ZnO/MgO Containing ZnO Nanoparticles as a Highly Effective Heterogeneous Base Catalyst for the Synthesis of 4H-Pyrans and Coumarins in [bmim]BF 4

Article abstract of DOI:10.1007/BF03246209

4H-Pyrans and coumarins were synthesized through one-pot reactions using ZnO/MgO solid sample containing ZnO nanoparticles as an efficient reusable catalyst in ionic liquid, [bmim]BF4. The catalyst is inexpensive and readily available, stable and storable

Full text of DOI:10.1007/BF03246209

J. Iran. Chem. Soc., Vol. 8, No. 1, March 2011, pp. 123-130.
JOUIRrNaAnLiaOnF THE
Chemical Society
ZnO/MgO Containing ZnO Nanoparticles as a Highly Effective Heterogeneous Base
Catalyst for the Synthesis of 4H-Pyrans and Coumarins in [bmim]BF4
H. Valizadeha,* and A.A. Azimib
aDepartment of Chemistry, Faculty of Sciences, Azarbaijan University of Tarbiat Moallem, Tabriz, Iran
bDepartment of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
(Received 26 January 2010, Accepted 6 May 2010)
4H-Pyrans and coumarins were synthesized through one-pot reactions using ZnO/MgO solid sample containing ZnO
nanoparticles as an efficient reusable catalyst in ionic liquid, [bmim]BF4. The catalyst is inexpensive and readily available, stable
and storable, easily recycled and reused for several cycles with consistent activity. The procedure offers advantages in terms of
high yields, short reaction times, and mild solvent-free reaction conditions.
Keywords: Coumarin, Heterogeneous catalyst, Pyran, ZnO/MgO, Ionic liquid
INTRODUCTION
bromide [8]. All these catalysts show limitations such as harsh
reaction conditions, low yields, tedious work-ups, and poor
recyclability.
Polyfunctionalized 4H-pyrans are interesting compounds
as they possess biological and pharmacological activities [1].
These compounds are used as anti-coagulants, anticancer
agents, spasmolytics, anti-anaphylactics, etc. [2-3]. 4H-Pyrans
containing heterocyclic rings are increasingly used for their
pharmacological activities [4]. These compounds can be used
for the treatment of neuro-degenerative diseases, including
Alzheimer’s disease, as well as for the treatment of
schizophrenia and myoclonus. Furthermore, a number of 2-
amino-4H-pyran derivatives are useful as photoactive
materials [5]. Thus, in view of their wide utility, researchers
have synthesized the 4H-pyran unit using different methods
including radioactive and non-radioactive techniques such as
microwave and ultrasonic irradiation [6]. In addition, the one-
pot synthesis of 4H-pyrans has been reported using
tetrabutylammonium bromide [7], (S)-proline, rare earth
Coumarins are an important class of natural products with
interesting biological and therapeutic properties [9]. Many
products, which contain the subunit of coumarin, exhibit
useful and diverse biological activity such as molluscacides,
anthelmintic, hypnotic, and so on, or serve as anticoagulant
agents and fluorescent brighteners [10-11]. Coumarins have
been synthesized using several synthetic routes such as
Pechmann, Perkin, Knoevenagel, Reformatsky and Wittig
reactions. However, due to simple and relatively inexpensive
starting materials, the Knoevenagel reaction has been widely
used for the synthesis of coumarins. We have already reported
the synthesis of coumarin derivatives via Knoevenagel
reaction of 2-hydroxybenzaldehyde derivatives with some
active methylene compounds in aqueous media and also in
ionic liquids [12-13].
perfluorooctanoates,
and
hexadecyltrimethylammonium
Recently, mineral oxides have proved to be useful to
chemists in the laboratory and industry due to the good
activation of adsorbed compounds and reaction rate
*Corresponding author. E-mail: h-valizadeh@azaruniv.edu

Valizadeh & Azimi
R1
CHO
EtO2C
CN
ZnO/MgO containing
NC
NC
EtO2C
+
ZnO nanoparticles
+
[bmim] BF4, r.t.
