A benign synthesis of 2-amino-4H-chromene in aqueous medium using hydrotalcite (HT) as a heterogeneous base catalyst

Article abstract of DOI:10.1039/c3cy20856g

A simple and environmentally benign synthesis of 2-amino-4H-chromene is described using hydrotalcite as a solid base catalyst in aqueous medium. The catalysts were prepared by a co-precipitation method and well characterized by various techniques such as

Full text of DOI:10.1039/c3cy20856g

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A benign synthesis of 2-amino-4H-chromene in
aqueous medium using hydrotalcite (HT) as a
Cite this: DOI: 10.1039/c3cy20856g
heterogeneous base catalyst†
Sandip R. Kale,a Sandeep S. Kahandal,a Anand S. Burange,a Manoj B. Gawandeb
and Radha V. Jayaram*a
A simple and environmentally benign synthesis of 2-amino-4H-chromene is described using hydrotalcite
as a solid base catalyst in aqueous medium. The catalysts were prepared by a co-precipitation method
and well characterized by various techniques such as XRD, FT-IR, SEM and the basicity was found using
the phenol adsorption method. The reusability of the catalyst, use of water as a green solvent and easy
isolation of the product along with good yields make the present protocol sustainable and
advantageous compared to conventional methods.
Received 12th December 2012,
Accepted 9th April 2013
DOI: 10.1039/c3cy20856g
www.rsc.org/catalysis
reported protocols require a long reaction time, high temperature,
and use of organic solvents, and have problems associated with
Introduction
It is well known that one pot multicomponent reactions are
always better than multistep syntheses,1 as they require mini-
mal workup and desired products can be obtained in one pot
often in quantitative yields. In addition, use of water for the
organic reactions has been an important and fertile area of
research in recent years.2
2-Amino-4H-chromenes and their derivatives are of significant
importance as they possess a wide range of biological activities
such as mutagenicity,3 antimicrobial,4 antiproliferative,5 sex
pheromone,6 antitumor,7 cancer therapy and central nervous
system activity.8 Several protocols have been reported for the
synthesis of 2-amino-4H-chromenes and their derivatives using
malononitrile, resorcinol and aldehyde. Various catalysts such as
piperidine,9 triethyl amine,10 aqueous K2CO3,11 cetyltrimethyl-
ammonium bromide (CTABr),12 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU),13 Ca(OH)2,14 HT/MW15 and basic ionic liquids16 have
been used for these reactions. Recently, electrochemically
induced multicomponent condensation of resorcinol, malono-
nitrile and aldehyde in propanol in an undivided cell in the
presence of NaBr as an electrolyte was reported.17 Most of these
the reusability of catalysts. In order to make the reaction simple
and green, it is important to use environmentally friendly
medium for the organic reactions, and combination of hetero-
geneous catalysts and water is the most demanding combination
for catalytic reactions.
In continuation of our efforts to develop efficient protocols
for the various organic transformations using heterogeneous
catalysts,18,19 we herein report Mg/Al hydrotalcite (HT) to
catalyze the three component reaction of resorcinol, aldehyde
and malononitrile under mild conditions (Scheme 1). The Mg–Al
hydrotalcite is found to be an efficient catalyst for the synthesis of
2-amino-4H-chromene in aqueous medium. Among the hetero-
geneous basic catalysts, hydrotalcite is a versatile material
used as a catalyst for several organic transformations such as
multicomponent reaction of malononitrile, aldehyde and nitro
methane,20a hydroxylation of phenol,20b aldol and Knoevenagel
condensation,21 polymerization,22 cycloaddition reaction of nitroso
a Department of Chemistry, Institute of Chemical Technology, N. M. Parekh Marg,
Matunga, Mumbai 400 019, India. E-mail: sandipict@gmail.com,
rv.jayaram@ictmumbai.edu.in
b Department of Chemistry, Faculty of Science and Technology, Universidade Nova
de Lisboa, 2829-516 Caparica, Portugal. E-mail: m.gawande@fct.unl.pt;
Fax: +351 21 2948550; Tel: +351 964223243, +351 21 2948300
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy20856g
Scheme 1 Synthesis of chromenes using hydrotalcite in aqueous medium.
c
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Table 1 Optimization of reaction parametersa
and azo-alkenes,23 homocoupling reaction,24 Michael reaction25
and Baeyer–Villiger oxidation.26
Entry
Catalystb (% w/w)
Reaction conditions
Yieldc (%)
The synthesized Mg/Al hydrotalcite heterogeneous catalysts
were characterized by XRD (X-ray diffraction), FT-IR (Fourier
transform Infrared spectroscopy) and SEM (scanning electron
microscopy). The basicity of hydrotalcite was measured by the
phenol adsorption method27 and it was found to show a
maximum basicity of 0.131 mmol gꢀ1 with a Mg/Al molar ratio
of 5.0. Thus this catalyst was chosen for optimizing the reaction
conditions (ESI†).
