Preparation and characterization of MNPs–PhSO3H as a heterogeneous catalyst for the synthesis of benzo[b]pyran and pyrano[3,2-c]chromenes

Article abstract of DOI:10.1007/s11164-019-04056-z

In this study, an efficient and novel procedure has been reported for loading sulfanilic acid on the surface of magnetite Fe₃O₄ nanoparticles using tris(hydroxymethyl) aminomethane and 1,2-dichloroethane. Next, the synthesized nanoca

Full text of DOI:10.1007/s11164-019-04056-z

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-04056-z
Preparation and characterization of MNPs–PhSO3H
as a heterogeneous catalyst for the synthesis of benzo[b]
pyran and pyrano[3,2‑c]chromenes
Homayoun Faroughi Niya1 · Nourallah Hazeri1 · Mahboobeh Rezaie Kahkhaie1 ·
Malek Taher Maghsoodlou1
Received: 6 September 2019 / Accepted: 6 December 2019
© Springer Nature B.V. 2019
Abstract
In this study, an efcient and novel procedure has been reported for loading sulfanilic
acid on the surface of magnetite Fe3O4 nanoparticles using tris(hydroxymethyl)
aminomethane and 1,2-dichloroethane. Next, the synthesized nanocatalyst was
fully characterized using FT-IR, XRD, TGA, VSM, SEM, and TEM. The results
show that the synthesis of magnetic nanocatalyst has been successful with a range
of 2–20 nm in size. Finally, the catalytic activity of this superparamagnetic nano-
catalyst was explored for the synthesis of tetrahydrobenzo[b]pyran and 2-amino-5-
oxo-4,5-dihydropyrano[3,2-c]chromene-3-carbonitrile derivatives. The synthesized
nanocatalyst has advantages such as non-toxicity, short reaction time, easy workup,
cleaner reaction profles under mild reaction conditions.
Keywords Catalyst design · Magnetic nanoparticles · Heterogeneous nanocatalyst ·
Tris(hydroxymethyl) aminomethane
Introduction
Nowadays, catalysis has received considerable attention because it has rep-
resented a new way to meet the challenges of sustainability and energy. Also,
employed catalysts are often very expensive. Recovery and reusability are two
noteworthy advantages of catalysts that make them economic in most of the mod-
ern catalytic processes [1–6]. Supported heterogeneous metal catalysts are widely
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-04056-z) contains supplementary material, which is available to authorized users.
*
Nourallah Hazeri
nhazeri@chem.usb.ac.ir; n_hazeri@yahoo.com
1
Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan, P.O.
Box 98135-674, Zahedan, Iran
Vol.:(0123456789)
1 3

H. Faroughi Niya et al.
used in industry such as environmental catalysis [7–10], electrocatalysis [11, 12],
and organocatalysis [13, 14]. Also, they have been widely reported such as Fe@
NC@Pd, CNT/PdFe/NC, and RGO@AC/Pd [15–17]. Among the various hetero-
geneous metal catalysts, magnetic nanoparticles (MNPs) received considerable
attention due to their recyclable and localizing properties [18–21]. Magnetic nan-
oparticles (especially superparamagnetic Fe3O4 nanoparticles) have been widely
employed in various applications in bioengineering and biomedical felds, such
as separation of biomedical products, sorting and labeling of cells, magnetic
resonance imaging (MRI), as well as cancer thermotherapy [22–24]. The most
commonly applied MNPs in medical applications include maghemite (γ-Fe2O3),
cobalt ferrite (Fe2CoO4), iron oxide (Fe3O4), and chromium dioxide (CrO2) [25].
The coating of Fe3O4 nanoparticles (NPs) prevents oxidation and aggregation
and improves their chemical stability [26]. Due to signifcant characteristics of
THAM such as non-corrosive, inert, and biodegradable [27], it can appear as a
good coating agent for Fe3O4 NPs. THAM was widely applied in medicine, phys-
iology, biology, and biochemistry, as a bufering agent capable of holding a pH in
the physiological pH range [28]. Also, THAM has been used in the production of
drugs as an excipient [27].
