A novel magnetically recyclable silver-loaded cellulose-based bionanocomposite catalyst for green synthesis of tetrazolo[1,5-a]pyrimidines

Article abstract of DOI:10.1007/s11164-017-2941-4

5-Methyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic ester derivatives were efficiently synthesized by reaction of methyl or ethyl acetoacetate, 5-aminotetrazole (produced from 2-cyanoguanidine and sodium azide), and various aromatic aldehydes in presence of a new cellulose-based Ag-loaded magnetic bionanostructure. This protocol offers many advantages such as short reaction time, high yield, the remarkable magnetic property of the nanocomposite, and easy separation of the nanocatalyst from the reaction mixture without considerable loss of catalytic activity. Furthermore, X-ray diffraction analysis, field-emission scanning electron microscopy, and energy-dispersive X-ray (EDX) analysis were used to characterize the prepared nanocatalyst.

Full text of DOI:10.1007/s11164-017-2941-4

Res Chem Intermed
DOI 10.1007/s11164-017-2941-4
A novel magnetically recyclable silver-loaded cellulose-
based bionanocomposite catalyst for green synthesis
of tetrazolo[1,5-a]pyrimidines
Ali Maleki1 Parisa Ravaghi1 Morteza Aghaei1
Hamed Movahed1
•
•
•
Received: 15 January 2017 / Accepted: 11 March 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract 5-Methyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic ester
derivatives were efficiently synthesized by reaction of methyl or ethyl acetoacetate,
5-aminotetrazole (produced from 2-cyanoguanidine and sodium azide), and various
aromatic aldehydes in presence of a new cellulose-based Ag-loaded magnetic bio-
nanostructure. This protocol offers many advantages such as short reaction time, high
yield, the remarkable magnetic property of the nanocomposite, and easy separation of
the nanocatalyst from the reaction mixture without considerable loss of catalytic
activity. Furthermore, X-ray diffraction analysis, field-emission scanning electron
microscopy, and energy-dispersive X-ray (EDX) analysis were used to characterize
the prepared nanocatalyst.
Keywords Cellulose/Fe2O3/Ag bionanostructure Á Magnetic nanocatalyst Á
Tetrazole Á NaN3 Á Multicomponent reaction
Introduction
Recently, the design and performance of organic–inorganic nanocomposite
materials have been widely developed, mainly focusing on the ability to control
their nanoscale structure via innovative synthetic approaches. Their properties
depend on their components, phases, internal interfaces, and morphological and
interfacial characteristics [1–4]. On the other hand, to reduce environmental
pollution, use of biopolymers is becoming popular since they are nontoxic,
ecofriendly, biodegradable, and biocompatible materials. However, some important
properties of biopolymers, e.g., mechanical, electrical, thermal, and optical, are not
&
Ali Maleki
maleki@iust.ac.ir
1
Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran
University of Science and Technology, Tehran 16846-13114, Iran
123

A. Maleki et al.
good enough to meet the requirements of real-world applications [5]. Therefore,
incorporation of nanoparticles (NPs) into biopolymer matrixes to fabricate
biopolymer-based nanocomposites—called bionanocomposites (BNCs)—is an
effective and practical approach to improve those properties [6]. Cellulose is
the best candidate BNC matrix, as it is the most abundant biopolymer on Earth
and offers various useful properties that synthetic polymers do not have, such as
being degradable [7], readily available, and environmentally benign [8]. BNCs
based on a cellulose matrix have been studied in literature [9–11]. Cellulose-based
hybrids and BNCs exhibit enhanced multifunctional properties as well as
synergetic effects between the biopolymer and inorganic materials. They can
also be used in biomedical devices, active wound-dressing agents, biofilms, and
biocoating materials, showing antimicrobial activities, etc. In addition, magnetic
nanomaterials can be easily separated from reaction media using an external
magnet. Ag NPs have been widely studied by chemists and physicists because of
their unique properties at the nanoscale and wide range of applications in optical,
catalytic, magnetic, electrical, and antimicrobial devices. Immobilization of NPs
of metals such as Ag onto polymer and biopolymer surfaces tremendously
improves the properties and applicability of such polymers and biopolymers,
turning them into BNCs [12–20].
