Application of a functionalized mesoporous silica catalyst to the synthesis of tetrazoles

Article abstract of DOI:10.1039/c5nj00211g

In the present study, well ordered MCM-41 was synthesized and functionalized with benzyl group and subsequently sulfonated with chlorosulfonic acid. The functionalized catalyst (MCMBSA) was characterized by XRD, TGA, FTIR, TEM, BET, and SEM. 5-Aryl-1H-tetrazoles were synthesized by the reaction of the corresponding aryl nitriles and sodium azide catalyzed by MCMBSA. The mole ratio of nitrile to sodium azide, the amount of catalyst, reaction times, and solvent type were optimized. The MCMBSA catalyst was more effective than the catalyst obtained from the direct sulfonation of MCM-41. The versatility of the method was investigated by using various nitriles, which showed reasonable yields of tetrazoles. The regeneration and reusability of catalyst was also examined.

Full text of DOI:10.1039/c5nj00211g

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High ordered MCM-41 synthesized and functionalized with benzylic groups. The solid was
used in the synthesis of aryl tetrazoles.

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Application of functionalized mesoporous silica catalyst for the synthesis of
tetrazoles
a
Alireza Najafi Chermahini,* Masoud Khani Omran, Hossein A. Dabbagh, Gholamhossein
a
a
a
Mohammadnezhad, Abbas Teimouri
b
a
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
e-mail:anajafi@cc.iut.ac.ir or najafy@gmail.com; Tel: +983133913256; Fax: +983133913250.
b
Chemistry Department, Payame Noor University (PNU), Tehran 19395-4697, Iran
1

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Abstract: In the present study, well ordered MCM-41 was synthesized and functionalized with
Benzyl group and subsequently Sulfonated using chlorosulfonic Acid (MCMBSA). The
functionalized catalyst (MCMBSA) was characterized by XRD, TGA, FTIR, TEM, BET and
SEM techniques. The 5-aryl-1H-tetrazoles was synthesised by the reaction of corresponding aryl
nitriles and sodium azide catalized by MCMBSA. The mole ratio of nitrile to sodium azide, the
amount of catalyst, reaction times, and solvent type were optimized. It was found that
MCMBSA catalyst is more effective than that catalyst which obtained from direct sulfonation of
MCM-41. The versatility of method was investigated by using various nitriles, which showed
reasonable yield of tetrazoles formation. The regeneration and reusability of catalyst was
examined.
Keywords: Sulfonated MCM-41, Tetrazole derivatives, Synthesis, Characterization.
2

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1
. Introduction
Tetrazoles are heterocyclic compounds with four nitrogen atoms which are explored extensively
due to their wide range of biological and industrial applications [1]. Tetrazoles have been widely
used as carboxylic acid isosteric pharmacophore in pharmaceutical chemistry. They have been
turn out to be potential TNF alpha inhibitors, P2X7-antagonists [2], and inhibitors of anandamide
cellular uptake [3]. Tetrazoles are screened for various biological activities such as antiulcer [4],
anticonvulsant [5], antiviral, antibacterial, antifungal, anti-inflammatory [6], antiallergic [7], and
antitubercular activities [8]. They have also significant roles in coordination chemistry as a
ligand for preparation of complex heterocyclic structures [9]. In addition, tetrazoles have been
applied in numerous material sciences and synthetic organic chemistry as analytical reagents [10,
11]. On the other hands, tetrazoles are also particularly interesting molecules with applications as
precursor of new classes of propellants and explosives [12, 13]. Available methods for the
synthesis of tetrazole ring include addition of sodium nitrite to aminoguanidines [14], addition of
sodium azide to carbodiimides [15], reactions of amines with a leaving group in the tetrazole’s 5-
position [16], and three component reaction of primary amines with sodium azide and triethyl
orthoformate [17] have been reported. On the other hands, many methods for the synthesis of 5-
substituted tetrazoles proposed based on [3+2] cycloaddition of azide ion to corresponding
organic nitriles. The reactions were carried out by using numerous homogenous catalysts such as
ZnBr , [18] BF -OEt , [19], Zn(OTf) [20], Fe(OAc) [21], AlCl [22], Cu (OTf) [23], AgNO
3
2
3
2
2
2
3
2
2
[
24], and by using some heterogeneous catalysts such as Cu O, [25] COY zeolites [26], Zn/Al
2
hydrotalcite [27], Montmorrilonite [28] and modified Montmorrilonite clays [29], sulfated
zirconia [30], AlPO-5 microporous molecular sieves [31], and mesoporous ZnS nanospheres
[
32].
3

