Equisetum arvense As an abundant source of silica nanoparticles. SiO2/H3PW12O40 nanohybrid material as an efficient and environmental benign catalyst in the synthesis of 2-amino-4H-chromenes under solvent-free c

Article abstract of DOI:10.1002/aoc.3924

Uniform SiO₂ nanoparticles were successfully prepared from Equisetum arvense obtained from the north-east of Iran. Then, surface modification of the extracted nanoparticles was performed with a methanol solution of H₃PW₁₂O

Full text of DOI:10.1002/aoc.3924

Received: 1 March 2017
DOI: 10.1002/aoc.3924
Revised: 12 May 2017
Accepted: 16 May 2017
F U L L PA P E R
Equisetum arvense As an abundant source of silica nanoparticles.
SiO /H PW O nanohybrid material as an efficient and
2
3
12 40
environmental benign catalyst in the synthesis of 2‐amino‐4H‐
chromenes under solvent‐free conditions
1,2,3
4
5
1
1
Reza Tayebee
| Akbar Pejhan | Hassan Ramshini | Behrooz Maleki | Nasrin Erfaninia |
2,3
2,3
Zohre Tabatabaie | Effat Esmaeili
1
Department of Chemistry, Hakim Sabzevari
Uniform SiO nanoparticles were successfully prepared from Equisetum arvense
University, Sabzevar 96179‐76487, Iran
2
2
Department of Chemistry, Payame Noor
University (PNU), Tehran 19395‐4697, Iran
obtained from the north‐east of Iran. Then, surface modification of the extracted
nanoparticles was performed with a methanol solution of H PW O via wet
3
12 40
3
Department of Chemistry, Payame Noor
impregnation method. The prepared nanocatalyst was characterized by XRD,
FESEM, ICP, UV–Vis, and FT‐IR spectroscopy. The supported heterogeneous
nanocatalyst was successfully applied as a Lewis/Bronsted acid catalyst in the
synthesis of a series of substituted 4H–chromenes via condensation of aromatic
aldehydes, malononitrile, and 4‐hydroxycoumarin under solventless conditions with
fine yields in appropriately short times.
University (PNU), Gonabad, Iran
4
Department of Physiology, Faculty of
Medicine, Sabzevar University of Medical
Sciences, Sabzevar, Iran
5
Department of Biology, Payam Noor
University, 19395‐4697 Tehran, Islamic
Republic of Iran
KEYWORDS
Correspondence
Reza Tayebee, Department of Chemistry,
Hakim Sabzevari University, Sabzevar, Iran.
Email:rtayebee@hsu.ac.ir
3
4H–coumarins, catalytic, H PW12O40, heterogeneous, multi‐component, nanomaterial, nano‐silica
Akbar Pejhan, Department of Physiology,
Faculty of Medicine, Sabzevar University of
Medical Sciences, Sabzevar, Iran.
Email: pejhana@medsab.ac.ir
Funding information
Research Councils of Sabzevar University of
Medical Sciences; Hakim Sabzevari University
1
| INTRODUCTION
synthesis strategies has confined their extensive applications
and unquestionably, preparing ultra‐fine silica powders with
high surface area bring about various technological
Nano‐sized catalysts impetuses keep on attracting enthusiasm
for different research areas due to their various physical and
chemical properties when contrasted to the bulk material.
When, the extremely small‐sized particles with expanded
surface area expose to the reactant molecules, allowing more
interactions to occur simultaneously; therefore, accelerating
the reaction. A few methodologies, including sol–gel, thermal
procedures and vapor‐phase reaction have been unveiled for
the preparation of nano‐silica. However, the high cost of the
[
1]
applications.
Catalysis by heteropolyacids and related compounds is a
[
2–4]
field of expanding significance in nanocatalysis.
They
are promising applicants for catalyst design due to their
controllable redox and acid properties and have been actually
[
5,6]
utilized in some automated procedures.
Although diverse
neutral or acidic solid supports such as zirconia and active
carbon have been resolved to immobilize heteropolyacids,
Appl Organometal Chem. 2017;e3924.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
1 of 10
https://doi.org/10.1002/aoc.3924

2
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TAYEBEE ET AL.
nanoscaled silica materials have been chosen as one of the
best supports.
active catalyst in the preparation of different substituted 2‐
amino‐4H‐chromenes through contraction of malononitrile,
aromatic aldehyde, and 4‐hydroxycoumarin under solvent-
less‐free conditions (Scheme 1).
