Magnetically separable Fe3O4@chitin as an eco-friendly nanocatalyst with high efficiency for green synthesis of 5-substituted-1H-tetrazoles under solvent-free conditions

Article abstract of DOI:10.1039/c6ra07252f

The present study describes an efficient, eco-friendly and simple method for the synthesis of 5-substituted-1H-tetrazoles catalyzed by magnetite-chitin (Fe₃O₄@chitin) as a green and recyclable catalyst. Fe₃O₄@chitin was initially prepared using hydrothermal synthesis. Subsequently, the structure, morphology, and magnetic properties of the prepared nanocatalyst were studied with some different spectroscopic, microscopic and thermogravimetric techniques such as FT-IR, XRD, SEM, TEM, VSM and TGA. Obtained results showed that Fe₃O₄@chitin nanoparticles exhibited uniform cubic shape and were well monodispersed. Also, magnetic measurement revealed that the synthesized nanocatalyst had superparamagnetic features. The application of this new nanocatalyst allows the synthesis of a variety of tetrazoles through the reaction of nitriles with 1-butyl-3-methylimidazolium azide ([bmim][N₃]) under solvent-free conditions. This synthetic pathway is a green protocol offering significant advantages, such as excellent yield of products in short reaction times, mild reaction conditions, minimization of chemical waste, easy preparation of the catalyst and its recyclability up to six cycles without any considerable loss of efficiency.

Full text of DOI:10.1039/c6ra07252f

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Magnetically separable Fe O @chitin as an eco-friendly
3
4
nanocatalyst with high efficiency for green synthesis of 5-
substituted-1H-tetrazoles under solvent-free conditions
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
DOI: 10.1039/x0xx00000x
Monireh Zarghani and Batool Akhlaghinia*
www.rsc.org/
The present study describes an efficient, eco-friendly and simple method for the synthesis of 5-substituted-1H-tetrazoles
catalyzed by magnetite-chitin (Fe @chitin) as a green and recyclable catalyst. Fe @chitin was initially prepared using
3
O
4
3 4
O
hydrothermal synthesis. Subsequently, the structure, morphology, and magnetic properties of the prepared nanocatalyst
were studied with some different spectroscopic, microscopic and thermogravimetric techniques such as FT-IR, XRD, SEM,
3 4
TEM, VSM and TGA. Obtained results showed that Fe O @chitin nanoparticles exhibited uniform cubic shape and were
well monodispersed. Also, magnetic measurement revealed that the synthesized nanocatalyst had superparamagnetic
feature. The application of this new nanocatalyst allows the synthesis of a variety of tetrazoles through the reaction of
3
nitriles with 1-butyl-3-methylimidazolium azide ([bmim][N ]) under solvent-free condition. This synthetic pathway is a
green protocol offering significant advantages, such as excellent yield of products in short reaction times, mild reaction
conditions, minimizing chemical waste, easy preparation of the catalyst and its recyclability up to six cycles without any
considerable loss of efficiency.
1
7-20
separated from the reaction mixtures by an external magnet.
1
. Introduction
Additionally, other important features of these nanoparticles are
high degree of chemical stability in various solvents, easy synthesis
and modification, high surface area, low toxicity, cost-effective and
During recent decades, there has been a considerable research
interest in the area of green chemistry using environmentally
benign reagents and clean synthetic procedures. In this context,
nanotechnology as one of the most fascinating developments can
be applied to produce safer and more sustainable nanostructure
2
1
benign character.
On the other hand, the recent scientific reports indicate that
natural polymers with many significant biological (biodegradable,
biocompatible and bioactive) and chemical properties (polycationic,
1
-5
material and more efficient chemical processes. Among the
various nanomaterial, magnetic nanoparticles as an eco-friendly
metal oxide, have received considerable attention because of their
hydrogel, reactive groups such as OH and NH
2
) can be used as
Chitin as linear
aminopolysaccharide to be found as skeletal scaffold in broad
22-25
heterogeneous catalytic systems.
a
6
potential applications, including magnetic fluids, magnetic
7
8
9
2
6
resonance imaging, biotechnology/biomedicine and data storage.
variety of fungi, protists, diatoms and invertebrates species, is
one of the most abundant biopolymer on earth after cellulose. This
biopolymer has been proven to be an extremely promising material
because of its many advantageous characteristics such as
biodegradability, biocompatibility, excellent chelation behavior, and
Moreover, recent search has been shown that magnetic
nanoparticles could be efficient and promising supports for
10-15
catalysts.
