In situ generated cetyltrimethylammonium bisulphate in choline chloride-urea deep eutectic solvent: A novel catalytic system for one pot synthesis of 1,3,4-oxadiazole

Article abstract of DOI:10.1007/s10562-014-1288-3

Cetyltrimethylammonium bisulphate ([CTA]HSO₄) catalysed one pot synthesis of 2,5-disubstituted-1,3,4-oxadiazoles from carboxylic acid and acid hydrazide in biodegradable deep eutectic solvent is investigated. [CTA]HSO ₄ is generated in situ from cetyltrimethylammonium peroxodisulphate. Cyclization of diacylhydrazide using [CTA]HSO₄ gives best alternative to traditional dehydration agents. Its remarkable features include a milder procedure, simplicity in workup and purification, good to excellent yields of products, use of inexpensive, recyclable reagents, and eco-friendly aspects by avoiding toxic catalysts/reagents and solvents. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-014-1288-3

Catal Lett
DOI 10.1007/s10562-014-1288-3
In Situ Generated Cetyltrimethylammonium Bisulphate
in Choline Chloride–Urea Deep Eutectic Solvent: A Novel
Catalytic System for One Pot Synthesis of 1,3,4-Oxadiazole
•
Priyanka Anant More Balu Laxman Gadilohar
•
Ganapati Subray Shankarling
Received: 19 March 2014 / Accepted: 25 May 2014
Ó Springer Science+Business Media New York 2014
Abstract Cetyltrimethylammonium bisulphate ([CTA]HSO4)
catalysed one pot synthesis of 2,5-disubstituted-1,3,4-oxadi-
azoles from carboxylic acid and acid hydrazide in biode-
gradable deep eutectic solvent is investigated. [CTA]HSO4 is
generated in situ from cetyltrimethylammonium peroxodi-
sulphate. Cyclization of diacylhydrazide using [CTA]HSO4
gives best alternative to traditional dehydration agents. Its
remarkable features include a milder procedure, simplicity in
workup and purification, good to excellent yields of products,
use of inexpensive, recyclable reagents, and eco-friendly
aspects by avoiding toxic catalysts/reagents and solvents.
applications in asymmetric organic synthesis [10], polymer
and material science [11]. 1,3,4-Oxadiazole derivatives are
employed in development of material for organic electrolu-
minescent devices since they possess high electron accepting
properties and display strong fluorescence with high quan-
tum yield [12]. Its derivatives are also found in excited-state
intramolecular proton transfer (ESIPT) material since it
enhances solid-state fluorescence [13].
Conventionally, 1,3,4-oxadiazoles have been synthe-
sized by variety of procedures as stated in the literature
[14–17]. The most general method for the synthesis of
1,3,4-oxadiazole is formation of diacylhydrazide from
condensation of carboxylic acids and acid hydrazides using
Keywords Cetyltrimethylammonium peroxodisulphate ꢀ
Deep eutectic solvent ꢀ Carboxylic acid ꢀ Acid hydrazide ꢀ
1,3,4-Oxadiazole
i
HATU [18], TBTU [19], Pr2Net [20] and later cyclode-
hydration of diacylhydrazide is accomplished typically
using Burgess reagent [18], p-toluenesulphonyl chloride
(TsCl) [19], Deoxo-Fluor [20], triflic anhydride [21],
orthophosphoric acid and phosphorus pentoxide [22],
thionyl chloride [23], or phosphorus oxychloride [24]. All
the procedures employing these reagents have limitations
such as low yield, high reaction temperature, and the use of
expensive, toxic catalysts in high equivalents.