To continue the development of synthesis of MNPs as the green catalyst for the
production of pharmaceutical compounds in one-pot synthesis, we wish to pro-
pose a novel magnetic nanocatalyst containing sulfuric acid-functionalized. Then,
the synthesized nanocatalyst was used as a recyclable catalyst for one-pot, three-
component synthesis of tetrahydrobenzo[b]pyran from reaction among aldehyde
1, malononitrile 2, and dimedone 3, and also efcient one-pot, three-component
method for synthesis of 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene-3-car-
bonitrile using aldehyde 1 malononitrile 2, and 4-hydroxycoumarin 5 (Scheme 1).
R1
O
O
CN
O
3
C
O
NH2
°
H
O
0
10
4a-m
CN
H
MNPs-PhSO3
OH
O
R1
H
2O,EtOH(1:1)
CN
R1
70
°
O
C
1
O
2
CN
O
5
O
NH2
6a-m
Scheme 1 MNPs–PhSO3H catalyzed one-pot synthesis of tetrahydrobenzo[b]pyran and 2-amino-5-oxo-
4,5-dihydropyrano[3,2-c]chromene-3-carbonitrile derivatives
1 3

Preparation and characterization of MNPs–PhSO3H as a…
Experimental
General
Melting points and FT-IR spectra of compounds were measured with an Electrother-
mal 9100 apparatus. The 1H NMR spectra were obtained on BRUKER DRX 300 and
400 MHz, using CDCl3 as a solvent. Ferrous chloride tetrahydrate (FeCl2·4H2O) and
ferric chloride hexahydrate (FeCl3·6H2O) were obtained from Aldrich. Other reagents
and solvents were obtained from Aldrich and Merck and were used without further
purifcation.
Synthesis of the magnetic Fe3O4 nanoparticles (MNPs)
Fe3O4 nanoparticles were prepared by chemical coprecipitation method. First,
FeCl3·6H2O (2.703 g, 10.0 mmol) and FeCl2·4H2O (0.994 g, 5.0 mmol) were dissolved
in 100 mL deionized water under N2 atmosphere. Then, concentrated ammonia (25%)
was immediately added to the mixture until the pH rise to 11 and a black suspension
was formed. Then, the mixture was stirred strongly for 1 h at r.t. The resulting black
precipitate was isolated and washed with distilled anhydrous ethanol and diethyl ether
(several times) and fnally dried in oven at 80 °C for 2 h.
Synthesis of magnetite nanoparticles coated by tris(hydroxymethyl)
aminomethane (Fe3O4@THAM)
Fe3O4 nanoparticles (1.0 g) were homogeneously dispersed in a mixture of deion-
ized water and ethanol by ratio (40:60 ml) by ultrasonic bath for 30 min. Then,
tris(hydroxymethyl) aminomethane [H2N–C–(CH2OH)3] (2.0 g, 16.5 mmol) was
added, and the resulting mixture was regularly stirred under refux condition and N2
atmosphere for 24 h. Finally, the resulting reaction mixture was cooled to room temper-
ature and separated by an external magnet, washed several times with water and dried
at 80 °C to give Fe3O4@tris(hydroxymethyl) aminomethane.
Synthesis of Fe3O4@THAM–CH2CH2–Cl
The Fe3O4@tris(hydroxymethyl) aminomethane (1.0 g) was dispersed in CH3CN
(50 mL) using an ultrasonic bath for 15 min. Subsequently, dichloroethane (1 mL,
12.63 mmol) and triethylamine (0.1 mL) were added dropwise to the reaction mixture.
The resulting mixture was regularly stirred under refux condition and N2 atmosphere
for 24 h. Finally, it was cooled to room temperature and washed with water/ethanol and
dried at 80 °C.
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H. Faroughi Niya et al.
Sulfanilic acid‑functionalized tris(hydroxymethyl) aminomethane‑coated
magnetite nanoparticles (MNPs–PhSO3H)
The Fe3O4@THAM–CH2CH2–Cl (1.0 g) was dispersed in dry toluene (50 mL)
using an ultrasonic bath for 30 min. Then, sulfanilic acid (2.0 g, 11.5 mmol) and
triethylamine (1.6 mL, 11.5 mmol) as a proton scavenger were added. The result-
ing mixture was mechanically stirred under refux condition and N2 atmosphere
for 48 h. Finally, it was cooled to room temperature and was washed with dry
toluene, dichloromethane, and ethanol and dried at 80 °C for 8 h.
General procedure for the synthesis of tetrahydrobenzo[b]pyran derivatives
MNPs–PhSO3H (0.01 g) was added to a mixture of dimedone (1.0 mmol), aro-
matic aldehydes (1.0 mmol) and malononitrile (1.0 mmol) in EtOH/H2O (1:1).
