Microwave-assisted expeditious synthesis of novel carbazole-based 1,3,4-oxadiazoles

Article abstract of DOI:10.1080/00397910600941141

A microwave-assisted expeditious synthetic route to novel carbazole-based 1,3,4-oxadiazoles is described. The reactions of 9-ethylcarbazol-3-carbaldehyde with aroylhydrazines under microwave irradiation first gave intermediates, l-aroyl-2-(9′-ethylcarbazo

Full text of DOI:10.1080/00397910600941141

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Microwave‐Assisted Expeditious Synthesis of
Novel Carbazole‐Based 1,3,4‐Oxadiazoles
Zheng Li a , Yulin Xing a , Xuelin Ma a , Shuxiu Xiao a & Zhong Lu a
a Gansu Key Laboratory of Polymer Materials, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou, Gansu, China
Version of record first published: 23 Nov 2006
To cite this article: Zheng Li, Yulin Xing, Xuelin Ma, Shuxiu Xiao & Zhong Lu (2006): Microwave‐Assisted
Expeditious Synthesis of Novel Carbazole‐Based 1,3,4‐Oxadiazoles, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:22, 3287-3295
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Synthetic Communicationsw, 36: 3287–3295, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600941141
Microwave-Assisted Expeditious Synthesis
of Novel Carbazole-Based
1,3,4-Oxadiazoles
Zheng Li, Yulin Xing, Xuelin Ma, Shuxiu Xiao, and Zhong Lu
Gansu Key Laboratory of Polymer Materials, College of Chemistry
and Chemical Engineering, Northwest Normal University, Lanzhou,
Gansu, China
Abstract: A microwave-assisted expeditious synthetic route to novel carbazole-based
1,3,4-oxadiazoles is described. The reactions of 9-ethylcarbazol-3-carbaldehyde with
aroylhydrazines under microwave irradiation first gave intermediates, l-aroyl-2-
(90-ethylcarbazol-30-yl-methylidene)hydrazines, which were further treated with
potassium permanganate in DMF under microwave irradiation to rapidly afford a
series of 2-aryl-5-(90-ethylcarbazol-30-yl)-1,3,4-oxadiazoles in excellent yield.
Keywords: Aroylhydrazine, carbazole, microwave irradiation, 1,3,4-oxadiazole
INTRODUCTION
1,3,4-Oxadiazoles are important heterocyclic compounds with a wide
range of pharmaceutical and biological activities, such as antibacterial,[1]
anti-inflammatory,[2] insecticidal,[3] anticonvulsant,[4] and antimitotic[5]
activities. However, heterocyclic compounds bearing the 1,3,4-oxadiazole
moiety have been used as a new p-conjugation relay to prepare a number
of donor–acceptor molecules carrying a p-electron-rich aromatic ring.[6] In
addition, carbazole-involved compounds have received increasing interest
Received in Japan April 18, 2006
Address correspondence to Zheng Li, Gansu Key Laboratory of Polymer Materials,
College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou, Gansu 730070, China. E-mail: lizheng@nwnu.edu.cn
3287