1 Introduction
1,3,4-Oxadiazoles are important class of heterocycles which
are widely used in many fields such as medicines [1], pesti-
cides [2], dyes [3], etc. It is used as an important tool in the
drug designing because of its anticancer [4], hypoglycaemic
[5], anti-inflammatory [6], antiarrhythmic [7], antioxidant
[8] and anthelmintic [9] activities. Examples of drugs con-
taining the 1,3,4-oxadiazole unit currently used in clinical
medicine are raltegravirÒ, an antiretroviral drug, nesapidilÒ,
an anti-arrhythmic drug, furamizoleÒ, a nitrofuran deriva-
tive that has strong antibacterial activity and tiodazosinÒ, an
antihypertensive drug. 1,3,4-Oxadiazole also finds its wide
DESs are known as solvents of green chemistry because
of their environmentally safe properties [25]. DESs based
on choline chloride have been applied for many organic
transformations [26, 27]. DES composed of choline chlo-
ride and urea (Fig. 1) is inexpensive, biodegradable and it
has low vapour pressure unlike conventional volatile sol-
vents. It is prepared by simple, green method which gives
DES with 100 % atom economy. On other hand cetyltri-
methylammonium peroxodisulphate (CTAPS) (Fig. 1) is a
persulphate functionalized surfactant. Till date many oxi-
dation reactions have been reported using CTAPS such as,
oxidation of aromatic hydrocarbons using micro-emulsions
of CTAPS [28], oxidation and deprotection of
P. A. More ꢀ B. L. Gadilohar ꢀ G. S. Shankarling (&)
Department of Dyestuff Technology, Institute of Chemical
Technology, N. P. Marg, Matunga, Mumbai 400019, India
e-mail: gsshankarling@gmail.com
123

P. A. More et al.
1H NMR (400 MHz; CDCl3; Me4 Si): d = 0.85 (t, 6H);
1.2–1.4 (m, 52H); 1.6 (m, 4H), 3.1–3.4 (m, 22H) ppm.
O
S
N
O
H
N
H
N
H
N
H
N
O
O
O
S
O
H
H
H
H
O
O
O
O
N
2.3 Preparation of Choline Chloride and Urea Deep
Eutectic Solvent (DES)
Cl
H
O
N
DES was synthesized according to the procedure reported
in this literature [33]. The preparation involved reaction of
choline chloride (1 mol) and urea (2 mol) at 70 °C till it
starts melting and forms a homogenous colourless liquid.
This method gives DES with 100 % atom economy which
was used directly for the reactions without purification.
(a)
(b)
Fig. 1 Structure of DES (ChCl–urea) (a) and CTAPS (b)
trimethylsilyl ethers [29], oxidative aromatization of 1,4-
dihydropyridines to pyridines [30], but no report found on
cyclization reaction using CTAPS. Here CTAPS decom-
poses to CTA-bisulphate, which is catalyst for cyclisation.
In most cases bisulphate is prepared using Conc. H2SO4
[31], while in our case it is generated in situ in the reaction,
thus avoiding direct use of strong acids. CTAPS can be
easily prepared in good yield. It is safe for handling unlike
conventional dehydrating agents and strong acids.
2.4 General Procedure for the Synthesis of 2,5-
Disubstituted-1,3,4-Oxadiazole (3a–l)
Carboxylic acid (0.003 mol) and acid hydrazide (0.003 mol)
were added to the DES (5.0 mL) and heated at 50 °C with
stirring to form clear solution. Then CTAPS (0.003 mol) was
added and reaction temperature was set at 70 °C. Reaction
was carried out until all the starting material was consumed
(TLC). Then, the reaction mass was quenched in water and
filtered to obtained precipitated solid. Solid was purified by
crystallization in ethanol to obtain the corresponding product.
Herein we report mild, simple procedure for one pot
synthesis of 2,5-disubstituted-1,3,4-oxadiazoles by con-
densation of carboxylic acid and acid hydrazide in DES
(choline chloride–urea) and subsequent cyclodehydration
of formed diacylhydrazide using [CTA]HSO4 in situ
formed from CTAPS. Here DES plays dual role of catalyst
as well as solvent.
2.5 Selected Spectral Data
3a: White solid; m.p.: 137–138 °C; IR m = 3063, 2919,
1549, 1490, 1450, 1275, 1165, 1070, 1025, 964, 922, 784,
2 Experimental Methods
714, 690 cm-1 1H NMR (400 MHz; CDCl3; Me4 Si):
;
2.1 General Information
d = 7.5–7.6 (m, 6H), 8.1–8.2 (m, 4H) ppm; 13C NMR
(100 MHz, CDCl3): d = 123.9, 126.9, 129.1, 131.7,
164.6 ppm; m/z = 223 (m ? 1).
The progress of the reaction was monitored by thin layer
chromatography (TLC) on silica gel plates. The com-
pounds were also checked for their melting points which
were in well accordance with the literature values. FT-IR
spectra were recorded on a Bomem Hartmann and Braun
MB-Series FT-IR spectrometer. 1H NMR spectra were
recorded on Varian 300 MHz Mercury plus spectrometer
and mass spectral data were obtained with a Micromass-Q-
TOF (YA105) spectrometer. Common reagent grade
chemicals were procured from M/s S. D. Fine Chemical
Ltd., India, and were used without further purification.
