Synthesis of thiotetrazoles and arylaminotetrazoles using rutile TiO 2 nanoparticles as a heterogeneous and reusable catalyst

Article abstract of DOI:10.3184/174751914X14061217395813

A novel method is described to prepare rutile TiO₂ nanoparticles which are reusable and efficient heterogeneous catalysts for the synthesis of thiotetrazoles and arylaminotetrazoles - compounds widely used in medicinal and coordination chemistry. This procedure has the advantages of good to excellent yields of products, elimination of homogeneous catalysts and toxic and explosive reagents, simple methodology, easy work up and the reusability of the heterogeneous catalyst.

Full text of DOI:10.3184/174751914X14061217395813

502 RESEARCH PAPER
VOL. 38 AUGUST, 502–506
JOURNAL OF CHEMICAL RESEARCH 2014
Synthesis of thiotetrazoles and arylaminotetrazoles using rutile TiO2
nanoparticles as a heterogeneous and reusable catalyst
Mehdi Mahama* and Mehdi Khalajb
aDepartment of Chemistry, Aliabad Katoul Branch, Islamic Azad University, Aliabad Katoul, Iran
bDepartment of Chemistry, Buinzahra Branch, Islamic Azad University, Buinzahra, Qazvin, Iran
A novel method is described to prepare rutile TiO nanoparticles which are reusable and efficient heterogeneous catalysts for
the synthesis of thiotetrazoles and arylaminotetr2azoles – compounds widely used in medicinal and coordination chemistry.
This procedure has the advantages of good to excellent yields of products, elimination of homogeneous catalysts and toxic and
explosive reagents, simple methodology, easy work up and the reusability of the heterogeneous catalyst.
Keywords: rutile TiO2 nanoparticles, thiotetrazole, arylaminotetrazole, sodium azide, heterogeneous catalyst
Tetrazoles are well‑documented compounds of synthetic
importance that have been widely used in coordination and
medicinal chemistry and in the synthesis of pharmaceutically
important compounds and nitrogen‑containing molecules.1–6
Tetrazoles are conventionally synthesised by the reaction of
harmful hydrazoic acid or azide ions with nitriles, thiocyanates
or cyanamides in the presence of homogeneous catalysts.3–11
However, these procedures suffer from disadvantages such
as environmentally unpleasant use of toxic reagents or
stoichiometric amounts of homogeneous mediators, long
reaction times, harsh reaction conditions, tedious isolation
steps and low yields of the products. This has limited the large‑
scale applications of tetrazoles in industry. Recently tetrazoles
have been prepared from the reaction between thiocyanates or
nitriles with NaN3 and NH4Cl as a substitute for HN3.1 However,
combining sodium azide with ammonium chloride as an acid
may yield gaseous HN3 which is potentially toxic.
Results and discussion
There are three mineral phases namely, anatase, rutile and
brookite for TiO2. TiO2 nanoparticles were prepared by the
Degussa Company with an average particle size of 30 nm
with an anatase/rutile phase ratio of 80/20. These data are in
good agreement with the X‑ray diffraction analysis (Fig. 1).
The phase transformation of TiO2 nanoparticles occurs at
temperatures higher than 600 °C. Clearly, heating the TiO2‑
Degussa nanoparticles up to 600 °C did not bring about any
phase transition. Rutile TiO2 nanoparticles can be obtained by
thermal treatment of TiO2‑Degussa nanoparticles at 900 °C.
As shown in Fig. 1, at 900 °C temperature the TiO2 phase is
completely rutile (Fig. 1). A typical TEM image of rutile TiO2
nanoparticles made from TiO2 nanoparticles with the thermal
method at 900 °C is shown in Fig. 2. We decided to study
the behaviour of rutile TiO2 nanoparticles in the synthesis of
thiotetrazoles and aminotetrazoles.
Heterogeneous catalysts have attracted attention in the past
few years due to their potential in organic synthesis12 and
they often permit easier separations from reaction mixtures.
