DABCO-modified super-paramagnetic nanoparticles as an efficient and water-compatible catalyst for the synthesis of pyrano[3,2-c:5,6-c']dichromene-6,8-dione derivatives under mild reaction conditions

Article abstract of DOI:10.1002/aoc.4561

In this paper, a novel catalyst is introduced based on the immobilization of 1,4-diazabicyclo[2.2.2] octane (DABCO) on super-paramagnetic iron oxide nanoparticles. For the synthesis of the catalyst, silica-magnetic iron oxide core-shell nanoparticles were

Full text of DOI:10.1002/aoc.4561

Received: 7 April 2018
DOI: 10.1002/aoc.4561
Revised: 24 July 2018
Accepted: 25 July 2018
F U L L P A P E R
DABCO‐modified super‐paramagnetic nanoparticles as an
efficient and water‐compatible catalyst for the synthesis of
pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione derivatives under
mild reaction conditions
Zahra Heydari1 | Saeed Bahadorikhalili2 | Parviz Rashidi Ranjbar1 | Mohammad Mahdavi3
1 School of Chemistry, College of Science,
In this paper, a novel catalyst is introduced based on the immobilization of 1,4‐
diazabicyclo[2.2.2] octane (DABCO) on super‐paramagnetic iron oxide nano-
particles. For the synthesis of the catalyst, silica‐magnetic iron oxide core‐shell
nanoparticles were first functionalized by (3‐chloropropyl)trimethoxysilane.
The functionalized silica‐magnetic iron oxide core‐shell nanoparticles then
reacted with DABCO to form the final catalyst. The catalyst was characterized
by scanning electron microscopy, vibrating‐sample magnetometer, thermogra-
vimetric analysis, X‐ray photoelectron spectroscopy and Fourier‐transform
infrared spectroscopy. This catalyst was used for the synthesis of pyrano[3,2‐
c:5,6‐c']dichromene‐6,8‐dione derivatives. Several starting materials were used,
and all of them led to the desired products in high isolated yields (19 exam-
ples). The catalyst also showed very good reusability in the reaction.
University of Tehran, P. O. Box 14155‐
6455 Tehran, Iran
2 Iranian Space Research Center, Institute
of Mechanics, Shiraz, Iran
3 Endocrinology and Metabolism Research
Center, Endocrinology and Metabolism
Clinical Sciences Institute, Tehran
University of Medical Science, Tehran
14176, Iran
Correspondence
Parviz Rashidi Ranjbar, School of
Chemistry, College of Science, University
of Tehran, P. O. Box 14155‐6455, Tehran,
Iran.
Email: prrashidi@khayam.ut.ac.ir
Mohammad Mahdavi, Endocrinology and
Metabolism Research Center,
KEYWORDS
DABCO, magnetic nanoparticles, nanocatalyst, pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione
Endocrinology and Metabolism Clinical
Sciences Institute, Tehran University of
Medical Science, Tehran, 14176, Iran.
Email: momahdavi@sina.tums.ac.ir
1 | INTRODUCTION
magnetic nanoparticles. Apart from all the desired proper-
ties of the MNPs, magnetic iron oxide nanoparticles are
Catalysts are an important part of many reactions, due to
their unique properties. These agents cause better reaction
performance in lower temperature and milder reaction
conditions to give products in higher isolated yields.[1]
For better productivity of the catalysts, it is critical to
recover them. To give the reusable properties to a catalyst,
a successful approach is to immobilize them on a solid
support.[2–4] Among different supports, magnetic nano-
particles (MNPs) are of high interest due to their thermal
and chemical stability, good functionalization possibility,
facile preparation and simple separation from the reaction
medium by an external magnet. Super‐paramagnetic iron
oxide nanoparticles (SPIONs) are of the most favorable
not expensive and can be synthesized from cheap reagents
by experimentally simple co‐precipitation in basic condi-
tions.[5] The low‐cost, simple and efficient synthesis of
SPIONs has made them an appropriate choice for being
used in a wide range of applications, especially as support
for catalysts in organic transformations.[6–11] Regarding
the importance of water as a green, low‐cost, available
and environmentally friendly solvent, various efforts have
been focused on the functionalization of the SPIONs, such
that make them more water‐compatible in the reac-
tions.[12–16] In this regard, modified SPIONs have been
used as support for the immobilization of both metallic
and non‐metallic catalysts.[17–20]
Appl Organometal Chem. 2018;e4561.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4561

