Silica-coated magnetic NiFe2O4 nanoparticles-supported H3PW12O40; Synthesis, preparation, and application as an efficient, magnetic, green catalyst for one-pot synthesis of tetrahydrobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives

Article abstract of DOI:10.1007/s11164-015-2198-8

A powerful, magnetic, supported, acid catalyst, NiFe₂O₄@SiO₂-H₃PW₁₂O₄₀, was prepared by chemical support of Keggin (H₃PW₁₂O₄₀) heteropolyacid (HPA) on silica-coated NiFe₂O₄ magnetic nanoparticles. XRD, TEM, SEM, VSM, and FTIR confirmed that the keggin HPA is well dispersed on the surface of silica-coated NiFe₂O₄ magnetic nanoparticles. The magnetically recoverable catalyst could be easily recycled at least six times without significant loss of catalytic activity. After full characterization, its catalytic activity was investigated in the synthesis of tetrahydrobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives. Graphical Abstract: (1) Novel silica-coated magnetic NiFe₂O₄ nanoparticles-supported H₃PW₁₂O₄₀ was fabricated and characterized. (2) Recyclability of the catalyst. (3) Avoiding use of corrosive acid catalysts. (4) Green chemistry.

Full text of DOI:10.1007/s11164-015-2198-8

Res Chem Intermed
DOI 10.1007/s11164-015-2198-8
Silica-coated magnetic NiFe2O4 nanoparticles-
supported H3PW12O40; synthesis, preparation,
and application as an efficient, magnetic, green catalyst
for one-pot synthesis of tetrahydrobenzo[b]pyran
and pyrano[2,3-c]pyrazole derivatives
Behrooz Maleki1 Hossein Eshghi2 Mohammad Barghamadi3
•
•
•
Negar Nasiri1 Amir Khojastehnezhad2
Samaneh Sedigh Ashrafi1 Omid Pourshiani4
•
•
•
Received: 8 June 2015 / Accepted: 29 July 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract
A
powerful, magnetic, supported, acid catalyst, NiFe2O4@SiO2–
H3PW12O40, was prepared by chemical support of Keggin (H3PW12O40) heteropolyacid
(HPA) on silica-coated NiFe2O4 magnetic nanoparticles. XRD, TEM, SEM, VSM, and
FTIR confirmed that the keggin HPA is well dispersed on the surface of silica-coated
NiFe2O4 magnetic nanoparticles. The magnetically recoverable catalyst could be easily
recycled at least six times without significant loss of catalytic activity. After full char-
acterization, its catalytic activity was investigated in the synthesis of tetrahy-
drobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives.
Graphical Abstract (1) Novel silica-coated magnetic NiFe2O4 nanoparticles-sup-
ported H3PW12O40 was fabricated and characterized. (2) Recyclability of the catalyst.
(3) Avoiding use of corrosive acid catalysts. (4) Green chemistry.
&
Behrooz Maleki
b.maleki@hsu.ac.ir
1
2
3
4
Department of Chemistry, Hakim Sabzevari University, Sabzevar 96179-76487, Iran
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran
Department of Chemistry, Sabzevar Branch, Islamic Azad University, Sabzevar, Iran
Department of Chemistry, Faculty of Science, University of Kurdistan, Sanandaj, Iran
123