However, heterogeneous catalysts are increasingly used in
the form of nanoparticles due to their small size and large
surface areas.9–13 Nanosized TiO2 has created much interest; its
small size and large specific surface area mean it has certain
unusual physico‑chemical properties.13 In continuation of our
research on natural products, tetrazoles and application of
heterogeneous catalysts,14,15 we now report a simple method for
the synthesis of thiotetrazoles and aminotetrazoles using rutile
TiO2 nanoparticles (NPs) as novel and stable heterogeneous
catalysts under thermal conditions (Scheme 1).
Initially, we employed benzylthiocyanate and sodium azide
as model substrates for the development of the optimised
conditions. Several solvents such as water, toluene, MeCN,
DMF and DMSO were examined. Control experiments show
that there is no reaction without a catalyst (Table 1, entry 1).
However, addition of a catalyst to the mixture rapidly increased
the formation of 5‑benzylthiotetrazole in high yields. The
results indicated that solvent had a significant effect on the yield
of product (Table 1, entry 2). A decrease in the catalyst loading
from 0.05 to 0.03 g afforded the 5‑benzylthiotetrazole in lower
yield (Table 1, entry 7). No significant improvement in the yield
was observed using higher amounts of the catalyst and 0.05 g
of the rutile TiO2 NPs was found to be optimum. The best result
R
NH2
N NH
N
or
HN
R
C
C
R
N
N
N
N
N
N
N
H
N
NPs
TiO2
DMF, 110 oC
B
Isomer
Isomer A
+
NaN3
S
N NH
N
R
R
S
N
Scheme 1 Synthesis of thiotetrazoles and aminotetrazoles.
* Correspondent. E‑mail: mehdimaham1@gmail.com
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JOURNAL OF CHEMICAL RESEARCH 2014 503
Fig. 1 X-ray diffraction of TiO2 nanoparticles at 400–1000 °C.
Table 1 Synthesis of 5-benzylthiotetrazole under different reaction
conditionsa
N
HN
N
NPs
TiO2
CN
NaN
+
3
N
S
S
Entry
Rutile TiO2 NPs/g
Solvent
Yield/%b
1
2
3
4
5
6
7
8
9
0
DMF
DMF
0
90
83
73
12
74
60
90
89
0.05
0.05
0.05
0.05
0.05
0.03
0.07
0.10
DMSO
CH3CN
H2O
Toluene
DMF
DMF
DMF
aReaction conditions: sodium azide (3 mmol), phenylthiocyanate (2 mmol),
DMF (6 mL), 110 °C, 20 h.
bIsolated yield.
Fig. 2 TEM image of rutile TiO2 NPs.
was obtained with 2.0 mmol of benzylthiocyanate, 3.0 mmol of
sodium azide, 0.05 g of catalyst and 6.0 mL of DMF as solvent
at 110 °C, which gave the product in an excellent yield.
From an economic point of view, the stability and sustained
activity of the catalysts are of great importance. Thus,
the recovery and reusability of the rutile TiO2 NPs were
examined by the analysis of the reaction of benzylthiocyanate
with sodium azide (Table 2, entry 3). After the first run was
completed, the rutile TiO2 nanoparticles were easily separated
by using centrifugation. Next, the catalyst was washed with
H2O and ethyl acetate and dried at 120 °C for 3 h in a hot
air oven and employed for the next cycle of the reaction. The
catalytic activity did not decrease considerably after five
catalytic cycles.
