Ce(III) immobilized on aminated poly(vinyl chloride): high-performance synergistic bifunctional acid–base catalyst for one-pot synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles

Article abstract of DOI:10.1007/s11164-018-3425-x

A bifunctional acid–base catalyst of Ce^III immobilized on ethylenediamine (EDA)-grafted poly(vinyl chloride) (PVC) has been prepared by a simple approach. First, PVC was treated with EDA to afford aminated poly(vinyl chloride) (PVC–EDA). Therea

Full text of DOI:10.1007/s11164-018-3425-x

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3425-x
Ce(III) immobilized on aminated poly(vinyl chloride):
high-performance synergistic bifunctional acid–base
catalyst for one-pot synthesis of 1,4-dihydropyrano
[
2,3-c]pyrazoles
1
1
1
1
Tianzhu Zhang
•
Junbin Zhou
•
Yang Chen
•
Yiqun Li
Received: 18 December 2017 / Accepted: 2 April 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
III
Abstract A bifunctional acid–base catalyst of Ce immobilized on ethylenedi-
amine (EDA)-grafted poly(vinyl chloride) (PVC) has been prepared by a simple
approach. First, PVC was treated with EDA to afford aminated poly(vinyl chloride)
(
obtain a bifunctional catalyst (PVC–EDA–Ce ) in which a harmonious coexistence
PVC–EDA). Thereafter, the PVC–EDA were used in the mobilization of CeCl to
3
III
3
of Lewis acid (Ce ) sites and Lewis base (amine) moieties on PVC was achieved.
?
III
The obtained PVC–EDA–Ce complex was characterized by EA, ICP–AES, FT-
IR, SEM, EDS, TGA, and DTG techniques. The as-prepared catalyst can efficiently
catalyze the one-pot four-component reactions of aromatic aldehydes, malononi-
trile, ethyl acetoacetate, and hydrazine in ethanol under mild conditions to afford
polyfunctionalized 1,4-dihydropyrano[2,3-c]pyrazoles with excellent yields. The
III
high catalytic performance of the PVC–EDA–Ce bifunctional catalyst is attrib-
III
uted to a synergistic effect of Lewis acid sites (Ce ) and Lewis base sites (amino
moieties). The significant features of the present protocol are environmentally
benign, simple operation, short reaction time, high yields, and without chromato-
graphic separation. Moreover, the catalyst can easily be separated by simple fil-
tration and reused for five runs without obvious decline or losing its catalytic
activity.
Keywords Poly(vinyl chloride) Á Cerium trichloride Á Acid–base bifunctional
catalyst Á Multicomponent reaction Á Dihydropyrano[2,3-c]pyrazole
&
Yiqun Li
tlyq@jnu.edu.cn
1
Department of Chemistry, Jinan University, Guangzhou 510632, China
123

