Simple preparation of cuprous oxide nanoparticles for catalysis of azide–alkyne cycloaddition

Article abstract of DOI:10.3184/174751918X15221562069666

Cuprous oxide (Cu₂O) nanoparticles were prepared by two simple methods and their structural, morphological and electronic properties were investigated by X-ray diffraction analysis, X-ray absorption near-edge structure, energy dispersive X-ray analysis and scanning electron microscopy. Cuprous oxide nanoparticles efficiently catalyse 1,3-dipolar cycloaddition of a variety terminal alkynes and organic azides producing the corresponding 1,2,3-triazole derivatives in excellent yields without use of any additives. Phenylacetylene and benzyl chloride were chosen as model starting compounds for the optimisation of the reaction conditions, such as effect of solvent, reaction temperature and time of reaction in the presence of a-Cu₂O nanoparticles as a catalyst. The results showed that using cuprous oxide nanoparticles (0.035 mmol) and heating at 70 °C in water, the reaction of phenylacetylene with benzyl chloride and sodium azide gave 1-benzyl-4-phenyl-1H-1,2,3-triazole almost quantitatively in 6 h of reaction time. A study of the recyclability and reusability of the catalyst revealed that a-Cu₂O could be reused at least five times without significant loss of yield.

Full text of DOI:10.3184/174751918X15221562069666

166 RESEARCH PAPER
VOL. 42 MARCH, 166–169
JOURNAL OF CHEMICAL RESEARCH 2018
Simple preparation of cuprous oxide nanoparticles for catalysis of
azide–alkyne cycloaddition
Mojtaba Aminia*, Sakine Ramezania, Alireza Pourvahabi Anbaria, Abolghasem Beheshtib,
Sanjeev Gautamc and Keun Hwa Chaed
aDepartment of Chemistry, Faculty of Science, University of Maragheh, Maragheh, Iran
bDepartment of Chemistry, Payame Noor University (PNU), P.O. Box 19395-3697, Tehran, Iran
cDr. S.S. Bhatnagar University Institute of Chemical Engineering and Technology, Panjab University, Chandigarh 160-014, India
dAdvanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea
Cuprous oxide (Cu O) nanoparticles were prepared by two simple methods and their structural, morphological and electronic properties
were investigated 2by X-ray diffraction analysis, X-ray absorption near-edge structure, energy dispersive X-ray analysis and scanning
electron microscopy. Cuprous oxide nanoparticles efficiently catalyse 1,3-dipolar cycloaddition of a variety terminal alkynes and organic
azides producing the corresponding 1,2,3-triazole derivatives in excellent yields without use of any additives. Phenylacetylene and benzyl
chloride were chosen as model starting compounds for the optimisation of the reaction conditions, such as effect of solvent, reaction
temperature and time of reaction in the presence of a-Cu2O nanoparticles as a catalyst. The results showed that using cuprous oxide
nanoparticles (0.035 mmol) and heating at 70 °C in water, the reaction of phenylacetylene with benzyl chloride and sodium azide gave
1-benzyl-4-phenyl-1H-1,2,3-triazole almost quantitatively in 6 h of reaction time. A study of the recyclability and reusability of the catalyst
revealed that a-Cu2O could be reused at least five times without significant loss of yield.
Keywords: nanoparticles, cuprous oxide, azide–alkyne cycloaddition
1,3-Cycloaddition reactions of azides with olefins provide direct
access to a wide range of 1,2,3-triazoles as useful heterocyclic
systems and a rich source of various biologically and
pharmacologically active molecules.1–5 The copper-catalysed
azide–alkyne cycloaddition is one of the most attractive and
fundamental reactions in synthetic organic chemistry.6–10
Cuprous oxide (Cu2O), a p-type semiconductor, is considered
to be one of the most attractive and promising inorganic