H3CC
R1
H3C
O
NH2
O
4a-j
3
1
2
O
R2
R2
W
O
ZnO/MgO containing
CO2R3
ZnO nanoparticles
H
W
+
[bmim] BF4, r.t.
O
OH
R1
R1
W = CN, CO2Me, CO2Et
7a-i
5
6
Scheme 1. Synthesis of 2-amino-4H-pyrans and coumarins in the presence of ZnO/MgO containing ZnO
nanoparticls in [bmim]BF4
enhancement, selectivity, easier work-up and recyclability of
the supports and the eco-friendly, green, reaction conditions
[14-19]. Thus, the heterogeneous solid base catalysts have
been recognized as potential alternatives to homogeneous
organic basic catalysts. In continuation of our interest in using
ILs, water or solventless systems as green reaction media [20-
26], herein we wish to report the synthesis of 4H-pyrane and
coumarins from one pot high surface area ZnO/MgO catalyzed
reactions in [bmim]BF4 (Scheme 1).
General Procedure for 4H-Pyrans (4a-j)
Benzaldehyde derivative (20 mmol), malononitril (20
mmol), ethylacetoacetate (20 mmol), and ZnO/MgO
containing ZnO nanoparticles (10 mol%) were added to
[bmim]BF4 (3 ml) and mixed thoroughly and stirred for 28-40
min at room temperature. After completion of the reaction, as
indicated by TLC (ethyl acetate/n-hexane, 5/1), the mixture
was extracted with ethylacetate. The extracts were
concentrated on a rotary evaporator and the crude mixture was
purified by silica gel (Merck 230-240 mesh) column
chromatography using (ethylacetate/n-hexane:1/5) mixtures as
eluent to give pure pyrans (4a-j).
EXPERIMENTAL
Chemicals and Apparatus
All reagents were purchased from Merck Company and
used without further purification. Infrared spectra were
recorded in KBr and were determined on a Perkin Elmer FT-
IR spectrometer. 1H and 13C NMR spectra were recorded on a
Bruker Avance AC-400 MHz using CDCl3 or DMSO-d6 as the
deuterated solvents and TMS as internal standard. All melting
points are uncorrected and measured in open glass-capillaries
using Stuart melting point apparatus.
General Procedure for Coumarins (7a-i)
Salicylaldehyde (20 mmol), active methylene compound
(20 mmol) and ZnO/MgO containing ZnO nanoparticles (10
mol%) were added to [bmim]BF4 (3 ml) and mixed thoroughly
and stirred for 25-40 min at room temperature. After
completion of the reaction, as indicated by TLC (ethyl
acetate/n-hexane, 5/1), the mixture was extracted with
ethylacetate. The extracts were concentrated on a rotary
124

ZnO/MgO Containing ZnO Nanoparticles as a Heterogeneous Base Catalyst
evaporator and the crude mixture was purified by silica gel
1.24 (t, J = 7.22 Hz, 3H), 2.29 (s, 3H), 4.21 (q, J = 7.22 Hz,
2H), 5.24 (s, 1H), 6.82 (br s, 2H, NH2), 7.23 (d, J = 2.31 Hz,
1H), 7.42 (m, 3H); 13C NMR (100 MHz, DMSO-d6) δ 21.16,
42.47, 60.63, 64.54, 115.08, 125.45, 127.12, 127.02, 128.08,
130.25, 131.15, 143.17, 148.90, 165.34, 169.85.
(Merck 230-240 mesh) column chromatography using
(ethylacetate/n-hexane:1/5) mixtures as eluent to give pure
coumarins (7a-i).
