzaldehydes with
amidines under ligand free condition. Various quinazolines could be
prepared in excellent yields within very short reaction duration.
Further, the Cu2O nanocatalyst was recycled and could be reused
consecutively up to four times without any significant loss of catalytic
activity.
technique from other conventional chemical methods, the
sonochemical technique is based on acoustic cavitation
phenomena. The ultrasound wave frequencies usually ranges
from 20 kHz to 10 MHz.[13] The ultrasound wave through a liquid
medium causes the formation and subsequent collapse of
acoustic cavitation micro bubbles. The constant collapse is an
adiabatic process; ultrasound wave generates localized hot spots
with high temperatures of 5000-25000 K, pressures of 1800 atm,
and cooling rates in 1010 K/s. These extreme surroundings help
in the collapse of bubbles and have been exploited to prepare
nanomaterials.[14,15] The sonochemical technique is favorable,
powerful, cost-effective and environment friendly and is easy to
develop reaction parameters that will help to increase selectivity
and yield of products.[13] The sonochemical technique has also
been employed to synthesize various shapes and size of CuO and
Cu2O nanoparticles. Ge et al. have synthesized uniform
hierarchical Cu2O hollow sub-microsphere using sonochemical
process at room temperature.[16] Wang et al. have synthesized
CuO and Cu2O nanoparticles having different shapes such as
truncated cubic, cubic, cubic octahedral and octahedral via green
sonochemical process without the use of any surfactant and
template.[12] Vijaykumar et al. have composite nanomaterial
containing Cu2O nanocrystalline and Cu nanoparticle embedded
in polyaniline matrices have been prepared by green
sonochemical process using aniline or 1% v/v aniline –water was
used as solvent.[17] Li et al. have synthesized Cu2O porous
microcube via green ultrasound rout using with and without
polyvinylpyrrolidone.[1] Huang et al. have synthesized different
crystal morphology of Cu2O nanoparticle using ethylene glycol
and various concentrations of polyvinylpyrrolidone.[18] Recently, K.
Kaviyarasan et. al. have documented the sonochemical synthesis
of Cu2O nanocubes by ultrasound irradiation of copper sulfate in
the presence of polyvinylpyrrolidone and ascorbic acid. The
polyvinylpyrrolidone acts as a stabilizing agent, whereas ascorbic
acid acts both as an antioxidant and reducing agent. The pH at
11 was maintained by aqueous NaOH solution.[19] Herein, we wish
to report a simple and green synthesis route for Cu2O nanocubes
using sonochemical energy. Further, an attempt was made to
check the catalytic activity of Cu2O nanoparticles for synthesis of
quinazolines via tandem cyclization of 2-halobenzaldehyde with
amidines.
Introduction
In recent years, synthesis of specific structure of nanomaterials in
material science has been of great interest to researchers.
Cuprous oxide (Cu2O) is a p-type semiconductor (a direct band
gap of about 2.17 eV)[1] that has received considerable attention
recently because of its potential applications in dye degradation,[2]
gas sensing, magnetic devices, solar energy conversion,[3,4]
catalysis,[5] lithium ion batteries,[6] CO oxidation,[7] and photo
activated water splitting into H2 and O2.[8] Cuprous oxide
nanoparticles synthesized through bottom-up approaches are
known to have different morphologies such as nanospheres,
hollow spheres, wires, cubes, pyramids, octahedral and
squares.[9] Generally, bottom-up approaches have been achieved
successfully by various methods which include hydrothermal, sol-
gel process, surfactant assisted route, chemical vapour
deposition.[10] The physical, chemical and optical properties such
as high surface area, low density, and tunable shell
structure/composition are found to be dependent on the shape
and size of nanomaterials. Therefore, it becomes very important
to explore and to control the growth mechanism that lead to a
particular shape and size of nanomaterials.[9] It has been well
documented that the morphology and microstructure of cuprous
oxide nanoparticles can be controlled by modifying the reaction
conditions and starting materials such as capping agents,
reducing agents, precursors, reaction temperature, pH value,
ultrasonication, and microwave. [1,6,11]
As a new strategy, the sonochemical technique has been given
much attention to prepare various inorganic nanomaterials such
as monometallic, bimetallic, carbide and oxide.[12] A different
Results and Discussion
The X-ray diffraction patterns of the inorganic compound of Cu2O
cubic nanoparticles prepared using copper salt are shown in
Figure 1. The XRD data of the prepared nanoparticles were found
to be in good agreement with those of Cu2O (JCPDS NO. 05–
0667). The six typical peaks located at 29.6°, 36.5°, 42.4°,
61.5°,73.6° and 77.3º which were attributed to the (110), (111),
[a] Amol B. Raut, Abhishek R. Tiwari, Prof. Bhalchandra. M. Bhanage
Institute of Chemical Technology, Matunga (East)
Mumbai, Maharashtra 400019, (India)
E-mail: bm.bhanage@gmail.com, bm.bhanage@ictmumbai.edu.in
Supporting information for this article is given via a link at the end of the
document
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10.1002/cctc.201601330
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Figure 1. XRD pattern of Cu2O cubic prepared by ultrasonic irradiation at room
temperature for 15 min.