materials and an excellent candidate for catalytic activity,
sensing, fuel cells and solar cells.11–14 Owing to its good stability,
non-toxicity and low cost, catalytic applications of Cu2O have
been reported and applied in various fields.15–20
Therefore, in this study, cuprous oxide nanoparticles
were prepared by two simple methods and their structural,
morphological and electronic properties were investigated
by X-ray diffraction analysis (XRD), X-ray absorption near-
edge structure (XANES), energy dispersive X-ray analysis
(EDX) and scanning electron microscopy (SEM). The catalytic
activity of Cu2O nanoparticles in the 1,3-dipolar cycloaddition
of organic azides and alkynes were investigated under air.
state, are consistent with reported values and thus signify the
formation of the Cu2O phase in the nanoparticles.22 Also the
EDX spectrum confirms the presence of copper and oxygen in
the cuprous oxide nanoparticles (Fig. 1c).
The morphological properties and structural information
about the cuprous oxide nanoparticles were investigated by
SEM, and the results are shown in Fig. 2. SEM images showed
a uniform size distribution of the cuprous oxide nanoparticles.
Catalytic effects
Phenylactelylene and benzyl chloride were chosen as model
starting compounds for the optimisation of the reaction
conditions in the presence of nanoparticles 1 as a catalyst.
Interestingly, from the different solvents tested, water was
found to be the most effective for this reaction (Table 1, entries
1–6). A significant improvement in the product yield was
observed at higher temperatures than room temperature (Table
1, entries 7–9). The optimisation of the reaction time showed
that when using cuprous oxide nanoparticles (0.035 mmol) and
Table 1 Effect of various reaction conditions on the azide–alkyne
cycloaddition
Results and discussion
Catalyst characterisation
Entry Catalyst Solvent Temperature (°C) Time (h)
Yield (%)a
94
69
19
35
25
23
18
49
77
65
88
87
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
2
–
H2O
70
70
70
70
70
70
r.t.
40
60
70
70
70
70
6
6
6
6
6
6
6
6
6
2
4
6
6
The indexed XRD peaks of the cuprous oxide nanoparticles
are shown in Fig. 1a to demonstrate the structures and phase
purities of Cu2O. The XRD peaks of the samples were found
to match with cubic bulk Cu2O (JC-PDS file no. 78–2076).21
The absence of any peaks corresponding to Cu(OH)2, Cu2O or
Cu confirmed the purity of the Cu2O phase in the two samples.
The sharp diffraction peaks clearly indicate high crystallinity
of sample 2. To probe the 3d character of Cu in the Cu2O
samples, the Cu L-edge XANES spectra were collected from
cuprous oxide nanoparticles. According to Fig. 1b, the two
distinct peaks at ~936.4 eV and ~956.5 eV, which are assigned
to the transitions of Cu2p3/2 (L3) and 2p1/2 (L2) electrons into the
empty d-states, respectively, signify the formation of the Cu2O
phase in these two samples.22 The energy separation between
the L3 and L2 edges, which is dependent on the copper oxidation
CH3OH
2-PrOH
CH3CN
Toluene
DMSO
H2O
H2O
H2O
H2O
H2O
9
10
11
12
13
H2O
H2O
0
aIsolated yield
* Correspondent. E-mail: mamini@maragheh.ac.ir

JOURNAL OF CHEMICAL RESEARCH 2018 167
Fig. 1 (a) XRD spectra, (b) copper L-edge XANES spectra and (c) EDX analysis of the cuprous oxide nanoparticles.
Fig. 2 SEM images of the cuprous oxide nanoparticles: (a) 1 and (b) 2.
heating at 70 °C in water, the reaction of phenylacetylene with
benzyl chloride and sodium azide gave 1-benzyl-4-phenyl-1H-
1,2,3-triazole almost quantitatively in 6 h of reaction time, but
for catalyst 2, 87% product was obtained (Table 1, entries 1 and
10–12). The Cu-catalysed 1,3-dipolar cycloaddition reaction
did not work in the absence of catalyst (Table 1, entry 13).

