An efficient synthesis of 2-amino-4-aryl-3,5-dicarbonitrile-6- ethoxypyridine derivatives by the solvent taking part in the reaction

Article abstract of DOI:10.1007/s11164-012-0829-x

An efficient and facile synthesis of 2-amino-4-aryl-3,5-dicarbonitrile-6- ethoxypyridine via reaction of aromatic aldehydes, malononitrile, and ethanol in the presence of NaOH has been developed, without using the classic reagents amines and 1,3-dicarbony

Full text of DOI:10.1007/s11164-012-0829-x

Res Chem Intermed
DOI 10.1007/s11164-012-0829-x
An efficient synthesis of 2-amino-4-aryl-3,
5-dicarbonitrile-6-ethoxypyridine derivatives
by the solvent taking part in the reaction
•
Suhui Wang Sheng Xia Shimin Tao
•
•
•
Yunyun Zha Youjian Feng Liangce Rong
•
Received: 6 July 2012 / Accepted: 20 September 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract An efficient and facile synthesis of 2-amino-4-aryl-3,5-dicarbonitrile-6-
ethoxypyridine via reaction of aromatic aldehydes, malononitrile, and ethanol in the
presence of NaOH has been developed, without using the classic reagents amines
and 1,3-dicarbonyl compounds. It is interesting that weak nucleophilic reagent
ethanol could take part in the reaction without using strong base catalyst sodium
ethylate. Compared with existing methods, the reported process has the advantages
of excellent yields, easily obtainable raw materials, and mild reaction conditions.
Keywords Pyridine Á Malononitrile Á Ethanol Á Synthesis Á Solvent effect
Introduction
The pyridine substructure is one of the most prevalent heterocycles found in natural
products, pharmaceuticals, and functional materials [1–3]. The Hantzsch reaction is
the classical method for synthesis of pyridine from 1,3-dicarbonyl compounds,
aldehyde, and ammonia [4, 5]. Many other powerful methodologies for synthesis of
these heterocycles rely on condensation of amines and carbonyl compounds or
cycloaddition reactions [6–13]. Cross-coupling chemistry also allows introduction
of substituents to activated heterocycles [14]. In reported methods, amines and 1,3-
dicarbonyl compounds are the most commonly used substrates. Herein, we report a
different method for preparation of pyridine derivatives without using amines and
1,3-dicarbonyl compounds.
S. Wang Á S. Xia Á S. Tao Á Y. Zha Á Y. Feng Á L. Rong (&)
College of Chemistry and Chemical Engineering, Jiangsu Normal University, Jiangsu Key
Laboratory of Green Synthetic Chemistry for Functional Materials, Xuzhou 221116,
Jiangsu, People’s Republic of China
e-mail: lcrong2005@yahoo.com
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S. Wang et al.
Results and discussion
We previously reported some 2-amino-1,3-dicarbonitrile compounds [acceptor–
donor–acceptor (A–D–A)] by the reaction of aromatic aldehydes, malononitrile, and
different ketones [15–17]. In our recent research, we wanted to use aromatic
aldehyde, malononitrile, and 2-methylcyclopentane-1,3-dione in ethanol (95 %) to
synthesize some new 2-amino-1,3-dicarbonitrile compounds. 4-Fluorobenzalde-
hyde, malononitrile, and 2-methylcyclopentane-1,3-dione were chosen to carry out
this reaction using Et3N as catalyst. Unfortunately, we did not get the anticipated
1
1,3-dicarbonitrile compound; however, a different product was obtained. In its H
nuclear magnetic resonance (NMR), two obvious signals could be found. One is a
triplet signal at d = 1.43 (3H), and the other is a quartet signal at d = 4.47 (2H).
The coupling constants of both are J = 7.2 Hz, clearly indicating mutual coupling
between them. Taking into account the reaction reagents and solvent, we believed
that the solvent, viz. ethanol, must take part in the reaction, and we could conclude
the structure is 2-amino-3,5-dicarbonitrile-6-ethoxy-4-(4-fluorophenyl)pyridine 4a.
