Copper (II) immobilized on aminated poly(vinyl chloride) as an efficient and retrievable catalyst for the CuAAC reaction in water under mild conditions

Article abstract of DOI:10.1007/s11164-017-3076-3

Ethylenediamine functionalized poly(vinyl chloride) (PVC–EDA) was conveniently synthesized by treatment of commercially available PVC powder with ethylenediamine solution. This amine-functionalized PVC was subsequently immersed in copper sulfate solution to form the supported copper catalyst [PVC–EDA–Cu(II)]. The as-prepared catalyst was well characterized with FT–IR, SEM, DSC, TGA, and ICP methods. It was found to catalyze efficiently the CuAAC reaction for the regioselective synthesis of 1,4-disubstituted-1,2,3-triazoles from benzyl halides, phenylacetylenes, and sodium azide in the presence of sodium ascorbate in water to give the corresponding products under mild and environmentally benign conditions. Various benzyl halides containing electron-withdrawing groups and electron-donating groups afforded the desired products in up to 98% yield. The present procedure has the merits of short reaction time and simple reaction work-up. Moreover, this catalyst can be recovered by simple filtration and reused for at least five consecutive runs without any loss of its catalytic activity.

Full text of DOI:10.1007/s11164-017-3076-3

Res Chem Intermed
DOI 10.1007/s11164-017-3076-3
Copper (II) immobilized on aminated poly(vinyl
chloride) as an efficient and retrievable catalyst
for the CuAAC reaction in water under mild conditions
Yizong Zhang1 Zhuan Zhang1 Yang Chen1 Yiqun Li1
•
•
•
Received: 25 December 2016 / Accepted: 12 July 2017
Ó Springer Science+Business Media B.V. 2017
Abstract Ethylenediamine functionalized poly(vinyl chloride) (PVC–EDA) was
conveniently synthesized by treatment of commercially available PVC powder with
ethylenediamine solution. This amine-functionalized PVC was subsequently
immersed in copper sulfate solution to form the supported copper catalyst [PVC–
EDA–Cu(II)]. The as-prepared catalyst was well characterized with FT–IR, SEM,
DSC, TGA, and ICP methods. It was found to catalyze efficiently the CuAAC
reaction for the regioselective synthesis of 1,4-disubstituted-1,2,3-triazoles from
benzyl halides, phenylacetylenes, and sodium azide in the presence of sodium
ascorbate in water to give the corresponding products under mild and environ-
mentally benign conditions. Various benzyl halides containing electron-withdraw-
ing groups and electron-donating groups afforded the desired products in up to 98%
yield. The present procedure has the merits of short reaction time and simple
reaction work-up. Moreover, this catalyst can be recovered by simple filtration and
reused for at least five consecutive runs without any loss of its catalytic activity.
Keywords Ethylenediamine Á Poly(vinyl chloride) Á Copper sulfate Á CuAAC Á
Triazole
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3076-3)
contains supplementary material, which is available to authorized users.
&
Yiqun Li
tlyq@jnu.edu.cn
1
Department of Chemistry, Jinan University, Guangzhou 510632, Guangdong province,
People’s Republic of China
123

Y. Zhang et al.
Introduction
1,2,3-Triazoles play a very important role in chemistry and medicine for the broad
spectrum of their biological and pharmaceutical properties such as antibacterial [1],
antiallergic [2], anticonvulsant [3], and anti-HIV activities [4] due to their unique
molecular structure. In 1963, Huisgen first reported the un-catalyzed 1,3-diplor
cycloaddition of azide and alkyne (known as Huisgen’s reaction) under thermal
condition to produce the approximate 1:1 mixture of 1,4- and 1,5-disubstituted
1,2,3-triazole without regioselectivity [5]. Following the Huisgen’s pioneering
work, Sharpless [6] and Meldal [7] independently in 2001 reported the copper-
catalyzed azide-alkyne cycloaddition to afford exclusive 1,4-disubstituted triazole
with high regioselectivity under mild condition. This copper-catalyzed version of
azide-alkyne cycloaddition also is called CuAAC or click reaction. Generally,
Cu(I) salts have been used as homogeneous catalytic systems [8]. Although the
Cu(I) homogeneous system has been widely used in this reaction, the Cu(I) species
are prone to Cu(0) and Cu(II) via disproportionation process, thereby decreasing of
their activity. Thus, there is need to protect and stabilize the active catalytic
Cu(I) species during the course of CuAAC. Various nitrogen- [9] or phosphorus-
[10] based ligands have been documented in the protection of the Cu(I) species from
oxidation and disproportionation, as well as enhancing its catalytic activity [11].
