Ultrasound-assisted one-pot synthesis of substituted coumarins catalyzed by poly(4-vinylpyridinium) hydrogen sulfate as an efficient and reusable solid acid catalyst

Article abstract of DOI:10.1016/j.ultsonch.2013.01.001

Poly(4-vinylpyridinium) hydrogen sulfate solid acid was found to be efficient 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. 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.

Full text of DOI:10.1016/j.ultsonch.2013.01.001

Ultrasonics Sonochemistry 20 (2013) 1062–1068
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Ultrasound-assisted one-pot synthesis of substituted coumarins catalyzed
by poly(4-vinylpyridinium) hydrogen sulfate as an efficient and reusable
solid acid catalyst
⇑
Nader Ghaffari Khaligh
House Research of Professor Reza. Education Guilan, District 1, 4156917139 Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly(4-vinylpyridinium) hydrogen sulfate solid acid was found to be efficient 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. This methodology offers momentous improvements
over various options for the synthesis of coumarins with regard to yield of products, simplicity in oper-
ation and green aspects by avoiding toxic catalysts and solvents. Further, the catalyst can be reused and
recovered for several times.
Received 8 September 2012
Received in revised form 28 December 2012
Accepted 8 January 2013
Available online 22 January 2013
Keywords:
Ó 2013 Elsevier B.V. All rights reserved.
Ultrasound-assisted synthesis
Poly(4-vinylpyridine)
Pechmann reaction
Solid acid
Solvent-free conditions
1. Introduction
crobial activity [12], serine protease inhibition [13] and
anticancer activity [14]. Coumarins could be synthesized by vari-
As a consequence of serious pollution problems, the adoption of
‘‘cleaner production’’ methods is an urgent priority. Four main ap-
proaches have recently been developed to synthesize compounds
in an environmentally friendly manner [1,2]. One approach to
increasing the environmental benign of a reaction is to conduct
them under solvent-free conditions. While reaction yields will al-
ways be important, now, its E factor [3] and the volume productiv-
ity [4] are considered. The largest contribution to the magnitude of
E factors comes from organic solvents. In industry, solvents are
recycled whenever possible; however, recycling is rarely accom-
plished with complete efficiency.
Advantages of solvent-free reactions include reduced energy
consumption, decreased reaction times; bring down handling costs
due to simplification of experimental procedure and a considerable
reduction in reactor size and, therefore, capital investment. These
would be especially important during industrial production and
have inspired a substantial research effort directed toward the
development of solvent-free reactions [2,5–7].
ous methods, such as Pechmann [15], Perkin [16], Knoevenagel
[17], Reformatsky [18], Witting [19], Claisen [20] and flash vacuum
pyrolysis reaction [21]. However, the Pechmann reaction is one of
the simplest and direct methods for the synthesis of coumarins
since it proceeds from very simple starting materials, namely, phe-
nols and b-keto esters or unsaturated carboxylic acids utilizing var-
ious catalysts, mineral acids like sulfuric acid [9], trifluoroacetic
acid [22], phosphorus pentoxide [23], Lewis acids such as ZrCl4
[24], TiCl4 [25], InCl3 [26] and BiCl3 [27]. In recent years, Lewis
acids such as AlCl3-nBPC, Yb(OTf)3, GaI3 and Sm(NO3)3 as well as
acidic ionic liquid were employed to catalyze Pechmann reactions
[28].
However, these catalysts have to be used in excess; for example,
sulfuric acid in ten to twelve equivalents [15], trifluoroacetic acid
in three to four equivalents [22] and phosphorous pentoxide is re-
quired in a fivefold excess [23]. Some of the Lewis acids such as
ZrCl4 are moisture sensitive, decomposes on storing and liberates
corrosive HCl fumes [24] and require special care in handling and
storage. Moreover, in some cases, mixtures of substituted phenols,
b-keto esters and the acidic catalyst required to stand for a number
of hours (depending on their reactivity) [29], high temperature
(150 °C) [29]b, also microwave irradiation [30] and undesired
side-products such as chromones, in addition to coumarins were
isolated. Furthermore, metal triflates are highly expensive.
Also there have been some attempts to find alternative, envi-
ronmentally benign synthesis routes. The use of heterogeneous
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 [8], anti-HIV (NNRTI) [9], antioxida-
tion [10], tumor necrosis factor-a (TNF-a) inhibition [11], antimi-
⇑
Tel.: +98 02166431738; fax: +98 02166934046.
