Introduction of poly(4-vinylpyridinium) perchlorate as a new, efficient, and versatile solid acid catalyst for one-pot synthesis of substituted coumarins under ultrasonic irradiation

Article abstract of DOI:10.1016/j.ultsonch.2012.07.013

Poly(4-vinylpyridinium) perchlorate, is a supported, recyclable, eco-benign catalyst for synthesis of substituted coumarins via Pechmann reaction using ultrasound irradiation at room temperature and neat condition in high yields with short reaction times. The catalyst was studied by FT-IR, X-ray diffraction, scanning electron microscope, thermo-gravimetric and energy dispersion X-ray analyses. All the products were extensively characterized by ¹H NMR, FT-IR, MS and melting point analyses. This methodology offers momentous improvements over various options for the synthesis of coumarins with regard to yield of products, simplicity in operation and green aspects by avoiding toxic catalysts and solvents. Further, the catalyst can be reused and recovered for several times without loss of activity.

Full text of DOI:10.1016/j.ultsonch.2012.07.013

Ultrasonics Sonochemistry 20 (2013) 26–31
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Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Short Communication
Introduction of poly(4-vinylpyridinium) perchlorate as a new, efficient,
and versatile solid acid catalyst for one-pot synthesis of substituted
coumarins under ultrasonic irradiation
⇑
Nader Ghaffari Khaligh , Farhad Shirini
Department of Chemistry, College of Science, University of Guilan, Rasht 41335-19141, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 March 2012
Received in revised form 19 April 2012
Accepted 20 July 2012
Available online 2 August 2012
Poly(4-vinylpyridinium) perchlorate, is a supported, recyclable, eco-benign catalyst for synthesis of
substituted coumarins via Pechmann reaction using ultrasound irradiation at room temperature and neat
condition in high yields with short reaction times. The catalyst was studied by FT-IR, X-ray diffraction,
scanning electron microscope, thermo-gravimetric and energy dispersion X-ray analyses. All the products
1
were extensively characterized by H NMR, FT-IR, MS and melting point analyses. This methodology
offers momentous improvements over various options for the synthesis of coumarins with regard to yield
of products, simplicity in operation and green aspects by avoiding toxic catalysts and solvents. Further,
the catalyst can be reused and recovered for several times without loss of activity.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Poly(4-vinylpyridinium) perchlorate
Ultrasonic irradiation
Supported catalyst
Pechmann
Coumarins
1
. Introduction
recyclability of the traditional acid catalyst is to chemically anchor
the reactive center onto a large surface area solid carrier [11]. In
As a consequence of serious pollution problems, the adoption of
‘cleaner production’’ methods is an urgent priority. Four main ap-
these types of solids, the reactive centers are highly mobile similar
to homogeneous catalysts and at the same time these species have
the advantage of being recyclable in the same fashion as heteroge-
neous catalysts. Functional polymers have the potential advanta-
ges of small molecules with the same functional groups [12].
Poly(4-vinylpyridine) is an interesting material because of its sta-
ble pyridyl group and ability to form charge transfer complex with
acidic dopants [13]. Poly(4-vinylpyridine) seems to an attractive
support to immobilize mineral acids because of the basic nature
of pyridyl group.
Coumarin and its derivatives form an important class of benzo-
pyrones found in nature. They are structural subunits in many
complex natural products and have shown numerous biological
activities, such as antitumor [14], anti-HIV (NNRTI) [15], antioxida-
tion [16], tumor necrosis factor-a (TNF-a) inhibition [17], antimicro-
bial activity [18], serine protease inhibition [19] and anticancer
activity [20]. Coumarins could be synthesized by various methods,
such as Pechmann [21], Perkin [22], Knoevenagel [23], Reformatsky
[24], Witting [25], Claisen [26] and flash vacuum pyrolysis reaction
[27]. However, the Pechmann reaction is one of the simplest and di-
rect methods for the synthesis of coumarins since it proceeds from
very simple starting materials, namely, phenols and keto esters or
unsaturated carboxylic acids utilizing various catalysts, such as sul-
furic acid [21], trifluoroacetic acid [28], phosphorus pentoxide [29],
‘
proaches have recently been developed to synthesize compounds
in an environmentally friendly manner [1]: use water or supercrit-
ical fluids instead of organic solvent; do the reaction under sol-
vent-free conditions; load catalytic amounts of reagent instead of
stoichiometric amounts, and use enzymatic and biosynthetic
processes.