The mixture was stirred at 100 °C for the appropriate time. After the completion
of the reaction, as monitored with TLC, the resulting mixture was diluted with hot
THF (10 mL), and the catalyst was easily separated by an external magnet and the
crude products were recrystallized in ethanol to aford pure tetrahydrobenzo[b]
pyran.
General procedure for the synthesis of 2‑amino‑5‑oxo‑4,5‑dihydropyrano[3,2‑c]
chromene‑3‑carbonitrile derivatives
MNPs–PhSO3H (0.01 g) was added to a mixture of 4-hydroxycoumarin
(1.0 mmol), aromatic aldehydes (1.0 mmol), and malononitrile (1.0 mmol) in
EtOH/H2O (1:1). The mixture was stirred at 70 °C for the appropriate time. After
the completion of the reaction, as monitored with TLC, the resulting mixture was
diluted with hot THF (10 mL) and the catalyst was easily separated by an exter-
nal magnet and the crude products were recrystallized in ethanol to aford pure
2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene-3-carbonitrile.
Characterization data of selected compounds
2‑amino‑5,6,7,8‑tetrahydro‑7,7‑dimethyl‑5‑oxo‑4‑phenyl‑4H‑chromene‑3‑carbonitrile
1
(4a) M.p. 228–230 °C; H NMR (400 MHz, CDCl3) δ=1.07 (s, 3H, CH3), 1.14
(s, 3H, CH3), 2.26 (dd, 2H, J=22 Hz, J=16.4 Hz), 2.49 (s, 2H, CH2), 4.44 (s, 1H,
CH), 4.55 (s, 2H, NH2), 7.21-7.34 (m, 5H).
2‑amino‑5,6,7,8‑tetrahydro‑4‑(4‑nitrophenyl)‑7,7‑dimethyl‑5‑oxo‑4H‑chromene‑3‑car-
1
bonitrile (4b) M.p. 179–180 °C; H NMR (300 MHz, DMSO-d6): δ=0.95 (s, 3H,
CH3), 1.027 (s, 3H, CH3), 2.11 (d, 1H, J=16.0), 2.26 (d, 1H, J=16.0), 2.51 (d, 2H
J=10.0), 4.36 (s, 1H, CH), 7.19 (s, 2H, NH2), 7.44 (d, 2H, Ar, J=8.7), 8.17 (d,2H,
Ar, J=8.7).
1 3

Preparation and characterization of MNPs–PhSO3H as a…
2‑amino‑5,6,7,8‑tetrahydro‑4‑(4‑methoxyphenyl)‑7,7‑dimethyl‑5‑oxo‑4H‑chromene‑3‑car-
bonitrile (4i) M.p. 202–204 °C; IR (KBr, ν, cm−1): 3465, 3320, 2955, 2190, 1676, 1247.
1H NMR (300 MHz, DMSO-d6): δ=0.96 (s, 3H, CH3), 1.04 (s, 3H, CH3), 2.10 (d, 1H,
J=16.0), 2.25 (d, 1H, J=16.0), 3.37 (d, 2H, J=7), 3.72 (s, 3H, OCH3), 4.13(s, 1H, CH),
6.85 (d, 2H, Ar, J=8.7 Hz), 6.98 (br, NH2), 7.06 (d, 2H, Ar, J=8.4 Hz).
2‑amino‑5,6,7,8‑tetrahydro‑4‑(4‑hydroxyphenyl)‑7,7‑dimethyl‑5‑oxo‑4H‑chromene‑3‑car-
bonitrile (4f) M.p. 268–270 °C; IR (KBr, ν, cm−1): 3285, 3160, 2960, 2185, 1675,
1209; 1H NMR (400 MHz, CDCl3): δ=1.05 (s, 3H, CH3), 1.12 (s, 3H, CH3), 2.25 (dd,
2H J=16.0 Hz, J=20.0 Hz), 2.46 (dd, 2H, J=16.0 Hz, J=20.0 Hz), 4.35 (s, 1H, CH),
4.61(s, 2H, NH2), 6.05 (s, 1H, OH), 6.67–7.28 (m, 4H, Ar).
