An efficient one-pot procedure for the preparation of 1,3,4-thiadiazoles in ionic liquid [Bmim]BF4 as dual solvent and catalyst

Article abstract of DOI:10.1002/hc.20432

The one-pot three component condensation of hydrazine hydrate with substituted phenylisothiocyanates followed by the addition of substituted benzaldehydes in the presence of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF₄)

Full text of DOI:10.1002/hc.20432

Heteroatom Chemistry
Volume 19, Number 3, 2008
An Efficient One-Pot Procedure for the
Preparation of 1,3,4-Thiadiazoles in Ionic
Liquid [Bmim]BF as Dual Solvent and Catalyst
4
Shahnaz Rostamizadeh,1 Reza Aryan,1 Hamid R. Ghaieni,1,2
and Ali M. Amani1
1Department of Chemistry, Faculty of Sciences, K. N. Toosi University of Technology,
P.O. Box 15875-4416, Tehran, Iran
2Faculty of Material and Chemical Engineering, Malek Ashtar University of Technology,
Tehran, Iran
Received 4 August 2007; revised 10 November 2007
thiadiazole is acetazolamide (Acetazola) [5], which
ABSTRACT: The one-pot three component con-
is a carbonic anhydrase inhibitor launched in 1954.
densation of hydrazine hydrate with substituted
In addition, its indications and usage include in
phenylisothiocyanates followed by the addition of
glaucoma, epilepsy, and congestive cardiac failure.
substituted benzaldehydes in the presence of ionic
Thiadiazole derivatives are also found to have anti-
liquid 1-butyl-3-methylimidazolium tetrafluoroborate
tumor [6a], hypoglycemic [6b], anticonvulsant [6c],
([bmim]BF4) and in the absence of any other catalyst
hypotensive [6d], antiproliferative [6e], and antitu-
under mild condition afforded 1,3,4-thiadiazoles in ex-
berculotic [6f] activities. As a result of these useful
cellent yields. The reaction workup is simple, and the
applications, chemists have been encouraged to de-
ionic liquid was easily recovered from the reaction and
sign new synthetic (both solution and solid phase)
ꢀ
C
reused. 2008 Wiley Periodicals, Inc. Heteroatom Chem
methodologies for the preparation of this medici-
nally important heterocyclic building block.
19:320–324, 2008; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20432
Literature survey of synthetic methods for these
interesting compounds indicates that there are
three main categories for the synthesis of 1,3,4-
INTRODUCTION
thiadiazoles: (1) cyclizations involving one-bond
formation [7a,b], (2) cyclizations involving forma-
tion of two bonds [7a,c], and (3) cyclizations in-
volving formation of three bonds [7a,d–h]. Solid-
phase organic synthesis is another synthetic route
to these compounds, which has been recently de-
veloped [8–10]. Furthermore, a stepwise method
toward some antituberculosis 2-phenylamino-5-
phenyl-1,3,4-thiadiazoles (the main targets of the
present work) has also been reported in the litera-
ture [6f]. Considering the green chemistry principles,
[11] most of these methods suffer from some limita-
tions, such as using volatile and hazardous organic
1,3,4-Thiadiazoles and related compounds have at-
tracted significant interest in medicinal chemistry
and many fields of technology. Some of the tech-
nological applications involve dyes [1], lubricating
compositions [2], optically active liquid crystals [3],
and photographic materials [4]. In the medicinal
field, one of the best-known drugs based on 1,3,4-
Correspondence
to:
Shahnaz
Rostamizadeh;
e-mail:
shrostamizadeh@yahoo.com.
c
ꢀ
2008 Wiley Periodicals, Inc.
320

Efficient One-Pot Procedure for the Preparation of 1,3,4-Thiadiazoles in Ionic Liquid [Bmim]BF4 321
TABLE 1 Evaluation of Other Green Reaction Conditions for
the Synthesis of Compound 4a
solvents, harsh reaction conditions, and stepwise
synthetic protocols, requiring laborious and time-
consuming purification of intermediates and more
importantly, low to moderate yields of products.