Chromenes are essential oxygenated heterocyclic
compounds endowed with different activities for example,
antioxidative, antihypertensive, antitumor and antiviral,
antidepressant, antileishmanial, antibacterial, diuretic,
anticoagulant, anti‐tubulin, antiallergenic, and hypotensive
2
2
| EXPERIMENTAL SECTION
.1 | Materials and methods
[
7]
activities. Chromenes are well known to activate potassium
channels and inhibit dihydrofolate reductases and phosphodi-
esterase IV and have been utilized as pigments, cosmetics,
potential biodegradable agrochemicals, optical brighteners,
Solvents and beginning materials were purchased from
commercial sources and handled as received. Scanning
electron micrograph (SEM) pictures were taken by XL‐30
Phillips (1992). The crystalline structures of the samples
were surveyed by X‐ray diffraction (XRD) investigation on
a PW1800‐PHILIPS diffractometer with Cu Kα radiation
(λ = 1.5418 Ǻ) at 20 keV. FT‐IR spectra were recorded on
an 8700 Shimadzu spectrophotometer in the area of 400 to
[8–13]
laser dyes and fluorescence markers.
In any case, a couple of methodologies have been accounted
for the preparation of these widely exploited pharmacophores
which suffer from disadvantages including relying on multi‐
step conditions, utilization of harmful organic solvents or
catalysts containing transition metals, tedious work‐up, trouble-
[14]
−1
1
some waste discarding, high reaction time and low yields.
4000 cm using KBr pellets. H NMR (300 MHz) spectra
were recorded on a Bruker‐Avance spectrometer. The
chemical composition of the analyzed materials was detected
by an inductively coupled plasma (ICP‐MS) model Vista‐pro
and samples were totally dissolved in a sensible hot acid
before analysis. A freeze dryer, Model FD‐10, Pishtaz Equip-
ment apparatus Engineering Co, Iran, was used for drying of
the prepared nanocatalyst. All products were perceived by
examination of their physical information with those already
announced. H PW O acid catalyst was set up according to
Considering critical preceding properties of chromene
derivatives, advancement of useful clean and uncomplicated
strategies for the efficient preparation of these compounds
by applying accessible reagents under catalytic conditions is
of great importance.
Performing organic reactions using solid acid catalysts is
associated with different advantages, for example, good
agreement with the environment, the appropriate thermal
stability of catalyst, catalyst efficiency, product isolation
and straightforward process, using mild conditions and
reducing the volume of wastes. In continuation of our efforts
to probe new and green methodologies based on solid acid
3
12 40
[
20]
the previously detailed methods.
2
.2 | Preparation of Nano‐silica from
[15]
catalysts under heterogeneous conditions,
a new routine
Equisetum arvense
fashion is disclosed for the production of reactive nano‐silica
from a commonplace plant Field Horsetail (Equisetum
arvense) which is a Perennial from creeping rhizomes and
often forming large colonies which is growing in nutritionally
poor soil. The stems of this plant contain more than 10%
silica, therefore, can be considered as a suitable environmen-
tal benign source of silica. The white ash obtained procure
from controlled burning and calcination at generally moder-
ate temperatures creates the main deposit containing silica
The stems of Equisetum arvense were washed altogether with
water to delete the dissolvable particles or other present
contaminants present and substantial impurities like sand.
At that point, it was dried in an oven at about 110°C for
24 h. The dried Equisetum arvense was refluxed with an
acidic solution (0.1 M HCl) under stirring for 90 min. Then,
it was cooled and kept around 20 h. Finally, it was isolated
and completely washed with hot, refined water until the rinse
turned free from acid. The wet solid was subsequently dried
in an oven at 110°C for 24 h and designated as Equisetum
arvense. 20 g of this material was mixed with 160 ml of
NaOH (2.5 M) solution. The mixture was heated for 3 h
with stirring. It was separated, and the remaining was
washed with 40 ml boiling distilled water. The acquired
viscous, transparent and colorless solution was permitted
to chill off to room temperature. Then, H SO (5.0 M)
[16–19]
in the appearance of an amorphous phase.
The small tiny particle diameter and high surface area of
the supplied nano‐silica powders gave rise to heterogenized
H PW O (HPA) to upgrade its catalytic execution. The
3
12 40
developed SiO /H PW O nanocatalyst endeavored as an
2
3
12 40
2
4
was included under steady stirring at controlled conditions
until pH 2, then NH OH was added until pH 8.5, and
4
permitted to room temperature for 3 h. Nano silica was
prepared by refluxing of the removed silica with HCl
(6.0 M) for 4 h; afterward, washed repeatedly with
SCHEME 1 General route for the preparation of 2‐amino‐4H‐
chromenes

TAYEBEE ET AL.
3 of 10
deionized water to make it acid‐free. It was then dissolved in
2.5 | Spectral data for some selected 2‐amino‐
4H‐chromenes
stirring NaOH (2.5 M). Thereafter, H SO was added until
2
4
pH 8. The precipitated silica was repeatedly washed with
warm deionized water to free it from alkali and was dried
at 50°C for 48 h in an oven.