(200), (220), (311) and (222) planes of Cu2O respectively. From
the XRD data, it is very clear that there was no formation of Cu,
Cu (OH)2 or CuO as the corresponding characteristic peaks were
absent thereby reveals that pure Cu2O nanoparticle could be
successfully obtained using copper salt. FEG-SEM was employed
to investigate the size and shape of Cu2O nanoparticles and the
results are shown in Figure 2 (a-e). The Cu2O nanoparticles
synthesized using copper chloride has a cubic shape; it can be
clearly observed that the surface morphology of Cu2O cubes are
very smooth with clear distinction of the edges and corner. The
average width of the Cu2O cubes is 100-200 nm. TEM images
show the well dispersion of cubic shaped Cu2O nanoparticle
(Figure 2f-h).The shape and particle size of Cu2O nanocubes in
TEM monographs show good similarity to the results obtained by
FEG-SEM. The purity of the synthesized Cu2O nanocubes was
further confirmed by energy dispersive X-ray spectrum (EDS)
figure 3a.The EDS shows the peaks only for copper and
oxygen.To resolve the interband transitions, the optical property
of Cu2O nanocubes sample was determined using UV−vis
spectroscopy. The absorption spectrum of prepared red-colored
Cu2O nanocubes was obtained by dispersing in ethanol. The
electronic spectral information clearly indicates that there is a
strong absorbance band at 550 nm for Cu2O nanocubes, which is
in good agreement with the previous report (figure 3b). The phase
purity of Cu2O nanocubes was confirmed by thermal analysis
using DSC/TGA measurement in N2 atmosphere (20 mL min-1) for
the range from room temperature to 600ºC with a ramp rate of
10ºC min-1 (Fig. 4a). A mass loss between 30ºC and 150ºC in the
TGA curve (Fig. 4a, red line) is due to the moisture content
present in the sample. The second mass loss observed in the
range of 200ºC to 300ºC is due to the evaporation of starch which
was used during the preparation of Cu2O nanocubes. The DSC
curve (Fig. 4a, blue line) shows thermal stability of Cu2O
nanocubes. The endothermic peak between 230ºC and 300ºC is
due to the thermal decomposition of starch. The synthesized
Cu2O nanocubes were further subjected to FTIR study (figure 4b).
A very strong band at 628 cm-1 is due to the stretching vibration
of copper (I)-O (Cu2O), which is in good agreement to the previous
Figure 2. (a-e) FEG-SEM images and (f-h) TEM images of Cu2O cubic prepared
by ultrasonic irradiation at room temperature for 15 min.
Figure 3: (a) EDS spectrum of as-prepared Cu2O cubic (b) Absorption spectrum
of as-prepared Cu2O cubic dispersed in ethanol.
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Table 1. Optimization of reaction parameters for the synthesis of 2-
phenylquinazoline [a]
O
NH.HCl
Catalyst, Base
N
H
NH2
+
Solvent, MW, Time
N
Br
1a
3aa
2a
Watt
[W]
Solvent
[ml]
Time
[min]
Yield
[%][b]
Entry
Catalyst
Cu2O
Base
1
2
160
260
EG
Cs2CO3
Cs2CO3
4
4
51
78
Figure 4: (a) DSC/TGA graph (b) FT-IR spectra of Cu2O cubic prepared by
ultrasonic irradiation at room temperature for 15 min.
Cu2O
EG
reports.[10] The spectra also confirm the absence of CuO, whose
band usually appears at around 530 cm-1. As the FTIR spectrum
contains only one peak, the synthesized product is pure Cu2O.