E-mail address: ngkhaligh@guilan.ac.ir
1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2013.01.001

N.G. Khaligh / Ultrasonics Sonochemistry 20 (2013) 1062–1068
1063
Table 1
acid catalysts offers advantages like ease of operation conditions,
Effect of temperature, solvent, amount of catalyst on the synthesis of Coumarins.a
reduced equipment corrosion and minimized contamination of
waste streams combined with reusability. Poly(4-vinylpyridine)
supported sulfuric acid [30]b, Nafion-H [31], zeolite H-BEA, modi-
fied zirconia [32], Amberlyst 15 [33] and other solid acids have
been employed as catalyst in the Pechmann reaction. In the case
of Poly(4-vinylpyridine) supported sulfuric acid and Amberlyst-
15, special equipment such as microwave oven was used for accel-
erating this reaction and the preparation of yttria–zirconia in-
volved use of sulfuric acid at 500 °C. The neutral ionic liquids
([bmim]PF6/POCl3) was reported for the synthesis of 7-hydroxy-
4-methylcoumarin, but it required hazardous POCl3 [28e].
Microwave irradiation has also been used for coumarin synthe-
sis via Pechmann reaction catalyzed by homogeneous liquid acids
such as sulfuric acid, p-toluene sulfonic acid and ionic liquid [34–
36]. However, sulfuric acid and p-toluene sulfonic acid are corro-
sive, hazardous and require careful handling in open chamber of
domestic microwave system. The separation of ionic liquid catalyst
to recover from reaction mixture by solvent extraction adds an ex-
tra step in synthesis.
Entry Amount of catalyst
(mg)
Temperature
(°C)
Solvent
Time
(min)
Yield
(%)b
1
2
3
4
5
6
7
8
9
–
60
Neat
C6H5CH3 60
CH3OH
C2H5OH
CH2Cl2
Neat
Neat
Neat
Neat
6 h
Trace
72
66
68
70
88
92
94
69
10
10
10
10
10
10
10
5
Reflux
Reflux
Reflux
Reflux
60
70
80
70
60
60
60
60
60
60
60
a
Reaction conditions: resorcinol, 1.0 mmol; EAA, 1.2 mmol (with solvent) and
2.0 mmol (under solvent-free conditions).
b
Determined by MS of the corresponding 7-hydroxy-4-methylcoumarin product.
Table 2
Reaction of resorcinol and EAA in the presence of different amount of PVPHS at
stirring at room temperature and ultrasonic irradiation under solvent-free
conditions.a
Ultrasound irradiation has been established as a significant
technique in synthetic organic chemistry [37–42]. Ultrasound-as-
sisted has been used for the synthesis of 2-amino-4H-chromenes,
coumarin thioglycopyranoside derivatives and hymecromone as
insecticide intermediate [28,42–44]. Shorter reaction time is the
main benefit of ultrasound-assisted reactions. Simple experimental
procedure, high yields, improved selectivity and clean reaction of
many ultrasound induced organic transformations offers addi-
tional convenience in the field of synthetic organic chemistry.
Poly(4-vinylpyridinium) hydrogen sulfate (PVPHS) is inexpen-
sive, insensitive to air and requires no special care during its han-
dling. The potential of this reagent for the chemoselective 1,1-
diacetate protection and deprotection of aldehydes [45], the syn-
thesis of 14-aryl-14H-dibenzo[a,j]xanthenes under conventional
heating and ultrasound irradiation [46], synthesis of 12-aryl-
12H-indeno[1,2-b]naphtho[3,2-e]pyran-5,11,13-triones [47], the
synthesis of coumarin derivatives under microwave irradiation
[30b] has been recently explored.
Entry Amonut of catalyst
(mg)
Room temperature
Ultrasonic irradiation
Time
(h)
Yield
(%)b
Time
(min)
Yield
(%)b
1
2
3
–
5
10
6
2
2
Trace
32
48
60
15
5
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
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 as well as solvent-free
system at variety of temperature with PVPHS as the catalyst. The
results are collected in Table 1.
The resorcinol conversion increased with increase in tempera-
ture up to 80 °C and thereafter it attained steady state over the cat-
alyst. There was no significant difference in conversion between 70
and 80 °C (Table 1, entries 6–8). The yield of 7-hydroxy-4-methyl-
coumarin decreased with decreasing of catalyst amount (Table 1,
entry 9). Interestingly, no reaction took place in the absence of cat-
alyst after 6 h of the reaction time (Table 1, entry 1). However, it
was noticed that the best result was achievable under solvent-free
conditions. The increased reactivity may be attributed to close con-
tact of reactants in solvent-free condition where the dilution factor
from solvent does not exist (viz. concentration effect). In some
cases, the solid state organic reaction occurred more efficiently
and more selectively than the solution reaction, since molecules
in a crystal were arranged tightly and regularly [49].
