Electrochemical oxidation of aldehyde-N-arylhydrazones into symmetrical-2,5-disubstituted-1,3,4-oxadiazoles

Article abstract of DOI:10.1007/s11164-012-1013-z

A convenient, efficient and one-pot synthesis of chemically and pharmaceutically interesting symmetrical-2,5-disubstituted-1,3,4-oxadiazoles is reported. The protocol involves anodic oxidation of aldehyde-N-arylhydrazones in anhyd. MeCN-LiClO₄. Constant potential electrolysis carried out in an undivided cell and platinum electrodes leads to the formation of the corresponding oxadiazoles under ambient condition and the mechanism was deduced from voltammetry studies. The reaction proceeded smoothly with high atom economy. Springer Science+Business Media Dordrecht 2013.

Full text of DOI:10.1007/s11164-012-1013-z

Res Chem Intermed
DOI 10.1007/s11164-012-1013-z
Electrochemical oxidation of aldehyde-
N-arylhydrazones into symmetrical-2,5-
disubstituted-1,3,4-oxadiazoles
•
Sushma Singh Laxmi K. Sharma
•
•
•
Apoorv Saraswat Ibadur R. Siddiqui
Rana K. Pal Singh
Received: 18 July 2012 / Accepted: 29 December 2012
Ó Springer Science+Business Media Dordrecht 2013
Abstract A convenient, efficient and one-pot synthesis of chemically and phar-
maceutically interesting symmetrical-2,5-disubstituted-1,3,4-oxadiazoles is repor-
ted. The protocol involves anodic oxidation of aldehyde-N-arylhydrazones in anhyd.
MeCN–LiClO4. Constant potential electrolysis carried out in an undivided cell and
platinum electrodes leads to the formation of the corresponding oxadiazoles under
ambient condition and the mechanism was deduced from voltammetry studies. The
reaction proceeded smoothly with high atom economy.
Keywords Aldehyde-N-arylhydrazones ꢀ Controlled potential electrolysis ꢀ
Cyclic voltammetry ꢀ Electrochemical oxidation ꢀ 1,3,4-Oxadiazoles ꢀ
Green chemistry
Introduction
Heterocycles form by far the largest of the classical divisions of organic chemistry
and are of immense importance biologically, industrially, and indeed to the
functioning of any developed human society. N-containing heterocycles, especially
five-membered rings, are of great interest as they are found in natural products [1]
and frequently used in medicinal chemistry [2]. Amongst these heterocycles, the
1,3,4-oxadiazole motif is of particular value in material science [3], agrochemistry
[4], and in pharmaceutical chemistry, as it has been used by medicinal chemists for
many years as a bioisosteric replacement of acid, ester and amide functionalities in
compounds exhibiting a wide range of biological activities (Fig. 1) [5–10].
S. Singh ꢀ L. K. Sharma ꢀ A. Saraswat ꢀ I. R. Siddiqui ꢀ R. K. P. Singh (&)
Electrochemical Laboratory of Green Synthesis, Department of Chemistry,
University of Allahabad, Allahabad 211002, India
e-mail: rkp.singh@rediffmail.com
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S. Singh et al.
N
N
H
N
O
N
N
F
O
O-K+
N
OMe
O
N
N
N
Me
H
H
N
NH
O
O
N
HO
O
O
N
N
N
O
O
Raltegravir
(Antiretroviral agent)
Nesapidil
(Anti-hypertensive agent)
ABT-751-Oxadiazoles
Fig. 1 Pharmaceuticals containing the 1,3,4-oxadiazole motif
Symmetrical-2,5-disubstituted-1,3,4-oxadiazoles have been synthesized by tra-
ditional methodology via several approaches [11–22], three of the more popular
being the cyclization of diarylhydrazines [11], cyclization of arylthiosemicarbazides
[12], and oxidation of arylhydrazones [13–15]. However, prolonged reaction time,
extended reaction period, at elevated temperature, and harsh reagents, e.g., BF3–
OEt2 [16], 1,1,1,3,3,3-hexamethydisilazane [17], triflic anhydride [18], phosphorus
pentoxide [19], polyphosphoric acid [20], thionyl chloride [21–23], phosphorus
oxychloride [24–26], and sulfuric acid [27–29], are usually encountered. These
liquid reagents not only make the reaction systems corrosive but also cause severe
environmental problems because of their difficult separation and recovery after
reactions.