3
4
5
Cu2O
Cu2O
Cu2O
360
460
360
EG
EG
EG
Cs2CO3
Cs2CO3
Cs2CO3
4
4
6
92
92
93
Evalution of the catalytic activity of Cu2O
nanocubes
6
7
Cu2O
Cu2O
Cu2O
CuCl2
CuBr
Cu2O
Cu2O
Cu2O
-
360
360
360
360
360
360
360
360
360
360
EG
Glycerol
H2O
EG
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
2
2
2
2
2
2
2
2
2
2
92
54
In order to check the catalytic activity of the prepared Cu2O
nanocubes. This nanocatalyst was employed for the tandem
cyclization of 2-halobenzaldehydes with amidines to synthesize
quinazolines, which are one of the most important central building
block of many organic medicinal compounds.[20] Attempts were
made to synthesize various derivatives of quinazoline under
ligand free condition, in green reaction media and for very short
reaction time. Previously known protocols for the tandem
cyclization of 2-halobenzaldehyde with amidines have involved
the use of non-recyclable copper catalyst with suitable ligand.
Further, they are limited by (i) the use of toxic solvents such as
DMF, DMSO, (ii) the need of high reaction time and (iii) good
yields.[21-22]
8
Trace
38
9
10
11
12
13
14
15
EG
47
EG
55
EG
Na2CO3
Cs2CO3
Cs2CO3
Cs2CO3
49
EG
69[c]
-
EG
Cu2O
EG
55 [d], 93[e]
The reaction was initiated by choosing 2-bromobenzaldehyde
(1a) and benzamidine hydrochloride (2a) as model substrates in
the presence of Cu2O nanocatalyst, base, solvent under domestic
[a] Reaction condition: 2-bromobenzaldehyde (1a) (1mmol), benzamidine
hydrochloride (2a) (1.1 mmol), catalyst (5 mol %), base (2 mmol), solvent (3
mL), [b] GC yield based on area %, [c] base (1 mmol), [d] catalyst (3 mol%),
[e] catalyst (8 mol%).
microwave for
4 minutes. The tandem cyclization of 2-
bromobenzaldehyde (1a) with benzamidine hydrochloride (2a) in
the presence of 5 mol% Cu2O nanocatalyst, 2.0 equivalent of
Cs2CO3 in ethylene glycol at 160W (domestic microwave) for 4
minutes furnished 2-phenylquinazoline (3aa) in 51% yield (Table
1, entry 1). This result clearly indicates that quinazolines could be
synthesized by reacting 2-halobenzaldehydes with amidine
hydrochlorides. However, in order to enhance the practicality and
applicability of this interesting synthetic method, there was a need
to optimize the reaction conditions. With this objective in mind,
next, attempts were made to increase the yield of the 3aa by
increasing the power of the domestic microwave from 160 W to
460 W (Table 1, entries 1-4). As per our expectation, increase in
the power from 160 W to 360 W resulted into a considerable
amount of increase in the yield of the desired product 3aa,
providing maximum 92% yield (Table 1, entries 2 and 3). Further
increase in power to 460 W did not affect the yield of the desired
product 3aa (Table 3, entry 4). Experiments with different time
interval were performed at 360 W (Table 1, entries 5 and 6) and
it was found that the reaction time could reduce to 2 minutes
(Table1, entry 6). However, no considerable increase in the yield
of 3aa could be noted when reaction carried out for 6 minutes
(Table 1, entry 5). Only 54% yield of 3aa was observed when
reaction was performed in glycerol (Table 1, entry 7) and trace
amount of desired product 3aa could be noted when reaction
performed in water (Table 1, entry 8). In order to compare the
catalytic activity of our prepared cubic shaped Cu2O nanocatalyst,
experiments were performed with different copper catalyst such
as CuCl2 and CuBr. However, the desired product 3aa could be
obtained in poor yields (Table 1, entries 9 and 10). The desired
product 3aa was noted in low yields when the reactions were
carried out with inorganic bases such as K2CO3 and Na2CO3
(Table 1, entries 11 and 12) and these two results indicate that
neutralization of amidine hydrochlorides is crucial for this organic
transformation. Reduction in the amount of Cs2CO3 to 1.0 mmol
resulted in substantial decreased in the yield of 3aa (Table 1,
entry 13). In the absence of Cu2O nanocatalyst, not even traces
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Table 2. Substrate study for the synthesis of quinazoline using Cu2O
nanocatalyst[a]
O
NH.HCl
12
1d
2b
90
Cu2O, Cs2CO3
N
H
NH2
+
EG, 360 W, 2 min
N
Br
1a
3aa
2a
Yield
[%]b
13
14
1a
1a
86
87
Entry
1
2
Product (3)
1
2
92
[a] Reaction condition: (1a) (1mmol), (2a) (1.1 mmol), Cu2O nanocatalyst (5
mol%), Cs2CO3 (2 mmol), EG (3 mL), [b] GC yield based on area %,, [c] Isolated
Yields.