2
.5.1 | 2‐Amino‐4‐(phenyl)‐3‐cyano‐4H,5H–
pyrano[3,2‐c] chromene‐5‐one (4a)
−
1
White solid, mp = 257–259°C. FT‐IR (KBr) (ν /cm ):
max
1
3
378, 3286, 3178, 2196, 1709, 1674, 1604. H NMR
(
(
300 MHz, DMSO‐d ): δ (ppm) 4.46 (1H, s, H‐4), 7.25
2H,d, J = 7.8 Hz, HAr), 7.28 (1H, brs, HAr), 7.33 (2H, t,
6
H
2
.3 | Surface modification of nano‐silica with
H PW O
3
12 40
J = 7.5 Hz, HAr), 7.42 (2H, brs, NH ), 7.45 (1H, d,
2
SiO /HPA nanohybrid material was supplied through the
well‐known wet impregnation strategy. Accordingly, 0.1 g
J = 8.4 Hz, HAr), 7.49 (1H, t, J = 7.6 Hz, HAr), 7.71 (1H,
t, J = 7.5 Hz, HAr), 7.91 (1H, d, J = 7.8 Hz, HAr). ppm.
2
of H PW O was dissolved in 5 ml methanol. Then, 0.9 g
3
12 40
nano‐silica was gradually added to the obtained solution dur-
ing 20 min. At that point, the acquired colloid mixture was
stirred at 30°C for 6 h until methanol vanished. The resulting
powder was dried at 80°C under air for 24 h. The amount of
2
.5.2 | 2‐Amino‐4‐(4‐cl‐phenyl)‐3‐cyano‐
H,5H–pyrano[3,2‐c] chromene‐5‐one (4b)
4
−1
White solid, mp = 263–265°C. FT‐IR (KBr) (ν /cm ):
max
1
tungsten in SiO /HPA was measured by ICP. For this, 2 ml of
2
3
383, 3314, 3189, 2194, 1715, 1675, 1607. H NMR
1
:1 concentrated HNO : HClO was added to an exact
3 4
(300 MHz, DMSO‐d ): δ (ppm) 4.50 (1H,s,H‐4), 7.31(2H,
6
H
amount of the nanocomposite in a polyethylene beaker and
warmed up to dried. This procedure was repeated three times
and afterward 2 ml HF was added and heated to demolish the
HPA framework. Eventually, 10 ml diluted HCl was
included, and the mixture was warmed for 20 min. The
received solution was filtered and diluted in a 50 ml volu-
metric flask and finally used for the description of tung-
d, J = 8.2 Hz,HAr), 7.36(2H,brs,NH ), 7.38 (2H,brs,HAr),
2
7
7
.44(1H,d, J = 8.2 Hz,HAr), 7.49(1H, t, J = 7.6 Hz, HAr),
.71(1H,t, J = 7.8 Hz, HAr),7.92 (1H,d, J = 7.8 Hz, HAr) ppm.
2.5.3 | 2‐Amino‐4‐(4‐nitrophenyl)‐3‐cyano‐
4H,5H–pyrano[3,2‐c] chromene‐5‐one (4e)
Pale Yellow Solid, mp = 251–253°C. FT‐IR (KBr) (νmax/
sten. The content of tungsten in the prepared SiO /HPA
2
was 4.4% as determined by ICP analysis. The last analysis
demonstrated that ~60% loading of the existing HPA
occurred onto the surface of nano‐silica. To study the effect
of drying conditions on the extent of loaded HPA, the
freeze dried nano‐silica was prone to the surface modifica-
tion. Results showed no obvious increment in HPA content
for the material prepared under the freeze drying conditions
compared with the sample dried under usual thermal
conditions.
−1
cm ): 3482, 3432, 3371, 3335, 2195, 1718, 1673, 1607,
1
1
4
506, 1374, 1306. H NMR (300 MHz, DMSO‐d ): δ (ppm)
.68 (1H, s, H‐4), 7.47 (1H,d, J = 8.3 Hz, H7), 7.52 (1H, t,
6
H
J = 7.7 Hz, H9), 7.57 (2H, br s, NH ),7.60 (2H, d,
J = 8.0 Hz, H2,6), 7.74 (1H, t, J = 7.8 Hz, H8), 7.91 (1H,
d, J = 7.8 Hz, H10), 8.18 (2H, d, J = 8.3 Hz, H3,5) ppm.