In recent years, the development of heterogeneous catalysts
have become a major area of research in synthetic organic chemis-
try due to some potential advantages of these material over homo-
geneous systems such as simplified recovery, reusability, enhanced
selectivity and reactivity, easy product isolation and incorporation
in continuous reactors and micro-reactors [2]. Application of solid
acids in organic transformation has important roles, because these
reagents do less harm to environment and have no corrosion or
disposal of effluent problems. As economically and ecologically be-
nign catalysts, their research and application have attracted much
attention in academia and industry [3–9]. There are more than 100
industrial processes using over 103 solid acids as catalysts [10].
It is clear that green chemistry not only requires the use of envi-
ronmentally benign reagents and solvents, but also the recovery
and reuse of the catalyst. One way to overcome the problem of
⇑
Corresponding author.
E-mail address: ngkhaligh@guilan.ac.ir (N.G. Khaligh).
ZrCl
4 4
[30], TiCl [31] and ionic liquids [32]. However, most of these
procedures required long duration (24 h [33] and 20 h [34]), high
1
350-4177/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2012.07.013

N.G. Khaligh, F. Shirini / Ultrasonics Sonochemistry 20 (2013) 26–31
27
temperature (150 °C) [34] and also microwave irradiation [35]. Also
there have been some attempts to find alternative, environmentally
benign synthesis routes. On those lines, Nafion-H [36], zeolite H-
BEA, modified zirconia [37], Amberlyst 15 [38], montmorillonite
clay [35a] and other solid acids have been employed for this purpose
in the Pechmann condensation.
Products were characterized by their physical constant and com-
parison with authentic samples. The purity determination of the
substrates and reaction monitoring were accompanied by TLC
using silica gel SIL G/UV 254 plates.
2.2. Instrumentation
Ultrasound irradiation has been established as a significant tech-
nique in synthetic organic chemistry [39–42]. Shorter reaction time
is the main benefit of ultrasound-assisted reactions. Simple experi-
mental procedure, high yields, improved selectivity and clean reac-
tion of many ultrasound induced organic transformations offers
additional convenience in the field of synthetic organic chemistry.
The MS were measured under GC (70 eV) conditions. The IR spec-
tra were recorded on a Perkin Elmer 781 Spectrophotometer. The
UV spectra were recorded on an Agilent 8453 UV–vis spectropho-
1
tometer at room temperature. In all the cases the H NMR spectra
were recorded with Bruker Avance 400 or 300 MHz instrument.
1
3
Herein, we have evaluated the catalytic activity of HClO
ported poly(4-vinylpyridine). We report the successful synthesis
of poly(4-vinylpyridinium) perchlorate [P(4-VPH)ClO ], the char-
4
sup-
C NMR data were collected on Bruker Avance 100 or 75 MHz
instrument. All chemical shifts are quoted in parts per million
(ppm) relative to TMS using deuterated solvent. Microanalyses were
performed on a Perkin–Elmer 240-B microanalyzer. Melting points
were recorded on a Büchi B-545 apparatus in open capillary tubes.
4
acterization and application as simple reusable solid phase catalyst
in synthesis of substituted coumarins via Pechmann reaction using
ultrasound irradiation at room temperature under solvent-free
conditions.
2.3. Catalysts preparation
HClO
to the suspension of powdered P(4-VP) (5.0 g) [Poly(4-vinylpyridine
O. The
mixture was stirred at room temperature for 8 h then the suspen-
sion was concentrated and the residue was successively washed
4
(0.6 ml, 6.95 mmol, as a 70% aqueous solution) was added
2
. Experimental
cross-linked with 2% DVB ꢀ60 mesh; Fluka Chemica] in Et
2
2.1. Material
All chemicals were purchased from Merck, Aldrich and Fluka
with Et
the solid powder was dried at 50 °C for 24 h under vacuum to afford
P(4-VPH)ClO 5.502 g as a pale brown powder (Scheme 1).