Selected Spectroscopic Data
5-Ethoxycarbonyl-2-amino-4-(4-hydroxyphenyl)-3-
5-Ethoxycarbonyl-2-amino-4-phenyl-3-cyano-6-
cyano-6-methyl-4H-pyrans (4f). IR (KBr) (νmax/cm-1): 3490-
methyl-4H-pyrans (4a). IR (KBr) (νmax/cm-1): 3428, 3331,
3150 (broad), 3448, 3275, 2216, 1718, 1679; H NMR (400
1
1
2220, 1712, 1645; H NMR (400 MHz, DMSO-d6) δ 1.11 (t,
MHz, DMSO-d6) δ 1.16 (t, J = 7.23 Hz, 3H), 2.39 (s, 3H),
4.16 (q, J = 7.23 Hz, 2H), 5.12 (s, 1H), 6.69 (br s, 2H, NH2),
7.21 (d, J = 8.01 Hz, 2H), 7.43 (d, J = 8.01 Hz, 2H), 8.32 (br s,
1H, OH); 13C NMR (100 MHz, DMSO-d6) δ 18.10, 42.23,
61.12, 63.11, 113.32, 127.32, 128.08, 128.28, 129.49, 133.18,
144.65, 148.54, 163.21, 164.22.
J = 7.21 Hz, 3H), 2.44 (s, 3H), 4.09 (q, J = 7.21 Hz, 2H), 5.09
(s, 1H), 6.77 (br s, 2H, NH2), 7.21 (m, 3H), 7.37 (m, 2H); 13C
NMR (100 MHz, DMSO-d6) δ 18.02, 38.00, 58.17, 62.70,
109.02, 123.06, 125.21, 126.32, 127.02, 129.41, 142.01,
148.40, 158.11, 161.63.
5-Ethoxycarbonyl-2-amino-4-(4-nitrophenyl)-3-cyano-
5-Ethoxycarbonyl-2-amino-4-(4-methylphenyl)-3-
6-methyl-4H-pyrans (4b). IR (KBr) (νmax/cm-1): 3449, 3291,
cyano-6-methyl-4H-pyrans (4g). IR (KBr) (νmax/cm-1): 3451,
1
1
2211, 1721, 1680, 1534, 1351; H NMR (400 MHz, DMSO-
3281, 2222, 1726, 1683; H NMR (400 MHz, DMSO-d6) δ
d6) δ 1.15 (t, J = 7.23 Hz, 3H), 2.39 (s, 3H), 4.05 (q, J = 7.23
Hz, 2H), 5.10 (s, 1H), 6.74 (br s, 2H, NH2), 7.25 (d, J = 7.98
Hz, 2H), 7.41 (d, J = 7.98 Hz, 2H); 13C NMR (100 MHz,
DMSO-d6) δ 19.12, 39.11, 58.12, 63.52, 110.32, 124.24,
126.01, 126.21, 127.42, 130.08, 142.86, 148.96, 159.00,
162.31.
1.19 (t, J = 7.26 Hz, 3H), 2.35 (s, 3H), 2.91 (s, 3H), 4.26 (q,
J = 7.26 Hz, 2H), 5.23 (s, 1H), 6.75 (br s, 2H, NH2), 7.18 (d,
J = 7.65 Hz, 2H), 7.39 (d, J = 7.65 Hz, 2H); 13C NMR (100
MHz, DMSO-d6) δ 19.87, 43.20, 45.36, 62.18, 64.54, 115.30,
127.42, 128.18, 128.76, 130.91, 134.23, 144.54, 148.55,
164.01, 164.87.
5-Ethoxycarbonyl-2-amino-4-(3-nitrophenyl)-3-cyano-
5-Ethoxycarbonyl-2-amino-4-(2-methoxyphenyl)-3-
6-methyl-4H-pyrans (4c). IR (KBr) (νmax/cm-1): 3456, 3287,
cyano-6-methyl-4H-pyrans (4h). IR (KBr) (νmax/cm-1): 3403,
1
1
2215, 1718, 1677, 1531, 1349; H NMR (400 MHz, DMSO-
3327, 2187, 1691, 1633; H NMR (400 MHz, DMSO-d6) δ
d6) δ 1.18 (t, J = 7.20 Hz, 3H), 2.35 (s, 3H), 4.11 (q, J = 7.20
Hz, 2H), 5.14 (s, 1H), 6.80 (br s, 2H, NH2), 7.17 (d, J = 2.61
Hz, 1H), 7.38 (m, 3H); 13C NMR (100 MHz, DMSO-d6) δ
20.6, 40.17, 59.89, 63.50, 111.30, 124.45, 127.42, 127.021,
128.78, 129.02, 130.65, 132.12, 142.16, 148.96, 159.74,
164.30.
