One-pot synthesis of 4H-benzo[b]pyrans and dihydropyrano[c]chromenes using inorganic-organic hybrid magnetic nanocatalyst in water

Article abstract of DOI:10.1016/j.molcata.2012.03.023

The synthesized nanocatalyst in this work provides a green and useful method to obtain 4H-benzo[b]pyrans and dihydropyrano[c]chromenes in aqueous media. The catalyst shows environmental benign character, which can be easily prepared, stored, recovered wit

Full text of DOI:10.1016/j.molcata.2012.03.023

Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
One-pot synthesis of 4H-benzo[b]pyrans and dihydropyrano[c]chromenes using
inorganic–organic hybrid magnetic nanocatalyst in water
a
a
b
b
a
Mehdi Khoobi , Leila Ma’mani , Faezeh Rezazadeh , Zeinab Zareie , Alireza Foroumadi ,
b
c,∗
Ali Ramazani , Abbas Shafiee
Pharmaceutical Sciences Research Center, Tehran University of Medical Sciences, Tehran 14176, Iran
Chemistry Department, Zanjan University, PO Box 45195-313, Zanjan, Iran
Department of Medicinal Chemistry, Faculty of Pharmacy and Pharmaceutical Sciences Research Center, Tehran University of Medical Sciences, Tehran 14176, Iran
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesized nanocatalyst in this work provides a green and useful method to obtain 4H-
benzo[b]pyrans and dihydropyrano[c]chromenes in aqueous media. The catalyst shows environmental
benign character, which can be easily prepared, stored, recovered without obvious significant loss of
activity. Due to water-resistant and superparamagnetic nano-nature of the catalyst, it could be easily
separated by the application of an external magnetic device and reused conveniently. The synthe-
sized inorganic–organic hybrid nanocatalyst has fully been characterized by magnetic, adsorptive and
thermal techniques (transmission and scanning electron microscopy (TEM & SEM), Fourier transform
spectroscopy infrared (FTIR), thermo gravimetric analysis (TGA), X-ray Diffraction (XRD), Brunauer
Emmett Teller (BET) and vibrational sampling magnetometer (VSM)), which reveal of the superparamag-
netic nano-nature of the particles. In summary, the magnetically inorganic–organic hybrid nanocatalyst
supported on hydroxyapatite encapsulated ꢀ-Fe2O3 was found to be quite excellent and clean
catalytic system for the synthesis of 4H-benzopyrans and 2-amino-5-oxo-4-aryl-4,5-dihydropyrano[3,2-
c]chromene-3-carbonitriles.
Received 1 October 2011
Received in revised form 13 February 2012
Accepted 25 March 2012
Available online 4 April 2012
Keywords:
Inorganic–organic hybrid nanocatalyst
Hydroxyapatite encapsulated maghemite
nanoparticles
Organocatalyst
4H-benzo[b]pyrans
Dihydropyrano[c]chromenes
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
as cognitive enhancers, for the treatment of neurodegenera-
tive diseases, including Parkinson’s disease, Down’s syndrome,
Alzheimer’s disease and AIDS associated dementia as well as for
the treatment of schizophrenia [10].
Consequently, several methods have been reported for the
promoting preparation of tetrahydrobenzo[b]pyrans or dihydropy-
rano[c]chromenes, for example applying of microwave [11] and
ultrasonic irradiation [12]. In addition, there are several modi-
fied procedures using a variety of reagents, including the use of
hexadecyldimethylbenzyl ammonium bromide (HDMBAB) [13],
tetrabutylammonium bromide (TBAB) [14,15], fluoride ion [16],
ionic liquids [17–20], rare earth perfluorooctanoate [RE(PFO)3]
The last decade has witnessed much advancement in
organocatalysis. Their stability to moisture and oxygen make
inconvenient air sensitive techniques unnecessary. The term
“
organocatalysis” has emerged during the last decade as one of
the major issues in the development of catalytic chemical tech-
nology [1]. Although many organocatalysts have been designed
and applied in this area, however, the design and development of
new, effective and easily accessible organic catalysts continue to
be a major challenge. In recent years, the utilization of secondary
amines-aminocatalysis as organocatalysts has emerged as a viable
strategy for different organic transformations [2]. For example, the
generally accepted enamine mechanism for ␣-functionalization is
an extension of the work by Stork et al. [3].
Tetrahydrobenzo[b]pyrans and dihydropyrano[c]chromenes
have recently attracted much attention as an important class of het-
erocycles having useful biological and pharmacological properties,
such as diuretic, anti-cancer, anticoagulant, and anti-anaphylactic
activities [4–9]. Also, they possess a wide range of application
[21], Na SeO4 [22], high surface area MgO [23,24], nanosized MgO
2
[25], solid acid [26,27], diammonium hydrogen phosphate [28,29],
silica bonded n-propyl-4-aza-1-azoniabicyclo[2.2.2]octane chlo-
ride [30], and DBU [31].
However aforementioned methods suffer from the disadvan-
tages, such as expensive or unavailability or toxicity of the reagent,
extended reaction times, toxic solvents, additionally the main
drawback of almost existing methods is that the catalysts are
decomposed under aqueous work-up conditions and their recover-
ies are often impossible. Therefore, to overcome these drawbacks
a great deal of efforts is directed to develop an efficient, cat-
alytic system for synthesis of these compounds. According to the
∗ Corresponding author. Tel.: +98 21 66406757; fax: +98 21 66461178.
E-mail address: ashafiee@ams.ac.ir (A. Shafiee).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.03.023