Recently, chemists have reported some greener approaches using polymer-
supported Burgess reagent [30–33], silica sulfuric acid [34, 35], and solid supported
Nafion NR50 [36] as dehydrants to the synthesis of 2,5-disubstituted-1,3,4-
oxadiazoles, which could efficiently avoid the corrosion and pollution caused by
other liquid homogeneous reagents because of the easy separation. Most of these
processes have some disadvantages such as use of expensive reagents, harsh
experimental conditions, long reaction times, and tedious work-up procedures that
generate large amounts of toxic waste. Hence the development of a synthetic
protocol that is nature friendly, simple, efficient, and cost-effective remains an ever-
challenging objective.
Among the most interesting and innovative chemical technologies, the electro-
chemical method provides a powerful means for the small-scale production of a
number of value-added and high purity compounds. Electroorganic reactions
[37–39] have found wide application in industrial processes [40] and are of
increasing interest with regard to the synthesis of complex molecules [41–44]. The
major advantages of electrochemical methods include mild reaction conditions, the
use of inexpensive electricity as a source of reduction equivalents, and high
selectivity, resulting from the possibility to precisely tune the electrochemical
potential at an electrode. Furthermore, in some cases, electrochemical reactions can
afford unexpected products that are difficult to prepare using chemical methods.
The principles of green chemistry dictate that chemical transformations should be
designed to minimize (1) required energy input, either mechanical or thermal, and (2)
the use of harmful organic solvents. Electrochemistry can be considered as a green
methodology in organic synthesis [38, 45–47], due to its non-polluting reagent,
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Electrochemical oxidation of aldehyde-N-arylhydrazones
the electron. A wide variety of reactions can be performed in all areas of organic
chemistry [48–50], but electrochemistry is more adapted to small-scale synthesis of
pharmaceutical [51] and other high-value, low-volume specialty products [52].
Results and discussion
Recently, we have reported a highly effective electrochemical technique for the
oxidative conversion of semicarbazones and arylthiosemicarbazones to several
useful heterocyclic compounds [53–56]. In continuation of our research applying
the electrochemical method as a useful and efficient alternative for the synthesis of
heterocyclic compounds, here we wish to report the electrochemical cyclization of
aldehyde-N-arylhydrazone 1a–q into corresponding symmetrical-2,5-disubstituted-
1,3,4-oxadiazoles 5a–q under mild conditions by combined electrolysis (Scheme 1).
Chiba and Okimoto [57] reported the synthesis of 1,3,4-oxadiazoles in low yield
using methanol and sodium acetate as solvent and electrolyte, respectively. The low
yield was attributed to the formation of acetate ion during the reaction which could
react with the radical cation of the species to simultaneously suppress the ionization
cyclization. However, in the present study, we have used lithium perchlorate as
supporting electrolyte which is inert and gives a good yield of product without
formation of any by-products at the platinum electrode.
Having these preliminary observations in hand, we have synthesized a series of
aldehyde-N-arylhydrazones 1a–q in order to generalize the scope of their oxidative
cyclization. These substrates are easily accessible in high yields and purity from
aroyl hydrazides and aldehydes.
In all cases, the conversion of the substrate 1a–q to desired oxadiazoles 5a–q was
completed within 3–4 h. Chromatographic analysis of the crude mixture indicated
the presence of the desired 1,3,4-oxadiazole as the major product usually with
excellent purity. The structures of the isolated oxidation products as oxadiazole
derivatives 5a–q were determined on the basis of their spectral analysis. The results
obtained are summarized in Table 1.