1.23 (t, J = 7.24 Hz, 3H), 2.32 (s, 3H), 3.78 (s, 3H), 4.24 (q,
J = 7.21 Hz, 2H), 4.98 (s, 1H), 6.64 (br s, 2H, NH2), 6.90-7.06
(m, 4H); 13C NMR (100 MHz, DMSO-d6) δ 18.02, 22.25,
36.24, 58.18, 60.21, 62.41, 109.40, 123.89, 124.21, 126.13,
126.06, 130.01, 141.71, 147.15, 159.55, 161.63, 167.23.
5-Ethoxycarbonyl-2-amino-4-(2-chlorophenyl)-3-
5-Ethoxycarbonyl-2-amino-4-(4-chlorophenyl)-3-
cyano-6-methyl-4H-pyrans (4i). IR (KBr) (νmax/cm-1): 3428,
cyano-6-methyl-4H-pyrans (4d). IR (KBr) (νmax/cm-1): 3457,
3332, 2195, 1686, 1643; H NMR (400 MHz, DMSO-d6) δ
1
1
3288, 2213, 1724, 1685; H NMR (400 MHz, DMSO-d6) δ
1.09 (t, J = 7.21 Hz, 3H), 2.43 (s, 3H), 4.06 (q, J = 7.21 Hz,
2H), 5.08 (s, 1H), 6.70 (br s, 2H, NH2), 7.19 (m, 3H), 7.38 (dd,
J = 7.60 and 2.35 Hz, 1H); 13C NMR (100 MHz, DMSO-d6) δ
16.32, 37.08, 58.18, 61.71, 108.42, 122.27, 123.01, 125.63,
126.56, 128.95, 129.21, 140.01, 148.45, 158.25, 160.16,
168.74.
1.19 (t, J = 7.25 Hz, 3H), 2.40 (s, 3H), 4.12 (q, J = 7.25 Hz,
2H), 5.09 (s, 1H), 6.71 (br s, 2H, NH2), 7.19 (d, J = 7.85 Hz,
2H), 7.38 (d, J = 7.85 Hz, 2H); 13C NMR (100 MHz, DMSO-
d6) δ 20.10, 41.52, 59.72, 63.51, 111.02, 126.24, 127.98,
128.21, 127.42, 131.78, 144.06, 148.16, 161.21, 162.32.
5-Ethoxycarbonyl-2-amino-4-(3-bromophenyl)-3-
5-Ethoxycarbonyl-2-amino-4-(2-nitrophenyl)-3-cyano-
cyano-6-methyl-4H-pyrans (4e). IR (KBr) (νmax/cm-1): 3451,
3282, 2212, 1726, 1686; 1H NMR (400 MHz, DMSO-d6) δ
6-methyl-4H-pyrans (4j). IR (KBr) (νmax/cm-1): 3453, 3295,
1
2209, 1719, 1684, 1530, 1353; H NMR (400 MHz, DMSO-
125

Valizadeh & Azimi
d6) δ 1.01 (t, J = 7.28 Hz, 3H), 2.44 (s, 3H), 3.99 (q, J = 7.28
pyran 4a and 3-ethoxycarbonyl coumarin 7b. When a mixture
of benzaldehyde (20 mmol), malononitril (20 mmol), ethyl
cyanoacetate (20 mmol) and ZnO/MgO containing ZnO
nanoparticles (10 mol %) was stirred at room temperature, the
product 4a was produced in 91% yield.
Hz, 2H), 5.24 (s, 1H), 6.75 (br s, 2H, NH2), 7.36 (m, 1H), 7.61
(dd, J = 7.81 and 2.66 Hz, 1H), 7.84 (m, 2H); 13C NMR (100
MHz, DMSO-d6) δ 15.25, 38.78, 57.58, 60.78, 106.67, 121.27,
123.32, 124.52, 126.23, 127.56, 128.21, 143.91, 149.45,
157.37, 159.06, 169.12.
The scope of this methodology was further extended by the
reaction of various benzaldehydes in this procedure. In short
reaction times at room temperature, different 4H-pyrans were
prepared in good yields in [bmim]BF4 in the presence of
ZnO/MgO containing ZnO nanoparticles (10 mol%) (Table 2).