From the viewpoint of green chemistry, it is useful to
develop new catalyst recycling methods to replace conventional
1
6
2
7
procedures such as centrifugation and filtration. Thus, magnetite
nanoparticles became the strong candidate due to unique
paramagnetic properties which allow them to be efficiently
the least toxicity. Chitin composed of 2-acetamide-2-deoxy-D-
glucopyranose, has highly crystalline structure due to strong
hydrogen bonding of two hydroxyl and an acetamide groups in its
linear structure. In addition, it has been found that hydroxyl groups
and N-acetyl amide group in the glucose ring of chitin have the
2
8
excellent chelation capacity with metal ions.
Other important feature of this biopolymer is high thermal
stability up to 360 °C. Literature survey shows that chitin is a stable
biopolymer under common hydrothermal conditions and could be
effectively used as a scaffold for hydrothermal mineral deposition.
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In this regard, there have been a variety of reports describing the
use of chitin as a template for the hydrothermal synthesis of
29-
nanocrystals such as ZnO, ZrO
2
, SiO
2 2 3 2
, and Fe O (hematite), GeO .
3
3
Recently, the chemistry of heterocycles has acquired immense
importance. Among this, 5-substituted-1H-tetrazoles which
represent an important class of heterocyclic compounds have
attracted great attention in recent years due to their wide range of
applications. Most important applications of these compounds
include their use in pharmaceuticals, material sciences,
coordination chemistry and recognized as a useful intermediate in
3
4-38
synthetic organic chemistry.
Up to now, numerous methods
have been reported for the preparation of 5-substituted-1H-
3
9-43
tetrazoles in the presence of various reactants and catalysts,
but each has its own disadvantage, such as the use of strong Lewis
acids, expensive reagents, longer reaction times, tedious separation
procedures and use of large amount of toxic solvents. Therefore, it
is of great practical importance to develop a more efficient and
environmentally benign method that avoids these drawbacks.
Consequently, due to our interest towards the extension of
44-47
environmentally friendly methods for the synthesis of tetrazoles
and based on our previous experience in this respect (that
hydrogen bonding of chitin could be catalyzed this cycloaddition
4
6
reaction), we decided to prepare and characterize magnetite-
chitin nanocatalyst as a novel, green and recyclable heterogeneous
catalyst (Scheme 1). Then synthesis of tetrazoles via reaction
between nitriles and azide ion was examined in solvent free
Scheme 1 Preparation of Fe O @chitin.
3
4
6 3
DMSO-d and CD CN. Elemental analysis was performed using a
3
condition. 1-Butyl-3-methylimidazolium azide [bmim][N ] was used
Thermo Finnigan Flash EA 1112 Series instrument. Mass spectra
were recorded with a CH7A Varianmat Bremem instrument at 70 eV
electron impact ionization, in m/z (rel %). TGA analysis was carried
out on a Shimadzu Thermogavimetric Analyzer (TG-50) in the
temperature range of 25-900 ᵒC at a heating rate of 10 ᵒC /min
under air atmosphere. Transmission electron microscopy (TEM) was
performed with a Leo 912 AB microscope (Zeiss, Germany) with an
accelerating voltage of 120 kV. SEM images were also recorded
using Leo 1450 VP scanning electron microscope operating at an
acceleration voltage 20 kV. The crystal structure of catalyst was
analyzed by XRD using a D8 ADVANCE–Bruker diffractometer
operated at 40 kV and 30 mA utilizing CuKɑ radiation (λ= 0.154 Aᵒ).
The magnetic property of catalyst was measured using a vibrating
sample magnetometer (VSM, 7400 Lake Shore). Inductively-coupled
plasma mass spectrometry (ICP-MS) was carried out on a Varian
Australia. All of the products were known compounds and they
as the azide ion source in the present study. The results showed
that magnetite-chitin nanocatalyst is highly efficient in this
cycloaddition reaction, and it can be easily separated from the
reaction mixture.
2
. Experimental
2.1. Chemicals and instruments
All chemical reagents and solvents were purchased from Merck
and Sigma-Aldrich chemical companies and were used as received
without further purification. Chitin was extracted from cuttlebone
46
which was taken out from cuttlefish (Sepia esculenta). It is
commonly found in saltwater beaches like Persian Gulf in Iran.
The purity determinations of the products were accomplished by
TLC on silica gel polygram STL G/UV 254 plates. The melting points
of products were determined with an Electrothermal Type 9100
melting point apparatus. The FTIR spectra were provided on
pressed KBr pellets using an AVATAR 370 FT-IR spectrometer
1
13
were characterized by the FT-IR spectroscopy, H NMR, C NMR
spectroscopy, and mass spectrometry and comparison of their
melting points with known compounds.