To show the merits of rutile TiO2 nanoparticles in comparison
with other reported catalysts, we summarised some of results
in Scheme 2, which show that rutile TiO2 is an equally or more
efficient catalyst with respect to reaction time and yield than
those previously reported. As shown in Scheme 2, a rutile
TiO2 nanoparticle is an effective catalyst since the products
are regiospecific, whereas with hydrazoic acid,16 FeCl3–SiO217
and glacial acetic acid,7 a mixture of isomers are produced. If
hydrazoic acid is used, care must be taken by monitoring the
Next, the reactivity of several thiocyanates and cyanamides
were tested in the cycloaddition reaction at 110 °C and the
results are indicated in Table 2. In all cases, tetrazoles were
prepared in good to excellent yields on heating (Table 2). As
shown in Table 2, cyanamides having the electron releasing
groups on the rings (entries 10–13) were completely reacted at
110 °C after 60 min, while the examples bearing the electron
withdrawing species (entries 5–9) require higher reaction
times. The influences of various substituents in different ortho,
meta or para positions on the type of products were examined.
Generally, when the substituent on the aryl ring of the
cyanamide is electron‑releasing, formation of a 1‑aryl‑5‑amino‑
1H‑tetrazole (isomer B) is favoured (entries 10–13, Table 2),
and as the electronegativity of the substituent is increased, the
product is shifted toward 5‑arylamino‑1H‑tetrazoles (isomers
A) (entries 5–9, Table 2).
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504 JOURNAL OF CHEMICAL RESEARCH 2014
Table 2 Formation of thiotetrazoles and aminotetrazoles
R
NH2
N NH
N
or
HN
R
C
C
R
R
N
N
N
N
N
N
N
H
N
NPs
TiO2
B
Isomer A
Isomer
+
NaN3
DMF, 110 oC
S
N NH
N
R
S
N
Entry
RXCN
Product
Time/h
Yield/%a
1
2
3
4
5
6
7
8
9
10
11
12
13
MeSCN
Me(CH2)3SCN
PhCH2SCN
4-OH,2-Me,5-IsopropylC6H2SCN
4-NO2C6H4NHCN
2-ClC6H4NHCN
2,5-(Cl)2C6H3NHCN
4-AcC6H4NHCN
3-CF3C6H4NHCN
2-MeC6H4NHCN
4-MeC6H4NHCN
MeSTet
Me(CH2)3STet
PhCH2STet
20
20
20
17
2
2
2
2
2
1
1
1
1
91
90
87, 84b
95
4-OH,2-Me,5-IsopropylC6H2STet
4-NO2C6H4NHTet (isomers A)
2-ClC6H4NHTet (isomers A)
2,5-(Cl)2C6H3NHTet (isomers A)
4-AcC6H4NHTet (isomers A)
3-CF3C6H4NHTet (isomers A)
2-MeC6H4TetNH2 (isomers B)
4-MeC6H4TetNH2 (isomers B)
4-OMeC6H4TetNH2 (isomers B)
2,6-(Me)2C6H3TetNH2 (isomers B)
80
83
80
81
82
84
87
90
4-OMeC6H4NHCN
2,6-(Me)2C6H3NHCN
81
aYields are after work-up.
bYield after the fifth cycle.
Product
A + B
Yield (%)
HN3
Low yields
Ethanol, xylene, reflux
FeCl3–SiO2
DMF, 110 oC, 75-125 min
A + B
A + B
73–76
72–87
HOAc
Glacial
+
NaN3
Ar–NH–CN
r.t., 20-30 h
ZnCl2
61–90
79–82
80–90
A or B
A or B
A or B
H2O, reflux, 15 h
Natrolite zeolite
DMF, 110-115 oC, 65-115 min
Rutile TiO2
DMF, 110 oC, 60-120 min
Ar
N
N
N
N
N
HN
N
Ar
N
N
NH2
H
Isomer A
B
Isomer
Scheme 2 Various methods for the synthesis of arylaminotetrazoles.
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JOURNAL OF CHEMICAL RESEARCH 2014 505
was heated and stirred at 110 °C for the appropriate time (Table 2).