T. Zhang et al.
Introduction
Multicomponent reactions (MCRs) are one-pot processes in which at least three or
more readily available reactants react to construct highly functionalized target
molecules which incorporate the majority of the atoms in the starting materials.
Simplicity, high selectivity, greater bond-forming efficiency, and high atom
economy are some of the main advantages of MCRs [1–6]. Hence, the development
of MCRs for the synthesis of heterocyclic compounds has attracted significant
interest from industry and academia. 1,4-dihydropyrano[2,3-c]pyrazoles are very
important heterocycles due to their biological activity and pharmaceutical interest.
Many of them have been found for applications such as a potential inhibitor of a
human Chk1 kinase, [7] as well as antitumor, [8] analgesic, [9] antibacterial, [10]
and anti-inflammatory agents [9–11]. Due to the biological activity of the 1,4-
dihydropyrano[2,3-c]pyrazole derivatives, various MCRs including three-compo-
nent reactions (3-CRs), four-component reactions (4-CRs), and even five-compo-
nent reactions (5-CRs) have been reported. 3-CRs of carbonyl compounds,
malononitrile, and pyrazolin-5-ones using both a basic catalyst like ionic liquid
(
(
[
[bmim][OH]), [12] ethylenediammonium diformate, [13] 4-dimethylaminopyridine
DMAP), [14] N-methylmorpholine, [15] triethanolamine, [16] Me N, [17] Et N,
3
3
17] electrogenerated base, [17, 18] etc. and an acidic catalyst including BF bonded
3
nano-Fe O , [19] [nano-Fe O @SiO @(CH ) -Imidazole-SO H]Cl, [20] NH H
4 2-
3
4
3
4
2
2 3
3
PO /Al O , [21] etc. have been reported. Among the recent developments and
4
2 3
expanding interest in MCRs, the use of the catalyst in 4-CRs of aldehydes,
malononitrile, ethyl acetoacetate, and hydrazine hydrate have been developed. The
basic catalysts have included ionic liquid [DBU][Ac], [22] Amberlyst A , [23]
2
1
Et N, [24] and TBD-anchored mesoporous silica nanoparticles [25], while the acidic
3
catalysts have included ionic liquid [DMDBSI]Á2HSO , [26] isonicotinic acid, [27]
4
and morpholine triflate [28]. More recently, we have established an overall 5-CRs
incoorporating Suzuki coupling of 4-bromobenzaldehyde and arylboronic acids with
4
-CRs of ethyl acetoacetate, malononitrile, and hydrazine hydrate to synthesize 1,4-
dihydropyrano[2,3-c]pyrazoles [29]. Although these reported MCRs have some
useful synthetic procedures, several limitations, including high homogeneous
catalyst loading, prolonged reaction time, harsh reaction conditions, poor recycla-
bility, low yields, and side reactions, have still remained. For example, the
employment of a basic catalyst for this type of MCRs has the potential to
accomplish various side reactions, such as the Aldol reaction and the Cannizzaro
condensation, to produce unexpected by-products. As a result, the development of
simple and efficient MCRs without the use of hazardous solvents and toxic catalysts
is highly desirable in the area of synthetic chemistry. Recently, heterogeneous
catalysis has appeared as an area of very rapid growth for organic synthesis because
it has the advantages of recoverability and recyclability of the catalyst, and is benign
to the environment.
Poly(vinyl chloride) (PVC) is one of the cheapest and most commonly used
synthetic polymers in the world. It is well known that PVC is easily functionalized
by nucleophilic groups via
a
displacement reaction directly without
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
chlorormethylation, which means it can be used as a polymeric matrix for a
heterogeneous catalyst [30]. Nevertheless, rare examples of using PVC as a support
have been reported in the literature. Recently, our group has reported the
preparation of Pd nanoparticles supported on 2-aminoethanol functionalized-PVC
[31] or a PVC-derived Schiff’s base [32] and their application in the Heck and
Suzuki reactions.
The above observations of MCRs promoted by acidic catalysts and/or basic
catalysts for the synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles has encouraged us
to envisage a single catalyst possessing both Lewis acid sites and Lewis base sites,
which can activate electrophiles and nucleophiles, respectively, to catalyze 4-CRs as
well as having many practical benefits like a synergic effect and so on. For this
purpose, we advocated a novel heterogeneous bifunctional acid–base PVC–EDA–
III
III
Ce catalyst, with amine-ligated Ce as the Lewis acid and amine moieties as the
Lewis base, for the 4-CRs to the synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles. To
III
the best of our knowledge, a PVC–EDA–Ce catalyst has not yet been used for this
purpose.
Experimental
Chemicals and characterizations
FT-IR spectra were recorded on a Nicolet 6700 spectrometer as KBr pellets in the
-1 1
range 4000–400 cm . H NMR spectra were obtained with a Bruker Advance
instrument (300 MHz) with DMSO-d as solvent and TMS as an internal standard.
6
The elemental Ce content of polymeric catalyst was determined by a Perkin Elmer
Optima 2000DV inductively coupled plasma (ICP). Scanning electron microscopy
(
SEM) was performed with a Philips XL 30ESEM instrument. X-ray photoelectron
spectroscopy (XPS) was performed on a Kratos Axis Ultra DLD spectrometer using
an Al X-ray source.
All chemicals are commercially available and used as received without further
purification. The PVC powder (MW 4800) was of industrial grade.
III
Preparation of PVC–EDA–Ce catalyst
Commercially available PVC powders (20 g, 310 mmol of Cl) were dispersed in
EDA (80 mL, 1200 mmol). The mixture was heated at 80 °C with constant stirring
for 48 h to afford the corresponding reddish-brown aminated PVC resin. The
resulting solid was filtered, washed thoroughly with deionized water until the filtrate
was colorless and pH value was neutral, then the powder was dried at 60 °C to a
constant weight.
The resulting reddish-brown PVC–EDA powder (2 g) was immersed completely
in CeCl Á7H O (745 mg, 2 mmol) aqueous ethanolic solution (20 mL), and the
III
3
2
resulting mixture was stirred for 48 h at room temperature. The resulting black
powder-anchored Ce was filtered and successively washed with deionic water and
III
ethanol and dried at 60 °C to a constant weight to afford the PVC–EDA–Ce as a
123