2
2
.5.4 | 2‐Amino‐4‐(3‐nitrophenyl)‐3‐cyano‐
H,5H–pyrano[3,2‐c] chromene‐5‐one (4f)
4
−
1
White solid, mp = 258–260°C. FT‐IR (KBr) (νmax/cm ):
2
.4 | General procedure for the synthesis of
−1 1
3
404, 3322, 3194, 2202, 1703, 1672, 1531, 1349 cm . H
substituted 2‐amino‐4H‐chromenes
NMR (300 MHz, DMSO‐d ): δ (ppm) 4.74 (1H, s, H‐4),
6
H
7
7
.44 (1H, d, J = 6.7 Hz, H7), 7.51(1H, t, J = 7.6 Hz, H9),
To a mixture of malononitrile (1 mmol), aromatic aldehydes
.56 (2H, br s, NH ), 7.64 (1H, t, J = 7.6 Hz,H5), 7.73
(1 mmol), and 4‐hydroxycoumarin (1 mmol) in a test tube
2
(1H, dt, J = 7.5, 1.3 Hz, H8), 7.82 (1H, d, J = 6.8 Hz,
was added SiO /HPA nanohybrid material. The resulting
2
H2’),7.92 (1H, dd, J = 6.8, 1.2 Hz, H10), 8.12 (1H, dd,
J = 8.4, 1.4 Hz, H4’),8.14 (1H, s, H6) ppm.
mixture was mixed at 80°C; a drop of ethanol was added
after solidification of the reaction mixture to maintain
liquidity. After fulfillment of the transformation as indicated
by TLC, the reaction mixture was cooled to room tempera-
ture. Then, chloroform (5 ml) was added, stirred for 1 min,
and separated to isolate the heterogeneous catalyst. Eventu-
ally, the filtrate was vanished, and the resulting unrefined
product was recrystallized from EtOH (96%) to afford the
pure product.
2
4
.5.5 | 2‐Amino‐4‐(4‐MeO‐phenyl)‐3‐cyano‐
H,5H–pyrano[3,2‐c] chromene‐5‐one (4 h)
−
1
White solid, mp = 236–238°C. FT‐IR (KBr) (ν /cm ):
3378, 3314, 3190, 2196, 1709, 1672, 1608. H NMR
(300 MHz, DMSO‐d6): δH(ppm) 3.72 (3H,s,OCH3),
max
1

4
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TAYEBEE ET AL.
4
J
.40(1H,s,H‐4),6.87(2H,d, J = 8.1 Hz,HAr),7.18 (2H,d,
means of UV–Vis, SEM, FT‐IR, ICP, and XRD. The most
informative technique for the examination of the surface
interaction amongst H PW O and nano‐SiO are FT‐IR
=
8.1 Hz,HAr), 7.37(2H,brs,NH2), 7.45(1H, d,
J = 8.3 Hz,HAr), 7.49(1H,t, J = 7.8 Hz, HAr),7.70 (1H,t,
J = 7.7 Hz, HAr),7.89(1H,d, J = 7.7 Hz, HAr) ppm.
3
12 40
2
and UV–Vis techniques.
3
| RESULTS AND DISCUSSION
3.1.1 | FT‐IR spectroscopy
FT‐IR spectra of the constructed samples in the mid‐infrared
region from 400 to 1400 cm are shown in Figure 1. To
Silica gel is an inorganic amorphous polymer that is built up
from the condensation of silicate tetrahedrons utilizing oxygen
as the coupling site, giving rise to the siloxane bond (Si‐O‐Si)
and, at last, nanometer‐sized particles. The siloxane groups
are on the inside of the particles, and the surface is made out
of silanol groups (Si‐OH). It is well known that solubility of
amorphous silica increases in solutions with pH ≥ 10; hence,
the amorphous silica contained in the Equisetum arvense can
be solubilized when treated with a sodium hydroxide solution
−1
ascertain binding of H PW O onto the surface of nano‐
3
12 40
SiO , FT‐IR of all fragments H PW O (A), nano‐SiO2
2
3
12 40
(
B), and SiO /H PW O (C) were assembled. The bands
2 3 12 40
−1
about 1125 and 616 cm in (B) were assigned to the surface
O–Si–O bending and Si–O stretching vibrations, respec-
[22]
tive.
The heteropolyacid (A) showed the four characteris-
−1 [23]
tic bands in the range 750–1100 cm .
around 3420 cm represented the vibration of water because
of the simple hydration of the materials. The observed bands
around 809 and 890 cm are due to W‐O‐W. The peaks
around 982 and 1080 cm were related to M = O and P‐O
An expansive
−1
[21]
with the above pH to form dissolvable silicate.
Further-
more, when sodium silicate is acidified, a supersaturated
−1
solution of Si(OH) monomers is shaped. The silica gel is
−1
4
formed is shaped during the gelification of a silica acid solu-
tion using applied a polymerization process, which is sepa-
rated into three phases including monomer polymerization to
form particles, particle growth, and particle union in ramified
is branched chains that extend throughout the solution and
brings about expanding viscosity and formation of a gel.
bonds, individually, which distinctively indicated the neat
structure of Keggin framework for the heteropolyacid.
