An environmentally benign one-pot synthesis of 3,4-dihydropyrimidin-2(1H)- ones and -thiones using tetrabutylammonium hexatungstate [TBA] 2[W6O19] as a recyclable catalyst

Article abstract of DOI:10.1080/15533174.2011.591358

Tetrabutylammonium hexatungstate [TBA]₂[W₆O ₁₉], an isopolytungstate, was found to be a highly efficient and recyclable heterogeneous catalyst for Biginelli reaction of ethyl acetoacetate, an aryl aldehyde, and urea or thi

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An Environmentally Benign One-Pot Synthesis of
3,4-Dihydropyrimidin-2(1H)-ones and -thiones Using
Tetrabutylammonium Hexatungstate [TBA] [W O ]
2
6 19
as a Recyclable Catalyst
a
a
Neda Mohammadzadeh-Dehsorkh , Abolghasem Davoodnia , Niloofar Tavakoli-Hoseini
a
a
&
Maryam Moghaddas
a
Department of Chemistry, Mashhad Branch , Islamic Azad University , Mashhad, Iran
Published online: 07 Nov 2011.
To cite this article: Neda Mohammadzadeh-Dehsorkh , Abolghasem Davoodnia , Niloofar Tavakoli-Hoseini & Maryam
Moghaddas (2011) An Environmentally Benign One-Pot Synthesis of 3,4-Dihydropyrimidin-2(1H)-ones and -thiones Using
Tetrabutylammonium Hexatungstate [TBA] [W O ] as a Recyclable Catalyst, Synthesis and Reactivity in Inorganic, Metal-
2
6 19
Organic, and Nano-Metal Chemistry, 41:9, 1135-1140, DOI: 10.1080/15533174.2011.591358
To link to this article: http://dx.doi.org/10.1080/15533174.2011.591358
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:1135–1140, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.591358
An Environmentally Benign One-Pot Synthesis of
3
,4-Dihydropyrimidin-2(1H)-ones and -thiones Using
Tetrabutylammonium Hexatungstate [TBA] [W O ] as a
2
6
19
Recyclable Catalyst
Neda Mohammadzadeh-Dehsorkh, Abolghasem Davoodnia, Niloofar
Tavakoli-Hoseini, and Maryam Moghaddas
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
Dihydropyrimidinones and -thiones have attracted consider-
2 6
Tetrabutylammonium hexatungstate [TBA] [W O
19], an able attention because of their wide spectrum of biological and
isopolytungstate, was found to be a highly efficient and recyclable therapeutic activities such as antiviral, antitumor, antibacterial,
heterogeneous catalyst for Biginelli reaction of ethyl acetoacetate,
an aryl aldehyde, and urea or thiourea under solvent-free
conditions, giving rise to 3,4-dihydropyrimidin-2(1H)-ones and
[6–8]
anti-inflammatory, anticancer, and antioxidant properties.
Additionally, their particular structure has been found in natural
marine alkaloid batzalladines, which are the first low-molecular-
and absence of any volatile and hazardous organic solvents are weight natural products reported in the literature to inhibit
just a few of the advantages of this procedure. Furthermore, the the binding of HIV gp-120 to the CD4 cell, thus disclosing
-
thiones. Good to high yields, short reaction times, easy workup,
catalyst can be recycled after a simple workup, and can be used
at least three times without substantial reduction in its catalytic
activity.