2‑Amino‑4,5‑dihydro‑4‑(3‑nitrophenyl)‑5‑oxopyrano[3,2‑c]chromene‑3‑carbonitrile
(6d) M.p. 261-263 °C; IR (KBr, ν, cm−1): 3402, 3322, 2202, 1703, 1670, 1606,
1
1538, 1380. H NMR (400 MHz, DMSO-d6): δ=4.75 (s, 1H, CH), 7.48-8.16 (m,
9H, Ar, NH2).
Results and discussion
Preparation and characterization of the catalyst
For the frst time, we prepared the heterogeneous nanocatalyst (MNPs–PhSO3H) by
following the method shown in Scheme 2. The magnetic Fe3O4 nanoparticles were
simply synthesized by chemical coprecipitation procedure. Then, sulfanilic acid was
coated on the surface of magnetite nanoparticles using tris(hydroxymethyl) ami-
nomethane and 1,2-dichloroethane. The prepared nanocatalyst was fully character-
ized by elemental, Fourier transform infrared (FT-IR), X-ray difraction (XRD),
thermogravimetric (TGA), vibrating sample magnetometer (VSM), feld emission
scanning electron microscopy (FE-SEM), X-ray spectroscopy (EDS), and TEM
analyses. The elemental analysis shows that nitrogen, hydrogen, and carbon contents
of Fe3O4@THAM were 0.23, 0.6, and 1.27 (wt %), respectively. This analysis con-
frms the loading of THAM on MNPs.
FT–IR
The FT–IR spectra of the Fe3O4 NPs, Fe3O4@THAM–CH2CH2–Cl, and the
MNPs–PhSO3H are shown in Fig. 1. The absorption peaks at around 582 and
631 cm−1 were attributed to Fe–O vibration bonds of uncoated Fe3O4 nanoparticles.
Also, the peaks at 3374 and 1618 cm−1 can be attributed to the asymmetric stretch-
ing vibrations of O–H group and the surface-adsorbed water of Fe3O4 NPs, respec-
tively. (curve a). Two peaks at 780 and 1200 cm−1 are attributed to C–Cl and CHCl
vibrations, respectively (curve b). The spectrum of MNPs–PhSO3H (Fig. 1c) shows
some bands at 3337 and 3423 cm−1, which are attributed to the stretching vibrations
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H. Faroughi Niya et al.
Scheme 2 Preparation of MNPs–PhSO3H
Fig. 1 FT‐IR spectra of Fe3O4 (a), Fe3O4@Fe3O4@THAM–CH2CH2–Cl (b), and MNPs–PhSO3H (c)
1 3

Preparation and characterization of MNPs–PhSO3H as a…
of two kinds of N–H bond of MNPs–PhSO3H. The absorption peaks at around 2923
and 3010 cm−1 were attributed to C-H stretching vibrations in the aliphatic and aro-
matic groups, respectively. Additionally, the presence of sulfanilic acid was afrmed
by the S=O stretching vibration band at 1000–1350 cm−1 in curve c.
XRD
Figure 2 shows the crystalline structure of MNPs and MNPs–PhSO3H nanoparti-
cles, which identifed with the XRD technique. Difraction peaks at 2θ=30.04°,
35.68°, 43.36°, 53.85°, 57.35°, 62.79°, and 74.43°, which could be indexed to (2 0
2), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (6 6 0) planes, are easily identifed in
the XRD pattern. The results indicate that the crystalline phase of Fe3O4 is increased
by the organic layer coating.
TGA and DTG
The stability of the MNPs–PhSO3H catalyst is determined through deriva-
tive thermogravimetry (DTG) and thermogravimetric analysis (TGA) analysis
(Fig. 3). The catalyst’s TGA curve shows three weight loss steps over the temper-
ature range. The frst step, including weight loss of 0.07% up to 100 °C, because
of the elimination of surface hydroxyl groups and physically adsorbed solvent, the
second weight loss of 12.51% at about 130 °C to nearly 220 °C is ascribed to the
decomposition of SO3H groups, and the third weight loss of 39.75% between tem-
peratures of 300–400 °C is ascribed to the breakdown of the decomposition of the
coating organic layer in the nanocatalyst. Due to this mass loss, it was calculated
that 1.5 mmol (125.19 mg) of SO3H groups and around 1.5 mmol (397.43 mg) of
the organic layer were coated on 1 g of MNPs– PhSO3H. In addition, the DTG
Fig. 2 XRD pattern of Fe3O4 (a) and MNPs–PhSO3H (b)
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H. Faroughi Niya et al.
