Nano-cellulose/BF3/Fe3O4: a magnetic bio-based nano-catalyst for the synthesis of pyrimido[2,1-b]benzothiazoles under solvent-free conditions

Article abstract of DOI:10.1007/s11164-018-3401-5

Abstract: Nano-cellulose/BF₃/Fe₃O₄ is a bio-based and eco-friendly catalyst with high catalytic activity. Easy separation with an external magnet and recyclability without significant loss of its activity are some advantag

Full text of DOI:10.1007/s11164-018-3401-5

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3401-5
Nano-cellulose/BF3/Fe3O4: a magnetic bio-based
nano-catalyst for the synthesis of pyrimido[2,1-
b]benzothiazoles under solvent-free conditions
Bi Bi Fatemeh Mirjalili1
Fatemeh Aref1
•
Received: 5 January 2018 / Accepted: 16 March 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Nano-cellulose/BF3/Fe3O4 is a bio-based and eco-friendly catalyst with high
catalytic activity. Easy separation with an external magnet and recyclability without
significant loss of its activity are some advantages of this catalyst. Nano-cellulose/BF3/
Fe3O4 was characterized by Fourier transform infrared spectroscopy, field emission
scanning electron microscopy, transmission electron microscopy, X-ray diffraction,
X-ray fluorescence, vibrating sample magnetometry, thermogravimetric analysis, and
BET theory. The catalyst was applied for the synthesis of 4H-pyrimido[2,1-b]benzoth-
iazole derivatives via one-pot condensation of aldehydes, 2-aminobenzothiazol and ethyl
acetoacetate, under solvent-free conditions at 100 °C. This method offers several
advantages including easy work-up, excellent yields and short reaction time.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3401-5) contains supplementary material, which is available to authorized users.
&
Bi Bi Fatemeh Mirjalili
fmirjalili@yazd.ac.ir
1
Department of Chemistry, College of Science, Yazd University, P.O. Box 89195-741, Yazd,
Islamic Republic of Iran
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B. B. F. Mirjalili, F. Aref
Graphical Abstract
Keywords Nano-cellulose/BF3/Fe3O4 Á Bio-based catalyst Á Magnetic
nano-catalyst Á 4H-pyrimido[2,1-b]benzothiazole
Introduction
Today, many organic reactions are synthesized using acidic catalysts, while liquid
acid catalysts are less used due to the difficulty in separation, and their corrosive and
highly toxic nature [1]. By bonding liquid acids to an efficient support, they can be
converted to solid acid catalysts which lack the problems associated with liquid acid
catalysts and play an important role in green chemistry [2].
Nano-cellulose is a unique natural polymer with considerable physical and
biological properties such as biocompatibility, biodegradability and low toxicity [3].
It has been used in numerous fields such as medicine [4, 5], electronics [6], tissue
engineering [7] and pharmacy [8]. Due to its high surface area, thermal stability, and
surface OH groups, it can be used as a suitable support for the preparation of nano-
catalysts based on biopolymers [9].
Nano-catalysts with diameters of less than 100 nm are difficult to separate from
the reaction mixture and the use of magnetic nanoparticles in the catalyst structure is
a logical solution to overcome this problem. Among them, Fe3O4 nanoparticles have
attracted much attention [10] as they have advantages such as good stability, easy
synthesis and functionalization, availability, easy separation and low toxicity
[11, 12].
Heterocyclic compounds play a significant role in medicinal chemistry, and
among them, pyrimido[2,1-b]benzothiazoles show numerous biological activities
including antituberculosis [13], antibacterial [14, 15], antiviral [16], antifungal
[17–19], antitumor [20], anti-allergic [21], and anti-inflammatory [22, 23], and the
inhibition of lung cancer [24].
Numerous methods have been developed for the synthesis of pyrimido[2,1-
b]benzothiazoles, via three-component condensation of b-keto ester, aromatic
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Nano-cellulose/BF3/Fe3O4: a magnetic bio-based nano…
aldehydes and 2-amino benzothiazole. Various catalysts, such as 1,1,3,3-N,N,N0,N0-
tetramethylguanidinium trifluoroacetate (TMGT) [25], FeF3 [26], thiamine
hydrochloride (VB1) [27], tetrabutylammonium hydrogen sulfate (TBAHS) [28],
kaolin [29], acetic acid [30] and AlCl3 [19], have been applied for the preparation of
pyrimido[2,1-b]benzothiazoles. Some of these catalysts are faced with drawbacks
such as long reaction times, low performance, toxicity and lack of recoverability.