1
2
3
4
5
6
7
8
No catalyst
H2O, 60 1C, 24 h
H2O, 60 1C, 4 h
H2O, 60 1C, 4 h
H2O, 60 1C, 4 h
H2O, 60 1C, 4 h
H2O, 60 1C, 4 h
H2O, 60 1C, 4 h
H2O, 60 1C, 4 h
No solvent, 60 1C, 4 h
Ethanol, 60 1C, 4 h
Isopropanol, 60 1C, 4 h
ACN, 60 1C, 4 h
DMF, 60 1C, 4 h
THF, 60 1C, 4 h
2-Me-THF, 60 1C, 4 h
1,4-Dioxane, 60 1C, 4 h
Hexane, 60 1C, 4 h
Diethyl ether, 60 1C, 4 h
Toluene, 60 1C, 4 h
H2O, 30 1C, 24 h
H2O, 80 1C, 4 h
0
65
72
78
95
32
56
95
0
25
19
Trace
18
Trace
Trace
0
Mg/Al: 2.0 HT (15)
Mg/Al: 3.0 HT (15)
Mg/Al: 4.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (5)
Mg/Al: 5.0 HT (10)
Mg/Al: 5.0 HT (20)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
Mg/Al: 5.0 HT (15)
9
10
11
12
13
14
15
16
17
18
19
20
21
Results and discussion
XRD of the Mg/Al: 5.0 hydrotalcite clearly indicates the crystalline
nature of the catalyst (Fig. 1). The presence of both sharp and
diffuse non-basal reflections was taken as an indication of a
partially disordered structure.
0
0
0
0
96
Initially, multicomponent reaction of benzaldehyde, resorcinol
and malononitrile was chosen as the model reaction. Effects of
various reaction parameters such as the influence of hydrotalcites
with various Mg/Al molar ratios, the influence of solvents, the
influence of catalyst concentration and the effect of temperature
were studied to optimize the reaction conditions (Table 1).
It is important to note that Mg/Al hydrotalcite with a molar
ratio of 5.0 was found to be the effective catalyst for the three
component reaction of benzaldehyde, resorcinol and malononitrile
affording a very good yield of the desired products. It was
noteworthy that, in the absence of the catalyst no significant
product formation was observed under similar reaction conditions
(Table 1, entry 1).
The solvent plays an important role in the catalyst activity
(Table 1). We have investigated the effect of various protic,
aprotic and non-polar solvents on the three component reaction of
resorcinol, malononitrile and benzaldehyde (Table 1, entries 8–18).
Under solvent free conditions the reaction did not take place even
after prolonged reaction time (Table 1, entry 9). In non-polar
solvents such as 1,4-dioxane, n-hexane, diethyl ether and toluene,
the reaction did not take place, whereas in the case of polar aprotic
solvents such as acetonitrile, THF and DMF, the yield of the
a
Reaction condition: benzaldehyde (3 mmol), resorcinol (3 mmol),
b
malononitrile (3 mmol). Weight percentage of the catalyst with
respect to resorcinol. Isolated yield after chromatography.
c
reaction was found to be very low (o20%, Table 1, entries 12–14).
In the case of polar protic solvents such as ethanol and isopropanol
(IPA) (Table 1, entries 10 and 11), the yield of the desired product
was moderate (19–25%). From Table 1 it’s clear that water was the
best choice as solvent (Table 1, entries 2–5 and 21).
Variation of the catalyst loading from 0 and 20 wt% had
different effects on the catalytic activity (Table 1 entries 5–8)
with the best performance being observed with an Mg/Al molar
ratio of 5 (Table 1, entries 2–5). The reaction was carried out
using 5 and 20 wt% of the catalysts, which provided the desired
product along with side products (not isolated).28 An increase
in the catalyst loading to 15 wt% resulted in an increase in
the yield (95%) of the desired product. Further increase in the
catalyst loading has no profound effect on the yield of the
desired product. The temperature has also a profound effect
with no reaction taking place at room temperature (30 1C).
Further increase in the temperature to 60 1C makes the reaction
almost quantitative (Table 1, entry 20). However a further
increase in the temperature did not show any significant
enhancement in the yield of the desired product (Table 1,
entry 20 vs. 21).