3
.1 | Characterization and physicochemical
properties of nano SiO / H PW O
2
3
12 40
The physicochemical and morphological properties of the
supported SiO /H PW O nanocatalyst were explored by
2
3
12 40
FIGURE 1 FT‐IR spectra of free H
3
PW12
O
40 (a), nano‐SiO
2
(b), and
2 2 3 40
FIGURE 2 The SEM images of nano‐SiO (a) and SiO /H PW12O
H
3
PW12
O
40 impregnated nano‐SiO (c)
2
nanoparticles (b)

TAYEBEE ET AL.
5 of 10
morphology with a restricted size distribution with an aver-
age diameter of <50 nm. However, functionalization and
anchoring of HPA had no critical impact on the structure of
nano‐SiO . The EDX in Figure 3 demonstrated the presence
2
of silica and the heteropolyacid in the prepared nanocompos-
ite. Figure 4 shows X‐ray diffractions for commercial
nanosilica and for SiO /H PW O . Although, the prepared
2
3
12 40
nanosilica seems to behave as amorphous, however, the three
weak characteristic signals for nanosilica are observed
between 30–40°. The distinct characteristic bands for HPA
at 2θ of 28, 31, and 36.2 confirmed that the Keggin structure
2 3 40
FIGURE 3 EDX spectrum of the prepared SiO /H PW12O
nanoparticles
[
24]
is retained after immobilization.
Although in the FT‐IR of SiO /H PW O , these four peaks
2
3
12 40
3
.1.3 | UV–vis and ICP analyses
marginally overlapped with SiO , in any case, the peak
2
around 982 cm− affirmed that the neat perfect structure of
1
The amount of loaded HPA on nano‐SiO was determined by
2
the Keggin heteropolyacid was preserved after immobiliza-
providing UV–Vis spectra of solutions containing HPA dur-
ing immobilization process. To discover the degree of
H PW O which can be loaded on the support, UV–Vis
tion onto the nano‐SiO particles.
2
3
12 40
spectra of solutions containing HPA before and after addition
of nano‐silica were attained. For this purpose, 0.5 g of SiO2
was suspended in a 5 ml methanol solution containing
100 mg HPA. Then, the suspension was stirred for 6 h
(Figure 5). Clearly, most of the heteropolyacid content
(~60%) was adsorbed on the silica surface after 90 min.
3
.1.2 | SEM, EDX, and XRD of nano‐SiO and
2
H PW O impregnated nano‐SiO
3
12 40
2
The morphology of SiO and SiO /HPA nanohybrid catalyst
was inspected by SEM as appeared in Figure 2. These micro-
graphs illustrated that the nanoparticles have a semi‐spherical
2
2
FIGURE 4 X‐ray diffraction patterns for
commercial nanosilica (a) and SiO
40 nanocatalyst (b). Asterisks show
heteropolyacid
2
/
3
H PW12O

6
of 10
TAYEBEE ET AL.
However, a higher amount of catalyst brought down yield from
94% to 88% at the same time (entry 7). In this manner, 0.2 g was
chosen as the optimal quantity to push the reaction forward.
3
.2.2 | Effect of various solvents on the reac-
tion progress
Matching to the fundamental concepts of changing volatile,
flammable, and poisonous solvents with less dangerous reac-
tion media, the efficacy of the solvent‐free condition was
contrasted to the solvent condition in the titled condensation
reaction. As a result, condensation of malononitrile with
benzaldehyde and 4‐hydroxycoumarin was done in various
solvents (Table 2). Fortunately, the solvent‐free case was
found as ideal condition and the best yield was accomplished.
In any case, a considerable amount of the intended product
was provided in H O, EtOH, CHCl , CH Cl , and toluene
FIGURE 5 UV–vis spectral changes during loading of HPA onto the
fabricated nanosilica
TABLE 1 The impact of catalyst amount on the condensation of
a
malononitrile, benzaldehyde, and 4‐hydroxycoumarin
2
3
2
2
Yield (%)b
as 84, 78, 33, 20, 60% yields, respectively. Noteworthy,
drinking water gave an acceptable result 84% and can be a
suitable choice for industrial purposes.
Entry
Nano‐SiO
2
/HPA (g)
Time (min)
1
2
3
4
5
6
7
0
120
60
40
30
20
20
20
>15
30
51
73
82
94
88
0.01
0.03
0.05
0.1
3.2.3 | Effect of reaction temperature and time
on the condensation reaction
0.2
To enhance the yield and accomplishing the best reaction
conditions, the impact of temperature was contemplated on
the condensation reaction of malononitrile with benzalde-
hyde and 4‐hydroxycoumarin (Figure 6). As is anticipated,
yield was improved with temperature and the best result
was acquired at 80°C after 20 min. Also, a further increase
in temperature didn't enhance yield. Consequently, the reac-
tion temperature 80°C was kept for all runs.
0.3
a
The desired amount of SiO
2
/HPA was added to a mixture of malononitrile
1 mmol), benzaldehyde (1 mmol), and 4‐hydroxycoumarin (1 mmol). Then,
the mixture was stirred at 80 °C in an oil bath and the reaction was followed by
TLC. After fulfillment of the reaction, the crude product was purified as depicted
in the experimental section.
