Tungstates: Novel heterogeneous catalysts for the synthesis of 5-substituted 1H-tetrazoles

Article abstract of DOI:10.1016/j.molcata.2009.01.037

Most heterogeneous catalysts utilized for the formation of 5-substituted 1H-tetrazoles contain zinc as the metal core at the catalytically active site. In this paper, we report that the tungstate MWO₄ (M = Ba, Ca, Zn, Cd, Cu, Na, H) can catalyz

Full text of DOI:10.1016/j.molcata.2009.01.037

Journal of Molecular Catalysis A: Chemical 304 (2009) 135–138
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Tungstates: Novel heterogeneous catalysts for the synthesis of 5-substituted
1
H-tetrazoles
∗
Jinghui He, Baojun Li, Fasheng Chen, Zheng Xu , Gui Yin
State Key Laboratory of Coordination Chemistry, School of Chemistry and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, Jiangsu, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Most heterogeneous catalysts utilized for the formation of 5-substituted 1H-tetrazoles contain zinc as
the metal core at the catalytically active site. In this paper, we report that the tungstate MWO4 (M = Ba, Ca,
Zn, Cd, Cu, Na, H) can catalyze the [2+3] cycloaddition reaction of nitriles with sodium azide to produce
Received 2 December 2008
Received in revised form 30 January 2009
Accepted 31 January 2009
Available online 10 February 2009
5-substituted 1H-tetrazoles in DMF. The catalyst is very efficient, affording good yield of aromatic nitriles
and can be reused for several cycles. The mono- and di-addition products from dicyanobenzene can be
selectively synthesized, which is a development being reported for the first time. The mechanism of the
catalysis may originate from the nitrile group coordinating with the unsaturated W atoms, formed by
oxygen vacancies on the surface of solid tungstates.
Keywords:
Tetrazoles
Tungstate
BaWO4
©
2009 Elsevier B.V. All rights reserved.
[2+3]-Cycloaddition
Heterogeneous catalyst
1
. Introduction
of the nitrogen atom with the Lewis acidic zinc ions [22–24]. How-
ever, it has been reported that most of other metallic ions showed
no catalytic activity in water [15,16]. The heterogeneous catalysts
effective in the synthesis of tetrazoles were believed to be limited
to the compounds containing zinc. In this paper, to extend the range
of high efficient heterogeneous catalyst for synthesis of tetrazoles,
Tetrazoles are a class of heterocyclic compounds that contain
nitrogen and are currently under intensive focus due to their
wide range of applications [1]. For example, they can function as
lipophilic spacers and carbonylic acid surrogates [2] in pharmaceu-
ticals, specialty explosives [3,4] and information recording systems
in materials [5], ligands, and precursors of a variety of nitrogen-
containing heterocycles in coordination chemistry [6,7].
The conventional method of synthesizing tetrazoles is by addi-
tion of azide ions to organic nitriles. Several methods of synthesis
are in use [8–14] but each has its own disadvantage, such as the
use of toxic metals, expensive reagents, explosive hydrazoic acid,
or severe reaction conditions. Recently, Sharpless and co-workers
have reported a method of synthesizing tetrazoles from nitriles and
sodium azide instead of using hydrozoic acid with stoichiometric
amounts of zinc bromidein water [15,16]. As the catalytic load on
these homogeneous catalysts is high, the separation of the products
and recycling of catalyst becomes difficult, shifting our preference
in favor of high efficient heterogeneous catalysts. Since then there
have been reports of the use of various heterogeneous catalysts
we report that a new family of salts without zinc: BaWO and other
4
tungstates MWO4 (M = Cu, Zn, Cd, Ca, Na, H) can be efficient cat-
alytic systems for the synthesis of tetrazoles with good yields being
obtained [25].
2. Experimental
2.1. Preparation of catalysts
Na WO ·2H O (99.55%), CaWO (99%) and H WO4 (99%) were
2
4
2
4
2
purchased and used without further purification. By heating
◦
◦
H WO4 at 400 C and 700 C for 2 h, WO ·xH O and monoclinic
2
3
2
WO3 were produced and were characterized by X-ray diffraction.