[9]
a new path toward the development of AIDS therapy. The first
procedure for the synthesis of dihydropyrimidinones, reported
by Biginelli in 1893, involves acid-catalyzed one-pot, three-
component condensation of β-dicarbonyl compounds with an
aromatic aldehyde and urea derivatives.[ A major drawback
to Biginelli’s original reactions, however, was poor to moderate
yields.[ There are several improved methods for the synthe-
sis of dihydropyrimidinones and -thiones by condensation of
β-dicarbonyl compounds with an aromatic aldehyde and urea
or thiourea in the presence of Lewis or protic acid promoters
Keywords Biginelli reaction, heterogeneous catalysis, multicompo-
10]
nent reactions, solvent-free conditions, tetrabutylammo-
nium hexatungstate [TBA] [W O ]
2 6 19
11]
INTRODUCTION
Multicomponent reactions (MCRs) have attracted the
attention of synthetic organic chemists for building highly func-
tionalized organic molecules and pharmacologically important
heterocyclic compounds in a very fast and efficient manner with-
out the isolation of any intermediate.[1] Multicomponent con-
densations involve three or more compounds reacting in a single
event, but consecutively to form a new product, which contains
the essential parts of all the starting materials. Therefore, MCRs
contribute to the requirements of an environmentally friendly
process by reducing the number of synthetic steps, energy con-
[
12]
[13]
such as concentrated H2SO4,
trichloroisocyanuric acid,
[14]
InBr3, N-butyl-N,N-dimethyl-α-phenylethylammonium bro-
[
15]
[16]
[17]
mide, [bmim][FeCl4], Ln(OTf)3, with Ln = Yb, Sc, La,
Amberlyst-70,[ and ZrCl4 or ZrOCl2. Among the available
methods applied for the preparation of dihydropyrimidinones
and -thiones, sometimes harsh reaction conditions, long reaction
times, low yields of the products, use of harmful organic sol-
vents, expensive reagents, and strongly acidic conditions limit
their uses for this purpose. In addition, some of the catalysts are
not recoverable and reusable materials. Therefore, the devel-
opment of simple, efficient, high-yielding, and environmentally
friendly methods using new catalysts for the Biginelli reaction
is still necessary.
18]
[19]
[
2]
[3]
[4]
sumption, and waste production. Bigenilli, Ugi, Passerini,
and Mannich[ reactions are some examples of MCRs.
5]
Polyoxometalates (POMs), constituting a large class of metal
oxide molecules, are known to have a variety of sizes, struc-
tures, electrochemical properties, and chemical reactivities.
Received 26 March 2011; accepted 22 April 2011.
The authors are thankful to Islamic Azad University, Mashhad
Branch, for financial support.
Address correspondence to Abolghasem Davoodnia, Department
of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad,
Iran. E-mail: adavoodnia@mshdiau.ac.ir
[20]
POMs have been extensively studied because they have many
[
21,22]
practical applications, such as catalysis,
molecular mate-
rials,[ and corrosion inhibition. In recent decades, uses of
23]
[24]
some POMs as catalysts for fine organic synthetic processes
1135

1136
N. MOHAMMADZADEH-DEHSORKH ET AL.
have been developed and are important for industries related to
TABLE 2
[
25]
fine chemicals, including flavors, pharmaceuticals, and food Synthesis of compound 4d in the presence of [TBA]2[W6O19]
industries.[ Furthermore, the Lindqvist-type metal–oxygen
clusters, M6O192 (M = W, Mo), and their functionalized com-
pounds have fascinating chemical properties and photophysical
26]
a
(0.1 g) at different temperatures in solvent-free conditions
−
◦
Yield (%)b
Entry
T ( C)
Time (h)
[
27,28]
properties, and diverse practical applications.
1
2
3
4
5
6
r.t
50
70
90
120
140
8
8
8
5
2
None
Trace
63
In continuation of our studies on the development of
novel synthetic methodologies in various organic reac-
tions,[ we wish to report here a solvent-free synthesis of
29]
78
74
93
3,4-dihydropyrimidin-2(1H)-ones and -thiones catalyzed by
tetrabutylammonium hexatungstate [TBA]2[W6O19], an
isopolytungstate, as a green and effective heterogeneous
catalyst (Scheme 1).
25 (min)
aEthyl acetoacetate (4 mmol), 4-chlorobenzaldehyde (4 mmol), and
urea (5 mmol).
b
Isolated yields.
Synthesis Of Tetrabutylammonium Hexatungestate,
[TBA]2[W6O19]
This catalyst was prepared according to the literature.[30]
A mixture of sodium tungestate dihydate, Na2WO4·2H2O,
(
99%, 0.1 mol, 33 g), acetic anhydride (40 mL), and N,N-
◦
dimethylformamide (30 mL) is heated at 100 C for 3 h to obtain
a white cream. Then a solution of acetic anhydride (20 mL) and
HCl (12 N, 18 mL) in DMF (50 mL) is added with stirring,
and the resulting mixture is filtered off to eliminate the undis-
solved white solid. A solution of tetrabutylammonium bromide
SCH. 1. Biginelli reaction catalyzed by [TBA]2[W6O19].