95[c]
1a
1a
of 3aa were formed (Table 1, entry 14). This result shows the
decisive role of the Cu2O nanocatalyst for this reaction. In the next
set of experiments, different concentration of Cu2O nanocatalyst
was checked (Table 1, entry 15). The yield of 3aa dropped
considerably when reaction was carried out with 3 mol% of Cu2O
nanocatalyst neither the yield could be improved further using 8
mol% of nanocatalyst. To summarize, the highest yield (92%) of
2-phenyl quinazoline (3aa) was obtained when the reaction
between 1a (1.0 mmol) and 2a (1.1mmol) was performed with 5
mol% Cu2O nanocatalyst, and 2.0 mmol of Cs2CO3 in EG (3 mL)
at 360 W for 2 minutes. It should be noted that the synthesis of
quinazolines presented here is the first examples of a Cu2O
nanocubes catalyzed synthesis of heterocycles in EG and
3
87[c]
1a
1a
1a
4
5
6
89
89
81
domestic microwave.
With these optimized reaction conditions in hand, the scope
of the developed new method was extended to other substrates
of 2-bromobenzaldehydes and amidines, results are summarized
in table 2. The reaction of 2-bromobenzaldehyde (1a) with
benzamidine hydrochloride (2a) produced 92% yield of 3aa
(Table 2, entry 1). Next, the reaction of 2a with amidines having
different electron donating and electron withdrawing groups were
employed and they provided the corresponding products 3ab-3af
in excellent to very good yields (Table 2, entries 2-6). In the next
set of experiments, 6-bromobenzo[d] [1,3]dioxole-5-carbaldehyde
(1b) and 2-bromo-5-chlorobenzaldehyde (1c) were made to react
with different amidines (2a, 2b and 2c), resulted into respective
products (3ba-3ca) in excellent yields (Table 2, entries 7-10).
Further, 1-(2-bromophenyl)ethanone (1d) could also be
converted into desired products 3da and 3db successfully (Table
2, entries 11-12). Interestingly, when the aliphatic amidines (2g
and 2h) were employed for this transformation they also resulted
into the formation of the desired products 3ag and 3ah (Table 2,
entries 13-14)
95[c]
7
8
9
2a
2b
2c
96
1b
1b
91[c]
10
11
90
88
2a
2a
Scheme 1. A plausible reaction mechanism.
A plausible reaction mechanism has been proposed as shown
in the scheme 1. The reaction starts with the neutralization of
benzamidine hydrochloric salt (2a) to give 2a’ which then reacts
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with 1a to give an imine intermediate (A). Next, Ullmann type [23]
N-arylation of A takes place to give the desired product 3a.
diffract meter (Shimadzu XRD-6100 using CuKα-1.54 Å) with
scanning rate 2θ per min and 2 theta (θ) angle ranging from 20 θ
to 80 θ with current 30 mA and voltage 40 kW, Field emission gun-
scanning electron microscopy (FEG-SEM) analysis was done by
Tescan MIRA 3 model with secondary electron (SE) detector
between 10.0 kV to 20.0 kV. The energy dispersive X-ray
spectrum (EDS) was recorded by using Oxford instrument (model
51-ADD0007). Fourier transform infrared spectroscopy (FT-IR)
was measured on Brucker Perkin Elmer-100 spectrometer.
Differential scanning calorimetry-Thermo gravimetric analysis
(DSC-TGA) was done on Perkin Elmer STA 6000. The isolated
products were confirmed by GC-MS, 1H NMR and 13C NMR
spectroscopic techniques. The properties of the GC-MS
(Shimadzu QP 2010) instrument (Rxt-17, 30 m×25 mm, film
thickness 0.25 µm. df) were: column flow=2 mL min, at 180 ºC
increasing to 240 ºC at 10 ºCmin-1. IR spectra were recorded with
a Shimadzu IR Affinity-1, Fourier-transform infrared spectroscopy
(FTIR) was recorded on a JASCO-FTIR-400 spectrometer. 1H
and 13C NMR spectra were obtained with a Bruker Avance 400
MHz NMR spectrometer with CDCl3 as the solvent system.