After completion of the reaction, rutile TiO2 NPs as catalyst was
separated via centrifugation. The centrifuged catalyst was washed
with H2O and EtOAc. Next, it was diluted with HCl (5 M, 20 mL) and
extracted three times with EtOAc (25 mL each). The organic layer
was concentrated under reduced pressure to afford the crude product
which was recrystallised from aqueous ethanol. Most tetrazoles
concentration of hydrazoic acid in the reaction mixture to avoid
an explosion. ZnCl218 is homogeneous and cannot be separated
from the reaction mixture, while rutile TiO2 nanoparticle is
heterogeneous and can easily be recovered and reused. Natrolite
zeolite19 is a good and local catalyst, but there are difficulties in
the preparation and availability of this catalyst.
The products were characterised by IR, 1H NMR and
13C NMR spectroscopy and melting points. The disappearance
of one strong and sharp absorption band (CN stretching band)
in the IR spectra, and the appearance of an NH stretching band
in the IR and 1H NMR spectra, were evidence for the formation
of tetrazoles. The 13C NMR spectra displayed signals about
δ=154–157.5 ppm for C5 of the tetrazole ring.19
1
were known and were characterised by melting points, IR, H NMR,
13C NMR and CHN analyses.17–20
5-(Methylthio)tetrazole (Table 2, entry 1): M.p. 150–152 °C (lit.20
150–151 °C); 1H NMR (DMSO‑d6, 400 MHz): δ 2.70 (s, 3H).
5-(Butylthio)tetrazole (Table 2, entry 2): M.p. 96–98 °C (lit.20
95–97 °C); 1H NMR (DMSO‑d6, 400 MHz): δ 0.89 (t, J=8.0 Hz, 3H),
1.40 (m, J=8.0 Hz, 2H), 1.83 (m, J=7.4 Hz, 2H), 3.33 (t, J=7.0 Hz,
2H).
The two A and B isomers, have different chemical
properties. 5‑arylamino‑1H‑tetrazoles, isomers (A) are acidic
substances, while 1‑aryl‑5‑amino‑1H‑tetrazoles, isomers (B)
have basic properties due to their NH2 functional group. On
the basis of 1H NMR spectra, we have considered two possible
structures A and B. A comparison of 1H NMR spectra revealed
that 5‑arylamino‑1H‑tetrazoles isomers (A) contain two NH
bonds (NH of the amine attached to the aryl group (NHA)
and NH of the tetrazole ring (NHT)) and 1‑aryl‑5‑amino‑1H‑
tetrazoles isomers (B) contain an NH2 bond. The free N–H
bond of tetrazoles (NHT) makes them acidic molecules and,
not surprisingly it has been shown that both the aliphatic and
aromatic heterocycles have pKa values that are similar to the
corresponding carboxylic acids, due to the ability of the moiety
to stabilise a negative charge by electron delocalisation.16–20 In
general, tetrazolic acids exhibit physical characteristics similar
to carboxylic acids. Thus, the signal of the NH proton of the
5-Benzylthiotetrazole (Table 2, entry 3): M.p. 132–134 °C (lit.20
1
132–133 °C); H NMR (DMSO‑d6, 400 MHz): δ 4.53 (s, 2H), 7.30 (t,
J=7.4 Hz, 1H), 7.34 (d, J=7.3 Hz, 2H), 7.42 (d, J=7.3 Hz, 2H).
5-(4′-Hydroxy-2′-methyl-5′-isopropylphenylthio)tetrazole (Table 2,
1
entry 4): M.p. 165–167 °C (lit.20 165–166 °C); H NMR (DMSO‑d6,
400 MHz): δ 1.13 (d, J=7.1 Hz, 6H), 2.23 (s, 3H), 3.13 (m, J=7.4 Hz,
1H), 6.81 (s, 1H), 7.31 (s, 1H), 9.88 (s, 1H).
5-(4-Nitrophenyl)amino-1H-tetrazole (Table 2, entry 5): M.p.
218–220 °C (lit.11 218–220 °C); 1H NMR (DMSO‑d6, 400 MHz): δ 7.78
(d, J=8.3 Hz, 2H), 8.22 (d, J=8.3 Hz, 2H), 10.97 (br, 1H).