H
N N
H O
1a-q
X
X
X
X
N
N
MeCN/LiClO4
O
electrolysis
Pt-electrode, RT
5a-q
X
H, p-Me, p-Cl, p-OMe, m-NO2, p-F, m-Cl, o-OMe, m, p-(OMe)2, p-OH,
p-NO2, p-NH2, m-Br, o-OH, o, p-dichloro, 1-Naphthol, 2-Naphthol
Scheme 1 One-pot synthesis of symmetrical-2,5-disubstituted-1,3,4-oxadiazoles 5a–q N0-benzylidene-
N-benzhydrazide 1a was chosen as a model substrate, in order to test the effectiveness of the process. We
found that placing 1a in pure MeCN at room temperature in the presence of LiClO4 resulted in the
formation of the desired 2,5-diphenyl-1,3,4-oxadiazole 5a with complete consumption of the starting
material in few hours. No significant by–products were detected by 1H and 13C NMR spectroscopy
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S. Singh et al.
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Electrochemical oxidation of aldehyde-N-arylhydrazones
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S. Singh et al.
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Electrochemical oxidation of aldehyde-N-arylhydrazones
Cyclic voltammetry of 1a
The cyclic voltammogram of 0.1 M substrate 1a (scan rate: 0.01 V s-1
;
t = 25 1 °C) exhibits two anodic peaks potential first at 0.90 V and the second
at 2.0 V without any reduction peak potential which proves the transformation of 1a
to 5a and within a two-electron oxidation process (Fig. 2).
Proposed reaction mechanism
In the proposed mechanism (Scheme 2), the aldehyde-N-arylhydrazone 1a–q,
undergoes electron oxidation to provide the radical cation intermediate 3.
Subsequent electron oxidation of 3 provides intermediate 4 which furnishes the
corresponding oxadiazoles after intramolecular cyclization and deprotonation steps
resulting in the formation of oxadiazoles 5a–q in 61–93 % yields.
In the process of our study regarding variation of the nature of aldehyde moiety
participating in the formation of oxadiazoles 5a–q, we have observed that the best
results were obtained with hydrazones derived from aromatic aldehydes possessing
electron-donating groups, although the presence of an electron-withdrawing group
on the aromatic ring decreased the yield. This is probably due to the fact that radical
cation intermediates 4 having electron–donating substituents are better stabilized in
comparison to that having electron-withdrawing substituents.
Experimental
Apparatus and reagents
Cyclic voltammetry was performed using ALSCH instruments electrochemical
analyzer Model 600A with a traditional three-electrode system. Cyclic voltammetry
Fig. 2 Cyclic voltammogram of 0.1 M solution of aldehyde-N-arylhydrazones 1a in 0.1 M (LiClO4–
MeCN) on a platinum electrode (area 2.5 mm2) at a scan rate of 0.01 V s-1, auxiliary electrode; platinum
mesh, reference electrode; SCE, t = 25 1 °C
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S. Singh et al.
H
N
X
X
X
X
N
H
N
N
O
O
1
5
-H+
-e-
Intramolecular
cyclization
H
X
X
X
X
X
X
N N
-e-
-H+
N N
H O
N N
O
H
O
H
2
3
4
Scheme 2 Proposed mechanism for the synthesis of symmetrical-2,5-disubstituted-1,3,4-oxadiazoles
5a–q
of substrate 1a were recorded using MeCN as solvent and LiClO4 as supporting
electrolyte at 25 °C. All experiments were carried out in a conventional
electrochemical cell, the electrode system containing a platinum electrode working
as well as the auxiliary electrode, and a saturated calomel electrode (SCE) as
reference electrode. The potential of the working electrode was measured versus
SCE as reference electrode at a scan rate of 0.01 V s-1
.
