Polyvinyl alcohol-stabilized cuprous oxide particles: efficient and recyclable heterogeneous catalyst for azide-alkyne cycloaddition in water at room temperature

Article abstract of DOI:10.1007/s13738-015-0599-7

Abstract The PVA-stabilized Cu₂O particles were prepared using a novel and simple rout in aqueous medium at room temperature under aerobic conditions. Initially, Cu(II) ions were fixed in PVA matrix via complex formation with OH groups, subsequ

Full text of DOI:10.1007/s13738-015-0599-7

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0599-7
ORIGINAL PAPER
Polyvinyl alcohol‑stabilized cuprous oxide particles: efficient
and recyclable heterogeneous catalyst for azide–alkyne
cycloaddition in water at room temperature
Abdol Reza Hajipour · Fatemeh Mohammadsaleh
Received: 8 August 2014 / Accepted: 23 January 2015
©
Iranian Chemical Society 2015
Abstract The PVA-stabilized Cu O particles were pre-
Keywords Polyvinyl alcohol · Cuprous oxide · Catalyst ·
2
pared using a novel and simple rout in aqueous medium at
room temperature under aerobic conditions. Initially, Cu(II)
ions were fixed in PVA matrix via complex formation with
OH groups, subsequently this PVA–Cu(II) macromolecu-
lar complex as a precursor reacted with ascorbic acid as a
Click reaction · 1,2,3-Triazoles
Introduction
reducing agent at pH = 12 to obtain the PVA–Cu O com-
An important goal in catalysis research is the develop-
ment of an efficient catalytic system that may be new, eco-
friendly, available, mild, and easily separable. Many efforts
have been directed toward the development of new catalytic
systems that increase activity and selectivity in chemical
synthesis and also minimize contamination in the reaction
conditions [1–5].
2
posite. The products were characterized using XRD, FE-
SEM, FTIR, and UV–Visible techniques. The as-prepared
PVA–Cu O composite was applied as an effective and recy-
2
clable heterogeneous catalyst for 1,3-dipolar cycloaddition
reactions between terminal alkynes and azides to prepare
1
,2,3-triazoles under an environmentally friendly reaction
conditions. The substituted triazoles were produced of vari-
ous aryl and benzyl azides using a catalytic amount of this
composite as efficient, stable, and air- and moisture-tolerant
catalyst in excellent yields in water at room temperature.
Cuprous oxide (Cu O) is a p-type semiconductor, and
2
recently, much attention has been paid to this and related
materials because of its potential applications in fields such
as solar cells [6], pigments [7], catalysts, and photocatalysts
[8]. Several successful methods have been reported for the
These PVA-stabilized Cu O particles could be recovered in
2
a facile manner from the reaction mixture and reused sev-
eral times without any loss of catalytic activity.
production of Cu O [9, 10]. Chemical reduction is a method
2
which is widely used for preparation of nanoparticles due
to its convenience and low cost, in which the preparation of
2
+
2+
Cu O from Cu salts involves the reduction of Cu ions
2
+
to Cu with a reducing agent. Ram et al. [11] have reported
a liquid-reduction method to make Cu O nanocrystals. They
2
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-015-0599-7) contains supplementary
material, which is available to authorized users.
used NaBH to reduce CuCl at 80–100 °C via ion exchange
4
2
2+
+
reaction of Cu → Cu → Cu . Also, the sequential reduc-
tion process CuO → Cu O → Cu has been reported by Pike
2
A. R. Hajipour (*) · F. Mohammadsaleh
Pharmaceutical Research Laboratory, Department of Chemistry,
Isfahan University of Technology, Isfahan 84156,
Islamic Republic of Iran
et al. [12] with CuO reducing completely to the intermediate
Cu O phase before further reduction to metallic copper using
2
temperature-programmed reduction of CuO nanoparticles.
e-mail: haji@cc.iut.ac.ir
Gou et al. [13] have prepared crystals of Cu O (200–450 nm)
2
via reduction of Cu(II) salt using ascorbic acid as reducing
agent and cetyltrimethylammonium bromide (CTAB) as sur-
A. R. Hajipour
Department of Neuroscience, Medical School, University
of Wisconsin, 1300 University Avenue, Madison 53706-1532,
WI, USA
factant. It is known that Cu O is chemically unstable in air
2
with respect to further oxidation to form CuO [14].