In comparison with the stirring at room temperature, PVPHS
employed under ultrasonic irradiation showed a more effective
catalytic activity than the other in terms of yield and reaction time
(Table 2, entries 2 and 3). Hence the ultrasonic irradiation which
required only 5 min duration to complete reaction of resorcinol
with EAA and the product yield was 96% is compared to the stirring
at room temperature which have taken 120 min for reaction and
yield was significantly less (48%) under solvent-free conditions
With these considerations in mind, we decided to evaluate our
solid acid catalyst for the Pechmann condensation under sonica-
tion and solvent-free conditions. To the best of our knowledge,
the coupling of ultrasonic irradiation with solid phase catalyst
PVPHS as an activator in Pechmann synthesis under solvent-free
conditions is now reported by us for the first time.
2. Results and discussion
In continuation of our ongoing research program on the devel-
opment of new catalyst and methods for organic transformations
[48] and energy-saving procedures developed in our laboratory,
in an earlier publication, we described Poly(4-vinylpyridinium)
hydrogen sulfate (PVPHS) as efficient catalyst for the synthesis of
14-aryl-14H-dibenzo[a,j]xanthenes under solvent-free conditions
and ultrasound irradiation [46]. In here we wish to apply this pro-
tocol to the Pechmann synthesis (Scheme 1).
To optimize the reaction conditions, the reaction between res-
orcinol (110 mg, 1 mmol) and 0.25 mL ethyl acetoacetate (EAA)
(260 mg, 2 mmol) was used as a model reaction. Before taking up
Scheme 1. The synthesis of substituted coumarins in presence PVPHS at room temperature under ultrasound irradiation and solvent-free conditions.

1064
N.G. Khaligh / Ultrasonics Sonochemistry 20 (2013) 1062–1068
Table 3
Pechmann synthesis of various coumarins in the presence of PVPHS under ultrasonicirradiation.a
Entry
1
Phenol
b-Keto ester
Time (min)
Yield (%)b
Ref.
EAA
MEAA
5
10
96
92
[54]
HO
OH
2
3
EAA
EAA
9
78
82
[55]
[56]
H2N
HO
OH
10
OH
OH
4
EAA
MEAA
8
12
92
86
[54]
OH
CH3
OH
5
6
EAA
MEAA
3
7
91
86
[57]
[58]
HO
HO
OH
OH
EAA
EAA
10
12
89
78
[55]
[55]
OCH3
HO
7
8
OH
H3COC
OH
EAA
MEAA
8
14
90
86
[59]
[58]
OH
OH
OH
9
EAA
EAA
12
10
78
72
[55]
[60]
10
OH
Cl
11
12
13
EAA
EAA
EAA
16
8
62
85
82
[57]
[59]
[28]
OH
O2N
H3CO
OH
10
OH
OCH3
OH
14
15
16
EAA
EAA
EAA
12
8
82
84
81
[61]
[62]
[54]
OCH2CH3
OH
CH3
OH
18
17
18
EAA
EAA
16
15
80
[55]
[63]
OH
72c
HO
OH
a
b
c
Reaction conditions: substrate, 1.0 mmol; EAA or MEAA, 2.0 mmol.
Isolated yield.
Obtained both a linear benzocoumarin (72%, m.p. 267 °C) and an angular benzocoumarin (14%, m.p. 276–277 °C) [63].

N.G. Khaligh / Ultrasonics Sonochemistry 20 (2013) 1062–1068
1065
(Table 2, entry 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 under stirring at room temperature and ultrasonic
conditions, respectively (Table 2, entry 1). In the absence of solid
acid, there is no solid phase, so this ultrasonic large acceleration
is not related to additional effects of the liquid–solid systems. It
can be explained that cavitational collapse in a liquid produces in-
tense local heating, high pressures, and enormous heating and
cooling rates. Therefore, acoustic cavitation provides a unique
interaction of energy and matter, and ultrasonic irradiation of liq-
uids causes high energy chemical reactions to occur.