(
Therma Nicolet spectrometer, USA) at room temperature in the
−1
range between 4000 and 400 cm . The NMR spectra were obtained
in Brucker Avance 100, 300 and 400 MHz instruments in CDCl
3
,
2.2. Extraction of ß-chitin from cuttlebone
2
| J. Name., 2012, 00, 1-3
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A cuttlebone (≈15 g) was cut into 1 × 1 cm pieces and soaked
PleaseRd So Cn oA t da vd aj un s ct ems argins
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DOI: 10.1039/C6RA07252F
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for 12 h in 500 mL of 2 M HCl under low vacuum. A low vacuum was
used to exclude CO bubbles, which formed during dissolution of
2
the aragonite component, from the internal chambers of the
organic matrix. The procedure was repeated overnight in fresh
aqueous 2 M HCl. The pieces were then washed with distilled water
until neutral pH and added to 500 mL of 1 M NaOH at a boiling
temperature for 4 h to remove associated proteins. The samples
were filtered, washed with copious amounts of water, and added to
9
5% ethanol for 3 h, followed by further washing with distilled
27a
water. The organic matrix was stored in water at 4 °C.
2
3 4
.3. Hydrothermal synthesis of the Fe O @chitin
Into a three-necked 500 mL round-bottom flask equipped with Fig. 1 FT-IR spectra of (a) Chitin and (b) Fe O @chitin.
3
4
argon gas inlet tube and dropping funnel, 2 g of chitin was
dispersed in 150 mL of deionized water. Then, FeCl .6H O (46
mmol, 12.4 g) and FeSO .7H O (23 mmol, 6.3 g) were added to the
white suspension. The resulting mixture was mechanically stirred
for min at room temperature under Ar atmosphere.
3
2
A mixture of benzonitrile (0.103 g, 1 mmol) and [bmim][N
3
] (0.216
4
2
g, 1.2 mmol) in the presence of 0.03 g of Fe O @chitin was stirred
3
4
o
at 110 C. The progress of the reaction was monitored by TLC.
When the reaction was completed, the catalyst was removed
magnetically and then 10 mL HCl (4 N) was added to the resulting
solution. Finally, the obtained aqueous solution was extracted with
ethyl acetate (2 × 10 mL) and the combined organic phase was
washed with distilled water (2 × 20 mL), dried over anhydrous
sodium sulfate and concentrated to obtain the the white
precipitate. The crude 5-phenyl-1H-tetrazole was recrystallized
from n-hexane / ethylacetate (1 : 1) obtaining 0.1386 g of colourless
crystals (95% yield).
3
Consequently, 20 mL ammonium hydroxide (25%) was dropped
very slowly into the mixture under vigorous stirring. The resulting
o
black mixture was continuously stirred for 5 h at 75 C under Ar
atmosphere. Finally, the mixture was permitted to cool at room
3 4
temperature and Fe O @chitin nanoparticles were separated by an
external magnet and washed with deionized water several times
before being dried under vacuo at 50 ˚C overnight.
2
.4. Synthesis of 1-butyl-3-methylimidazolium chloride
1
-Chlorobutane (10.8 g, 0.8 mol) was added to 1-methyimidizole
(
8.2 g, 0.1 mol), in a round bottomed fask equipped with a reflux 3. Results and Discussion
o
condenser. The mixture was stirred at 70 C for 48 h. After
formation of two phases, the top layer which contains unreacted
starting material was decanted. Then ethyl acetate (30 mL) was
added with entire mixing followed by its decantation and this step
was repeated three times. After the third decanting of ethyl
3
.1. Characterization of catalyst
The Fe @chitin as a magnetically heterogeneous nanocatalyst
was prepared by hydrothermal method which consists of the
3
O
4
2
+
3+
4
reaction between Fe and Fe ions and NH OH in the presence of
chitin (see Scheme 1). The composition of the synthesized
nanocatalyst was analyzed by some technical methods such as
Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction
o
acetate, remaining solvent was removed by heating at 70 C. The
obtained pale yellow liquid was vacuum distilled and the resulting
1
-butyl-3-methylimidazolium chloride was placed for vacuum drying
(
XRD), transmission electron microscopy (TEM), scanning electron
o
48
at 80 C in a vacuum drying oven.
microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX),
thermogravimetric analysis (TGA) and vibrating sample
magnetometry (VSM).