(
0.047 mol, 15.1 g) in methanol (50 mL) is added with rapid
stirring to give a white precipitate. This suspension is stirred for
min and the product is filtered. Recrystallization from a mini-
mum amount of hot dimethyl sulfoxide (DMSO) gives colorless
EXPERIMENTAL
5
Chemicals and Apparatus
All chemicals were available commercially and used without diamond-shaped crystals.
additional purification. The catalyst was synthesized according
General Procedure for the Synthesis of
to the literature. Melting points were recorded on an electrother-
mal type 9100 melting point apparatus. The infrared (IR) spec-
tra were obtained using a 4300 Shimadzu spectrophotometer
3
4
,4-Dihydropyrimidin-2(1H)-ones and -thiones
a–n Using [TBA]2[W6O19] as Catalyst
1
as KBr disks. The H-nuclear magnetic resonance (NMR; 500
A mixture of ethyl acetoacetate 1 (4 mmol), an aryl aldehyde
MHz) spectra were recorded with a Bruker DRX500 spectrom- 2 (4 mmol), urea or thiourea 3 (5 mmol), and [TBA]2[W6O19]
◦
eter.
(0.1 g, 0.053 mmol) was heated on the oil bath at 140 C for
TABLE 1
TABLE 3
Effect of solvent on the model reaction
Effect of [TBA]2[W6O19] amount on the model reaction in
a
a
solvent-free conditions
◦
Yield (%)b
Entry
Solvent
EtOH
H2O
CH2Cl2
CH3CN
Cyclohexane
Solvent-free
T ( C)
Time (h)
Entry
Catalyst (mol%)
Time (min)
Yield (%)b
1
2
3
4
5
6
Reflux
Reflux
Reflux
Reflux
Reflux
140
8
8
8
8
8
10
None
12
60
None
93
1
2
3
4
5
6
None
0.7
0.9
1.3
2.0
120
25
25
25
25
25
None
40
68
93
80
25 (min)
2.6
60
aEthyl acetoacetate (4 mmol), 4-chlorobenzaldehyde (4 mmol), urea
(5 mmol), and [TBA] [W O ] (0.1 g).
a
Ethyl acetoacetate (4 mmol), 4-chlorobenzaldehyde (4 mmol), and
◦
urea (5 mmol) at 140 C.
2
6
19
b
b
Isolated yields.
Isolated yields.

[TBA]2[W6O19]-CATALYZED BIGINELLI REACTION
1137
TABLE 4
a
Synthesis of 3,4-dihydropyrimidin-2(1H)-ones and -thiones 4a–n catalyzed by [TBA]2[W6O19]
◦
Melting point ( C)
Entry
1
Ar
X
O
Productb
Time (min)
30
Yield (%)c
87
Found
Reported
195–196
Reference
[16]
Br
O
3-BrC6H4
192–193
O
EtO
Me
NH
N
H
4a
2
4-BrC6H4
O
30
88
186–188
197
[15]
3
4
2-ClC6H4
4-ClC6H4
O
O
40
25
85
93
218–220
216–217
222–224
212–214
[14]
[14]
5
4-FC6H4
O
36
86
178–180
173–176
[19a]
6
7
3-HOC6H4
4-HOC6H4
O
O
38
35
80
86
175–177
236–238
163–165
230–232
[14]
[14]
(Continued on next page)

1138
N. MOHAMMADZADEH-DEHSORKH ET AL.
TABLE 4
a
Synthesis of 3,4-dihydropyrimidin-2(1H)-ones and -thiones 4a–n catalyzed by [TBA]2[W6O19] (Continued)
◦
Melting point ( C)
Entry
8
Ar
X
O
Productb
Time (min)
25
Yield (%)c
90
Found
Reported
198–200
Reference
[19a]
4-MeOC6H4
200–203
9
4-MeC6H4
O
25
92
209–212
214–216
[13]
1
0
1
2-ClC6H4
4-ClC6H4
S
S
35
35
80
83
166–168
190–193
168–169
192–194
[19b]
[14]
1
12
13
14
4-FC6H4
S
S
S
38
30
37
81
90
80
190–191
151–152
180–183
191–192
150–152
189–192
[18]
[14]
[13]
4-MeOC6H4
4-MeC6H4
a
◦
Ethyl acetoacetate (4 mmol), aryl aldehyde (4 mmol), urea or thiourea (5 mmol), and [TBA]
2
[W
6
O
19] (0.1 g) at 140 C.
b
All the products were characterized by IR spectral data and comparison of their melting points with those of authentic samples. Also, the
1
structures of some products were confirmed by H-NMR spectral data.
c
Isolated yields.