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HEYDARI ET AL.
Pyranochromene is a very important skeleton, which
develop a novel catalyst based on the functionalization
of DABCO on magnetic nanoparticles. The catalyst is
used for efficient and simple synthesis of pyrano[3,2‐
c:5,6‐c']dichromene‐6,8‐dione derivatives. For the synthe-
sis of DABCO‐SPION catalyst, magnetic iron oxide nano-
particles encapsulated by silica were used as support for
the immobilization of DABCO as the active catalytic
agent. Encapsulation of SPIONs with a silica shell is to
make them more stable, and to supply more reactive
hydroxyl groups on the surface of the NPs. The active
DABCO was covalently attached to the SPIONs using
(3‐chloropropyl)trimethoxysilane. Finally, the counter
ion was replaced by BF4−. The synthetic steps for the syn-
thesis of DABCO‐SPION catalyst are presented in
Scheme 2.
can be found in various structures. Different derivatives
of this structure are shown to have several biological
activities.[21–24] Due to the wide applications and various
activities of pyranochromene derivatives, several efforts
have been focused on the synthesis of these compounds.
The most efficient synthetic routes contain the reaction
of 4‐hydroxycoumarine with hydrobenzoin[25] or alde-
hyde
derivatives.[26–28]
The
reaction
of
4‐
hydroxycoumarine with mandelic acid in acidic condi-
tions was also shown to lead to the pyranochromene
derivatives.[29] Regarding the importance of pyrano[3,2‐
c:5,6‐c']dichromene‐6,8‐dione derivatives in various bio-
logical applications, the synthesis of this class of com-
pounds is of high interest.[25,30]
1,4‐Diazabicyclo[2.2.2] octane (DABCO) is a very
common base, which is used as a catalyst in various reac-
tions.[31–35] Continuing our previous efforts in the immo-
bilization of the catalysts on nanoparticles for the
synthesis of reusable catalysts on one hand,[36–43] and
the synthesis of biologically active organic compounds
on the other hand,[44–52] in this report a novel catalyst is
introduced for the synthesis of pyrano[3,2‐c:5,6‐c']
dichromene‐6,8‐dione derivatives. The catalyst is pre-
pared based on the immobilization of DABCO on mag-
netic nanoparticles (DABCO‐SPION). The reaction
scheme is presented in Scheme 1.
The catalyst was characterized by several methods.
Fourier‐transform infrared (FT‐IR) spectroscopy was used
for studying the covalent attachment of DABCO to
SPION@SiO2. The FT‐IR spectrum is presented in
Figure 1. The FT‐IR spectrum SPION@SiO2 is shown in
Figure 1a, and that of DABCO‐SPION can be seen in
Figure 1b. In Figure 1a, two bands at 3430 and 1246
cm−1 are correlated to the stretching and bending vibra-
tions of hydroxyl groups of SPION@SiO2. In addition,
vibrations at 570 cm−1 are correlated to Fe‐O bonds.
Si‐O vibrations can be seen as an intense bond at 1106
cm−1. After the modification of SPION@SiO2 by DABCO,
additional bonds are presented.
Structure, particle size and the morphology of
DABCO‐SPION catalyst is very important in its role as
catalyst in the reaction. The structure and the particle size
of the nanoparticles were studied using scanning electron
microscopy (SEM). SEM images can be seen in Figure 2.
In this figure, the images of DABCO‐SPION catalyst can
be observed in two different magnifications. The mor-
phology of the catalyst can clearly be observed in the
SEM image to be spherical. The particles showed narrow
dispersity with particles size between 20 and 25 nm.
The magnetic behavior of the catalyst is a significant
feature, especially in its recovery. The magnetic catalyst
can be simply isolated from the reaction mixture by an
external magnet, to be used in the next reaction. There-
fore, the magnetic behavior of SPION@SiO2 and
2 | RESULTS AND DISCUSSION
Novel methods for green synthesis of biologically active
chemicals have been of high interest in our group.
Regarding the benefits of nanocatalysts, in this paper we
SCHEME 1 The preparation of pyrano[3,2‐c:5,6‐c']dichromene‐
6,8‐diones in the presence of DABCO‐SPION catalyst
SCHEME 2 The preparation steps of
DABCO‐SPION catalyst