Another simple and practical alternative method to generate Cu(I) species is the
in situ reduction of copper sulfate with excessive sodium ascorbate (3–10-fold
excess) in reaction mixture. The excessive sodium ascorbate can efficiently render
the reaction much less susceptible to oxygen and water, and thus such reactions can
process smoothly in air conditions. Apart from these, homogeneous catalysis suffers
from many drawbacks such as cumbersome separation and recycling of the
expensive catalyst, metal contamination of products, and environmental pollution.
Alternatively, heterogeneous catalysts can mitigate these problems. In the last few
years, a variety of heterogeneous catalytic systems were prepared by immobilization
of copper ions onto different solid supports such as silica [12], alumina [13], zeolite
[14], polymer [15], and organic/inorganic hybrid [16]. More recently, several
articles have thoroughly reviewed the progress on heterogeneous catalysis in
CuAAC reactions [17, 18]. Although this class of heterogeneous catalysts has
operated well in most cases, they still suffer from one or more disadvantages such as
multiple-step tedious preparation of support matrix, low loading amount of catalyst,
lower catalytic activity, high leaching of catalyst, and low thermal stability.
Therefore, developing a simple and efficient heterogeneous copper catalyst with
high stability, low leaching, high loading, low costing, and a cleaner catalytic
system is still an interesting challenge.
The increasing attention for environmental protection has led both the modern
academic and industrial communities to develop chemical processes using nontoxic
reagents, solvents, and catalysts to obtain products with maximum yield and
minimal cost. Water used as a reaction media has a strong impact on the rate of
some organic transformations due to its hydrophobic, hydrogen-bonding, and
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Copper (II) immobilized on aminated poly(vinyl chloride)…
polarity effects [19]. Thus, there has been a growing recognition that water is an
attractive medium for many organic reactions.
PVC is one of the most versatile polymers with good flexibility, chemical
resistance, film forming property, and cost effectiveness; it is also easy to modify.
PVC functionalized with a functional group can be easily achieved by a
displacement reaction without chloromethylation, which makes it a low cost and
practical application as new materials for heterogeneous catalysts. As part of our
continuing interest in heterogeneous palladium catalyzed carbon–carbon cross-
coupling reactions, our group recently demonstrated that the Pd nanoparticles
immobilized on functionalized PVC is an efficient catalyst for Heck [20] and Suzuki
[21] couplings. However, as far as we know, no CuAAC reaction catalyzed by a
functionalized PVC-anchored Cu(II) complex has yet been reported. In order to
improve further the practicality and efficiency of the CuAAC reaction, we report
here the preparation and characterization of a new ethlylenediamine-functionalized
PVC anchored Cu(II) complex [PVC–EDA–Cu(II)], and its performance on
CuAAC in water under aerobic conditions. Compared with other supported copper
catalysts, the ease of preparation, low cost, its stability toward air and moisture, its
compatibility with various substrates, and high catalytic activity make it an ideal
catalyst for these reactions.
Experimental section
Chemicals and characterizations
FT–IR spectra were recorded on a Bruker Equinox-55 spectrometer as KBr pellets
in the range of 4000–400 cm-1.1H NMR spectra were obtained with a 300 MHz
Bruker Avance instrument with CDCl3 as solvent and TMS as internal standard. The
elemental palladium content of polymeric catalysts was determined by Perkin Elmer
Optima 2000DV inductively coupled plasma (ICP). Shimadzu AA6680 atomic
absorption spectrometer (AAS) was used to measure residual metal ion concentra-
tions. Scanning electron microscopy (SEM) and energy-dispersive X-ray spec-
troscopy (EDS) were performed with a Philips XL 30ESEM instrument.
Thermogravimetric analysis (TGA) was operated on a Netzsch STA449F3 in the
range of 25–700 °C under nitrogen atmosphere.
Poly(vinyl chloride) powder (MW 4800) is industrial grade. All chemicals are
commercial available and used as received without further purification.
Preparation of PVC–EDA–Cu(II) catalyst
PVC powder (20 g, 310 mmol Cl) and EDA (80 mL, 1200 mmol) were added into
a round-bottomed flask equipped with a magnetic stirrer bar and stirred at 80 °C for
48 h. The resulting solid was separated from the mixture and washed thoroughly
with deionized water then dried in air to constant weight.
The resulting reddish-brown PVC–EDA powder (2 g) was immersed completely in
CuSO4 aqueous solution (20 mL, 2 mmol), and the resulting mixture was stirred for
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Y. Zhang et al.