[TBA]2[W6O19]-CATALYZED BIGINELLI REACTION
1139
an appropriate time. The progress of the reaction was moni-
The catalyst has a significant effect on the yield of the reaction
tored by thin-layer chromatography (TLC). Upon completion, (Table 1). It could be seen that no product was produced in
the reaction mixture was cooled to room temperature and hot the absence of [TBA]2[W6O19] even after 2 h (entry 1), while
ethanol was added. The catalyst was insoluble in hot ethanol good results were obtained in the presence of [TBA]2[W6O19].
and it could therefore be recycled by a simple filtration. The Increasing the amount of the catalyst to 0.1 g, the yield of the
product was collected from the filtrate after cooling to room product could be increased to 93% within the same time. Greater
temperature and recrystallized from ethanol to give compounds amounts of the catalyst were found to have an inhibitory effect
4
a–n in good to high yields.
on the formation of the product (entries 5 and 6). Therefore, the
optimal amount of the catalyst was 0.1 g (1.3 mol% based on
ethyl acetoacetate) (entry 4).
Recycling and Reusing of the Catalyst
Due to the fact that the catalyst was insoluble in hot ethanol, it
The same model reaction in the presence of 0.1 g of the cata-
could therefore be recycled by a simple filtration. The separated lyst was carried out at different temperatures in solvent-free con-
◦
catalyst was washed with cold ethanol, dried at 60 C under ditions to assess the effect of temperatures on the reaction yield.
vacuum for 1 h, and was reused in another reaction. The catalyst It was observed that yield is a function of temperature since
could be used at least three times with only slight reduction in the yield was increased as the reaction temperature was raised
◦
its catalytic activity.
(Table 2). At 140 C, the product 4d was obtained in high yield.
Also, the reaction was carried out in various solvents
(Table 3). As shown in Table 3, in comparison to conventional
methods the yield of the reaction under solvent-free conditions
is higher and the reaction time is shorter.
1
Selected H-NMR and IR Data
1
Compound 4a (Ar = 3-BrC6H4, X = O): H-NMR (500
MHz, DMSO-d6, δ ppm): 1.10 (t, 3H, J = 7.0 Hz, CH3), 2.25
In order to evaluate the generality of this model reaction we
then prepared a range of 3,4-dihydropyrimidin-2(1H)-ones and
thiones under the optimized reaction conditions. In all cases,
(
s, 3H, CH3), 3.95–4.05 (m, 2H, CH2), 5.14 (d, 1H, J = 3.2 Hz,
CH), 7.23 (d, 1H, J = 7.7 Hz, arom-H), 7.31 (t, 1H, J = 7.8 Hz,
arom-H), 7.39 (s, 1H, arom-H), 7.45 (dt, 1H, J = 7.9, 0.9 Hz,
arom-H), 7.77 (s, 1H, NH), 9.25 (s, 1H, NH). IR (KBr disk): υ
-
aromatic aldehydes with substituents carrying either electron-
donating or electron-withdrawing groups reacted successfully
and gave the expected products in good to high yields in short
reaction times. The kind of aldehyde had no significant effect
on the reaction. The results are shown in Table 4.
Reusability of the catalyst was also investigated. The catalyst
was recovered according to the procedure mentioned in exper-
imental section and reused for next reactions. The obtained
results are summarized in Table 5. As shown in this table, the
catalyst could be used at least three times with only slight re-
duction in the catalytic activity.
−
1
3
238 (NH), 3114 (NH), 1706 (C = O), 1654 (C = O) cm .
1
Compound 4d (Ar = 4-ClC6H4, X = O): H-NMR (500
MHz, DMSO-d6, δppm): 1.09 (t, 3H, J = 7.1 Hz, CH3), 2.25
(
s, 3H, CH3), 3.95–4.05 (m, 2H, CH2), 5.15 (d, 1H, J = 3.2 Hz,
CH), 7.25 (d, 2H, J = 7.7 Hz, arom-H), 7.39 (d, 2H, J = 7.7 Hz,
arom-H), 7.75 (s, 1H, NH), 9.22 (s, 1H, NH). IR (KBr disk): υ
−1
3
233 (NH), 3114 (NH), 1703 (C = O), 1650 (C = O) cm .