Very recently, the use of room temperature
ionic liquids (RTILs) as green solvents for or-
ganic synthesis has emerged and gained consid-
erable importance due to their solvating ability,
trace to negligible vapor pressure, easy recyclability,
and repeated reusability [12]. Among ionic liquids,
those with the Brønsted acidity such as 1-butyl-3-
methylimidazolium tetrafluoroborate ([bmim]BF4)
have attracted further interest because of their dual
solvent and catalyst role in promoting organic pro-
cesses [13].
According to the vast number of applications and
the environmental demands, search for new meth-
ods to replace the commonly known catalysts is one
of our targets to provide a greener synthetic method-
ology. Most of the methods reported in the litera-
ture including cyclization need to be catalyzed with
acidic (the Bro¨nsted or Lewis) catalysts [8,10]. So, in
our investigation to provide a green protocol toward
1,3,4-thiadiazoles, the Bro¨nsted acidic ionic liquid,
[bmim]BF4 (one of the most important room tem-
perature ionic liquids), was chosen as both the reac-
tion medium and promoter.
Temperature
( ◦C)
Time
(h)
Yield
(%)
Medium
Catalyst
TBAB
TBAB
TBAB
[bmim]PF6
[bmim]BF4
[bmim]Br
–
100
70
70
50
30
>10
>5
>5
3
0.5
>10
10
20
20
50
85
15
NaHSO4
Na2HPO4
–
–
–
100
TBAB: Tetra -n-butylammonium bromide.
were considered. In the case of [bmim]Br (tetra-n-
butylammonium bromide (TBAB)), two mild, safe,
green salts, NaHSO4 and NaH2PO4, were used as an
extra catalyst in this study. The best result was ob-
tained within 10–30 min when [bmim]BF4 was used
as a solvent and catalyst at 30–40◦C. The results are
summarized in Table 2. The products were obtained
in relatively short reaction time, and no impurities
were observed by TLC. Using simple workup, the
products were then isolated and no further chro-
matographic purifications were performed because
no impurities were observed by NMR.
Excellent isolated yields and short reaction
times, as shown in Table 2, clearly indicate the ef-
ficiency and versatility of ionic liquid [bmim]BF4
for the synthesis of substituted 2-phenylamino-5-
phenyl-1,3,4-thiadiazole derivatives. Repeating the
experiment showed that even after third run and
measuring reaction time and yield, ionic liquid’s ac-
tivity did not show any significant decrease (Table 3).
The reason for dual catalysis and mediation of
[bmim]BF4 for this reaction can be elicited by com-
parison of the results in Tables 1 and 2. Reactions
were carried out in the ionic liquids without any ex-
tra catalyst, and the rates of reactions are remarkably
high. Also, the lack of catalytic effect while reaction
was run in TBAB without any catalyst (knowing that
this kind of ionic liquid is not acidic itself ) clearly
suggests the catalytic role of the ionic liquid chosen.
This enhanced reactivity in imidazolium ionic liq-
uids may be attributed to the inherent Bro¨nsted and
Lewis acidity of the ring hydrogens H2, H4, and H5
of the imidazolium cation. Previous studies involv-
ing multinuclear NMR spectroscopy and conductiv-
ity measurements for the imidazolium ions corre-
lating their acidity characteristics support the above
observations [14]. Furthermore, hydrogen bonding
between ion pair of [bmim]BF4, proved by NMR [15]
and theoretical studies [16], can also be another im-
portant explanation for the reactivity and rate ac-
celeration observed. According to these studies, the
RESULTS AND DISCUSSION
Scheme 1 shows the course of the process applied
for the efficient synthesis of antituberculotic 1,3,4-
thiadiazoles under one-step and one-pot reaction
condition.
