Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient and recyclable magnetic nanocatalyst for rapid eco-friendly synthesis of 2-amino-4-substituted-1,4-dihydrobenzo[4, 5]imidazo[1,2-a]pyrimidine-3-carbonitriles

Article abstract of DOI:10.1007/s11164-018-3456-3

Abstract: A one-pot, multi-component, synthetic method for preparation of 2-amino-4-substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles has been developed. It was accomplished by mixing 2-aminobenzimidazole, aldehyde and malononitril

Full text of DOI:10.1007/s11164-018-3456-3

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3456-3
Sulfonated chitosan-encapsulated HAp@Fe3O4:
an efficient and recyclable magnetic nanocatalyst
for rapid eco-friendly synthesis of 2-amino-4-
substituted-1,4-dihydrobenzo[4, 5]imidazo[1,2-a]
pyrimidine-3-carbonitriles
Vilas N. Mahire1,2 Girish P. Patil3,4 Amol B. Deore3
•
•
•
Padmakar G. Chavan3 Harishchandra D. Jirimali1
Pramod P. Mahulikar1
•
•
Received: 13 February 2018 / Accepted: 24 April 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A one-pot, multi-component, synthetic method for preparation of 2-amino-4-
substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles has been
developed. It was accomplished by mixing 2-aminobenzimidazole, aldehyde and
malononitrile in the presence of S-Chit-HAp@Fe3O4 as a nanocatalyst in aqueous
medium. The catalyst (S-Chit-HAp@Fe3O4) was synthesized using a co-precipitation
method and further characterized by using FT-IR, XRD, FE-SEM, EDX, HR-TEM, TGA
and VSM analysis. It was found that the catalyst exhibits magnetic properties and
showed high catalytical activity for the synthesis of 2-amino-4-substituted-1,4-dihy-
drobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3456-3) contains supplementary material, which is available to authorized users.
&
Pramod P. Mahulikar
mahulikarpp@gmail.com
1
2
School of Chemical Sciences, North Maharashtra University, Jalgaon, Maharashtra, India
Department of Chemistry, Bhusawal Arts, Science and P. O. Nahata Commerce College,
Bhusawal, Maharashtra, India
3
4
School of Physical Sciences, North Maharashtra University, Jalgaon, Maharashtra, India
SVKM’s Institute of Technology, Dhule, India
123

V. N. Mahire et al.
Graphical Abstract S-Chit-HAp@Fe3O4 nanoparticles catalyzing one-pot, multi-
component synthesis of 2-amino-4-substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-
a]pyrimidine-3-carbonitriles in aqueous medium has been reported herein.
Keywords Green chemistry Á S-Chit-HAp@Fe3O4 Á Chitosan Á Hydroxyapatite Á
Fe3O4 nanoparticles Á 2-Aminobenzimidazole Á Malononitrile
Introduction
The development of greener protocols for the synthesis of highly functionalized
motifs having medicinal value is always a welcome in pharmaceutical science and is
an attractive research thrust area. In organic synthesis, multi-component reactions
are highly desirable because the rapid generation of complex molecules is achieved
in a single reaction step through the appropriate combination of three or more
substrates in a single reaction flask and by avoiding the intermediate steps. The
diversity-oriented organic synthesis or drug designing are the best examples of
multi-component reactions [1, 2]. It is well known that pyrimidinobenzimidazoles
have a wide range of biological activities such as antimicrobial [3], antiulcer [4],
anti-HIV [5] and antiviral activity [6], etc. Furthermore, pyrimidinobenzimidazoles
are also found to be better analgesic, anti-inflammatory and antiamoebic drugs [7].