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Z. Li et al.
because of their unique optical properties and strong hole-transporting ability
in opto-electronic devices.[7,8] Therefore, the compounds bearing both
carbazole and 1,3,4-oxadiazole moieties may be good candidates for optical
materials or biologically active chemicals.
The common synthetic route to 1,3,4-oxidiazoles includes reactions of acid
hydrazides (or hydrazine) with acid chlorides[9,10] (or carboxylic acids[11]) or
direct cyclization of diacylhydrazines by use of a variety of anhydrous reagents
such as thionyl chloride,[12] phosphorous pentoxide,[13] phosphorous oxychlor-
ide,[14] triflic anhydride,[15] and polyphosphoric acid.[16] The acylation of tetra-
zoles[17] and oxidation of acylhydrazones[18,19] are additional synthetic routes
to these compounds. Recently, solid-phase syntheses of these compounds were
also reported.[20–22] However, these methods often require long reaction times
and high temperature, and the reagents used in the reactions are usually
corrosive and environmentally unfriendly.
In this article, we report a microwave-assisted rapid synthetic method for
novel carbazole-based 1,3,4-oxadiazoles by using potassium permanganate as
an efficient cyclization reagent. To the best of our knowledge, this is the first
exampleofthesynthesisofsmallmoleculesofcarbazole-based1,3,4-oxadiazole.
RESULTS AND DISCUSSION
The reaction of carbazole (1) with bromoethane in the presence of potassium
hydroxide first gave 9-ethylcarbazole (2) in high yield. Compound 2 on
treatment with phosphorus oxychloride and N,N-dimethylformamide in the
solution of 1,2-dichloroethane at 908C for 6 h afforded 9-ethylcarbazol-3-car-
baldehyde (3) in high selectivity (no 3,6-dicarbaldehyde by-product was
observed). Compound 3 was irradiated with various aroylhydrazines for 3–6
min at 150 W of microwave power to efficiently give 1-aroyl-2-(90-ethylcarba-
zol-30-ylmethylidene)hydrazines (4a–i) after simple washing with ethanol. The
solution of compounds 4a–i and potassium permanganate in DMF were
subjected to microwave irradiation at 70 W of power for only 5–8 min to
readily afford 2-aryl-5-(90-ethylcarbazol-30-yl)-1,3,4-oxadiazoles (5a–i) in
excellent yield Scheme 1, Table 1. It is noteworthy to mention that the
attempt to synthesize compounds 5a–i using other organic solvents, such as
ethanol, acetone, and acetonitrile, did not succeed because of the insolubility
of potassium permanganate and formation of many by-products. In addition,
the same reactions were also investigated by a conventional heating method.
However, it took at least 10 h to complete the reactions with satisfactory yield.
The resulting compounds 5a–i are highly soluble in common
organic solvents including CHC13, CH2C12, DMSO, DMF, and EtOH. The
1
structures of new compounds 4a–i and 5a–i were identified by H NMR,
IR, and elemental analyses. The 1H NMR spectra of compounds 4a–i
in DMSO-d6 show –CH55 group proton peaks at d ¼ 8.60–8.63 ppm and
–NH– group proton peaks at d ¼ 11.75–11.91 ppm. These two kinds of

Carbazole-Based 1,3,4-Oxadiazoles
3289
Scheme 1.
1
signals thoroughly disappear in the H NMR spectra of compounds 5a–i,
which indicate the formation of oxadiazole ring. The IR spectra of
compounds 4a–i show characteristic absorptions at 3171–3196 cm21, and
1630–1654 cm21, attributable to the imino and carbonyl groups, respectively.
These adsorptions vanish in the IR spectra of compounds 5a–i, which also
mean the formation of the oxadiazole ring.
In conclusion, we have described a rapid and highly efficient method for
the synthesis of a new series of carbazole-based 1,3,4-oxadiazoles under
microwave irradiation. This protocol has rapid reaction rate, short reaction
time, and high reaction yield.
Table 1. Microwave-assisted synthesis of 4a–i and 5a–i
Compd.
R
Time (min)
Mp (8C)
Yield (%)a
4a
4b
4c
4d
4e
4f
H
4-Cl
4
5
6
6
6
4
3
5
3
5
5
8
5
5
6
6
8
7
237–239
246–248
221–222
253–254
218–220
220–221
240–241
238–240
296–298
158–159
204–206
148–150
236–238
192–194
140–142
168–170
198–200
296–298
97
95
90
96
91
92
95
95
94
92
89
84
86
79
88
75
90
83
2-Cl
4-NO2
3-NO2
2-CH3
3-CH3
4-Br
4g
4h
4i
4-HO
H
4-Cl
5a
5b
5c
5d
5e
5f
2-Cl
4-NO2
3-NO2
2-CH3
3-CH3
4-Br
5g
5h
5i
4-HO
aYields refer to the isolated products.