With the optimal catalytic system in hand, we investigated
the Knoevenagel reaction of various salicylaldehydes 5 with
active methylene compounds 6 (Scheme 1). A variety of
structurally diverse salicyaldehydes reacted favorably with
active nucleophilic reagents to give the desired coumarins in
good to excellent yields. Results are presented in Table 3. It
can be easily seen that the condensation of a series of
salicyaldehydes and active methylene compounds were
conveniently catalyzed by this catalyst in [bmim]BF4 at room
temperature and the reaction completed in short time.
RESULTS AND DISCUSSIONS
The reaction between benzaldehyde, malononitril and ethyl
cyanoacetate was chosen as the test reaction for the synthesis
of 4H-pyran 4a, and the reaction between salicylaldehyde and
diethylmalonate was chosen as the test reaction for the
synthesis of coumarin 7b because of the large body of
literature data available for these reaction systems. The
reaction conditions were very mild and carried out in
[bmim]BF4 and the progress of the reaction was monitored by
TLC. In the first set of experiments, the catalytic potential of
some metal oxides was investigated for this procedure in
[bmim]BF4 at room temperature (Table 1). According to the
results obtained, of the metal oxides tested, Al2O3, Ag2O, CaO
and SiO2 were unable to promote the reaction. ZnO
nanoparticles were prepared and loaded on MgO to prepare
high surface area ZnO/MgO containing ZnO nanoparticles
solid sample according to the literature [27]. Using this high
surface area solid sample as the catalyst gave large amounts of
5-ethoxycarbonyl-2-amino-3-cyano-6-methyl-4-phenyl-4H-
Additionally, the catalytic activity of the reused catalyst,
which was readily recovered by extraction of the reaction
mixture with ethyl acetate (15 ml × 3), was investigated. 4H-
Pyran and coumarin derivatives, with catalyst recycled for
four times, were synthesized in comparable yields to the fresh
catalyst (Table 4).
The SEM images of the catalyst were obtained before the
Table 1. Synthesis of 4H-Pyran 4a and Coumarin 7b in the Presence of Various Metal Oxidesa
Entry
Metal oxide (10 mol%)
Product 4a
Time (min)
Product 7b
Time (min)
Yield (%)c
Yield (%)c
1
2
3
4
5
6
7
8
-
ZnO
MgO
SiO2
Al2O3
CaO
50
35
35
40
45
45
45
30
40
84
81
45
42
40
42
91
60
40
40
45
45
50
50
35
42
80
78
38
35
35
32
90
Ag2O
ZnO/MgOb
aThe reactions were made at room temperature in [bmim]BF4. bZnO/MgO solid sample containing
ZnO nanoparticles was prepared according to the procedure reported in the literature [27] at room
Temperature in the PH 6. cIsolated yield.
126

ZnO/MgO Containing ZnO Nanoparticles as a Heterogeneous Base Catalyst
a
Table 2. ZnO/MgO Containing ZnO Nanoparticles Catalyzed Synthesis of 4H-Pyrans in [bmim]BF4
Entry
1
2
3
4
5
6
7
R1
Ph
4-NO2
3-NO2
4-Cl
Product
4a
Time (min)
Yield (%)b
30
32
30
28
40
30
30
32
33
30
81
79
82
80
89
90
91
83
85
88
4b
4c
4d
4e
4f
4g
4h
4i
4j
3-Br
4-OH
4-Me
2-OMe
2-Cl
8
9
10
2-NO2
aThe reaction was made at room temperature. bIsolated yield.
a
Table 3. ZnO/MgO Containing ZnO Nanoparticles Catalyzed Synthesis of Coumarins in [bmim]BF4
Entry R1
R2
R3
W
Product
Time
(min)
25
28
40
38
32
30
30
Yield
(%)b
85
91
82
83
80
83
84
M.p. (°C)
Found
264-267
93-96
232-234
131-134
77-78
270-273
188-191
87-90
Reported [Ref.]