The white precipitate of BaWO was synthesized by mixing 100 mL
4
of 0.1 mol/L sodium tungstate (AR, Aldrich) with 100 mL of 0.1 M
BaCl2 (AR, Aldrich) in 100 mL of ethanol with vigorous stirring. The
product was separated by centrifugation and washed several times
with deionized water until no chloride ions were detected in the
[
17–21], but most of these catalysts contain zinc. Beside the innova-
tion of synthetical methods concentrating on zinc salt, theoretical
calculations also indicate that the catalytic activity originates from
activation of the CN triple bond of the nitrile by the coordination
◦
centrifugate solution. After drying at 50 C for 5 h the sample was
denoted as BaWO . The remaining tungstates were synthesized
4
by the same method by changing the chloride solution accord-
∗ Corresponding author. Tel.: +86 25 83593133; fax: +86 25 83314502.
E-mail address: zhengxu@netra.nju.edu.cn (Z. Xu).
ingly. Another BaWO sample with larger surface area was prepared
4
in an aqueous medium and was noted as BaWO –water. All the
4
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.01.037

136
J. He et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 135–138
Table 2
catalysts used in the reaction contained an identical amount of
a
WO42 . The BET surface area was measured on ASAP2020 equip-
−
BaWO4 mediated preparation of 5-substituted 1H-tetrazoles .
Entry Substrate
1
NaN3 (mmol) Time (h) Yieldb (%) TOF(10−1
h
−1
)
ment (Micromeritics, USA).
2
1
.2. Typical procedure for preparation of 5-substitited
H-tetrazole
5.3
5.3
24
24
90
93
3.61
3.73
All nitriles were purchased from Aldrich. Barium tungstate
2
3
(
0.1 g, 0.26 mmol) was added to a mixture of benzonitrile (0.257 g,
2
.5 mmol) and sodium azide (0.350 g, 5.4 mmol) in DMF (5 mL) and
◦
the resulting composition stirred constantly for 24 h at 120 C. After
the completion of the reaction, the catalyst was separated by cen-
trifugation, washed with 4 mL water twice, and the centrifugate
was treated with 6 M HCl (20 mL) and while being stirred vigor-
ously. The aqueous solution finally obtained was extracted twice
with ethyl acetate; the combined organic phase was washed with
water and concentrated to precipitate the crude solid crystalline
5.3
24
85
3.41
3.53
4
5
5.3
5.3
24
24
88
85
5-phenyltetrazole. To obtain pure 5-phenyltetrazole from it, col-
3.41
3.73
umn chromatography was performed using silica gel (100–200
mesh) (0.28 g, 75%), 1H NMR (300 MHz, CDCl ): ı = 8.00 (m, 2H),
3
+
7
.59 (m, 3H), MS (70 eV): m/z (%) = 146.1 (M , 6.87%), 118.1 (M-N ,
2
6
7
5.3
5.3
24
24
93
<5
1
00.0%), 103.1(M-HN , 10.3%), 91.1 (M-HCN , 30.35%), 77.1 (M-
3
3
HCN , 14.93%), 63.0(M-H C N , 9.72%), and 51.0(M-H C N , 5.73%).
4
3
2
4
3
3
4
<0.2
3. Results and discussion
8
9
5.3
5.3
24
<5
<0.2
As a part of the effort to develop a new catalytic system,
BaWO4 was first evaluated as the catalyst for the preparation of
-phenyltetrazole using various reaction parameters. The results
are summarized in Table 1. Generally, with the catalysis of BaWO4
in DMF (Table 1, entries 4–7), the reaction gave moderate to good
yields, whereas no products were obtained without catalytic load-
ing (Table 1, entry 8). The solvent had significant effect on the
5
24
12
6
84c
3.37
24c + 61d
10
2.6
–
–
catalytic performance of the BaWO catalyst in the reaction (Table 1,
4
entries 1–3 and 6), in which DMF afforded good yield (75%) (Table 1,
entry 6), and dimethyl sulfoxide (DMSO) moderate yield (55%)
2.6e
6c + 70d
11
(
Table 1, entry 2). Water and tetrahydrofuran (THF) are not suit-
a
Reaction conditions: nitrile (2.5 mmol), NaN3 (2.6 or 5.3 mmol), BaWO4
able solvents for the reaction. The quantities of NaN3 and catalyst
used also affect the yield significantly. On using twice the amount of
◦
(
0.26 mmol), DMF (5 mL), 120 C.
b
Average of yields of two parallel experiments.
NaN , the yield increased from 27% (Table 1, entry 4) to 75% (Table 1,
3
entries 4 and 5; entries 6 and 7). An abnormal phenomenon can be
observed from Table 1, which is, keeping the amount of NaN3 used
constant, the yields decreased with increasing amounts of the cat-
alyst (Table 1, entries 4 and 5, 6 and 7). The possible reason for
this occurrence might be the adsorption of the product occasioned
by the increased loading of the catalyst. By simple centrifugation,
c
Di-addition product:
, determined by H NMR.
