Chloroaluminate ionic liquid-modified silica-coated magnetic nanoparticles: Efficient and reusable catalyst for selective synthesis of mono- and bis-dihydropyrano[3,2-b]chromenediones

Article abstract of DOI:10.1002/aoc.3799

The synthesis, characterization and catalytic activity of chloroaluminate ionic liquid-modified silica-coated magnetic nanoparticles ([SiPrPy]AlCl₄@MNPs) are described. The prepared magnetic nanocatalyst was characterized using Fourier transfor

Full text of DOI:10.1002/aoc.3799

Received: 27 October 2016
DOI: 10.1002/aoc.3799
Revised: 22 January 2017
Accepted: 14 February 2017
F U L L PA P E R
Chloroaluminate ionic liquid‐modified silica‐coated magnetic
nanoparticles: Efficient and reusable catalyst for selective
synthesis of mono‐ and bis‐dihydropyrano[3,2‐b]chromenediones
Ehsan Estakhri | Mahboobeh Nasr‐Esfahani | Iraj Mohammadpoor‐Baltork
|
Shahram Tangestaninejad
| Majid Moghadam | Valiallah Mirkhani
Department of Chemistry, Catalysis
Division, University of Isfahan, Isfahan
81746‐73441, Iran
The synthesis, characterization and catalytic activity of chloroaluminate ionic
liquid‐modified silica‐coated magnetic nanoparticles ([SiPrPy]AlCl4@MNPs) are
described. The prepared magnetic nanocatalyst was characterized using Fourier
transform infrared spectroscopy, elemental analysis, vibrating sample magnetome-
try, scanning and transmission electron microscopies, X‐ray diffraction and
inductively coupled plasma analysis. The results showed that the ionic liquid had
been successfully immobilized onto the magnetic support, and the resulting
nanoparticles exhibited high catalytic activity for the synthesis of a diverse range
of dihydropyrano[3,2‐b]chromenediones via a one‐pot, three‐component and
solvent‐free reaction of aromatic aldehydes, 1,3‐diones and kojic acid. This catalytic
system also showed excellent activity in the selective synthesis of mono‐ and bis‐
dihydropyrano[3,2‐b]chromenediones from dialdehydes. The procedure gave the
products in excellent yields and in very short reaction times. Moreover, the catalyst
could be reused eight times without loss of its catalytic activity.
Correspondence
Iraj Mohammadpoor‐Baltork and Shahram
Tangestaninejad, Department of Chemistry,
Catalysis Division, University of Isfahan,
Isfahan 81746‐73441, Iran.
Email: imbaltork@sci.ui.ac.ir; stanges@sci.
ui.ac.ir
Funding Information
Research Council of the University of
Isfahan
KEYWORDS
chromenediones, magnetic nanoparticles, multicomponent reaction, supported ionic liquid catalysis
1 | INTRODUCTION
1,3‐dialkylimidazolium or 1‐alkylpyridinium cations. In
spite of extensive applications of chloroaluminate ILs, they
The development of cost‐effective and environmentally
benign catalytic systems is one of the main themes of
contemporary organic synthesis.[1] Ionic liquids (ILs) have
attracted considerable interest as environmentally benign
reaction media due to their unique and intriguing properties
such as high thermal and chemical stability, negligible
vapour pressure, controlled miscibility, nonflammability,
nonvolatility and antiwear performance.[2–5] In particular,
highly Lewis acidic chloroaluminate ILs are well known to
have potential applications in catalysis,[6–8] electrochemis-
try,[9] extractions,[10] absorptions[11] and so on.[12]
Chloroaluminate ILs are easily prepared by the addition of
AlCl3 to the chloride salt of an organic ingredient, e.g.
suffer from disadvantages such as difficulties in product
isolation and in catalyst recovery and also using large
amounts of catalyst, which limit their usefulness on a large
scale and make them economically and environmentally
unfavourable. Furthermore, they are particularly hygroscopic
and unstable towards hydrolysis.[13] Recently, a concept
involving supported ionic liquid catalysis (SILC) has been
proposed,[14] which combines the benefits of ILs and hetero-
geneous catalysts such as their easy recovery and reusability,
noncorrosive and non‐hazardous nature, low cost and
operational simplicity. In the SILC systems, the IL is
immobilized on the surface of a high‐area and porous
support material. Among them, nano‐sized materials have
Appl Organometal Chem. 2017;e3799.