Melting points were obtained using a capillary melting point apparatus (Mel-
TempÒ) and are uncorrected. IR spectra were registered with a Shimadzu 8201 PC
spectrometer in KBr pellets. 1H and 13C NMR spectra were recorded with a Bruker
DRX 400 (400 MHz) spectrometer in CDCl3. Chemical shifts are reported in ppm
downfield from Me4Si as internal reference. Mass spectra were obtained directly
with a Shimadzu GCMS-QP5050A (EI) spectrometer.
Chemicals used in reaction were reagent-grade, from Merck and Fluka. These
chemicals were used without further purification. Water used for the experiment was
double-distilled.
General procedure for the synthesis of aldehyde-N-arylhydrazones 1a–q
A mixture of 3 mmol of aroyl hydrazines and 3 mmol of aryl aldehydes was
refluxed in 40 mL of ethanol for 7 h. Then, the reaction mixture was cooled to r.t.,
and the precipitates were collected by filtration and recrystallized from ethanol to
give the pure aldehyde-N-arylhydrazones 1a–q. The structures of the products 1a–
q were confirmed from their spectral data and comparison of melting points with
literature values [58].
Characterization data of representative compound 1a
Melting point 207–208 °C [14]. IR (KBr) m: 3,236 (N–H), 1,667 (C=O), 1,640
1
(C=N), 1,472 (C=C aromatic), 3,060 (C–H aromatic) cm-1. H NMR (400 MHz,
CDCl3): d 10.5 (bs, 1H), 8.8 (s, 1H) 7.29–7.96 (m, 10H). 13C NMR (100 MHz,
123

Electrochemical oxidation of aldehyde-N-arylhydrazones
CDCl3): d 127.2, 128.5, 129.0, 130.7, 131.1, 132.0, 133.4, 154.6, 170.1. EIMS
(m/z): 224(M?).
Controlled potential electrolysis (CPE)
Preparative-scale [59] electrolysis were carried out in a 250-mL three-electrode cell
assembly with a platinum plate (flattened sheet of dimension 1.0 9 1.0 cm),
working as well as the auxiliary electrode, and SCE [53–56, 60] as reference
electrode. Electrooxidation of the substrates were as follows: a solution of substrate
1a–q (10 mmol) and powdered LiClO4 (5 mmol) in MeCN (100 mL) was
electrolyzed in an undivided cell equipped with a magnetic stirrer under constant
potential electrolysis indicated in Table 1. After the electrolysis was finished, the
MeCN was evaporated using the rotator evaporator, extracted with CHCl3, and after
removal of the solvent, the residue was purified by EtOH and recrystallization from
small amounts of EtOH. The structures of the synthesized products 5a–q were
1
clearly determined on the basis of their IR, H NMR, 13C NMR, and MS spectra.
The results obtained are summarized in Tables 1 and 2.
Characterization data of representative compounds (5a–q)
2,5-Bis-(phenyl)-1,3,4-oxadiazole (5a)
IR (KBr) m: 1,066 (C–O–C), 1614 (C=N), 1,465 (C=C aromatic), 3,045 (C–H
1
aromatic) cm-1. H NMR (400 MHz, CDCl3): d 7.52–7.58 (m, 6H), 8.14–8.16
Table 2 Physical and yield
data of synthesized compound
5a–q
Compound
Empirical
formula
Melting point (°C)
Yield
(%)
5a
5b
5c
5d
5e
5f
C14H10N2O
135–137 (137–138) [61]
175–177 (175) [62]
239–240 (241–242) [62]
159–161 (160) [63]
135–137 (136–138) [64]
200–202 (200) [63]
156–158 (154–156) [16]
99–101 (98–99) [62]
176–179 (179–181) [61]
349–351 (350) [62]
314–316 (314) [62]
254–256 (253–254) [16]
199–202 (200–201) [62]
119–122 (121) [62]
194–197 (194–195) [64]
198–200
77
86
69
90
63
65
69
90
93
81
61
83
74
78
67
78
82
C16H14N2O
C14H8N2OCl2
C16H14N2O3
C
C
C
C
C
C
C
C
C
C
C
C
C
14H8N4O5
14H8N2OF2
14H8N2OCl2
16H14N2O3
18H18N2O5
14H10N2O3
14H8N4O5
5g
5h
5i
5j
5k
5l
14H12N4O
5m
5n
5o
5p
5q
14H8N2OBr2
14H10N2O3
14H6N2OCl4
22H14N2O3
22H14N2O3
199–201
123