Since the structure of the product has been identified, we wanted to know
whether other methods have been reported for the synthesis of these compounds.
Investigating the literature, we found that few methods to prepare these compounds
´
´
have been reported. Only in 1970, Alvarez-Insua reported synthesis of similar
structures under rigorous conditions using strong base sodium alkoxide and absolute
alcohol, but with very low yields (3–49 %). In our research, we found Et3N could
also catalyze this reaction, so we reasoned that this method may offer better
conditions for synthesis of these compounds.
To identify other efficient conditions for this reaction, 4-fluorobenzaldehyde and
malononitrile were chosen also as starting substrates, and different catalysts were
used in 95 % ethanol. After the reactions were completed, different results were
obtained as listed in Table 1. Table 1 shows that, when strong organic base, such as
NaOEt and KOBut, was used in reaction, the yields were below average. The
catalytic effect of benzylamine, DBU, and piperidine is similar to that of Et3N, with
21, 19, and 18 % yield, respectively. However, when inorganic base NaOH was
used in the reaction, the reaction could proceed smoothly with the best yield. We
also tested the reaction time, the loading amount of NaOH, and the reaction
temperature for the model reaction. The results showed that NaOH is the preferred
catalyst for this synthesis (Scheme 1).
Under this optimized condition, different aromatic aldehydes were chosen to
react with malononitrile and 95 % ethanol, and corresponding 2-amino-4-aryl-3,5-
dicarbonitrile-6-ethoxypyridine products could be obtained in good yield
(Scheme 2). In these reactions, we found that aromatic aldehydes bearing
electron-donating substituents, such as methyl, methoxy, were more efficient
reagents for synthesis of target products compared with electron-withdrawing
counterparts. The results of the reactions are summarized in Table 2.
According to the product, a possible mechanism is presented in Scheme 3.
Firstly, the aromatic aldehyde and malononitrile react together to give condensation
compound cinnamonitrile 5, then Michael addition occurs between 5 and
malononitrile. Ethanol as a nucleophilic reagent reacts with 6 to give 7. Finally,
123

An efficient synthesis
Table 1 Effect of catalyst and temperature on the model reactiona
Entry
Catalyst (mmol)
Temperature (°C)
Time (h)
Yieldb (%)
1
PhCH2NH2 (1)
NaOEt (1)
KOBut (1)
80
80
80
80
80
80
80
80
80
80
80
80
80
70
70
60
20
6
21
28
25
19
18
20
54
53
54
25
20
52
53
35
30
25
2
3
6
4
DBU (1)
20
20
20
6
5
Piperidine (1)
Et3N (1)
6
7
NaOH (1)
8
NaOH (0.8)
NaOH (0.5)
NaOH (0.2)
NaOH (0.1)
NaOH (0.5)
NaOH (0.5)
NaOH (0.5)
NaOH (0.5)
NaOH (0.5)
6
9
6
10
11
12
13
14
15
16
a
6
6
5
4
6
6
6
Reagents and conditions: 4-fluorobenzaldehyde 1a (2 mmol), malononitrile 2 (4 mmol), EtOH (10 mL)
b
Isolated yields
Bold indicates the optimized reaction condition
F
O
O
NC
O
H2N
CN
F
F
CN
+
CN
C2H5OH
CHO
1a
NC
CN
OC2H5
2
3
H2N
N
4a
Scheme 1 Reaction of 4-fluorobenzaldehyde and malononitrile
cyclization reaction, tautomerism, and oxidation reaction are carried out to give
products 4.