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https://doi.org/10.1002/aoc.3799

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ESTAKHRI ET AL.
recently attracted much attention due to the size and high
surface area of materials causing reactions to occur
efficiently. Magnetic nanoparticles (MNPs),[15] with
properties of respectable stability, easy production and
functionalization, facile separation by magnetic forces, and
low toxicity and cost, have led to important developments
in catalysis.[16–18]
In view of the pharmacological importance of chromenes
(a class of fused oxygenated heterocycles), their synthesis has
been of interest in medicinal chemistry studies. A number of
chromenes have been shown to possess numerous biological
properties such as antioxidant,[19] antiviral,[20] antitumour,[21]
2 | EXPERIMENTAL
2.1 | General
The chemicals used in this work were purchased from
Fluka and Merck. Melting points were determined with a
Stuart Scientific SMP2 apparatus. Fourier transform infra-
red (FT‐IR) spectra were recorded with a Nicolet Impact
400D instrument in the range 400–4000 cm−1. H NMR
1
and 13C NMR spectra (400 and 100 MHz) were recorded
with a Bruker Avance 400 MHz spectrometer using CDCl3
as solvent. Elemental analysis was conducted with a LECO
CHNS‐932 analyser. The Al content of the catalyst was
measured using inductively coupled plasma optical
emission spectrometry using a Jarrell‐Ash 1100 ICP
analyser. Field emission scanning electron microscopy
(FE‐SEM) measurements were performed with a Hitachi
S‐4700. Transmission electron microscopy (TEM) measure-
ments were carried out using a Philips CM10 analyser
operating at 100 kV. Magnetic measurements were carried
out in with a vibrating sample magnetometer (Meghnatis
Daghigh Kavir Co.) at room temperature. X‐ray diffraction
patterns were obtained with a Bruker XRD D8 Advance
instrument with Co Kα radiation at 40 kV.
antimicrobial,[22]
antifungal,[23]
anti‐inflammatory,[24]
estrogenic,[25] anticancer,[26] antitubercular[27] and analgesic
activities.[28] Chromene derivatives also perform a significant
role in the production of extremely effective fluorescent dyes
for synthetic fibres, fluorescent pigments, electrophotogra-
phy and electroluminescent devices.[29] In spite of the
applications of chromene derivatives, little attention has been
paid
to
the
synthesis
of
dihydropyrano[3,2‐b]
chromenediones and only few methods are accessible dealing
with the synthesis of the essential heterocycles.[30–33]
Recently, we reported a highly efficient synthesis of
dihydropyrano[3,2‐b]chromenediones via three‐component
reactions of aromatic aldehydes, 1,3‐diones and kojic acid
in the presence of Bi(OTf)3.[34] Although the reported
methods are useful, the development of additional efficient
methods with easily recoverable catalysts is still in demand.
As part of our ongoing research on the preparation of
useful heterogeneous catalysts and also on the synthesis
of fine chemicals,[35,36] herein, we report novel
chloroaluminate IL‐modified silica‐coated magnetic nano-
particles ([SiPrPy]AlCl4@MNPs) as a highly efficient and
easy recoverable catalyst for the one‐pot multicomponent
selective synthesis of substituted as well as mono‐ and
bis‐dihydropyrano[3,2‐b]chromenediones (Scheme 1).
2.2 | Synthesis of [SiPrPy]AlCl4@MNPs
(Scheme 2)
2.2.1 | Preparation of MNPs1
MNPs1 was prepared according to a reported procedure.[37]
In this manner, FeCl3⋅6H2O (11.0 g) and FeCl2⋅4H2O
(4.0 g) were dissolved in 250 ml of deionized water under
nitrogen with vigorous stirring at 25°C, during which the
pH of the solution was adjusted to 9 with conc. NH3⋅H2O.
The mixture was further stirred for 4 h and the resulting
MNP precipitates were washed with deionized water and
ethanol until the pH reached 7. The black precipitate
(MNPs1) was collected with a permanent magnet.
2.2.2 | Preparation of MNPs2
The coating of a layer of silica on the surface of naked Fe3O4
(MNPs1) was achieved using the sol–gel method.[38] MNPs1
(3.0 g) was dispersed in ethanol (200 ml) by sonication.