Nowadays, magnetic nanoparticles have opened a new avenue as powerful and
clean recoverable supports for a variety of organic transformations, since they could
be easily separated by an external magnetic field [8]. The Fe3O4 nanoparticles are
hydrophilic in nature and are able to aggregate into large clusters. Therefore, surface
modification is an appropriate strategy to prevent agglomeration of nanoparticles
[9–12]. As per the literature, it is found that hydroxyapatite [Ca10(PO4)6(OH)2] is an
ideal material for encapsulating iron oxide nanoparticles. It is a biocompatible
inorganic nanomaterial used for preparation of a high-performance heterogeneous
catalyst [13, 14]. Recently, nanoparticles made from natural materials, particularly
polysaccharides, have attracted attention as environmentally benign polymeric
supports [15–18]. Among biopolymers, chitosan is an excellent support material for
catalytic applications in heterogeneous catalysis reaction. Moreover, chitosan is
used as an environmentally benign polymeric support due to its excellent chelating
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Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient…
and mechanical properties. The terminal hydroxyl and amino groups of chitosan can
be suitably modified to anchoring sites for catalytical applications [19–21].
As per the reports, non-magnetic nanoparticles have disadvantages like loss of
catalytic ability during centrifugation and filtration processes. Hence, we decided to
synthesize the magnetic nanoparticles since they can be simply separated from a
reaction mass by an external magnet and recycled for subsequent reactions. Fe3O4
nanoparticles are easily oxidized in atmosphere; hence, modification of their surface
is an essential task. Therefore, hydroxyapatite and then chitosan were grafted over
the surface of Fe3O4 nanoparticles to achieve additional porosity and stability.
Furthermore, the catalyst was then sulfonated using 1, 3-propane sultone to obtain
additional active sites for the catalyst. The strategy for preparing the catalyst is
summarized in Scheme 1. The synthesized nanocatalyst was found to be efficient
and a recyclable support for one-pot, multi-component synthesis of 2-amino-4-
substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles in aque-
ous media (Scheme 2). In continuation of our work on synthesis of bioactive
compounds using green chemistry [22–24], we have attempted in the present work a
cleaner synthesis using newly developed magnetic catalyst.
Results and discussion
FT-IR analysis
In FT-IR spectral study (Fig. 1), Fig. 1a represents FT-IR spectrum of Fe3O4
nanoparticles, in which the characteristic vibration band observed at 583 cm-1 was
due to the Fe–O vibration and the bands at 3391 and 1622 cm-1 were due to
adsorbed water molecules. The FT-IR spectrum of HAp@Fe3O4 nanoparticles
Scheme 1 Schematic visualization for preparation of S-Chit-HAp@Fe3O4 nanoparticles
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V. N. Mahire et al.
Scheme 2 Synthesis
carbonitrile derivatives
of
2-amino-4-substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-
Fig. 1 FT-IR spectra of (a) Fe3O4, (b) HAp@Fe3O4, (c) Chit-HAp@Fe3O4 and (d) S-Chit-HAp@Fe3O4
(Fig. 1b) showed broad bands at 3424 and 1636 cm-1 due to stretching and bending
vibrations of molecularly adsorbed water molecules. Also, the stretching vibration
at around 3567 cm-1 confirmed the presence of hydroxyl groups. The symmetric
3-
stretching vibrations of the PO4
ion were also found at around 1043 and
962 cm-1. The absorption bands at 603 and 563 cm-1 were attributed to
3-
asymmetric stretching and bending vibration frequencies of the PO4
ion,
respectively. The above results indicated successful grafting of HAp over Fe3O4
nanoparticles. Figure 1c shows the FT-IR spectrum of Chit-HAp@Fe3O4 nanopar-
ticles in which broad peaks near 3424 cm-1 could be attributed to the N–H and O–H
stretching vibrations, as well as inter- and extra-molecular hydrogen bonding
existing in chitosan skeletons [25]. There could also the contribution of an –NH2
absorption band in the region of 3400–3000 cm-1, which was found to be merged
by the O–H stretching vibrations. The weak band near 2884 cm-1 was attributed to
the C–H stretching vibration. The band at 1647 cm-1 was assigned to N–H bending
vibration and that of 1423 cm-1 to C–O stretching of primary alcoholic groups in
123

Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient…
chitosan. Figure 1d represents FT-IR spectrum of S-Chit-HAp@Fe3O4 nanoparti-
cles with absorption bands at 1377 and 1035 cm-1 corresponding to the stretching
vibration of the S=O group, and the bands appearing at 2881 cm-1 were
characteristic of C–H stretching vibrations, indicating successful sulfonation of
Chit-HAp@Fe3O4 nanoparticles. Therefore, FT-IR spectral data confirmed the
successful encapsulation of Fe3O4 nanoparticles with hydroxyapatite, chitosan and
1, 3-propane sultone layers.