, determined by H NMR.
d
e
Mono-addition product:
NaN3 added gradually over 12 h period.
BaWO4 can be quantitatively recovered and reused for three con-
secutive cycles (Table 1, entry 6).
A variety of organic nitriles was used as substrates to evalu-
ate the catalytic performance of BaWO4. The reactions proceeded
under the optimum reaction conditions as shown above. The results
show that aromatic nitriles give good yields (Table 2, entries 1–6).
Among them, three nitrobenzolenitriles offered the yields higher
than benzolenitrile with no substitutions; furthermore, the yields
for nitrobenzolenitrile with 2-substitutions or 4-substitutions were
higher than for the isomer with 3-substitutions, indicating possi-
ble contributions from the electronic effects of the aromatic ring.
This is also proved by the results obtained from the heteroaromatic
nitriles such as 2-pyridinecarbonitrile whose yield was similar to
that of nitrobenzolenitrile (Table 2, entry 5). However, in com-
parison with Zn-containing catalysts, tungstates catalysts exhibit
negligible activity for alkyl nitriles (Table 2, entries 7 and 8) with
the exception of phenylacetonitrile (Table 2, entry 6).
Table 1
a
Formation of 5-phenyltetrazole using various reaction parameters .
Yield b (%)
TOF (10
c
−1 −1
)
h
Entry
Solvent
NaN3 (mmol)
BaWO4 (mmol)
1
2
3
4
5
6
7
8
Water
DMSO
THF
DMF
DMF
DMF
DMF
DMF
5.3
5.3
5.3
2.6
2.6
5.3
5.3
5.3
0.26
0.26
0.26
0.26
0.78
0.26
0.78
0
0d
0
55
2.20
0
1.08
0.23
3.00, 2.92
0.91
0
e
0
27
17
75 , 73
f
g
68
0
a
Reaction conditions: nitrile (2.5 mmol), NaN3 (2.6–5.3 mmol), BaWO4
◦
(
0.26–0.78 mmol), DMF (5 mL), reaction time (24 h) and 120 C.
b
Average of yields of two parallel experiments.
c
Turnover frequency of nitrile.
d
e
f
◦
Reaction using reflux of water (100 C).
Reaction using reflux of THF.
Yield for first use.
Dicyanobenzene is an interesting substrate for study of mono-
addition reactions or di-addition reactions. It was reported that the
di-addition product was obtained using soluble Zn(II) salts [15,16],
g
Yield for the third cycle.

J. He et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 135–138
137
Table 3
Preparation of 5-substituted 1H-tetrazoles with various catalysts .
The non-stoichiometric hydrate of tungsten oxide shows the high-
est reactivity (Table 3, entry 5). There are two possibilities that could
a
Entry
Catalystb
BET surface Area (m /g)
2
Yieldsc (%)
TOF (10
−1 −1
h
)
2−
serve as the catalytically active sites: (1) the free WO4 ions or
(
2) the solid-state WOx polyhedron. Na WO ·2H O was selected
2
4
2
1
2
3
4
5
6
7
8
9
BaCl2
Dissolving
Dissolving
Partly dissolving
38
0
0
0
0
Ba(NO3)2
Na2WO4·2H2O
H2WO4
as a catalyst because it has moderate solubility in DMF. If the free
WO42 ions are the reactive sites, then Na WO ·2H O should dis-
−
36
89
95
47
75
52
74
75
88
93
1.44
3.57
3.81
1.88
3.00
2.08
2.96
3.00
3.53
3.73
2
4
2
play much higher activity than the solid tungstate catalyst due to
WO3·xH2O
12
2−
its higher concentration of WO4 . In fact, its catalytic activity is
WO3
0.88
17.0
5.7
remarkably lower (36%, Table 3, entry 3) than that of the other solid
tungstates (Table 3, entries 7–11). Although the reason for low activ-
ity of Na WO ·2H O is unclear, it nevertheless proves that WO
BaWO4
BaWO4–water
CaWO4
ZnWO4
CdWO4
1.4
2
4
2
x
1
0
14.0
57.4
109.2
polyhedrons in solid tungstate are the catalytically reactive sites.