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Mesoporous materials such as MCM-41 and SBA-15 due to their excellent properties such
as high surface area, regular pore shape, large pore volume, narrow pore size distribution,
tunable pore size and good thermal stability, has been utilized for several applications such as an
adsorbent of metal cations, support of various catalysts, environmental science, sensors, and etc.
[
33-36]. Because of the low acidity of MCM-41 in compare to mineral acids, its modification
reported as a good solution [37-40]. In this paper, we report a new process for the synthesis of 5-
substituted 1H-tetrazoles using modified MCM-41 via [3+2] cycloaddition from nitriles and
sodium azide.
2. Experimental
2.1. Chemicals and instruments
Tetraethyl orthosilicate (TEOS, > 98%), and Cetylterimethylammonium Bromide (CTAB,
99%) purchased from Dae-Jung, S. Korea. All nitriles and solvents purchased from Merck and
>
sodium azide were purchased from Sigma–Aldrich Chemicals. Double-distilled water was used
for the preparation of all aqueous solutions. All Chemicals were used as-received. A JASCO
FT/IR-680 PLUS spectrometer was used to record IR spectra using KBr pellets. NMR spectra
were recorded on a Bruker 400 Ultrasheild NMR, and DMSO-d was used as a solvent. Melting
6
points reported were determined by open capillary method using a Galen Kamp melting point
apparatus and are uncorrected. Progress of reactions monitored with a Hewlett Packard 1090
HPLC using a C18 analytical column (eluting with methanol:water 30:70 as mobile phase).
Small-angle X-ray diffraction (XRD) patterns were recorded with a Brucker D8 Advance
diffractometer using Co radiation, Kα, (40 kV and 40 mA). The surface topography of the
4

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catalysts was evaluated using SEM analysis on a Philips XI30 microscope and TEM was
performed on a Philips CN120 device. Thermogravimetric analysis (STA 503) was carried out
under nitrogen atmosphere in the temperature range 25-800 ˚C with a heating rate of 10˚C/min.
The nitrogen adsorption-desorption isotherm of the sample was carried out by using a
Quantachrome Auto sorb IQ MP device.
2.2. Preparation of modified MCM-41 with sulfonic acid
The MCM-41 hollow tubular material was prepared based to the published procedure with
slightly modification [41]. In order to synthesize MCM-41, the amount of 2.4 g
cetyltrimethylammonium bromide (CTAB) surfactant was added to 120 g of deionized water and
stirred for an hour until a clear solution was obtained. In the next stage, 10 mL of ammonia is
added to the solution and stirring was continued for 5 minutes. Then, 10 ml of tetraethyl
orthosilicate was added to the solution and the mixture kept for 24 h at room temperature under
stirring. After that, the resulting white gel was transferred to a teflon lining steel reactor and
heated for 48 hours in the oven at 70 ˚C. The reactor was removed from the oven and brought to
room temperature, and the product was filtered and washed with water and ethanol. Finally, to
remove the organic template by calcination the sample was heated in an electric furnace at 550
˚C for 8 h. For benzyl grafting of MCM-41 material, 1 g of MCM-41 was added to benzyl
alcohol solution (5 mL) in toluene (20 mL) and resulting mixture kept for 12 hours under reflux
conditions. After filtration, washing and drying the sample, the white solid, was sulfonated with
chlorosulfonic acid in dry chloroform (20 mL) under reflux condition for 4 hours. Then, the
mixture is filtered and washed.
2.3. Synthesis of Tetrazoles
5