B. Maleki et al.
H3
C
N
R
O
CH3
NC
2N
O
N
N
H
N
5
6
RCHO
1
EtOH/Reflux
XCH2CN
R
R
O
O
CH3
NC
2N
X
NFS-PWA
2
R1
N
H
O
N
NH2
R1
4
H
7
O
O
O
NH2NH2.H2
O +
OEt
R1
O
R1
R1=CH3, H
3
Keywords Nanomagnetic catalyst Á Heteropolyacid Á Keggin Á Tetrahydrobenzo
[b]pyran Á Pyrano[2,3-c]pyrazole
Introduction
Among a variety of solid acids, HPA have unique physicochemical properties. Their
acidity is substantially higher than that of traditional mineral acids. Furthermore,
HPA are capable of protonating and activating substrates, and are sometimes more
effective than conventional inorganic acids and traditional acid catalysts. Because
the high solubility of these materials often makes their separation from the reaction
mixture difficult, immobilization of HPA on supports with high surface area
facilitates their use as catalysts in a variety of heterogeneous reactions [1–3].
Heterogeneous application of HPA to the surface of silica-coated magnetic
nanoparticles (MNP) has been an interesting approach in recent years. This strategy
has enabled research groups to overcome problems with the separation and
recycling of homogeneous catalysts [4–10]. Thus, in this study, an efficient
magnetic supported acid catalyst NiFe2O4@SiO2–H3PW12O40 (NFS–PWA) was
prepared by chemical support of Keggin HPA on silica-coated NiFe2O4 magnetic
nanoparticles.
Pyrans, a group of heterocyclic compounds, are extremely important in the
pharmaceutical and agrochemical industries and in synthetic chemistry. They have a
wide range of biological and pharmacological properties, for example anti-allergy,
anti-inflammatory, anti-tumor, spasmolytic, diuretic, anti-cancer, anti-coagulant,
and anti-anaphylactic activity [11, 12]. Also, 4H-benzo[b]pyrans can be used as
cognitive enhancers, for treatment of neurodegenerative diseases, including
Alzheimer’s disease, amyotrophic lateral sclerosis, AIDS-associated dementia,
and Down’s syndrome, and for treatment of schizophrenia and Huntington’s
diseases [13].
In the literature several methods are reported for synthesis of tetrahy-
drobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives. The most convenient
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
methods are bicomponent condensation of 1,3-dicarbonyl compounds (1,3-cyclo-
hexanedione or 5,5-dimethyl-1,3-cyclohexanedione) with a-cyanocinnamonitrile
[14–18], or one-pot, three-component reaction of aldehydes, malonitrile, and 1,3-
dicarbonyl compounds (1,3-cyclohexanedione or 5,5-dimethyl-1,3-cyclohexane-
dione) or 3-methyl-1-phenyl-2-pyrazoline-5-one, in the presence of a variety of
catalysts [19]. Highly efficient synthesis of 4H-pyrano[2,3-c]pyrazole by four-
component cyclocondensation of hydrazine hydrate or phenylhydrazine, ethyl
acetoacetate, aldehydes, and malonitrile in the presence of L-proline and
[Bimim]BF4 at 50 °C has also been reported [20]. Catalysts used for one-pot,
three-component reactions include piperidine [21], hexadecyltrimethyl ammonium
bromide (HTMAB) [22], sodium bromide [23], ionic liquids [24–29], tetramethy-
lammonium hydroxide [30], diammonium hydrogen phosphate [31], organocata-
lysts [32], sodium selenite [33], tetrabutylammonium bromide (TBAB) [34, 35],
polyaniline–silica gel [36], alum [KAl(SO4)2Á12H2O] [37], rare earth perfluorooc-
tanoates, e.g. Re(PFO)3 [38], Caro’s acid [39], amines or amino acids [40],
potassium phosphate [41], PPA–SiO2 [42], microwave irradiation [43], BF3ÁOEt2
[44], magnetic core–shell titanium dioxide nanoparticles (Fe3O4@SiO2@TiO2)
[45], NH4H2PO4–Al2O3 [46], DABCO–EtOH [47], TPPA [48], t-BuOK–t-BuOH
[49], L-proline under ultrasound irradiation [50], and silica coated magnetite-
polyoxometalate nanoparticles (Fe3O4@SiO2@NH-NH2–H3PW12O40) [51]. Many
of these methods suffer from one or more limitations, for example low yields, use of
expensive reagents, long reaction times, tedious work-up procedures, and co-
occurrence of several side reactions.
Experimental
Apparatus and analysis
All reagents were purchased from Merck and Sigma–Aldrich and used without
further purification. IR spectra were recorded on a Shimadzu 435-U-04 spectropho-
tometer (KBr pellets). 1H NMR spectra were obtained by use of a Bruker 300 MHz
spectrometer in DMSO-d6 or CDCl3 as solvent and with TMS as internal reference.
Melting points were determined in open capillary tubes in a Stuart BI Branstead
Electrothermal apparatus (cat no: IA9200) and are uncorrected. The particle size
and morphology of the synthesized catalyst were characterized by transmission
electron microscopy (TEM; Philips CM-200 and Titan Krios) and by scanning
electron microscopy (SEM; Philips XL 30 and S-4160) after gold coating. The size
dispersion of the samples was obtained by use of a laser particle size analyzer
(Cordouan Vasco3). X-ray diffraction (XRD) measurements were performed with a
Bruker Axs (Germany) D8 Advance diffractometer.
Preparation of NiFe2O4@SiO2–H3PW12O40 (NFS–PWA)
Silica-coated MNP (NFS) were synthesized as reported elsewhere [62, 63]. Then,
for immobilization of PWA on the NFS, 0.75 g PWA was dissolved in 5 ml
123