5-(2-Chlorophenyl)amino-1H-tetrazole (Table 2, entry 6): M.p.
228–230 °C (lit.11 228–230 °C); 1H NMR (DMSO‑d6, 400 MHz): δ 7.05
(t, J=7.9 Hz, 1H), 7.34 (t, J=8.5 Hz, 1H), 7.50 (d, J=8.1 Hz, 1H), 8.04
(d, J=8.3 Hz 1H), 9.13 (s, 1H), 14.87 (br, 1H).
5-(2,5-Dicholorophenyl)amino-1H-tetrazole (Table 2, entry 7): M.p.
273–275 °C (lit.11 272–274 °C); 1H NMR (DMSO‑d6, 400 MHz): δ 7.09
(d, J=8.8 Hz, 1H), 7.50 (d, J=8.8 Hz, 1H), 8.20 (s, 1H), 9.59 (br, 1H),
14.50 (br, 1H).
1
tetrazole ring (NHT) is shifted downfield. Indeed, H NMR
spectra showed signals at δ=9–10 ppm indicative of NHA in
5‑arylamino‑1H‑tetrazoles isomers (A), whereas 1H NMR
spectra of 1‑aryl‑5‑amino‑1H‑tetrazoles isomers (B) showed
one peak at δ=5–7 ppm indicative of the NH2 group.
5-(4-Acetylphenyl)amino-1H-tetrazole (Table 2, entry 8): M.p.
216–218 °C, IR (KBr disk, cm–1) 3403, 3282, 3177, 3024, 2978, 2839,
2780, 2440, 1651, 1627, 1605, 14579, 1470, 1426, 1364, 1285, 1258,
1
1195, 1055, 1024, 962, 885, 865, 835, 733, 592; H NMR (DMSO‑d6,
300 MHz): δ 10.38 (s, 1H), 7.92 (d, J=7.9 Hz, 2H), 7.14 (d, J=7.6 Hz,
1H,), 1.10 (s, 1H); 13C NMR (DMSO‑d6, 75 MHz): δ 196.7, 145.3, 130.4,
130.1, 116.2, 116.1, 26.8. Anal. calcd for C9H9N5O: C, 53.20; H, 4.46; N,
34.47; found: C, 53.25; H, 4.51; N, 34.52%.
Conclusion
In conclusion we have shown that the rutile TiO2 nanoparticles,
easily prepared in the laboratory, can afford an effective solid
catalyst for the synthesis of thiotetrazoles and aminotetrazoles.
The catalyst was characterised by XRD and TEM. The protocol
offers several merits such as generality and simplicity, high
yields and elimination of homogeneous catalysts and dangerous
and toxic reagents. Furthermore, the catalytic activity of rutile
TiO2 nanoparticles did not decrease considerably after five
catalytic cycles.
5-[3-Trifluoromethyl)phenyl]amino-1H-tetrazole (Table 2, entry 9):
M.p. 70–72 °C, IR (KBr, cm–1) 3296, 3170, 3024, 2978, 2839, 2781,
2440, 1660, 1620, 1601, 1556, 1470, 1407, 1334, 1260, 1234, 1186,
1175, 1129, 1099, 1072, 1057, 1024, 984, 917, 885, 867, 797, 782, 762,
1
728, 697, 675, 665, 612; H NMR (300 MHz, DMSO‑d6) δH 7.23 (d,
J=7.4 Hz, 1H), 7.49 (t, J=7.4 Hz, 1H), 7.74 (d, J=7.6 Hz, 1H), 8.70 (s,
1H), 10.40 (s, 1H); 13C NMR (75 MHz, DMSO‑d6) δC 141.65, 130.63,
130.43, 130.01, 120.76, 117.63, 112.89, 34.58. Anal calcd for C8H6F3N5:
C, 41.93; H, 2.64; N, 30.56; found: C, 41.99; H, 2.69; N, 30.63%.