T. Zhang et al.
III
black powder. The N contents in the PVC–EDA and PVC–EDA–Ce by elemental
analysis were 6.96 and 6.54 mmol/g, respectively. The Ce content of the PVC–
III
EDA–Ce was determined to be 0.401 mmol/g by ICP–AES. The molar ratio of N
III
to Ce in the PVC–EDA–Ce was 16.3:1.
The synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles catalyzed
III
by PVC–EDA–Ce
A mixture of carbonyl compounds (1 mmol), malononitrile (1 mmol), ethyl
acetoacetate (1 mmol), and 80% hydrazine (1 mmol) was stirred in ethanol
(
5 mL) in a 50-mL round-bottomed flask at 78 °C (reflux temperature) in the
III
presence of PVC–EDA–Ce (40 mg, 1.6 mol%) for a specific time. The progress of
the reaction was monitored by TLC. Upon the completion of the reaction, hot
ethanol was added and the mixture was filtered to recover the catalyst. The crude
product was obtained after removing the ethanol with a rotary evaporator, and was
further purified by recrystallization with absolute ethanol to obtain the pure product.
1
All the products were characterized by melting point and H-NMR, which were
consistent with the data reported in the literature.
Results and discussion
III
Synthesis and characterization of PVC–EDA–Ce
Ethylenediamine-functionalized PVC (PVC–EDA) was obtained by the treatment of
commercially available PVC with EDA, which has previously been reported by us
[
32]. Complexation of the as-prepared PVC–EDA with CeCl Á7H O solution
3
2
III
provides the desired PVC–EDA–Ce catalyst. The synthetic pathway is depicted in
Scheme 1.
III
Then, PVC–EDA–Ce was characterized by FT-IR, XRD, SEM, EDS, TGA,
DTG, and XPS techniques.
III
Scheme 1 Schematic route for the preparation of PVC–EDA–Ce
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
FT-IR spectroscopy is an important tool to investigate the catalyst structure. The
III
FT-IR spectra of PVC (a), PVC–EDA (b), and PVC–EDA–Ce (c) are shown in
-
1
Fig. 1. The adsorption peaks located at 611.3 and 692.3 cm in curve (a) are
assigned to the C–Cl stretching vibration of PVC. As can be seen from curve (b),
-
peaks at 1656.6 cm are attributed to the stretching vibration of secondary N–H
1
-
stretching. These peaks negatively shift to 1639.2 cm in curve (c), implying the
1
III
coordination of Ce with the nitrogen atoms of the amine. The characteristic peaks
of C–Cl become weaker in curve (b) and (c) owing to the chlorine atoms on the
PVC molecular chain being replaced by EDA.
SEM images of the fresh and reused catalyst are depicted in Fig. 2. The rough
surface of the catalyst can be seen, which facilitates the contact of the catalyst and
the substrate. It can be clearly observed that the recovered catalyst after the 5th run
is broken into pieces might be caused by the stirring during the course of the
reaction.
Energy dispersive X-ray spectroscopy (EDS) analysis provides local information
of various elements. The existence of the metallic elements Ce and non-metallic
III
elements C and Cl in the PVC–EDA–Ce is clearly shown by the EDS (Fig. 3)
associated with the SEM analysis. As a result, it is reasonable to assume that the
III
Ce species was successfully anchored on the PVC–EDA.
The thermal stability of the catalyst has a significant effect on its catalytic
activity and recyclability because this MCR is usually performed under heating
conditions. Figure 4 shows the TGA (solid line) and DTA (dashed line) curves of
III
PVC–EDA–Ce under N atmosphere varying from 25 to 900 °C. As can be seen
2
from the plot, there is three-step thermal decomposition. The first weight loss was
observed around 100 °C responding to the removal of loosely physically adsorbed
water. Subsequently, the weight loss which began near 200 °C was likely due to the
III
Fig. 1 FT-IR spectra of PVC (a), PVC–EDA (b), PVC–EDA–Ce (c)
123

