Efficient and green catalytic synthesis of dihydropyrimidinone (thione) derivatives using cobalt nitrate in solvent-free conditions

Article abstract of DOI:10.4067/S0717-97072014000100015

A series of 3,4-dihydropyrimidin-2(1H)-one(thione) derivatives was synthesized using Co(NO3)2.6H2O in solvent-free condition. Avoiding organic solvents during the chemical reactions leading to an economic approach is effective. The reaction is characteriz

Full text of DOI:10.4067/S0717-97072014000100015

J. Chil. Chem. Soc., 59, Nº 1 (2014)
EFFICIENT AND GREEN CATALYTIC SYNTHESIS OF DIHYDROPYRIMIDINONE (THIONE) DERIVATIVES
USING COBALT NITRATE IN SOLVENT-FREE CONDITIONS
MASOUD NASR-ESFAHANI,* MORTEZA MONTAZEROZOHORI, MARYAM AGHEL-MIRREZAEE AND
HASSAN KASHI
Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran
(Received: May 14, 2013 - Accepted: December 14, 2013)
ABSTRACT
A series of 3,4-dihydropyrimidin-2(1H)-one(thione) derivatives was synthesized using Co(NO3)2.6H2O in solvent-free condition. Avoiding organic solvents
during the chemical reactions leading to an economic approach is effective. The reaction is characterized by high efficiency, short reaction time, high yields, simple
experimental procedure, availability of catalyst and environmentally friendly reaction conditions.
Keywords: Dihydropyrimidinone, Dihydropyrimidinthione, Cobalt(II)Nitrate, Solvent-free, One-pot synthesis
an important subject 18. Solid acids have emerged as potential alternate catalysts
INTRODUCTION
to the common liquid acids due to their safe natures, enhanced selectivity,
requirements in catalytic amounts and easier work up 19
.
Synthesis of 3,4-dihydropyrimidine-2(1H)-Ones (DHPMs) through one-
pot reaction of aromatic aldehyde, urea and ethyl acetoacetate in acid ethanol
Solid catalysts are harmless to the environment due to safety, no corrosion
and reduction of the amount of waste residuals. So many reactions in organic
chemistry of the solid catalyst are used. Following our interest in producing
3,4-dihydropyrimidine-2(1H)-ones 20, a study to revisit this reaction in a
parallel combinatorial fashion using Co(NO3)2.6H2O in a solvent-free synthesis
approach was initiated.
1
solution was initiated by Biginelli in 1893 . These compounds occupied an
important place in medicinal and synthetic organic chemistry, mainly because
of their wide range of biological activities 2.
Notably, monastrol (1) is the only cell-permeable molecule that blocks
normal bipolar spindle assembly in mammalian cells causing cell cycle arrest
3,and is considered a lead for the development of new anticancer drugs 4, while
the appropriately functionalized DHPM analogs have emerged as orally active
antihypertensive agents (2, 4)5.
EXPERIMENTAL
Chemicals were purchased from Merck, Fluka and Aldrich chemical
companies. Melting points were determined using a Barnstead/Electothermal
(BI) capillary apparatus and are uncorrected. IR spectra were recorded from
KBr discs on a JASCO FT-IR-680. 1H NMR spectra were recorded with
Brucker ultrasheilld NMR 400 machines. NMR spectra were obtained on
solution in DMSO-d6 using TMS as internal standard.
Many biological activities such as anticancer, antifungal, anti HIV have
been exhibited using the representatives such as batzelladines, ptilomycalines