HEYDARI ET AL.
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DABCO‐SPION catalysts were studied using the vibrat-
ing‐sample magnetometer (VSM) method. The VSM
results can be seen in Figure 3. The super‐paramagnetic
behavior of SPION@SiO2 and DABCO‐SPION was clearly
observed in the VSM results.[53] The S‐type magnetization
hysteresis loop of SPION@SiO2 and DABCO‐SPION cata-
lyst proves the super‐paramagnetic characters of these
nanoparticles. In DABCO‐SPION catalyst the magnetiza-
tion magnitude is decreased in comparison with
SPION@SiO2, but it still shows the magnetic behavior
in an external magnetic field.
For studying the thermal stability of the catalyst, ther-
mogravimetric (TG) analysis was used. The weight loss in
TG analysis was utilized to determine the amount of
loaded DABCO on the surface of SPION@SiO2 NPs. The
TG analysis (TGA) curves can be seen in Figure 4. By
studying the TGA curves, it can be seen that no signifi-
cant weight loss has been observed in temperatures lower
than 300°C. By this observation it can be concluded that
DABCO‐SPION catalyst is thermally stable up to 300°C.
The TG curve of DABCO‐SPION shows a sharp decrease
in weight and was observed at 300–700°C. The amount of
DABCO on the surface of the catalyst was measured by
acid–base titration. The titration results showed that the
amount of the attached DABCO in the catalyst is 1.9 g
FIGURE 1 Infrared (IR) spectrum of (a) 1,4‐diazabicyclo[2.2.2]
octane (DABCO), (b) super‐paramagnetic iron oxide nanoparticle
(SPION)@SiO2 and (c) DABCO‐SPION catalyst
mol−1
.
For exact characterization of the catalyst, the X‐ray
photoelectron spectroscopy (XPS) method was applied
(Figure 5). The XPS results proved the formation of
FIGURE 3 Vibrating‐sample magnetometer (VSM) diagram of
(a) super‐paramagnetic iron oxide nanoparticle (SPION)@SiO2
and (b) 1,4‐diazabicyclo[2.2.2] octane (DABCO)‐SPION catalyst
DABCO ionic liquids on the surface of the catalyst. The
existence of Fe (2p), O (1s), Si (2p), Si (2s) and O (2s) sig-
nals in the spectrum confirms the presence of both iron
oxide and silica in the structure of the SPION@SiO2
core‐shell. In addition, N (1s) and C (1s) signals confirm
FIGURE 2 Scanning electron microscopy (SEM) images of 1,4‐
diazabicyclo[2.2.2] octane (DABCO)‐super‐paramagnetic iron
oxide nanoparticle (SPION) catalyst in different magnifications

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HEYDARI ET AL.
low isolated yields. We note that blank runs without
any amount of catalyst did not lead to the desired prod-
uct. This proves that the presence of the catalyst is neces-
sary for the reaction performance. The effects of different
bases on the reaction time and the isolated yield are pre-
sented in Table 1.
Then the effect of the amount of catalyst on the reac-
tion was investigated, and different quantities of catalyst
ranging from 1 to 50 mol% were tested. The best results
were observed when 10 mol% of the catalyst has been
applied, while using more quantities of the catalyst did
not improve the reaction performance. We note that in
the absence of the catalyst, no product was formed, which
proves that the presence of the catalyst is critical for the
reaction performance.
FIGURE
4
Thermogravimetric analysis (TGA) super‐
paramagnetic iron oxide nanoparticle (SPION)@SiO2 of 1,4‐
diazabicyclo[2.2.2] octane (DABCO)‐SPION catalyst
TABLE 1 Effect of different bases on the reaction time and the
isolated yielda
Entry
Base
Time (h)
Yield (%)
1
No base
DIPEA
6
0
62
58
67
75
71
66
71
64
84
71
2
4
3
KOH
3
4
K2CO3
6
5
Pyridine
NEt3
4
6
4
FIGURE 5 X‐ray photoelectron spectroscopy (XPS) spectrum of
1,4‐diazabicyclo[2.2.2] octane (DABCO)‐super‐paramagnetic iron
oxide nanoparticle (SPION) catalyst
7
NaOH
4
8
Pyridine
DABCO
DABCO‐SPION
DABCO‐SPIONb
4
9
2
10
11
1.5
1.5
the presence of DABCO in the structure of the catalyst.
−
The presence of BF4 counter ion can also be confirmed
by observing the F (1 s) and B (1 s) signals. We note that
N (1 s) spectra shows two peaks with their binding ener-
gies of 399.7 and 401.9 eV, which belong to amine and
ammonium groups, respectively.[54]
aReaction conditions: benzaldehyde (1 mmol), 4‐hydroxycumarine (2 mmol)
in EtOH/H2O (10 ml, 50% v/v) in the presence of different bases as catalyst
(10 mol%) at 80°C.
bDABCO‐SPION with Cl‐counter ion.
The aim of this paper was the synthesis of pyrano[3,2‐
c:5,6‐c']dichromene‐6,8‐dione derivatives using DABCO‐
SPION catalyst. Therefore, the activity of the catalyst in
the synthesis of pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐
diones was studied. For this purpose, the optimal condi-
tions were chosen by changing different variables and
studying the effect of each variable on the reaction perfor-
mance. The reaction was performed under different con-
ditions, and the best one was selected as the optimal
conditions. To do this, a series of bases including NaOH,
KOH, K2CO3, pyridine, NEt3 and non‐immobilized
DABCO were used as catalyst, and their effect was
observed on the isolated yields of the products. It can be
seen that the desired products have been obtained when
non‐immobilized DABCO has been applied as catalyst
in very lower yields. Moreover, other bases have given
TABLE 2 Effect of different solvents on the reaction time and the
yielda
Entry
Solvent
Time (h)
Isolated yield (%)
1
2
3
4
5
6
7
H2O
4
41
56
62
83
48
55
41
DMF
2
DMSO
EtOH/H2Ob
EtOH
2
1.5
3
CH2Cl2
CH3CN
4
4
aReaction conditions: benzaldehyde (1 mmol), 4‐hydroxycumarine (2 mmol)
in the presence of catalyst (10 mol%) at 80°C.
b50:50 (v/v) mixture.