In this work, we report on nano-cellulose/BF3/Fe3O4 as a bio-based, magnetic
and biodegradable nano-catalyst for the synthesis of 4H-pyrimido[2,1-b]benzoth-
iazole derivatives via one-pot three-component condensation of ethyl acetoacetate,
aromatic aldehydes and 2-amino benzothiazole.
Experimental
General
All compounds were purchased from Merck, Aldrich and Fluka and used without
any additional purification. A refrigerated centrifuge (Eppendorf Centrifuge 5417R)
was used for the preparation of nano-cellulose. Fourier transform infrared (FT-IR)
spectra were run on a Bruker, Equinox 55 spectrometer. A Bruker (DRX-400
Avance) NMR was used to record the 1H-NMR spectra. Melting points were
determined by a Buchi melting point B-540 B.V.CHI apparatus and were
uncorrected. X-ray diffraction (XRD) patterns were obtained with a Philips Xpert
˚
MPD diffractometer equipped with a Cu Ka anode (k = 1.54 A) in the 2h range
from 10° to 80°. Field emission scanning electron microscopy (FESEM) was carried
out on a Mira 3-XMU, and transmission electron microscopy (TEM) using a Philips
CM120 with a LaB6 cathode and an accelerating voltage of 120 kV. X-ray
fluorescence (XRF) analysis was carried out with a Bruker S4 Explorer instrument.
Vibrating sample magnetometry (VSM) measurements were performed by using a
vibrating sample magnetometer (Meghnatis Daghigh Kavir, Kashan Kavir, Iran).
Brunauer–Emmett–Teller (BET) surface area analysis of the catalyst was carried out
with a Micromeritics, Tristar II 3020 analyzer.
Preparation of nano-cellulose from cotton
A sample of 4 g of cotton fibers was washed several times with distilled water, and
then to remove other compounds present in the fibers, such as lignin, hemicellulose,
wax and organic acids, with 160 ml of 17.5% NaOH solution for 24 h under reflux
conditions. After that, the fibers were washed with distilled water several times in
order to obtain a neutral pH. Then, to bleach the cotton fibers, 20 ml of sodium
hypochlorite solution plus 60 ml of distilled water were added to the cotton fibers
under reflux conditions for 2 h. Subsequently, the cotton fibers were again washed
with distilled water. In order for acid hydrolysis and the preparation of nano-
cellulose, 80 ml of the 35% sulfuric acid aqueous solution was added to the cotton
fibers under reflux conditions. After 6–7 h, the resulting suspension was diluted with
water, then centrifuged at 4000 rpm. To separate the resulting nano-cellulose from
123

B. B. F. Mirjalili, F. Aref
the acidic solution and to remove any remaining free acid, washing with water and
centrifugation were repeated several times.
Preparation of nano-cellulose/BF3/Fe3O4
An amount of 0.5 ml of BF3 was added dropwise to a mixture of nano-cellulose
(0.5 g) in 5 ml of dichloromethane and stirred for 1 h at room temperature. Then,
the resulting mixture was filtered and washed with dichloromethane and dried at
room temperature. Subsequently, the resulting nano-cellulose/BF3 with 5 g of Fe3O4
nanoparticles were dispersed in 5 ml of dichloromethane under ultrasound
irradiation at room temperature for 1 h. Then, the resulting suspension was filtered
and washed with dichloromethane and dried at room temperature so that he nano-
cellulose/BF3/Fe3O4 catalyst was obtained.
General procedure for synthesis of 4H-pyrimido[2,1-b]benzothiazole
derivatives
Nano-cellulose/BF3/Fe3O4 (0.06 g) as a catalyst was added to a mixture of aromatic
aldehyde (1 mmol), ethyl acetoacetate (1 mmol), and 2-aminobenzothiazole
(1 mmol). The mixture was stirred at 100 °C. After completion of the reaction
(monitored by TLC), the reaction mixture was dissolved in ethanol and the catalyst
was separated by an external magnet and reaction mixture was filtered. Then, by
adding water to the filtrate, the product appeared.
Results and discussion
In this work, nano-cellulose/BF3/Fe3O4 was prepared in several steps. At first, nano-
cellulose was obtained from the acid hydrolysis of cotton. Then, BF3 was added to
the mixture of nano-cellulose and CH2Cl2. The OH groups present in the nano-
cellulose act as a nucleophile and a B–O–C bond was formed by the interaction
between the BF3 and the OH groups. Then, the resulting nano-cellulose/BF3 was
added to the mixture of Fe3O4 nanoparticles/CH2Cl2. Some of the fluorine atoms
remaining in the BF3 were replaced by surface OH groups in the Fe3O4
nanoparticles, and nano-cellulose/BF3/Fe3O4 was prepared (Scheme 1).