M. Khoobi et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
75
successful usage of magnetic catalytic systems as vastly pow-
erful and clean recoverable supports for a variety of catalysts,
nowadays their applying is growing [32–41]. In other hand, sci-
ence and technology are shifting emphasis on economically and
environmentally benign and sustainable processes. In this regard,
various magnetic nanoparticles including ꢀ-Fe O3 coating with
2
hydroxyapatite has been considered as ideal biocompatible and
high-performance heterogeneous catalytic supports [42–45] owing
to its excellent thermal and chemical stability, high surface area
and their ability to design new functionalized materials consid-
ered as “inorganic–organic hybrids” [46–49]. Bearing in mind
the usefulness and efficiency of organocatalysts and in connec-
tion with our previous works using hydroxyapatite-encapsulated
ꢀ
-Fe O nanocrystallites [50–56] and functionalized iron oxide
2 3
Fig. 1. VSM curve of [ꢀ-Fe2O3@HAp] (line) vs. [ꢀ-Fe2O3@HAp Si (CH2)3 AMP)]
nanoparticle with amino pyridine moiety [57], we decided to
explore on novel magnetically immobilized organocatalyst fabri-
cated by covalently anchoring (2-aminomethyl)phenol moiety on
the surface of hydroxyapatite encapsulated maghemite nanopar-
ticles (denoted as [ꢀ-Fe O @ HAp Si (CH2)3 AMP]), for the
nanocrystallites (dotted line).
of the reaction, the reaction mixture was allowed to cool at room
temperature. The reaction mixture was diluted with ethyl acetate
and the catalyst was easily separated from the reaction mixture
with an external magnet and washed twice with ethyl acetate. The
combined organic layers were concentrated in vacuum and the
resulting residue was purified by recrystallization from ethanol.
2
3
preparation of tetrahydrobenzo[b]pyrans and dihydropyrano[c]
chromenes.
1
13
2
. Experimental
All compounds gave satisfactory spectral data ( H NMR and
C
NMR) and they were identical with those reported in the literature
[17–31].
2
.1. Synthesis of inorganic–organic hybrid magnetic
nanocatalyst: (2-aminomethyl)phenol supported on
HAp-encapsulated-ꢀ-Fe O ([ꢀ-Fe O @HAp Si (CH2)3 AMP])
2
3
2
3
3. Result and discussion
A sample of magnetic hydroxyapatite [ꢀ-Fe O @HAp] [55]
2
3
Magnetic hydroxyapatite encapsulated ꢀ-Fe O3 supported
2
(
500 mg) was pre-activated by heating under vacuum for 48 h
(
2-aminomethyl) phenol ([ꢀ-Fe O @HAp Si (CH2) AMP]) was
◦
2 3 3
at 120 C and suspended in a mixture of 150 ml of dry toluene
containing a stoichiometric amount (92 mg, 0.5 mmol) of 3-
aminopropyltrimethoxysilane. The amounts of this agent were
estimated by assuming two hydroxyl groups per formula of phos-
phate. The mixture was refluxed under Ar atmosphere for 48 h. The
product was separated by filtration, washed with ethanol, and dried
synthesized according to the procedure shown in Scheme 1.
ꢀ-Fe O @HAp] were synthesized via co-precipitation of
[
2
3
2+
3+
ferrous (Fe ) and ferric (Fe ) ions followed by coating of
the ꢀ-Fe O3 nanoparticles with hydroxyapatite [55]. The
2
nanocrystallities of [ꢀ-Fe O @HAp] were functionalized by 3-
2
3
aminopropyltrimethoxysilane to produce an organic–inorganic
hybrid. Then, this synthesized hybrid was reacted with 2-hydroxy
benzaldehyde followed by sodium cyanoborohydride reduction
to obtain the (2-aminomethyl) phenol functionalized core-shell
magnetic nanocrystallities of [ꢀ-Fe O @HAp Si (CH2)3 AMP)].
◦
under vacuum for 24 h at 50 C after Soxhelet extraction with hot
ethanol to give the solid surface bonded amine group at a load-
−
1
ing 0.75 mmol g (calculated by the back-titration analysis). The
−
1
resulting [ꢀ-Fe O @HAp Si (CH2)3 NH ] (0.75 mmol g , 10 g)
2
3
2
2
3
was allowed to react with equimolar of 2-hydroxybenzaldehyde
in super dry EtOH for 24 h under Ar atmosphere. Then, the solid
product was magnetically separated by an external magnet and
washed with dry EtOH. The resulting solid was allowed to react
[
ꢀ-Fe O @HAp Si (CH2)3 AMP)] nanocrystallites were fully
2 3
characterized by VSM (Fig. 1), SEM, TEM, XRD and TGA (Fig. 2). Due
to the superparamagnetic nature of the nanoparticle core, nuclear
magnetic resonance (NMR) technique could not be used to confirm
surface modification of the Si (CH ) NHCH PhOH.
with an excess NaBH CN in super dry MeOH for 72 h. The used sol-
3
2
3
2
vent was evaporated and dried over Mg for subsequent application.
The product was magnetically separated by an external magnet
and washed with diethyl ether and after Soxhelet extraction by
It is important that the core-shell material should possess
sufficient magnetic and superparamagnetic properties for prac-
tical applications. Magnetic hysteresis measurements for the
◦
hot ethanol dried under vacuum for 24 h at 70 C to give the ([ꢀ-
[
ꢀ-Fe O @HAp] and [ꢀ-Fe O @HAp Si (CH2)3 AMP] nanocrys-
−
1
2 3 2 3
Fe O @HAp Si (CH2)3 AMP]) at a loading 0.72 ± 0.01 mmol g
2
3
tallites were done in an applied magnetic field at r.t., with the
field sweeping from −8000 to +8000 Oe. As shown in Fig. 1, the
M(H) hysteresis loop for the samples was completely reversible,
showing that the nanoparticles exhibit superparamagnetic char-
acteristics. The hysteresis loops of them reach saturation up to
the maximum applied magnetic field. The magnetic saturation val-
ues of the [ꢀ-Fe O @HAp] and [ꢀ-Fe O @HAp Si (CH2)3 AMP)]
[
ꢀ-Fe O @HAp] (determined by TGA analysis).
2 3
2
2
[
.2. General procedure for the synthesis of
-amino-5-oxo-4-aryl-4, 5-dihydropyrano
3,2-c]chromene-3-carbonitrile or tetrahydrobenzo[b]pyran
derivatives
2
3
2
3
−
1
nanocrystallites are 7.24 and 4.37 emu g
at r.t., respectively.
A
stirring mixture of an aromatic aldehyde (1 mmol),
Lower magnetic saturation of later nanoparticles could be due
to the influence of the functional group. Both particles shown
high permeability in magnetization and their magnetization is
sufficient for magnetic separation with a conventional magnet.
The reversibility in hysteresis loop confirms that no aggregation
imposes to the nanoparticles in the magnetic fields.
malononitrile (1.2 mmol), magnetic catalytic system ([ꢀ-
Fe O @HAp Si (CH2)3 AMP], 1.5 mol%) and water (5 mL)
2
3
were heated under refluxing conditions for a few minutes. To this
stirred mixture, 4-hydroxycoumarin or dimedone (1 mmol) was
added. The reaction mixture was refluxed for an appropriate time
as mentioned in Table 1 or Table 2. The progress of the reaction was
monitored by TLC, for disappearance of aldehyde. After completion
The SEM and TEM showed that the average size of synthesized
encapsulated nanoparticles is less than 100 nm and they present

