2W10O32. 2H2O: A novel and powerful catalyst for the synthesis of 4-arylidene-2-phenyl-5(4)-oxazolones under ultrasonic condition

Article abstract of DOI:10.1016/j.crci.2011.02.003

Di[1,6-bis(3-methylimidazolium-1-yl)hexane] decatungstate dihydrate ([C₆(MIm)₂]₂W₁₀O₃₂. 2H₂O) as a new family of polyoxometalate-based dicationic ionic liquids (POM-DIL) is synthesized and employed as a novel and powerful heterogeneous catalyst in the synthesis of 4-arylidene-2-phenyl-5(4)-oxazolones (azlactones) under ultrasound-assisted solvent-free condition. On the basis of the results, the products were obtained in excellent yields under mild condition. Utilization of easy work-up and purification make it very interesting from an economic perspective. Moreover, a recycling study confirmed that the catalyst can be reused multiple times without significant loss of its activity.

Full text of DOI:10.1016/j.crci.2011.02.003

C. R. Chimie 14 (2011) 869–877
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Preliminary communication/Communication
[C6(MIm)2]2W10O32. 2H2O: A novel and powerful catalyst for the
synthesis of 4-arylidene-2-phenyl-5(4)-oxazolones under ultrasonic
condition
*
*
Mahboubeh Rostami, Ahmad R. Khosropour , Valiollah Mirkhani ,
Iraj Mohammadpoor-Baltork, Majid Moghadam, Shahram Tangestaninejad
Catalysis Division, Department of Chemistry, Faculty of Science, University of Isfahan, Isfahan 81746-7344, Iran
A B S T R A C T
A R T I C L E I N F O
Article history:
Di[1,6-bis(3-methylimidazolium-1-yl)hexane] decatungstate dihydrate ([C6(MIm)2]2
W10O32. 2H2O) as a new family of polyoxometalate-based dicationic ionic liquids (POM-
DIL) is synthesized and employed as a novel and powerful heterogeneous catalyst in the
synthesis of 4-arylidene-2-phenyl-5(4)-oxazolones (azlactones) under ultrasound-assisted
solvent-free condition. On the basis of the results, the products were obtained in excellent
yields under mild condition. Utilization of easy work-up and purification make it very
interesting from an economic perspective. Moreover, a recycling study confirmed that the
catalyst can be reused multiple times without significant loss of its activity.
Received 9 October 2010
Accepted after revision 10 February 2011
Available online 24 March 2011
Keywords:
Heterogeneous catalysis
Polyoxometalates
Green chemistry
Azlactones
´
ß 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
POM-based ionic liquid and employed it as catalyst for the
esterification of alcohols [9]. However, a limited number of
The development of hybrid organic-inorganic frame-
works due to replacing toxic metal catalysts with
degradable organic compounds has emerged as one of
the most potentially significant fields of investigation in
modern chemistry [1]. Recently, hybrids of ionic liquids
with polyoxometalates (POMs) have grown tremendously,
not just as novel inorganic materials [2–4], but also as
green catalysts and anti-tumor agents [5], have been
increasingly known. For example, Tandon et al. [6]
reported a new catalyst consisting of imidazolium and
tungstate ions—tris(imidazolium) tetrakis-(diperoxotung-
sto)phosphate—for the oxidation of alcohols. Kumar et al.
[7] reported that 1-methyl-3-butylimidazolium decatung-
state catalyzed the selective oxidation of benzylic and
secondary alcohols with hydrogen peroxide. Zhao et al. [8]
utilized Keggin POMs [bmim]3[PW12O40] to catalyze olefin
epoxidation. Very recently, Hou et al. synthesized a new
these compounds have been investigated catalytically in
heterocyclic synthesis [4]. Considering the number of
structures discovered, it must be realized that the studying
of reactions using organic cations bonded with POMs as
Keggin anions as heterogeneous catalysts especially in
ultrasonic-assisted organic synthesis (UAOS), is still in an
immature state and being one of the underdeveloped areas
research.
In recent years, solvent-free reactions in order to meet
environmental demands have become one of the most
important aspects in organic chemistry and great efforts
have been and still are being made to find and develop new
ones [10]. Moreover, the strategy of one-pot condensation
reactions leading to interesting heterocyclic scaffolds is
particularly useful for the creation of diverse drug-like
molecules for biological screening [11]. One of the most
attractive one-pot condensation reactions is the Erlen-
meyer-Plo¨chl reaction that has been used for the synthesis
of 4-arylidene-2-phenyl-5(4)-oxazolone derivatives [12]
which occupy an important place in the realm of biological
and pharmaceutical sciences [13–17]. Recently, almazo-
lone as an alkaloid bearing azlactone unit, was isolated
* Corresponding authors.
E-mail addresses: khosropour@chem.ui.ac.ir (A.R. Khosropour),
mirkhani@sci.ui.ac.ir (V. Mirkhani).
1631-0748/$ – see front matter ß 2011 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2011.02.003