B. Maleki et al.
methanol. This solution was added dropwise to a suspension of 1.0 NFS in methanol
(50 ml) and the mixture was then heated at 70 °C for 48 h, under vacuum, with
mechanical stirring, to give the supported magnetic nano catalyst. The catalyst was
collected by use of a permanent magnet, dried under vacuum overnight, then
calcined at 250 °C for 2 h.
General procedure for synthesis of tetrahydrobenzo[b]pyrans
and pyrano[2,3-c]pyrazoles
A mixture of aldehyde (1, 1 mmol), malononitrile or ethyl cyanoacetate (2,
2 mmol), 1,3-dicarbonyl compound (1,3-cyclohexanedione or 5,5-dimethyl-1,3-
cyclohexanedione) (3, 1 mmol) or 3-methyl-1-phenyl-2-pyrazoline-5-one (5,
1 mmol) and NFS–PWA (0.02 g) in EtOH (96 %, 3 ml) was stirred on an oil
bath under reflux condition for the appropriate time (Table 1). The reaction was
monitored by TLC (n-hexane–ethyl acetate, 4:1). After completion of the reaction,
the nanomagnetic catalyst was separated from the reaction mixture by use of an
external magnetic field. The resulting crude product was poured into crushed ice,
and the solid product which separated was isolated by filtration and recrystallized
from ethanol (2 ml) to afford tetrahydrobenzo[b]pyrans and pyrano[2,3-c]pyrazoles.
Spectra data for selected compounds
2-Amino-3-cyano-4-(phenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (4a)
1
IR (KBr) vmax: 3314, 3202, 2214, 1688, 1624, 1507, 1482, 1370 cm-1; H NMR
(300 MHz, CDCl3) dH: 1.04 (s, 3H), 1.13 (s, 3H), 2.11–2.21 (m, 2H), 2.42 (s, 2H),
Table 1 Optimization of the reaction conditions for synthesis of 2-amino-3-cyano-4-(phenyl)-7,7-
dimethyl-5-oxo-4H-5,6,7,8- tetrahydrobenzo[b]pyran (4a)
Entry
Catalyst (g)
Conditions
Time (min)
Yielda (%)
1
NFS–PWA (0.03)
NFS–PWA (0.03)
NFS–PWA (0.03)
NFS–PWA (0.03)
PWA (0.03)
EtOH, reflux
CH3OH, reflux
H2O, reflux
5
5
94
2
87
3
15
10
5
79
4
Solvent-free, 100 °C
EtOH, reflux
EtOH, reflux
EtOH, reflux
EtOH, reflux
EtOH, reflux
EtOH, reflux
EtOH, 25 °C
83
5
74
6
SiO2 (0.03)
60
60
60
10
5
48
7
–
32
8
NFS (0.03)
54
9
NFS–PWA (0.02)
NFS–PWA (0.04)
NFS–PWA (0.03)
72
10
11
a
92
60
Impure
Isolated yields
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
4.67 (s, 1H), 6.52 (brs, 2H, D2O exchangeable), 7.14–7.42 (m, 5H) ppm; 13C NMR
(75 MHz, CDCl3) dC: 26.32, 27.65, 31.24, 35.09, 39.08, 49.98, 59.74, 113.09,
118.42, 125.86, 126.63, 127.54, 142.68, 158.54, 162.32, 194.24 ppm.
2-Amino-3-cyano-4-(4-methylphenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (4c)
1
IR (KBr) vmax: 3392, 3205, 2190, 1670, 1630, 1598, 1486, 1410 cm-1; H NMR
(300 MHz, CDCl3) dH: 0.97 (s, 3H), 1.05 (s, 3H), 2.14 (d, 1H), 2.21 (d, 1H),
2.47–2.76 (m, 2H), 4.45 (s, 1H), 6.92 (brs, 2H, D2O exchangeable), 7.04 (d, 2H),
7.09 (d, 2H) ppm; 13C NMR (75 MHz, CDCl3) dC: 26.98, 28.34, 30.65, 33.98,
36.67, 39.08, 51.61, 61.54, 112.93, 118.82, 125.44, 126.61, 126.96, 142.64, 153.67,
162.32, 194.72 ppm.
2-Amino-3-cyano-4-(3-methoxyphenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (4e)
1
IR (KBr) vmax: 3370, 3180, 2185, 1660, 1624, 1509, 1492, 1380 cm-1; H NMR
(300 MHz, DMSO-d6) dH: 0.92 (s, 3H), 1.01 (s, 3H), 2.05 (d, 2H), 2.66 (d, 2H), 3.45
(s, 3H), 4.17 (s, 1H), 7.06 (brs, 2H, D2O exchangeable), 7.13–7.26 (m, 3H), 7.34 (s,
1H) ppm; 13C NMR (75 MHz, DMSO-d6) dC: 27.30, 28.78, 32.33, 35.52, 35.58,
38.43, 50.44, 58.41, 112.74, 120.19, 128.91, 129.63, 131.80, 144.16, 158.98,
163.61, 197.00 ppm.
2-Amino-3-cyano-4-(4-nitrophenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (4h)
1
IR (KBr) vmax: 3401, 3307, 2200, 1690, 1598, 1498, 1468, 1375 cm-1; H NMR
(300 MHz, CDCl3) dH: 1.04 (s, 3H), 1.18 (s, 3H), 2.26–2.51 (m, 2H), 2.59 (s, 2H),
5.26 (s, 1H), 6.12 (brs, 2H, D2O exchangeable), 7.56 (d, 2H), 8.09 (d, 2H) ppm; 13
C
NMR (75 MHz, CDCl3) dC: 26.98, 27.44, 31.08, 35.87, 40.18, 51.88, 62.54, 113.67,
119.02, 127.74, 127.97, 129.02, 144.32, 161.61, 165.78, 197.24 ppm.
2-Amino-3-cyano-4-(3-nitro phenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (5i)
1
IR (KBr) vmax: 3346, 3297, 2189, 1680, 1588, 1498, 1478, 1389 cm-1; H NMR
(300 MHz, CDCl3) dH: 1.02 (s, 3H), 1.06 (s, 3H), 2.16–2.24 (m, 2H), 2.39 (s, 2H),
4.84 (s, 1H), 6.09 (brs, 2H, NH2, D2O exchangeable), 7.19–7.32 (m, 3H), 7.44 (s,
1H) ppm; 13C NMR (75 MHz, CDCl3) dC: 26.32, 27.98, 31.53, 35.05, 39.73, 51.24,
59.74, 113.04, 118.19, 126.21, 126.89, 130.65, 141.76, 156.08, 161.91, 193.65 ppm.
123