Amounts of 9 ml of concentrated NH3⋅H2O and 8 ml of
tetraethoxysilane (TEOS) were added to the well‐dispersed
MNP solution and stirred for 24 h at 40 °C. The resulting
MNPs2 was collected using an external magnet and washed
with water until the solution was neutral. Then, it was
washed with ethanol and diethyl ether successively and dried
under vacuum.
SCHEME
catalysed by [SiPrPy]AlCl4@MNPs
1
Synthesis of dihydropyrano[3,2‐b]chromenediones

ESTAKHRI ET AL.
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added and the catalyst was easily separated using a perma-
nent magnet and washed with chloroform (10 ml × 2). The
solvent was evaporated and the resulting crude product was
purified by chromatography on silica gel to afford the
desired mono‐ and bis‐dihydropyrano[3,2‐b]chromenediones
(Scheme 3).
2.2.3 | Preparation of MNPs3
MNPs2 (2.0 g) was dispersed in 50 ml of dry toluene by
sonication for
1 h, and then 10.0 mmol of 3‐
chloropropyltriethoxysilane (CPTES) was added to the solu-
tion. After mechanical stirring under nitrogen atmosphere at
reflux conditions for 24 h, the solid was magnetically
separated, washed with diethyl ether and dried under vacuum
to afford MNPs3.
3 | RESULTS AND DISCUSSION
2.2.4 | Preparation of [SiPrPy]cl@MNPs
3.1 | Preparation and characterization of
[SiPrPy]AlCl4@MNPs
MNPs3 (2.0 g) was dispersed in 50 ml of dry toluene by
sonication for 1 h, and then pyridine (5.0 mmol, 0.4 g) was
added to the solution. After mechanical stirring under
nitrogen atmosphere at reflux conditions for 24 h, the solid
was magnetically separated, washed with diethyl ether and
dried under vacuum to afford [SiPrPy]Cl@MNPs.
The catalyst was prepared by covalent attachment of the IL
onto the surface of the MNPs in multiple steps from several
commercially available chemicals (Scheme 2). First, Fe3O4
(MNPs1) was synthesized by a chemical co‐precipitating
method reported in the literature.[37] Fe3O4 was then coated
with TEOS in order to prevent the aggregation and also the
oxidation of the MNPs as well as to provide suitable surface
Si─OH groups for further functionalization.[38] Usage of
MNPs2 with CPTES in dry toluene afforded silica‐coated
MNPs functionalized with chloropropyl groups (MNPs3).
After that, pyridine was refluxed with MNPs3 in dry toluene
under nitrogen atmosphere to give MNPs modified with
pyridinium chloride IL ([SiPrPy]Cl@MNPs). Finally, a
solution of anhydrous AlCl3 in dry toluene was stirred with
[SiPrPy][Cl]@MNPs for 3 h, then separated, washed and
dried to afford [SiPrPy]AlCl4@MNPs as a magnetically
recoverable nanocatalyst.
Successful functionalization of MNPs can be verified
from the FT‐IR spectra shown in Figure 1. The strong band
at 579 cm−1 is assigned to the characteristic Fe─O vibrations
and confirms the existence of Fe3O4 component in all sam-
ples. A broad band at around 3400 cm−1 and weak band at
1620 cm−1 are attributed to the Fe─OH group (MNPs1;
Figure 1a). For silica‐coated magnetite (MNPs2; Figure 1b),
the band at 1100 cm−1 represents Si─O bonds, while bands
at 3427 and 1631 cm−1 correspond to the stretching and
bending vibrations of Si─OH. The weak band at 798 cm−1
is characteristic of Si─O─Fe, that suggests that SiO2 is chem-
ically bonded to Fe3O4.[39] The FT‐IR spectra of [SiPrPy]
Cl@MNPs (Figure 1c) and [SiPrPy]AlCl4@MNPs
(Figure 1d) show the characteristic bands at 2940 and
2.2.5 | Preparation of [SiPrPy]AlCl4@MNPs
[SiPrPy]Cl@MNPs (1.0 g) in 20 ml of dry toluene was
sonicated for 20 min at room temperature. Then a solution
of anhydrous AlCl3 (5.0 mmol) in dry toluene was added
and the resulting mixture was stirred for 3 h. Finally, the black
particles were collected, washed with hot dichloromethane
and dried to afford [SiPrPy]AlCl4@MNPs.