7
6
M. Khoobi et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
Table 1
Synthesis of dihydropyrano[c]chromenes (via condensation of aldehydes, 4-hydroxy coumarin and malononitrile using magnetic nano-organocatalyst).
Entry
Product
Yielda
Time
m.p.
Ref.
NH
2
CN
CN
CN
O
O
O
O
(
4a)
10 hb
1
2
3
10
10
78
253–255
253–255
253–255
[20]
[20]
[20]
NH
2
O
O
O
(
4a)
10 hc
NH
2
O
O
(
4a)
10 mind
NH
2
CN
O
NO
2
O
O
O
(
4b)
10 mind
4
5
84
78
257–258
220–222
[20]
[20]
NH
2
CN
O
O
OMe
(
4c)
10 mind
NH2
CN
O
S
O
O
O
(
4d)
10 mind
6
7
75
79
255–256
255–257
[59]
NH
2
CN
CN
N
O
O
O
(
4e)
10 mind
–
NH2
N
O
O
(
4f)
10 mind
8
70
270–272
[60]
a
Yields refer to isolated pure products.
No catalyst, H2O, reflux, 10 h.
b
c
[
[
ꢀ-Fe2O3@ HAp], H2O, reflux, 10 h.
ꢀ-Fe2O3@ HAp Si (CH2)3 AMP], H2O, reflux, 10 min.
d
Scheme 1. Preparation of the magnetic hydroxyapatite encapsulated ꢀ-Fe2O3 supported (2-aminomethyl)phenol ([ꢀ-Fe2O3@HAp Si (CH2)3 AMP]).