XRD analysis
The XRD analysis was carried out to gain evidence of Chit-HAp as well as Fe3O4
nanoparticles and also for determining the crystallographic features. In XRD
patterns (Fig. 2), the diffraction peaks of Fe3O4 were noted at 30.4°, 35.5°, 43.5°,
53.7°, 57.5° and 63°. These peaks were indexed to the cubic crystalline structure of
˚
Fe3O4 with an Fd3/m space group and lattice parameter a = b = c = 8.398 A
(JCPDS card no. # 79-0418), which confirmed the formation of Fe3O4 phase.
Additionally, the diffraction peaks found in case of HAp@Fe3O4, Chit-HAp@Fe3-
O4 and S-Chit-HAp@Fe3O4 were indexed to the hydroxyapatite with an P63/m
˚
space group and lattice parameters a = 9.439 A and c = 6.886 A (JCPDS card no. #
˚
Fig. 2 XRD patterns of
(a) Fe3O4, (b) HAp@Fe3O4,
(c) Chit-HAp@Fe3O4 and
(d) S-Chit-HAp@Fe3O4
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V. N. Mahire et al.
896438). In the XRD patterns of Chit-HAp@Fe3O4 and S-Chit-HAp@Fe3O4, the
small diffraction peaks of chitosan centred at 20.4° were observed [26].
FE-SEM and EDX analysis
FE-SEM images of Fe3O4, HAp@Fe3O4, Chit-HAp@Fe3O4 and S-Chit-HAp@Fe3-
O4 are depicted in Fig. 3. Figure 3a shows the FE-SEM image of Fe3O4
nanoparticles with an average diameter of 11 nm. An FE-SEM image of
HAp@Fe3O4 nanoparticles is shown in Fig. 3b which visualises the formation of
nanoparticle as well as the nanorod-like morphology of HAp@Fe3O4 (shown by the
arrow). Figure 3c indicates the gel-like coating of chitosan over the HAp@Fe3O4
nanoparticles/nanorods, confirming the formation of Chit-HAp@Fe3O4 nanoparti-
cles. Figure 3d shows thr explored S-Chit-HAp@Fe3O4 configuration formed after
sulfonation. Figure S22 shows the EDX spectrum of S-Chit-HAp@Fe3O4 nanopar-
ticles in which the characteristic peaks of Fe, Ca, N, O and S were observed and that
confirmed the successful grafting of layers over Fe3O4 nanoparticles.
TEM analysis
To study the morphology and crystalline nature of S-Chit-HAp@Fe3O4 matrix,
detailed TEM characterizations were carried out. The bright field images of S-Chit-
HAp@Fe3O4 matrix are shown in Fig. 4a and b. The close-up image of S-Chit-
HAp@Fe3O4 matrix is shown in Fig. 4b, from which we noted the formation of the
Fig. 3 FE-SEM images of a Fe3O4, b HAp@Fe3O4, c Chit-HAp@Fe3O4 and d S-Chit-HAp@Fe3O4
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Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient…
Fig. 4 a and b TEM images of S-Chit-HAp@Fe3O4 matrix, c HRTEM images of S-Chit-HAp@Fe3O4
matrix, d selected area electron diffraction pattern of S-Chit-HAp@Fe3O4 matrix
rod-like structure of HAp@Fe3O4 with an average diameter of 10 nm (shown by the
arrow). Figure 4c depicts an HR-TEM image of S-Chit- HAp@Fe3O4 matrix, which
revealed that an individual Fe3O4 nanoparticle has a single-crystalline nature with a
fringe spacing of 0.48 nm [27]. The selected area electron diffraction (SAED)
pattern is shown in Fig. 4d which indicates the polycrystalline nature of the Fe3O4.