2.3 | General procedure for synthesis of
Dihydropyrano[3,2‐b]chromenediones
A mixture of aromatic aldehyde (1 mmol), 1,3‐dione
(1 mmol), kojic acid (1 mmol) and [SiPrPy]AlCl4@MNPs
(40 mg, 0.1 mmol) was stirred at 110 °C for the required time.
The progress of the reaction was monitored by TLC (eluent:
n‐hexane–EtOAc, 1:2). After completion of the reaction, the
mixture was cooled to room temperature, chloroform
(10 ml) was added and the catalyst was easily separated using
a permanent magnet and washed with chloroform (10 ml × 2).
The solvent was evaporated and the resulting crude product
was purified by recrystallization from EtOH to afford the
pure product.
2.4 | General procedure for selective synthesis
of mono‐ and bis‐dihydropyrano[3,2‐b]
chromenediones
A mixture of dialdehyde (1 mmol), 1,3‐dione (1 or 2 mmol),
kojic acid (1 or 2 mmol) and [SiPrPy]AlCl4@MNPs (0.1 or
0.2 mmol) was stirred at 110 °C for 0.5–1.0 h. The progress
of the reaction was monitored by TLC (eluent: n‐hexane–
EtOAc, 1:2). After completion of the reaction, the mixture
was cooled to room temperature, chloroform (10 ml) was
SCHEME 2 Preparation of [SiPrPy]AlCl4@MNPs nanocatalyst

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ESTAKHRI ET AL.
(220), (311), (400), (422), (511) and (440) reflections,
respectively, of the crystalline structure of standard naked
Fe3O4 sample (JCPDS 19–0629). A broad peak from
2θ = 18.0° to 28.0° in Figure 2(b) is attributed to the
amorphous silica shell formed surrounding the Fe3O4
core.[40] As can be seen, the phase composition of MNPs
remains intact after silica coating of the nanoparticles.
The magnetic properties of neat MNPs1, MNPs2 and
[SiPrPy]AlCl4@MNPs were investigated at room tempera-
ture (Figure 3). The saturation magnetization (Ms) values of
samples are 69.40, 63.84 and 36.96 emu g−1, respectively.
The decrease of Ms from MNPs to [SiPrPy]Cl@MNPs could
be attributed to the coating of iron oxide MNPs with silica
and IL. Also, the magnetization curves of the MNPs exhibit
no hysteresis before and after functionalization. These results
evidently establish their superparamagnetic characteristics[37]
which are further reinforced by the easy separation of these
MNPs with an external magnetic field.
The FE‐SEM images of MNPs2 (Figure 4a) and [SiPrPy]
AlCl4@MNPs (Figure 4b) illustrate that the surface morphol-
ogy of the two samples are different, which is a good indica-
tion of the IL being supported on the nanoparticles. The
energy‐dispersive X‐ray (EDX) analysis of [SiPrPy]
AlCl4@MNPs indicates the presence of all the expected
elements (Figure 4c). The presence of Fe, Si, C, N and Al
in the EDX spectrum demonstrates that silica‐coated MNPs
are grafted with IL and also the exchange of anion (Cl to
AlCl4) had been achieved. The size and morphology of the
resultant [SiPrPy]AlCl4@MNPs catalyst were also investi-
gated using TEM (Figure 4d). The TEM image of catalyst
displays dark nano‐Fe3O4 cores restricted by grey silica
shells. The histogram of size distribution shows that the aver-
age diameter of nanoparticles is about 12–14 nm (Figure 4e).
The nitrogen content of the catalyst, measured by CHNS
analysis, presents a value of about 2.35%. Based on this
FIGURE 1 FT‐IR spectra: (a) MNPs1; (b) MNPs2; (c) [SiPrPy]
cl@MNPs; (d) [SiPrPy]AlCl4@MNPs
1456 cm−1 which are attributed to the C─H stretching vibra-
tion of CPTES linker anchored on the surface of MNPs. Also
observed are the characteristic bands at 1486 cm−1 of C═N
vibration and at 3088 cm−1 of C─H stretching, which indi-
cate the pyridine ring is attached to MNPs3.