48 h at rt. The resulting black powder anchored copper was filtered and successively
washed with deionized water and ethanol to afford the PVC–EDA–Cu(II) as black
powder. The Cu content was determined to be 1.692 mmol/g by ICP-AES.
Experimental procedure for copper sorption
Twenty grams of copper solution (0.1 mmol/mL) at neutral condition was mixed
with 2.0 g of PVC–EDA in an agitated system at room temperature. Five milliliter
samples were withdrawn at specified times and filtered through a 1.2 mm filtration
membrane. The Cu2? concentration was determined with AAS.
General procedure for the catalytic CuAAC reactions
A mixture of sodium azide (1.2 mmol), sodium ascorbate (0.1 mmol), alkyne
(1.1 mmol), benzyl compound (1.0 mmol), PVC–EDA–Cu(II) (0.015 g, mol%),
and water (3 mL) was vigorously stirred at 70 °C under for specific time under
aerobic condition. The reaction process was monitored by TLC. Upon completion of
the reaction, the mixture was filtered to recover the catalyst. The catalyst was
washed with water and ethanol, dried, and stored for subsequent runs. The filtrate
was extracted with ethyl acetate (10 mL 9 3) and washed with brine. After
removing the organic solvent using a rotary evaporator, the crude product was
further purified by recrystalization using ethanol as the solvent to afford the pure
1
product. All of the products were characterized by H NMR and melt point, which
are consistent with those data reported in the literature.
Results and discussion
Equilibrium time for the preparation of catalyst
The time needed to reach the equilibrium was evaluated for Cu(II) after
impregnating 2.0 g of PVC–EDA in aqueous solutions of the copper sulfate
(0.1 mmol/mL), and the metal remaining in the aqueous phase at different times was
measured by AAS. The results are presented in Fig. 1. The process can be divided
into three phases: (a) a very fast sorption within the 4 h (almost linear plot), (b) a
slower mass transfer step (between 4 and 40 h), and (c) the equilibrium step (after
40 h). This results suggested that the specific time (48 h) for the preparation of
PVC–EDA–Cu(II) ensures the adsorption equilibrium of Cu2? can be reached.
Synthesis and characterization of PVC–EDA–Cu(II)
PVC-supported ethylenediamine (PVC–EDA) was prepared by treatment of
commercially available PVC, which was reported by us previously [21]. Complex-
ation of the as-prepared PVC–EDA with copper sulfate solution provide the desired
PVC–EDA–Cu(II) catalyst, respectively. The synthetic pathway is depicted as
Scheme 1.
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Copper (II) immobilized on aminated poly(vinyl chloride)…
Fig. 1 Sorption kinetic of copper(II) ion
NH2
H2N
n
n
Cl
Cl
Cl
Cl
NH NH
NH2 H2N
n
Cu2+
NH NH
Cu2+
NH2 H2N
Scheme 1 Schematic route for preparation of PVC–EDA–Cu(II)
To verify formation of the ethylenediamine-functionalized PVC and its
corresponding Cu(II) complex, more detailed characterization of the catalyst is
conducted by FT–IR, ICP–AES, SEM, EDS, TGA, and DSC techniques
(Scheme 2).
FT–IR spectroscopy is an important tool to investigate the polymer structure.
Figure 2 shows the FT–IR spectra for the PVC, PVC–EDA, and PVC–EDA–Cu(II).
As can be seen from curve (A), the adsorption peaks at 608 and 688 cm-1 was
attributed to the C–Cl stretching vibration. By comparing with the FT–IR spectra of
curve (A) and curve (B), the characteristic peaks of C–Cl become weaker in the
spectra of PVC–EDA owing to the introduction of EDA. As can be seen from FT–
IR of curve (B), the adsorption at 3236 cm-1 is assigned to the stretching vibration
of N–H, the adsorption at 1639.6 cm-1 is assigned to the stretching vibration of N–
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Y. Zhang et al.
PVC-EDA-Cu(II)
sodium
N
N
ascorbate
N
R2
R1
H
R1
H+
R2
Cu EDA-PVC
H
PVC-EDA-Cu(I)
R1
Cu EDA-PVC
PVC-EDA-Cu(I)
A
N
N
N
R1
Cu(I)-EDA-PVC
R2
N
C
R1
Cu PVC
N
N
R2
N
Cu(I)-EDA-PVC
Cu EDA-PVC
R2
+
X
N
R2 N3
N
PVC-EDA-Cu(I)
Cu PVC
R1
NaN3
R1
Cu PVC
B
Scheme 2 Proposed mechanism for the PVC–EDA–Cu(II) catalyzed CuAAC reaction
Fig. 2 FT–IR spectra of PVC (a), PVC–EDA (b), PVC–EDA–Cu(II) (c)
H of secondary amine, and 1106 cm-1 is assigned to the bending vibration of N–H
of primary amine, respectively, which negatively shifts to 1627 cm-1 and
1091 cm-1 , respectively, in curve (C). This is probably caused by the coordination
of Cu(II) with the nitrogen atoms of amine.