The one-pot three component condensation re-
action of hydrazine hydrate with phenylisothio-
cyanates and benzaldehydes, in the presence of
small amounts of 1-butyl-3-methylimidazolium salts
([bmim]X, X = Br, PF6, BF4), easily produced 1,3,4-
thiadiazoles 4 in high yields in the absence and/or the
presence of an added catalyst (comparative results
are given in Table 1). To study the scope and limi-
tations of the reaction, some other green conditions
SCHEME 1
Heteroatom Chemistry DOI 10.1002/hc

322 Rostamizadeh et al.
TABLE 2 Results of the Reaction Specified in Scheme 1
Entry
R1
R2
Reaction Time (min)
Yield a (%)
M. P. (◦C) (Lit.) [Ref.]
4a
4b
4c
4d
4e
4f
H
H
H
H
H
4-NO2
4-NO2
4-NO2
4-NO2
4-Me
4-Me
4-Me
4-Me
3-Me
H
4-Br
4-Cl
4-F
4-NO2
H
4-Br
4-Cl
4-NO2
H
4-Cl
4-F
30
20
20
20
15
30
20
20
15
25
15
15
10
25
85
95
92
87
93
90
95
92
90
80
89
86
90
95
200–202 (199–200) [6f]
320–323 (338–339) [6f]
217–219 (216–217) [6f]
252–254 (258–262) [6f]
267–269 (275–277) [6f]
207–211 (216–220) [6f]
281–284 (293–294) [6f]
288–290 (300–302) [6f]
343–347 (368) [6f]
177–179 (176–180) [6f]
220–224 (213–214) [6f]
203–207 (210–214) [6f]
270–273 (280–284) [6f]
180–182
4g
4h
4i
4j
4k
4l
4m
4-NO2
H
4nb
aIsolated yield.
bNew compound.
TABLE 3 Reusability of [bmim]BF4 Ionic Liquid for the Syn-
thesis of Compound 4a
In this proposed mechanism, it can be observed
that after formation of 4-phenylthiosemicarbazide,
the ionic liquid amplifies the partial positive
charge on carbon in the carbonyl group, producing
thiosemicarbazone intermediate (A). Then, the ionic
liquid accelerates the cyclization to form a cyclic in-
termediate (B) in the next step, followed by aroma-
tization to the final 1,3,4-thiadiazole product, affect-
ing its activity and the rate enhancement role in this
process.
In conclusion, we have developed a new and ef-
ficient method for the one-pot synthesis of 1,3,4-
thiadiazoles in excellent, isolated yields and short
reaction times, using a room temperature ionic liq-
uid, viz. [bmim]BF4, as a reaction medium. More
importantly, the ionic liquid acts not only as a sol-
vating medium but also as a promoter and catalyst
for the reaction, giving rise to advantage of both mild
temperature conditions and the non-requirement of
a catalyst. Easy workup, the absence of a catalyst,
and short reaction times in the non-volatile ionic liq-
uid used as the reaction medium make the method
amenable for scale-up operations.
Run No.
Yield of 4a (z)
1
2
3
85
82
80
ionic liquid [bmim]BF4 has two strong intramolec-
ular hydrogen bonds between its cation and anion
[15,16]. This kind of hydrogen bonding can be dis-
rupted by solutes possessing sites capable of making
such bonds. The proposed mechanism is represented
in Scheme 2 to further illustrate the possible role of
hydrogen bonds in catalyzing and accelerating this
reaction.
EXPERIMENTAL
Melting points were measured on a Buchi B-540 ap-
paratus and are uncorrected. IR spectra were mea-
sured on Bomem FTIR ABB FTLA200-100 spectrom-
1
eter. H and 13C NMR spectra were measured with
a Bruker DRX-300 Avance spectrometer at 300 and
75 MHz, using TMS as an internal standard. Chem-
ical shifts are reported (δ) relative to TMS, and cou-
pling constants (J) are reported in hertz (Hz). Mass
spectra were recorded on a high resolution Agilent
SCHEME 2
Heteroatom Chemistry DOI 10.1002/hc

Efficient One-Pot Procedure for the Preparation of 1,3,4-Thiadiazoles in Ionic Liquid [Bmim]BF4 323
technology EX mass spectrometer. Chemicals
(t, 2H, J = 9.0), 7.56 (d, 2H, J = 6.0), 7.98 (m, 2H),
were obtained from Merck Darmstadt, Germany
and Sigma-Aldrich Saint Quentin Falavier, Cedex,
France and were used without further purification.
10.12 (s, 1H, NH).