Characterization of the nano-cellulose/BF3/Fe3O4 structure was performed using
various techniques such as FT-IR, FESEM, TEM, XRD, XRF, VSM, TGA and
BET. The FT-IR spectra of (a) nano-cellulose, (b) Fe3O4, (c) nano-cellulose/BF3/
Fe3O4 and (d) recovered nano-cellulose/BF3/Fe3O4 are shown in Fig. 1. The FT-IR
spectrum of nano-cellulose shows the stretching vibrations of the OH group at
3339 cm-1 and the stretching vibrations of the C–O group at around 1059 and
1162 cm-1. In the FT-IR spectrum of nano-cellulose/BF3/Fe3O4, in addition to the
above-mentioned bands, the stretching vibrations of the Fe/O bond appears at
560 cm-1. The band at 1371 cm-1 displays the stretching vibrations of the B–O
bond, indicating that the BF3 is supported on the nano-cellulose.
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Nano-cellulose/BF3/Fe3O4: a magnetic bio-based nano…
Scheme 1 Preparation of nano-cellulose/BF3/Fe3O4
(a)
(b)
(c)
(d)
Fig. 1 FT-IR spectra of a nano-cellulose, b Fe3O4, c nano-cellulose/BF3/Fe3O4 and d recovered nano-
cellulose/BF3/Fe3O4
The particle sizes of the nano-cellulose/BF3/Fe3O4 were investigated using
FESEM and TEM. The images in Fig. 2 indicate that the nano-cellulose/BF3/Fe3O4
particles are on average about 20 nm.
XRD patterns of nano-cellulose/BF3/Fe3O4 and Fe3O4 are shown in Fig. 3.
According to the XRD pattern of nano-cellulose/BF3/Fe3O4 (Fig. 3a), signals in 2h
equal to 23° and 13° are related to the cellulose. The signals in 2h = 30°, 35.5°, 43°,
53.5°, 57° and 62.9° show the existence of Fe3O4. Other peaks at around 2h = 13
and 28 are, it seems, related to the bonding of B on the surface of nano-cellulose.
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B. B. F. Mirjalili, F. Aref
Fig. 2 a FESEM image of nano-cellulose/BF3/Fe3O4, b TEM image of nano-cellulose/BF3/Fe3O4
Fig. 3 XRD patterns of the a fresh nano-cellulose/BF3/Fe3O4, b reused nano-cellulose/BF3/Fe3O4 and
c Fe3O4
The XRF method was used for elemental analysis of nano-cellulose/BF3/Fe3O4.
To determine the B:F ratio in the catalyst, the kilo counts per second (KCPS) of
elements in the catalyst was compared with the KCPS of elements in the pure
H3BO3 and NaF (Table 1). The amounts obtained for B and F are 3.496 g
(0.32 mol) and 1.112 g (0.059 mol), respectively. Thus, the B:F ratio, in the catalyst
is approximately 5:1.
Table 1 Results of XRF analysis of catalyst and standard samples of H3BO3 and NaF
Elemental component
Nano-cellulose/BF3/Fe3O4
NH3BO3
B%
NaF
F%
B%
F%
KCPS
KCPS
2.5
KCPS
40.7
B
F
3.496
–
–
0.5
1
17.48
–
–
1.112
45.25
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Nano-cellulose/BF3/Fe3O4: a magnetic bio-based nano…
The magnetic properties of Fe3O4 and nano-cellulose/BF3/Fe3O4 were charac-
terized at 300 K using VSM (Fig. 4). According to the hysteresis loop magnetism,
there is no remnant magnetization and also the coercivity value is zero for both
samples, which confirms the super-paramagnetic properties of the samples at room
temperature. The amounts of saturation magnetization for Fe3O4 and nano-
cellulose/BF3/Fe3O4 are * 49 and * 13 emu g-1, respectively. The results show
that the bonding of Fe3O4 to nano-cellulose/BF3/Fe3O4 causes a decrease in
saturation magnetization, but, despite this reduction, the catalyst can still be easily
separated from the reaction mixture by using an external magnet with 0.2-T
magnetic inductance.