1
Compound 4e (Ar = 4-FC6H4, X = O): H-NMR (500 MHz,
DMSO-d6, δ ppm): 1.09 (t, 3H, J = 7.2 Hz, CH3), 2.25 (s, 3H,
CH3), 3.95–4.00 (m, 2H, CH2), 5.14 (d, 1H, J = 3.2 Hz, CH),
Although we did not investigate the reaction mechanism,
based on Kantevari’s suggestion,
7.10-7.30 (m, 4H, arom-H), 7.73 (s, 1H, NH), 9.20 (s, 1H, NH).
[
31]
the tetrabutyl ammonium
IR (KBr disk): υ 3238 (NH), 3079 (NH), 1703 (C = O), 1650
+
−
1
ion [(n-Bu)4N ] probably induces polarization of carbonyl
group in aldehydes. On the other hands, Reinheimer and co-
workers[ reported that terminal oxygen atoms or the bridging
oxygen atom in the polyoxometalate anion W6O19 are slightly
basic and therefore can promote the reactions.
(
C = O) cm .
Compound 4m (Ar = 4-MeOC6H4, X = S): H-NMR (500
1
32]
MHz, DMSO-d6, δppm): 1.11 (t, 3H, J = 7.1 Hz, CH3), 2.28 (s,
2−
3
5
7
1
H, CH3), 3.72 (s, 3H, CH3O), 4.00 (q, 2H, J = 7.1 Hz, CH2),
.11 (d, 1H, J = 3.6 Hz, CH), 6.89 (d, 2H, J = 8.6 Hz, arom-H),
.12 (d, 2H, J = 8.6 Hz, arom-H), 9.57 (s, 1H, NH), 10.26 (s,
H, NH). IR (KBr disk) υ 3314 (NH), 3169 (NH), 1667 (C =
TABLE 5
−1
O) cm .
Comparison of efficiency of [TBA]2[W6O19] in the synthesis of
3,4-dihydropyrimidin-2(1H)-ones and -thiones after three uses
RESULTS AND DISCUSSION
Yield (%)a
The catalyst [TBA]2[W6O19] was prepared according to
[
30]
Entry
Ar
X
First
Second
Third
Fournier’s method.
We first selected the synthesis of
compound 4d as a model reaction to optimize the reaction
conditions. The reaction was carried out by heating a mix-
ture of ethyl acetoacetate (4 mmol), 4-chlorobenzaldehyde (4
mmol), and urea (5 mmol) in the presence of various amounts
of [TBA]2[W6O19].
4
4
4
a
d
m
3-BrC6H4
4-ClC6H4
4-MeOC6H4
O
O
S
87
93
90
86
92
89
86
90
88
aIsolated yields.

1140
N. MOHAMMADZADEH-DEHSORKH ET AL.
CONCLUSION
TBA]2[W6O19], a solid organic salt of isopolytungstate,
showed high catalytic activity in the synthesis of 3,4-
10. Biginelli, P. Gazz. Chim. Ital. 1893, 23, 360.
1
1. Barluenga, J.; Thomas, M.; Ballesterus, A.; Lopez, A. Tetrahedron Lett.
989, 30, 4573.
2. Hassani, Z.; Islami, M.R.; Kalantari, M. Bioorg. Med. Chem. Lett. 2006,
6, 4479.
[
1
1
dihydropyrimidin-2(1H)-ones and -thiones. This procedure of-
1
fers several advantages including mild reaction conditions, high 13. Bigdeli, M.A.; Jafari, S.; Mahdavinia, G.H.; Hazarkhani, H. Catal. Com-
yields, and ease of workup, which makes it a useful and attrac-
tive protocol for the synthesis of these compounds. Furthermore,
the catalyst can be recycled after a simple workup, and used at
least three times with only slight reduction in its catalytic activ-
mun. 2007, 8, 1641.
4. Fu, N.Y.; Yuan, Y.F.; Cao, Z.; Wang, S.W.; Wang, J.T.; Peppe, C. Tetrahe-
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1
ity. It has also all advantages devoted to solvent-free reactions, 16. Chen, X.; Peng, Y. Catal. Lett. 2008, 122, 310.
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17. Ma, Y.; Qian, C.T.; Wang, L.M.; Yang, M. J. Org. Chem. 2000, 65, 3864.
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