3b: White solid; m.p.: 172–174 °C; IR m = 3062, 2923,
2853, 1602, 1544, 1473, 1447, 1402, 1072, 1009, 728, 704,
689 cm-1
;
1H NMR (400 MHz, CDCl3; Me4 Si):
d = 51–7.58 (m, 3H), 7.68 (d, 2H), 8.01 (d, 2H), 8.13 (dd,
2H) ppm; 13C NMR (100 MHz, CDCl3): d = 123.13,
124.01, 126.65, 127.21, 128.55, 129.35, 132.09, 132.68,
164.11,165.00 ppm; m/z = 300 (m?).
3c: White solid; m.p.: 150–152 °C; IR m = 3070, 2929,
1610, 1550, 1500, 1225, 1150, 1068, 848, 735, 710,
690 cm-1
;
1H NMR (400 MHz; CDCl3; Me4 Si):
d = 7.18–7.26 (m, 2H), 7.49–7.56 (m, 3H), 8.05–8.16 (m,
4H) ppm, 13C NMR (100 MHz, CDCl3): d = 121.3, 126.1,
130.1, 138.4, 163.9 ppm; m/z = 241 (m ? 1).
2.2 Preparation of Cetyltrimethylammonium
Peroxodisulphate (CTAPS)
3d: White solid; m.p.: 160–162 °C; IR m = 3061, 2920,
1605, 1550, 1478, 1406, 1086, 1074, 839, 730, 702,
CTAPS was prepared as reported in literature [32]. Briefly,
an aqueous solution of potassium persulphate was added
dropwise into the aqueous solution of cetyltrimethylammo-
nium bromide (CTAB). The white precipitate formed was
filtered, washed with distilled water and dried under vacuum.
687 cm-1
;
1H NMR (400 MHz; DMSO; Me4 Si):
d = 7.50–7.58 (m, 5H), 8.06–8.14 (m, 4H) ppm; 13C NMR
(100 MHz, CDCl3): d = 122.69, 124.03, 127.20, 128.41,
123

Catalytic System for One Pot Synthesis of 1,3,4-Oxadiazole
129.34, 129.71, 132.08, 138.24, 164.02, 164.97 ppm; m/
z = 256 (m?).
carried out in DES in the absence of any catalyst, N0-
benzoyl-4-chlorobenzohydrazide was obtained and the
formation of final product was not observed (Table 1, entry
1). This explains cyclization of diacylhydrazine does not
occur in reported DES (Scheme 1). DES catalyses forma-
tion of the N,N0-diacylhydrazide intermediate as well as
acts as a solvent in this process. Progress in reaction was
observed after addition of metal peroxodisulphate. How-
ever, excellent yield was obtained with CTAPS (Table 1,
entry 5). This is because CTAPS wraps the diacylhydrazide
molecule by its aliphatic chain and brings it in the vicinity
of peroxodisulphate [34] (Fig. 2). This action is absent in
case of other peroxodisulphates used in this study (Table 1,
entries 2–4). When the reaction was carried out using
CTAB, formation of final product was not observed
(Table 1, entry 6). This explains the catalytic role of
CTAPS and efficiency over other peroxodisulphates for the
synthesis of 1,3,4-oxadiazole.
3e: White solid; m.p.: 148–150 °C; IR m = 2960, 2923,
2835,1619, 1510, 1262, 1069, 1017, 825, 740, 710,
690 cm-1; 1H NMR (400 MHz; CDCl3; Me4 Si): d = 3.78
(s, 3H), 6.79 (d, 2H), 7.13–7.15 (m, 2H), 7.25–7.29 (m,
2H), 7.32–7.35 (m, 3H), 7.39–7.48 (m, 5H) ppm; 13C NMR
(100 MHz, CDCl3): d = 55.23, 113.82, 119.31, 125.68,
127.06, 127.85, 128.31, 128.74, 129.47, 129.88, 130.18,
135.35, 154.51,154.64, 160.50 ppm; m/z = 252 (m?).