TGA
TGA analysis of the newly synthesized material, S-Chit-HAp@Fe3O4 and its step-by-
step buildup from Fe3O4, is depicted in Fig. 5. The TGA curves of Fe3O4,
HAp@Fe3O4, Chit-HAp@Fe3O4 and S-Chit-HAp@Fe3O4 nanoparticles were
obtained in N2 atmosphere in the dynamic mode starting from room temperature to
800 °C. TGA curves of Fe3O4 and HAp@Fe3O4 nanoparticles (Fig. 5a, b) did not
show any considerable weight loss. The small amount of weight loss was observed due
to the elimination of adsorbed moisture and condensation of the peripheral hydroxyl
groups, which was around 3–4%. The Chit-HAp@Fe3O4 (Fig. 5c) was prepared by
grafting of organic bio-polymer chitosan on the inorganic HAp@Fe3O4 nanoparticles
and, hence, the grafted chitosan polymer showed elimination of free NH2 and OH
terminal groups of chitosan and the degradation started at around 180–420 °C with
123

V. N. Mahire et al.
Fig. 5 TGA thermograms of (a) Fe3O4, (b) HAp@Fe3O4, (c) Chit-HAp@Fe3O4 and (d) S-Chit-
HAp@Fe3O4
total weight loss of 25.29%. The degradation observed at 420–765 °C was due to the
pyrolysis of the cross-linked sugar units and of which the weight loss amounts to
21.90%. The TGA curve of S-Chit-HAp@Fe3O4 (Fig. 5d) in the temperature range of
180–500 °C showed the elimination of the sulfonated free chains of the modified
chitosan with a smaller weight loss as compared to the Chit-HAp@Fe3O4 due to the
increased extent of organic content of the material. The amount of residue of the
pyrolyzed material remaining at 500–700 °C was found to be more in case of S-Chit-
HAp@Fe3O4 than that for Chit-HAp@Fe3O4.
VSM analysis
Figure 6 shows VSM curves in which Fe3O4 showed the highest saturation
magnetization. The drastic decrease in saturation magnetization from 1.712 to 0.157
memu was observed when HAp was coated over Fe3O4. Furthermore, HAp@Fe3O4
was coated with chitosan and its saturation was found to decreases again to 0.061
memu. On sulfonation of Chit-HAp@Fe3O4, the magnetization value decreased to
0.056 memu. In summary, the continuous decreasing trend in magnetization value
upon coating confirmed the successful coating of Fe3O4 and the coating thickness
was found to be increasing with coatings of different coaters.
Optimization of reaction conditions
In the preliminary attempts at optimizing the reaction conditions, we initially carried
out a model reaction of 2-aminobenzimidazole 1 (2 mmol), 4-chlorobenzaldehyde
2a (2 mmol) and malononitrile 3 (2 mmol) in the presence of S-Chit-HAp@Fe3O4
(5 mol%) as a catalyst in water (5 mL) under reflux condition for 15 min, and we
123

Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient…
Fig. 6 Magnetization curves of
HAp@Fe3O4
a Fe3O4, b HAp@Fe3O4, c Chit-HAp@Fe3O4 and d S-Chit-
obtained product 5a with a 76% yield (Table 1, entry 2). In order to optimize the
reaction conditions, the model reaction was screened for the effect of solvents and
quantity of catalyst loading. The reaction in water using 10 mol% of catalyst was
found to offer the optimized reaction condition for synthesis of pyrimidinobenz-
imidazoles (Table 2, entry 10). It was found that there was non-significant loss in
catalytic activity of the catalyst even after a third reaction cycle.
Initially, model reactions were performed in solvents like dichloromethane,
dimethylformamide and chloroform in reflux conditions; however, no satisfactory
yield of 5a was obtained (Table 2, entries 1, 2 and 4). Furthermore, the reactions in
acetonitrile and ethanol resulted in moderate yields (5a, Table 2, entries 3 and 5).