(3) and crambescidines (Fig. 1) 6.
General procedure for the synthesis of 3,4-dihydropyrimidin-2(1H)-
ones(thiones)
A
mixture of aldehyde (1.0 mmol), β-dicarbonyl (1.0 mmol),
Co(NO3)2.6H O (0.15 mmol) and urea or thiourea (1.5 mmol) was magnetically
stirred at 80 2°C in solvent-free condition. After completion of the reaction,
as indicated by TLC (EtOAc/n-hexane, 1:4), the reaction mixture was filtered
and the residue recrystallized from ethanol to afford the pure product. All
products were characterized by mp, IR and 1H NMR spectra. The physical and
spectroscopic data of new compounds is given bellow:
Methyl 4-(2,6-dichlorophenyl)-6-methyl-2-oxo-1,2,3,4 tetrahydropy-
rimidine-5-carboxylate (8q): Mp: 292-294 ˚C; Rf = 0.43 (n-hexane:ethyl
acetate = 4:1); IR (ῡ, cm–1): 3320, 3227, 2976, 1698, 1652, 1661, 1577, 1497,
1
1284, 1236; H NMR (400 MHz, DMSO-d ) d(ppm) : 2.06 (s, 3H), 2.19 (s,
3H), 6.18 (s, 1H), 7.26 (t, 1H, J = 7.6 Hz ),67.40 (d, 2H, J = 8.0 Hz), 7.62 (s,
1H), 9.25 (s, 1H); Anal. Calcd. for C13H12Cl2N2O3: C, 49.54; H, 3.84; Cl, 22.50;
N, 8.89; O, 15.23; found: C 49.45, H 3.90, N 8.80.
Methyl 4-(2-bromophenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydropyrimi-
dine-5-carboxylate (8r): Mp: 249-251 ˚C; Rf = 0.33 (n-hexane:ethyl acetate =
4:1); IR (ῡ, cm–1): 3363, 3249, 2929, 1703, 1623, 1458, 1381, 1229; 1H NMR
(400 MHz, DMSO-d6) d (ppm) : 2.05 (s, 3H), 2.34 (s, 3H), 5.63 (s, 1H), 7.20
(t, 1H, J = 6.0 Hz ), 7.26 (d, 1H, J = 6.4 Hz), 7.37 (t, 1H, J = 7.2 Hz ), 7.61 (d,
1H, J = 7.2 Hz), 7.71 (s, 1H), 9.30 (s, 1H); Anal. Calcd. for C13H13BrN O3: C,
48.02; H, 4.03; Br, 24.57; N, 8.62; O, 14.76; found: C 47.93, H 4.10, N28.54.
Methyl 4-(2,4-dichlorophenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydro-
pyrimidine-5-carboxylate (8y): Mp: 214-215 ˚C; Rf = 0.50 (n-hexane:ethyl
acetate = 4:1); IR (ῡ, cm–1): 3406, 3179, 2949, 1678, 1652, 1563, 1464, 1203;
1H NMR (400 MHz, DMSO-d ) d(ppm) : 2.30 (s, 3H), 3.34 (s, 3H), 5.58 (s,
1H), 7.28 (d, 1H, J = 8.8 Hz), 67.43 (d, 1H, J = 8.0 Hz ), 7.58 (s, 1H, J = 7.2
Hz), 9.65 (s, 1H), 10.45 (s, 1H); Anal. Calcd. for C13H12Cl2N2O2S: C, 47.14;
H, 3.65; Cl, 21.41; N, 8.46; O, 9.66; S, 9.68; found: C 47.03, H 3.72, N 8.39.
Methyl 4-(3-chlorophenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydropy-
rimidine-5-carboxylate (8z): Mp: 248-249 ˚C; Rf = 0.35 (n-hexane:ethyl
acetate = 4:1); IR (ῡ, cm–1): 3316, 3176, 2968, 1678, 1662, 1574, 1434, 1385,
<<Fig. 1>>
7
The Biginelli method was developed and many catalysts such as CaF2
8, Sr(OTf)2 9, PPh3 10, tetrabutylammonium bromide (TBAB) 11, copper(II)
sulfamate 12, phenyl phosphonic acid 13, g-Fe2O3/CuO (on the nanoscale)
14
and so on have been used in the Biginelli reaction. Also microwave irradiation
15, ultrasound irradiation and/or ionic liquids have been used. However,
many of these methods suffer from drawbacks such as the use of expensive
reagents, strong acidic conditions, long reaction times and use of expensive
and poisonous solvents. Therefore, the introduction of a more efficient and
milder methods accompanied with higher yields are in demand yet. In recent
years with the development of green chemistry technology, multi components
reactions under solvent-free conditions with a solid catalyst are considered as
16
17
e-mail: manas@mail.yu.ac.ir
2311

