Poly(4-vinylpyridine)-supported dual acidic ionic liquid: An environmentally friendly heterogeneous catalyst for the one-pot synthesis of 4,4'-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols)

Article abstract of DOI:10.3906/kim-1206-28

A poly(4-vinylpyridine)-supported Bronsted ionic liquid was easily prepared from its starting materials and used as a novel, highly efficient, and reusable heterogeneous catalytic system for the synthesis of 4,4'-(arylmethylene)bis(3- methyl-1-phenyl-1H-p

Full text of DOI:10.3906/kim-1206-28

Turk J Chem
Turkish Journal of Chemistry
(2013) 37: 756 – 764
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1206-28
Research Article
Poly(4-vinylpyridine)-supported dual acidic ionic liquid: an environmentally
friendly heterogeneous catalyst for the one-pot synthesis of
4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols)
Kaveh PARVANAK BOROUJENI,∗ Pegah SHOJAEI
Department of Chemistry, Shahrekord University, Shahrekord, Iran
Received: 14.06.2012
•
Accepted: 13.04.2013
•
Published Online: 16.09.2013
•
Printed: 21.10.2013
Abstract: A poly(4-vinylpyridine)-supported Brønsted ionic liquid was easily prepared from its starting materials and
used as a novel, highly efficient, and reusable heterogeneous catalytic system for the synthesis of 4,4’-(arylmethylene)bis(3-
methyl-1-phenyl-1H-pyrazol-5-ols) from the condensation reaction between aromatic aldehydes and 2 equivalents of 3-
methyl-l-phenyl-5-pyrazolone.
Key words: 3-Methyl-l-phenyl-5-pyrazolone, aldehydes, ionic liquid, one-pot synthesis, heterogeneous catalysis
1. Introduction
Recently, various ionic liquids have attracted significant attention as an alternative reaction medium for homoge-
neous catalysis. One type is Brønsted acidic ionic liquids. These ionic liquids are of special importance because
they possess simultaneously proton acidity and the characteristic properties of an ionic liquid.1,2 Among them,
SO3 H-functionalized ionic liquids with a hydrogen sulfate counteranion are of particular value as a class of
dual acidic functionalized ionic liquids, because the existence of both SO3 H functional groups and hydrogen
sulfate counteranions can enhance their catalytic acidities.3−5 Thus, they are designed to replace traditional
mineral liquid acids such as sulfuric acid and hydrochloric acid in chemical procedures. However, despite having
widespread application in organic synthesis, most of them suffer from one or more of the following drawbacks: la-
borious work-up procedures, difficulty of recovery and recycling, disposal of spent catalyst, difficulty of handling,
and corrosion problems. Thus, these shortcomings make them a prime target for heterogenization.6−9
2,4-Dihydro-3H-pyrazol-3-one derivatives including 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-
5-ols) are known to possess a wide range of biological activities and are used as gastric secretion stimulatory,10
anti-inflammatory,11 antidepressant,12 antibacterial,13 antifilarial,14 antitumor,15 and antiviral agents.16 More-
over, the corresponding 4,4’-(arylmethylene)bis(1H-pyrazol-5-ols) are used as insecticides,17 pesticides,18 dyes-
tuffs,19 and the chelating and extracting reagents for different metal ions.20 One-pot tandem Knoevenagel-type
condensation/Michael reaction between aromatic aldehydes with 2 equivalents of 3-methyl-l-phenyl-5-pyrazolone
is one of the most pivotal preparation methods of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols).
In the absence of catalyst, this reaction is very slow (4–24 h in refluxing ethanol, benzene, or water and a further
24 h under ambient temperature) and the products are obtained in moderate yields.21−23 Sodium dodecyl sulfate
∗Correspondence: parvanak-ka@sci.sku.ac.ir
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
as a surfactant catalyst has been employed to accomplish this reaction in aqueous medium at 100 ◦ C.24 Fur-
thermore, Elinson et al. utilized an electrocatalytic procedure for the preparation of 4,4’-(arylmethylene)bis(1H-
pyrazol-5-ols).25 In addition, ceric ammonium nitrate,26 tetramethyl-tetra-3,4-pyridinoporphyrazinato copper
(II) methyl sulfate,27 silica-bonded S-sulfonic acid,28 ([3-(3-silicapropyl)sulfanyl]propyl) ester,29 silica sulfuric
acid,30 zanthan sulfuric acid,31 and 3-aminopropylated silica gel32 have been reported as catalysts for this reac-
tion to date. Although these methods are suitable for certain synthetic conditions, there exist some drawbacks
such as low yields, high reaction temperature, long reaction times, drastic reaction conditions, tedious work-up
leading to the generation of large amounts of toxic waste, and the use of unrecyclable, hazardous, or difficult to
handle catalysts. In view of this, there is a demand for clean processes utilizing ecofriendly and green catalysts
for this useful reaction.