(
b
Isolated yields.
The amount of loaded HPA onto the support was in good
concurrence with the result of ICP.
Effect of reaction time was likewise researched to explore
minimum time required to acquire the maximum yield. As
Figure 7 shows, 20 min was adequate to acquire 94% yield.
No apparent increment in yield was attained after extending
reaction time. Conclusively, reactions at various conditions
and different molar ratios of substrates in the presence of
the heterogeneous SiO /H PW O uncovered that the best
3
3
.2 | Catalytic tests
.2.1 | Studying the impact of catalyst amount
on the condensation reaction
2
3
12 40
A model reaction was disclosed by applying 0–0.3 g of SiO2/
HPA for the preparation of 2‐amino‐4H‐chromenes (Table 1).
Firstly, the reaction was non‐beneficial without catalyst and
only 15% of product was accomplished after 2 h (entry 1);
demonstrating that presence of catalyst was intransitive in
the condensation reaction. It was found that 0.01 g of catalyst
prompted 30% yield after 60 min (entry 2). This perception
TABLE 2 Effect of various solvents on the condensation of
malononitrile, benzaldehyde and 4‐hydroxycoumarin
a
Temp. (°C) (under
reflux)
Time
(min)
Entry
Solvent
Yield (%)b
1
2
3
4
5
6
H
2
O
100
78
20
20
60
60
30
20
84
78
33
20
60
94
EtOH
CHCl
3
61
established that SiO /HPA displayed high catalytic activity
2
CH
2
Cl
2
40
in the favorite transformation. The reaction of malononitrile
with benzaldehyde and 4‐hydroxycoumarin in the presence
of 0.2 g catalyst gave the corresponding chromene in 94%
yield (Table 1, entry 6). As is expected, yield% was expanded
with improving catalyst amount and the greatest yield of 94%
was accomplished with 0.2 g of the catalyst after 20 min.
Toluene
110
80
‐
a
Reaction conditions are described below Table 1. 0.2 g of the nanocatalyst was
used under reflux conditions. 3 ml solvent was used in each case.
b
Isolated yields.

TAYEBEE ET AL.
7 of 10
speared over oxygen atoms, structural composition distortions,
and approach of the substrate molecule into the bulk of the
heteropolyacid would contribute to the catalytic efficiency of
the examined heteropolyacids under the reaction conditions.
3
.2.5 | Studying generality of the desired con-
densation reaction
Since, pharmacological and biological activities of chromenes
depend on the nature of substituents being either on the
4
H–pyran or on the adjacent rings, extension and generality
FIGURE 6 Effect of reaction temperature on the condensation of
malononitrile, benzaldehyde, and 4‐hydroxycoumarin in the presence
of 0.2 g of SiO /H PW12O
2 3 40
of the present methodology were assessed by applying the
condensation reaction under the achieved perfect conditions
with different aromatic aldehydes bearing different substitu-
ents (Table 4). Selective and efficient generation of a variety
of chromenes with the assembled nano‐hybrid catalyst was
studied by considering the condensation reaction under the
optimized conditions by using different aromatic aldehydes
bearing electron withdrawing (donating) groups and
halogen‐substituted counterparts. Table 4 shows most of the
electron‐rich and electron‐poor aromatic aldehydes reacted
well and produced high yields. Aromatic aldehydes including
electron‐withdrawing groups gave the respective addition
excess, products in good yields; while, the others containing
electron‐donating groups afforded moderate yields.
FIGURE 7 Effect of reaction time on the condensation of
benzaldehyde, malononitrile, and 4‐hydroxycoumarin
3.2.6 | Comparison of the viability of the
present approach with some reported protocols
a
TABLE 3 Studying the catalytic activity of different heteropolyacids
The productivity of the SiO /H PW O catalytic system
2
3
12 40
Amount
(g)
Time
(min)
Yield
(%)
Mol
(%)
was contrasted to some reported catalysts for the synthesis
of 2‐amino‐4H‐chromenes. The reaction of malononitrile
with benzaldehyde and 4‐hydroxycoumarin was chosen in
the presence of different catalysts as organized in Table 5.
Clearly, SiO /H PW O is more effective compared to the
reported catalysts respecting the yield. Likewise, the present
methodology employed a low cost and easily prepared cata-
lyst under solvent‐free conditions. Albeit, a portion of the
introduced catalysts pushed the reaction even though at better
reaction conditions, nonetheless, they needed costly and
harmful reagents and solvents and higher amounts of catalyst.
The present catalytic framework conveys distinct advantages
in competition with the already revealed strategies; regarding
yield, straightforward work‐up, mild reaction conditions and
lack of toxicity.