1-(2-Methylphenyl)-5-amino-1H-tetrazole (Table 2, entry 10): M.p.
191–192 °C (lit.19 191–192 °C); 1H NMR (300 MHz, DMSO‑d6): δH
7.52–7.34 (m, 4H), 6.79 (s, 2H), 2.05 (s, 3H).
1-(4-Methylphenyl)-5-amino-1H-tetrazole (Table 2, entry 11): M.p.
177–179 °C (lit.19 178–179 °C); 1H NMR (500 MHz, DMSO‑d6): δH 7.45
(d, J=8.1 Hz, 2H), 7.40 (d, J=8.1 Hz, 2H), 6.82 (s, 2H), 2.37 (s, 3H).
1-(4-Methoxyphenyl)-5-amino-1H-tetrazole (Table 2, entry 12): M.p.
212–214 °C (lit.19 211–213 °C); 1H NMR (500 MHz, acetone‑d6): δH
7.50 (d, J=8.8 Hz, 2H), 7.16 (d, J=8.8 Hz, 2H), 6.20 (s, 2H), 3.85 (s,
3H).
Experimental
All the purchased solvents and reagents were of the highest
commercial quality and were used without further purification. All
reaction mixtures were stirred magnetically and were monitored
by TLC using 0.25 mm E‑Merck silica gel 60 F254 pre‑coated glass
plates, which were visualised with UV light and then developed by
using iodine mixed with silica gel 60–120 mesh. Melting points were
recorded on a Buchi R‑535 apparatus and are uncorrected. IR spectra
were recorded on a PerkinElmer FT‑IR 240‑C spectrophotometer
using KBr optics. NMR spectra were recorded on Bruker Avance
300, 400 and 500 MHz spectrometers in acetone and DMSO using
TMS as the internal standard, with chemical shifts being given in ppm
with respect to internal TMS and J values quoted in Hz. The catalyst
was prepared by thermal treatment of TiO2‑Degussa nanoparticles at
900 °C.
1-(2,6-Dimethyphenyl)-5-amino-1H-tetrazole (Table 2, entry 13):
M.p. 147–149 °C; FT‑IR (KBr, cm–1): 3441, 3383, 3351, 2952, 2921,
1697, 1651, 1604, 1583, 1558, 1526, 1486, 1442, 1247, 1228, 1194, 1168,
1
1032, 987, 938, 780; H NMR (500 MHz, DMSO‑d6): δH 7.05 (s, 3H),
5.26 (s, 2H), 2.24 (s, 6H); 13C NMR (125 MHz, DMSO‑d6): δC 157.4,
137.0, 136.5, 128.2, 126.2, 18.5. Anal calcd for C9H11N5: C, 57.13; H,
5.86; N, 37.01; found: C, 57.16; H, 5.92; N, 37.06%.
Synthesis of thiotetrazoles and aminotetrazoles; general procedure
Rutile TiO2 NPs (0.05 g) was added to the mixture of thiocyanates or
cyanamide (2 mmol) and NaN3 (3 mmol) in DMF (6 mL). The mixture
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506 JOURNAL OF CHEMICAL RESEARCH 2014
7
8
9
A.R. Modarresi‑Alam and M. Nasrollahzadeh, Turk. J. Chem., 2009, 33,
267.
M. Nasrollahzadeh, Y. Bayat, D. Habibi and S. Moshaee, Tetrahedron
Lett., 2009, 50, 4435.
D. Habibi, M. Nasrollahzadeh, L. Mehrabi and S. Mostafaee, Monatsh.
Chem., 2013, 144, 725.
We gratefully acknowledge the Iranian Nano Council and
Islamic Azad University, Aliabad Katoul Branch for the
support of this work.
Received 22 May 2014; accepted 11 July 2014
Paper 1402675 doi: 10.3184/174751914X14061217395813
Published online: 12 August 2014
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