Recycle study
The reusability of catalytic system makes the developed protocol
more economical and eco-friendly process. Hence, we tested the
recycle catalytic activity and stability of Cu2O nanocatalyst for the
synthesis
of
2-phenylquinazoline
(3aa)
using
2-
bromobenzaldehyde (1a) and benzamidine hydrochloride (2b).
After the completion of reaction, the reaction mixture was allowed
to cool at room temperature. Then, the catalyst was recycle by
simple filtration. The catalyst was washed with distilled water (5
mL) and ethanol (5 mL) consecutively 3 to 4 times to remove the
other organic and inorganic impurities, followed by catalyst drying
under vacuum rotary evaporator. Then it was further used as
catalyst for next run. The Cu2O nanocatalytic system could be
reused for four time successive recycles without significant loss
in its catalytic activity (Figure 5).
Procedure for the preparation of Cu2O-nanocubes
In
a typical experiment procedure to synthesize Cu2O
nanoparticle, 1.0 g starch was dissolved in 80 mL hot deionized
water (80 ºC), then 0.1 M copper precursors dissolved in
deionized water was added to starch solution under stirring for 10
min. Then, 10 mL of 0.1 M sodium borohydride solution dissolved
in deionized water was added drop wise to the above mixture
under stirring to form a black solution. The solution was
transferred to ultrasonic irradiation under 20 kHz ultrasonic waves
at the output power 5 seconds on and 5 seconds off mode for 15
min at room temperature. The reaction progress was observed by
change in color for black to brick red, indicating the formation of
Cu2O nanoparticles. Finally, the brick red product were collected
by centrifugation, washed with distilled water and dried under
vacuum at 80 °C for 8 h.
Figure 5. Recyclability study of synthesis of Cu2O nanocatalyst. Reaction
conditions: Reaction condition: (1a) (1mmol), (2a) (1.1 mmol), Cu2O
nanocatalyst (5 mol%), Cs2CO3 (2 mmol), EG (3 mL).
General procedure for the synthesis of quinazolines (3)
To an ovan dried reaction vial 2-bromobenzaldehydes (1, 1.0
mmol), amidine (2, 1.1 mmol), Cu2O nanocatalyst and (5 mol%),
Cs2CO3 (2.0 mmol) were added followed by addition of ethylene
glycol (3.0 mL). This reaction mixture was stirred at room
temperature for 1 minute. Thereafter, it was subjected to domestic
microwave irradiation using power of 360 W for 2 minutes. After
the completion of reaction, the reaction mixture was allowed to
cool at room temperature. The reaction mixture was diluted with
water (10 mL) and diluted reaction mixture was extracted with
ethyl acetate (3 × 10 mL), dried over Na2SO4 and concentrated
on the rotary evaporator. The obtained crude product was purified
using column chromatography over silica gel using pet ether/ethyl
acetate as eluent.
Conclusions
In summary, a facile, green ultrasonic irradiation assisted
synthesis of Cu2O-nanocubes at room temperature has been
achieved successfully. The Cu2O nanocubes were used a
heterogeneous nanocatalyst for the synthesis of quinazolines via
tandem cyclization of 2-bromobenzaldehydes with amidines.
Various quinazolines could be prepared in excellent yields under
ligand free condition and for very short reaction time. Further, the
Cu2O nanocatalyst was reused consecutively up to four times
without any significant loss of catalytic activity.
Experimental section
Acknowledgements
The chemicals and reagents were purchased from firms of repute
in the highest purity available and were used without further
purification. The reactions were monitored by using thin layer
chromatography with Merck silica gel 60 F254 plates. The yields
for the quinazolines as well were confirmed by using Perkin Elmer
Clarus 400 gas chromatography equipped with a flame ionization
detector and a capillary column (elite-1, 30 m×0.32 µm×0.25 µm).
The prepared Cu2O nanomaterial was characterized by X-ray
The Authors ABR and ART are greatful to University Grant
Comission (UGC) for providing financial aid under Basic Science
Research (BSR) programme.
Keywords: Cuprous oxide • nanoparticles • quinazolines • recycle
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10.1002/cctc.201601330
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A facile, green ultrasonic irradiation
assisted synthesis of Cu2O-nanocubes
at room temperature and their
Amol B. Raut, Abhishek R. Tiwari and
Bhalchandra M. Bhanage*
Page No. – Page No.
application
in
synthesis
of
quinazolines.
Ultrasonic
irradiation
assisted
preparation of Cu2O-nanocubes and
their high catalytic activity in
synthesis of quinazolines
This article is protected by copyright. All rights reserved.