The structures of all the products were confirmed on the basis of spectroscopic
data, particularly 1H NMR and 13C NMR analysis, and high-resolution mass
123

S. Wang et al.
R
CHO
CN
CN
NC
CN
C2H5OH
+
+
R
H2N
N
OC2H5
1
2
3
4
Scheme 2 Synthesis of 2-amino-6-ethoxy-4-arylpyridine-3,5-dicarbonitrile
Table 2 Synthetic results of compounds 4
Entry
Ar
Product
Yielda (%)
1
2
3
4
5
6
7
8
9
10
a
4-FC6H4
4a
4b
4c
4d
4e
4f
53
58
60
63
61
63
65
71
68
62
2-FC6H4
4-CH3C6H4
2-CH3OC6H4
3-CH3OC6H4
4-CH3OC6H4
3,4-(CH3)2C6H3
3,4-(CH3O)2C6H3
3,4-OCH2OC6H3
3,4,5-(CH3O)3C6H2
4g
4h
4i
4j
Isolated yields
Ar
CN
CN
NC
CN
Ar
HC
CN
C
CN
OH-
CH3
O
CN
CN
ArCHO
+
H
CN
-H2O
N
6
5
1
2
Ar
Ar
Ar
NC
HN
CN
NC
N
CN
OCH2CH3
NC
CN
CN
HN
N
H
OCH2CH3
H2N
OCH2CH3
8
9
7
Ar
Ar
[O]
NC
CN
OCH2CH3
NC
H2N
CN
OCH2CH3
N
H2N
N
4
10
Scheme 3 Possible reaction mechanism
1
spectroscopy (HRMS). We take 4a as an example for structural analysis. The H
NMR spectrum of 4a revealed a triplet signal at d = 1.43 ppm (3H, d = 7.2 Hz),
and a quartet signal at d = 4.47 ppm (2H, d = 7.2 Hz) due to CH3– and –CH2– of
123

An efficient synthesis
ethoxy (CH3CH2O–). A singlet signal at 5.65 ppm is due to the two protons of NH2.
The triplet signal at d = 7.22 (2H, J = 9.6 Hz) and doublet signals at 7.53 (2H,
J = 5.4 Hz, J = 6.6 Hz) are due to four phenyl protons. The 13C NMR spectrum of
4a revealed signals at 14.49, 64.64, 84.07, 114.73, 115.78, 116.37, 116.66, 116.90,
130.86, 130.98, 159.89, 161.04, 165.99, and 166.60, respectively. In the HRMS
spectrum, the calculated m/z (C15H11FN4O [M ? Na]?) of 4a is 305.0815, and the
found m/z is 305.0820.
Conclusions
We have successfully developed an efficient and facile method to prepare pyridine
derivatives via reaction of different aromatic aldehydes, malononitrile, and solvent
ethanol (95 %), without using the classic reagents amines and 1,3-dicarbonyl
compounds. It is interesting that the weak nucleophilic reagent ethanol could take
part in the reaction in the presence of NaOH, without using strong base catalyst such
as sodium ethylate. This protocol has the advantages of mild conditions, easily
obtainable raw materials, and good yields.
Experimental
Melting points were determined on XT-5 microscopic melting-point apparatus and
were uncorrected. Infrared (IR) spectra were recorded on a FT Bruker Tensor 27
1
spectrometer. H and 13C NMR spectra were obtained from solution in dimethyl
sulfoxide (DMSO)-d6 with Me4Si as internal standard using a Bruker-400 and
Bruker-300 spectrometer. HRMS spectra were obtained with a Bruker microTOF-Q
134 instrument.
General procedure for the synthesis of 2-amino-4-aryl-3,5-dicarbonitrile-6-
ethoxypyridine
A mixture of aromatic aldehydes 1 (2 mmol), malononitrile 2 (2 mmol),
CH3CH2OH (10 mL), and NaOH (0.5 mmol) was put in a reaction flask at 80 °C
for about 4 h. After completion, the reaction mixture was poured into water, filtered,
and then washed with water thoroughly. The products were dried and recrystallized
from 95 % ethanol.