The thermal stability of nano-cellulose/BF3/Fe3O4 was investigated using TGA in
the temperature range of 35–770 °C (Fig. 5). The TGA curve illustrates three stages of
weight loss. The first weight loss in the range of 95–150 °C (6% weight loss) is related
to the removal of moisture from the catalyst. The next weight loss (33%) appears in the
range of 263–335 °C and relates to the decomposition of cellulosic units in the
nanocomposite. Finally, the main weight loss (60%) is observed in the range of
690–720 °C. The char yield of the catalyst at 770 °C is * 78%.
The specific surface area of catalyst was measured by BET theory (Fig. 6). The
single point surface area at P/Po = 0.0685 is 65.9890 m2 g-1 and BET surface area
is 74.2480 m2 g-1. The N2 adsorption isotherm of the catalyst is depicted in Fig. 6a.
After characterization of nano-cellulose/BF3/Fe3O4, the catalytic activity was
investigated for the synthesis of 4H-pyrimido[2,1-b]benzothiazole derivatives via
the three-component reaction of aldehydes, 2-amino benzothiazole, and ethyl
acetoacetate. In order to optimize the reaction conditions, the model reaction of the
4-nitrobenzaldehyde, 2-amino benzothiazole, and ethyl acetoacetate under various
conditions was investigated (Table 2). According to Table 2, the best condition for
Fig. 4 Magnetization loops of a Fe3O4 and b nano-cellulose/BF3/Fe3O4
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B. B. F. Mirjalili, F. Aref
Fig. 5 Thermal gravimetric analysis pattern of nano-cellulose/BF3/Fe3O4
Fig. 6 a Nitrogen adsorption isotherms on nano- cellulose/BF3/Fe3O4 and b BET of nano-cellulose/BF3/
Fe3O4
the reaction is a solvent-free condition using 0.06 g of the catalyst at 100 °C
(Table 2, entry 14).
Based on optimal reaction conditions, 4H-pyrimido[2,1-b]benzothiazole derivatives
were synthesized using the three-component condensation of aldehydes (I, 1 mmol),
2-aminobenzothiazole (II, 1 mmol), and ethyl acetoacetate (III, 1 mmol) (Table 3).
Electronic effects and the position of the substituent on the aldehyde have considerable
effects on the yield and speed of the reaction. Electron-withdrawing groups have higher
activityandyieldcomparedtoelectron-donating groups. Thestructureoftheseproducts
was characterized by melting point, FT-IR, and 1H-NMR spectra.
In order to investigate the recoverability and reusability of the catalyst, the
above-mentioned model reaction in the presence of nano-cellulose/BF3/Fe3O4 at
100 °C under solvent-free conditions was repeated. After the completion of the
reaction, the catalyst was separated by an external magnet and washed several times
with dichloromethane and dried at room temperature. The quantity of the recovered
123

Nano-cellulose/BF3/Fe3O4: a magnetic bio-based nano…
Table 2 The reaction of 4-nitrobenzaldehyde (1 mmol), 2-aminobenzothiazole (1 mmol), and ethyl
acetoacetate (1 mmol) in the presence of nano-cellulose/BF3/Fe3O4 under various conditions
Entry
Solvent
Catalyst (g)
Condition
Time
Yield (%)a
1
–
–
R.T.c
7 h
–
2
–
–
100 °C
100 °C
100 °C
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
R.T.c
5 h
53
35
38
55
40
43
50
48
45
0
3
–
Fe3O4
3 h
4
–
Fe3O4@NCs
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.06)
Catalystb (0.04)
Catalystb (0.05)
Catalystb (0.07)
3 h
5
H2O
1.8 h
2 h
6
EtOH
7
CH3CN
2.5 h
2.5 h
1.7 h
3.5 h
5.3 h
70 min
60 min
45 min
45 min
70 min
60 min
65 min
8
H2O/EtOH
9
n-Hexane
10
11
12
13
14
15
16
17
18
CH3CO2Et
–
–
–
–
–
–
–
–
80 °C
90 °C
68.5
63
98
98
56
78
84
100 °C
110 °C
100 °C
100 °C
100 °C
aIsolated yield
bNano-cellulose/BF3/Fe3O4
cRoom temperature
catalyst is 97–99% of the original catalyst weight. The XRD pattern (Fig. 3b) and
FTIR (Fig. 1d) of the recovered catalyst show no significant change of its structure
under the reaction conditions. The recovered catalyst was then used in the model
reaction under the same conditions for four consecutive times without a significant
reduction in catalytic activity (Fig. 7).