T. Zhang et al.
Fig. 2 SEM images of the fresh catalyst (a), and the recovered catalyst after 5 runs (b)
Fig. 3 EDS data for the fresh catalyst
releasing of HCl from the damaged PVC. The final weight loss occurred around
3
80 °C and can be attributed to the decomposition of the catalyst. TGA analysis
III
indicates that PVC–EDA–Ce is stable up to 200 °C. The thermal stability of
PVC–EDA–Ce can satisfy the thermal conditions in this MCR. The DTG curve of
III
III
PVC–EDA–Ce shows three peaks at around 100, 200 and 380 °C, which are
assigned to the loss of adsorbed water, the releasing of HCl, and the decomposition
of catalyst, respectively.
X-ray photoelectron spectroscopy (XPS) measurements were performed to
III
confirm the chemical oxidation state of the fresh and used PVC–EDA–Ce catalyst.
Figure 5a and b shows the binding energy located at 898.4, 901.1, 903.5, and
9
07.7 eV belonging to the Ce(III) oxidation state, [33] and the peak at around
17 eV without overlap with any other peaks, which is a fingerprint of the Ce(IV)
9
oxidation state [34]. The results of the XPS analysis suggest that a mixture of
Ce(III) and Ce(IV) oxidation states exists in the catalyst, which may be caused by
Ce(IV) impurities trapped in the CeCl or the oxidation of a small part of Ce(III)
3
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
III
Fig. 4 TGA and DTG curves of PVC–EDA–Ce
III
Fig. 5 XPS spectra for Ce3d (a) in fresh PVC–EDA–Ce and Ce3d (b) in the used catalyst
ions to Ce(IV) in the process of the catalyst preparation in water under air. The lack
of changes in the chemical oxidation states of cerium in the fresh (Fig. 5a) and used
catalyst (Fig. 5b) indicates that the reaction has no effect on the valence of the
catalyst.
III
Performance of PVC–EDA–Ce on multicomponent reaction
for the synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles
In search of the optimal conditions for the synthesis of 1,4-dihydropyrano[2,3-
c]pyrazoles, the influences of solvent, base, temperature, and amount of catalyst
were carefully explored using the 4-CR of 4-chlorobenzaldehyde, malononitrile,
ethyl acetoacetate, and hydrazine as a model reaction (Scheme 1). The results are
summarized in Table 1.
123