1
1
The specific surface area of the solid catalyst usually plays an
important role in catalytic activity. However, this correlation was
12
CuWO4·2H2O
a
Reaction conditions: nitrile (2.5 mmol), NaN3 (5.3 mmol), catalyst (0.26 mmol),
2
not observed in our study. For example, both WO ·xH O (12 m /g,
◦
3
2
DMF (5 mL), 120 C, 24 h.
2
b
Amount of Ba2+ or WO42− catalyst used is equivalent to that of 0.1 g BaWO4
Table 3, entry 5) and CuWO4·2H2O (109.2 m /g, Table 3, entry 12)
afforded excellent yields, although their surface areas differ by the
order of 10, which suggests that oxygen vacancies in solid tungstate
may play a key role in catalytic activity [26–28].
(
0.26 mmol).
c
Average value of two parallel experiments.
Based on a previous report on the role of surface oxygen vacan-
cies in many chemical catalytic reactions [29,30], we could conclude
that the unsaturated coordination sites on the surface of the solid
whereas the mono-addition product was achieved with the use of
solid zinc oxide catalyst [8]. We found that mono- and di-addition
products could be selectively synthesized by simply adjusting the
molar ratio of NaN3 to nitriles and the rate of addition of NaN3
to the reaction solution. The di-addition product was obtained
when the molar ratio of NaN3 to nitrile was about 2 (molar ratio:
tungstates form WO -like structures via oxygen vacancies. The
3
unsaturated W atoms are supposed to activate the nitriles and
enhance their reactivity with azides. However, further experiments
are necessary to gain a clearer insight into these reactions.
5
.3:2.5) (Table 2, entry 9); whereas when the molar rate was about 1
2.6:2.5), the methodology of adding NaN3 to the reaction solution
had a crucial influence on the formation of mono- and di-addition
(
4. Conclusions
products. If NaN is entirely added into the reaction solution before
the reaction, both adducts are formed with similar yields (61:24,
Table 2, entry 10). This indicates that the substrate and mono-
3
In summary, we report that tungstates are effective heteroge-
neous catalysts for the [2+3] cycloaddition of azides with a wide
variety of nitriles to form 5-substituted 1H-tetrazoles with good
yields. The selective synthesis of mono- or di-addition product is
addition products show similar reaction activity with NaN . On the
3
other hand, if NaN3 was added gradually over a period of 6 h, the
dominant product was mono-addition (Table 2, entry 11). The above
results reveal that the dicyanobenzene is highly reactive, which is
related to the withdrawal of the two electron groups (–CN) attached
to the aromatic ring. In short, using tungstates for catalysis the yield
of the mono- or di-addition products can be fine-tuned by adjust-
ing the molar ratio of NaN3 to the nitriles, the rate of addition of
achieved by adjusting the adding rate and amounts of NaN as well
3
as the reaction time. This methodology may find widespread use in
organic synthesis involving tetrazoles and inspire the exploring of
new heterogeneous catalyst for synthesis of tetrazole.
Acknowledgements
NaN , and the reaction time.
3
We thank Tan Wee Boon for assistance rendered in language
editing. This work was supported by NNSFC under major project of
nanoscience and nanotechnology No. 90606005; major project No.
In order to probe into the reactive sites of barium tungstate,
several inorganic salts were used as catalysts in control experi-
ments (Table 3). First, both barium nitrate (0.26 mmol) and barium
2
0490210, and surface program No. 20571040.
2
+
chloride (0.28 mmol) with similar Ba contents as 0.1 g BaWO
4
0.26 mmol) were selected as the catalysts to investigate if Ba2 is
+
(
References
the active site. The results showed that they cannot catalyze the
formation of tetrazoles; but all the tungstates exhibited moderate
to good catalytic activity, regardless of whether the cations were
[
1] R.N. Butler, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven (Eds.), Comprehensive
Heterocyclic Chemistry, vol. 4, Pergamon, Oxford, UK, 1996.
2+
2+
2+
+
transition metal ions (Zn , Cd , Cu , entries 10–12) or not (Na ,
[2] H. Singh, A.S. Chala, V.K. Kapoor, D. Paul, R.K. Malhotra, Prog. Med. Chem. 17
(1980) 151.
2+
2+
+
Ba , Ca , H , entries 3–9), regardless of whether the tungstate
was of scheelite type (CaWO , BaWO ) or of wolfram type (ZnWO ,
[
3] V.A. Ostrovskii, M.S. Pevzner, T.P. Kofmna, M.B. Shcherbinin, I.V. Tselinskii, Tar-
gets Heterocycl. Syst. 3 (1999) 467.