265-267 [28]
94-97 [28]
233-234 [28]
132-133 [29]
77-78 [30]
270-271 [29]
189-190 [29]
88-90 [31]
1
2
3
4
5
6
7
8
9
7-OH
H
H
H
8-OH
H
H
8-Br
8-Br
8-OMe
8-OMe
Me
Et
Et
Et
Et
Et
Et
Et
Me
CO2Me
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CN
7a
7b
7c
7d
7e
7f
7g
7h
7i
7-OH
7-OMe
7-Et2N
7-OH
7-OMe
H
35
25
89
78
H
224-226
224-225 [32]
aThe reaction was made at room temperature. bYield of isolated product.
Table 4. Reusing of the High Surface Area Basic ZnO/MgO Containing ZnO Nanoparticles Catalyst for
the Synthesis of 4a, 4b, 7a and 7b
Run
Yield (%)a
Yield (%)a
Yield (%)a
Yield (%)a
Product 4a
Product 4b
Product 7a
Product 7b
1
2
3
4
81
80
79
78
79
79
79
78
77
78
85
83
84
83
82
91
89
89
88
89
5
aIsolated yield.
127

Valizadeh & Azimi
Fig. 1. SEM image of (a) ZnO nanoparticles (b) ZnO nanoparticles in the ZnO/MgO solid before reaction (c) ZnO
nanoparticles in the ZnO/MgO solid after reaction to synthesis of 4a (d) ZnO nanoparticles in the ZnO/MgO
solid after the three times recycled to the synthesis of 4a (e) ZnO nanoparticles in the ZnO/MgO solid after
reaction to the synthesis of 74a (f) ZnO nanoparticles in the ZnO/MgO solid after the three times recycled
to the synthesis of 7a.
128

ZnO/MgO Containing ZnO Nanoparticles as a Heterogeneous Base Catalyst
reaction and compared with the SEM images of the catalyst
[10] R.D.H. Murray, J. Medez, S.A. Brown, The Natural
Coumarins: Occurrence, Chemistry and Biochemistry;
Wiley, New York, 1982.
[11] A. Schönberg, N. Latif, J. Am. Chem. Soc. 76 (1954)
6208.
[12] H. Valizadeh, M. Mamaghani, A. Badrian, Synth.
Commun. 35 (2005) 785.
[13] H. Valizadeh, S. Vaghefi, Synth. Commun. 39 (2009)
1666.
[14] H.H. Kang, H.S. Rho, D.H. Kim, S.-G. Oh, Tetrahedron
Lett. 42 (2003) 7225.
[15] H. Sharghi, M. Hosseini, Synthesis (2002) 1057.
[16] S.M. Hosseini, H. Sharghi, J. Org. Chem. 69 (2004)
6953.
recycled for one and three times for the synthesis of 4a and 7b
(Fig. 1). To the best of our knowledge, this is the first
heterogeneous high surface area basic ZnO/MgO containing
ZnO nanoparticles catalyst for the synthesis of 4H-pyrans and
coumarins in [bmim]BF4.
In conclusion, we have developed a facile and effective
procedure for the synthesis of highly substituted 4H-pyrans
and coumarins in the presence of the high surface area basic
ZnO/MgO solid sample containing ZnO nanoparticles as
recyclable catalyst in [bmim]BF4. Compared with other
methods in existence, it has the advantages of better yields,
inexpensive operation, and environmental friendliness.
ACKNOWLEDGMENTS
[17] S.F. Yin, B.Q. Xu, S.J. Wang, C.F. Ng, C.T. Au, Catal.
Lett. 96 (2004) 113.
The partial financial assistance from the Research Vice
Chancellor of Azarbaijan University of Tarbiat Moallem is
gratefully acknowledged.
[18] M.T. Drexler, M.D. Amiridis, Catal. Lett. 79 (2002)
175.
[19] R.S. Varma, Green. Chem. 1 (1999) 43.
[20] H. Valizadeh, M.M. Heravi, M. Amiri, Mol. Divers. 14
(2010) 575.
[21] H. Valizadeh, M. Amiri, H. Gholipour, J. Heterocycl.
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[22] H. Valizadeh, A. Shockravi, Tetrahedron Lett. 46 (2005)
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