J. Chil. Chem. Soc., 59, Nº 1 (2014)
1285; 1H NMR (400 MHz, DMSO-d6) d(ppm) : 2.29 (s, 3H), 3.56 (s, 3H), 5.17
(s, 1H), 7.17-7.36 (m, 4H), 9.71 (s, 1H), 10.45 (s, 1H); Anal. Calcd. for C13H-
ClN O2S: C, 52.61; H, 4.42; Cl, 11.95; N, 9.44; O, 10.78; S, 10.80; found: C
1532.532, H 4.50, N 9.37.
temperature were optimized. The results showed that use of 1mmol
benzaldehyde, 1 mmol ethylacetoacetate, 1.5 mmol of urea or thiourea and
1.5 mmol catalyst under solvent-free condition at 80 °C, the product of
dihydropyrimidinone (thiones) with 90% efficiency was achieved. Then
under this condition, dihydropyrimidinones and dihydropyrimidinthiones by
various aromatic and aliphatic aldehydes were synthesized. The results were
presented in table 3. As shown, various aromatic aldehydes bearing either
electron-releasing or electron-withdrawing substituents can lead to high yields.
The use of methyl acetoacetate or acetylacetone as 1,3-dicarbonyl moiety in
place of ethyl acetoacetate also gave similar results (Table 3, entries 16–21,
25-27). Furthermore, substrate with two formyl group (entry 10) was produced
the corresponding bis-dihydropyrimidinone in short time and excellent yield
(Scheme 2).
RESULTS AND DISCUSSION
In this paper, we report synthesis of the dihydropyrimidinones and
dihydropyrimidinthiones using cobalt nitrate hexahydrate as catalyst at 80 °C
in solvent-free condition (Scheme 1).
Scheme 1. Synthesis of DHPMs 8a-x.
Effect of catalyst concentration and solvent
The catalyst concentration was varied over a range of 5–25 mol% of
Co(NO3) .6H2O on the basis of the total volume of the reaction mixture. Table
1 shows 2the effect of catalyst concentration on the reaction of benzaldehyde,
ethylacetoacetate and urea. The yield of the corresponding dihydropyrimidinone
increased with increasing catalyst concentration from 5 to 15 mol%. Further
addition of catalyst had no noticeable effect on the yield. This was due to over
an obvious concentration, there have an excess of catalyst sites beyond what
is actually required by the reactant substrates, and the additional catalyst does
not increase the rate of the reaction. Therefore, in all further reactions 15 mol%
were used.
Scheme 2. Synthesis of ethyl-4-(3-(5-(ethoxycarbonyl)-1,2,3,4-
tetrahydro-6-methyl-2-oxopyrimidine-4-yl)-Phenyl)-1,2,3,4-tetrahydro-6-
methyl-2-oxopyrimidine-5-carboxylate.
The reactions of acid-sensitive substrates such as 2-thiophenecarbaldehyde
and cinnamaldehyde also proceeded well to give the dihydropyrimidinone
without any side products (entry 11 and 12). However, aliphatic aldehydes
such as butanal, as observed previously, reacted over longer times with a
reduced yield (entry 13, 80 min time, 35% yield) compared with the aromatic
compounds under our reaction conditions 20
.
Table 1. Investigation of catalyst effects in the synthesis of
5-(Ethoxycarbonyl)-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-one
under solvent-free conditionsa
To use of Co(NO3)2.6H2O in large scale synthesis especially in chemical
laboratory, a typical reaction was performed for synthesis of 8a with tenfold
amounts of reactants and catalyst with respect to one mentioned in the
experimental section. The result showed that the yield of 87% in these
conditions that is comparable with one in table 3.
Amount of catalyst
Entry
Time (min)
Yields (%)b
(mol%)
Three possible mechanisms are proposed for Biginelli reaction according
to the literature but generally accepted reaction mechanism includes the acid-
catalyzed formation of C-N bond from the benzaldehyde and urea (pathway
A, Scheme 3) 36. According to reported results the pathway A is characteristic
for the Brønsted type of catalysts whereas Lewis acid type of catalysts follows
the pathway B (ureido-crotonate mechanism) 37. For clarifying the role of the
catalyst, three separated reactions were performed (pathways A-C, Scheme
3) under the optimized reaction conditions (15 mol% of cobalt(II) nitrate at
80 ºC in solvent.free condition and reflux temperature in acetonitrile during
30 min and 20 h, respectively). The prolonged heating of benzaldebyde and
ethyl acetoacetate (pathway C) or benzaldehyde and urea (pathway A) did not
undergo the expected reactions to yield of products whereas the reaction of
ethyl acetoacetate and urea furnished the ureido-crotonate (pathway B). These
observations clearly indicate that Biginelli reaction catalyzed by cobalt(II)
nitrate proceeds predominately.
1
2
3
4
5
5
80
65
40
40
50
85
90
93
94
94
10
15
20
25
bIsolated yields
Then, the solvent effect in the condensation of benzaldehyde (1 mmol), urea
(1.5 mmol) and ethyl acetoacetate (1mmol) in the presence of Co(NO3)2.6H O
(0.15 mmol) as a model has been studied. As shown in Table 2, among t2he
tested solvents, such as ethanol, methanol, acetonitrile, water, chloroform and
a solvent-free system, the best result was obtained after 40 min under solvent-
free conditions in excellent yield (90%).
Table 2. Solvent effect on the reaction of benzaldehyde, urea and ethyl
acetoacetate catalyzed by Co(NO3)2.6H2O.
Entry
Solvent (reflux)
CH3OH
Time(h)
20
Yielda(%)
1
2
3
4
5
6
68
70
60
55
65
90
CH3CH2OH
CH3CN
20
20
CH3Cl
20
H2O
Solvent-freeb
20
0.6
a Isolated yields
b At 80 0C
We began our studies with the reaction of benzaldehyde (5a),
ethylacetoacetate (6a) and urea (7a) as a model reaction. For this purpose,
various parameters such as molar ratio of reactants, catalyst and reaction
Scheme 3. Three proposed possible mechanisms for Biginelli reaction.
2312