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M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
from the Senegalese alga Haraldiophyllum sp. and synthe-
sized experimentally [18]. These systems also play a
crucial role for the synthesis of hydrogel supports for in
vivo cell growth [19]. Hence, synthesis of this heterocyclic
nucleus is very important. Different methods have been
reported for the synthesis of azlactones [20–26]. However,
most of them require refluxing for hours in toxic organic
solvents, the use of expensive or corrosive catalysts and a
tedious work-up. As a consequence, implementation of
ecofriendly and straightforward chemical methods
remains actually a particularly interesting task.
clic compounds and also in continuation of our study on
developing new and ecofriendly methodologies for the
preparation of fine chemicals [31–34], herein we would
like to report [C6(MIm)2]2W10O32. 2H2O as a new hybrid of
dicationic ionic liquid-polyoxometalate which incorpo-
rates 1,6-bisÀ(34-methylimidazolium-1-yl)hexane as cation
and W10O32 as anion. Next, the novel POM-based DIL
served as an active catalyst for room temperature
synthesis of 4-arylidene-2-phenyl-5(4)-oxazolones under
solvent-free and ultrasonic irradiation (US) conditions
(Scheme 1).
As increasing environmental consciousness in chemical
research and industry, the challenge for a sustainable
environment calls for clean procedures. UAOS, as a green
synthetic approach, is a powerful technique that is being
used more and more to accelerate organic reactions [27].
UAOS can be extremely efficient and it is applicable to a
broad range of practical syntheses. The notable features of
the ultrasound approach are enhanced reaction rates,
formation of purer products in high yields, easier
manipulation and it is considered a processing aid in
terms of energy conservation and waste minimization
compared with traditional methods. So, this technique is
more convenient, taking green chemistry concepts into
account [28].
This synthetic strategy combining the merits of solvent-
free, green heterogeneous catalyst and US realized the
environmentally friendly synthesis of a series of azlac-
tones. To our knowledge, it is the first catalytic application
of a hybrid of dicationic ionic liquid-polyoxometalate
under ultrasonic condition for the synthesis of hetero-
cycles.
To evaluate the catalytic activity of [C6(MIm)2]2W10O32
.
2H2O, this POM-DIL was first put to use in the condensa-
tion reaction of 4-chlorobenzaldehyde (2a), hippuric acid
(b) and acetic anhydride (c) at room temperature under US,
as a standard model reaction. Under optimum reaction
conditions, it was found that high catalytic activity for this
transformation was obtained and an excellent conversion
rate and satisfactory yield (92%) in the presence of only 5
mol% of the catalyst affording the desired product
(Table 1).
2. Results and discussion
The result revealed that, in the presence of 3, 4 and 6
mol% of the catalyst, it gave lower yields of the product
even after prolonged reaction times (Table 1, entries 1, 2
and 4). Moreover, the catalyst is essential and in the
absence of it, the reaction progressed sluggishly (Table 1,
entry 5). We have also investigated the effect of solvents
and revealed that under the solvent-free condition, the
excellent yield of the product was obtained (Table 1,
entry 3).
According to the reports of Armstrong’s group, germinal
dicationic ionic liquids (DILs) showed high thermal
stability in high temperature organic reactions [29,30].
Then, we decided to utilize this kind of cation to make the
catalyst stable enough during reaction. Moreover, to our
knowledge, almost all of the so-called ‘‘ionic liquids’’ based
on POMs needed to be dissolved in organic solvents or
another ionic liquid for catalytic applications. So, to expand
the application of new POMs in the synthesis of heterocy-
O
Ph
O
[C6(MIm)2]2W10O32. 2H2O
r. t.,
ArCHO + PhCONHCH2CO2H + Ac2O
N
ArCH
1
2
3
Scheme 1. Ultrasound-assisted one-pot synthesis of azlactones catalyzed by [C6(MIm)2]2W10O32. 2H2O.
Table 1
Synthesis of 4-(4’-chlorobenzylidene)-2-phenyl-5(4)-oxazolone as a model catalyzed by ([C6(MIm)2]2W10O32. 2H2O) under different conditionsa.
Entry
Amount of catalyst (mol%)
Solventb
Method
Temperature (8C)
Yield (%)c
1
2
3
4
5
6
7
8
9
10
3
4
5
6
0
5
5
5
5
5
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
CH3CN
Ultrasound
Ultrasound
Ultrasound
Ultrasound
Ultrasound
Ultrasound
Ultrasound
Ultrasound
Stirring
25–35
25–35
25–35
25–35
25–35
25–35
25–35
25–35
25–35
80
60
75
92
91
< 5
20
CHCl3
25
Toluene
40
Solvent-free
Solvent-free
< 10
60
Stirring
a
After 18 min.
b
c
Using 2 ml.
Isolated Yields.