2
O. After removal of water and ether in a rotary evaporator,
Chemical Companies and used without further purification.
4
2.4. Characterization of catalyst
Fig. 1 presents the FTIR spectra of P(4-VP) and P(4-VPH)ClO
4
.
Scheme 1. Preparation of the catalyst.
FTIR spectra can be used as a powerful technique for the investiga-
4
Fig. 1. FTIR spectra of P(4-VP)(bottom) and P(4-VPH)ClO (top).

2
8
N.G. Khaligh, F. Shirini / Ultrasonics Sonochemistry 20 (2013) 26–31
Table 1
Table 3
Effect of temperature, solvent, amount of catalyst on the synthesis of 7-hydroxy-4-
Reaction of resorcinol and EAA in the presence of P(4-VP), HClO
under solvent-free conditions and ultrasonic irradiation.
4
and P(4-VPH)ClO
4
methylcoumarin.a
a
Entry Amount of catalyst
mg)
Temperature
(°C)
Solvent Time
(min)
Yield
(%)
Entry Reagent
Amount of Time
Yield (%)b
b
(
reagent
Poly(4-vinylpyridine) 50 mg
HClO (70%)
P(4-VPH)ClO
(min)
1
2
3
4
5
6
7
8
9
50
50
50
50
50
25
50
50
-
Reflux
Reflux
Reflux
Reflux
60
60
40
80
60
C
CH
C
6
H
5
CH
OH 60
OH 60
3
60
72
66
68
70
88
69
80
92
1
2
3
60
62
3
4
0.25 mmol 60
50 mg 15
Mixture of products
96
H
2 5
4
CH
-
-
-
-
2
Cl
2
65
40
45
54
16
6 h
a
Reaction conditions: resorcinol, 1.0 mmol; EAA, 2.0 mmol.
Determined by GC–MS of the corresponding 7-hydroxy-4-methylcoumarin
product.
b
-
trace
tion. The filtrate was concentrated in vacuum, and the crude prod-
uct was washed with water, dried and slowly re-crystallized in
ethanol or ethanol–water system. The melting point, IR, 1H NMR
and mass spectroscopic techniques were used to analyze the
products and compared with the authentic samples.
a
Reaction conditions: resorcinol, 1.0 mmol; EAA, 1.2 mmol (with solvent) and
.0 mmol (under solvent-free conditions).
Determined by GC–MS of the corresponding 7-hydroxy-4-methylcoumarin
product.
2
b
The spectral data of some representative products:
tion of interaction between perchloric acid and P(4-VP) support. As
shown in this figure, the presence of an extra hydrogen atom on the
5
-Hydroxy-3,4,7-trimethylcoumarin (Table 3, entry 4): white
crystals, mp: 249–251 °C (ethanol) (lit. mp: 249 °C) [50]; IR
KBr): = 3216, 3028, 2986, 1675, 1620, 1283, 1120,
062 cm ; H NMR (300 MHz, DMSO-d6) d = 2.04 (s, 3H, CH3),
4
pyridine nitrogen in the P(4-VPH)ClO increases the number of
(
1
m
vibrational modes and brought completely different FTIR spectrum
À1
1
[
1
43–46]. The characteristic absorption bands at 1100 and
À1
2.27 (s, 3H, CH3), 2.53 (s, 3H, CH3), 6.57 (s, 2H, ArH), 10.32
(brs,1H, OH). MS (EI, 70 eV): m/z = 204 [M+].