Fig. 3 TGA and DTG curves of MNPs–PhSO3H
curve shows that the decomposition of the organic structure mainly happened at
200 °C. So, the MNPs–PhSO3H is stable below 150 °C.
VSM
The magnetic properties of Fe3O4 NPs and MNPs–PhSO3H were studied by VSM
at room temperature (Fig. 4). The saturation magnetization value (Ms) of Fe3O4
NPs and MNPs–PhSO3H was found to be 116.6 and 20.4 emu g−1. The decrease
in saturation magnetization value of MNPs–PhSO3H can be attributed to the
organic layer coating on the surface of the Fe3O4 NPs.
Fig. 4 VSM spectra of Fe3O4 (a) and MNPs–PhSO3H (b)
1 3

Preparation and characterization of MNPs–PhSO3H as a…
FE‑SEM and EDS
As shown in Fig. 5, the FE-SEM image shows morphology and size of
MNPs–PhSO3H catalyst. The FE-SEM images confrmed the spherical morphol-
ogy of MNPs–PhSO3H. The average size of nanoparticles was determined from 9
to 23 nm (Fig. 5). Also, the FE-SEM images show that the size and morphology
of nanoparticles are uniform in both Fe3O4 (a) and MNPs–PhSO3H (d). The EDS
results of MNPs–PhSO3H are shown in Fig. 6. The existing elemental composi-
tion in MNPs–PhSO3H (C, N, O, S, Fe) is clearly displayed.
Fig. 5 SEM images of Fe3O4 (a), Fe3O4@THAM (b), Fe3O4@THAM–CH2CH2–Cl (c), and MNPs–
PhSO3H (d)
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H. Faroughi Niya et al.
Fig. 6 EDS spectrum of MNPs–PhSO3H
TEM
The spherical morphology of MNPs–PhSO3H was confrmed by TEM (Fig. 7). The
average size of MNPs–PhSO3H was measured using the histogram curve, and it is
about 9 nm (Fig. 8), which confrmed the achieved SEM data.
Fig. 7 TEM images of MNPs–PhSO3H
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Preparation and characterization of MNPs–PhSO3H as a…
Fig. 8 The size histogram of MNPs–PhSO3H
Catalytic application of MNPs–PhSO3H
After the characterization of MNPs–PhSO3H, its catalytic activity was studied for the
synthesis of tetrahydrobenzo[b]pyran and 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]
chromene-3-carbonitrile derivatives (Scheme 1).
For the optimization of the reaction conditions in the synthesis of
tetrahydrobenzo[b]pyran, the reaction of benzaldehyde (1), malononitrile (2), and
dimedone (3) was selected as a model. Initially, the model reaction was optimized
in several solvents as well as under solvent-free conditions, and the results are sum-
marized in Table 1. The best result was achieved in H2O/EtOH (1:1) in the presence
of 0.01 g of MNPs–PhSO3H catalyst at 70 °C. In these conditions, the desired prod-
uct was obtained at 76% yields (Table 1, entry 2). Next, the amount of catalyst was
investigated that the highest yield of product was obtained using 0.01 g of catalyst
(Table 2, entry 1). Finally, the efect of temperature was investigated, and the results
are shown in Table 2. It was found that conventional heating at 100 °C in the pres-
ence of 0.01 g of MNPs–PhSO3H is an efcient condition (Table 2, entry 8).
After optimizing the reaction conditions, in order to investigate the catalytic
activity of MNPs–PhSO3H, various types of aldehyde carrying either electron-with-
drawing or electron-donating groups were reacted with malononitrile (2) and dime-
done (3) under refux condition at 100 °C with 0.01 g of MNPs–PhSO3H in H2O/
Table 1 Optimization of solvent
Entry
Solvent
Time (min)
Isolated
at 70 °C
yield (%)
1
2
3
4
Solvent-free
H2O/EtOH (1:1)
EtOH
100
35
45
76
65
58
50
H2O
55
Bold indicates Optimized conditions
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H. Faroughi Niya et al.