SEM images of the fresh catalyst and reused catalyst are shown in Fig. 3. They
clearly show that the fresh catalyst has a rough surface, which facilitates the contact
123

Copper (II) immobilized on aminated poly(vinyl chloride)…
Fig. 3 SEM images of fresh catalyst (left) and recovered catalyst (right)
of catalyst and substrates. It is obvious that the surface of the recovered catalyst
after the 3rd run is not as rough as the fresh catalyst.
To examine the existence of elements in the catalyst, the energy-dispersive X-ray
spectroscopy (EDS) was also conducted. The presence of Cu is clearly shown in the
EDS spectrum, together with other elements including carbon and chloride (Fig. 4)
associated with SEM analysis. As a result, it is reasonable to assume that Cu(II)
species is anchored on the PVC–EDA matrix.
We conducted thermogravimetric analysis (TGA) of PVC–EDA–Cu(II) to
characterize the thermostability of the catalyst (Fig. 5). The initial weight loss
happened around 100 °C, which probably was due to the loss of water. As the
temperature increased, the second weight loss of catalyst occurred at 280 °C due to
decomposition, leaving a residue of 60% of the initial weight of the sample. This
result showed that the catalyst had good stability up to 280 °C. The curve of DSC
Fig. 4 EDS data for the fresh
catalyst
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Y. Zhang et al.
Fig. 5 TGA and DSC of PVC–EDA–Cu(II)
also indicated that the two strong peaks at around 100 and 280 °C were related to
the loss of water and the decomposition, respectively.
Performance of catalyst for the CuAAC reactions
To obtain optimal conditions, the influence of the amount of catalyst and
temperature on the CuACC reaction was explored using the model reaction of
phenylacetylene, benzyl bromide, and sodium azide. The results are list in Table 1.
The catalyst has a significant effect on the product yields. The model reaction
was explored with the amount of catalyst ranging from 1.5 to 7.0 mol% (Table 1). It
was found that this reaction could be conducted efficiently even with a low amount
of the catalyst (1.5 mol% Cu) with 75% yield at 70°C (Table 1, entry 1). The yields
increased from 75 to 94% by increasing the amount of catalyst from 1.5 to
5.0 mol% (Table 1, entries 1–4). Further increasing the amount of catalyst to
7.0 mol% had no apparent effect on the product yield (Table 1, entry 5). Therefore,
2.5 mol% of the catalyst was enough to push the reaction to completion. The
reaction temperature has great influence on the model reaction. It can be seen from
Table 1 that decreasing the temperature from 70 to 25 °C led to lower yields and
prolonged time (Table 1, entries 6–7).
Once optimized reaction conditions were reached, the coupling reactions of
sodium azide, a variety of benzyl halides, and phenylacetylenes were performed to
explore the general effectiveness of the PVC–EDA–Cu(II) catalyst. The results are
listed in Table 2. As shown in Table 2, the reaction proceeds well and affords the
desired products in good yields. Because of the high reactivity of aryl bromides, the
123

Copper (II) immobilized on aminated poly(vinyl chloride)…
Table 1 Opitimization of reaction conditions
Entry
Amount of catalyst (mol% of Cu)
T (°C)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
1.5
2.5
3.0
5.0
7.0
2.5
2.5
70
70
70
70
70
25
50
120
70
75
93
93
94
95
62
75
60
50
50
[120
90
Reaction conditions: benzyl bromide (1.0 mmol), phenylacetylene (1.1 mmol), sodium ascorbate
(0.020 g, 0.1 mmol), sodium azide (1.2 mmol), catalytic amount of PVC–EDA–Cu(II), water (3 mL)
a
Isolated yields
benzyl bromides gave higher yields than that of benzyl chlorides. The reactions of
electron-rich benzyl halides proceeded well and gave high yields for the
corresponding products compared with electron-deficient ones. However, the
substituent effects in phenylacetylene had not obviously influenced the reaction time
and the yield of products.
Heterogeneity of the catalysis
To determine the heterogeneous nature of the catalytic system, we have also
performed a hot filtration test using model reaction. During this test, the catalyst was
removed from the reaction mixture by filtration after 5 min using a hot filtration,
and the filtrate was then allowed to react continually for 2 h under the optimal
condition. The results show that after removal of the catalyst, the reaction did not
proceed, indicating that no catalytically active Cu2? remained in the filtrate. This
finding suggests that the catalyst is characteristically heterogeneous in nature.