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Z. Li et al.
EXPERIMENTAL
IR spectra were recorded using KBr pellets on a Digilab FTS 3000 FTIR
1
spectrophotometer and H NMR spectra on a Mercury Plus-400 instrument
using (CD3)2SO or CDCl3 as solvents and Me4Si as internal standard.
Elemental analyses were performed on a Vario E1 elemental analysis instru-
ment. Mass spectra were recorded on a QP-1000A GC-MS using the impact
mode (70 eV). Melting points were observed in an electrothermal melting-
point apparatus. Microwave reactions were conducted in a modified micro-
wave oven fitted with a condenser (LG-WP650, China). Aroylhydrazines,[23]
9-ethylcarbazole (2),[24] and 9-ethyl-carbazol-3-carbaldehyde (3)[25] were
prepared according to literature procedures.
General Procedure for Preparation of 1-Aroyl-2-(90-ethylcarbazol-
30-yl-methylidene)hydrazines (4a–i)
The aroylhydrazine (6 mmol) and 9-ethylcarbazole-3-carbaldehyde (6 mmol)
were thoroughly mixed in a mortar with a pestle, and then the mixture was
transferred to a flask and subjected to microwave irradiation at 150 W of
power. The completion of the reaction was monitored by thin-layer chromato-
graphy (TLC) (ethyl acetate–petroleum ether ¼ 1 : 2 as eluent). Then the
resulting mixture was washed with ethanol, and the solid was recrystallized
from DMF and water to give the product.
Analytical Data for Compounds 4a–i
4a: White solid. IR (KBr pellet, cm21): 3176 (vN-H), 1634 (vC55O), 1599, 1553,
1
1469, 1303, 1232 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d: 1.32
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.46 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.22–8.47
(m, 12H, Ar-H), 8.61 (s, 1H, –CH55), 11.79 6 (s, 1H, –NH–). MS: m/z,
341 (Mþ). Anal. calcd. for C22H19N3O: C, 77.40; H, 5.61; N, 12.31. Found:
C, 77.31; H, 5.54; N, 12.24.
4b: White solid. IR (KBr pellet, cm21): 3186 (VN-H), 1644 (VC55O), 1597,
1
1554, 1473, 1299, 1234 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d:
1.34 (t, J ¼ 6.8 Hz, 3H, –CH3), 4.49 (q, J ¼ 6.8 Hz, 2H, –CH2–),
7.24–8.48 (m, 11H, Ar–H); 8.62 (s, lH, –CH55), 11.87 (s, lH, –NH–).
MS: m/z, 375 (Mþ). Anal. calcd. for C22H18ClN3O: C, 70.30; H, 4.83; N,
11.18. Found: C,70.22; H, 4.90; N, 11.26.
4c: White solid. IR (KBr pellet, cm21): 3182 (VN–H), 1646 (vC55O), 1596,
1
1553, 1474, 1300, 1235 (carbazole ring). H NMR (DMSO-d6, 400 MHz)