Encouraged by these results, we extended the scope of this
reaction to a range of aldehydes (Table 2, entries 1–14) under
optimized reaction conditions and the corresponding 2-amino-
4H-chromenes 4a–n were obtained in excellent yields (Table 2).
As expected, functional groups such as nitro (Table 2, entries 7
and 8) which have a strong electron withdrawing inductive
effect (ꢀI) as well as a mesomeric effect (ꢀR) give excellent
yields (91–95%) of the desired products, whereas electron
donating groups such as –OH, –OMe, –Me, –Cl, –Br, –F
(Table 2, entries 2–6, 12–14) at the ortho or para position of
the aldehyde group give slightly lower yields. The electron
withdrawing group makes the carbonyl more vulnerable toward
Fig. 1 XRD profile of Mg/Al HT.
c
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Table 2 2-Amino-4H-chromene synthesis in aqueous mediuma
Aldehyde
R
Product
Structure
m.p. (1C)
Entry
1
Code
Time (h)
Yieldb (%)
Found
Reported
–H
4a
4
95
234–235
159–161
96–98
234–23611b
2
3
4
–4-Cl
–2-Cl
–4-Br
4b
4c
4d
5
90
87
85
160–16216
4.5
96–9816
5
224–226
225–22716
5
6
7
–4-OMe
4e
4f
6
4
3
75
95
96
112–114
176–178
210–212
112–11416
–3-Cl
NRc
–4-NO2
4g
NRc
8
9
–3-NO2
4h
4i
4
92
90
188–190
216–218
NRc
Thiophene substituted benzaldehyde
4.5
NRc
c
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Table 2 (continued )
Aldehyde
Product
m.p. (1C)
Entry
10
R
Structure
Code
Time (h)
Yieldb (%)
Found
Reported
NRc
–2,5-(OMe)2
4j
6
86
198–200
11
–3,4-(OMe)2
4k
6
5
88
91
215–217
186–188
NRc
12
13
–4-F
4l
187–18916
–4-Me
–4-OH
4m
4n
6
87
83
180–182
248–250
182–18416
14
5.5
NRc
All products were characterized by H NMR, 13C NMR, IR and MS data. Yields refer to pure products after silica gel column chromatography.
a
1
b
c
Not reported (melting point not available in the literature).
nucleophilic attack. The probable reaction mechanism for the completed, the catalyst was filtered under vacuum, washed with
synthesis of 2-amino-4H-chromene using hydrotalcite is given ethyl acetate, and dried at 60 1C. The recovered catalyst could be
in Scheme 2. The first step of the mechanism involves the reused more than four reaction cycles with significant loss in
Knoevenagel condensation between aldehyde and malono- catalytic activity (Fig. 2, for the first cycle 95% and for the fourth
nitrile (basic sites of the catalyst abstract an acidic proton of cycle 61% yield).
malononitrile and then subsequent attack on the carbonyl
group furnishes condensed product I).29 In the second step
Michael addition of resorcinol and condensed product I affords
Conclusion
the desired product after rearomatization and intramolecular In conclusion, we have reported an efficient protocol for one
cyclization. During the formation of the 2-amino-4H-chromenes, pot multicomponent reaction of resorcinol, malononitrile and
the formation of 2-amino-5-hydroxy-4H-chromeme cannot be aldehyde for the synthesis of 2-amino-4H-chromenes catalyzed
ruled out, but the product was not observed, this may be due by hydrotalcite using water as solvent. The present protocol was
to the steric hindrance of the phenyl group and the hydroxyl found to be efficient providing higher yields of the desired
group which makes the molecule less stable.
products. This procedure offers several advantages including
An important norm for heterogeneous catalysis is the reusability mild reaction conditions, reusability of the catalyst, high yield
of the catalyst. To study the catalyst reusability, reaction of of products as well as a simple experimental procedure, which
resorcinol, benzaldehyde and malononitrile was carried out on make it an attractive process for the synthesis of 2-amino-4H-
Mg/Al: 5.0 hydrotalcite several times. After the reaction was chromenes and their derivatives. The catalyst has the advantage
c
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points were determined using a digital melting point apparatus
and are uncorrected. The FTIR spectra were recorded using
a Perkin Elmer (Spectra 100) spectrometer by a KBr pellet
1
technique. H NMR spectra were recorded on a JEOL-300 MHz
NMR spectrometer using TMS as an internal standard.