1
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J IRAN CHEM SOC
Polyvinyl alcohol (–[CH –CHOH]–n/PVA) is recog-
stirring. A green gel was produced when CuSO solution
2
4
nized as a water-soluble and crystalline vinyl polymer with
hydroxyl (–OH) groups bonded to carbon atoms in the back
bone of long-chain molecule.
was added and the mixture was heated at 90 °C overnight.
Afterward, the reaction mixture was cooled down to room
temperature and followed by reduction stage.
1,2,3-triazoles [15] are basically five-membered nitro-
gen heterocyclic compounds, which are commonly prepared
through Copper (Cu) Catalyzed Alkyne–Azide Cycloaddition
Reduction of PVA–Cu(II) complex by ascorbic acid
(
CuAAC) reaction that is the most well-documented exam-
Ascorbic acid (0.89 g) was dissolved in 10 ml of distilled
water and was added dropwise into the PVA–Cu(II) reac-
tion mixture at room temperature, under very slow stirring
[the mole ratio ascorbic acid:Cu(II) was 1:1]. After several
seconds, the orange-red particles appeared in the reaction
mixture. The resulting mixture was then aged in air at room
temperature, overnight. A reddish product obtained which
was then washed with distilled water and ethanol several
times to remove impurities and dried in air.
ple of “Click” reactions. The ‘Click Chemistry’ concept was
developed by Sharpless and co-workers [16] and CuAAC
reaction is done via [3 + 2] cycloaddition of terminal alkynes
with azides. 1,2,3-triazoles are associated with a wide range
of biological properties such as antiallergic [17], anticancer
[18], anti HIV [19], and antimicrobial activities [20].
Much efforts have been devoted to the synthesis of
,2,3-triazoles through cycloaddition reaction between
1
alkynes and azides using various Cu-based catalysts. Sarkar
et al. [21] used PVP-stabilized copper nanoparticles as cat-
alyst to synthesize 1,2,3-triazoles. Molteni et al. [22] have
reported the use of mixture of Cu/Cu-oxide nanoparticles
as catalyst for the ‘‘click’’ [3 + 2] cycloaddition between
azides and terminal alkynes. Zhang et al. [23] have pre-
General procedures for the PVA–Cu O-catalyzed [3 + 2]
2
cycloaddition of azides and phenylacetylene
Aryl azide as azide‑precursor
pared polyvinylpyrrolidone (PVP)-coated Cu O nanopar-
A solution of tetrabutylammonium bromide (0.016 g),
aryl azide (0.1 mmol), phenylacetylene (0.12 mmol), and
2
ticles as an efficient catalyst for azide–alkyne click chem-
istry in water at 37 °C. Wang and co-workers [24] have
reported catalytic system of Cu O/H O. They found that
PVA‒Cu O (0.005 g) in water (2 mL) was stirred at room
2
temperature and monitored by TLC until conversion of the
starting materials was complete. After completion, the reac-
tion mixture was decanted and washed with water followed
by dissolving in boiling ethanol and filtering to remove the
2
2
Cu O as a catalyst in water could be quite robust for the
2
azide–alkyne cycloaddition reaction.
In this work, we describe a facile, environmentally
benign, and simple route for the synthesis of Cu O particles
PVA–Cu O catalyst. The ethanol was evaporated and the
2
2
embedded in polyvinyl alcohol matrix using PVA–Cu(II)
composite as precursor and ascorbic acid as reducing agent
in aqueous medium at pH = 12 and room temperature. The
residue was crystallized by acetonitrile and water to give
the corresponding pure triazole.
as-prepared PVA–Cu O composite was used as an efficient
Multicomponent synthesis of 1,2,3‑triazoles
2
and recyclable heterogeneous catalyst for azide–alkyne
click reaction in water at room temperature.
Tetrabutylammonium bromide (0.016 g), benzyl bromide
or chloride (0.1 mmol), NaN (0.15 mmol), and phenylacet-
3
ylene (0.12 mmol) were stirred in water (2 mL) in a 10-ml
round-bottomed flask at room temperature. Sequentially,
Experimental
PVA‒Cu O (0.005 g) was added and the reaction mixture
2
General procedure for the synthesis of PVA-stabilized
was monitored by TLC until satisfactory conversion of the
starting materials. Then the reaction mixture was decanted
and washed with water followed by dissolving in boiling
Cu O particles
2
Preparation of PVA–Cu(II) complex
ethanol and filtering to remove the PVA/Cu O catalyst.
2
The ethanol was evaporated, and the residue was purified
by column chromatography (Hexane:EtOAc, 90:10) to give
the corresponding pure triazole.