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The procedure for the synthesis of the 5-phenyl-1H-tetrazole, 1a, is a representative one. In a
round-bottom flask, benzonitrile (0.2 g, 2 mmol), sodium azide (0.4 g, 6 mmol), modified MCM-
41 (5 mg), and DMF (10 mL) were charged and reaction mixture was heated at 100 ºC. The
progress of reaction was followed by HPLC. After completion of the reaction, the reaction was
cooled to room temperature and insoluble material was filtered and washed several times with
doubly distilled water and acetone to separate the catalyst. The solution was acidified with HCl
(2 mL, 12 M). The precipitate was collected, dried, and recrystallized from water/ethanol to
obtain pure 5-phenyltetrazole as a white powder, yield: 85%.
3. Results and discussion
3.1 Preparation and characterization of catalysts
Highly ordered MCM-41 was prepared according to the method of Kumar et al. [42] with slight
modification. For benzyl anchoring, benzyl alcohol and activated MCM-41 kept under reflux
condition for 12 h in toluene. The functionalized MCM-41 then sulfonated with chlorosulfonic
acid as sulfonating agent to obtain (MCMBSA).
Please insert Scheme 1
To obtain solid acid catalysts with different acidity, values of ClSO H:MCM-41 ratio changed.
3
In addition, for comparison of effect of aryl group on reactivity of sulfonyl group, three samples
were prepared by direct addition ClSO H to MCM-41 (MCMSA). (Scheme 1).
3
6

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Please insert Figure 1 here
Preparation of MCM-41-SO H catalysts was monitored using FT-IR spectroscopy via the KBr
3
salt dilution technique. Fig. 1 shows the FT-IR spectra of synthesized MCM-41 (a), benzylated
MCM-41 (b), MCM-41 after direct sulfonation with chlorosulfonic acid (MCMSA catalyst) (c),
benzylated MCM-41 catalyst after sulfonation (MCMBSA) (d), and recycled MCMSBA
catalyst. In the case of MCMSA catalyst, (Fig. 1c and 1d), the sulfonic acid bonds can be
−
1
observed at ∼1200–1250, 1010–1100 and 650 cm , which are attributed to the O=S=O
asymmetric and symmetric stretching modes and S=O stretching vibration mode of the sulfonic
groups, respectively [42].
Please insert Figure 2 here
However, in the FT-IR spectra of sulfonated nanoparticles, such bands could not be observed
because they are probably overlapped by the bands of SiO . In addition, as can be seen from Fig.
2
−
c, the bands at 3000–3500 cm confirm that existence of OH groups on the mesoporous
1
1
material surface after the sulfonation. Incorporation of sulfonic acid to the aromatic ring was
−
confirmed by observation of CS stretching band at 729 cm that presented in Fig. 2.
1
7

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Please insert Figure 3 here
Fig. 3 shows the TEM and SEM images for the synthesized catalyst. It shows clearly the
hexagonal pore arrangement of the synthesized MCM-41 (Fig. 3a). After entering the sulfonic
groups, the hexagonal structure still retained (Fig. 3b), which indicated that the present
preparation method had little effect on the uniform structure of MCM-41. In addition the Fig. 3c
shows the SEM of MCMBSA catalyst after sulfonation. The SEM photograph shows that
prepared catalysts are still in the tubular morphology.
Please Insert Figure 4 here
The X-ray difraction (XRD) pattern (Fig. 4) illustrates the typical low angle (100), (110), and
(200) reflections that are related to a well-ordered MCM-41- type molecular sieve. With
application of brag equation, the unit cell parameters for the MCM-41 and MCMBSA catalyst
obtained 4.1 and 3.77 nm, respectively. The values indicate the unit cell parameters slightly
decreased after sulfonation. With sulfonation modification, the signals shifted toward higher
diffraction angles that this shift can be explained by a slight decrease in the pore size resulting
from the insertion of sulfonic acid groups into the MCM-41 pores [43].
Moreover, less intense and broadened diffraction peaks were also observed for the SO H-
3
MCM-41 solid acid catalyst, showing that the structural integrity in MCM-41 was slightly
degraded after modification. Fig. 5 shows the representative N adsorption–desorption isotherms
2
of hollow tubular pure MCM-41 and MCMBSA catalyst. All solids exhibited type IV isotherms
8