The model reaction was further examined for effect of temperature on rate of
reaction and it was observed that the period and yield of reactions were dependent
Table 1 Reaction of 2-aminobenzimidazole 1, 4-chlorobenzaldehyde 2a and malononitrile 3 in the
presence of S-Chit-HAp@Fe3O4
Entry
Catalyst (mol%)
Time (min)
Yield of 5aa (%)
1
None
5
8 (h)
15
52
2
76
3b
10
15
88, 83, 83
4
15
15
79
65
5
20
15
Conditions: 2-aminobenzimidazole 1 (2 mmol), 4-chlorobenzaldehyde 2a (2 mmol), malononitrile 3
(2 mmol)
aIsolated yields
bCatalyst was reused three times
123

V. N. Mahire et al.
Table 2 Reaction of 2-aminobenzimidazole 1, 4-chlorobenzaldehyde 2a and malononitrile 3 in different
solvents
Entry
Solvent
Temp (°C)
Time (min)
Yield of 5aa (%)
1
2
3
4
5
6
7
8
9
10
CH2Cl2
Reflux
Reflux
Reflux
100
240
240
180
240
60
37
CHCl3
52
CH3CN
62
DMF
43
C2H5OH
Reflux
Reflux
Reflux
RT
73
C2H5OH: H2O (50:50)
60
58
C2H5OH: H2O (20:80)
30
81
H2O
H2O
H2O
180
180
15
Trace
72
60
Reflux
88
Conditions: 2-aminobenzimidazole 1 (2 mmol), 4-chlorobenzaldehyde 2a (2 mmol), malononitrile 3
(2 mmol) and S-Chit-HAp@Fe3O4 nanoparticles (10 mol%)
aIsolated yields
on solvent rather than temperature (Table 2, entries 8 and 9). Shaabani et al.
attempted synthesis of pyrimidinobenzimidazoles in water [28] and reported that
reactions required 7–12 h for completion with only 70% yield of 5a; however, in the
present work, the same reaction catalyzed by 10 mol% of catalyst (S-Chit-
HAp@Fe3O4) in water was completed within only 15 min with better yield of 5a
(88%). This result confirmed an anchoring support of catalyst to promote the
reaction within a shorter period with maximum yield of product.
Once reaction was identified, the scope and limitations of the developed synthetic
protocol were further explored under the optimized reaction conditions with
aldehydes having different substituents (Table 3). Among all these studied cases,
better yields of the desired products were obtained within short reaction time. It was
observed that an electron-withdrawing group in the aldehyde boosted the reaction
with higher yield, while the electron-donating group reflected a somewhat opposite
effect.
It is well known that water is non-toxic, inexpensive, universal as well as green
solvent and, thus, reactions in water medium have gained considerable interest. The
present study also reinforced the importance of water as a solvent in green
chemistry. The catalyst exhibited remarkable activity irrespective of the groups in
aldehydes to boost the reactions towards better yields within a short period
(Scheme 3).
Proposed mechanism
The reaction presumably proceeds in three steps (Scheme 4), initial condensation of
aldehyde 2 and malononitrile 3 by Knoevenagel condensation to form an
intermediate called arylidene malononitrile (I). It is then followed by Michael type
123

Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient…
Table 3 Synthesis
carbonitrilea
of
2-amino-4-substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-
Entry
R
Product Time (min) Yield (%)b M.p. (°C)
References
Found
Reported
1
4-ClC6H4
2-FC6H4
5a
5b
5c
5d
5e
5f
10
10
08
10
15
30
90
15
15
20
15
12
88
75
83
79
70
91
90
91
85
89
81
70
226
234–236 [30–33]
2
208–210 224–226 [30–33]
222–224 236–238 [30–33]
3
2-ClC6H4
4-FC6H4
4
218
200
212
216
214
208
210
218
222
266–268 [30–33]
5
4-CH3C6H4
2-F, 4-ClC6H3
–
–
–
–
–
–
–
–
6
7
2,5-(OCH3)2C6H3 5g
8
2-BrC6H4
3-BrC6H4
4-BrC6H4
3-NO2C6H4
4-CNC6H4
5h
5i
9
238–240 [30–33]
317–319
10
11
12
5j
5k
5l
236
–
[28]
–
aGeneral reaction conditions: 2-aminobenzimidazole 1 (2 mmol), aldehydes 2a-l (2 mmol), malononitrile
3 (2 mmol), S-Chi-HAp@Fe3O4 nanoparticles (10 mol%) and H2O (5 mL)
bIsolated yields
l
C
H
l
C
O
-
-
e
F
@
mo
10
l
%
hit HA
S C
3O4
p
N
N
(
)
C
N
C
NH2
N
e ux
H2 R fl
m n
i
15
O
,
,
N
H
N
C
N
N
H
NH2
C
a
2
1
3
a
5
Scheme 3 Optimization of reaction conditions
addition between 2-aminobenzimidazole 1 and arylidene malononitrile I to give
unstable intermediate II, which undergoes intramolecular cyclization affording the
desired product 2-amino-4-substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrim-
idine-3-carbonitriles.