Due to their diverse biological and medicinal properties, in recent years,
growing attention has been focused on analogs of purines and nucleosides,
including azolopyrimidines containing a bridgehead nitrogen atom and their
dihydro derivatives. The tetrazole moiety has been identified as an active
pharmaceutical scaffold and thus is an important structural descriptor in the
methodology of new drug design [21, 22]. Compounds containing the tetrazole
unit have hypotensive, antimicrobial, antiviral, antiallergic, cytostatic, nootropic,
and various other biological activities. Additionally, 1,5-disubstituted tetrazoles
have received notable recognition as drugs [23–25]. On the other hand, organic
compounds containing pyrimidine heterocycle are known to display different
biological and pharmaceutical activities [26]. Therefore, synthesis of such
complex heterocyclic scaffolds is considered to be one of the most interesting
areas by both organic and medicinal chemists. In this regard, three-component
reactions of 5-aminotetrazole, active methylene compounds, and aldehydes offer
improved efficiency for synthesis of tetrazolopyrimidines. However, these
protocols suffer from drawbacks such as high temperature, long reaction time,
low yield, and use of stoichiometric amounts of corrosive protonic acids, limiting
their use in large-scale synthesis and also in contrast to green chemistry principles.
Multicomponent reactions (MCRs) are ideal synthetic strategies to construct
diverse molecular scaffolds starting from a few simple materials or intermediates.
MCRs have various advantages such as intrinsic convergence, complexity-
generating power, operational simplicity, resource and energy efficiency, atom
economy, and sustainable technology, offering large chemical libraries of drug-
like compounds [27, 28].
In continuation of our previous work on magnetic nanocomposite catalysts
[29–33], in this study, c-Fe2O3 NPs were supported on cellulose layers and Ag NPs
were immobilized on the surface of cellulose to yield a new magnetic BNC. We
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A novel magnetically recyclable silver-loaded cellulose…
then developed the application of the presented BNC in a simple, efficient, green
protocol for preparation of 5-methyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-
6-carboxylic ester derivatives 4a–s via an MCR strategy. This procedure offers
various further advantages such as easy catalyst recycling, inexpensive catalyst,
environmentally friendly procedure, short reaction time, avoidance of hazardous
organic solvents, and high product yield (Scheme 1).
Experimental
General
All solvents, chemicals, and reagents were purchased from Merck, Fluka, and
Aldrich. Melting points were measured on an Electrothermal 9100 apparatus and
are uncorrected. Fourier-transform infrared (FT-IR) spectra were recorded on a
1
Shimadzu IR-470 spectrometer using KBr pellets. H and 13C nuclear magnetic
resonance (NMR) spectra were recorded on a Bruker DRX-500 Avance spectrom-
eter at 500 and 125 MHz, respectively. The magnetic property of the samples were
determined at room temperature by using a vibrating sample magnetometer (VSM),
Meghnatis Kavir Kashan Co., Kashan, Iran. Field-emission scanning electron
microscopy (FE-SEM) images were obtained on a Zeiss device. Energy-dispersive
X-ray (EDX) spectra were recorded on a Numerix DXP-X10P. X-ray diffraction
(XRD) measurements were carried out using a JEOL JDX-8030.
Synthesis of magnetic cellulose/Ag BNC
A mixture of polyethylene glycol (PEG)-2000 (0.5 g) and NaOH (4.0 g) was
prepared in 45 mL distilled water to obtain aqueous solution of PEG/NaOH. Then,
1.0 g cellulose was added to the mixture and swelled for 3 h at room temperature.
Next, as-prepared suspension was cooled to -15 °C overnight. The frozen solid
mass was vigorously stirred at room temperature to obtain homogeneous cellulose
solution. After that, 2.25 g FeCl3Á6H2O and 0.75 g FeCl2Á4H2O were dissolved in
15 mL distilled water, and the resulting solution was stirred vigorously at room
temperature. Afterwards, the iron solution was added dropwise into the cellulose
solution during 2.5 h. Then, the magnetized cellulose was filtered and washed with
distilled water. The obtained composite was mixed with 50 mL distilled water and
stirred for 2 h to disperse the composite. Moreover, AgNO3 solution was prepared
Scheme 1 Synthesis of 5-methyl-7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic esters 4a–
s in presence of magnetic cellulose/Ag BNC
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A. Maleki et al.
by mixing 0.1 g poly(vinylpyrrolidinone) (PVP) as stabilizer in 10 mL AgNO3
solution (0.05 M), for addition to the previous solution with stirring for 30 min.