General procedure for synthesis of chromene
A mixture of resorcinol (0.330 g, 3 mmol), malononitrile (0.198 g,
3 mmol), and aldehyde (3 mmol) was taken in a 25 mL round
bottom flask containing 10 mL of water. The Mg/Al-HT (15 wt%)
catalyst with respect to resorcinol was then added to the reaction
flask and the contents were stirred. The reaction mixture was
heated for an appropriate time as mentioned in Table 1. The
progress of the reaction was monitored by thin layer chromato-
graphy (ethyl acetate/pet ether: 30%). After reaction was com-
pleted, the reaction mixture was allowed to cool at room
temperature. The crude product was extracted with ethyl acetate.
The organic layer was washed with water (25 mL), dried with
anhydrous Na2SO4, the solvent evaporated under vacuum and
the crude product recrystallized from ethanol. Characterization
data for selected compounds are provided below:
2-Amino-3-cyano-7-hydroxy-4-(3-chloro)-4H chromene (4f).
m.p.: 176–178 1C. IR (KBr): 3440 (OH), 3246 (NH2), 2970 (ArCH),
2235 (CN), 1645, 1580 (aromatic), 1149 (C-O) cmꢀ1 1H NMR
.
(300 MHz DMSO-d6) d: 4.68 (s, 1H, CH), 6.41 (d, 1H, J = 2.2 Hz,
ortho to Ar-OH), 6.51 (dd, 1H, J = 8.4 and 2.2 Hz, ortho to Ar-OH),
6.81 (d, 1H, J = 2.2 Hz, meta to Ar-OH), 6.94 (s, 2H, NH2), 7.14–
7.37 (m, 4H, H-ArCl), 9.77 (s, 1H, OH). 13C NMR (75 MHz
DMSO-d6) d ppm: 160.82, 157.76, 149.30, 133.62, 131.03,
130.35, 127.57, 127.15, 126.64, 120.95, 113.41, 113.02, 102.73,
62.50; MS m/z (%): 300, 299, 282, 263, 232, 187 (100).
Scheme 2 Plausible mechanism for the chromene synthesis.
2-Amino-3-cyano-7-hydroxy-4-(4-nitrophenyl)-4H chromene
(4g). Yellow solid. Yield 96%, m.p. 210–212 1C. IR (KBr):
3474 (OH), 3336 (NH2), 2980 (ArCH) 2188 (CN), 1645, 1580
(aromatic), 1459 (NO2), 1150, 1112 (C-O) cmꢀ1 1H NMR
;
(300 MHz DMSO-d6) d: 4.90 (s, 1H, CH), 6.44 (d, 1H, J =
2.5 Hz, ortho to Ar-OH), 6.51 (dd, 1H, J = 8.4 and 2.5 Hz, ortho to
Ar-OH), 6.79 (d, 1H, J = 8.4 Hz, meta to Ar-OH) 7.04 (s, 2H, NH2)
7.46 (d, 2H, J = 8.8 Hz, meta to Ar-NO2), 8.18 (d, 2H, J = 8.8 Hz,
ortho to Ar-NO2), 9.82 (s, 1H, OH); 13C NMR (75 MHz DMSO-d6)
d ppm: 160.87, 158.01, 154.20, 149.34, 146.74, 130.38, 129.14,
124.43, 120.79, 113.10, 112.73, 102.86, 55.58; MS m/z (%): 308
(M + 30), 293, 278, 263, 262 (100), 251, 252, 243, 187.
Fig. 2 Reusability of the catalyst.
of being inexpensive, non-hazardous, easily prepared and hetero-
geneous in nature, which makes the protocol economically viable
for the synthesis of titled compounds.
2-Amino-3-cyano-7-hydroxy-4-(3-nitrophenyl)-4H chromene
(4h). m.p.: 188–192 1C. IR (KBr): 3451 (OH), 3336 (NH2), 3243,
2985 (ArCH), 2190 (CN), 1625, 1550 (aromatic), 1353, 1450
(NO2), 1150, 1054 (C-O) cmꢀ1 1H NMR (300 MHz DMSO-d6)
.