2
.5. Synthesis of 1-butyl-3-methylimidazolium azide
Freshly prepared 1-butyl-3-methylimidazolium chloride (3.49 g, 20
mmol) and NaN3 (1.30 g, 20 mmol) were added into 25 mL
deionized water, and the mixture was stirred for 24 h at room
FT-IR spectroscopy is an appropriate technique to identify
surface chemical structure of nanocatalyst. The FT-IR spectra of
chitin as well as the magnetite-chitin nanocatalyst in the range of
−1
temperature. The solvent was removed under reduced pressure at
4
000–400 cm have been depicted in Fig. 1a and 1b respectively.
o
5
0 C. To separate 1-butyl-3 methylimidazolium azide from the
The FT-IR spectrum of chitin was established by the following
crude product (which contains azide ionic liquid and NaCl), the
resulting mixture was washed with acetonitrile (3 ×10 mL).
absorption bands: O–H and N–H stretching vibrations around 3460-
−1
−1
3
250 cm , C–H stretching vibrations at 2977 and 2870 cm , N–H
−1
The remaining acetonitrile was removed under high vacuum to yield
yellow transparent liquid that became more viscous upon extensive
bending vibrations at 1680 cm . Furthermore, the specific amide
−1
−1
bonds were detected at 1662 cm (amide I), 1550 cm (amide II)
4
9
−1
−1
drying. Isolated yield was 92% (3.33 g).
and 1315 cm (amide III). The intense band located at 1475 cm
(
C–N stretching vibrations) and the broad bands ranging from 1160
−1
2
.6. Typical procedure for synthesis of 5-phenyl-1H-tetrazole in
to 1010 cm (C–O–C and C–O stretching vibrations) are indicative
-1
the presence of the Fe @chitin
3
O
4
of chitin. In addition, the other strong chitin band at 860 cm
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(
related to –CH
3
in acetylamide groups) can be clearly observed in This fact suggested that the coating process did not result in the
phase change of Fe nanoparticles.
To elucidate the morphology and size of the synthesized
nanoparticles was confirmed by nanocatalyst, Fe @chitin was analyzed by SEM and TEM (Fig. 3).
5
0
Fig. 1a.
3 4
O
The FT-IR spectrum of Fe O @chitin was shown as Fig. 1b. As
3
4
can be seen, the presence of Fe
3
O
4
3 4
O
-1
detecting absorption band at around 603 cm which corresponding It is evident from SEM image (Fig. 3a) that the shape of
51
to the Fe-O stretching bond of magnetite nanoparticles. On the Fe O @chitin nanocatalyst is not clearly defined, however,
3
4
other hand, although the all characteristic bands of chitin were synthesized nanocatalyst is well dispersed. The TEM micrograph of
present in the FT-IR spectrum of nanocatalyst but some of them the Fe @chitin shown in Fig. 3b revealed that the synthesized
exhibited noticeable changes. For instance, after incorporation of nanoparticles existed as cubicles with a mean edge length of 25 nm
3
O
4
52
5
5
magnetite in the chitin matrix, hydroxyl and acetyl groups and were nearly monodisperse. Obtained results indicated that
frequencies (which related to absorption bands appearing around the presence of chitin during the synthesis of Fe nanoparticles,
460 and 3250 cm in chitin) shifted to 3420 and 3260 cm in leads to improvement of the nanoparticles size and distribution.
Fe @chitin, respectively. Its occurrence may indicate an Energy-dispersive X-ray analysis (EDX), indicated the presence of C,
interaction between the Fe nanoparticles and the –OH and –NH O and Fe elements (Fig. 4), which further supported the conclusion
stretching vibration of chitin. Splitting of the absorption bands at that Fe nanoparticles have been synthesized successfully in the
(amide II) (due to –NH bending presence of chitin.
vibrations) and 857-876 cm (–CH in acetylamide groups) also
3
O
4
-1
-1
3
3 4
O
3
O
4
3
O
4
−1
1
620 (amide I) and 1550 cm
-1
3
Thermogravimetric analysis (TGA) of the Fe
used to determine the content of organic materials of the
nanocatalyst, as shown in Fig. 5. Upon heating, Fe @chitin
showed a weight loss of about 6.24 % at temperatures ranging from
0 to 140 °C, mainly due to the loss of water, because
3 4
O @chitin was also
confirmed these interactions. Thus, overall the observation
revealed that Fe O @chitin as a green nanocatalyst was successfully
3
4
O
4
synthesized via an environmentally friendly process.
3
Also, XRD analysis was applied to detect the crystallinity of the
synthesized nanocatalyst (Fig. 2). As shown in Fig. 2a, the
characteristic peaks with strong intensities appeared at angles
corresponding to (2, 2, 0), (3, 1, 1), (4, 0, 0), (4, 2, 2), (5, 1, 1) and (4,
5
polysaccharides usually have a strong affinity for water and
therefore may be easily hydrated. The second weight loss (3.02 % in
the range of 160–340 °C) was attributed to the partial
decomposition of chitin chain. After that, there was a significant
4
, 0) reflections were in good agreement with the standard pattern
5
3
for crystalline magnetite with cubic structure (JCPDS 19-0629).
o
o
weight loss (about 9.01 %) from 350 C to 650 C corresponding to
3
Moreover, in comparison with the standard pattern of pure Fe O
4
the complete decomposition of biopolymer backbone. In
MNPs, two additional small and weak peaks (at 9.6 and 19.6 angles)
were found in the XRD pattern of Fe @chitin which could be
attributed to the characteristic peaks of chitin (Fig. 2b).