Then, 5 mL NaBH4 was added to the mixture dropwise within 2 h. Finally,
magnetic cellulose/Ag BNC was obtained by washing with water for several times
and drying at room temperature.
Typical procedure for synthesis of 5-aminotetrazole
First, 0.084 g 2-cyanoguanidine and 0.13 g sodium azide (NaN3) were dissolved in
3 mL distilled water. Then, the mixture was heated at 50 °C to produce transparent
solution. After that, 0.1 g cellulose/Fe2O3/Ag BNC was added to the mixture
followed by vigorous stirring for an appropriate time. Finally, the produced
precipitate was filtered and simply purified only by washing with distilled water to
yield pure product.
General procedure for synthesis of 5-methyl-7-aryl-4,7-
dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic esters (4a–s)
An aryl aldehyde 1 (1 mmol), an acetoacetate ester 2 (1 mmol), and 5-aminote-
trazole 3 (1 mmol) in presence of magnetic cellulose/Ag BNC (0.1 g) were
dissolved in 2 mL EtOH. Then, the mixture was stirred at 40 °C for an appropriate
time. Reaction progress was monitored by thin-layer chromatography (TLC). After
reaction completion, the catalyst was separated by using an external magnet, and the
rest of the reaction mixture was filtered, washed with ethanol and water, dried, and
purified by recrystallization to yield target products. It was observed that the BNC
catalyst could be washed with ethanol, dried, and reused in subsequent reactions at
least six times without any significant reduction in yield.
All products were known compounds that were confirmed by comparison with
authentic literature samples.
Selected spectral data of synthesized compounds
Methyl-7-(4-bromophenyl)-5-methyl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-
carboxylate (4g)
IR (KBr): 3255, 3058, 2949, 1710, 1648, 1574, 1488, 1432, 1413, 1385, 1302, 1278,
1
1227, 1190, 1153, 1103, 1072, 1012, 839, 813, 772, 725, 686 cm-1. H NMR
(DMSO-d6): 2.54 (s, 3H, CH3), 3.61 (s, 3H, OCH3), 6.64 (s, 1H, CH), 7.24 (d,
J = 7.5 Hz, 2H, ArH), 7.48 (d, J = 7.5 Hz, 2H, ArH), 11.19 (s, 1H, NH). 13C NMR
(DMSO-d6) d: 170.1, 153.4, 152.3, 144.4, 136.5, 136.3, 127.2, 102.1, 60.6, 56.1,
30.5. Anal. Calcd. for C13H12BrN5O2: C, 44.59; H, 3.45; N, 20.00; Found: C, 44.60;
H, 3.41; N, 20.15.
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A novel magnetically recyclable silver-loaded cellulose…
Results and discussion
Characterization of magnetic cellulose/Ag BNC
The morphology and size distribution of the NPs were determined by FE-SEM
image analysis. As seen in Fig. 1a, b, NPs with spherical morphology were
dispersed properly on the cellulose surface. To determine the size of the NPs, 90
particles were used randomly. It was found that the particle size distribution was
narrow and the sizes of most particles lay in the range of 20–25 nm (Fig. 1c). In
addition, the average size of the NPs was about 27.56 nm.
Then, EDX was used to carry out elemental analysis of the magnetic cellulose/Ag
BNC. Figure 2 illustrates the percentages of carbon, oxygen, iron, silver, and gold
as obtained by EDX analysis. The presence of Au in the nanocomposite according
to this measurement is because of the Au coating used for material conductivity,
Fig. 1 a, b FE-SEM images and c particle size distribution of BNC
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A. Maleki et al.