HEYDARI ET AL.
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TABLE 3 Synthesis of pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione
derivatives in the presence of DABCO‐SPION catalysta
TABLE 3 (Continued)
Entry
Product
Time (h)
Isolated yield (%)
Entry
Product
Time (h)
Isolated yield (%)
9
1.5
85
1
1.5
83
10
1.5
91
2
3
4
1.5
81
85
82
2
11
12
1.5
1.5
86
84
1.5
13
2
86
5
6
2
81
84
2.5
14
15
2
1
88
83
7
8
1.5
1.5
81
16
2
89
88
(Continues)
(Continues)

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HEYDARI ET AL.
TABLE 3 (Continued)
can be seen in Table 2 that the synthesis of pyrano[3,2‐
c:5,6‐c']dichromene‐6,8‐dione derivatives was performed
in different solvents. But a 50:50 (v/v) mixture of H2O/
EtOH is the best solvent, and the highest isolated yield
is obtained in this green mixture as solvent. The effect
of temperature on the reaction performance is very
important. Performing the reaction under different tem-
perature conditions proved 80°C to be the best tempera-
ture for yielding the desired product.
Using the optimized conditions, the efficiency of the
DABCO‐SPION catalyst in the synthesis of pyrano[3,2‐
c:5,6‐c']dichromene‐6,8‐dione derivatives was studied. As
shown in Table 3, DABCO‐SPION successfully catalyzed
the reaction to give the desired products. The correspond-
ing pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione derivatives
were obtained in good isolated yields for various exam-
ined substrates at 80°C in the presence of DABCO‐SPION
catalyst. As illustrated, a wide range of starting materials
can be efficiently produced with this catalytic system. All
substrates possessing electron‐rich as well as electron‐
poor substituents underwent intramolecular cyclization
catalyzed by DABCO‐SPION catalyst.
Entry
Product
Time (h)
Isolated yield (%)
17
2
85
18
19
1.5
83
84
2
aReaction conditions: benzaldehyde (1 mmol), 4‐hydroxycumarine (2 mmol)
in EtOH/H2O (10 ml, 50% v/v) in the presence of catalyst (10 mol%) at 80°C.
A proposed mechanism is suggested for DABCO‐
SPION‐catalyzed
synthesis
of
pyrano[3,2‐c:5,6‐c']
Then, the effect of different solvents on the perfor-
mance of the reaction was studied. For studying the effect
of different solvents on the reaction performance, various
protic and aprotic solvents with different polarities were
used as solvent, and the isolated yields of the products
were compared. The results are presented in Table 2. It
dichromene‐6,8‐dione derivatives. The suggested mecha-
nism can be seen in Scheme 3. In the suggested mecha-
nism, the catalyst first activated the hydroxyl group of
the coumarin. The activated coumarin reacts with alde-
hyde and forms an intermediate that again reacts with
another activated coumarin. The product of this step
SCHEME 3 Suggested mechanism for
DABCO‐SPION‐catalyzed synthesis of
pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione
derivatives