062 cm were assigned to the asymmetric ClO
4
stretching vibra-
À1
tions and the bands at 628 and 463 cm were attributed to the
8
-hydroxy-4-methyl-2H-naphtho[2,3-b]pyran-2-one (Table 3,
symmetric ClO
4
stretching and bending vibrations, respectively
À1
entry 18): yellow powders, mp: 275–277 °C (ethanol/water)
[
47–49]. In addition, the absorption bands at 1554 and 1450 cm
(
lit. mp: 267 °C) [60]; IR (KBr)
m
= 3360, 3031, 1705, 1564,
corresponding to C@C and C@N stretching vibration of P(4-VP) dis-
À1
1
À1
1238, 1062 cm ;
H NMR (400 MHz, DMSO-d6) d = 2.46(s,
appeared and a band at 1639 cm appeared. The appearance of this
3
H, CH3), 6.30(s, 1H, ArH), 7.08 (dd, J = 8.8 and 1.8 Hz, 1H,
transition can be attributed to poly(4-vinylpyridinium) cation.
ArH), 7.14 (d, J = 1.8 Hz, 1H, ArH), 7.54 (s, 1H, ArH), 7.88 (d,
J = 8.8 Hz, 1H, ArH), 8.21 (s, 1H, ArH), 10.15 (brs, 1H, OH). MS
The capacity of P(4-VPH)ClO
and potentiometric titration methods and it was found to be
.82–0.86 mmol ClO per gram of poly(4-vinylpyridinium) per-
chlorate polymer [50,51].
4
was determined by gravimetric
+
(
EI, 70 eV): m/z = 226 [M ].
0
4
3
. Results and discussion
2.5. General procedure for the preparation of coumarins
Part of our research is aiming to explore the potential ability of
In a 25 mL batch reactor equipped with a distillation condenser
the ultrasound irradiation as an energy source for organic synthe-
sis under solvent-free conditions. In continuation of our ongoing
research program on the development of new catalysts and
methods for organic transformations [52–56], in our previous
works, we reported the environmentally benign synthesis of poly
(4-vinylpridinium) hydrogen sulfate and the application in protec-
tion hydroxyl groups with HMDS [52], synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes under ultrasound irradiation [55a], and also
synthesis of 12-aryl-12H-indeno[a,j]naphtha[3,2-e]pyran-5,11,13-
the mixture of phenols (1 mmol), b-keto esters (2 mmol) and P(4-
VPH)ClO (50 mg) was stirred and irradiated under ultrasonic irra-
4
diation (with a frequency of 35 kHz and a nominal power 200 W)
at ambient temperature for the time mentioned in the Table 1.
Ethyl acetoacetate was taken 2 equiv. for the proper solubility of
phenol and ease of proper stirring of the reaction mixture. After
completion of the reaction (monitored by TLC), ethanol was added
to the reaction mixture and the catalyst was recovered by filtra-
Scheme 2. The synthesis of substituted coumarins in presence P(4-VPH)ClO
4
at room temperature under ultrasound irradiation and solvent-free conditions.
Table 2
Reaction of resorcinol and EAA in the presence of different amount of P(4-VPH)ClO
4
at soft and ultrasonic irradiation under solvent-free conditions.a
Heating condition (60 °C) Ultrasonic irradiation
Entry
Amount of catalyst (mg)
Soft condition
Time (min)
Yield (%)b
Time (min)
Yield (%)b
Time (min)
Yield (%)b
1
2
3
–
25
50
6 h
120
80
N.R.
32
48
6 h
45
40
Trace
69
88
60
25
15
44
86
96
a
Reaction conditions: resorcinol, 1.0 mmol; EAA, 2.0 mmol.
Determined by GC–MS of the corresponding 7-hydroxy-4-methylcoumarin product.
b

N.G. Khaligh, F. Shirini / Ultrasonics Sonochemistry 20 (2013) 26–31
29
Table 4
Table 4 (continued)
Pechmann synthesis of various coumarins in the presence of P(4-VPH)ClO4 under
ultrasonic irradiation.a
Entry Phenol
b-Keto
ester
Time
(min)
Yield
(%)
Ref.
b
Entry Phenol
b-Keto
ester
Time
(min)
Yield
(%)
Ref.
75c
18
EAA
25
[70]
b
HO
OH
1
EAA
MEAA
15
20
96
84
[60]
HO
OH
OH
a
b
c
Reaction conditions: substarte, 1.0 mmol; EAA or MEAA, 2.0 mmol.