d: 4.90 (s, 1H, CH), 6.44 (d, 1H, J = 2.5 Hz, ortho to Ar-OH), 6.50
(dd, 1H, J = 8.4 and 2.5 Hz, ortho to Ar-OH), 6.83 (d, 1H, J =
Experimental section
All chemicals were obtained from Sigma-Aldrich Company and 8.4 Hz, meta to Ar-OH), 7.04 (s, 2H, NH2), 7.60–8.8 (m, 4H, Ar-NO2),
used as received. Powder X-ray diffraction (XRD) patterns of the 9.82 (s, 1H, OH); 13C NMR (75 MHz DMSO-d6) d ppm: 160.91,
catalyst were recorded using a Philips 1050 diffractometer 157.91, 149.33, 149.09, 148.37, 134.73, 130.78, 130.39, 122.28,
using graphite monochromatized Cu-Ka radiation over a 2y 122.17, 120.76, 113.10, 112.97, 102.80, 62.44; MS m/z (%): 308
range of 10–901. BET surface area measurements were carried (M + 30), 293, 278, 263, 262 (100), 251, 252, 243, 187 (100).
out by the acetic acid adsorption method. Basicity of the catalyst
2-Amino-3-cyano-7-hydroxy-4-(2-thiophene)-4H chromene (4i).
was found using the phenol adsorption method. The melting Yellow solid. Yield 90%, m.p. 216–218 1C. IR (KBr): 3448 (OH),
c
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(4j). Light yellow solid. Yield 86%, m.p. 198–200 1C. IR (KBr):
3448 (OH), 3345 (NH2), 2963 (aromatic CH), 2194 (CN), 1643,
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(s, 3H, OMe), 4.90 (s, 1H, CH), 6.37 (d, 1H, J = 2.2 Hz, ortho
to Ar-OH), 6.44 (dd, 1H, J = 8.4 and 2.5 Hz, ortho to Ar-OH), 6.50
(d, 1H, J = 2.5 Hz, 6H-Ar-OMe), 6.73–6.93 (m, 5H, NH2, 1H-meta
to ArOH and 2H-ortho to -OMe), 9.67 (s, 1H, OH); 13C NMR
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149.53, 135.76, 129.61, 121.11, 115.48, 114.20, 113.29, 112.58,
112.01, 102.50, 67.84, 56.72, 55.66; MS: m/z (%): 324, 323, 322,
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2-Amino-3-cyano-7-hydroxy-4-(3,4-dimethoxy)-4H chromene
(4k). Light yellow solid. Yield 88%, m.p. 217–219 1C. IR (KBr):
3448 (OH), 3345 (NH2), 2963 (ArCH), 2194 (CN), 1643, 1508
(aromatic) cmꢀ1, 1130, 1149, 1242, 1265 (C-O); 1H NMR
(300 MHz DMSO-d6) d: 3.70 (s, 6H, OMe), 4.56 (s, 1H, CH),
6.39 (d, 1H, J = 2.2 Hz, ortho to Ar-OH), 6.48 (dd, 1H, J = 8.4 and
2.2 Hz ortho to Ar-OH), 6.64–6.89 (m, 6H, NH2, 1H-meta to ArOH
and 3H-Ar(OMe)2), 9.78 (s, 1H, OH); 13C NMR (75 MHz DMSO-d6)
d ppm: 160.59, 157.38, 149.12, 139.31, 130.29, 121.12, 119.82,
114.38, 112.71, 112.47, 111.75, 108.36, 106.72, 102.52, 55.91,
56.34; MS: m/z (%): 324, 323, 322, 294, 258, 216, 187 (100).
2-Amino-3-cyano-7-hydroxy-4-(4-hydroxy)-4H chromene (4n).
Yellow solid. Yield 83%, m.p. 248–250 1C, IR (KBr): 3479 (OH),
3349 (NH2), 3218, 2980 (ArCH), 2184 (CN), 1645, 1587 (aromatic),
1
1459 (NO2), 1152 (C-O) cmꢀ1; H NMR (300 MHz, DMSO-d6) d:
4.40 (s, 1H, CH), 6.37 (d, 1H, J = 2.2 Hz, ortho to Ar-OH), 6.46 (dd,
1H, J = 8.4 and 2.2 Hz, ortho to Ar-OH), 6.68 (d, 2H, J = 8.4 Hz,
ortho to Ar0-OH), 6.92 (d, 2H, J = 8.4 Hz, meta Ar0-OH), 6.75–6.78
(m, 3H, NH2 and meta to Ar-OH), 9.27 (s, 1H, OH). 9.82 (s, 1H,
OH); 13C NMR (75 MHz DMSO-d6) d ppm: 160.49, 157.33, 156.49,
149.19, 137.25, 130.34, 128.82, 121.24, 115.67, 114.75, 112.71,
102.50, 57.18; MS m/z (%): 281, 280, 279, 264, 252, 215, 187
(100), 171.
Acknowledgements
S.R.K. is grateful to CSIR, New Delhi, India, for the award of
Senior Research Fellowships.
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