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with H1 hysteresis loops. Upon functionalization of the parent material with chlorosulfonic acid,
changes in the N sorption isotherm curves and consequently decreases in the gas uptake were
2
2
observed. Therefore the BET surface area decreased from 1056 to 496 and 330 g/m for the
MCMBSA and MCMSA catalysts, respectively. This could be attributed to the attachment and
occupation of sulfonic groups onto the surface of the inner pores.
Please insert Fig. 5 here
The calcined MCMBSA and MCMSA catalysts were run on a TGA system for knowing the
existence of sulfonyl groups. The results presented in supplementary section (Fig. S1). A one-
◦
step weight loss at ~120 C was observed for both catalysts which are due to water desorption.
◦
For the MCMBSA catalyst, two weight loss stages were observed at ~150-200 and ~520 C. (See
Fig. S2 in SI). Based on TGA analysis a 32% total organic content assigned to MCMBSA
catalyst. In addition, for more clarification, an inverse acid-base titration of the catalyst achieved
and acid content found to be 6.5 mmol/100g of MCMBSA solid acid catalyst.
3.2 Catalytic Tests
To optimize the reaction conditions and to explore the catalytic activity of sulfonated MCM-41
catalysts, benzonitrile (1a) and sodium azide were used as test substrates to obtain 5-phenyl-1H-
tetrazole (1b). It was confirmed by the results tabulated in Table 1 that no reaction was observed
without the catalyst, despite prolonged reaction times, when the reaction is carried out without
catalyst (Table 1, entry 1). In addition, when pristine MCM-41 was used as the catalyst, the
9

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reaction progress was less than 10%. These results indicate that the catalyst exhibits a high
catalytic activity in this transformation. In the first stage and in the reaction between benzonitrile
1a and sodium azide (Scheme 2), the effect of the catalyst amount was investigated.
The percentage yield of the product was 75, 85 and 78 % using 2, 5 and 10 mg of catalyst, in the
DMF as the solvent, respectively. (Table 1 entries 4, 9, and 10). It is evident that a low loading of
catalyst still effective, although the reactivity was decreased consequently. We used high
performance liquid chromatography as a powerful tool to probe the reaction progress in all runs.
For example, chromatogram of reaction for conversion of compound 1a to 1b after 2 and 6h is
presented in Figure S3 in supplementary section.
Please insert scheme 2 here
In addition, the sodium azide:benzonitrile ratio is an important factor. With application of 1:1,
3
:1 and 5:1 ratios, the yields of tetrazole formation was 30, 85 and 80 % after 22, 8 and 10 hour,
respectively. (Table 1, entries 6, 8, 11). The effect of different sulfonic acid loading (0.2, 0.4 and
.6 ml chlorosulfonic acid) on MCM-41 surface was examined (Table 1, entries 3-5). The
0
increase of acid loading from 0.2 to 0.6 ml, the reaction time was decreased from 22 to 10 h,
respectively. The effect of temperature also was investigated and an increase in temperature from
80 to 110 ºC accelerates the reaction time (Table 1 entries 8, 13 and 4). The effect of aryl group
anchored to silica surface may be interesting. As we mentioned in previous section with direct
sulfonation of MCM-41, MCMSA catalysts obtained and used in conversion of benzonitrile to
5-phenyl-1H-tetrazole. As you can see, direct sulfonation of MCM-41 decreased the reaction
rate.
1
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The solvent has a prominent influence on the reaction times (Table 2). We also examined
reaction in different solvents, but the yields of the desired products were inferior (Table 2, entries
1-6). Among the different solvents were examined, DMF gave the product in good yield at 100
ºC and 8 h. In addition, DMSO was effective at similar conditions but protic solvents such as 1-
propanol and non-polar solvents such as toluene gave low product yields. It was observed that,
application of 5 mg MCMBSA catalyst in solution of benzonitrile and sodium azide in DMF at
100 ºC is the optimized condition for conversion of aryl nitriles to the corresponding 5-
substituted-1H-tetrazoles.
Please insert Table 1 here
To generalize the scope and versatility of this method, different aryl nitriles are used for the
synthesis of these products and results tabulated in Table 3. The results indicate heteroaromatic
nitriles such as 2, and 3-pyridinecarbonitriles and 2-thiophenecarbonitrile give the corresponding
tetrazoles with good yields (Table 3 entries 3, 4, 7). In addition, aryl nitrile with electron
donating groups gives lower yields. However, it is notable that the yields in Table 3 are reported
after recrystalization. Since the solubility of tetazoles with functional groups such as COOH and
COH increase in water, the overall yields slightly decreased.
Please insert Table 2 here
One of the most important advantages of heterogeneous catalysis is the recovery and reusability
of the catalysts by simple filtration. In the present study, the reusability of MCMBSA catalyst
1
1