Isolated yield.
2
EAA
EAA
20
20
76
80
[61]
[62]
H N
2
Obtained both a linear benzocoumarin (72%, m.p. 267 °C) and an angular ben-
70
zocoumarin (14%, m.p. 276–277 °C)
.
3
OH
OH
triones [55b]. Herein, we planned to evaluate the catalytic activity
HClO supported poly(4-vinylpyridine) for the synthesis of substi-
tuted coumarins under ultrasonic irradiation (Scheme 2).
To optimize the reaction conditions, the reaction between res-
orcinol (110 mg, 1 mmol) and ethyl acetoacetate [EAA] (0.25 mL,
4
4
EAA
MEAA
18
22
90
88
[60]
HO
OH
OH
OH
CH3
OH
2
60 mg, 2 mmol) was used as a model reaction. Before taking up
5
EAA
MEAA
13
17
90
88
[63]
[64]
HO
the reaction using ultrasonic irradiation, it was tried out using dif-
ferent solvents such as toluene, methanol, ethanol and dichloro-
methane under reflux reaction conditions (Table 1, entries 1–4)
as well as under solvent-free conditions at different temperatures
6
EAA
EAA
20
22
87
80
[61]
[61]
HO
4
with P(4-VPH)ClO as the catalyst (Table 1, entries 5–8). It was no-
ticed that the best results were achievable under solvent-free con-
ditions (Table 1, entries 5–8). This may be explained due to (I) the
decreased diffusion of the phenol and b-keto-ester molecules in
the presence of the solvent and (II) the interference of solvent mol-
OCH3
7
HO
OH
ecules with the active sites of the P(4-VPH)ClO
In comparison with soft conditions (stirring at room tempera-
ture), clearly P(4-VPH)ClO employed under heating conditions
and ultrasonic irradiation showed a more effective catalytic activ-
ity in terms of yield and reaction time (Table 2, entries 2 and 3).
However, in the absence of catalyst, the reaction yielded only trace
and 44% of 7-hydroxy-4-methylcoumarin after 6 h and 60 min un-
der soft, heating conditions and ultrasonic irradiation, respectively
4
.
H COC
3
8
EAA
MEAA
18
24
88
88
[65]
[64]
OH
4
OH
OH
OH
9
1
1
EAA
EAA
EAA
22
20
26
76
74
64
[61]
[66]
[63]
(Table 2, entry 1). Sonochemical rate enhancement is a known phe-
nomenon in organic reactions [57]. Hence the ultrasonic irradia-
tion required only 15 min duration to complete the model
reaction and the yield was 96% compared to the soft and heating
conditions which have taken 80 and 40 min for reaction times
and yields were 48 and 88%, respectively (Table 2, entry 3). The
higher yield and lesser reaction time during ultrasonic irradiation
in presence of the catalyst can be attributed to implosive collapse
of the cavitations period of the sound waves. When the formed
bubbles burst, it results in high temperature and high pressure
which facilitate the intermolecular reaction.
0
1
OH
Cl
OH
O N
2
1
1
2
3
EAA
EAA
18
20
88
85
[65]
[67]
OH
H CO
3
OH
4
To illustrate the catalytic activity of P(4-VPH)ClO , in the con-
trolled blank experiments, under similar reaction conditions the
model reaction was also carried out in the presence of the starting
materials used for the preparation of the reagent (i.e. P(4-VP),
OCH3
OH
1
1
4
5
EAA
EAA
22
18
84
82
[68]
[69]
HClO
was carried out in the presence of P(4-VPH)ClO
er yield at shorter reaction time (Table 3). The model reaction cata-
lyzed by poly(4-vinylpyridine) and aqueous HClO (70%) led to 62%
and the mixture of products, respectively. We reasoned that the
large negative H value [58] might be the reason for the potential
4
). Obviously the results showed when the model reaction
4
, we observed high-
OCH CH3
2
4
OH
0
competitive side reactions with substrates during the use of per-
chloric acid. On the other hand, when the catalyst was in the solid
phase, the ultrasonic irradiation has several additional enhance-
CH3
1
1
6
7
EAA
EAA
24
22
84
82
[60]
[61]
OH
ment effects [59]. The microjects of EAA bombard the P(4-VPH)ClO
4
surface that cause the exposition of unreacted surface of solid cata-
lyst and increase the chances of interphase surface able to react.