J. Chil. Chem. Soc., 59, Nº 1 (2014)
Comparative results
In order to show the ability of our method with respect to previous reports, our result for synthesis of 5-(ethoxycarbonyl)-6-methyl-4-phenyl-3,4-
dihydropyrimidin-2(1H)-ones (8a) in comparison to other methods for preparation of this compound have been summarized in table 4. As shown in table 4, the
yield/time ratio of the present method is better or comparable with the other reported results.
Table 3. Synthesis of 3,4-dihydropyrimidin-2-(1H)-ones(thiones) Catalyzed by Co(NO3)2.6H2O under solvent-free Conditionsa
entry
1
R1
R2
X
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
Product
8a
8b
8c
Time (min)
40
Yield(%)b
Mp (ºC)
208-209
205-207
215-217
185-187
212-214
257-259
216-218
207-209
208-209
212-214
213-215
230-232
178-180
251-252
227-228
191-193
292-294
249-251
232-234
250-253
171-169
191-194
197-199
200-202
254-255
248-249
241-243
Ref
21
13
13
22
13
24
13
13
25
26
23
27
23
22
28
29
-
C6H5
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OMe
OMe
OMe
Me
90
2
4-CH3OC6H4
4-CH3C6H4
3-BrC6H4
4-ClC6H4
2-CH3OC6H4
2-ClC6H4
4-NO2C6H4
2-NO2C6H4
3-CHOC6H4
2-Thienyl
C6H5-CH=CH
n-Pr
40
92
3
50
90
4
8d
8e
65
79
5
65
80
6
8f
55
85
7
8g
8h
8i
70
85
8
75
80
9
35
88
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
8j
24
90
8k
8l
65
85
40
75
8m
8n
8o
8p
8q
8r
80
35
2,4-Cl2C6H3
3-NO2C6H4
C6H5
50
80
14
93
40
88
2,6-Cl2C6H3
2-BrC6H4
C6H5
45
80
50
85
-
8s
45
90
30
31
30
32
33
34
-
4-CH3C6H4
4-CH3OC6H4
4-CH3OC6H4
4-HOC6H4
2-CH3OC6H4
2,4-Cl2C6H3
3-ClC6H4
4-ClC6H4
Me
8t
45
85
Me
8u
8v
8w
8x
8y
8z
50
90
OEt
OEt
OEt
OMe
OMe
OMe
70
90
S
80
85
S
30
90
S
55
90
S
60
84
-
S
8a’
60
85
35
a Reaction conditions: aldehyde (1 mmol), β-dicarbonyl (1 mmol), urea or thiourea (1.5 mmol),
Co(NO3)2.6H2O (0.15 mmol), 80 °C; b Isolated yield
Table 4. Comparison of efficiency of various catalysts in synthesis of 5-(Ethoxycarbonyl)-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-ones
Entry
Catalyst
Sr(OTf)2
Amount of catalystb
Conditions
Solvent-free/70 0C
Yield/Timea
Ref
38
39
40
41
42
43
44
45
46
47
-
1
2
5
10
7
97/4
[Al(H2O)6](BF4)3
Zr(H2PO4)2
LiBr
CH3CN (reflux)
81/20
88/1
3
Solvent-free/90 0C
CH3CN (reflux)
4
10
100
10
10
40
10
10
15
92/3
5
SbCl3
CH3CN (reflux)
90/18
98/2
6
CaF2
C2H5OH/(reflux)
Solvent-free/900C
Solvent-free/100 0C
Solvent-free/100 0C
Solvent-free/100 0C
Solvent free/80 0C
7
Chloroacetic acid
NH4Cl
92/3
8
90/3
9
Triphenylphosphine
TiCl4-MgCl2
Co(NO3)2.6H2O
70/10
90/3
10
11
90/0.6
a Values refer to yield(%)/time(h)
b Amount of catalysts are in mol%
2313

J. Chil. Chem. Soc., 59, Nº 1 (2014)
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CONCLUSIONS
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In summary, we have described an improved procedure for the synthesis
of dihydropyrimidinones and dihydropyrimidinthiones using cobalt(II) nitrate
hexahydrate as heterogeneous catalyst. For clarifying the role of the catalyst,
three separated reactions were performed that the results clearly indicated that
the reaction catalyzed by Co(NO3)2 proceeds predominately through ureido-
crotonate intermediate, which this achievement well supports the necessity of
Lewis type catalyst for the Biginelli reaction. The mild reaction conditions,
rapid conversion, high yields, simple experimental procedure, availability
of catalyst are some notable advantages of the present method. Moreover,
compatibility with the environment, more efficiency and easy separation after
synthesis are considered as another advantages of this catalyst loading. Most
importantly, absence of organic solvents in this method contributes it to the
development of green technology.
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ACKNOWLEDGMENTS
We are grateful to the Yasouj University for financial assistance.
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