It was clear that the ultrasound irradiation could accelerate
Pechmann reaction. The following is expected to be plausible rea-
son for the higher yields and shorter reaction times during ultra-
sonic irradiation. The chemical effects of ultrasounds have been
attributed to implosive collapse of the cavitations period of the
sound waves. The bubbles are generated at localized sites in the li-
quid mixture that contain small amounts of dissolved gases. When
these bubbles burst, it results in high temperature and high pres-
sure which facilitate the intermolecular reaction. When one of
the phases is a solid, the ultrasonic irradiation has several addi-
tional enhancement effects, and this is especially useful when
the solid acts as a catalyst [50]. When cavitation occurs near an ex-
tended solid acid catalyst surface, cavity collapse is non-spherical
and drives high-speed jets of ethylacetoacetate (EAA) into the
PVPHS surface. These jets and associated shock waves can cause
substantial surface damage and expose fresh, highly heated sur-
faces. This fact increases the chances of interphase surface able
to react [51].
yields under ultrasonic irradiation in a short reaction time. More-
over, the presence of electron-donating group in the meta position
it decrease the further hydroxyalkylation site which leads to a low-
er rate of attack of nucleophile to electrophile. Phenol (Table 3, en-
try 9) required longer reaction duration, as no electron-donating
group is present. Similarly, naphthols (Table 3, entries 16, 17) re-
quired long reaction times due to presence of another phenyl moi-
ety. Alkoxyphenols (Table 3, entries 12–14) showed no detectable
dealkylation under the reaction conditions. When the condensa-
tion of meta-aminophenol (Table 3, entry 2) with ethyl acetoace-
tate was carried out, the major product is the coumarin and only
trace of quinoline was obtained [52]. A literature survey revealed
that, resacetophenone (Table 3, entry 7) failed to react to give a
coumarin derivative in the presence of sulfuric acid as the catalyst
[53]. However, the reaction was observed under aluminium chlo-
ride catalysts [28], but required a temperature of 130 °C in nitro-
benzene as the solvent. In contrast, the PVPHS furnished good
yields of the product under ultrasonic irradiation. In all instances
except two, coumarins are obtained. As exceptions, 2,7-dihydroxy-
naphthalene and 2-naphthol give a mixture of a linear and angular
coumarin and a chromone. From these experiments, it is clearly
demonstrated that the synthesis of coumarin derivatives using
PVPHS under ultrasonic irradiation is indeed an effective method
and is undoubtedly superior to other procedures with respect to
reaction time, availability of catalyst, work-up procedure and
yields.
Table 3 also compares the activities of various b-keto esters
with various phenols. Reactivity order for b-keto esters was found
to be EAA > MEAA (Table 3, 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 [28,54–63].
The possible mechanism for the synthesis of 7-hydroxy-4-
methylcoumarin in the presence of PVPHS as a promoter under
ultrasound irradiation is shown in Scheme 2. On the basis of this
mechanism, the implosive collapse of the cavitations period of
the sound waves generates the bubbles at localized sites in the li-
quid phase (EAA). When these bubbles burst, it results in high tem-
perature and high pressure which facilitate the hydroxyalkylation
reaction. When the catalyst is a solid, the ultrasonic irradiation
has several additional enhancement effects [50]. The transesterifi-
cation and dehydration occur concomitantly condensing together
the two reactants at two sites to form coumarin product and regen-
erates PVPHS in the reaction.
In order to evaluate the generality of the process, several diver-
sified examples illustrating the present method for the synthesis of
substituted coumarins was studied. The reaction of EAA with var-
ious phenols was carried out in the presence of PVPHS as catalyst
under ultrasonic irradiation. The yields obtained were good to
excellent and these results are illustrated in Table 3.
The shorter reaction times were observed for the reaction of
phloroglucinol (Table 3, entry 5) with ethyl acetoacetate (EAA)
and
a-methyl ethyl acetoacetate (MEAA). This was mainly due to
the presence of three hydroxyl groups that cooperated in activating
the aromatic ring for hydroxyalkylation. The reactions of pyrogallol
with EAA and MEAA (Table 3, entry 8) were slightly slower than
that of the reactions of resorcinol and phloroglucinol presumably
due to steric hindrance of hydroxyl groups. Substrates (Table 3, en-
tries 1, 2, 4, 5 and 13) having electron-donating groups in the para
position to the site of electrophilic substitution gave maximum
We compared the obtained results (reaction conditions, time,
isolated yield) with some of those reported in the literature (Ta-
Scheme 2. Proposed mechanism for Pechmann reaction of resorcinol with EAA at room temperature under ultrasonic irradiation.