B. Maleki et al.
2-Amino-3-cyano-4-(2,4-dichlorophenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (4k)
1
IR (KBr) vmax: 3388, 3289, 2187, 1698, 1624, 1595, 1489, 1379 cm-1; H NMR
(300 MHz, CDCl3) dH: 1.01 (s, 3H), 1.12 (s, 3H), 2.13–2.22 (m, 2H), 2.47 (s, 2H),
4.45 (s, 1H), 5.85 (brs, 2H, D2O exchangeable), 7.15–7.38 (m, 3H) ppm; 13C NMR
(75 MHz, CDCl3) dC: 26.06, 26.42, 33.67, 35.12, 40.79, 51.45, 60.23, 113.12,
118.47, 122.12, 129.13, 130.18, 142.12, 155.45, 160.78, 193.14 ppm.
2-Amino-3-ethylacetato-4-(phenyl)-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzo[b]pyran (4n)
1
IR (KBr) vmax: 3423, 3265, 3020, 2980, 1667, 1614, 1520, 1462, 1378 cm-1; H
NMR (300 MHz, CDCl3) dH: 1.01 (s, 3H), 1.08 (s, 3H), 1.24 (t, 3H), 2.16–2.25 (m,
2H), 2.34 (s, 2H), 4.24 (q, 2H), 4.37 (s, 1H), 5.52 (brs, 2H, D2O exchangeable),
7.12–7.42 (m, 5H) ppm; 13C NMR (75 MHz, CDCl3) dC: 19.12, 26.52, 27.65, 31.44,
35.56, 39.68, 50.15, 60.63, 71.47, 113.09, 126.42, 126.86, 127.03, 142.54, 156.68,
161.87, 172.42, 196.01 ppm.
2-Amino-3-cyano-4-(4-chlorophenyl)-oxo-4H-5,6,7,8-tetrahydrobenzo[b]pyran (4t)
1
IR (KBr) vmax: 3320, 3192, 2193, 1680, 1619, 1576, 1462, 1376 cm-1; H NMR
(300 MHz, CDCl3) dH: 2.09–2.21 (m, 2H), 2.41–2.54 (m, 2H), 2.80–2.88 (m, 1H),
2.93–3.00 (m, 1H), 5.58 (s, 1H), 6.09 (brs, 2H, D2O exchangeable), 7.15 (d, 2H),
7.44 (d, 2H) ppm; 13C NMR (75 MHz, CDCl3) dC: 20.50, 27.55, 33.15, 37.09,
50.45, 59.13, 113.24, 118.67, 126.45, 126.06, 130.14, 140.05, 159.05, 162.34,
192.46 ppm.
6-Amino-3-methyl-5-cyano-4-(4-bromophenyl)1,4-dihydropyrano[2,3-c]pyrazole
(6c)
IR (KBr) vmax: 3392, 3278, 2221, 1610, 1568, 1445, 1346 cm-1 1H NMR
;
(300 MHz, DMSO-d6) dH: 2.01 (s, 3H), 4.89 (s, 1H), 6.05 (brs, 2H, D2O
exchangeable), 6.96–7.47 (m, 9H) ppm; 13C NMR (75 MHz, DMSO-d6) dC: 25.01,
40.76, 51.05, 61.63, 113.96, 119.04, 125.46, 126.09, 126.42, 128.01, 129.76,
130.05, 131.57, 142.42, 151.52, 161.43 ppm.
6-Amino-3-methyl-5-cyano-4-(4-methoxyphenyl)1,4-dihydropyrano[2,3-c]pyrazole
(6e)
IR (KBr) vmax: 3389, 3292, 2197, 1618, 1576, 1454, 1372 cm-1 1H NMR
;
(300 MHz, DMSO-d6) dH: 1.68 (s, 3H), 3.89 (s, 3H), 5.76 (s, 1H), 6.45 (brs, 2H,
D2O exchangeable), 7.09–7.98 (m, 9H) ppm; 13C NMR (75 MHz, DMSO-d6) dC:
25.32, 36.12, 40.04, 51.05, 59.61, 113.78, 119.14, 125.79, 126.54, 127.02, 128.38,
129.11, 130.18, 131.67, 140.67, 151.67, 158.52 ppm.
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
6-Amino-3-methyl-5-cyano-4-(4-hydroxyphenyl)1,4-dihydropyrano[2,3-c]pyrazole
(6h)
1
IR (KBr) vmax: 3654, 3379, 3289, 2198, 1610, 1576, 1471, 1382 cm-1; H NMR
(300 MHz, DMSO-d6) dH: 1.89 (s, 3H), 5.17 (s, 1H), 6.15 (brs, 2H, D2O
exchangeable), 7.13–7.87 (m, 9H), 8.24 (brs, 1H, D2O exchangeable) ppm; 13C
NMR (75 MHz, DMSO-d6) dC: 25.78, 41.96, 51.02, 60.08, 113.54, 118.43, 125.92,
126.09, 126.59, 128.44, 129.32, 130.32, 131.49, 140.42, 152.21, 160.22 ppm.
6-Amino-3-methyl-4-(4-methoxyphenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-
carbonitrile (7a)
IR (KBr) vmax: 3483, 3254, 2191, 1641, 1608, 1492, 1390 cm-1 1H NMR
;
(300 MHz, DMSO-d6) dH: 1.79 (s, 3H), 3.45 (s, 3H), 4.56 (s, 1H), 6.88 (brs, 2H,
D2O exchangeable), 7.10 (d, 2H), 7.19 (d, 2H), 12.01 (s, 1H, D2O exchangeable)
ppm; 13C NMR (75 MHz, DMSO-d6) dC: 10.22, 35.17, 36.01, 57.64, 97.82, 121.31,
126.04, 128.64, 136.19, 136.07, 141.64, 155.22, 161.35 ppm.
H3C
R
N
CH3
NC
O
N
N
H2N
O
N
5
6
RCHO
1
EtOH/Reflux
XCH2CN
R
R
O
O
CH3
NC
X
NFS-PWA
2
R1
N
H2N
O
N
NH2
R1
4
H
7
OO
O
NH2NH2.H2O
+
OEt
R1
O
R1
R1=CH3, H
3
Scheme 1 One-pot synthesis of tetrahydrobenzo[b]pyrans and pyrano[2,3-c]pyrazoles
123