S. Singh et al.
(m, 4H). 13C NMR (100 MHz, CDCl3): d 123.9, 126.9, 129.0, 131.7, 164.6. EIMS
(m/z): 222 [M?].
2,5-Bis-(p-methylphenyl)-1,3,4-oxadiazole (5b)
IR (KBr) m: 1,075 (C–O–C), 1,617 (C=N), 1475 (C=C aromatic), 3,035 (C–H
aromatic), 3,010 (C–H methyl), 985, 890, 755 (substituted benzene) cm-1. 1H NMR
(400 MHz, CDCl3): d 2.40 (s, 6H), 7.32 (d, 4H), 8.01(d, 4H). 13C NMR (100 MHz,
CDCl3): d 21.1, 121.2, 126.7, 129.6, 142.0, 164.4. EIMS (m/z): 250 [M?].
2,5-Bis-(p-chlorophenyl)-1,3,4-oxadiazole (5c)
IR (KBr) m: 1,065 (C–O–C), 1,605 (C=N), 1462 (C=C aromatic), 3,052 (C–H
aromatic), 689 (C–Cl), 960, 865, 750 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 7.45 (d, 4H), 8.00 (d, 4H). 13C NMR (100 MHz, CDCl3): d
122.0, 128.1, 129.4, 138.0, 167.8. EIMS (m/z): 290 [M?].
2,5-Bis-(p-methoxyphenyl)-1,3,4-oxadiazole (5d)
IR (KBr) m: 1,076 (C–O–C), 1,640 (C=N), 1472 (C=C aromatic), 3,060 (C–H
1
aromatic), 2,815 (O–CH3), 980, 810, 765 (substituted benzene) cm-1. H NMR
(400 MHz, CDCl3): d 3.88 (s, 6H), 7.01–7.03 (m, 4H), 8.04–8.06 (m, 4H). 13C
NMR (100 MHz, CDCl3): d 55.5, 114.5, 116.5, 128.6, 162.2, 164.1. EIMS (m/z):
282 (M?).
2,5-Bis-(m-nitrophenyl)-1,3,4-oxadiazole (5e)
IR (KBr) m: 1,080 (C–O–C), 1,622 (C=N), 1,456 (C=C aromatic), 3,059 (C–H
aromatic), 1,550 (N=O), 915, 870, 775 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 7.58–8.61 (m, 8H). 13C NMR (100 MHz, CDCl3): d 122.0,
123.5, 129.8, 133.0, 137.3, 148.7, 163.6, 172.7. EIMS (m/z): 312 [M?].
2,5-Bis-(p-fluorophenyl)-1,3,4-oxadiazole (5f)
IR (KBr) m: 1058 (C–O–C), 1622 (C=N), 1465 (C=C aromatic), 3048 (C–H
aromatic), 830 (C–F), 984, 832, 765 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 6.99–7.15 (m, 4H), 7.96–8.16 (m, 4H). 13C NMR (100 MHz,
CDCl3): d 116.0, 128.5, 132.1, 162.1, 167.8, 172.6. EIMS (m/z): 258 [M?].