Table 2 Efect of the catalyst amount and temperature on the model reaction
Entry
Catalyst (g)
Temperature (°C)
Time (min)
Isolated
yield (%)
1
2
3
4
5
7
8
9
0.01
0.02
0.03
0.01
0.01
0.01
0.01
0.01
70
70
70
r.t
50
80
100
120
100
35
60
60
260
140
35
10
10
120
76
75
60
25
45
80
91
85
35
10
No catalyst
Bold indicates Optimized conditions
EtOH (1:1) to produce tetrahydrobenzo[b]pyran derivatives. The respective results
are summarized in Table 3. All the desired products were obtained in short reaction
times and good-to-excellent yields (65–95%). The fact that the electronic charac-
ter of substituents of the aromatic aldehyde has no clear efect on the reaction is
signifcant.
A proposed mechanism for the synthesis of tetrahydrobenzo[b] pyran is pro-
vided in Scheme 3. In the frst step, the electrophilicity of the carbonyl group of
aldehyde (1) was increased by the proton from MNPs–PhSO3H. Next, the Knoev-
enagel condensation of malononitrile (2) with activated aldehyde generates interme-
diate (A) that with dehydration, give 2-benzylidenemalononitrile intermediate (B).
In the second step, the dimedone (3) was converted to the enolizable dimedone (C)
in the presence of MNPs–PhSO3H. Subsequently, Michael addition of the enoliz-
able dimedone (C) to 2-benzylidene malononitrile intermediate (B), followed by
continuous intramolecular cyclization happen to give the intermediate (E). Finally,
tautomerization afords the corresponding tetrahydrobenzo[b]pyran.
In another study for the synthesis of 2-amino-5-oxo-4,5-dihydropyrano[3,2-
c]chromene-3-carbonitrile derivatives, we selected the reaction of benzalde-
hyde (1), malononitrile (2), and 4-hydroxycoumarin (5) as a model reaction.
Initially, the model reaction was optimized in several solvents as well as under
solvent-free conditions, and the results are summarized in Table 4. The best result
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Preparation and characterization of MNPs–PhSO3H as a…
1 3

H. Faroughi Niya et al.
Scheme 3 Proposed mechanism for the synthesis of tetrahydrobenzo[b]pyran
Table 4 Optimization of solvent
Entry
Solvent
Time (min)
Yield (%)
at 70 °C
1
2
3
4
Solvent-free
H2O/EtOH (1:1)
EtOH
50
10
40
30
42
87
50
40
H2O
Bold indicates Optimized conditions
was achieved in H2O/EtOH (1:1). In these conditions, the desired product was
obtained at 87% yields (Table 4, entry 2). Next, the amount of catalyst was inves-
tigated that the highest yield of product was obtained using 0.01 g of catalyst
(Table 5, entry 1). Finally, the efect of temperature was investigated, and the
results are shown in Table 5. It was found that conventional heating at 70 °C in
the presence of 0.01 g of MNPs–PhSO3H is an efcient condition (Table 5, entry
1).
After optimizing the reaction conditions, in order to investigate the
catalytic activity of MNPs–PhSO3H, various types of aldehyde carry-
ing either electron-withdrawing or electron-donating groups were reacted
with malononitrile (2) and 4-hydroxycoumarin (5) under refux condi-
tion at 70 °C with 0.01 g of MNPs–PhSO3H in H2O/EtOH (1:1) to produce
1 3

Preparation and characterization of MNPs–PhSO3H as a…
Table 5 Efect of the catalyst amount and temperature on the model reaction
Entry
Catalyst (g)
Time (min)
Temperature (°C)
Isolated
yield (%)
1
2
3
4
5
7
8
9
10
0.01
10
10
10
65
50
10
10
10
60
70
70
70
30
50
87
49
45
50
67
74
65
60
45
0.02
0.03
0.01
0.01
0.01
0.01
0.01
No catalyst
80
100
120
70
Bold indicates Optimized conditions
Table 6 Synthesis of 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene-3-carbonitrile by reaction of
4-hydroxycoumarin, aromatic aldehyde, malononitrile in the presence of MNPs–PhSO3H (0.01 g) in
water/ethanol (1:1) at 70 °C
Entry
R1
Product
Time
Isolated yield M.p (°C)
(%)
M.p (°C) [Ref.]