Entry
Catalyst
1
2
3
4
H
H
H
H
3
6
3
5
PW12
O
40
62
40
20
0.02
0.03
0.01
0.02
20
20
20
20
88
84
80
75
0.7
0.7
0.7
0.7
P
2
W
18
O
PMo12
PW10
O
2
3
12 40
V
O
40
a
The desired amount of different of HPA was added to a mixture of malononitrile
1 mmol), benzaldehyde (1 mmol) and 4‐hydroxycoumarin (1 mmol). Then, the
mixture was stirred at 80°C in an oil bath and the reaction was followed by
TLC. After completion of the reaction, the crude product was purified as
described in the experimental part.
(
condition was solvent‐free at 80°C in the presence of 0.2 g of
the nanocatalyst.
3
.2.4 | Comparison of the catalytic activities of
different heteropolyacids
3
.2.7 | Studying stability and reusability of
Catalytic activities of H PW O , H P W O , H PMo O
12 40
3
12 40
6
2
18 62
3
nano‐SiO /H PW O
2
3
12 40
and H PW V O were compared in the desired condensa-
5
10 20 40
tion reaction (Table 3). Although, all of the examined catalysts
were successful and provided 75–88% yield after 20 min,
however, the first showed the best yield. Many variables for
example, acidity of the heteropolyacid, negative charge density
Comparison of the infrared spectra of nano‐SiO /H PW O
2
3
12 40
before and after condensation reaction (regenerated after 5
runs) and observation of the characteristic bands around
800–1100 cm− clearly confirmed preservation of the Keggin
1

8
of 10
TAYEBEE ET AL.
a
TABLE 4 Synthesis of different substituted chromenes in the presence in the nearness of SiO
2
/HPA under solvent‐free conditions
b
c
Entry
Ar‐
Time (min)
Yield (%)
94
M.p.(°C)
257–259
263–265
265–267
254–256
251–253
258–260
244–245
236–238
224–226
260–262
244–246
Lit. M.p. (°C)
[256–258]
[262–264]
[266–268]
[254–255]
[252–254]
[260–262]
[241–243]
[238–240]
[220–222]
[260–262]
[246–248]
Ref
4
4
4
4
4
4
4
4
4
4
4
a
b
c
d
e
f
Phenyl
20
15
22
25
20
30
25
25
20
20
20
[14e]
[14e]
[14a]
[14a]
[14e]
[14e]
[14e]
[14e]
[14e]
[14a]
[14j]
4‐Cl‐phenyl
2‐Cl‐ phenyl
4‐Me‐ phenyl
‐ phenyl
‐ phenyl
3‐MeO‐ phenyl
4‐MeO‐ phenyl
91
88
90
4‐NO
3‐NO
2
89
2
86
g
87
h
88
i
j
4‐N(CH
4‐F‐phenyl
3‐Cl‐phenyl
3
)
2
–phenyl
89
92
k
85
a
Reaction conditions are described below Table 1.
b
Yields refer to the separated pure products. The desired pure products were described by comparison of their physical information with those of known compounds.
c
Melting points are uncorrected and refer to the desired products.
TABLE 5 Comparison of the efficacy of nano‐SiO
pyrano[3,2‐c] chromene‐5‐one (4a)
2 3
/H PW12O40 with some catalysts for the synthesis of 2‐amino‐4‐(phenyl)‐3‐cyano‐4H,5H–
Entry
Catalyst and conditions
Time(min)
Yield (%)
Ref
‐‐
1
2
3
4
5
6
7
In this work
Triethylbenzylammonium chloride (TEBA), H
20
480
50
94
95
85
92
78
85
92
2
O, 90 °C
[14a]
[14e]
[14f]
[14g]
[14j]
[14k]
o‐Benzenedisulfonimide (OBS), solvent‐free, 120 °C
Hexamethylenetetramine, EtOH/reflux
15
[Fe
2
O
3
@ HAp Si (CH
)
2 3
‐AMP], H
2
O, reflux
10
Sodium dodecyl sulfate (SDS), H
NaBr, electrolysis, EtOH, 60 °C
2
O, 60 °C
120
60
structure (Figure 8). What's more, to guarantee reproducibility
of the condensation reaction, repeated repetitious typical
experiments were performed under same reaction conditions.
The acquired yields were found to be reproducible within
appeared in Figure 9. After fulfillment of the primary run,
chloroform was added and the catalyst, as the sole insoluble
material, was recovered from the reaction by filtration. The
separated catalyst was totally washed with chloroform; then,
dried and tested in another run. The catalyst was tried for
subsequent five runs. Very good reusability was watched
for the catalyst.
±
3% variation.
Reproducibility and reusability of nano‐SiO /H PW O
2
3
12 40
were examined in the synthesis of 2‐amino‐4H‐chromenes, as
2
FIGURE 8 FT‐IR spectra of new (a) and regenerated (b) SiO /
H
3
PW12
O
40
2 3 40
FIGURE 9 Yield% as a function of reusability of SiO /H PW12O

TAYEBEE ET AL.