2
7b, 29-33
comparison with literature,
it can be concluded that chitin
3
O
4
5
4
has somehow greater stability due to interaction with Fe
3 4
O
nanoparticles.
The magnetic property of Fe O @chitin, which was critical to
3
4
ensure its application, was measured using VSM. The hysteresis
loop of synthesized nanocatalyst is shown in Fig. 6. The results of
VSM showed that the magnetic saturation value of Fe O @chitin
3
4
-1
was 32.5 emμ·g , which was lower than the reference value for
5
3
pure Fe
3 4
O MNPs. It may be ascribed to the cover of chitin
polymeric matrix on the surface of magnetic Fe O MNPs.
3
4
Moreover, it is clearly seen from this figure that no remanence and
coercivity was observed, which indicated that the nanoparticles
obtained in this study are superparamagnetic. These magnetic
properties allow easily and quickly separation of the nanocatalyst
from the reaction mixture with the help of an external magnet.
3 4
Fig. 2 The XRD patterns of (a) Chitin and (b) Fe O @chitin.
4
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Fig. 6 (a) Magnetization curve of Fe
3
O
4
@chitin (b) Separation of
nanocatalyst using an external magnet.
3 4
Fig. 3 (a) SEM image of Fe O @chitin, (b) TEM image of
3 4
Fe O @chitin.
3
.2. Catalytic synthesis of 5-substituted-1H-tetrazoles
Recently, we reported an efficient and green synthesis of 5-
substituted-1H-tetrazoles from organic nitriles and azide ion using
highly porous expanded perlite as a natural heterogeneous catalyst
4
7
in high yields. Accordingly, 1-butyl-3-methylimidazolium azide
(
[bmim][N ]) ionic liquid was used as an azide ion source instead of
3
3 3
the highly toxic reagents such as NaN or TMSN . Also, the
cycloaddition reaction was performed in solvent free condition that
in comparison with the most reported literature, it was very eco-
friendly procedure. These advantages become even more attractive
if such reactions can be catalyzed by using easily recyclable catalyst
such as magnetic nanoparticles. Hence, according to our previous
experience and in order to extend the range of high efficient
heterogeneous catalyst, we now report a new method for the
preparation of 5-substituted-1H-tetrazoles via reaction between a
3 4
Fig. 4 The EDX analysis of Fe O @chitin.
wide variety of nitriles and [bmim][N
free condition by using Fe @chitin as a simple, green, and
recyclable catalyst (Scheme 2).
3
] as an azide source in solvent
3
O
4
To obtain the best conditions for the synthesis of 5-substituted-
1
H-tetrazoles, initial studies were performed upon the reaction of
benzonitrile with 1-butyl-3-methylimidazolium azide as a model
reaction and the effects of different conditions were investigated
for this reaction (Table 1). According to Table 1, when the model
o
reaction was performed under catalyst-free conditions at 100 C,
the desired product was obtained in low yield (Table 1, entry 1),
3 4
whereas, using 0.03 g of Fe O @chitin in [3 + 2] cycloaddition
reaction as a nanocatalyst afforded high yield of the product (90%)
in short reaction time (20 min) (Table 1, entry 2). To study the effect
of different molar ratios of reactants, [3 + 2] cycloaddition reaction
was performed with different molar ratios of benzontrile /
3 4
Fig. 5 TGA thermogram of Fe O @chitin.
[
bmim][N ] (Table 1, entries 2, 3 and 4). As shown, increasing the
3
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Journal Name
R
N
Fe O @chitin
nanoparticles as catalyst, the desired product was obtained in low
yield and in long reaction time (50 %, 1h), while, the same reaction
in the presence of 0.03 g chitin gave admirable yield (70 %) in short
reaction time (30 min) (Table 1, entry 10). In comparison with
3
4
N
R
C
N
[
bmim][N ], 110 °C
HN
1
N
3
-18
[bmim][N3]:
N
N
N3
catalytic performance of Fe
3 4
O @chitin (95% yield in 20 min vs 70 %
in 30 min), it could be concluded that Lewis acidity of Fe
3 4
O
R
=
Ph, 4ꢀBrC H , 4ꢀClC H , 4ꢀNO C H , 4ꢀCNC H , 2ꢀNH ꢀ5ꢀNO C H ,
56
6
4
6
4
2
6
4
6
4
2
2 6 3
nanoparticles may promote the [3 + 2] cycloaddition reaction.