2-Amino-3,5-dicarbonitrile-6-ethoxy-4-(4-fluorophenyl)pyridine (4a)
M.p. 163–164 °C; IR (KBr, v, cm-1): 3,482, 3,337, 3,221, 2,225, 1,624, 1,559,
1,508, 1,387, 1,341, 1,303, 1,185, 1,162, 1,020, 857, 784 cm-1 1H NMR
;
(300 MHz, CDCl3) (d, ppm): 1.43 (3H, t, J = 7.2 Hz, CH3), 4.47 (2H, q,
J = 7.2 Hz, CH2), 5.65 (2H, s, NH2), 7.22 (2H, t, J = 9.6 Hz, ArH), 7.53 (2H, dd,
J = 5.4 Hz, J = 6.6 Hz, ArH). 13C NMR (300 MHz, CDCl3) (d, ppm): 14.49,
64.64, 84.07, 114.73, 115.78, 116.37, 116.66, 116.90, 130.86, 130.98, 159.89,
123

S. Wang et al.
161.04, 165.99, 166.60. HRMS m/z calculated for C15H11FN4O [M ? Na]?:
305.0815, found: 305.0820.
2-Amino-3,5-dicarbonitrile-6-ethoxy-4-(2-fluorophenyl)pyridine (4b)
M.p. 203–204 °C; IR (KBr, v, cm-1): 3,471, 3,329, 3,327, 2,227, 1,627, 1,589,
1,555, 1,498, 1,457, 1,384, 1,347, 1,301, 1,270, 1,230, 1,189, 1,105, 1,019,
1
769 cm-1; H NMR (300 MHz, CDCl3) (d, ppm): 1.43 (3H, t, J = 7.5 Hz, CH3),
4.47 (2H, q, J = 7.2 Hz, CH2), 5.68 (2H, s, NH2), 7.27 (2H, dt, J = 9.9 Hz,
J = 7.5 Hz, ArH), 7.40 (1H, t, J = 7.5 Hz), 7.53 (1H, q, J = 6.6 Hz, ArH). 13C
NMR (300 MHz, CDCl3) (d, ppm): 14.48, 64.63, 85.08, 114.31, 115.27, 116.71,
124.96, 130.44, 130.47, 133.00, 133.11, 155.58, 157.33, 160.80, 166.29. HRMS
m/z calculated for C15H11FN4O [M ? Na]?: 305.0815, found: 305.0817.
2-Amino-3,5-dicarbonitrile-4-(4-methylphenyl)-6-ethoxypyridine (4c)
M.p. 184–185 °C; IR (KBr, v, cm-1): 3,330, 2,227, 1,622, 1,583, 1,556, 1,516,
1
1,472, 1,435, 1,418, 1,382, 1,339, 669 cm-1; H NMR (400 MHz, DMSO-d6) (d,
ppm): 1.35 (3H, t, J = 6.8 Hz, CH3), 2.40 (3H, s, CH3), 4.44 (2H, q, J = 6.8 Hz,
CH2), 7.39 (4H, q, J = 8.0 Hz, ArH), 7.96 (2H, s, NH2). 13C NMR (300 MHz,
CDCl3) (d, ppm): 14.52, 20.07, 64.41, 114.62, 115.67, 116.25, 116.29, 125.76,
131.44, 131.57, 160.68, 162.13, 163.18, 166.72. HRMS m/z calculated for
C16H14N4O [M ? Na]?: 301.1065, found: 301.1071.
2-Amino-3,5-dicarbonitrile-6-ethoxy-4-(2-methoxyphenyl)pyridine (4d)
M.p. 185–186 °C; IR (KBr, v, cm-1): 3,438, 3,338, 3,226, 3,012, 2,221, 1,635,
1,550, 1,496, 1,429, 1,382, 1,346, 1,298, 1,279, 1,254, 1,177, 1,113, 1,014, 916,
1
784, 759 cm-1; H NMR (300 MHz, CDCl3) (d, ppm): 1.43 (3H, t, J = 6.9 Hz,
CH3), 3.87 (3H, s, OCH3), 4.46 (2H, q, J = 6.9 Hz, CH2), 5.56 (2H, s, NH2), 7.07
(2H, dd, J = 7.5 Hz, J = 14.1 Hz, ArH), 7.27 (1H, d, J = 7.2 Hz, ArH), 7.47
(1H, t, J = 7.2 Hz, ArH); 13C NMR (300 MHz, CDCl3) (d, ppm): 14.54, 55.85,
64.34, 85.47, 111.92, 114.85, 115.85, 121.14, 122.72, 130.12, 132.43, 156.28,
158.79, 160.72, 166.27. HRMS m/z calculated for C16H14N4O2 [M ? Na]?:
317.1014, found: 317.1028.