B. Maleki et al.
6-Amino-3-methyl-4-(4-chlorophenyl)-1,4-dihydro- pyrano[2,3-c]pyrazole-5-
carbonitrile (7b)
IR (KBr) vmax: 3481, 3252, 2187, 1643, 1593, 1492, 1410 cm-1 1H NMR
;
(300 MHz, DMSO-d6) dH: 1.78 (s, 3H), 4.86 (s, 1H), 6.78 (brs, 2H, D2O
exchangeable), 7.18 (d, 2H), 7.45 (d, 2H), 12.22 (s, 1H, D2O exchangeable) ppm;
13C NMR (75 MHz, DMSO-d6) dC: 10.02, 33.62, 55.17, 96.38, 120.71, 128.05,
129.36, 132.68, 133.30, 135.19, 140.64, 155.64, 161.08 ppm.
6-Amino-3-methyl-4-(4-bromophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-
carbonitrile (7c)
IR (KBr) vmax: 3444, 3238, 2195, 1637, 1600, 1491, 1394 cm-1 1H NMR
;
(300 MHz, DMSO-d6) dH: 1.75 (s, 3H), 4.81 (s, 1H), 6.48 (brs, 2H, D2O
exchangeable), 7.09 (d, 2H), 7.32 (d, 2H), 12.12 (s, 1H, D2O exchangeable) ppm;
13C NMR (75 MHz, DMSO-d6) dC: 10.12, 33.62, 55.69, 96.23, 120.01, 128.45,
129.79, 132.54, 133.30, 135.21, 140.54, 155.89, 161.98 ppm.
Result and discussion
In continuation of our previous work on the development of new synthetic methods
[52–63], in this research, an advance in the synthesis of biologically important
materials, use of a separable magnetic nano catalyst, NFS–PWA, is reported. This
new environmentally benign, heterogeneous, and highly re-usable catalyst had very
good catalytic activity in the preparation of tetrahydrobenzo[b]pyrans and
pyrano[2,3-c]pyrazoles (Scheme 1).
The catalyst was prepared by use of simple chemical methods. As shown in
Scheme 2, first, NiFe2O4 nanoparticles were prepared by reaction of nickel and iron
chloride under basic conditions. The NiFe2O4 nanoparticles were then coated with
silica to form NiFe2O4@SiO2 (NFS) and, finally, Keggin HPA (PWA) was
supported on the surface of silica-coated magnetic nanoparticles to prepare the final
catalyst NFS–PWA.
To determine the properties of this acidic, supported, nanomagnetic catalyst, it
was characterized by use of a variety of techniques: FTIR (Fig. 1), SEM and TEM
(Fig. 2), determination of particle size dispersion (Fig. 3), VSM (Fig. 4), and XRD
(Fig. 5). The FTIR spectra of NFS, PWA, and NFS–PWA are compared in Fig. 1.
The FTIR spectrum of NFS (Fig. 1a) contains highly intense absorption peaks at
NiCl2
NH
4OH
PWA
TEOS
+
H
2O
FeCl3
NiFe
2O4
NFS-PWA
NFS
Scheme 2 Preparation of NiFe2O4@SiO2–H3PW12O40 (NFS–PWA)
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Fig. 1 FTIR spectra of a NiFe2O4@SiO2 (NFS), b H3PW12O40 (PWA), and c NiFe2O4@SiO2–
H3PW12O40 (NFS–PWA)
Fig. 2 a TEM and b SEM images of NiFe2O4@SiO2 (NFS)
1200 and 1100 cm-1; these peaks were assigned to the longitudinal and transverse
stretching vibration modes, respectively, of the Si–O–Si asymmetric bond.
Additional bands at 812 and 470 cm-1 were identified as characteristic peaks of
the Si–O–Si bond. Other peaks at 950 cm-1 were assigned to SiO-3 2 vibrations, and
are indicative of the presence of non bridging oxygen ions [64]. The spectrum of
PWA (Fig. 1b) contains typical absorption bands at 1080 (P–O), 982 (W=O), 890
and 802 (W–O–W) cm-1 [65]. In the FTIR spectrum of NFS–PWA (Fig. 1c) peaks
at approximately 984, 888, and 807 cm-1 confirm successful immobilization of the
PWA on the surface of silica-coated nickel ferrite NPs.
123