T. Zhang et al.
Table 1 Optimization of the reaction
a
Entry
Solvent
Catalyst (mol% of Ce)
T (°C)
Time (min)
Yield (%)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
PVC-EDA-Ce (1.6)
III
PVC–EDA–Ce (1.6)
1
2
3
4
5
6
7
8
9
Solvent-free
78
30
20
15
15
15
20
25
60
60
60
30
90
65
80
50
40
25
15
45
80
120
120
65
64
95
77
73
88
80
66
73
59
65
45
75
86
59
76
83
94
83
71
41
26
H
2
O
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
78
EtOH
75% EtOH (v/v)
50% EtOH (v/v)
MeOH
i-PrOH
CH
DMSO
CH Cl
3
CN
III
PVC-EDA-Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
10
11
12
13
14
15
16
17
18
19
20
21
22
2
2
Reflux
78
Solvent-free
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Catalyst-free
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
60
CeCl
3
Á7H
2
O (2.0)
PVC–EDA (32.0 of N)
III
PVC–EDA–Ce (0.4)
III
PVC–EDA–Ce (0.8)
III
PVC–EDA–Ce (1.2)
III
PVC–EDA–Ce (2.0)
III
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
III
50
PVC–EDA–Ce (1.6)
III
PVC–EDA–Ce (1.6)
40
r. t.
Reaction condition: 4-chlorobenzaldehyde (1 mmol), malononitrile (1 mmol), hydrazine hydrate
(1 mmol), ethy1 acetoacetate (1 mmol) were stirred in the presence of catalyst in the solvent (5 mL)
a
Isolated yield
Initially, various protic and aprotic solvents with varying polarities were explored
in the model reaction. As shown in Table 1, the rate of reaction and the yield of the
product were significantly influenced by the used solvent. Among the solvents
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
screened, we found that the use of EtOH as the solvent gave the highest yield of
9
5% (Table 1, entry 3).
Subsequently, we examined the effect of the amount of catalyst ranging from 0.0
to 2.0 mol% of Ce on the model reaction. It was found that this reaction could
proceed efficiently even with a low amount of the catalyst (0.4 mol% Ce) with 59%
yield at 78 °C (Table 1, entry 15). With increasing the amount of Ce from 0.4 to
1
.6 mol%, the reaction yield increased from 59 to 95% (Table 1, entries 15–17, 3).
Further increasing the amount of catalyst to 2.0 mol% had no apparent effect on the
III
product yield (Table 1, entry 18). Therefore, 1.6 mol% of Ce is sufficient to drive
the reaction to completion in a short time. Recall that, in the absence of a catalyst,
the model reaction proceeded with a prolonged reaction time and a lower yield
(
Table 1, entry 12).
Then, the effect of reaction temperature was evaluated on the model reaction. As
shown from Table 1, the product yield decreased from 95 to 26% and the reaction
time increased from 15 to 120 min when the temperature was dropped from reflux
to room temperature (Table 1, entries 18–22). Therefore, the reflux temperature was
the optimum temperature for this reaction.
III
Finally, to further evaluate the catalytic activity of the PVC–EDA–Ce catalyst,
a comparison was made with the CeCl Á7H O (Table 1, entry 13) and PVC–EDA
3
2
III
Table 1, entry 14) in the model reaction. It can be seen that the PVC–EDA–Ce is
(
superior over the CeCl Á7H O Lewis acid catalyst and the PVC–EDA Lewis base
3
2
catalyst in terms of product yields and reaction times under similar reaction
III
conditions. The enhanced catalytic activity of PVC–EDA–Ce may be attributed to
the synergic effect of Lewis acidic sites and Lewis basic sites formed in the PVC–
III
EDA–Ce catalyst. Moreover, the elemental analysis result (molar ratio of N/Ce is
III
6.3/1) indicates that the number of amino groups is sufficient to anchor Ce and to
1
serve as a Lewis base in the reaction.
Synthesis of dihydropyrano[2,3-c]pyrazoles with different substrates
Under the optimum conditions, the scope and generality of the 4-CRs of various
carbonyl compounds, hydrazine, ethyl acetoacetate, and malononitrile were
investigated. As shown in Table 2, in all cases, the nature of functional groups
on the aryl ring of aldehyde exerted a slight influence on the reaction. A series of
substituted aromatic aldehydes carrying either electron–donating or electron–
withdrawing groups proceeded smoothly to afford the desired products with
excellent yields (75–95%) in short times (15–25 min) (Table 2, entries 1–15, 17,
22). Notably, the aromatic ketones showed sluggish reactions and gave slightly
lower yields with prolonged reaction times (35–48 min) (Table 2, entries 18–21).
III
The cyclic performance of PVC–EDA–Ce catalyst
For practical applications, the reusability of a catalyst is highly desirable for a
catalytic process. To explore this aspect of the catalyst, catalytic recycling
experiments were performed using the model reaction under the optimized
conditions. The cyclic recovery of the catalyst was simple. After the completion
123

T. Zhang et al.
Table 2 Synthesis of pyrano[2,3-c]pyrazoles
Carbonyl
compound
a
Entry
Product
Time (min)
Yield (%)
M.p. (°C)
1
20
93
95
88
91
246
233
243
228
–
–
–
–
247
234
245
230
1
a
5
a
2
3
4
15
25
20
1
b
5
b
1
c
5
c
1
d
5
d
5
15
92
179
–
180
1
e
5
e
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
Table 2 continued
Carbonyl
compound
a
Entry
Product
Time (min)
Yield (%)
M.p. (°C)
6
20
85
88
259
222
–
–
261
224
1
f
5
f
7
8
20
20
1
g
5
g
93
209
–
210
1
h
5
h
9
20
25
90
91
214
227
–
–
215
229
1
i
5
i
1
0
1
j
5
j
11
15
95
247
–
249
1
k
5
k
123