2-Amino-3,5-dicarbonitrile-6-ethoxy-4-(3-methoxyphenyl)pyridine (4e)
M.p. [280 °C; IR (KBr, v, cm-1): 3,430, 3,340, 3,238, 2,985, 2,216, 1,648, 1,570,
1,436, 1,381, 1,340, 1,300, 1,260, 1,180, 1,033, 883, 807 cm-1 1H NMR
;
(300 MHz, CDCl3) (d, ppm): 1.43 (3H, t, J = 7.2 Hz, CH3), 3.86 (3H, s, OCH3),
4.48 (2H, q, J = 7.2 Hz, CH2), 5.62 (2H, s, NH2), 7.03–7.11 (3H, m, ArH), 7.44
(1H, t, J = 7.8 Hz, ArH); 13C NMR (300 MHz, CDCl3) (d, ppm): 14.51, 55.67,
64.55, 84.95, 104.32, 113.94, 116.93, 120.87, 120.92, 130.34, 130.37, 130.42,
134.78, 143.18, 160.95, 161.40, 164.51. HRMS m/z calculated for C16H14N4O2
[M ? H]?: 295.1195, found: 295.1198.
123

An efficient synthesis
2-Amino-3,5-dicarbonitrile-6-ethoxy-4-(4-methoxyphenyl)pyridine (4f)
M.p. [280 °C; IR (KBr, v, cm-1): 3,415, 3,342, 3,241, 2,967, 2,936, 2,842, 2,214,
1,652, 1,608, 1,543, 1,517, 1,381, 1,340, 1,299, 1,244, 1,180, 1,021, 924, 840, 786,
663, 611 cm-1; 1H NMR (300 MHz, DMSO-d6) (d, ppm): 1.43 (3H, t, J = 7.2 Hz,
CH3), 3.86 (3H, s, OCH3), 4.46 (2H, q, J = 7.2 Hz, CH2), 5.64 (2H, s, NH2), 7.03
(2H, d, J = 8.7 Hz, ArH), 7.51 (2H, d, J = 8.7 Hz, ArH); 13C NMR (300 MHz,
CDCl3) (d, ppm): 14.53, 55.64, 64.45, 83.90, 114.60, 114.65, 115.23, 116.28,
125.75, 130.42, 130.47, 160.66, 161.16, 161.78, 166.70. HRMS m/z calculated for
C16H14N4O2 [M ? Na]?: 317.1014, found: 317.1016.
2-Amino-3,5-dicarbonitrile-4-(3,4-dimethylphenyl)-6-ethoxypyridine (4g)
M.p. 198–200 °C; IR (KBr, v, cm-1): 3,472, 3,324, 3,219, 2,980, 2,922, 2,226,
1,623, 1,581, 1,552, 1,436, 1,378, 1,343, 1,301, 1,252, 1,178, 1,029, 833,
1
784 cm-1; H NMR (300 MHz, CDCl3) (d, ppm): 1.43 (3H, t, J = 7.2 Hz, CH3),
2.32 (3H, s,CH3), 2.33 (3H, s,CH3), 4.47 (2H, q, J = 7.2 Hz, CH2), 5.60 (2H, s,
NH2), 7.28 (3H, s, ArH); 13C NMR (300 MHz, CDCl3) (d, ppm): 14.53, 20.08,
64.43, 84.11, 115.05, 126.08, 129.60, 130.36, 130.49, 131.16, 137.55, 140.09,
161.03, 161.28. HRMS m/z calculated for C17H16N4O [M ? Na]?: 315.1222,
found: 315.1228.