Carbazole-Based 1,3,4-Oxadiazoles
3291
d: 1.33 (t, J ¼ 6.8 Hz, 3H, –CH3), 4.58 (q, J ¼ 6.8 Hz, 2H, –CH2–),
7.25–8.49 (m, 11H, Ar–H), 8.61 (s, 1H, –CH55), 11.85 (s, 1H, –NH–).
MS: m/z, 375 (Mþ). Anal. calcd. for C22H18ClN3O: C, 70.30; H, 4.83; N,
11.18. Found: C, 70.21; H, 4.90; N, 11.11.
4d: Yellow solid. IR (KBr pellet, cm21): 3195 (vN–H), 1654 (vC55O), 1602,
1
1559, 1478, 1305, 1239 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d:
1.41 (t, J ¼ 7.2 Hz, 3H, –CH3), 4.53 (q, J ¼ 7.2 Hz, 2H, –CH2–),
7.29–8.54 (m, 11H, Ar–H); 8.63 (s, 1H, –CH55), 11.91 (s, 1H, –NH–).
MS: m/z, 386 (Mþ). Anal. calcd. for C22H18N4O3: C, 68.38; H, 4.70; N,
14.50. Found: C, 68.47; H, 4.77; N, 14.41.
4e: Yellow solid. IR (KBr pellet, cm21): 3196 (vN–H), 1653 (vC55O), 1604,
1
1559, 1479, 1304, 1238 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d:
1.40 (t, J ¼ 7.2 Hz, 3H, –CH3), 4.52 (q, J ¼ 7.2 Hz, 2H, –CH2–),
7.26–8.53 (m, 11H, Ar–H); 8.63 (s, 1H, –CH55), 11.90 (s, 1H, –NH–).
MS: m/z, 386 (Mþ). Anal. calcd. for C22H18N4O3: C, 68.38; H, 4.70; N,
14.50. Found: C, 68.29; H, 4.65; N, 14.42.
4f: White solid. IR (KBr pellet, cm21): 3l72 (vN–H), 1631 (vC55O), 1593, 1550,
1
1467, 1290, 1228 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d: 1.30
(t, J ¼ 6.8 Hz, 3H, –CH3), 2.14 (s, 3H, –CH3), 4.44 (q, J ¼ 6.8 Hz, 2H,
–CH2–), 7.20–8.45 (m, 11H, Ar–H); 8.60 (s, 1H, –CH55), 11.76
(s, 1H, –NH–). MS: m/z, 354 (Mþ). Anal. calcd. for C23H20N3O: C,
77.94; H, 5.69; N, 11.86. Found: C, 77.86; H, 5.73; N, 11.77.
4g: White solid. IR (KBr pellet, cm21): 3171 (vN–H), 1630 (vC55O), 1592,
1
1551, 1466, 1291, 1227 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d:
1.29 (t, J ¼ 6.8 Hz, 3H, –CH3), 2.15 (s, 3H, –CH3), 4.42 (q, J ¼ 6.8 Hz,
2H, –CH2–), 7.19–8.44 (m, 11H, Ar–H); 8.60 (s, 1H, –CH55), 11.75
(s, 1H, –NH–). MS: m/z, 354 (Mþ). Anal. calcd. for C23H20N3O: C, 77.94;
H, 5.69; N, 11.86. Found: C, 78.06; H, 5.60; N, 11.93.
4h: Yellow solid. IR (KBr pellet, cm21): 3185 (vN–H), 1649 (vC55O), 1599,
1
1556, 1476, 1300, 1237 (carbazole ring). H NMR (DMSO-d6, 400 MHz) d:
1.38 (t, J ¼ 7.2 Hz, 3H, –CH3), 4.51 (q, J ¼ 7.2 Hz, 2H, –CH2–),
7.26–8.50 (m, 11H, Ar–H); 8.63 (s, 1H, –CH55), 11.90 (s, 1H, –NH–).
MS: m/z, 419 (Mþ). Anal. calcd. for C22H18BrN3O: C, 62.87; H, 4.32; N,
10.00. Found: C, 62.98; H, 4.24; N, 10.10.
4i: Yellow solid. IR (KBr pellet, cm21): 3477 (vO–H), 3183 (vN–H), 1645
1
(vC55O), 1597, 1554, 1475, 1298, 1236 (carbazole ring). H NMR (DMSO-
d6, 400 MHz) d: 1.37 (t, J ¼ 7.2 Hz, 3H, –CH3), 4.50 (q, J ¼ 7.2 Hz,
2H, –CH2–), 7.25–8.51 (m, 11H, Ar–H); 8.62 (s, 1H, –CH55), 11.88