2-Phenylamino-5-(4-nitrophenyl)-1,3,4-thiadia-
zole (4e). IR (cm−1): 3347 (NH), 3137 (NH), 2978
(CH), 1596 (C N), 695 (C S C); 1H NMR (300
MHz, DMSO-d6, δ): 7.1 (t, 1H, J = 7.5), 7.40 (t, 2H,
J = 7.5), 7.77 (d, 2H, J = 8.4), 8.18 (d, 2H, J = 8.4),
8.38 (d, 2H, J = 8.4).
General Procedure for the Synthesis of
1,3,4-Thiadiazoles in [bmim]BF4 Ionic Liquid
Hydrazine hydrate (1 mmol) was added dropwise
to substituted phenylisothiocyanate (1 mmol) dis-
solved in appropriate amount of ionic liquid (1–
2 mL), then, only after 5–10 min (monitored by TLC),
the substituted benzaldehyde (1 mmol) was added,
and the reaction mixture was stirred at 30–40◦C un-
til completion (once again as monitored by TLC).
After completion of reaction, water was added to the
reaction mixture and stirred for 30 min at ambient
temperature. Then, the precipitated product was iso-
lated with a simple filtration and recrystallized from
ethanol or methanol. The ionic liquid was recovered
with evaporation of water in vacuo and reused three
times.
2-(4-Nitrophenylamino)-5-phenyl-1,3,4-thiadia-
zole (4f). IR (cm−1): 3311 (NH), 3154 (NH), 2980
(CH), 1601 (C N), 690 (C S C); 1H NMR (300
MHz, DMSO-d6, δ): 7.44 (m, 3H), 7.91 (m, 2H), 8.06
(d, 2H, J = 9.0), 8.40 (d, 2H, J = 9.0), 10.42 (s, 1H,
NH).
2-(4-Nitrophenylamino)-5-(4-bromophenyl)-1,3,4-
thiadiazole (4g). IR (cm−1): 3301 (NH), 3147 (NH),
3003 (CH), 1598 (C N), 695 (C S C); 1H NMR (300
MHz, DMSO-d6, δ): 7.71 (d, 2H, J = 8.1), 7.82 (d,
2H, J = 8.1), 8.15 (m, 2H), 8.69 (br. s, 2H), 10.47
(br. s, 1H, NH).
Analytical Data for Substituted
1,3,4-Thiadiazoles
2-(4-Nitrophenylamino)-5-(4-chlorophenyl)-1,3,4-
thiadiazole (4h). IR (cm−1): 3311 (NH), 3167 (NH),
3014 (CH), 1611 (C N), 690 (C S C); 1H NMR (300
MHz, DMSO-d6, δ): 7.51 (d, 2H, J = 9.0), 7.95 (d,
2H, J = 9.0), 8.04 (d, 2H, J = 9.0), 8.24 (d, 2H, J =
9.0), 10.45 (s, 1H, NH).
2-Phenylamino-5-phenyl-1,3,4-thiadiazole (4a).
IR (cm−1): 3297 (NH), 3148 (NH), 1606 (C N), 680
1
(C S C); H NMR (300 MHz, DMSO-d6, δ): 7.21 (t,
1H, J = 9.0, para to thiadiazole ring), 7.41 (m, 4H,
ortho to thiadiazole ring), 7.57 (d, 2H, J = 9.0, meta
to thiadiazole ring), 7.90 (m, 2H, meta to thiadiazole
ring), 8.17 (s, 1H, ortho to thiadiazole ring), 10.12
(1H, NH), 11.84 (1H, NH).
2-(4-Nitrophenylamino)-5-(4-nitrophenyl)-1,3,4-
thiadiazole (4i). IR (cm−1): 3302 (NH), 3128 (NH),
2990 (CH), 1596 (C N), 695 (C S C); 1H NMR (300
MHz, DMSO-d6, δ): 8.02 (d, 2H, J = 9.0), 8.19 (d,
2H, J = 9.0), 8.24 (d, 2H, J = 4.2), 8.28 (d, 2H, J =
4.2), 10.47 (br. s, 1H, NH).