4
4
4
CdWO , CuWO ), or even tungsten oxide. It is evident that the ori-
[4] M. Hiskey, D.E. Chavez, D.L. Naud, S.F. Son, H.L. Berghout, C.A. Bome, Proc. Int.
Pyrotech. Semin. 27 (2000) 3.
4
4
gins of catalytic activity whether from WOx polyhedrons (x = 4 or 6)
or the counter ion seem to play no part in the catalytic activity.
Secondly, two catalysts, ZnWO4 (Table 3, entry 10) and BaWO4
[
[
5] G.I. Koldobskii, V.A. Ostrovskii, Usp. Khim. 63 (1994) 847.
6] R. Huisgen, J. Sauer, H.J. Sturn, J.H. Markgraf, Chem. Ber. 93 (1960) 2106.
[7] D.J. Moderhack, Prakt. Chem. 340 (1988) 687.
(
Table 3, entry 7), which have similar BET surface areas afforded
[8] S.J. Wittenberger, Org. Prep. Proc. Int. 26 (1994) 499.
[9] J.V. Dunica, M.E. Pierce, J.B. Santella III, J. Org. Chem. 56 (1991) 2395.
10] B.E. Huff, M.A. Staszak, Tetrahedron Lett. 34 (1993) 8011.
[11] S.J. Witternberger, B.G. Donner, J. Org. Chem. 58 (1993) 4139.
[12] K. Koguro, T. Oga, S. Mitsui, R. Orita, Synthesis 6 (1998) 910.
2+
similar yields. It is notable to mention that Zn (such as ZnBr2 in
aqueous solution [15,16]) and solid ZnO in DMF [17] can catalyze
[
[
2+3] cycloaddition reactions independently, but ZnWO4 does not
[13] D.P. Curran, S. Hadida, S.Y. Kim, Tetrahedron 55 (1999) 8997.
[14] R.J.P. Corriu, R. Perz, C. Reye, Tetrahedron 39 (1983) 999.
[15] Z.P. Demko, K.B. Sharpless, J. Org. Chem. 66 (2001) 7945.
exhibit an extra catalytic activity compared with BaWO4 (Table 3,
entry 7). It further confirms that WOx polyhedrons are the catalytic
active sites.
The high levels of catalytic activity of tungsten oxide and its
hydrate also indicate that WOx polyhedrons are the reactive sites,
as there are no metal cations in these species (Table 3, entries 4–6).
[16] Z.P. Demko, K.B. Sharpless, Org. Lett. 4 (2002) 2525.
[
[
17] M.L. Kantam, K.B.S. Kumar, C. Sridhar, Adv. Synth. Catal. 347 (2005) 1212.
18] M.L. Kantam, V. Balasubrahmanyam, K.B.S. Kumar, Synth. Commun. 36 (2006)
1809.
[19] M.L. Kantam, K.B.S. Kumar, K.P. Raja, J. Mol. Catal. Chem. 247 (2006) 186.

138
J. He et al. / Journal of Molecular Catalysis A: Chemical 304 (2009) 135–138
[
[
20] B. Schmidt, D. Meid, D. Kieser, Tetrahedron 63 (2007) 492.
21] D. Amantini, R. Beleggia, F. Fringuelli, F. Pizzo, L. Vaccoro, J. Org. Chem. 69 (2004)
896.
[26] M. Gillet, C. Lemire, E. Gillet, K. Aguir, Surf. Sci. 532 (2003) 519.
[27] L. Nagornaya, S. Burachas, Y. Vostretsov, V. Martynov, V. Ryzhikov, J. Cryst.
Growth 199 (1999) 877.
2
[
[
22] F. Himo, Z.P. Demko, L. Noodleman, J. Org. Chem. 68 (2003) 9076.
23] F. Himo, Z.P. Demko, L. Noodleman, K.B. Sharpless, J. Am. Chem. Soc. 124 (2002)
[28] V.B. Mikhailik, H. Kraus, G. Miller, M.S. Mykhaylyk, D. Wahl, J. Appl. Phys. 97
(2005) 083523.
1
2210.
24] F. Himo, Z.P. Demko, L. Noodleman, K.B. Sharpless, J. Am. Chem. Soc. 125 (2003)
983.
25] Z. Xu, J.H. He, B.J. Li, P.R.C. Patent CN 200810020682, 7, 2008.
[29] F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, R. Rosei,
Science 309 (2005) 752.
[30] A. Gora, E. Broclawik, M. Najbar, Comput. Chem. 24 (2000) 405.
[
[
9