B. Maleki et al.
Fig. 3 Particle size dispersion of NFS
Fig. 4 VSM curve of NiFe2O4 at room temperature
The morphological features and distribution of the NiFe2O4@SiO2 magnetic
nanoparticles (NFS MNP) were investigated by use of TEM and SEM (Fig. 2a, b).
The TEM and SEM photographs reveal that the NFS MNP are narrowly distributed,
well dispersed, and also are almost spherical and regular in shape. Also, the particle
size dispersion diagram (Fig. 3) obtained from the NFS NP shows that the size of
these magnetic nanoparticles (MNP) is between 25 and 97 nm and the mean
diameter is 53 nm.
It is of great importance that core/shell materials should have magnetic and
superparamagnetic properties suitable for practical applications. Magnetic hysteresis
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Fig. 5 XRD patterns of a prepared NiFe2O4 MNP, b NiFe2O4@SiO2–H3PW12O40 (NFS–PWA), and
c standard pattern of bulk NiFe2O4 (JCPDS 10-325)
measurements of the NiFe2O4 were conducted in an applied magnetic field at r.t, with
the field sweeping from -10,000 to ?10,000 Oersted. As shown in Fig. 4, the
M(H) hysteresis loop of the sample was completely reversible, showing that the
nanoparticles had superparamagnetic properties. The hysteresis loop reached
saturation at the maximum applied magnetic field. The magnetic saturation value
for NiFe2O4 was 16.71 emug-1 at r.t. These MNP had high-permeability magneti-
zation, and their magnetization was sufficient for magnetic separation with a
conventional magnet.
The XRD diffraction patterns of NiFe2O4 and NFS–PWA and standard pattern of
NiFe2O4 are shown in Fig. 5. The diffraction peaks in NiFe2O4 and NFS–PWA
(Fig. 5a, b) indicate that these MNP have the spinel structure, with all the major
peaks matching the standard pattern of bulk NiFe2O4 (Fig. 5c) (JCPDS 10-325)
[65]. It should also be pointed out that no separate crystal phase characteristic of
bulk PWA existed in the NFS–PWA, which confirms the high dispersion of PWA
on the support NFS.
Our initial attempt started with reaction of benzaldehyde (1 mmol), dimedone
(1 mmol), and malononitrile (2 mmol) in the presence of NFS–PWA under reflux in
ethanol (Table 1, entry 1). The effect of different solvents was then investigated;
EtOH was the optimum solvent (Table 1, entries 2 and 3). The reaction was also
conducted in the absence of solvent with 0.03 g catalyst at 100 °C; a yield of 83 %
was achieved in 10 min (entry 4). It was found that use of PWA, SiO2, and
NiFe2O4@SiO2 (NFS) without catalyst resulted in poor product yields, irrespective
of the conditions (Table 1, entries 5–8). To determine the optimum amount of
catalyst (Table 1, entries 9, 10) we performed the reaction with different amounts of
123

B. Maleki et al.
Table 2 One-pot synthesis of tetrahydrobenzo[b]pyran and pyrano[2,3-c]pyrazole derivatives by use of
NiFe2O4@SiO2–H3PW12O40 (NFS–PWA)
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
4a
5
94
227–239 227–239 [22]
O
CN
H3C
NH2
O
CH3
Cl
4b
5
95
209–211 206–207 [22]
O
CN
H3C
NH2
O
CH3
CH3
4c
15
89
212–214 214–216 [22]
O
CN
H3C
NH2
Cl
O
CH3
4d
5
91
232–235 233–234 [22]
O
CN
H3C
NH2
O
CH3
OCH3
4e
5
90
197–199 196–198 [58]
O
CN
H3C
NH2
O
CH3
4f
15
82
211–212 212–214 [32]
O
CH3
CN
H3C
NH2
O
CH3
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
CN
4g
10
82
225–228 225–228 [32]
O
CN
H3C
NH2
O
NO2
CH3
4h
10
85
180–182 177–179 [32]
O
CN
H3C
NH2
NO2
O
CH3
4i
10
80
210–211 210–212 [32]
O
CN
H3C
NH2
O
CH3
Br
4j
5
87
195–197 196–198 [32]
O
CN
H3C
NH2
O
CH3
Cl
4k
10
84
189–190 189–191 [37]
O
Cl
CN
H3C
NH2
O
CH3
123