One gram of polyvinyl alcohol (crystalline powder, high
molecular weight) was dissolved in 50 ml distilled water.
In another beaker, a solution of NaOH (4.0 g) in 25 ml
distilled water was prepared, and this solution was added
to the PVA solution under stirring and then the mix-
ture was heated at 70 °C for 3 h. After cooling, the solu-
tion of CuSO ·5H O (1.25 g) in 20 ml distilled water was
Results and discussion
The first objective of this work was the synthesis of the
4
2
added dropwise to the above PVA solution under vigorous
Cu O particles embedded in polyvinyl alcohol matrix. For
2
1
3

J IRAN CHEM SOC
this, initially, a complex of PVA–Cu(II) was prepared; sub-
sequently, this product was used as a precursor reacting
with ascorbic acid as a reductant to produce the PVA-stabi-
Chemical phase analysis was carried out by X-ray pow-
der diffractogram, and Fig. 1 demonstrates XRD patterns
of precursor PVA–Cu(II) complex and final product PVA–
lized Cu O crystals. During the reduction process the color
Cu O composite.
2
2
of reaction mixture changed from deep green to a reddish
appearance. Here the reduction stage takes place in PVA
network, and the copper(II) ions which are chelating with
The pattern of PVA–Cu(II) complex is fully amorphous;
therefore, it may be impossible to determine the constituent
phases of this compound. However, XRD pattern of PVA–
Cu O composite indicates diffractions of Cu O particles,
OH groups of PVA are reduced and PVA–Cu O composite
2
2
2
is formed.
containing six peaks that are clearly distinguishable and all
of them can be perfectly indexed to crystalline Cu O parti-
2
cles. The peak positions are in good agreement with those
for Cu O powder reported in references (JCPDS 5-667)
2
[25]. These results confirm that cupric ions in precursor
PVA–Cu(II) composite are reduced to crystalline Cu O
2
particles embedded in PVA matrix. The color changes dur-
ing reaction, provides substantial evidence for reduction of
the precursor (green) to the reddish product PVA–Cu O.
2
The phase morphology of PVA–Cu(II) complex and
PVA–Cu O composite was studied using field emission
2
scanning electron microscopy (FE-SEM). As shown in
Fig. 2, the FE-SEM micrograph of PVA–Cu(II) complex
clearly shows particles have been well distributed in the
PVA matrix. After the reduction stage, the surface morphol-
ogy of prepared PVA–Cu O composite compared with the
2
PVA–Cu(II) complex has changed.
Fig. 1 X-ray diffraction patterns of PVA–Cu (II) complex (a) and
PVA–Cu O composite (b). The color exchange of reaction mixture
Figure 3a and b shows FTIR spectra of the PVA–Cu(II)
2
has been shown in the image
complex and the PVA–Cu O composite. The both spectra
2
Fig. 2 FE-SEM images: PVA–Cu (II) complex (a, b), PVA–Cu O composite (c, d)
2
1
3

J IRAN CHEM SOC
Fig. 3 FTIR spectra of PVA–Cu(II) complex (a) and PVA–Cu O
2
composite (b)
Fig. 5 Sequential reactions of the reduction stage to produce the
Cu O particles
2
Table 1 Optimization of the catalyst loading
Fig. 4 Visible spectra of PVA–Cu(II) complex (a) and PVA–Cu O
Entry
PVA–Cu O (g)
Yield (%)a
2
2
composite (b)
1
2
3
4
0.001
0.003
0.005
0.01
35
60
95
96
exhibit characteristic bands of stretching and bending vibra-
tions of O–H, C–H, and C–O groups attributed to PVA. The
−
1
peak at 3,350 cm is the characteristic band of the O–H
stretching vibration of PVA. The bands observed at 1,640–
Reaction conditions 3-nitrophenyl azide (0.1 mmol), Phenylacetylene
(0.12 mmol), TBAB (0.016 g), H O (2 mL), PVA–Cu O, r.t, 1 h
2
2
−
1
1
,585 cm have been assigned to stretching mode of C=O
a
Isolated yield
and C=C groups in polymer chain. Two strong bands
−
1
observed at 1,422 and 1,347 cm have been attributed to
CH and combination frequency of (CH + OH) groups,
and the maximum absorption wavelength is observed at
491 nm. In the interpretation of the visible spectra it should
be noted that the size, morphology, and crystallinity of par-
ticles would affect the optical absorption peaks [29].