HEYDARI ET AL.
7 of 9
undergoes an intermolecular reaction, which is activated
by DABCO‐SPION catalyst to form the desired
pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione.
observation proves the stability of the catalyst in the reac-
tion conditions.
A very important advantage of the current catalyst
is its reusability. The separation of the catalyst from
the reaction mixture is simply possible by a magnet,
due to the magnetic nature of the catalyst. The
recycling of DABCO‐SPION catalyst was tested in 10
sequential reactions for the preparation reaction of
pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione. The results
are presented in Figure 6. It is obvious in Figure 6 that
no significant loss in catalyst activity can be observed
after the 10th run.
For better studying of the stability of the catalyst, the
morphology of the recovered DABCO‐SPION catalyst was
analyzed by SEM microscopy. The SEM image of the
recovered catalyst after the 5th run is presented in
Figure 7. It can be seen in Figure 7 that the morphology
and the structure of the recovered catalyst has not signif-
icantly changed after being used in the reaction. This
3 | CONCLUSIONS
In summary, we have introduced DABCO‐catalyzed syn-
thesis of pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione deriva-
tives. To do this, DABCO was successfully immobilized
on the surface of SPION@SiO2 nanoparticles. The cata-
lyst was characterized by various methods. After the char-
acterization of the catalyst, it was used for the synthesis of
pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione derivatives. Dif-
ferent pyrano[3,2‐c:5,6‐c']dichromene‐6,8‐dione deriva-
tives were synthesized in the presence of DABCO‐
SPION catalyst from a wide range of starting materials.
Various starting materials were used and, in all cases,
the desired products were obtained in good isolated
yields. The catalyst also showed very good reusability in
the reaction.
4 | EXPERIMENTAL
4.1 | Preparation of DABCO‐SPION
catalyst
Super‐paramagnetic iron oxide nanoparticles were pre-
pared according to our previously reported paper.[35]
SPIONs were synthesized by co‐precipitation of Fe2O3
and FeO in basic conditions. SPIONs were then coated
by tetraethyl orthosilicate according to a previously
[35]
reported method to form SPION@SiO2.
SPION@SiO2
nanoparticles were functionalized by (3‐chloropropyl)
trimethoxysilane. For this purpose, 10 mmol of (3‐
chloropropyl)trimethoxysilane in 30 ml of EtOH was
added drop‐wise to a vigorous stirring solution of 25 mg
of Fe3O4@SiO2 nanoparticles in 30 ml EtOH/H2O (1:2)
and HCl (pH 4). After vigorous stirring for 24 h, the prod-
uct was isolated by an external magnet and washed using
ethanol (3 × 10 ml). The product was dried at 100°C in
FIGURE
6
Reusability of 1,4‐diazabicyclo[2.2.2] octane
(DABCO)‐super‐paramagnetic iron oxide nanoparticle (SPION)
catalyst after 10 runs
vacuum
for
12 h
to
obtain
(3‐chloropropyl)
trimethoxysilane modified magnetic nanoparticles.
Then, DABCO (0.4 g, 5 mmol) was poured into a mix-
ture of (3‐chloropropyl)trimethoxysilane modified mag-
netic nanoparticles (2 g) in 50 ml toluene, and the
mixture was stirred at 90°C overnight. Then the solid res-
idue was magnetically separated, washed with toluene
and Et2O, and dried in a vacuum oven at 50°C for 24 h.
The product was added to a solution containing NaBF4
(0.55 g, 5 mmol) in dry acetonitrile (15 ml) and stirred at
room temperature for 72 h. The product was magnetically
isolated from the reaction mixture, washed and dried
under vacuum to give the catalyst.
FIGURE 7 Scanning electron microscopy (SEM) image of the
recovered 1,4‐diazabicyclo[2.2.2] octane (DABCO)‐super‐
paramagnetic iron oxide nanoparticle (SPION) catalyst after the 5th
run

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HEYDARI ET AL.
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4.2 | General procedure for the
preparation of pyrano[3,2‐c:5,6‐c']
dichromene‐6,8‐dione
[18] S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander
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Benzaldehyde (1 mmol) and 4‐hydroxycumarine (2
mmol) were added to a mixture of ethanol and water
(10 ml, 50% v/v) in a round flask equipped with con-
denser, heater and magnetic stirrer. Then, DABCO‐
SPION catalyst (10 mol%) was added to the reaction mix-
ture. The reaction mixture was stirred at 80°C for 90 min.
The reaction performance was controlled by thin‐layer
chromatography. After the reaction was completed, the
solvent was eliminated using a rotary evaporator and
the product was recrystallized in ethanol to obtain pure
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13C NMR, FT‐IR and mass spectroscopy.
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SUPPORTING INFORMATION
Additional supporting information may be found online
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