1
2
3
4
5
6
7
8
H
4-Cl
4-OMe
3-NO2
2-Cl
6a
6b
6c
6d
6e
6f
6g
6h
6i
10
5
10
10
20
5
15
10
5
15
10
30
10
87
98
90
92
97
98
89
90
89
84
85
79
88
252-255
250–252
232–234
261–263
268–270
249–251
248–251
258–260
226–228
257–259
284–287
270–271
260–262
255–257 [38]
251–253 [39]
232–234 [39]
263–265 [40]
266–268 [40]
250–251 [41]
252–254 [41]
259–261 [38]
223–226 [38]
261–263 [42]
285–259 [43]
268–270 [41]
258–260 [44]
4-Br
4-NO2
4-Me
4-CN
2-NO2
4-F
3-OH
4-OH
9
10
11
12
13
6j
6k
6l
6m
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H. Faroughi Niya et al.
2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene-3-carbonitrile
derivatives.
The respective results are summarized in Table 6. All the desired products were
obtained in short reaction times and good-to-excellent yields (79–98%). The fact
that the electronic character of substituents of the aromatic aldehyde has no clear
efect on the reaction is signifcant.
A proposed mechanism for the synthesis of tetrahydrobenzo[b] pyran is provided
in Scheme 4. In the frst step, the electrophilicity of the carbonyl group of aldehyde
(1) was increased by the proton from MNPs–PhSO3H. Next, the Knoevenagel con-
densation of malononitrile (2) with activated aldehyde generates intermediate (A)
that with dehydration, gives 2-benzylidene malononitrile intermediate (B). In the
second step, Michael addition of the active methylene of 4-hydroxycoumarin reacts
to 2-benzylidene malononitrile intermediate (B), followed by continuous intramo-
lecular cyclization happen to give the intermediate (D). Finally, the corresponding
product (6) is aforded by tautomerization.
Comparison of the catalytic activity of MNPs–PhSO3H with some
reported catalytic systems
In order to show the catalytic activity of MNPs–PhSO3H for the synthesis of
tetrahydrobenzo[b]pyran and 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene-
3-carbonitrile derivatives, this system was compared with the known data from the
other literature. For comparison, the derivatives 4a and 6a were selected, as shown
in Table 7. It is clearly demonstrated that short reaction times and high yields make
Scheme 4 Proposed mechanism for the synthesis of 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene-
3-carbonitrile
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Preparation and characterization of MNPs–PhSO3H as a…
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H. Faroughi Niya et al.
Fig. 9 Reusability of the nanocatalyst in the synthesis of tetrahydrobenzo[b]pyran (a) and synthesis of
2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene 3-carbonitrile (b)
this route as a useful protocol. Also, the new catalyst is comparable in non‐toxicity,
stability, and easy separation.
Catalyst recovery and reuse
One of the most important advantages of this superparamagnetic nanocatalyst is
its reusability and recovery that makes it valuable in commercial applications.
Thus, the recovery and reusability of the MNPs–PhSO3H were analyzed in the
synthesis of tetrahydrobenzo[b]pyran via the reaction of dimedone (1.0 mmol),
benzaldehyde (1.0 mmol), and malononitrile (1.0 mmol) in EtOH/H2O (1:1) at
100 °C and synthesis of 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene
3-carbonitrile via the reaction of 4-hydroxycoumarin (1.0 mmol), benzaldehyde
(1.0 mmol), and malononitrile (1.0 mmol) in EtOH/H2O (1:1) at 70 °C. The cata-
lyst was reused for at least six runs in the synthesis of tetrahydrobenzo[b]pyran
and synthesis of 2-amino-5-oxo-4,5-dihydropyrano[3,2-c]chromene 3-carboni-
trile, without any decrease in its catalytic activity (Fig. 9). This small decrease
can be attributed to the loss of catalyst after every recycling.
Conclusions
In this study, we were able to prepare a new nanocatalyst using a very easy and
inexpensive method. The synthesized nanocatalyst has characterized by FT-IR,
TGA, XRD, SEM, VSM, and TEM analyses. This nanocatalyst has been success-
fully used in the synthesis of tetrahydrobenzo[b] pyran and 2-amino-5-oxo-4,5-
dihydropyrano[3,2-c]chromene-3-carbonitrile derivatives that all the derivatives
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Preparation and characterization of MNPs–PhSO3H as a…
have fully synthesized in good-to-excellent yields. The nanocatalyst was simply
separated using an external magnetic and reused several times without a consider-
able decrease in its catalytic activity.
Acknowledgements The authors gratefully appreciate the partial support from the Research Council of
University of Sistan and Baluchestan.
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