M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
871
To demonstrate the effect of sonication, the synthesis of
2c as a model was investigated with and without US. When
the reaction was carried out in the absence of US with
stirring at room temperature, the yield of 2c was very low
(< 5%, Table 1, entry 9), while at the same temperature
under the influence of US, it gives excellent yield of the
corresponding product in a short reaction time (92%, Table
1, entry 3). Moreover, under heating condition (at 80 8C)
and in the absence of US, the corresponding product was
obtained only in 60% yield (Table 1, entry 10). The reason
may be the phenomenon of cavitations produced by
ultrasound.
To further optimize the reaction, the procedure was
carried out at different output power of the ultrasound
irradiation. It was found that, the yield reached a plateau at
140 W power. Using the optimized reaction conditions, we
synthesized a series of 4-arylidene-2-phenyl-5(4)-oxazo-
lones in the presence of [C6(MIm)2]2W10O32. 2H2O under
US. These compounds were synthesized in excellent yields
with various aromatic/heteroaromatic aldehydes contain-
ing electron donating or electron withdrawing functional
groups at different positions but it did not show any
remarkable differences in the yields of the products and
reaction times (Table 2).
Table 2
Synthesis of 4-arylidene-2-phenyl-5(4)-oxazolones catalyzed by ([C6(MIm)2]2W10O32. 2H2O) under ultrasound irradiation at room temperature.
Entry
Ar
Product
Time (min)
Yield (%)a
O
O
1
C6H5
3a
20
80
N
O
O
2
4-ClC6H4
3b
18
92
N
Cl
Cl
O
O
O
3
2-ClC6H4
3c
12
88
N
O
Cl
Cl
4
2,4-Cl2C6H3
3d
18
89
N
O
O
5
4-BrC6H4
3e
15
91
N
Br
Br
O
O
6
2-BrC6H4
3f
14
94
N

872
M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
Table 2 (Continued )
Entry
Ar
Product
Time (min)
Yield (%)a
O
O
7
3-BrC6H4
3g
15
89
N
Br
O
O
N
8
4-FC6H4
3h
10
92
F
O
O
N
9
4-O2NC6H4
3i
24
85
NO2
O
O
10
3-O2NC6H4
3j
25
87
N
NO2
O
O
N
11
4-NCC6H4
3k
24
85
CN
O
O
N
12
4-CH3OC6H4
3l
8
93
OCH3
O
O
N
13
4-(CH3)2NC6H4
3m
8
92
N(CH3)2