In order to evaluate the generality of the process, several diver-
sified examples illustrating the present method for the synthesis of
OH

3
0
N.G. Khaligh, F. Shirini / Ultrasonics Sonochemistry 20 (2013) 26–31
Scheme 3. Proposed mechanism for Pechmann reaction of resorcinol with EAA at room temperature under ultrasonic irradiation.
substituted coumarins was studied. The reaction of EAA with
various phenols was carried out in the presence of P(4-VPH)ClO
under ultrasound irradiation is shown in Scheme 3. On the basis
of this mechanism, P(4-VPH)ClO catalyzes the reaction of hydrox-
4
4
as catalyst under ultrasonic irradiation. In all instances except
two, coumarins were obtained in good to excellent yields (Table
yalkylation by the activation of the EAA, making the carbonyl
group susceptible to nucleophilic attack by resorcinol. The transe-
sterification, and dehydration occur concomitantly condensing to-
gether the two reactants at two sites to form coumarin and
4
). The 2,7-dihydroxynaphthalene and 2-naphthol gave a mixture
of a linear and angular coumarin and a chromone (Table 4, entries
7 and 18). From these experiments, it is clearly demonstrated that
the synthesis of coumarin derivatives using P(4-VPH)ClO under
1
4
regenerate P(4-VPH)ClO in the reaction.
4
The recovered catalyst was reused four consecutive times giving
excellent yields (Fig. 2). Use of solid acid catalysts suffers from
some disadvantages such as ‘leaching’ that leads to loss of activity.
The leaching of perchloric acid into the liquid phase was rigorously
checked by potentiometric and conductometric techniques [50,51]
after every run and also we compare FTIR of fresh catalyst with
recovered catalyst after 4th run. The results showed no significant
leaching of HClO4 during the reaction.
ultrasonic irradiation is indeed an effective method with respect
to reaction time, availability and reusability of catalyst, work-up
procedure and yields.
Different types of b-keto esters were used namely, ethyl aceto-
acetate [EAA] and
a-methyl ethyl acetoacetate [MEAA] (Table 4,
entries 1, 4, 5 and 8). Reactions of resorcinol and phloroglucinol
with EAA and MEAA (Table 4, entry 1 and 5) took place fast. This
was mainly due to the presence of the hydroxyl groups meta to
each other, that cooperated in activating the aromatic ring for
hydroxyalkylation. The reaction of orcinol with EAA and MEAA (Ta-
ble 4, entry 4) took place somewhat slower than that of the reac-
tion of resorcinol (Table 4, entry 1) presumably due to the steric
hindrance of the methyl group ortho to the position of hydrox-
ylalkylation. Also reactivity order for b-keto esters was found to
4
. Conclusions
In summary, to the best of our knowledge the coupling of ultra-
sonic irradiation with solid phase catalyst P(4-VPH)ClO as an acti-
4
vator in Pechmann synthesis under solvent-free conditions is now
reported by us for the first time. The methodology has several
advantages such as: green aspects by avoiding toxic catalysts and
solvents, high reaction rates and excellent yields, no side reactions,
ease of preparation and handling of the catalyst, effective reusabil-
ity of the catalyst, use of inexpensive catalyst with lower loading
and simple experimental procedure. Further work to explore this
novel catalyst in other organic transformations is in progress.
be EAA > MEAA (Table 4, entries 1, 4, 5 and 8). All products were
characterized by comparison of their melting point, IR, 1H NMR
and mass spectra with those of authentic samples [60–70].
The possible mechanism for the synthesis of 7-hydroxy-4-
methylcoumarin in the presence of P(4-VPH)ClO as a promoter
4
Acknowledgment
The author is thankful to the Guilan University Research Coun-
cil for partial support of this work.
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