168 JOURNAL OF CHEMICAL RESEARCH 2018
From these results, nanoparticles 1 were demonstrated to
be a very efficient catalyst in the multicomponent 1,3-dipolar
cycloaddition of terminal alkynes and organic azides generated
in situ from sodium azide and different organic halides. The
same methodology was successfully applied for the reaction
of various alkynes and alkyl halides (Table 2). Cycloaddition
of phenyl acetylenes with benzyl halides bearing both
electron-withdrawing and electron-releasing groups attached
to the aromatic ring, and also steric hindrance of the ortho
substituents on the benzyl chloride, do not greatly affect the
yield of cycloaddition products (Table 2, entries 1–17). Owing
to the weak C–Br bond compared with C–Cl, replacement of
benzyl chloride with benzyl bromide facilitated the reaction
under the same conditions (Table 2, entries 9–12). In the
presence of aliphatic alkynes (Table 2, entries 7, 8, 16 and 17)
and aliphatic halides (Table 2, entries 13–15), a decrease in the
yield of cycloaddition products was observed.
The recyclability of a catalyst is an important and critical
topic in the field of heterogeneous catalysis. In a final set of
experiments, the recyclability of 1 as a catalyst was surveyed
(Fig. 3). After completing one run of the reaction, the reaction
mixture was centrifuged and washed with water and ethanol
and, after drying, another cycle of the reaction was carried out
using the same particles. The results revealed that 1 could be
reused at least five times without significant loss of yield.
Based on the nature of the product and analogous mechanisms
in previous reports,23,24 a reaction mechanism for the Cu2O-
catalysed 1,3-dipolar cycloaddition reactions was proposed, as
shown in Scheme 1. After formation of a Cu–acetylide complex
via addition of Cu2O to the phenylacetylene, subsequently,
the azide coordinates to the copper acetylide to form a six-
membered copper metallacycle, which, with protonolysis of the
copper triazole complex in the final step, creates the triazole
product and regenerates the Cu2O catalyst. To confirm that
the rate-limiting process is the formation of the benzyl azide
or the Cu2O-catalysed cycloaddition reaction, the reaction of
phenylacetylene and benzyl azide was studied in water with
0.035 mmol of 1 and the corresponding product was obtained
Table 2 Cycloaddition of benzyl halides with terminal alkynes in the
presence of cuprous oxide nanoparticles
R1
R1
R2
N
N
1
R3
X
+
NaN3
+
N
0
H2O, 70 C
R2
R3
Entry R1
R2
X
R3
Yield (%)a
94
96
85
94
94
93
78
65
97
97
97
92
71
74
75
85
1
Ph
H
H
H
H
H
H
H
H
H
H
H
Ph
H
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Ph
Ph
Ph
Ph
2
3
4
5
4-Me-Ph
2-Me-Ph
4-NO2-Ph
Ph
4-MeO-Ph
6
Ph
4-Me-Ph
(HO)CH2
(HO)(H3C)2CH2
Ph
4-MeO-Ph
4-Me-Ph
Ph
7
Ph
8
Ph
9
Ph
10
11
12
13
14
15
16
17
Ph
Ph
Ph
CH3(CH2)3
CH3(CH2)3
CH3(CH2)3
Ph
Ph
4-MeO-Ph
4-Me-Ph
(HO)CH2
(HO)(H3C)2CH2
Ph
79
aIsolated yield.
Fig. 3 Recycling study of catalyst 1 in the azide–alkyne cycloaddition.
N
Cu2O Nanoparticles
R'
N
N
H
R
H
R
R
[Cu2O]
R' N3
R'
N
N
N
+
R' X
NaN3
[Cu2O]
R
R
[Cu2O]
R
N
N
N
[Cu2O]
R'
N
N
N
R'
Scheme 1 Proposed mechanism of cuprous oxide catalytic cycle for the formation of 1,2,3-triazoles.

JOURNAL OF CHEMICAL RESEARCH 2018 169
Acknowledgements
M. Amini thanks the Research Council of the University of
Maragheh for financial support of this work.
in 96% yield in 6 h, while under similar conditions, a blank
experiment provided only 3% of the product; therefore, the rate-
limiting process is the Cu-catalysed cycloaddition reaction.
Conclusions
Received 7 December 2017; accepted 22 March 2018
Paper 1805333
https://doi.org/10.3184/174751918X15221562069666
Published online: 29 March 2018
We have developed a simple and efficient protocol for the
synthesis of the corresponding 1,2,3-triazoles in excellent
yields through 1,3-dipolar cycloaddition of terminal alkynes
with organic azides generated in situ from sodium azide and
different organic halides using cuprous oxide nanoparticles as
an efficient and recyclable catalyst.
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