M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
873
Table 2 (Continued )
Entry
Ar
Product
Time (min)
Yield (%)a
O
O
N
14
4-PhCH2OC6H4
3n
20
86
OCH2
O
O
15
3,5-(CH3)2-4-(OH)C6H2
3o
10
90
N
OCH3
CH3O
OH
N
O
O
N
O
O
16
1,4-(CHO)2C6H4
3p
22
88
O
N
O
17
1-naphthyl
3q
8
91
O
O
18
3-indolyl
3r
13
89
N
N
H
O
O
19
5-CH3-2-thiophenyl
3s
12
93
N
S
CH3
a
Yields refer to isolated pure products.
On the other hand, bis-azalctone 3p was obtained
in excellent yield while using 1,4-phthalaldehyde
(Scheme 2).
The present method is convincingly superior to
the reported methods with respect to yield, reaction
time, simplicity and safety. Also, to the best of
our knowledge, no report is available in the
literature using a combination of polyoxometalate as
catalyst and ultrasound irradiation for heterocyclic
synthesis.
any toxic organic reagents/solvents and generates virtually
no byproducts. Equally important is the high selectivity,
and nearly quantitative yields of this transformation,
which lead to significant structural diversity in the
products. All the synthesized compounds were character-
ized by spectral data (IR, NMR, and MS) and compared with
authentic samples.
Another advantage of this catalyst is its recycleability.
Thus [C6(MIm)2]2W10O32. 2H2O can be separated by
filtration and washing with hot ethanol, dried at 80 8C
under reduced pressure and reused in at least five runs
without any loss of its activity (Table 3).
Generally, the reaction is experimentally excellent and
proceeded under US at room temperature without using

874
M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
O
CHO
CHO
O
N
[C6(MIm)2]2W10O32
O
N
O
+ 2 PhCONHCH2CO2H + Ac2O
r. t.,
Scheme 2. Synthesis of (4Z)-4-{4-[(7Z)-(5-Oxo-2-phenyloxazol-4(5H)-ylidene)methyl]benzylidene}-2-phenyloxazol-5(4H)-one.
Table 3
was synthesised according to the literature [35]. All known
Results obtained using recycled [C6(MIm)2]2W10O32. 2H2Oa.
organic products were identified by comparison of their
physical and spectral data with those of authentic samples.
Thin layer chromatography (TLC) was performed on UV-
active aluminum-backed plates of silica gel (TLC Silica gel
60 F254). 1H, and 13C NMR spectra were measured on a
Bruker DPX 400 MHz spectrometer in DMSO-d6 with
Entry
Cycle no.
Time (min)
Yield (%)b
1
2
3
4
5
1
2
3
4
5
18
18
18
18
18
92
92
90
86
84
chemical shift (d) given in ppm relative to TMS as internal
a
Based on synthesis of 4-(4’-chlorobenzylidene)-2-phenyl-5(4)-oxa-
standard. Coupling constants are given in Hz. Low-
resolution mass spectra (LRMS) were recorded on a Bell
and Howell 21–490 spectrometer. IR spectra were taken on
a FT-IR-Tensor 27 spectrometer in KBr pellets and reported
in cmÀ1. Melting points were determined using Stuart
Scientific SMP2 apparatus. Sonication was performed in a
UP 400S ultrasonic processor equipped with a 3 mm wide
and 140 mm long probe, which was immersed directly into
the reaction mixture. The operating frequency was 24 kHz
and the output power was 0–400 W through manual
adjustment. Thermogravimetric analysis (TG) was carried
out with a Seiko TG-DTA 6200-S under atmospheric
zolone.
b
Isolated yield.
Although the detailed mechanism of the reaction is not
clear, this condensation most probably proceeded via two
step reactions (Scheme 3).
In the first step, [C6(MIm)2]2W10O32. 2H2O through
activation of acetic anhydride, engages hippuric acid to
produce intermediate A. In the next step, tautomerization
of A in the presence of the catalyst and the following
condensation reaction produces the corresponding prod-
uct. It seems that US affects the stability of the
intermediates and therefore, accelerates the reaction
accordingly.
condition. This measurement was performed with
a
temperature ramp of 10 8C/min between 25 and 600 8C.
3.2. Preparation and characterization of [C6(MIm)2]2W10O32
.
2H2O
3. Experimental
To a solution of Na2WO4.2H2O (5 mmol) in H2O (10 ml)
was added aqueous HCl (9 ml, 3 M) and boiled until a clear
yellow solution was obtained. A solution of 1,6-bis(3-
methylimidazolium-1-yl)hexane chloride [C6(MIm)2Cl2]
(10 mmol; dried at 80 8C) was added to this solution and
3.1. General
All chemicals purchased from Aldrich-Sigma and Merck
chemical companies and used as received. 1,6-bis(3-
methylimidazolium-1-yl)hexane chloride [C6(MIm)2Cl2]
Ph
N
Ph
N
Ph
N
Ph
H
Ph
H
O
O
+
OCOCH3
O
Cat.
OH
+
O
O
-H
Ac2O
Cat.
O
N
N
H
O
O
OH
O
-
CH3CO2
Cat. =
C6(MIm)2 2W10O32
Ph
Ph
Ph
O
O
+ H+
- H+
O
N
N
N
H
O
O
O
CHAr
H
CHAr
H
Cat. + H2O
OH
ArCH=O
Cat.
Cat.
Scheme 3. Plausible mechanism.