T. Zhang et al.
Table 2 continued
Carbonyl
compound
a
Entry
Product
Time (min)
Yield (%)
M.p. (°C)
1
1
2
3
15
92
90
195
220
–
–
196
221
1
l
5
l
25
20
1
m
5
m
1
4
92
222–224
1
n
5
n
15
20
91
234–236
1
o
p
q
5
o
1
6
45
25
87
89
168
228
–
–
170
231
1
5
5
p
q
1
7
1
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
Table 2 continued
Carbonyl
compound
a
Entry
Product
Time (min)
Yield (%)
M.p. (°C)
1
1
2
8
9
0
48
75
91
85
175
287
245
–
–
–
176
289
247
1
r
5
r
35
40
1
s
5
s
1
t
5
t
2
1
45
20
87
94
245
–
246
1
u
5
u
2
2b
> 300
1
v
Reaction conditions: carbonyl compound (1 mmol), hydrazine hydrate (1 mmol), malononitrile (1 mmol),
III
and ethyl acetoacetate (1 mmol) were stirred in the presence of PVC–EDA–Ce (40 mg) in EtOH at
7
8 °C
a
Isolated yield
b
Two equivalents of hydrazine hydrate, malononitrile, and ethyl acetoacetate were used
of the reaction, hot ethanol was added to the reaction mixture and the catalyst was
easily recovered from the mixture by filtering, and then could be used directly in a
consecutive run after washing with ethanol and drying at room temperature. As
demonstrated in Fig. 6, the catalyst could be reused for at least five runs without
obvious loss of its catalytic activity.
123

T. Zhang et al.
III
Fig. 6 Recyclability of PVC–EDA–Ce
Scheme 2 Plausible synergic catalytic mechanism for the synthesis of pyrano[2,3-c]pyrazoles
1
23

Ce(III) immobilized on aminated poly(vinyl chloride)…
Plausible mechanism for synthesis of dihydropyrano[2,3-c]pyrazoles
III
derivatives catalyzed by PVC–EDA–Ce
We have proposed a plausible synergic catalytic mechanism for the synthesis of 1,4-
III
dihydropyrano[2,3-c]pyrazole derivatives in the presence of PVC–EDA–Ce in
accordance with the literature shown in Scheme 2 [35, 36]. First, the pyrazolone
A was produced via the condensation between hydrazine and ethyl acetoacetate
III
activated by Ce . Simultaneously, the formation of 2-benzylidene malononitrile
B from a Knoevenagel condensation of malononitrile and aryl aldehyde catalyzed
by the amino groups of the catalyst occurred. Then, Michael addition of pyrazolone
A onto a,b-unsaturated compound B facilitates the formation of Michael adduct
C which subsequently transformed to intermediate D via intramolecular cyclization
in the presence of the catalyst. Finally, intermediate D undergoes tautomerization to
give the desired product pyrano[2,3-c]pyrazole.
Conclusions
III
The catalyst PVC–EDA–Ce was synthesized in a simple and efficient way by the
treatment of CeCl Á7H O with PVC–EDA which could be achieved by a
3
2
III
substitution reaction of PVC and EDA. It has been proven that PVC–EDA–Ce
has a good performance to catalyze 4-CRs of carbonyl compounds, ethyl
malononitrile, acetoacetate, and hydrazine for the synthesis of 1,4-dihydropy-
rano[2,3-c]pyrazoles with excellent yields. The high catalytic activity of PVC–
III
EDA–Ce can be attributed to the synergic catalytic effect of its Lewis acid sites
and Lewis base sites. High reactivity, wide substrate scope, short reaction time, mild
conditions, and reusability of the catalyst are the merits of this practical catalytic
process for the synthesis of structurally diverse 1,4-pyrano[2,3-c]pyrazole
derivatives.
Acknowledgements We are grateful to the National Natural Science Foundation of China (No
21372099) for financial support.
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