1066
N.G. Khaligh / Ultrasonics Sonochemistry 20 (2013) 1062–1068
Table 4
Comparison of our results with results obtained by other groups.
Entry
Catalyst
Reaction conditions
Time (h)
Yield (%)a
Ref.
1
2
3
4
5
6
7
8
p-TsOH
PVPHS
Sulfamic acid
K5CoW12O40.3H2O
I2
Neat/125 °C
Neat/MW (560 W)/65–70 °C
Neat/125 °C
Neat/125 °C
Neat/90 °C
Neat/130 °C
Neat/125 °C
Neat/110 °C
Neat/120 °C
Neat/100 °C
Neat/120 °C
Neat/MW (250 W)/150 °C
Neat/160 °C
Neat/MW (600 W)/160 °C
CH3CN/Reflux
4
89
85
93
91
90
82
94
81
80
88
87
99
94
97
95
92
96
[29]
[30]b
[37]
[52]
[53]
[54]
[55]
[56]
[58]
[64]
[65]
[66]
[67]
7 min
8
2
2.5
1
2
1.5
5 min
9
2.38
15 min
4
15
1
1
LiBr
Amberlyst-15
Cellulose sulfuric acid
SiO2/H2SO4
Zr-TMS-TFA-25
Sulfate CexZr1-xO2
Nano-crystalline sulfate-Zr
Mesoporous phosphate-Zr
9
10
11
12
13
14
15
SiO2/NaHSO4
PVPHS
[68]
This work
Neat/70 °C
Neat/US (35 kHz, 200 W)
5 min
a
Isolated yield.
ble 4). As can be seen, our method was simpler, more efficient, and
used no toxic solvents.
3. Experimental
3.1. Materials
Unless specified, all chemicals were analytical grade and pur-
chased from Merck, Aldrich and Fluka Chemical Companies and
used without further purification. Products were characterized by
their physical constant and comparison with authentic samples.
The purity determination of the substrates and reaction monitor-
ing were accompanied by TLC using silica gel SIL G/UV 254 plates.
Fig. 1. Recovery and reuse of the catalyst on Pechmann reaction of resorcinol with
EAA at room temperature under ultrasonic irradiation.
3.2. Instrumentation
The IR spectra were recorded on a Perkin Elmer 781 Spectro-
photometer using KBr pellets for solid and neat for liquid samples
in the range of 4000–400 cmÀ1. The UV spectra were recorded on
an Agilent 8453 UV–vis spectrophotometer at room temperature.
Mass spectra were recorded on a FINNIGAN MAT 8430 mass spec-
trometer operating at an ionization potential of 70 eV. In all the
cases the 1H NMR spectra were recorded with Bruker Avance 400
or 300 MHz instrument using. 13C NMR data were collected on Bru-
ker 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. Bandelin Sonorex (with a fre-
quency of 35 kHz and a nominal power 200 W) ultrasonic bath
was used for ultrasonic irradiation, built-in heating, 30–80 °C ther-
mostatically adjustable. The reaction vessel placed inside the ultra-
sonic bath containing water.
completion of the reaction (monitored by TLC), ethanol was added
to the reaction mixture and the catalyst was recovered by filtra-
tion. The filtrate was concentrated in vacuum, and the crude prod-
uct was washed with water, dried and slowly recrystallized in
ethanol or ethanol–water system. The melting point, IR, 1H NMR
and mass spectroscopic techniques were used to analyze the prod-
ucts and compared with the authentic samples.
The recovered catalyst, after drying, was reused for four more
consecutive pechmann reactions of resorcinol (1.0 mmol) and
EAA (2.0 mmol) affording 96%, 96%, 94%, and 94% isolated yields,
respectively, in 5, 5, 6, and 8 min (Fig. 1).
Of course, the SEM images of the fresh and recycled catalyst
demonstrated a very interesting effect of ultrasonic irradiation on
the surface morphology and particle size of solid acid catalyst
(Fig. 2). Upper image is fresh catalyst and lower is recycled catalyst
after first and 4th run of Pechmann reaction. High-velocity inter-
particle collisions caused by ultrasonic irradiation are responsible
for the agglomeration of particles into extended aggregates. This
process can effectively remove the inactive and passivating surface
catalyst dramatically improves reaction rates. Scanning electron
microscopy of the solid acid catalyst PVPHS showed likely that
the origin of the observed rate enhancements comes from in-
creased effective surface area and to a lesser extent from surface
damage.