M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
875
Fig. 1. Overlap of FT-IR Spectrums.
Fig. 2. TG-DTG diagrams of [(C14H24N4)2W10O32]. 2H2O.
the precipitate was filtered, washed with water and dried
overnight at 80 8C in vacuum. Tungsten content in the
catalyst was confirmed by ICP-AES analysis (Shimadz, ICP-
1000 IV). The particular characteristics of the POM-based
DIL catalyst during the reaction have been examined in
detail. In the FT-IR spectrum of POM-based DIL, the well-
known characteristic bands at 891and 796 cmÀ1, which
resulted from the W = O and W–O–W vibrations, respec-
tively, proved the presence of the core POM clusters. Sharp
bands at 3435, 3076, 2933, 1566 and 1167 cmÀ1, arising
from the dicationic lionic liquid, can also be seen clearly
(Fig. 1).
POM-DIL showed an obvious weight loss of nearly 15.3%
between 300 and 425 8C, which corresponded to the
proportion of the DIL.
So we concluded that this loss resulted from the
decomposition of 1,6-bis(3-methylimidazolium-1-yl)hex-
ane ion. At around 450 8C, another obvious weight loss was
detected, corresponding to the onset of the thermal
À4
decomposition of W10O32 (Scheme 4).
TGA-DTG analysis (Fig. 2) showed about 1.32% weight
loss below 150 8C, suggesting that 1 mol of POM-DIL still
contained about 2 mol of H2O. In addition, the TGA trace
Scheme 4. [C6(MIm)2]2W10O32. 2H2O.