In a continuation of our work on the development of efficient and environmentally benign procedures using
poly(vinylpyridine)-supported reagents and catalysts,33 herein we report the synthesis of poly(4-vinylpyridine-
co-1-sulfonic acid butyl-4-vinylpyridinium)hydrogen sulfate ([P4 VPy-BuSO3 H]HSO4) from the reaction of
poly(4-vinylpyridine) (P4 VPy, 2% divinylbenzene) with 1,4-butane sultone/H2 SO4 . [P4 VPy-BuSO3 H]HSO4
was used as a dual acidic ionic liquid catalyst for the synthesis of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-
pyrazol-5-ols) by 2-component 1-pot tandem Knoevenagel-type condensation/Michael reaction between various
aromatic aldehydes with 3-methyl-l-phenyl-5-pyrazolone.
2. Experimental
2.1. General
The chemicals were either prepared in our laboratory or were purchased from Merck and Fluka. Reaction
monitoring and purity determination of the products were accomplished by GLC or TLC on silica-gel polygram
SILG/UV254 plates. Gas chromatography was recorded on a Shimadzu GC 14-A. IR spectra were obtained
by a Shimadzu model 8300 FT-IR spectrophotometer. NMR spectra were recorded on a Bruker Avance DPX-
300 spectrometer. A Leco sulfur analyzer was used for the measurement of sulfur in catalyst. Melting points
were determined on a Fisher-Jones melting-point apparatus and are uncorrected. Thermal gravimetric analysis
(TGA) was performed by a Stanton Redcraft STA-780 with a 20 ◦ C/min heating rate in N2 . The shape and
surface morphology of the samples were examined on a scanning electron microscope (SEM) (Hitachi S-3400N,
Japan).
2.2. Synthesis of [P4 VPy-BuSO3 H]HSO4
In a round bottomed flask (50 mL) equipped with a reflux condenser was added 1 g of the P4 VPy (2% DVB)
to 1,4-butane sultone (1.5 mL) and the mixture was stirred at 100 ◦ C for 30 h, filtered, washed with distilled
water (20 mL), and dried at 80 ◦ C overnight. Afterwards, H2 SO4 (3 M, 5 mL) was added to the obtained
resin and the mixture was stirred at room temperature for 2 h, filtered, washed with distilled water (20 mL),
and dried at 80 ◦ C overnight to give [P4 VPy-BuSO3 H]HSO4 .
2.3. Typical procedure for synthesis of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-
ols)
To a solution of an aldehyde (1 mmol), 3-methyl-l-phenyl-5-pyrazolone (2 mmol), and ethanol (3 mL) was
added [P4 VPy-BuSO3 H]HSO4 (0.1 mmol) and the resulting mixture was magnetically stirred under reflux
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
conditions. The progress of the reaction was monitored by TLC. After the completion of the reaction, the
catalyst was filtered off and washed with ethanol (2 × 5 mL), and the filtrate was concentrated on a rotary
evaporator under reduced pressure to give the crude product. Whenever required, the products were purified
by column chromatography on silica gel (n-hexane/EtOAc) or by recrystallization from ethanol.
Representative spectral data of some of the obtained compounds are given below.
4,4’-(Phenylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (Table 1, entry 1a): IR (KBr,
cm−1): 3410, 3080, 2910, 2895, 1599, 1499, 1480, 1418, 1283, 1025, 739, 700; 1 H NMR (300 MHz, DMSO-d6):
δ = 2.35 (6H, s), 4.99 (1H, s), 7.21–7.29 (7H, m), 7.47 (4H, t, J = 7.78 Hz), 7.72 (4H, d, J = 7.97 Hz), 13.99
(2H, br, OH).