2-Amino-3,5-dicarbonitrile-4-(3,4-dimethoxyphenyl)-6-ethoxypyridine (4h)
M.p. 213–215 °C; IR (KBr, v, cm-1): 3,443, 3,334, 3,231, 2,222, 1,634, 1,567,
1,519, 1,435, 1,384, 1,342, 1,325, 1,265, 1,177, 1,144, 1,021 cm-1 1H NMR
;
300 MHz, CDCl3) (d, ppm): 1.43 (3H, t, J = 6.9 Hz, CH3), 3.94 (3H, s, OCH3),
3.94 (3H, s, OCH3), 4.47 (2H, q, J = 6.9 Hz, CH2), 5.61 (2H, s, NH2), 7.00
(1H, d, J = 8.4 Hz, ArH), 7.07 (1H, s, ArH), 7.17–7.20 (1H, m, ArH); 13C NMR
(300 MHz, CDCl3) (d, ppm): 14.56, 56.20, 56.32, 64.47, 83.92, 111.32, 111.85,
115.23, 116.30, 122.09, 125.78, 149.12, 151.33, 160.57, 161.14, 161.16. HRMS
m/z calculated for C17H16N4O3 [M ? Na]?: 347.1120, found: 347.1132.
2-Amino-3,5-dicarbonitrile-4-(benzo[d][1,3]dioxol-4-yl)-6-ethoxypyridine (4i)
M.p. 189–191 °C; IR (KBr, v, cm-1): 3,456, 3,356, 3,237, 2,908, 2,210, 1,648,
1,637, 1,573, 1,544, 1,504, 1,449, 1,429, 1,384, 1,384, 1,345, 1,252, 1,187, 1,101,
;
1,036, 932, 876, 821, 781 cm-1 1H NMR (300 MHz, CDCl3) (d, ppm): 1.43
(3H, t, J = 7.2 Hz, CH3), 4.46 (2H, q, J = 7.2 Hz, CH2), 5.63 (2H, s, NH2), 6.06
(2H, s, CH2), 6.94 (1H, d, J = 7.8 Hz, ArH), 6.99 (1H, s, ArH), 7.03 (1H, dd,
J = 1.2 Hz, J = 7.8 Hz, ArH); 14.51, 64.52, 84.08, 102.05, 104.32, 109.04,
109.09, 114.96, 116.02, 123.40, 123.45, 127.14, 148.28, 150.05, 160.55, 161.07,
166.63. HRMS m/z calculated for C16H12N4O3 [M ? Na]?: 331.0807, found:
331.0810.
123

S. Wang et al.
2-Amino-3,5-dicarbonitrile-6-ethoxy-4-(3,4,5-trimethoxyphenyl)pyridine (4j)
M.p. 207–209 °C; IR (KBr, v, cm-1): 3,330, 2,940, 2,208, 1,633, 1,568, 1,512,
1
1,384, 1,318, 1,252, 1,175, 1,127, 895, 669 cm-1; H NMR (300 MHz, CDCl3)
(d, ppm): 1.44 (3H, t, J = 7.2 Hz, CH3), 3.91 (9H, s, 3XOCH3), 4.48 (2H, q,
J = 7.2 Hz, CH2), 5.66 (2H, s, NH2), 6.77 (2H, s, ArH); 13C NMR (300 MHz,
CDCl3) (d, ppm): 14.52, 56.54, 61.23, 64.55, 83.94, 106.27, 115.04, 116.12, 128.55,
140.17, 140.18, 153.52, 160.63, 161.12, 166.70. HRMS m/z calculated for
C18H18N4O4 [M ? H]?: 355.1406, found: 355.1420.
Acknowledgments We are grateful to the National Natural Science Foundation of China (NSFC)
(21172188), the Foundation of Xuzhou Normal University (10XLS02), and the Priority Academic
Program Development of Jiangsu Higher Education Institutions for financial support.
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