The proposed mechanism for the synthesis of 4H-pyrimido[2,1-b]benzothiazoles
(IV), isshowninScheme 2. Onthe basisofthismechanism,first, ethyl acetoacetate(III)
reacts with benzaldehyde (I) according to the Knoevenagel condensation reaction, and
then the product thus obtained reacts with 2-aminobenzothiazole (II) through Michael
123

B. B. F. Mirjalili, F. Aref
Table 3 Synthesis of 4H-pyrimido[2,1-b]benzothiazole derivatives (IVa–l) in the presence of nano-cel-
lulose/BF3/Fe3O4 under solvent-free conditions at 100°C
Entry
R
Product
Time (min)
Yield (%)a
Melting point, °C [Refs.]
1
H
IVa
IVb
IVc
IVd
IVe
IVf
IVg
IVh
IVi
IVj
IVk
IVl
65
45
45
50
70
60
75
60
50
60
80
75
80
98
92
97
92
95
91
92
91
95
85
85
175–177 [28]
155–157 [28]
140–142 [27]
107–109 [25]
210–212 [19]
218–220 [31]
259–261 [32]
170–172
2
4-NO2
4-Cl
3
4
4-Br
5
4-OH
6
3-NO2
3-OH
7
8
2-NO2
2-Cl
9
125–127 [33]
104–106
10
11
12
2,4-(Cl)2
2,4-(OMe)2
3,4-(OH)2
164–166 [32]
227–229 [31]
The ratio of I (mmol):II (mmol):III (mmol):nano-cellulose/BF3/Fe3O4 (g) is equal to 1:1:1:0.06
aIsolated yield
Fig. 7 Catalyst recycling
experiments
96
95
93
91
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
Run
addition. Subsequently, by cyclization and water removal, 4H-pyrimido[2,1-b]ben-
zothiazole (IV) derivatives are synthesized. The catalyst used in this reaction acts as a
Lewis acid and accelerates the reaction by activating the carbonyl groups.
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Nano-cellulose/BF3/Fe3O4: a magnetic bio-based nano…
Scheme 2 Proposed mechanism for the synthesis of 4H-pyrimido[2,1-b]benzothiazole derivatives
(IVa–l
)
The performance of the nano-cellulose/BF3/Fe3O4 catalyst in the model reaction
was compared with the other reported catalysts in the synthesis of 4H-pyrimido[2,1-
b]benzothiazole derivatives and the results are summarized in Table 4. The results
show that with the nano-cellulose/BF3/Fe3O4 catalyst, 4H-pyrimido[2,1-b]benzoth-
iazole derivatives can be synthesized in a shorter time and with higher yields. The
magnetic property of the catalyst and the greenness of the process are its advantages
compared to other catalysts and so are important from an environmental standpoint.
123

B. B. F. Mirjalili, F. Aref
Table 4 Comparison of nano-cellulose/BF3/Fe3O4 catalyst with reported catalysts for the synthesis of
4H-pyrimido[2,1-b]benzothiazoles
Entry Catalyst
Condition Solvent
Time
Yield (%) [Refs.]
1
2
3
4
5
6
7
8
TMGTa (0.08 g)
100 °C
120 °C
Reflux
60–65 °C
80 °C
–
5 h
53 [25]
72 [28]
62 [30]
70 [19]
85 [33]
TBAHSb (30 mol%)
Acetic acid (20 mol%)
AlCl3 (10 mol%)
Ethylene glycol 2 h
Methanol
18 h
–
1.8 h
2 h
FeF3 (10 mol%)
–
(Mg–Al–CO3) (0.05 g)
VB1c (10 mol%)
70 °C
Reflux
–
45 min 79 [34]
Water
–
75 min 92 [27]
Nano-cellulose/BF3/Fe3O4 (0.06) 100 °C
45 min 98 [This work]
a1,1,3,3-N,N,N0,N0-tetramethylguanidinium trifluoroacetate
bTetrabutylammonium hydrogen sulfate
cThiamine hydrochloride
Conclusion
Given the importance of green chemistry, we have reported the synthesis and
characterization of nano-cellulose/BF3/Fe3O4 as a bio-based and eco-friendly
catalyst with high catalytic activity. It was applied for the synthesis of 4H-
pyrimido[2,1-b]benzothiazole derivatives via the condensation reaction of aromatic
aldehydes, 2-aminobenzothiazole, and ethyl acetoacetate. Easy work-up, excellent
yield, short reaction time, and reusability of the catalyst are some of the advantages
of this novel protocol.
Acknowledgement The Research Council of Yazd University is gratefully acknowledged for the
financial support for this work.
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