3 Results and Discussion
Considering the aim of the development of an efficient and
toxic reagent free protocol led us to synthesize 2,5,-
disubstituted-1,3,4-oxadiazoles from cyclodehydration of
carboxylic acids and acid hydrazide in DES and CTAPS as
a catalyst.
3.1 Catalytic Performance
H
H
N
N
CTAPS prepared was characterised by FT-IR and 1H
NMR. To check the acidity of the catalyst CTAPS was
dispersed in water and heated to dissolve. pH of the solu-
tion was found to be 1.8. It shows that CTAPS decomposes
in presence of water to generate [CTA]HSO4. Initially, the
reaction between benzoic acid and 4-chloro-benzohydraz-
ide was chosen as a model reaction. When the reaction was
R
R`
O
O
O
N
HO
S
O
O
Fig. 2 Wrapping of diacylhydrazide by hydrocarbon chain of CTA
Table 1 Comparison of yield in different persulphate catalysts
Table 2 Optimization of reaction temperature for synthesis of 1,3,4-
oxadiazole
Entry
Catalyst
Time (h)
Yielda (%)
Entry
Temperature (°C)
Time (h)
Yielda (%)
1
2
3
4
5
6
DES
24
8
–
DES/K2S2O8*
DES/Na2S2O8*
DES/(NH4)2S2O8*
DES/CTAPS*
DES/CTAB*
36
40
48
90
–
1
2
3
4
RT
50
70
90
24
11
5
–
9
52
90
90.5
12
5
5
24
Reaction condition benzoic acid (0.003 mol), 4-chloro-benzohydraz-
ide (0.003 mol), CTAPS (0.003 mol), DES (5 mL)
Reaction conditions benzoic acid (0.003 mol), 4-chloro-benzohyd-
razide (0.003 mol), * (0.003 mol), DES (5 mL), 70 °C
RT room temperature
a
Isolated yield
a
Isolated yield
DES
No product
H
N
O
OH
O
Cl
NH2
O
H
DES
70oC
N
+
N
H
CTAPS, DES
O
O
70oC
Cl
N
N
Cl
Scheme 1 DES catalysed formation of diacylhydrazide
123

P. A. More et al.
3.2 Effect of Temperature
Table 3 Mole equivalent study of CTAPS for synthesis of 1,3,4-
oxadiazole
Subsequently to optimise the reaction temperature the same
model reaction was carried out at various temperatures
using CTAPS (1.0 equiv.) (Table 2). Initially, the reaction
was stirred at room temperature (30 °C) (Table 2, entry 1).
It was observed that reaction does not proceed at room
temperature. The highest yield was obtained in 5 h when
the reaction was carried out at temperature 70 °C (Table 2,
Entry
CTAPS (equiv.)
Time (h)
Yielda (%)
1
2
3
0.5
1.0
1.5
24
5
32
90
91
4.5
Reaction condition benzoic acid (0.003 mol), 4-chloro-benzohydraz-
ide (0.003 mol), CTAPS, DES (5 mL), 70 °C
a
Isolated yield
Table 4 CTAPS catalysed synthesis of 2,5-disubstituted-1,3,4-oxadiazoles
H
O
OH
O
N
R1
NH2
CTAPS (1.0 Equi), DES
+
O
N
70oC
R2
N
R1
R2
Entry
R1
H
R2
Product
Time (h)
5.5
Yielda (%)
92
3a
H
H
H
Cl
O
N
N
Br
F
3b
3c
3d
Br
F
7
7
5
81
90
90
O
N
N
O
N
N
H
O
Cl
N
N
3e
3f
H
OCH3
OCH3
4
85
84
O
OCH3
N
N
N
Br
Br
6.5
O
N
OCH3
Cl
3g
3h
Cl
Cl
Cl
6
82
91
O
N
Cl
N
Cl
OCH3
5.5
O
OCH3
N
N
H3CO
3i
OCH3
OCH3
4
87
O
N
OCH3
N
Reaction condition carboxylic acid (0.003 mol), acid hydrazide (0.003 mol), CTAPS (0.003 mol), DES (5 mL), 70 °C
a
Isolated yield
123

Catalytic System for One Pot Synthesis of 1,3,4-Oxadiazole
entry 3). Completion of reaction was confirmed by the
absence of reactants on TLC. Further increase in the tem-
perature did not show any improvement in yield (Table 2,
entry 4).
completion even after 24 h (monitored on TLC) and the
corresponding product was obtained with 32 % yield
(Table 3, entry 1). When the reaction was carried out using
1.0 equivalent of CTAPS, the product was obtained with
90 % isolated yield (Table 3, entry 2). Further increase in
mole equivalents of CTAPS did not increase the yield
(Table 3, entry 3).