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was investigated in the tetrazole formation reaction with benzonitrile under optimized condition.
After completion of the reaction, the catalyst was separated by simple centrifugation, washed
with 5 mL acetone, then with doubly distilled water, and dried at 80 ºC. The results of FT-IR
analysis show sulfonic acid bonds can be observed in the recovered catalyst (Fig. 1d). The
recovered catalyst was used in the next runs. The results of three successive runs showed that the
catalyst can be reused several times without significant loss of activity (see Table 3). Partial loss
of the catalyst activity is probably due to breaking of O-S bonds and copper leaching of sulfonic
acid groups.
Please insert Table 3 here
In order to evaluate the efficiency of this protocol, activity of the various reagents was compared
in the synthesis of 5-phenyl-1H- tetrazole (Table 3). In addition, the results of tetrazole synthesis
with application of MCMSA catalyst are presented in Table S1 in supplementary section.
Please Insert Table 4 here
As shown in Table 5, phenyl tetrazole was produced in 8 h when our catalyst was used, whereas
nano-ZnO and ZnBr were used as catalyst, more reaction time was needed for reaction to
2
complete. In addition, with application of Sb O or FeCl as the catalyst, the higher temperatures
2
3
3
needed for completion of reaction.
1
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Please Insert Table 5 here
To account for the formation of tetrazole derivatives, the following mechanism proposed
Scheme 3). In the first stage, the aryl nitrile is activated by proton of sulfonic acid group. This
activated fragment after attack by azide ion subsequently cyclisize to tetrazole ring.
(
Please insert Scheme 3 here
4. Conclusion
In summary, we report MCM-41 grafted with sulfonic acid groups is an effective heterogeneous
catalyst for the one-pot synthesis of 5-substituted 1H-tetrazoles in various substituted nitriles and
sodium azide in DMF in good to excellent yields. The results show that application a
benzonitril:sodium azide 1:3 ratio and 5 mg MCMBSA catalyst has the best reaction condition
to formation of tetrazole derivatives. Effect of solvent was studied and the best results observed
for aprotic polar solvents such as DMF. The reusability of catalyst was examined and the results
of three successive runs show that the catalyst can be reused without significant loss of its
activity. The easy work-up, lower catalyst loading, mild reaction conditions, clean reaction
profiles, and cost efficiency render this approach as an interesting alternative to the existing
methods.
Acknowledgement
We would like to thank Isfahan University of Technology (IUT) for the financial support
(Research Council Grant).
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Table captions in sequence
Table 1
Initial screening of reaction parameters for the formation of 5-1H-phenyl tetrazole in DMF as the solvent
Entry
Catalyst
Solvent
Catalyst
amount (mg)
Benzonitrile/ T ºC
Sodium azide
Time (h) Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
-
MCM-41
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
-
5
2
2
2
5
10
5
10
2
5
10
5
5
2
2
1:5
1:3
1:3
1:3
1:3
1:1
1:1
1:3
1:3
1:5
1:5
1:5
1:3
1:3
1:3
1:3
1:3
1:3
100
24
24
20
12
10
22
20
8
-
10>
68
75
80
30
35
85
78
70
80
75
70
90
60
70
77
83
100
100
100
100
100
100
100
100
100
100
100
80
110
100
100
100
100
MCMBSA-0.2
MCMBSA-0.4
MCMBSA-0.6
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMBSA-0.4
MCMSA-0.2
MCMSA-0.4
MCMSA-0.6
MCMSA-0.4
8
0
1
2
3
4
5
6
7
8
14
10
8
14
8
22
14
12
10
2
5
Table 2
The effect of different solvents on reaction times of tetrazole formation
Entry Solvent
Catalyst
(
T ºC
Ph-CN : NaN3
Time (h)
mg)
1
2
DMF
H O
100
100
3 : 1
3 : 1
5
5
8
12
a
2
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3
4
5
DMSO
NMP
n-Propanol
Toluen
100
100
100
100
3 : 1
3 : 1
3 : 1
3 : 1
5
5
5
5
10
12
16
-
6
a
few droplets of DMF was added
Table 3
Reusability of MCMBSA catalyst in the model reaction under optimized condition
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Time (h)
Yield (%)
8
85
8
81
8
77
8
70
Table 4
Preparation of tetrazole derivatives under optimized condition using MCMBSA catalyst
entry
1
Reactant
Product
Time (h)
Yield (%)
Mp (ºC)
218-216
N
N
N
CN
8
85
80
73
NH
2
3
4
N
N
O N
2
CN
O N
2
12
220-218
211-209
NH
N
N
N
CN
CN
1
0
NH
N
N
N
N
N
N
12
12
75
70
239-237
251-249
NH
N
N
5
N
N
N
HOOC
HO C
2
CN
N
H
1
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6
7
N
N
H C
3
14
8
80
90
254-252
204-202
H C
3
CN
N
N
H
H
N
CN
N
S
S
N
N
8
9
1
N
N
Br
Cl
Br
Cl
CN
12
12
12
82
78
77
269-267
264-262
184-182
N
N
H
N
N
N
CN
N
H
0
N
N
N
OHC
OHC
CN
N
H
Table 5
Comparison of protocols for the synthesis of 5-phenyl-1H-tetrazole
Entry Catalyst Time (h) Temperature (°C)
Yield (%)
Reference
44
1
2
ZnBr2
24
5
100
refluxing temperature
at DMF
120
120
80
76
88
Silica sulfuric acid
45
3
4
5
6
7
8
9
1
1
AgNO3
Nano ZnO
Fe(OAc) , TMS-N3
2
5
83
72
56
86
91
86
79
84
85
46
47
22
48
49
50
51
26
14
24
18
6
TMSN3-TBAF
CdCl2
Sb O3
FeCl -SiO2
3
Zn/Al hydrotalcite
MCMBSA
85
80
120
120
120
100
8
2
12
12
8
0
1
This work
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Captions of figures and schemes
Scheme 1. Schematic illustration of the catalyst preparation
N
N
Catalyst
N
CN NaN3
DMF, Reflux
NH
1
a
1b
Scheme 2 The synthesis of 5-substituted 1H-tetrazoles from nitriles and sodium azide catalyzed by MCMBSA
2
1