M. Khoobi et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
77
Table 2
Synthesis of 4H-benzo[b]pyrans (via condensation of aldehydes, dimedone and malononitrile using [ꢀ-Fe2O3@HAp Si (CH2)3 AMP] as recoverable nanocatalyst in water).
Entry
Product
Yielda
Time
m.p.
Ref.
[61]
[61]
O
CN
O
NH2
NO2
(
6a)
1
2
80
10 min
10 min
225–226
198–201
O
CN
O
OMe
NH2
(
6b)
84
O
O
CN
O
O
NH2
(
6c)
3
4
87
74
10 min
10 min
195–198
191–194
[61]
[62]
S
CN
NH2
(
6d)
O
H N
2
NC
O
O
N
H
(
6e)
5
55
10 min
295–297
[61]
a
Yields refer to isolated pure products.
as uniform particles SEM (Fig. 2a and b). The obtained histogram
confirmed the fact that the size of distribution for 75 observed
nanoparticles was a narrow normal one with a 69 nm average value
and a 2.1 nm standard deviation. The theoretical curve of standard
distribution from our studies (Fig. 2c) was calculated by means of
origin program.
the immobilized (2-aminomethyl) phenol moiety was performed
and showed a first peak due to desorption of the water (centered at
◦
◦
95 C). This is followed by a second peak around 300 until 500 C,
corresponding to the loss of the organic spacer group (Fig. 2f).
In connection with our previous work using synthesized
magnetic nano-catalysts, and our interest in multi-component
reactions, herein the facile synthesis of dihydropyrano[c]chromene
and 4H-drobenzo[b]pyran derivatives in the presence of catalytic
amounts of [ꢀ-Fe O @HAp Si (CH2)3 AMP)] nanocrystallites in
The XRD pattern of [ꢀ-Fe O @HAp] nanocrystallities agrees well
2
3
with that of the tetragonal structure of ꢀ-Fe O3 (1999 JCPDS file
2
No 13-0458). No other phase except the maghemite is detectable
2
3
◦
◦
◦
◦
[
5
58] (Fig. 2d). Diffraction peaks at around 18.4 , 30.2 , 35.7 , 43.6 ,
aqueous medium is reported. On the basis of the optimal con-
◦
◦
6.2 and 63.1 corresponding to the (111), (220), (311), (400),
ditions established (1.5 mol%, [ꢀ-Fe O @HAp Si (CH2)3 AMP]),
2
3
(
440) and (511) are readily recognized from the XRD pattern. Nitro-
we examined the condensing of malononitrile, 4-hdroxycoumarine
and various aromatic aldehyde in aqueous medium. As shown
in Scheme 2 the reactions proceed smoothly and correspond-
ing 2-amino-5-oxo-4-aryl-4,5-dihydropyrano[3,2-c]chromene-3-
carbonitrile derivatives were obtained in high yields. Furthermore,
the three-component reaction of dimedone, aldehydes, and
malononitrile for preparation of tetrahydrobenzo[b]pyrans was
investigated (Scheme 2).
Subsequently, a series of differently substituted dihydropy-
rano[c]chromene and tetrahydrobenzo[b]pyran derivatives was
prepared successfully under aqueous conditions. The results are
listed in Tables 1 and 2. The results clearly indicate that reactions
can tolerate a wide range of differently substituted aldehydes. The
reactions were complete within 10–15 min and excellent yields
of products were obtained by this method. However, in case of
gen adsorption–desorption isotherms are shown in Fig. 2e and
reveal that the adsorption–desorption process is not reversible.
This is a result of the hysteresis loops due to capillary condensa-
tion. The surface area was calculated using BET method, and that
was found 122 m g for synthetic hydroxyapatite coated mag-
netic nanoparticle ([ꢀ-Fe O @HAp]). This average was reduced
to 87 m g
nanoparticle powder [ꢀ-Fe O @HAp Si (CH2)3 AMP)]. The syn-
thetic powder showed a type (IV) nitrogen adsorption–desorption
isotherm (Fig. 2e).
2
−1
2
3
2
−1
for functionalized hydroxyapatite coated magnetic
2
3
Quantitative determination of the functional group contents of
the surface-bonded [ꢀ-Fe O @HAp Si (CH2)3 AMP)] nanocrys-
2
3
tallites was performed using thermo-gravimetric analysis (TGA)
and a loading of 0.72 ± 0.01 mmol g−1 was obtained. TGA analysis of