3.3. General procedure for the preparation of coumarins
In a 25-mL batch reactor equipped with a distillation condenser
the mixture of phenols (1.0 mmol), b-keto esters (2.0 mmol) and
PVPHS (10 mg, 0.02 mmol) was stirred and irradiated with ultra-
sonic of low power (with a frequency of 35 kHz and a nominal
power 200 W). The temperature of the reaction mixture started
to rise. After 2 min of irradiation, the ultrasound source was
switched off. Since the Pechmann reaction proved to be exother-
mic, the reaction mixture continued to rise in temperature. After
The SEM micrographs of recycled catalyst after first and 4th run
showed that the structure of catalyst not changed noticeable
(Fig. 2, lower images). Also the FTIR spectra of recycled catalyst

N.G. Khaligh / Ultrasonics Sonochemistry 20 (2013) 1062–1068
1067
Fig. 2. SEM images of fresh PVPHS (upper image) and PVPHS after first and 4th runs of Pechmann reaction (lower images).
is same as after first and 4th run catalyst, therefore the composi-
tion of solid acid catalyst was not changed by ultrasonic
irradiation.
(KBr): m ;
= 3216, 3028, 2986, 1675, 1620, 1283, 1120, 1062 cmÀ1
1H NMR (300 MHz, DMSO-d6) d = 2.04 (s, 3H, CH3), 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+].
7,8-Benzo-4-methylcoumarin (Table 3, entry 16) white pow-
3.4. The spectral data of selected compounds are below
ders, mp: 155–156 °C (ethanol) (lit. mp: 153–154 °C); [57] IR
(KBr):
m ;
= 2920, 1675, 1615, 1561, 1285, 1167, 1056 cmÀ1 1H
7-Hydroxy-4-methylcoumarin (Table 3, entry 1): off-white
powders, mp: 184–185 °C (ethanol) (lit. mp. 182–184 °C) [57]; IR
(KBr):
NMR (300 MHz, DMSO-d6) d = 2.24 (s, 3H, CH3), 5.98 (s, 1H, ArH),
6.57 (s, 1H, ArH), 6.72 (d, J = 8.8 Hz, 1H, ArH), 7.46 (d, J = 8.8 Hz,
1H, ArH), 10.56 (brs, 1H, OH) ppm. MS (EI, 70 eV) m/z: 176 [M+].
5-Hydroxy-3,4,7-trimethylcoumarin (Table 3, entry 4) white
crystals, mp: 249–251 °C (ethanol) (lit. mp: 249 °C) [57]; IR
NMR (400 MHz, DMSO-d6) d = 2.56 (s, 3H, CH3), 6.53 (s, 1H, ArH),
7.72–7.75 (m, 2H, ArH), 7.83 (d, J = 8.8 Hz, 1H, ArH), 7.90 (d,
J = 8.8 Hz, 1H, ArH), 8.06–8.08 (m, 1H, ArH), 8.38–8.41 (m, 1H,
ArH). MS (EI, 70 eV): m/z = 210 [M+].
8-hydroxy-4-methyl-2H-naphtho[2,3-b]pyran-2-one (Table 3,
entry 18) yellow powders, mp: 275–277 °C (ethanol/water) (lit.
m ;
= 3450, 3028, 1675, 1585, 1400, 1230, 1054 cmÀ1 1H
mp: 267 °C) [66]; IR (KBr)
m = 3360, 3031, 1705, 1564, 1238,

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N.G. Khaligh / Ultrasonics Sonochemistry 20 (2013) 1062–1068
[29] (a) J.C. Rodriguez-Dominguez, G. Kirsch, Tetrahedron Lett. 47 (2006) 3279;
1062 cmÀ1
;
1H NMR (400 MHz, DMSO-d6) d = 2.46(s, 3H, CH3),
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4. Conclusions
In conclusion, Pechmann synthesis of coumarins is efficiently
catalyzed in the presence of PVPHS under ultrasound and sol-
vent-free conditions. The methodology has several advantages
such as: green aspects by avoiding toxic solvents, high reaction
rates and excellent yields, no side reactions, ease of preparation
and handling of the catalyst, effective recovering and reusability
of the catalyst, use of inexpensive catalyst with lower loading
and simple experimental procedure. The use of ultrasound offers
some hope of activating less reactive, but also less costly, catalysts.
Further work to explore this catalyst for the other organic transfor-
mations is in progress.
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