2-Phenylamino-5-(4-bromophenyl)l-1,3,4-thiadia-
zole (4b). IR (cm−1): 3302 (NH), 3122 (NH), 1595
(C N), 679 (C S C); 1H NMR (300 MHz, DMSO-d6,
δ): 7.21 (t, 1H, J = 6.5, para on thiadiazole ring),
7.37 (t, 2H, J = 6.5, meta to thiadiazole ring), 7.60
(m, 4H, meta to thiadiazole ring), 7.88 (m, 2H, meta
to thiadiazole ring), 8.12 (s, 1H, ortho to thiadiazole
ring), 10.17 (1H, NH), 11.88 (1H, NH).
2-(4-Methylphenylamino)-5-phenyl-1,3,4-thiadia-
zole (4j). IR (cm−1): 3260 (NH), 3200 (NH), 1615
1
(C N), 699 (C–S–S); H NMR (300 MHz, DMSO-d6,
δ): 2.27 (s, 1H, CH3), 2.30 (s, 2H, CH3), 7.15 (m,
2H para to thiadiazole ring), 7.41 (m, 4H, meta to
thiadiazole ring), 7.89 (ortho to thiadiazole ring),
8.14 (s, 1H, ortho to thiadiazole ring), 10.04 (1H,
NH).
2-Phenylamino-5-(4-chlorophenyl)-1,3,4-thiadia-
zole (4c). IR (cm−1): 3306 (NH), 3132 (NH), 1586
(C N), 690 (C S C); 1H NMR (300 MHz, DMSO-d6,
δ): 7.30 (t, 1H, J = 6.9), 7.42 (m, 4H), 7.65 (t, 4H, J
= 7.5), 7.98 (bs, 1H), 9.18 (s, 1H), 10.47 (s, 1H).
2-(4-Methylphenylamino)-5-(4-chlorophenyl)-1,3,
4-thiadiazole (4k). IR (cm−1): 3342 (NH), 3142
(NH), 2978 (CH), 1596 (C N), 685 (C S C); 1H
NMR (300 MHz, DMSO-d6, δ): 2.30 (s, 3H, CH3),
7.16 (d, 2H, J = 8.1), 7.39 (d, 2H, J = 8.1), 7.46 (d,
2H, J = 6.9), 7.93 (d, 2H, J = 6.9), 10.08 (s, 1H,
NH).
2-Phenylamino-5-(4-fluorophenyl)-1,3,4-thiadia-
zole (4d). IR (cm−1): 3322 (NH), 3132 (NH), 1606
(C N) 690 (C S C); 1H NMR (300 MHz, DMSO-d6,
δ): 7.19 (t, 1H, J = 6.0), 7.25 (t, 2H, J = 6.0), 7.36
Heteroatom Chemistry DOI 10.1002/hc

324 Rostamizadeh et al.
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2-(4-Methylphenylamino)-5-(4-fluorophenyl)-1,3,
4-thiadiazole (4l). IR (cm−1): 3322 (NH) 3132 (NH)
1606 (C N) 690 (C S C); 1H NMR (300 MHz,
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7.39 (d, 2H, J = 8.1), 7.56 (d, 2H, J = 6.0) 7.98 (m,
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2988 (CH), 1591 (C N), 690 (C S C); 1H NMR
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2-(3-Methylphenylamino)-5-phenyl-1,3,4-thiadia-
zole (4n). IR (cm−1): 3312 (NH), 3163 (NH), 1602
(C N), 699 (C S C); 1H NMR (300 MHz, DMSO-d6,
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to thiadiazole ring), 7.24 (t, 1H, J = 6.9, para to
thiadiazole ring), 7.42 (m, 5H, meta and ortho to
thiadiazole ring), 7.90 (m, 2H, ortho to thiadiazole
ring), 8.16 (s, 1H, ortho to thiadiazole ring), 10.04
(1H, NH), 11.8 (1H, NH); 13C NMR (75 MHz,
DMSO-d6, δ): 21.26, 123.12, 126.23, 126.44, 127.91,
128.14, 128.92, 130.30, 134.34, 137.60, 139.26,
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