7
8
M. Khoobi et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
Fig. 2. (a) SEM image; (b) TEM micrograph; (c) histogram of sizes using SEM image; (d) XRD spectra; (e) the isotherm plot of [ꢀ-Fe2O3@HAp] and (f) TGA diagram of
ꢀ-Fe2O3@HAp Si (CH2)3 AMP] nanocrystallites.
[
Scheme 2. The one-pot synthesis of dihydropyrano[c]chromene-3-carbonitrile and 4H-benzo[b]pyran derivatives.

M. Khoobi et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
79
Fig. 3. The recycling of catalyst (left) and catalyst ability to effective recovery at the end of reactions (right).
Scheme 3. Possible mechanism for one-pot synthesis of 2-amino-5-oxo-4-aryl-4,5-dihydropyrano[3,2-c]chromene-3-carbonitrile.
aliphatic aldehydes, reactions were not complete under the above
conditions and desired products were obtained in low yields.
In continuation to this research, a simple and an efficient one-
pot synthetic approach was used for the preparation of biologically
interesting spirooxindole derivatives in good yields by means
of three-component reactions of isatin, malononitrile, and dime-
done catalyzed by mentioned magnetic catalytic system in water
A
possible mechanism for the reaction using [ꢀ-
Fe O @HAp Si (CH2)3 AMP] nanocrystallities as clean catalyst
2
3
is shown in Scheme 3. According to the mechanism [ꢀ-
Fe O @HAp Si (CH2)3 AMP] nanocrystallities catalyzed the
2
3
readily in situ formation of Knoevenagel product 8. The higher
reactivity of the iminium ion compared to the carbonyl species
facilitates Knoevenagel reaction between aryl aldehyde and
malononitrile. In the presence of magnetic catalyst, enamine 9
is formed from amine functional group and 4-hydroxycoumarin.
Reaction of compound 9 with 8 followed by hydrolysis afforded
compound 4a. The substituent on the aromatic ring did not show
any electronic effects in terms of yields under these reaction
conditions.
(
Table 2, entry 5).
Finally, to determine the applicability of catalyst recovery, we
decanted the vessel by use of an external magnet and remained
catalyst was washed with diethyl ether to remove residual product,
dried under vacuum and reused in a subsequent reaction. In 10 con-
secutive runs, the conversion stayed with no detectable loss, higher
than 90%. To rule out any contribution of homogeneous catalysis,
we tested the reaction leached by a catalytic amount of catalyst
4
. Conclusion
(
after 2 h stirring in H O and removal of the catalyst) and observed
2
that the reaction did not carried out even after 12 h. This clearly
confirmed that no active species were present in the supernatant
In summary, we showed that (2-aminomethyl)phenol
moiety
supported
on
HAp-encapsulated-ꢀ-Fe O3
[ꢀ-
2
(
Fig. 3).
Fe O @HAp Si (CH2)3 AMP] was
a
novel and effective
2
3

8
0
M. Khoobi et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 74–80
heterogeneous
of 4H-benzo[b]pyran
dihydropyrano[3,2-c]chromene-3-carbonitrile derivatives from
commercially available starting materials. The present method
requires remarkably small amounts of non-toxic and environ-
mentally friendly [ꢀ-Fe O @HAp Si (CH2)3 AMP] as catalyst
catalyst
for
and
the
one-pot
synthesis
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2 3
5
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Appendix A. Supplementary data
Supplementary
can be
data
found,
associated
in the
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