B. Maleki et al.
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
OH
CN
4l
15
82
236–238 236–238 [37]
O
O
O
H3C
H3C
H3C
NH2
O
CH3
4m
10
80
211–212 212–214 [40]
OMe
CN
NH2
O
CH3
4n
30
90
158–160 158–160 [31]
CO2Et
NH2
O
CH3
Cl
4o
45
86
151–153 153–155 [31]
O
CO2Et
NH2
H3C
O
CH3
CH3
CH3
4p
20
87
208–210 210–212 [41]
S
O
O
CN
H3C
NH2
O
4q
25
84
216–217 217–219 [45]
O
CN
H3C
NH2
O
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
NO2
4r
20
90
230–232 234–235 [43]
O
CN
NH2
O
NO2
4s
25
85
237–239 239–241 [43]
O
CN
NH2
O
Cl
4t
25
82
224–226 226–228 [43]
O
CN
NH2
O
6a
5
84
165–166 168–170 [33]
CH3
NC
N
H2N
O
N
Cl
6b
5
88
182–184 186–187 [20]
CH3
NC
N
H2N
O
N
123

B. Maleki et al.
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
Br
6c
5
90
187–188 183–184 [20]
CH3
NC
N
H2N
O
N
CH3
6d
10
81
174–176 174–175 [20]
CH3
NC
N
H2N
O
N
OMe
6e
10
82
170–172 171–172 [20]
CH3
NC
N
H2N
O
N
NO2
6f
15
94
190–192 188–190 [20]
CH3
NC
N
H2N
O
N
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
NO2
6g
6h
7a
15
20
45
89
76
82
199–197 194–196 [20]
208–210 206–207 [20]
213–215 212–213 [20]
CH3
NC
N
H2N
O
N
OH
CH3
NC
N
H2N
O
N
OMe
CH3
NC
N
H2N
O
N
H
Cl
7b
45
88
234–236 234–235 [20]
CH3
NC
N
H2N
O
N
H
Br
7c
45
90
244–246 249–250 [20]
CH3
NC
N
H2N
O
N
H
123

B. Maleki et al.
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
7d
45
90
244–246 245–246 [20]
CH3
NC
N
H2N
O
N
H
CH3
7e
60
86
196–198 197–198 [20]
CH3
NC
N
H2N
O
F
N
H
7f
45
50
45
90
90
89
245–247 247–248 [20]
251–253 251–252 [20]
222–224 223–224 [20]
CH3
NC
N
H2N
O
N
H
NO2
7g
CH3
NC
N
H2N
O
N
H
Br
7h
CH3
NC
N
H2N
O
N
H
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Table 2 continued
Entry Tetrahydrobenzo[b]pyran or pyrano[2,3-
c]pyrazole
Time
(min)
Yielda
(%)
M.p. (°C)
Found
Reported
NO2
7i
45
91
231–233 232–233 [20]
CH3
NC
H2N
N
O
N
H
a
Isolated yields
catalyst; we found that 0.03 g NFS–PWA effectively catalyzed the reaction and
resulted in the highest yield (Table 1, entry 1). Moreover, this reaction occurred at
room temperature (25 °C) and impure product was achieved (Table 1, entry 11). We
believe that, at room temperature, the reaction was not complete, and stopped
because the aldehyde and malonitrile reacted with each other, resulting in impurities
plus the desired product.
Under the optimized reaction conditions, the scope of one-pot synthesis of
tetrahydrobenzo[b]pyran was investigated by use of different aldehydes and 1,3-
dicarbonyl compounds (1,3-cyclohexanedione or 5,5-dimethyl-1,3-cyclohexane-
dione) as substrates (Table 2).
To broaden the scope of the method, replacement of malononitrile with ethyl
cyanoacetate was examined. Successful condensation was achieved under similar
reaction conditions, affording the corresponding tetrahydrobenzo[b]pyran deriva-
tives in high yields (Table 2, entries 4n–o).
Encouraged by these results, we replaced the cyclic 1,3-dicarbonyl compounds
(1,3-cyclohexanedione or 5,5-dimethyl-1,3-cyclohexanedione) with 3-methyl-1-
phenyl-2-pyrazoline-5-one (Table 2, entry 5) under the same conditions (Scheme 1).
Reaction of 3-methyl-1-phenyl-2-pyrazoline-5-one (Table 2, entry 5) with different
aldehydes and malononitrile was performed for synthesis of 6-amino-5-cyano-4-aryl-
1,4-dihydropyrano[2,3-c]pyrazoles derivatives in EtOH under reflux (Table 2).
Finally, we developed this synthetic method for efficient, one-pot synthesis of
6-amino-5-cyano-4-aryl-1,4-dihydropyrano[2,3-c]pyrazoles by polycondensation of
aldehydes with hydrazine hydrate, malononitrile, and ethyl acetoacetate. To
determine the optimum reaction conditions, reaction of benzaldehyde (1 mmol)
with hydrazine hydrate (1.2 mmol), malononitrile (1.2 mmol), and ethyl acetoac-
etate (1.2 mmol) in the presence of NFS–PWA was selected as model reaction and
the effects of such reaction conditions as amount of the catalyst and temperature
were studied in detail. The best result was obtained with 0.02 g NFS–PWA in EtOH
under reflux. Several structurally diverse aldehydes (Scheme 3; Table 2) were
subjected to condensation with hydrazine hydrate, malononitrile, and ethyl
acetoacetate with NFS–PWA (0.03 g) as catalyst.
The practical synthetic efficiency of this reaction was emphasized by reaction of
terephthalaldehyde with 3-methyl-1-phenyl-2-pyrazoline-5-one and malononitrile to
123