4,4’-[(4-Chlorophenyl)methylene]bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (Table 1, entry 1d):
IR (KBr, cm−1): 3440, 3050, 2930, 2895, 1589, 1499, 1420, 1298, 8104, 749, 698; 1 H NMR (300 MHz, DMSO-
d6): δ = 2.35 (6H, s), 4.99 (1H, s) 7.29, (4H, d, J = 8.10 Hz), 7.37 (2H, d, J = 7.90 Hz), 7.45 (4H, t, J =
7.01 Hz), 7.75 (4H, d, J = 7.50 Hz), 13.95 (2H, br, OH).
Table 1. Synthesis of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols).
Ar
Me
Me
Me
[P4VPy-BuSO3H]HSO4
(0.1 mmol)
H
ArCHO
+
2
N
N
N
Ethanol / Reflux
O
N
N
N
OH
HO
Ph
Ph
Ph
Entry
1
ArCHO
Time (h)
Yield (%)a
M.p. (oC) (Lit.Ref
)
G
O
H
a: G= H
0.7
0.8
0.9
0.8
0.9
1
95
92
92
95
92
92
97
97
171-173 (170-17228)
204-206 (202-20428)
177-179 (173-17523)
216-218 (215-21729)
235-237 (236-23724)
152-154 (149-15023)
226-228 (230-23224)
211-213 (210-21229)
b: G= 4-CH3
c: G=4-OCH3
d : G= 4-Cl
e : G= 2-Cl
f: G= 4-OH
g: G= 4-NO2
h: G= 4-CN
0.6
0.6
2
3
0.8
0.8
90
90
190-192 (189-19026)
CHO
O
184-185 (181-18328)
CHO
S
a Isolated yields. All products are known compounds and were identified by comparison of their physical and spectral
data with those of the authentic samples.
4,4’-[(4-Nitrophenyl)methylene]bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (Table 1, entry 1g):
IR (KBr, cm−1): 3450, 3080, 2930, 2898, 1600, 1499, 1420, 1348, 749, 698; 1 H NMR (300 MHz, DMSO-d6):
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
δ = 2.41 (6H, s), 5.16 (1H, s), 7.29–7.32 (2H, m), 7.45 (4H, t, J = 6.91 Hz), 7.58 (2H, d, J = 8.01 Hz),
7.71–7.78 (4H, d, J = 7.51 Hz), 8.15 (2H, d, J = 8.01 Hz), 13.88 (2H, br, OH).
4,4’-[(4-Cyanophenyl)methylene]bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (Table 1, entry 1h):
IR (KBr, cm−1): 3430, 3098, 2928, 2895, 2238, 1595, 1499, 1420, 1299, 820, 760, 700; 1 H NMR (300 MHz,
DMSO-d6): δ = 2.35 (6H, s), 5.11 (1H, s), 7.28 (2H, t, J = 7.37 Hz), 7.42 (4H, d, J = 7.88 Hz), 7.45–7.50
(6H, m), 7.79 (2H, d, J = 8.38 Hz), 13.91 (2H, br, OH).
4,4’-[(2-Furyl)methylene]bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (Table 1, entry 2): IR (KBr,
cm−1): 3430, 3090, 2930, 2895, 1595, 1498, 1418, 1288, 785, 698; 1 H NMR (300 MHz, DMSO-d6): δ = 2.36
(6H, s), 5.11 (1H, s), 6.14–6.19 (1H, m), 6.39–7.11 (1H, m), 7.28 (2H, t, J = 6.05 Hz), 7.48–7.56 (5H, m), 7.78
(4H, d, J = 8.06 Hz), 13.89 (2H, br, OH).
4,4’-[(2-Thienyl)methylene]bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (Table 1, entry 3): IR
(KBr, cm−1): 3430, 3086, 2930, 2895, 1599, 1498, 1420, 1300, 1289, 785, 700; 1 H NMR (300 MHz, DMSO-d6):
δ = 2.38 (6H, s), 5.18 (1H, s), 6.79–6.83 (1H, m), 6.95–7.08 (1H, m), 7.29–7.33 (3H, m), 7.49 (4H, t, J = 7.88
Hz), 7.81 (4H, d, J = 7.88 Hz), 13.94 (2H, br, OH).
3. Results and discussion
The synthetic routes for [P4 VPy-BuSO3 H]HSO4 are shown in Scheme 1. At the first stage, commercially
available P4 VPy (2% DVB) reacted with 1,4-butane sultone to give poly(4-vinylpyridine-co-1-sulfonate butyl-
4-vinylpyridinium) ([P4 VPy-BuSO3 ]). The resulting pale yellow solid was analyzed by elemental analysis to
quantify the percentage loading of the sulfonate moiety by measuring the sulfur content, giving 0.9 mmol
sulfonate moiety per gram. In a second step, the [P4 VPy-BuSO3 ] was further treated with H2 SO4 to form
[P4 VPy-BuSO3 H]HSO4 as a white cream solid. The acidic sites loading in [P4 VPy-BuSO3 H]HSO4 obtained
by means of acid–base titration was found to be 1.7 mmol/g.