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M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
3.3. General procedure for the synthesis of 4-arylidene-2-
phenyl-5(4)-oxazolones
4. Conclusions
In conclusion, we have developed a novel, highly
effective and entirely green procedure for the synthesis
of 4-arylidene-2-phenyl-5(4)-oxazolones at room temper-
ature under solvent-free and US conditions in the presence
of [C6(MIm)2]2W10O32. 2H2O. To the best of our knowledge,
this is the first application of hybrid of organic-inorganic
catalyst in heterocyclic synthesis under sonication condi-
tion that bear dicationic ionic liquid and polyoxometalate.
Moreover, other noteworthy features of this approach as
follows: (i) the catalyst could be easy synthesysed, (ii)
[C6(MIm)2]2W10O32 could be reused for five times after
simple treatment and (iii) the corresponding azlactone can
be separated by easy work-up. The mild reaction condi-
tions, clean reaction profiles, operational and experimental
simplicity, enhanced reaction rates, and with options of
further transformations of the resulting 4-arylidene-2-
phenyl-5(4)-oxazolones into synthetically interesting bio-
logically active compounds, this synthetic methodology
ideally suited for automated applications in organic
synthesis.
A mixture of arylaldehyde (1 mmol), hippuric acid
(1.2 mmol) and acetic anhydride (5 mmol) in the presence
of [(C6(MIm)2)2W10O32]. 2H2O (0.05 mmol) was exposed to
US at room temperature for 5–25 min. The output power of
140 W was chosen after optimization. After completion of
the reaction as indicated by TLC, ethanol (5 ml) was added
to it and stirred for 10 min until a yellow solid precipitated.
The mixture was allowed to stand overnight, and then it
was cooled in an ice bath. An aqueous solution (20%) of
NaHCO3 (10 ml) was added, the solid products and the
catalyst were filtered. The solid materials were dissolved in
Et2O to remove the catalyst. The solvent was evaporated
and the pure products were recrystallized from ethanol
(10)/water (5).
The structures of all the products were unambiguously
established on the basis of their spectral analysis (FT-IR, 1H
NMR, 13C NMR and mass spectra).
3.4. Spectral data for selected compounds
(4Z)-4-(4-Hydroxy-3,5-dimethoxybenzylidene)-2-pheny-
Acknowledgments
loxazol-5(4H)-one (3o): yellow solid, mp 217–219 8C.
FTIR(KBr)
y
(cmÀ1): > 3000, 2922, 2848, 1791, 1761,
max
The authors are grateful to the Center of Excellence of
Chemistry of University of Isfahan (CECUI) and also the
Research Council of the University of Isfahan for financial
support of this work.
1654, 1595, 1201, 1130; 1HNMR (400 MHz, acetone-d6),
d
(ppm): 3.91 (s, 1H), 3.95 (s, 6H), 7.28 (s, 1H), 7.64 (t,
J = 8 Hz, 2H), 7.73 (t, J = 8 Hz, 1H), 7.83 (s, 2H), 8.22 (d, 2H);
13CNMR (100 MHz, acetone-d6),
d(ppm): 55.79, 109.12,
116.48, 125.57, 128.12, 129.28, 130.58, 131.77, 133.64,
136.84, 152.64, 166.82, 174.66; MS(EI): m/z (%) = 325.44,
201.25, 105.11, 77.05, 57.18.
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d
(ppm): 112.23, 112.40, 119.62, 121.63, 123.24, 125.65,
126.45, 127.08, 127.52, 127.55, 129.13, 132.60, 134.48,
137.02, 160.03, 166.99. FTIR(KBr)
(cmÀ1): 3250,
y
max
> 3000, 2922, 2852, 1726, 1639, 1230, 1132. MS (EI): m/z
(%) = 288.04, 154.87, 104.88, 76.87, 56.99.
(4Z)-4-[(5-Methylthiophen-2-yl)methylene]-2-phenylox-
azol-5(4H)-one (3 s): yellow solid; melting point: 145–
147 8C. FTIR(KBr)
1645, 1548, 1325, 1151.1HNMR (400 MHz, CDCl3),
y
max (cmÀ1): > 3000, 2920, 2852, 1788,
d
(ppm): 2.63(s, 3H), 6.897(s, 1H), 7.44(m, 2H), 7.54(m, 2H),
7.62(m, 1H), 8.17(m, 2H). 13CNMR (100 MHz, CDCl3),
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d(ppm): 16.20, 125.55, 125.89, 126.86, 127.60, 128.22,
128.91, 133.00, 135.95, 136.35, 151.58, 160.56, 167.27. MS
(EI): m/z (%) = 269.12, 207.11, 105.14, 77.08, 51.14.

M. Rostami et al. / C. R. Chimie 14 (2011) 869–877
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