Experimental
Materials and methods
All the solvents and reagents were purchased from commercial suppliers, Sigma-
Aldrich Ltd., S. D. Fine Chemicals Ltd., Spectrochem India Ltd., and were used
123

V. N. Mahire et al.
Scheme 4 Proposed
dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitrile derivatives
mechanism
for
synthesis
of
2-amino-4-substituted-1,4-
without further purification. The progress of each reaction was monitored by thin
layer chromatography (TLC) on 0.2-mm precoated silica gel 60 F254 plates (Merck,
Germany) with spots located using UV light as the visualizing agent. Melting points
were obtained by an open capillary method and are uncorrected. The IR spectra
(KBr pellets) were recorded on Perkin Elmer FT-IR C88522 spectrophotometer.
1
The H NMR and 13C NMR spectra were recorded on FT-NMR Bruker Avance-II
spectrometer at 400 and 100 MHz, respectively, using DMSO-d6 as a solvent. The
mass spectra were recorded on a Waters Q-Tof Micromass (LCMS) spectrometer in
electrospray ionization (ESI) mode. The X-ray diffractograms (XRD) of the catalyst
were recorded in the range of 20–80 on a Bruker D8-Advance instrument. The FE-
SEM analysis of the catalyst was performed on a Hitachi S-4800 scanning electron
microscope (Japan) with an accelerating voltage of 30 kV. The EDX analysis of
catalyst was performed by using Bruker EDX flash detector mode 5030. The field
emission gun transmission electron microscopy (FEG-TEM, 300 kV) of the catalyst
was performed on an FEI Tecnai G2 F30 instrument. TGA data were obtained with
a Perkin Elmer instrument in the temperature range of 29–800 °C at a constant
heating (10 °C/min) in a nitrogen atmosphere (20 mL/min). The magnetization
curves of the samples were measured at room temperature with a Lake Shore 7410
vibrating sample magnetometer (VSM).
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Sulfonated chitosan-encapsulated HAp@Fe3O4: an efficient…
Preparation of Fe3O4 nanoparticles
Fe3O4 magnetic nanoparticles were synthesized by co-precipitation of FeCl3Á6H2O
and FeCl2Á4H2O in ammonia solution, according to the reported procedure [29].
Typically, FeCl3Á6H2O (2.7 g, 10 mmol) and FeCl2Á4H2O (0.99 g, 5 mmol) were
dissolved in 100 mL of distilled water at 80 °C under N2 atmosphere and vigorous
stirring. Then, 7.5 mL of 25% NH4OH solution was added to the reaction mixture in
one portion with vigorous mechanical stirring. Addition of NH4OH to the Fe2?/Fe3?
salt solution resulted in immediate precipitation of the ferric hydroxide. After 1 h,
the mixture was cooled to room temperature, and the resulting black precipitate was
separated by applying an external magnet, washed four times with distilled water
and heated in vacuum at 110 °C for 4 h to obtain the particles in Fe3O4 form.