3.3 Effect of Quantity of Catalyst
To optimise the mole equivalents of CTAPS required for
the reported reaction, same model reaction was carried out
using 0.5 equivalents of CTAPS. Reaction did not go to
3.4 Substrate Generality of the Catalyst
Under the above optimised conditions, 2,5-disubstituted-
1,3,4-oxadiazoles were synthesized to find out the gener-
ality of the proposed synthesis plan. Various substituted
benzoic acid derivatives and acid hydrazides were selected
for this study. Reactions were conducted at 70 °C using 1.0
equivalent of CTAPS and DES. Reaction mass was quen-
ched with water, after completion of reaction (monitored
on TLC). As DES is miscible with water it goes into the
water and the product gets precipitated. It was observed
that all these reactions proceeded smoothly affording the
corresponding 2,5-disubstituted-1,3,4-oxadiazoles in good
to excellent yields (Table 4). It is a very clean route for
synthesizing 2,5-disubstituted-1,3,4-oxadiazoles as the
products are purified by simple crystallization technique
and avoids tedious column chromatographic purification.
The study indicate that the proposed synthetic process is
efficient to synthesize various 1,3,4-oxadiazole derivatives
with good yield in shorter reaction time.
100
90
89
90
80
70
60
50
40
30
20
10
86
83
79
0
Recycle Number
Fig. 3 Recyclability study of DES/CTA-HSO4
Scheme 2 A plausible reaction
mechanism
Ar
O
C
NH
H
N
O
Ar
O
H
N
NH2
O
Ar
O
C
C
H
N
H
Ar
NH
H
O
H
H
H
+
2O
l-
NH
O
C
C
-
TA
C
[
+S2O8
]
2
NH2
H
O
N
Ar
Ar
Ar
Ar
O
S
H
H
H
TA + -O
]
O
N
N
O
O
2
C
[
O
Ar
Ar
Ar
Ar
H
H
H+
H+
-
H
2O
H
O
N
N
N
N
-
N
N
H
N
N
O
O
N
O
O
O
N
O
H
H
H
Ar
H
O
Ar
Ar
2
Ar
123

P. A. More et al.
Acknowledgments P. A. M. is thankful to UGC-CAS for providing
financial assistance and SAIF IIT Bombay for recording NMR and
mass analysis.
3.5 Recycling of Catalytic System
After completion of reaction, water was added to the
reaction mass. As DES and in situ generated CTA-bisul-
phate is soluble in water, the product gets precipitated.
After separation of the product by filtration, the water was
evaporated from DES–CTA bisulphate catalytic system.
This catalytic system comprised of DES and in situ gen-
erated CTA bisulphate was recycled further up to four runs.
It gave good yield of product without significant loss in
catalytic activity (Fig. 3).
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mechanism was illustrated (Scheme 2). Carboxylic acid
reacted with acylhydrazide in the presence of DES forming
diacylhydrazide and water. Generally persulphate decom-
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sulphate anion radical formation [35] (Eq. 1). Similarly
CTAPS decomposes in presence of water to give CTA-
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-
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catalyses the cyclodehydration of diacylhydrazide to form
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which eliminates the direct use of strong acids such as
Conc. H2SO4 or dehydrating agents such as phosphoric
acid.
-
S2O82- can be decomposed as following giving HSO4
:
ꢁ2
ꢀꢁ
S2O8 ! 2SO4
ð1Þ
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ð2Þ
ð3Þ
ð4Þ
ꢁ
_
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ꢁ2
S2O8 þ OH ! HSO4ꢁ þ SO4ꢀꢁ þ 1=2O2
_
ꢀꢁ
SO4 þ OH ! HSO4ꢁ þ 1=2O2
_
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good to excellent yield. It provides a better and practical
alternative to the existing procedures. Use of biodegradable,
non-volatile deep eutectic solvent eliminates the use of
hazardous organic solvents and makes the process greener
and economically viable. Thus, this method offers
improvements in terms of simple reaction conditions, gen-
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