2,5-Bis-(m-chlorophenyl)-1,3,4-oxadiazole (5g)
IR (KBr) m: 1,065 (C–O–C), 1612 (C=N), 1,462 (C=C aromatic), 3,052 (C–H
aromatic), 690 (C–Cl), 985, 830, 758 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 7.30–7.60 (m, 6H), 7.89–7.92 (m, 2H). 13C NMR (100 MHz,
CDCl3): d 125.1, 127.4, 128.8, 130.3, 134.2, 137.8, 167.2, 171.4. EIMS (m/z):
293 [M?].
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Electrochemical oxidation of aldehyde-N-arylhydrazones
2,5-Bis-(o-methoxyphenyl)-1,3,4-oxadiazole (5h)
IR (KBr) m: 1,056 (C–O–C), 1,617 (C=N), 1,465 (C=C aromatic), 3,049 (C–H
1
aromatic), 2,815 (O-CH3), 965, 892, 750 (substituted benzene) cm-1. H NMR
(400 MHz, CDCl3): d 3.95 (s, 6H), 7.03–7.07 (m, 4H), 7.45–7.49 (m, 2H),
7.97–7.99 (m, 4H). 13C NMR (100 MHz, CDCl3): d 56.2, 114.5, 121.2, 122.0,
128.0, 129.4, 160.4, 167.4, 172.7. EIMS (m/z): 282 [M?].
2,5-Bis-(m,p-dimethoxyphenyl)-1,3,4-oxadiazole (5i)
IR (KBr) m: 1,050 (C–O–C), 1,621 (C=N), 1,459 (C=C aromatic), 3,045 (C–H
1
aromatic), 2,810 (O–CH3), 978, 866, 751 (substituted benzene) cm-1. H NMR
(400 MHz, CDCl3): d 3.95 (s, 6H), 3.98 (s, 6H), 6.96 (d, 2H), 7.63–7.68 (m, 4H).
13C NMR (100 MHz, CDCl3): d 56.0, 56.2, 109.5, 111.1, 116.6, 120.3, 149.4, 151.9,
164.2. EIMS (m/z): 342 [M?].
2,5-Bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (5j)
IR (KBr) m: 1,046 (C–O–C), 1,606 (C=N), 1,465 (C=C aromatic), 3,052 (C–H
aromatic), 3,594 (O–H), 967, 885, 785 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 6.95 (d, 4H), 7.91 (d, 4H), 10.27 (s, 2H, OH). 13C NMR
(100 MHz, CDCl3): d 114.2, 116.1, 128.4, 160.6, 163.5. EIMS (m/z): 254 [M?].
2,5-Bis-(p-nitrophenyl)-1,3,4-oxadiazole (5k)
IR (KBr) m: 1,057 (C–O–C), 1610 (C=N), 1,463 (C=C aromatic), 3,056 (C–H
aromatic), 1,555 (N=O), 974, 872, 790 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 8.41 (d, 4H), 8.56 (d, 4H). 13C NMR (100 MHz, CDCl3): d
124.4, 126.9, 129.3, 149.3, 162.6. EIMS (m/z): 312 [M?].
2,5-Bis-(p-aminophenyl)-1,3,4-oxadiazole (5l)
IR (KBr) m: 1,062 (C–O–C), 1,618 (C=N), 1,468 (C=C aromatic), 3,058 (C–H
aromatic), 3,416 (N–H), 985, 890, 755 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 6.61–6.66 (m, 4H), 7.85–7.87 (m, 4H), 7.16 (s, 4H). 13C
NMR (100 MHz, CDCl3): d 115.5, 126.4, 127.7, 146.6, 164.3, 171.5. EIMS (m/z):
252 [M?].
2,5-Bis-(m-bromophenyl)-1,3,4-oxadiazole (5m)
IR (KBr) m: 1056 (C–O–C), 1621 (C=N), 1461 (C=C aromatic), 3032 (C–H
aromatic), 736 (C–Br), 985, 890, 755 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 7.75 (d, 2H), 7.54 (br s, 2H), 7.35 (t, 4H). 13C NMR
(100 MHz, CDCl3): d 123.5, 126.0, 130.2, 131.1, 131.7, 138.6, 162.5, 172.8. EIMS
(m/z): 380 [M?].