However, it should be mentioned that the aim of supporting chitin
3
,5ꢀMeOC H , 3ꢀMeC H , 4ꢀOHC H , 2ꢀOHC H , 4ꢀEtOC H , 9ꢀphenanthrene,
6 3 6 4 6 4 6 4 6 4
4
ꢀpyridine, 2ꢀpyridine, 2ꢀthiophene, PhCH , (CH ) CHCH , (CH ) CH(CH )
2 2
2
3 2
2
3 2
Scheme 2 Synthesis of 5-substituted-1H-tetrazoles in the presence on the surface of magnetic nanoparticles was its easy magnetically
of Fe
3
O
4
@chitin under solvent-free conditions.
separation, its recovery and its reusability.
To assess the efficiency and scope of the present methodology,
various 5-substituted-1-H-tetrazoles were synthesized by the [3 + 2]
cycloaddition reaction between a wide range of structurally
3
amount of [bmim][N ] was found to have no effect on the yield
3 3 4
divergent nitriles and [bmim][N ] in presence of Fe O @chitin
(90%) of [3 + 2] cycloaddition reaction. In contrast, decreasing the
under the optimized reaction conditions. The representative results
are summarized in Table 2. Generally, the aromatic nitriles
containing electron withdrawing and electron-donating substituent
were well tolerated under the reaction conditions and gave the
corresponding tetrazoles in good to excellent yields. However, as it
is shown in Table 2, reactions of electron poor nitrile compounds
were completed within a short reaction time, whereas electron rich
nitriles required more reaction time to proceed properly (compare
entries 1–5 with entries 6–11). So, it can be concluded that the
electronic character of substituents on the aromatic rings of the
nitriles had a noteworthy effect on the efficiency of the reaction
which demonstrated that electron donating substituents on
aromatic rings decrease the electrophilic character of nitrile. In the
same way, the reaction was examined using phenanthrene-9-
carbonitrile in which the desired product was obtained in good
yield, although longer reaction time was required which may be due
to the steric hindrance (Table 2 entry 12).
amount of [bmim][N ] dropped down the yield reaction a lot. The
3
effect of temperature on the reaction was also evaluated (Table 1,
entries 5, 6). The best result was achieved by carrying out the
o
reaction at 110 C, affording 95 % of 5-phenyl 1H- tetrazole in 20
min. Different amount of the catalyst (0.035, 0.03 and 0.02 g) was
also optimized and it was found that 0.03 g of catalyst gave the
maximum yield of the product (Table 1, entries 7, 8). An increase in
the amount of catalyst did not improve the results to any great
extent. However using 0.02 g of catalyst could not provide
satisfactory yield. Comprehensively, the optimum reaction
conditions for synthesis of 5-phenyl-H-tetrazole were as follows: 1 :
o
1
.2 molar ratio of reactants at 110 C in the presence of 0.03 g of
Fe O @chitin as catalyst, under solvent free conditions.
3
4
3 4
Consequently, we checked the effect of Fe O nanoparticles and
chitin on the [3 + 2] cycloaddition reaction, by carrying out the
reaction under the optimized reaction conditions, separately (Table
3 4
1, entries 9 and 10). It was observed that, by using 0.03 g of Fe O
Table 1 Optimization of various reaction parameters for the synthesis of 5-phenyl-1H-tetrazole
Entry
Molarratio
Catalyst (g) Temperature (ᵒC)
Time
(min)
Isolated yield (%)
(
3
Benzontrile/[bmim][N ])
1
2
3
4
5
6
7
8
1:1.2
1:1.5
1:1.2
1:1
-
100
100
100
100
120
110
110
110
110
110
1(h)
20
10
90
90
70
95
95
85
95
50
70
0.03
0.03
0.03
0.03
0.03
0.02
0.035
0.03
0.03
20
20
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
20
20
20
20
a
9
1(h)
30
b
10
a
b
The reaction was performed in the presence of Fe O NPs. The reaction was performed in the presence of chitin.
3
4
6
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
Furthermore, when heteroaromatic nitriles such as thiophene-2- afford the desired products in admirable yields (Table 2 entries 16,
carbonitrile, 4-pyridinecarbonitrile and 2-pyridinecarbonitrile were 17 and 18). These results demonstrated the superior properties of
used as substrate, due to the strong electron withdrawing effect of Fe
the heteroatoms, excellent yields of the corresponding 5- catalysts for the synthesis of 5-substituted-1-H-tetrazoles by [3 + 2]
substituted-1-H-tetrazoles were obtained in a short reaction time cycloaddition from nitriles and [bmim][N ].