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O
C
2
SO H
3
H
+
RC
N
RC NH
RC
NH
OH
O
H2
C
SO
H
3
Na+
N3
H
N
R
NH
RC
N
N
N
N
N
N
Scheme 3 A plausible mechanism for the conversion of aryl nitriles to tetrazole derivatives
2
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Figure 1 FT-IR spectra of MCM-41-SO H (a) pristine MCM-41, (b) MCM-41 after benzylation (c) MCM-41 after
3
direct sulfonation with ClSO H (MCMSA catalyst), (d) benzylated MCM-41 after sulfonation (MCMBSA
3
catalyst), (e) MCMBSA catalyst after recycling.
2
3

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-
Figure 2. Expanded FTIR spectra of recycled MCMBSA and MCMSA in the 600-800 cm region. The target
1
indicates the stretching mode of CS bond.
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a
b
c
Figure 3 (a) The TEM image of MCM-41 (b) The TEM image of MCMBSA catalyst (c) The SEM image of
MCMBSA catalyst
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Figure 4 XRD patterns of MCM-41 and MCMBSA catalyst
1
000
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
MCMBSA
MCM
0
.00
0.20
0.40
0.60
0.80
1.00
Relative Presure (P/P0)
Fig 5. The N -sorption isotherm of MCM-41 and MCMBSA catalyst at 77 K
2
2
6