Preparation of magnetic hydroxyapatite nanoparticles (HAp@Fe3O4)
In a 250-mL round-bottom flask, Fe3O4 nanoparticles (409 mg) were added to a
solution of Ca(NO3)2Á4H2O (7.59 g, 33.7 mmol) in 50 mL of water, and the
reaction mass was stirred at room temperature. (NH4)2HPO4Á4H2O (2.64 g,
20 mmol) in 50 mL of water was added drop-wise (1 mL/min) to the above
solution and the pH of the resulting solution was adjusted to 11. The total mass was
stirred for 2 h and then aged for 24 h. The obtained precipitate was separated by a
magnet, washed repeatedly with deionized water to make it neutral and then dried in
oven at 110 °C overnight to obtain HAp@Fe3O4 nanoparticles.
Preparation of chitosan-coated magnetic hydroxyapatite nanoparticles
(Chit-HAp@Fe3O4)
In order to coat chitosan onto the surface of HAp@Fe3O4, 1 g of chitosan was
dissolved in 100 mL of 1% acetic acid solution in a 250-mL beaker containing 1 g
of prepared HAp@Fe3O4. Subsequently, 2.6 mL of 25% glutaraldehyde solution
was added into the mixture. The mixed solution was stirred continuously by a
homogenizer for 30 min in order to disperse HAp@Fe3O4 nanoparticles into
chitosan solution. Chitosan reacts with glutaraldehyde to form cross-link hydrogel.
The pH in the system was adjusted to 9 using 1% NaOH solution. The mixture was
stirred and homogenized for 2 h at room temperature, resulting in the formation of
Chit-HAp@Fe3O4 nanoparticles. The synthesized nanoparticles were filtered,
washed with distilled water and dried in an oven at 110 °C overnight to obtain
Chit-HAp@Fe3O4 nanoparticles.
Sulfonation of chitosan-coated magnetic hydroxyapatite nanoparticles (S-
Chit-HAp@Fe3O4) (Scheme 1)
Chit-HAp@Fe3O4 nanoparticles (750 mg) were added to the 1% NaOH solution
(100 mL) in a 250-mL round-bottom flask and stirred for 10 min. To this solution,
1, 3-propane sultone (305 mg) was added and refluxed for 6 h. The resulting
123

V. N. Mahire et al.
product was filtered, washed with distilled water and dried in an oven at 110 °C for
4 h.
General procedure for synthesis of 2-amino-4-substituted-1,4-
dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitrile (Scheme 2)
In a 50-mL round-bottom flask, a mixture of 2-aminobenzimidazole 1 (2 mmol),
aldehyde 2a–l (2 mmol), malononitrile 3 (2 mmol) and 10 mol% of catalyst in
water (5 mL) was heated at reflux for a specified time (Table 3). After completion
of the reaction (monitored by TLC, hexane: ethyl acetate), the mixture was cooled
to room temperature and then diluted with ethanol to separate catalyst from the
reaction mixture by an external magnet, followed by decantation of the organic
solution. The organic solution was then poured onto crushed ice to obtain solid
product, which was filtered and dried. The recrystallization with ethanol afforded
pure products in quantitative yields (Table 3).
Conclusion
We have developed an environmentally friendly synthetic approach for the water-
mediated synthesis of 2-amino-4-substituted-1,4-dihydrobenzo[4,5]imidazo[1,2-
a]pyrimidine-3-carbonitriles using newer magnetic nanocatalyst S-Chit-HAp@Fe3-
O4. Heterogeneous reaction conditions, easy separation of the catalyst after reaction
by an external magnet, shorter reaction period, quantitative product yields,
successful easy recovery and efficient recycling of the catalyst are worthy
advantages of this greener approach.
Supplementary data
Supplementary data for this article can be accessed on the publisher’s website.
Acknowledgements The authors are thankful to UGC, New Delhi, for financial assistance under a one-
time grant [F. no. 19-110/2013 (BSR)] and major research project [F. no. 41-302/2012 (SR)]. the authors
are also thankful to SAIF, IIT Bombay, IIT Madras and Panjab University, Chandigarh, for providing
instrument facilities. Author VNM is very much grateful to the UGC, New Delhi, for awarding the Rajiv
Gandhi National Fellowship.
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