9 of 10
SCHEME 2 Proposed route for the
synthesis 2‐amino‐4H‐chromenes
[
14h]
3
.2.8 | Hot filtration test
was separated (mp. 81–82°C).
Then, the Michael‐type
addition of the 4‐hydroxycoumarin with the Knoevenagel
intermediate gave rise to the in situ formation of
benzylidinemalononitrile. Thereafter, intramolecular nucleo-
philic cyclization (Thorpe–Ziegler type reaction) and
A hot filtration test was arranged to be sure that the
catalytic activity of the nanohybrid catalyst was originated
from the conjugated composite SiO /HPA and not from
the drained segments, especially H PW O , in the
reaction mixture. Thus, to a mixture of malononitrile
2
3
12 40
tautomerization of this intermediate afforded the final
[14d, g–h]
product (Scheme 2).
It is plausible to consider both
(
(
1 mmol), aldehyde (1 mmol), and 4‐hydroxycoumarin
1 mmol) was added 0.02 g of SiO /H PW O and the
Lewis/ Bronsted acid functions for the nanocatalyst, which
activates both carbonyl group of aldehyde and increases
nucleophilic attributes of malononitrile on the other.
2
3
12 40
reaction was started as depicted in the experimental part
for 10 min. At this stage, the product yield was 63%. At
that point, hot chloroform was added and the catalyst was
separated off and with the filtrate, the reaction proceeded
for another 10 min at 80°C. Yield% was enhanced to
4
| CONCLUSION
6
7%; consequently, no significant increase was observed.
The amorphous nano‐silica powder was prepared with the
small grain size, expansive surface area, and high degree
of purity from Equisetum arvense. Then, SiO /H PW O
This obviously affirmed the satisfactory steadiness of
nano‐SiO /HPA in the condensation reaction and no
significant destruction of the catalyst over the span of the
reaction. This result was furthermore asserted by reproduc-
ibility and reusability investigations of the catalyst as
specified previously.
2
2
3
12 40
was incorporated by the impregnation strategy and the
prepared nanocatalyst was characterized and used as a
powerful and reusable strong solid acid catalyst in the
synthesis of 2‐amino‐4H‐chromenes under solvent‐free
conditions. According to SEM, the mean grain size of
about 50 nm was estimated. FT‐IR of SiO /H PW O
2
3
12 40
3
.2.9 | Suggesting a plausible reaction
exhibited that the primary Keggin structure of the loaded
heteropolyacid was preserved after supporting onto SiO2.
The entire synthetic sequence possesses convenient
generality, is cost‐effective and environmentally friendly,
pathway
The reaction is anticipation to proceed via the Knoevenagel
condensation of an aromatic aldehyde with malononitrile.
This step can be catalyzed by SiO /H PW O via the
which made the method honorable as
a
reliable
2
3
12 40
elimination of an acidic hydrogen from the active nitrile
and formation of an intermediate nitrile. In a model reac-
tion, the knoevenagel condensation of malononitrile and
benzaldehyde was performed and the produced intermediate
contribution to the current procedures in the field of
2‐amino‐4H‐chromenes synthesis. This safe and clean
method would be an acceptable applicant for the automated
applications.

1
0 of 10
TAYEBEE ET AL.
ACKNOWLEDGEMENTS
Arman, F. Rigi, J. Mol. Liq. 2011, 158, 145. (i) K. Tabatabaeian,
H. Heidari, M. Mamaghani, N. O. Mahmoodi, Appl. Organometal.
Chem. 2012, 26, 56. (j) H. Mehrabi, H. Abusaidi, J. Iran. Chem.
Soc. 2010, 7, 890. (k) Z. Vafajoo, H. Veisi, M. T. Maghsoodlou,
H. Ahmadian, C. R. Chim. 2014, 17, 301.
Partial financial support from the Research Councils of
Sabzevar University of Medical Sciences and Hakim
Sabzevari University is greatly appreciated.
[
15] (a) B. Maleki, S. Barat Nam Chalaki, S. Sedigh Ashrafi, E. Rezaei
Seresht, F. Moeinpour, A. Khojastehnezhad, R. Tayebee, Appl.
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Tayebee, Appl. Organometal. Chem. 2015, 29, 408. (c) R. Tayebee,
M. Jarrahi, B. Maleki, M. K. Razi, Z. B. Mokhtari, S. M.
Baghbanian, RSC Adv. 2015, 5, 10869. (d) B. Maleki, M.
Baghayeri, S. A. J. Abadi, R. Tayebee, A. Khojastehnezhad, RSC
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How to cite this article: Tayebee R, Pejhan A,
Ramshini H, et al. Equisetum arvense As an abundant
source of silica nanoparticles. SiO /H PW O
nanohybrid material as an efficient and environmental
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0.1002/aoc.3924