3292
Z. Li et al.
(s, 1H, –NH–). MS: m/z, 357 (Mþ). Anal. calcd. for C22H19N3O2: C, 73.93;
H, 5.36; N, 11.76. Found: C, 73.81; H, 5.40; N, 11.83.
General Procedure for Preparation of 2-Aryl-5-
(90-ethylcarbazol-30-yl)-1,3,4-oxadiazoles (5a–i)
The mixture of compounds 4a–i (3 mmol) and potassium permanganate
(3 mmol) was dissolved in 2 mL of DMF. Then the mixture was subjected
to microwave irradiation at the 70 W of power for the appropriate time
indicated in Table 1. The completion of the reaction was monitored by TLC
(ethyl acetate–petroleum ether ¼ 1 : 2 as eluent). Then the resulting mixture
was filtered, and the filtrate was evaporated to remove the solvent in vacuo.
The residue was recrystallized from ethanol to afford the desired product.
Analytic Data for Compounds 5a–i
5a: White solid. IR (KBr pellet, cm21): 1598, 1551, 1460, 1332, 1235
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.44
(t, J ¼ 6.8 Hz, 3H, –CH3), 4.36 (q, J ¼ 6.8 Hz, 2H, –CH2–), 7.24–8.73
(m, 12H, Ar–H). MS: m/z, 339 (Mþ). Anal. calcd. for C22H17N3O: C,
77.86; H, 5.05; N, 12.38. Found: C, 77.78; H, 5.12; N, 12.44.
5b: White solid. IR (KBr pellet, cm21): 1595, 1540, 1477, 1346, 1260
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.48
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.39 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.25–8.81
(m, 11H, Ar–H). MS: m/z, 373 (Mþ). Anal. calcd. for C22H16ClN3O: C,
70.68; H, 4.31; N, 11.24. Found: C, 70.74; H, 4.24; N, 11.30.
5c: White solid. IR (KBr pellet, cm21): 1595, 1541, 1475, 1344, 1261
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.47
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.38 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.22–8.80
(m, 11H, Ar–H). MS: m/z, 373 (Mþ). Anal. calcd. for C22H16ClN3O: C,
70.68; H, 4.31; N, 11.24. Found: C, 70.77; H, 4.22; N, 11.16.
5d: Yellow solid. IR (KBr pellet, cm21): 1601, 1555, 1465, 1337, 1239
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.54
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.45 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.28–8.87
(m, 11H, Ar–H). MS: m/z, 384 (Mþ). Anal. calcd. for C22H16N4O3: C,
68.74; H, 4.20; N, 14.58. Found: C, 68.69; H, 4.30; N, 14.49.
5e: Yellow solid. IR (KBr pellet, cm21): 1600, 1554, 1461, 1336, 1237
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.53
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.43 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.27–8.86

Carbazole-Based 1,3,4-Oxadiazoles
3293
(m, 11H, Ar–H). MS: m/z, 384 (Mþ). Anal. calcd. for C22H16N4O3: C, 68.74;
H, 4.20; N, 14.58. Found: C, 68.64; H, 4.13; N, 14.62.
5f: White solid. IR (KBr pellet, cm21): 1591, 1548, 1453, 1326, 1227
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.41
(t, J ¼ 6.8 Hz, 3H, –CH3), 2.18 (s, 3H, –CH3), 4.32 (q, J ¼ 6.8 Hz, 2H,
–CH2–), 7.19–8.68 (m, 11H, Ar–H). MS: m/z, 353 (Mþ). Anal. calcd. for
C23H19N3O: C, 78.16; H, 5.42; N, 11.89. Found: C, 78.20; H, 5.35; N, 11.97.
5g: White solid. IR (KBr pellet, cm21): 1590, 1547, 1452, 1325, 1228
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.40
(t, J ¼ 6.8 Hz, 3H, –CH3), 2.18 (s, 3H, –CH3), 4.30 (q, J ¼ 6.8 Hz, 2H,
–CH2–), 7.17–8.67 (m, 11H, Ar–H). MS: m/z, 353 (Mþ). Anal. calcd. for
C23H19N3O: C, 78.16; H, 5.42; N, 11.89. Found: C, 78.09; H, 5.31; N, 11.96.
5h: Yellow solid. IR (KBr pellet, cm21): 1597, 1553, 1462, 1334, 1236
(carbazole and oxadiazole rings). 1H NMR (CDCl3, 400 MHz) d: 1.53
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.43 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.26–8.84
(m, 11H, Ar–H). MS: m/z, 417 (Mþ). Anal. calcd. for C22H16BrN3O: C,
63.17; H, 3.86; N, 10.05. Found: C, 63.25; H, 3.91; N, 10.12.
5i: Yellow solid. IR (KBr pellet, cm21): 3489 (vO–H), 1594, 1549, 1460, 1331,
1
1233 (carbazole and oxadiazole rings). H NMR (CDCl3, 400 MHz) d: 1.51
(t, J ¼ 7.2 Hz, 3H, –CH3), 4.40 (q, J ¼ 7.2 Hz, 2H, –CH2–), 7.25–8.82
(m, 11H, Ar–H). MS: m/z, 355 (Mþ). Anal. calcd. for C22H17N3O2: C,
74.35; H, 4.82; N, 11.82. Found: C, 74.27; H, 4.76; N, 11.89.
ACKNOWLEDGMENT
The authors thank the Chunhui Project of the Ministry of Education of China
(Z2004-1-62034) for financial support of this work.
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