2
−
1
respectively [26]. The strong band at 1,114 cm has been
assigned to the stretching mode of C–O. In the spectrum
of PVA–Cu(II) complex, the absorption peaks at about 677,
−1
6
09 and 486 cm indicate the vibrations of Cu(II)–O bond
Cu O is a p-type semiconductor with a direct band gap of
2
[
27], and Fig. 3b represents Cu(I)–O vibration for PVA–
2.17 eV [30]. In this study, the calculated band gap for as-
−
1
Cu O composite at 625 cm [28].
prepared PVA–Cu O composite obtained about 1.88 eV. The
2
2
To investigate the optical properties, the visible spectra of
smaller band gap value of PVA–Cu O, as compared to that
2
PVA–Cu(II) complex and PVA–Cu O were examined and
of Cu O, is attributed to the effects of PVA and particle size.
2
2
are shown in Fig. 4. For measurements, solid pellet samples
of the isolated products were used in the 350–750 nm range.
The spectrum of PVA–Cu(II) complex shows two broad
absorption bands at 617 nm and 407 nm. However, the
Since PVA is a hydrophilic and environmentally benign
polymer, here we chose PVA as a stabilizing and dispers-
ing matrix to make a stable Cu O-polymer system that
2
can be well adapted to aqueous solutions and biological
environments.
PVA–Cu O has an intense absorption in the visible range
2
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J IRAN CHEM SOC
Table 2 Optimization of the phase-transfer additives
Embedding cupric ions in the PVA template makes dif-
fusion and migration of ions in solution become more dif-
ficult. Many researches have studied the molecular struc-
ture of PVA–Cu(II) complexes, and various structures have
been reported for these compounds [31]. Isolated Cu=O
molecules and/or –(Cu–O)n– chains in PVA network are
represented as possible structures.
Conversion (%)a
Entry
phase transfer additive
Tween 40
Time (h)
1
2
4
4
95
40
Here the reduction process carried out at pH = 12, and
3
4
4
3
95
95
in this system, the OH‾ is a key factor for the production
of Cu O particles. In the absence of OH‾, the XRD pattern
2
of reduction stage product showed diffractions of metallic
copper (Fig. S1).
For the Cu(I) ions, there are three possible reactions that
are competing with each other [32]:
+
Cu → Cu
0
(1)
(2)
(3)
+
Cu → Cu + Cu
0
2+
2
5b
–
4
80
+
−
2Cu + 2OH → 2CuOH → Cu O + H O
2 2
Reaction conditions 4-azidoacetophenone (0.1 mmol), Phenylacety-
lene (0.12 mmol), phase-transfer additive (50 mol %), H O (2 mL),
2
In the presence of OH–, the reaction corresponding to
Eq. (3) probably occurs, and only the reaction (3) leads
to the formation of cuprous oxide. Under our reduction
PVA–Cu O, r.t
2
a
TLC conversion
b
In the absence of phase-transfer additive
Table 3 Scope of aryl azides
and benzyl halides in the PVA–
Cu O-catalyzed click reactions
2
Entrya, b
1
Ar
Ar
-
Time (h)
Yield
%)
1
2
(
3-NO
2
C
6
H
4
1
1
95
95
95
94
94
85
72
90
90
70
68
85
70
85
Run 2
Run 3
1
Run 4
1
2
3
4
5
6
7
8
9
4-PhCOC
6
H
4
-
-
-
-
-
4
4-AcC
6
H
4
3
4-NO
2-NO
2
2
C
C
6
6
H
H
4
4
1.5
8
4-CNC
6
H
4
2.5
3
-
-
-
-
-
C
6
H
5
(X=Br)
(X=Cl)
C
6
H
5
3
a
4-NO
2-NO
3-BrC
2
C
6
H
4
(X=Br)
(X=Cl)
4
Isolated yield after
purification
1
1
0
1
2
C
6
H
4
9
b
TBAB (0.016 g) was added
6
H
4
(X=Br)
8
as phase-transfer additive
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J IRAN CHEM SOC
Table 4 Comparison of results for the preparation of 1-(4-nitrobenzyl)-4-phenyl-1H-1,2,3-triazole (Table 2, entry 9) through the various proce-
dures reported in the literatures
Entry Catalyst
Reaction conditions
Yield (%) [Ref.]
75 [33]
1
2
3
Copper–aluminum hydrotalcite (3:1)
/
/
/
phenylacetylene/CH CN/r.t/12 h.