Fig. 2 EDX analysis of magnetic cellulose/Ag BNC
Fig. 3 XRD patterns of a magnetic cellulose/Ag BNC, b JCPDS of Ag, and c JCPDS of c-Fe2O3
whereas carbon and oxygen originate from cellulose, and Ag and Fe come from the
Ag NPs and iron oxide, respectively.
The XRD pattern of the magnetic cellulose/Ag BNC is shown in Fig. 3a. As can
be seen, the peaks of the Ag NPs at 2h = 38.1° (1 1 1), 44.2° (2 0 0), 64.4° (2 2 0),
and 77.4° (3 1 1) are in good agreement with well-crystallized silver in Joint
Committee on Powder Diffraction Standards (JCPDS) card no. 04-0783 (Fig. 3b).
Furthermore, in the XRD pattern of the prepared c-Fe2O3, the position of all
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A novel magnetically recyclable silver-loaded cellulose…
diffraction peaks matched with the pattern for cubic c-Fe2O3 (JCPDS no. 25-1402,
Fig. 3c) at 2h = 30.2° (2 0 6), 35.6° (1 1 9), 57.4° (1 1 15), and 63.0° (4 0 12). The
peak at around 2h = 20° is related to cellulose.
Application of nanocatalyst in organic synthesis
Application of the prepared BNC catalyst was investigated for synthesis of methyl-
7-aryl-4,7-dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic esters from 3-nitroben-
zaldehyde 1 (1 mmol), ethyl acetoacetate 2 (1 mmol), and 5-aminotetrazole 3
(1 mmol) as starting materials in ethanol. To determine the optimum reaction
conditions, different catalysts and temperatures were used and the duration of the
reaction determined by TLC (Table 1) [34–37]. To optimize the type of catalyst,
CH3C6H4SO3H, FeCl3, H3NSO3, and cellulose/Fe2O3/Ag catalysts were used in this
experiment. The best results were obtained when cellulose/Fe2O3/Ag BNC was used
in the reaction. Also, to determine the effect of different parts of this catalyst,
cellulose and magnetic cellulose were used in the reaction. The results indicated low
efficiency for cellulose and 35 % for magnetic cellulose, confirming that cellulose
was used as the matrix for the catalyst. To optimize the temperature, the model
reaction was carried out in ethanol at room temperature, 40 °C, and refluxing
conditions. The results illustrated that 90 % yield was obtained at 40 °C, and on
increasing the temperature, there was no significant change in the efficiency of the
reaction.
Reaction of various types of aromatic aldehyde (1) substituted with ethyl or
methyl acetoacetate (2) and 5-aminotetrazole (1H-tetrazol-5-amine, 3) in presence
of cellulose/Fe2O3/Ag in ethanol provided the corresponding methyl-7-aryl-4,7-
dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic ester derivatives. The results are
presented in Table 2. Various types of aromatic aldehydes substituted with different
electron-withdrawing and electron-releasing groups were chosen and used under
optimal conditions, indicating that aldehydes bearing electron-withdrawing groups
reacted more quickly than their electron-donating aldehyde counterparts. In
Table 1 Comparison of efficiency of present BNC versus other catalysts and effects of different solvents
and temperatures on yield of product 4s
Entry
Catalyst (mol %)
Solvent
Temp. (°C)
Time (h)
Yield (%)
Ref.
–
EtOH
–
Reflux
85
2
7
9
9
1
1
1
1
1
Trace
54
This work
[34]
2
3
4
5
6
7
8
9
H2SO4
Sulfamic acid
FeCl3
EtOH
–
Reflux
85
85
[34]
46
[34]
Cellulose/Fe2O3/Ag
Cellulose/Fe2O3/Ag
Cellulose/Fe2O3/Ag
Cellulose
EtOH
EtOH
EtOH
EtOH
EtOH
Reflux
40
90
This work
This work
This work
This work
This work
90
20
78
Reflux
Reflux
Trace
35
Cellulose/Fe2O3
123