The X‐ray diffraction patterns of MNPs1 and MNPs2
have the same typical peaks with respect to their positions
and comparative intensities (Figure 2). The peaks at 30.06°,
35.42°, 43.05°, 53.39°, 56.94° and 62.52° correspond to the
FIGURE 3 Vibrating sample magnetometry results: (a) MNPs1; (b)
FIGURE 2 X‐ray diffraction patterns: (a) MNPs1; (b) MNPs2
MNPs2; (c) [SiPrPy]AlCl4@MNPs

ESTAKHRI ET AL.
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FIGURE 4 FE‐SEM images: (a) MNPs2; (b) [SiPrPy]AlCl4@MNPs. (c) SEM–EDX spectrum of [SiPrPy]AlCl4@MNPs. (d) TEM image of
[SiPrPy]AlCl4@MNPs. (e) particle size distribution for [SiPrPy]AlCl4@MNPs catalyst
value, the amount of IL that is supported on the silica‐coated
MNPs is about 1.68 mmol g−1. The Al content of [SiPrPy]
AlCl4@MNPs is 2.55 mmol per gram of catalyst as obtained
using inductively coupled plasma analysis.
chosen as a model reaction for the optimization of parame-
ters such as the amount of catalyst and temperature
(Table 1). Initially, the reaction was investigated in the
absence of the catalyst at 110°C; no desired product was
obtained indicating that the catalyst is vital to achieve this
reaction. Then, the model reaction was examined in the
presence of various catalysts: AlCl3, BiCl3, FeCl3, ZnCl2,
3.2 | Synthesis of Dihydropyrano[3,2‐b]
MnCl2,
[PrPy]Cl,
[PrPy]AlCl4,
[HSO3PMIM]OTf‐
chromenediones
SiO2@MNPs,[35] MNPs2, [SiPrPy]Cl@MNPs and [SiPrPy]
AlCl4@MNPs. The best result was obtained with [SiPrPy]
AlCl4@MNPs (Table 1, entries 2–13). The activity of
The reaction of 3‐methoxybenzaldehyde (1f; 1mmol),
dimedone (2a; 1mmol) and kojic acid (3; 1mmol) was

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ESTAKHRI ET AL.
TABLE 1 Optimization of conditions for synthesis of dihydropyrano[3,2‐b]chromenedionesa
Entry
Catalyst (mol%)
Temp. (°C)
Yield (%)b
1
—
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
r.t.
Trace
65
2
AlCl3 (10)
3
BiCl3 (10)
45
4
FeCl3 (10)
30
5
ZnCl2 (10)
40
6
MnCl2 (10)
40
7
[PrPy]Cl (10)
25
8
[PrPy]AlCl4 (10)
48
9
[HSO3PMIM]OTf‐SiO2@MNPs (40 mg)
MNPs2 (40 mg)
69
10
11
12
13
14
15
16
17
18
19
20
21
22
Trace
45
[SiPrPy]Cl@MNPs (40 mg)
[SiPrPy]Cl@MNPs (80 mg)
[SiPrPy]AlCl4@MNPs (10)
[SiPrPy]InCl4@MNPs (10)
[SiPrPy]HSO4@MNPs (10)
[SiPrPy]AlCl4@MNPs (5)
[SiPrPy]AlCl4@MNPs (7)
[SiPrPy]AlCl4@MNPs (15)
[SiPrPy]AlCl4@MNPs (10)
[SiPrPy]AlCl4@MNPs (10)
[SiPrPy]AlCl4@MNPs (10)
[SiPrPy]AlCl4@MNPs (10)
57
95
86
58
40
70
95
Trace
30
70
100
120
85
95
aReaction conditions: 3‐methoxybenzaldehyde (1f, 1 mmol), dimedone (2a, 1 mmol) and kojic acid (3, 1 mmol) under solvent‐free conditions.
bIsolated yield.