Table 2 entries 13, 14 and 15). It is noteworthy that not only
aromatic nitriles but also aliphatic ones could also be employed to
3 4
O @chitin as an eco-friendly and effective heterogeneous
3
(
Table 2 Synthesis of different structurally 5-substituted-1H-tetrazoles in the presence of Fe O @chitin under solvent-free condition.
3
4
R
N
Fe
3 4
O @chitin
N
R
C
N
[
bmim][N ], 110 °C
3
HN
N
Entry
R
Product
Time
Isolated Yield
(%)
(
min)
1
C
6
H
5
20
95
2
3
4
4-BrC
6
H
4
30
20
15
90
95
98
6 4
4-ClC H
4-NO C H
2
6
4
5
4-CNC
6
H
4
15
95
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Journal Name
6
2-NH
2
-5-NO
2
C
6
H
3
30
94
7
3,5-Di-MeOC H
120
90
6
4
8
3-MeC
6
H
4
120
92
9
4-OHC
2-OHC
6
H
4
60
45
85
92
1
0
1
6
H
4
1
4-EtOC
6
H
4
45
88
1
2
9-Phenanthrene
120
70
8
| J. Name., 2012, 00, 1-3
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Journal Name
PleaseRd So Cn oA t da vd aj un s ct ems argins
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ARTICLE
1
3
4-Pyridine
30
95
1
4
2-Pyridine
60
87
1
1
5
6
2-Thiophene
30
95
85
C H CH
2
120
6
5
1
1
7
8
(CH
3
)
2
CHCH
2
120
90
85
94
3 2 2 2
(CH ) CH(CH )
In order to establish the structure of synthesized tetrazoles, their
The catalytic activity of Fe O @chitin in [3 + 2] cycloaddition
3 4
melting points were initially compared with the known compounds. reaction was proved by the fact that a long reaction time and
The data were found to be in good agreement with expected comparatively lower yield of product was observed when the model
values. Then to further elucidate the structure of these compounds, reaction was carried out in absence of Fe O @chitin which do not
3
4
spectroscopic methods such as FT-IR spectroscopy, mass afford formation of desired product (Table 2, entry 1). Based on the
1
13
spectrometry, H NMR and C NMR spectroscopy were recorded. literature, as well as our findings, plausible reaction pathway is
4
6, 57
Key features in the FT-IR spectra of the products include the shown in Scheme 3.
Initially, we assume that nitrile
-1
absorption bands at: 3485–3329 cm (related to the N–H stretching functionality is activated through hydrogen bond formation
-1
vibration), 1469–1430 cm (due to scissoring bending C–H), 1293– between nanocatalyst hydroxyls and nitrogen atom of nitrile group,
-1
-1
1
233 cm (due to N–N=N–), 1189–1110 and 1106–1041 cm (due so facilitate the attack of azide ion which leads to the formation of
1
13
to tetrazole ring). Also, in HNMR and C NMR spectra, a signal at intermediate
0.98–7.80 and 164–151 ppm which correspond to the NH and the [3 + 2] cycloaddition reaction between activated nitrile and
quaternary carbon NH–C=N, respectively, can be beneficial for azide ion. At last, protonolysis of the intermediate II by HCl (4 N)
I. In the next step, the intermediate II is afforded from
1
further confirming the structure of desired products.
This journal is © The Royal Society of Chemistry 20xx
causes to produce III and IV. However, the more stable tautomer IV
is achieved due to the equilibrium (5-substituted 1H-tetrazole). The
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Journal Name
nanocatalyst re-enters to the catalytic cycle by making the active
sites for further turnovers.
One of the main advantages associated with supporting a
3 4
catalyst onto Fe O MNPs is the possibility of recovery by simple
magnetic decantation and reuse without loss of activity. So, in
relevance with this, the activity of the recycled catalyst was
3
examined using the reaction of benzonitrile and [bmim][N ] under
the optimized conditions. In order to regenerate the nanocatalyst,
upon completion of reaction, it was separated magnetically and
washed several times with deionized water and ethyl acetate to
remove residual compounds. The resultant nanocatalyst was dried
o
at 50 C under vacuum, and reused for subsequent runs. As shown
in Fig. 7, the recycled catalyst could be reused up to six runs
without any noteworthy loss of activity, which demonstrates the Fig. 7 Recovery and reuse of Fe
practical recyclability of the synthesized nanocatalyst. Moreover, in cycloaddition reaction.
order to investigate the amount of Fe leaching in these reactions,
3 4
O @chitin catalyst for [3 + 2]
after magnetically separation of the nanocatalyst, the resulting
mixture was analyzed by ICP-MS before and after the six cycles. The
results showed the absence of Fe in the first run, while a very small
amount of Fe (0.0021 mmol ) was detected after six runs. It
indicates that 95 % of the nanocatalyst could be recovered in this
method.