B. Maleki et al.
R
CH2 (CN)2
NH2NH2.H2O
CH3
N
NC
EtOH/Reflux
O
N
H2N
NFS-PWA
H
OO
CHO
R
7
OEt
Scheme 3 Preparation of 6-amino-5-cyano-4-aryl-1,4-dihydropyrano[2,3-c]pyrazoles by four-component
condensation
CHO
O
O
N
NH2
CN
N
CHO
CHO
H3C
H3C
CH3
NC
NFS-PWA
N
+
+
+
CH2 (CN)2
O
N
N
EtOH/Reflux
H2N
O
N
CH3
NC
N
H2N
N
2 mmol
1 mmol
2.5 mmol 1 mmol
2.5 mmol 1 mmol
0%
0%
60 min, 95% yield
60 min, 74% yield
Scheme 4 Reaction between terephthalaldehyde, 3-methyl-1-phenyl-2-pyrazoline-5-one, and malononitrile
give a bispyrano[2,3-c]pyrazole derivative (Scheme 4). Use of 2 equiv. 3-methyl-1-
phenyl-2-pyrazoline-5-one, one equiv. terephthalaldehyde, and 2.5 equiv. malonon-
itrile with NFS–PWA (0.02 g) afforded only 4,40-(1,4-phenylene)-bis (6-amino-3-
methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile) in 95 % yield.
When 3-methyl-1-phenyl-2-pyrazoline-5-one (1 mmol), terephthalaldehyde
(1 mmol), malononitrile (2.5 mmol), and NFS–PWA (0.02 g) were used under
the same conditions we expected one of the formyl groups on the aromatic ring of
terephthalaldehyde would react with 3-methyl-1-phenyl-2-pyrazoline-5-one and
malononitrile. However, both formyl groups condensed with methyl-1-phenyl-2-
pyrazoline-5-one and malononitrile [66].
The catalyst could be easily separated without noticeable reduction in activity.
For this purpose, the same model reaction was again studied under the optimized
conditions. After completion of the reaction, the nanomagnetic catalyst was
separated from the reaction mixture by use of an external magnetic field, dried at
100 °C for 2 h, and re-used in the same reaction. The catalyst was recovered in
excellent yields and used six times without significant loss of activity (Fig. 6). The
FTIR spectra of NFS–PWA catalyst before use (fresh) and after re-use six times
123

Silica-coated magnetic NiFe2O4 nanoparticles-supported…
Fig. 6 Re-use of the NiFe2O4@SiO2–H3PW12O40 (NFS–PWA) for synthesis of 4a
NFS-PWA
H+
NFS-PWA
H+
O
H
O
O
O
R
R1
R1
R
H
N
H
OH
R1=CH3, H
O
CN
R1
R1
1
R1=CH3, H
X
CN
3
H
R
H+
C
X
H
H
H3C
H3C
[A]
C
H+
C
X
NH
N
N
OH
O
N
N
2
5
H+
O
R
O
O
R
R
O
R
H
H
CH3
CH3
X
X
N
H+
CN
NH
NC
HN
or
or
N
N
O
N
R1
R1
N
N
R1
R1
O
[B]
[C]
Tautomeric Proton Shift
R
R
O
O
CH3
or
NC
X
N
R1
H2N
O
N
NH2
R1
6
4
Scheme 5 Proposed mechanism
123

B. Maleki et al.
(recovered) were studied. The spectra showed that the structure of the recovered
NFS–PWA catalyst remained almost the same after re-use six times. In addition, the
weight of the recovered catalyst was the same as that of the fresh catalyst used for
the first time in the reaction.
A plausible mechanism for the one-pot synthesis of tetrahydrobenzo[b]pyrans
and pyrano[2,3-c]pyrazoles is given in Scheme 5. The first step involves Knoeve-
nagel condensation of the alkyl nitrile (2) with the aldehyde (1) to furnish the cyano
olefin [A]. Subsequently, enolates of 3 and 5 add to [A] by Michael addition to give
the intermediates [B] and [C]. The nitrile group is then attacked by the enolate
oxygen, and this is followed by tautomeric proton shift to give the target compounds
4 and 6.
Conclusion
We have developed a simple and efficient catalytic method for synthesis of
tetrahydrobenzo[b]pyrans and pyrano[2,3-c]pyrazoles by one-pot condensation of
aldehydes, malononitrile, and 1,3-dicarbonyl compounds (1,3-cyclohexanedione or
5,5-dimethyl-1,3-cyclohexanedione) or 3-methyl-1-phenyl-2-pyrazoline-5-one, with
NFS–PWA as a re-usable, safe, and green heterogeneous catalyst. The notable
advantages of this method are operational simplicity, generality, availability of
inexpensive reactants, short reaction times, and easy work-up.
Acknowledgments The author thanks the Research Council of Hakim Sabzevari University for partial
support of this work.
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