O
S O
O
n
n
n
H2SO4
100 oC / 30 h
r.t. / 2 h
HSO4
N
N
N
CH2(CH2)3SO3
CH2(CH2)3SO3H
[P4VPy-BuSO3]
[P4VPy-BuSO3H]HSO4
Scheme 1. Synthesis of [P4 VPy-BuSO3 H]HSO4 .
For comparison, FT-IR spectra of the P4 VPy and [P4 VPy-BuSO3 H]HSO4 are presented in Figure 1.
As can be seen in the spectrum of [P4 VPy-BuSO3 H]HSO4 , new peaks appeared at 1160, 1200, and 1220 cm−1
,
which can be assigned to S=O stretching vibration.9 A new peak also appeared at 1645 cm−1 , which is ascribed
to the C-N (pyridine-CH2 -) bond absorption. This observation confirms the N-alkylation of the pyridine ring.
Figure 2 shows the TGA curves of P4 VPy and [P4 VPy-BuSO3 H]HSO4 . A weight loss was observed in
each case at around 100 ◦ C due to loss of moisture. In the case of [P4 VPy-BuSO3 H]HSO4 , the second weight
loss started at about 200 ◦ C, and is mainly assigned to the decomposition of alky-sulfonic acid groups and
hydrogen sulfate counteranions. In TGA curves of P4 VPy and [P4 VPy-BuSO3 H]HSO4 the last weight losses
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
were observed at about 350 ◦ C and 420 ◦ C, respectively, which are attributed to the degradation of polymer
backbone.
Figure 1. FT-IR spectra of (a) P4 VPy and (b) [P4 VPy-BuSO3 H]HSO4 .
Figure 2. TGA curves of P4 VPy (a) and [P4 VPy-BuSO3 H]HSO4 (b).
The SEM images of P4 VPy and [P4 VPy-BuSO3 H]HSO4 are provided in Figure 3. From the image of
P4 VPy, the surface of P4 VPy is somewhat coarse and irregular with many pores on the surface, whereas the
SEM photograph of [P4 VPy-BuSO3 H]HSO4 shows that with chemical modification the primary structure of
P4 VPy was changed and the polymer support survived the sequence of functionalization steps.
In order to explore the catalytic activity of [P4 VPy-BuSO3 H]HSO4 , we studied the condensation reac-
tion of aromatic aldehydes with 3-methyl-l-phenyl-5-pyrazolone (Table 1). The best results in terms of yield as
well as reaction time were obtained by refluxing ethanol, which proved to be the solvent of choice among the
other organic solvents. The optimum molar ratio of [P4 VPy-BuSO3 H]HSO4 to aldehyde was found to be 0.1:1.
Various types of substituted benzaldehydes (entries 1a–h) reacted with 3-methyl-l-phenyl-5-pyrazolone to give
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
the corresponding 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols). It was pleasing to observe that
even acid sensitive aldehydes, such as 2-furyl and 2-thienyl carbaldehyde, were smoothly converted into the cor-
responding products, a conversion that is otherwise problematic in the presence of strong acid catalysts (entries
2 and 3). As shown in Table 1 (entry 1), the aromatic aldehydes with electron-withdrawing groups reacted faster
than the aromatic aldehydes with electron-releasing groups. This observation can be rationalized on the basis of
the mechanistic details of the reaction (Scheme 2). An aldehyde was first activated by [P4 VPy-BuSO3 H]HSO4 .
Nucleophilic addition of 3-methyl-l-phenyl-5-pyrazolone to activated aldehyde was followed by the loss of H2 O
generated benzylidene intermediate (I), which was further activated by [P4 VPy-BuSO3 H]HSO4 . Then the 1,4-
nucleophilic addition of a second molecule of 3-methyl-l-phenyl-5-pyrazolone on activated intermediate I, in the
Michael addition fashion, afforded the synthesis of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols).