Additionally, this nanocatalyst promoted the synthesis of 5-
substituted-1H-tetrazole in solvent-free conditions instead of toxic
solvents, which follows along the line of green chemistry.
In addition, a comparative study of the reported methods in the 4. Conclusions
literature on [3 + 2] cycloaddition reaction was compiled in Table 3
In summary, with the aim of have a good contribution in innovation
to highlight the advantages of this nanocatalyst. Table 3 clearly
demonstrates that lower temperature and shorter reaction time
were required for this cycloaddition reaction, while, several of the
of green chemistry, we have reported an operationally simple, clean
and highly efficient procedure for the synthesis of 5-substituted-1H-
tetrazoles using Fe
3
O
4
@chitin nanocatalyst in a green media. As a
@chitin was
5
8-
previously reported procedures did not provide these conditions.
novel and eco-friendly heterogeneous catalyst, Fe
3 4
O
6
2
However, recently, we have been succeeded to perform this [3 +
] cycloaddition reaction in a very short time by applying new
easily prepared and characterized by spectroscopic, magnetic and
thermal techniques (TEM, SEM, FT-IR, XRD, VSM and TGA). The
2
4
6, 47
natural heterogeneous catalyst.
Similarly, the catalytic system
3 4
ultimate material was found to be Fe O @chitin nanocatalyst with a
presented in the present study offers excellent yields in short
reaction times with the difference that the Fe @chitin can be
mean edge length of 25 nm, cubic shape, and strong magnetic
property. Then synthesized nanocatalyst has been used as an
efficient catalyst for the preparation of various 5-substituted-1-H-
3
O
4
easily separated from the reaction mixture by using an external
magnet.
3
tetrazoles via reaction between nitriles and [bmim][N ] as the azide
ion source under solvent-free condition. Excellent results were
obtained in each case affording the corresponding tetrazoles in
short reaction times and high yields. Furthermore, this process
avoids problems associated with organic solvent and toxic reagent
use, which makes it a useful and attractive protocol in view of the
economic and environmental advantages. Apart from this,
simplicity of product separation and effortless reusability of
nanocatalyst are the important features of this method.
O
OH
O
HN
O
[
bmim]Cl
+
OH
O
O
NH
O
O
O
HO
H
N
N
N
N
N
R
R
3 4
Fe O NP
NH
N
[bmim]N
+
N
3
IV
III
R
C
N
HCl
N
R
N
N [bmim]
N
N
N
R
[bmim]
N
C
N
Acknowledgements
O
O
OH
OH
O
HN
O
HN
O
OH
OH
O
The authors gratefully acknowledge the partial support of this study
by Ferdowsi University of Mashhad Research Council (Grant no.
p/3/29760).
O
O
O
O
NH
O
O
O
NH
O
O
O
HO
HO
Fe
3
O
4
NP
Fe
3
O
4
NP
I
II
Scheme 3 Plausible mechanism for the formation of 5-substituted-
1
H-tetrazoles in presence of Fe O @chitin.
3 4
1
0 | J. Name., 2012, 00, 1-3
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ARTICLE
Table 3 Comparison between efficiency of Fe O @chitin and some other catalysts for the synthesis of 5-phenyl-1H-tetrazole.
3
4
Entry
Catalyst
Solvent
Temperature (°C)
Time (h)
Yield (%)
58
1
2
Fe O @SiO /Salen Cu(II)
DMF
DMF
120
120
7
90
82
3
4
2
5
9
CuFe
2
O
4
12
6
0
3
4
5
6
7
8
Ag NPs
DMF
DMF
120
120
110
140
130
120
8
92
90
93
98
95
98
6
2
1
Cyanuric chloride
2
6
CAN/HY-Zeolite
DMF
4
4
4
CuSO
4
.5H
2
O
DMSO
DMSO
DMSO
1
1
4
5
CAES
4
6
Cuttlebone
20 (min)
4
7
9
Expanded Perlite
Fe @chitin
Solvent-free
Solvent-free
110
110
10 (min)
20 (min)
95
95
10
3 4
O
2
0
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2 | J. Name., 2012, 00, 1-3
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Graphical Abstract
Magnetically separable Fe O @chitin as an eco-friendly nanocatalyst with high efficiency for
3
4
green synthesis of 5-substituted-1H-tetrazoles under solvent-free conditions
O
OH
O
HN
O
OH
O
NH
O
O
O
HO
O
Monireh Zarghani, Batool Akhlaghinia *
Fe O
3
4
N
HN
R
N
[
bmim]N3
N
R
C
N
o
Solvent free, 110 C
1
. Eco-friendly Fe O @chitin nanocatalyst was prepared, characterized and applied for
3 4
synthesis of 5-substituted-1H-tetrazoles in a green media.