[SiPrPy]AlCl4@MNPs was also compared with those of
similar heterogeneous catalysts such as [SiPrPy]
InCl4@MNPs and [SiPrPy]HSO4@MNPs, and the results
show that [SiPrPy]AlCl4@MNPs is a more effective catalyst
for this purpose (Table 1, entries 13–15). In order to
determine the optimum amount of [SiPrPy]AlCl4@MNPs
catalyst, the reaction was examined with various amounts
of the catalyst (5, 7, 10 and 15 mol%) under solvent‐free
conditions at 110 °C. The best yield of the desired product
was achieved with 10 mol% catalyst; using a smaller amount
of catalyst resulted in lower yield, while a greater amount
did not affect the reaction time and yield.
incomplete at lower temperature. Thus, 10 mol% of the
[SiPrPy]AlCl4@MNPs catalyst at 110°C under solvent‐free
conditions were chosen as the optimized reaction conditions
(Table 1, entry 13).
The general applicability of this catalyst was evaluated
using a variety of aldehydes under the optimized reaction
conditions (Table 2). The reaction of dimedone and kojic acid
with benzaldehyde and substituted benzaldehydes bearing
electron‐donating groups (methoxy and methyl) and
electron‐withdrawing groups (halogens and nitro) produced
the corresponding dihydropyrano[3,2‐b]chromenediones
(4aa–4ka) in high yield. Thiophene‐2‐carbaldehyde as an
acid‐sensitive aldehyde also displayed good reactivity and
gave the desired product (4la) in 86% yield. Polycyclic
aldehyde 1‐naphthaldehyde was also transformed to its
To investigate the influence of temperature, the model
reaction was performed in the range 70–120 °C. It was found
that 110°C was the optimal temperature and the reaction was

ESTAKHRI ET AL.
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TABLE 2 Synthesis of dihydropyrano[3,2‐b]chromenediones via one‐pot reaction of aromatic aldehydes, dimedone and kojic acid catalysed by
[SiPrPy]AlCl4@MNPsa
Entry
R
Product
Time (min)
Yield (%)b
1
C6H5
4aa
4ba
4ca
4da
4ea
4fa
20
30
20
30
25
20
30
30
30
40
35
35
35
95
90
95
91
94
95
92
90
92
87
85
86
90
2
3‐NO2C6H5
4‐ClC6H5
3
4
4‐OMeC6H5
3‐BrC6H5
5
6
3‐OMeC6H5
4‐NO2C6H5
4‐MeC6H5
7
4ga
4ha
4ia
8
9
2‐ClC6H5
10
11
12
13
2,4‐(Cl)2C6H3
3,4‐(OMe)2C6H3
Thiophen‐2‐yl
1‐Naphthyl
4ja
4ka
4la
4ma
aReaction conditions: aldehyde (1a–m, 1 mmol), dimedone (2a, 1 mmol), kojic acid (3, 1 mmol), [SiPrPy]AlCl4@MNPs (10 mol%), 110°C under solvent‐free conditions.
bIsolated yield.
corresponding dihydropyrano[3,2‐b]chromenedione (4ma)
in 90% yield.
It is important to mention that selective synthesis of
mono‐ and bis‐dihydropyrano[3,2‐b]chromenediones from
dialdehydes can be achieved using this method (Scheme 3).
Using a 1:1:1 molar ratio of dialdehyde (isophthaldialdehyde
or terephthaldialdehyde), dimedone and kojic acid, only one
formyl group responded selectively and the corresponding
mono‐dihydropyrano[3,2‐b]chromenediones (4na, 4pa) were
obtained in good yields. Moreover, using cyclohexane‐1,3‐
In order to demonstrate the efficiency of this catalytic
system further, the present methodology was used with
cyclohexane‐1,3‐dione (2b) in place of dimedone. Under
these conditions, the corresponding dihydropyrano[3,2‐b]
chromenediones were obtained in high yields and short
reaction times (Table 3).
TABLE 3 Synthesis of dihydropyrano[3,2‐b]chromenediones via one‐pot reaction of aromatic aldehydes, cyclohexane‐1,3‐dione and kojic acid
catalysed by [SiPrPy]AlCl4@MNPsa
Entry
R
Product
Time (min)
Yield (%)b
1
2
3
4
C6H5
4ab
4bb
4cb
4db
20
25
30
30
95
95
92
85
3‐NO2C6H5
4‐ClC6H5
4‐OMeC6H5
aReaction conditions: aldehyde (1a–d, 1 mmol), cyclohexane‐1,3‐dione (2b, 1 mmol), kojic acid (3, 1 mmol) and [SiPrPy]AlCl4@MNPs (10 mol%), 110°C under solvent
free conditions.
bIsolated yield.