The electron-withdrawing groups substituted on aromatic aldehyde in intermediate I increase the rate of the
1,4-nucleophilic addition reaction because the alkene LUMO is at lower energy in their presence compared with
electron-donating groups.34
Figure 3. SEM photographs of P4 VPy (a) and [P4 VPy-BuSO3 H]HSO4 (b).
O
O
S
O
S
O
Py
O
O
H
O
Ph
N
H
O
OH
H
O S O-H
H
Me
Me
Ar
O
N
O
O-H
(I)
O
H
N
N
O S O
Ar
Ar
H O
O
Me
- H2O
N
N
Ph
Ph
H
Py
Ar
Ar
O
Me
H
Me
Me
Me
Me
H
H
N
N
N
N
N
N
H O
N
N
OH
OH
N
HO
N
Ph
Ph
Ph
Ph
Ph
Scheme 2. Suggested mechanism for the preparation of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols)
catalyzed by [P4 VPy-BuSO3 H]HSO4 .
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
In order to confirm the true heterogeneity of the catalytic systems (i.e. the absence of leaching of the acidic
sites into the reaction mixture), [P4 VPy-BuSO3 H]HSO4 was added to ethanol and the mixture was stirred for
2 h under reflux conditions. Then the catalyst was filtered off and the filtrate was analyzed for its acid content,
which showed a negligible release of the acidic sites. The filtrate was found to be inactive for the condensation
of 3-methyl-l-phenyl-5-pyrazolone with aldehydes. These observations indicate that [P4 VPy-BuSO3 H]HSO4 is
stable under the reaction conditions, and there is no leaching of acid moieties during reactions.
[P4 VPy-BuSO3 H]HSO4 recovered after a reaction can be washed with ethanol and used again at least
5 times without any noticeable loss of catalytic activity (Scheme 3).
Ph
Me
Me
Me
[P4VPy-BuSO3H]HSO4
(0.1 mmol)
H
PhCHO
+
2
N
N
N
Ethanol / 0.7 h / Reflux
O
N
N
N
OH
HO
Ph
Ph
Ph
Run No.
Isolated Yield (%)
1
2
3
4
5
95
93
92
90
90
Scheme 3. Recyclability of [P4 VPy-BuSO3 H]HSO4 in the preparation of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-
1H-pyrazol-5-ols).
A comparison of the efficiency of [P4 VPy-BuSO3 H]HSO4 catalyst with some of those reported in the
literature is given in Table 2. As seen, in addition to having the general advantages attributed to solid catalysts,
[P4 VPy-BuSO3 H]HSO4 has good efficiency compared to many of those reported catalysts in the condensation
of benzaldehyde with 2 equivalents of 3-methyl-l-phenyl-5-pyrazolone.
Table 2. Comparison of the efficiencies of a number of different reported catalysts with that of [P4 VPy-BuSO3 H]HSO4
in the condensation of benzaldehyde with 2 equivalents of 3-methyl-l-phenyl-5-pyrazolone.
Entry
Cat./Solv./Temp.
Sodium dodecyl sulfate/H2O/Reflux
Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
1
86.824
9226
9527
8028
9029
9330
9531
9832
95
Ceric ammonium nitrate/H2O/r.t.◦
0.25
0.50
2
[Cu(3,4-tmtppa)](MeSO4)4/H2O/90 C
Silica-bonded S-sulfonic acid/EtOH/Reflux
[3-(3-Silicapropyl)sulfanyl]propyl)ester/EtOH/Reflux
3
1
◦
Silica sulfuric acid/EtOH/H2O/70 C
Zanthan sulfuric acid/EtOH/Reflux
3-Aminopropylated silica gel/CH3CN/r.t.
[P4VPy-BuSO3H]HSO4/EtOH/Reflux
0.25
0.16
0.70
4. Conclusion
We synthesized [P4 VPy-BuSO3 H]HSO4 as a novel heterogeneous acid catalyst that favorably combines the
properties of ionic liquids and the advantages of solid supports. This solid catalyst can act as an efficient catalyst
for the synthesis of 4,4’-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols). The significant advantages of
this methodology are high yields, short reaction times, simple work-up procedure, and easy preparation and
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PARVANAK BOROUJENI and SHOJAEI/Turk J Chem
handling of the catalyst. In addition, the use of this catalyst resulted in a reduction in the unwanted and
hazardous waste that is produced during conventional homogeneous processes. Finally, this solid catalyst can
be recovered unchanged and used again.
Acknowledgment
The authors thank the Research Council of Shahrekord University for its partial support of this work.
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