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S. Singh et al.
2,5-Bis-(o-hydroxyphenyl)-1,3,4-oxadiazole (5n)
IR (KBr) m: 1,035 (C–O–C), 1,609 (C=N), 1,449 (C=C aromatic), 3,018 (C–H
aromatic), 983, 891, 753 (substituted benzene) cm-1. 1H NMR (400 MHz, CDCl3):
d 6.97 (d, 2H), 7.44–7.51 (m, 4H), 7.94 (d, 2H), 10.91 (s, 1H). 13C NMR (100 MHz,
CDCl3): d 116.1, 121.5, 123.6, 128.3, 129.8, 155.7, 167.2, 172.6. EIMS (m/z): 254
[M?].
2,5-Bis-(o,p-dichlorophenyl)-1,3,4-oxadiazole (5o)
IR (KBr) m: 1,066 (C–O–C), 1,611 (C=N), 1,465 (C=C aromatic), 3,054 (C–H
aromatic), 695 (C–Cl), 985, 890, 757 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 7.29 (dd, 2H), 7.44 (d, 2H), 7.89 (d, 2H). 13C NMR
(100 MHz, CDCl3): d 120.9, 127.4, 130.8, 131.6, 133.5, 138.0, 162.3. EIMS (m/z):
357 [M?].
2,5-Bis-(naphthalen-1-ol)-1,3,4-oxadiazole (5p)
IR (KBr) m: 1,059 (C–O–C), 1,606 (C=N), 1,469 (C=C aromatic), 3,057 (C–H
aromatic), 690 (C–Cl), 980, 895, 760 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 5.4 (s, 2H), 7.34–8.09 (m, 6H), 8.45 (d, J = 7.7 Hz, 2H), 7.46
(d, J = 8.6 Hz, 2H). 13C NMR (100 MHz, CDCl3): d 116.7, 121.1, 121.4, 124.2,
124.9, 126.1, 126.4, 127.6, 134.7, 150.1, 168.2. EIMS (m/z): 354 [M?].
2,5-Bis-(naphthalen-2-ol)-1,3,4-oxadiazole (5q)
IR (KBr) m: 1,064 (C–O–C), 1,609 (C=N), 1,474 (C=C aromatic), 3,045 (C–H
aromatic), 696 (C–Cl), 976, 890, 765 (substituted benzene) cm-1 1H NMR
.
(400 MHz, CDCl3): d 5.4 (s, 2H), 7.03–7.94 (m, 10H), 7.89 (d, J = 7.7 Hz, 2H).
13C NMR (100 MHz, CDCl3): d 109.8, 123.6, 125.5, 126.3, 126.4, 124.9, 127.7,
129.4, 128.3, 134.6, 151.8, 169.8. EIMS (m/z): 354 [M?].
Conclusion
In summary, we have developed a convenient and efficient one-pot electrochemical
synthesis of 2,5-disubstituted-1,3,4-oxadiazole derivatives at ambient temperature.
The novelty of this methodology can be summarized as follows:
(1) We first synthesized a series of 2,5-disubstituted-1,3,4-oxadiazole derivatives
in good to excellent yield by utilizing inexpensive reagents, simple equipment,
and undivided cell.
(2) In our study, we have used lithium perchlorate as supporting electrolyte which
is inert and gives good yield of product without formation of any by-product at
the platinum electrode.
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Electrochemical oxidation of aldehyde-N-arylhydrazones
Further expansion of the scope and application of this methodology for the
synthesis of bioactive compounds is currently underway and will be reported in due
course.
Acknowledgments The authors are thankful to the Head, Department of Chemistry and University of
Allahabad for providing necessary facilities, SAIF, Punjab University, Chandigarh, for providing spectra
and also CST-UP for financial support for accomplishment of the study.
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