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ESTAKHRI ET AL.
FIGURE 5 Reusability of [SiPrPy]AlCl4@MNPs catalyst in the
synthesis of 4fa
Based on the experimental results, a plausible mechanism
is proposed (Scheme 4). At first, [SiPrPy]AlCl4@MNPs
activates aldehyde 1 and also catalyses the keto–enol
tautomerization of 1,3‐dione 2 to give A and B, respectively.
Then, the reaction of A and B affords the heterodyne C. The
Diels–Alder reaction of kojic acid 3 with C gives intermedi-
ate D, which on further dehydration in the presence of the
catalyst provides the desired product 4 and releases the
catalyst for the next catalytic cycle.
SCHEME 3 Selective synthesis of mono‐ and bis‐dihydropyrano
[3,2‐b]chromenediones catalysed by [SiPrPy]AlCl4@MNPs
dione in place of dimedone produced favoured products
(4nb, 4pb) in good yields. Such selectivity can be a useful
applied achievement in the synthesis of chromenediones.
Subsequently the remaining formyl group can be transformed
to some other important functional groups. When applying a
1:2:2 molar ratio of dialdehyde (isophthaldialdehyde or
terephthaldialdehyde), dimedone (or cyclohexane‐1,3‐dione)
and kojic acid in the presence of [SiPrPy]AlCl4@MNPs
catalyst, both formyl groups reacted and the corresponding
bis‐dihydropyrano[3,2‐b]chromenediones (5na, 5pa, 5nb
and 5pb) were obtained in good to high yields. To the best
of our knowledge, this is the first report of the selective
synthesis of mono‐ and bis‐dihydropyrano[3,2‐b]
chromenediones, which shows the efficiency and applicabil-
ity of the catalytic system under investigation.
3.3 | Recyclability of catalyst
One of the main advantages of supported catalysts is their
recyclability. The possibility of recycling and reusing the
[SiPrPy]AlCl4@MNPs catalyst was examined in the three‐
component reaction between 3‐methoxybenzaldehyde,
dimedone and kojic acid (Figure 5). When the reaction was
completed, the mixture was diluted with chloroform. The cat-
alyst was simply separated using an external magnet, washed
with chloroform, dried and reused in the next run. The
catalyst was reused for eight runs and all reactions led to
the desired products without notable changes in the reaction
time and yield (95 to 92%). As can be seen, the loss of the
yield is really negligible, and may be due to slight deactiva-
tion of catalyst surface during the reaction and also the
work‐up process.
4 | CONCLUSIONS
[SiPrPy]AlCl4@MNPs as an inexpensive, stable and green
catalyst was designed, prepared and characterized using a
variety of techniques. This catalyst was successfully applied
for the synthesis of dihydropyrano[3,2‐b]chromenediones
via a one‐pot three‐component reaction of a wide variety
of aldehydes (substituted benzaldehydes, polycyclic and
heteroaromatic), kojic acid and 1,3‐diones under solvent‐
free conditions. Efficient and selective synthesis of mono‐
SCHEME 4 Plausible mechanism for the synthesis of dihydropyrano
[3,2‐b]chromenediones catalysed by [SiPrPy]AlCl4@MNPs

ESTAKHRI ET AL.
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2006, 54, 391.
and bis‐chromenediones was also performed using this
catalytic system. The catalyst is stable under the reaction
conditions and can be recycled and reused several times
without a significant loss in its activity. Furthermore,
simplicity of preparation and handling of the catalyst, easy
experimental procedure, no side reactions, high atom‐econ-
omy, use of low‐cost and reusable catalyst with lower
loading and solvent‐free conditions are other advantages
of this method in the synthesis of these important heterocy-
clic compounds.
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The authors are grateful to the Research Council of the
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How to cite this article: Estakhri E, Nasr‐Esfahani
M, Mohammadpoor‐Baltork I, Tangestaninejad S,
Moghadam M, Mirkhani V. Chloroaluminate ionic
liquid‐modified silica‐coated magnetic nanoparticles:
Efficient and reusable catalyst for selective
synthesis of mono‐ and bis‐dihydropyrano[3,2‐b]
chromenediones. Appl Organometal Chem. 2017;
e3799. https://doi.org/10.1002/aoc.3799
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