Recycling of the catalyst for the CuAAC reaction
For practical applications, reusability of catalyst is another important parameter. In
order to make this issue clear, catalytic recycling was performed using the model
reaction. The results are listed in Table 3. After completion of the reaction, the
catalyst was easily recovered by filtering and directly used in the next run after
washing with water, ethanol, and drying. The results demonstrated that this catalyst
can be reused at least five times with comparative loss of its activity. The SEM of
reused catalyst showed that the changes of surface and size of particles of the
catalyst, which probably decrease the activity of catalyst.
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Y. Zhang et al.
Table 2 The CuAAC reaction based on the different benzyl halides and alknynes
Entry
R1
R2
X
Time (min)
Yielda (%)
Product
1
H
Ph
Br
Cl
Cl
Cl
Cl
Cl
Br
Cl
Cl
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Br
Br
Br
Br
70
100
110
120
165
155
240
180
210
120
120
150
270
120
140
280
210
150
300
120
160
200
93
90
82
80
86
85
87
78
80
84
90
89
85
98
88
81
77
86
94
88
85
83
a
a
b
c
d
e
f
2
H
Ph
3
4-F
Ph
4
3-F
Ph
5
4-Cl
3-Cl
4-NO2
4-CH3
3-CH3
4-CH3
H
Ph
6
Ph
7
Ph
8
Ph
g
h
g
i
9
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
Ph
4-Me-C6H4
4-Me-C6H4
4-Me-C6H4
4-Me-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
4-MeO-C6H4
2-Py
4-Br
4-NO2
4-CH3
H
j
k
l
m
n
o
m
p
q
r
4-F
3-F
H
4-CH3
H
4-Br
4-CN
2-Py
2-Py
s
Reaction conditions: benzyl bromide (1.0 mmol), phenylacetylene (1.1 mmol), sodium ascorbate
(0.020 g, 0.1 mmol), sodium azide (1.2 mmol), PVC–EDA–Cu(II) (2.5 mol%), and water (3 mL) at
70 °C
a
Isolated yields
Plausible mechanism for CuAAC reaction
Many explorations on prediction of the mechanism of the CuAAC have been
reported. On the basis of a dinuclear copper intermediate reported by Fokin et al.
[22], we propose the possible mechanism for the regioselective synthesis of 1,4-
disubstituted-1,2,3-triazole from benzyl halide, phenylacetylene, sodium azide, and
sodium ascorbate in the presence of PVC–EDA–Cu(II).
Initially, the PVC–EDA–Cu(I) species, generated in situ by reduction of PVC–
EDA–Cu(II) and sodium ascorbate, reacts with alkyne to form an acetylide–copper
complex (A). Secondly, the Cu(I)-activated intermediate A interacts with the azide
activated by PVC–EDA–Cu(I) to produce the dinuclear copper intermediate (B).
123

Copper (II) immobilized on aminated poly(vinyl chloride)…
Table 3 The reusability of PVC–EDA–Cu(II) for repeat experiments
Entry
Yielda (%)
Time (min)
1
2
3
4
5
93
89
86
80
77
70
90
95
100
120
Reaction conditions: benzyl bromide (1.0 mmol), phenylacetylene (1.1 mmol), sodium ascorbate
(0.1 mmol), sodium azide (1.2 mmol), PVC–EDA–Cu(II) (2.5 mol%), and water (3 mL) at 70 °C
a
Isolated yields
Next, rearrangement of intermediate B leads to the formation of the copper
metalated triazolide (C) and releases PVC–EDA–Cu(I). Finally, the intermediate C
captures a proton to produce the target triazole and regenerate the PVC–EDA–
Cu(I) complex. In addition, PVC–EDA in this catalytic cycle serves as an efficient
ligand to coordinate to the Cu species on the catalyst.
Conclusions
In conclusion, PVC–EDA–Cu(II) was conveniently synthesized in a simple and
efficient way by the treatment of CuSO4 with PVC–EDA, which was achieved by a
substitution reaction of PVC and EDA. It was proven that PVC–EDA–Cu(II) has
good performance to catalyze the CuAAC reaction with excellent yields. Moreover,
the catalyst can be conveniently separated and recovered from the reaction system,
and it can be reused at least five times.
Acknowledgements We are grateful to the National Natural Science Foundation of China (Grant Nos.
21372099 and 21072077) and the Natural Science Foundation of Guangdong Province (Grant Nos.
10151063201000051 and 8151063201000016) for financial support.
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