3
Copper(I) Zeolite [Cu(I)–USY]
76 [34]
97 [35]
phenylacetylene/toluene/r.t/15 h.
phenylacetylene/neat/r.t/0.5 h/N atmosphere.
2
4
5
CuFe O4
4-Nitrobenzyl bromide/NaN /phenylacetylene/H O/70 °C, 5 h.
86 [36]
94 [37]
2
3
2
4-Nitrobenzyl bromide/NaN /phenylacetylene/PEG 400-H O (1:1)/Et N (1
3
2
3
equv.)/r.t/8 h.
6
Present work
4-Nitrobenzyl bromide/NaN /phenylacetylene/H O/TBAB/r.t/4 h.
85
3
2
Supplementary data, XRD patterns and 1 HNMR spectra of triazole products have been prepared and provided as a separate file
conditions, at pH = 12, the cuprous oxide particles were
In both protocols, phenylacetylene was used as alkyn-
precursor and as shown in Table 3, both aromatic and ali-
phatic azides worked well in these reactions, and the corre-
sponding 1,2,3-triazoles were isolated in good to excellent
yields.
obtained and no impurity of metallic copper was observed.
We also found that H O plays an important role in the
2
process of reduction in this system. When reduction stage
carried out in absolute ethanol as solvent in the presence
of ascorbic acid, no color exchange of reaction mixture
The PVA–Cu O composite was handled in air, and all
2
was observed; however, as soon as H O was added into the
the experiments were carried out under aerobic conditions.
It is important to investigate the stability of as-prepared
2
reaction mixture, the reddish particles appeared.
+
It can be understood that the formation process of Cu
PVA-protected Cu O particles. This product was applied as
2
ions and final Cu O particles composes the sequential reac-
tions demonstrated in Fig. 5.
a heterogeneous catalyst, and the reusability of catalyst is
an important topic in the field of heterogeneous catalysis
that is commonly attributed to its stability. We examined
2
We next examined the catalytic activity of as-prepared
PVA–Cu O in the model Huisgen reaction between termi-
the possibility of recovery and reuse of PVA–Cu O cata-
2
2
nal alkyne and organic azides, employing two protocols:
lyst using the CuAAC reaction between phenylacetylene
and 3-nitrophenyl azide as the substrates under the same
conditions reported in Table 3. After each run, the catalyst
was separated by decantation, followed by washing with
ethanol several times, and after drying, the catalyst was
reused directly for the next run without further purification.
The catalyst could be recovered and reused at least 3 times
without any drop in the product yield.
The present catalytic method was compared with the
some procedures reported for reaction of 4-Nitrobenzyl
azide with phenylacetylene in the literatures, and the results
have been shown in Table 4. The most important advan-
tages of this method are mild and eco-friendly reaction
conditions, availability, simplicity, short reaction times, and
good yields.
(1) the use of aryl azides as azide-precursor and (2) in situ
generation of aliphatic azides from the nucleophilic substi-
tution reaction of organic halide with sodium azide (multi-
component synthesis).
Different amounts of PVA–Cu O were applied for the
2
reaction mentioned in Table 1, to evaluate the catalyst load-
ings in this reaction system. A catalyst loading of 0.005 g
was found to be optimal with respect to yield and short
reaction times (Table 1, entry 3). The low catalyst concen-
tration usually led to a long period of reaction as increasing
the amount of catalyst shortened the reaction time.
In order to follow the majority of the principles of the
Green Chemistry [16] framework and eliminate the organic
solvents, we used H O as solvent in our reaction system.
2
Since reactants are organic phase in aqueous medium, a
phase-transfer additive was added to all reaction mixtures
to facilitate the movement of reactants. Among the differ-
ent phase-transfer additives tested such as choline chlo-
ride, tween 40, tetrabutylammonium bromide (TBAB), and
methyltriphenylphosphonium bromide, TBAB gave the
best result (Table 2).
Conclusions
In summary, PVA-stabilized Cu O particles were prepared
2
using PVA‒Cu(II) macromolecular complex as a precur-
sor and ascorbic acid as a reducing agent in the aqueous
1
3

J IRAN CHEM SOC
medium at pH = 12 and room temperature. The as-pre-
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2
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Acknowledgments We gratefully acknowledge the funding support
received for this project from the Isfahan University of Technology
(2008)
(IUT), IR Iran and Isfahan Science & Technology Town (ISTT), IR,
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sor and Green Chemistry Research (IUT) is gratefully acknowledged.
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