A. Maleki et al.
Table 2 Cellulose/Fe2O3/Ag BNC-catalyzed synthesis of 5-methyl-7-aryl-4,7-dihydrotetrazolo[1,5-
a]pyrimidine-6-carboxylic ester derivatives 4a–s
Entry R1
R2
Product Time (h) Yield (%)a M.p. (°C)
Found
Reported
1
p-MeC6H4
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
4a
4b
4c
4d
4e
4f
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
88
87
89
85
85
89
88
90
91
93
90
90
92
89
87
92
91
90
90b
190–195 194–196 [35]
219–221 218–220 [34]
230–233 230–232 [34]
225–229 226–228 [34]
200–204 200–202 [35]
250–253 253–255 [34]
220–224 220–222 [34]
184–189 184–186 [34]
179–181 178–180 [34]
199–201 197–198 [34]
248–250 247–249 [34]
2
3,4-(CH3O)2C6H3
p-NO2C6H4
m-NO2C6H4
3-OCH3C6H4
2,4-Cl2C6H3
4-BrC6H4
3
4
5
6
7
4g
4h
4i
8
3-ClC6H3
9
3-FC6H4
10
11
12
13
14
15
16
17
18
19
a
4-FC6H4
4j
3-NO2-4-HOC6H3
3-(CHO)C6H4
C6H5
4k
4l
310
[300 [34]
4m
188–192 189–191 [34]
256–258 257–259 [34]
200–204 199–201 [34]
180–185 184–186 [34]
203–208 206–208 [34]
238–242 237–238 [34]
200–205 204–205 [34]
2,4-Cl2C6H4
C2H5 4n
3,4,5-(CH3O)3C6H2 C2H5 4o
3-ClC6H4
3-NO2C6H4
4-BrC6H4
C6H5
C2H5 4p
C2H5 4q
C2H5 4r
C2H5 4s
Isolated yield
b
Yield in six subsequent runs using the same recovered catalyst were 90, 89, 86, 86, 83, and 80 %,
respectively
addition, this methodology could be applied to aromatic aldehydes with either
electron-withdrawing groups (such as nitro, halogen) or electron-donating groups
(such as methoxy) in moderate to excellent yield under the same conditions.
A possible mechanism for the reaction is shown in Scheme 2. In the first step, the
nanocatalyst activates the carbonyl group of the aldehyde (1) to produce
intermediate 5 through a Knoevenagel condensation reaction with CH-acid (2). In
the second step, 5-aminotetrazole (3) attacks 5 by imine–enamine tautomerization to
Scheme 2 Plausible reaction pathway for synthesis of dihydrotetrazolopyrimidines
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A novel magnetically recyclable silver-loaded cellulose…
Fig. 4 Recyclability of cellulose/Fe2O3/Ag BNC nanocatalyst in synthesis of 4s
form intermediate 6. Then, it can undergo intramolecular nucleophilic attack to
keto-carbonyl site to yield cyclic compound 7. Finally, release of an H2O molecule
from 7 gives the product (4a–s).
Catalyst recyclability
To study catalyst reusability, a model reaction was carried out using 0.1 g BNC
catalyst to form 4s starting from benzaldehyde 1 (1.0 mmol), ethyl acetoacetate 2
(1 mmol) and 5-aminotetrazole 3 (1.0 mmol) in EtOH (2 mL) at 40 °C. After
reaction completion, NPs were separated by using an external magnet and washed
with ethanol and water, dried, and reused in subsequent reactions. It was observed
that the catalyst could be reused for at least six times without any significant
decrease in reaction yield (Fig. 4).
Conclusions
We report a simple and efficient procedure for synthesis of 5-methyl-7-aryl-4,7-
dihydrotetrazolo[1,5-a]pyrimidine-6-carboxylic ester derivatives via one-pot three-
component reaction of methyl or ethyl acetoacetate, 5-aminotetrazole, and aromatic
aldehydes using a new magnetic cellulose/Ag BNC catalyst in ethanol at 40 °C.
This composite nanocatalyst can be efficiently recycled from the reaction mixture
and reused several times. The present protocol offers several advantages such as use
of ecofriendly catalyst, safe and green reaction medium, short reaction time, high
yield, simple workup procedure, simple isolation of catalyst and products, low cost,
and readily available starting materials.
Acknowledgements The authors gratefully acknowledge partial support from the Research Council of
the Iran University of Science and Technology.
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A. Maleki et al.
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