The reaction of (N-isocyanimino) triphenylphosphorane with (E)-3-aryl-2- propenoic acid derivatives: One-pot synthesis of 2-[(E)-2-aryl-1-ethenyl]-1,3,4- oxadiazoles via intramolecular aza-Wittig reaction

Article abstract of DOI:10.1002/hc.20701

The reaction of (E)-3-aryl-2-propenoic acid derivatives with (N-isocyanimino) triphenylphosphorane proceeds smoothly at room temperature to afford the corresponding 2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazole via an intramolecular aza-Wittig reaction in goo

Full text of DOI:10.1002/hc.20701

Heteroatom Chemistry
Volume 22, Number 5, 2011
The Reaction of (N-Isocyanimino)
triphenylphosphorane with (E)-3-Aryl-2-
propenoic Acid Derivatives: One-Pot Synthesis
of 2-[(E)-2-Aryl-1-ethenyl]-1,3,4-oxadiazoles
via Intramolecular aza-Wittig Reaction
Hamideh Ahankar,1 Ali Ramazani,2 Issa Amini,3 Yavar Ahmadi,4
and Ali Souldozi5
1Chemistry Department, Abhar Branch, Islamic Azad University, P.O. Box 22, Abhar, Iran
2Chemistry Department, Zanjan Branch, Islamic Azad University, P.O. Box 49195-467, Zanjan,
Iran
3Chemistry Department, Payame Noor University, P.O. Box 97, Abhar, Zanjan, Iran
4Chemistry Department, Zanjan University, P.O. Box 45195-313, Zanjan, Iran
5Chemistry Department, Urmia Branch, Islamic Azad University, P.O. Box 969, Urmia, Iran
Received 26 November 2010; revised 31 January 2011
reagents as well as ligands in a number of tran-
ABSTRACT: The reaction of (E)-3-aryl-2-
sition metal catalysts [1]. Iminophosphoranes are
propenoic acid derivatives with (N-isocyanimino)
a special type of zwitterions, which bear strongly
triphenylphosphorane proceeds smoothly at room
nucleophilic electron-rich nitrogen. The electron
temperature to afford the corresponding 2-[(E)-2-aryl-
distribution around the P+–N− bond and its conse-
1-ethenyl]-1,3,4-oxadiazole via an intramolecular
quent chemical implications were probed and as-
aza-Wittig reaction in good yields under neutral
sessed through theoretical, spectroscopic, and crys-
conditions. The structures of the products were
tallographic investigations [1–3]. They are excellent
deduced from their IR, 1H NMR, and 13C NMR spectra
ligands and excel in their ligating functions of the un-
ꢀ
C
and mass spectrometry.
2011 Wiley Periodicals, Inc.
stabilized iminophosphoranes because of their am-
bidentate and chemically differentiating character.
Proton affinity of these iminophosphoranes can be
used as a molecular guide to assess their utility as
synthetic reagents and their function as ligands in
coordination and organometallic chemistry [1–3].
The intramolecular version of the aza-Wittig-
type reaction has attracted considerable attention
recently because of its high potential for the syn-
thesis of a wide variety of nitrogen heterocycles,
which can be attributed in good measure to the
rapid progress in the preparation of functional-
ized iminophosphoranes, and several interesting
Heteroatom Chem 22:612–616, 2011; View this article
online at wileyonlinelibrary.com. DOI 10.1002/hc.20701
INTRODUCTION
Organophosphorus compounds [1–3] have been ex-
tensively employed in organic synthesis as useful
Correspondence to: Ali Ramazani; e-mail: aliramazani@gmail
.com.
c
ꢀ
2011 Wiley Periodicals, Inc.
612

Reaction of (N-Isocyanimino)triphenylphosphorane with (E)-3-Aryl-2-propenoic Acid Derivatives 613
N
O
N
CH2Cl2
r. t.
H
O
N
C
N
PPh3
OH
+
Ph3PO
+
X
X
R
R
2
4
3
1
SCHEME 1 Synthesis of 2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazoles 3 (see Table 1).
heterocyclization reactions involving iminophos-
phoranes have been reviewed [3]. These compounds
can be easily converted through the aza-Wittig
reaction with isocyanates, carbon dioxide, or car-
bon disulfide into functionalized heterocumulenes,
which exhibit a rich chemistry of unusual synthetic
promise [3]. The nucleophilicity at the nitrogen
is a factor of essential mechanistic importance
in the use of these iminophosphoranes as aza-
Wittig reagents. Iminophosphoranes are important
reagents in synthetic organic chemistry, especially
in the synthesis of naturally occurring products
and compounds with biological and pharmacolog-
ical activity [3]. There are several reports for the
with a variety of reagents, such as thionyl chloride,
phosphorous oxychloride, or sulfuric acid, usually
under harsh reaction conditions. Few reliable and
operationally simple examples have been reported
for the one-step synthesis of 1,3,4-oxadiazoles, es-
pecially from readily available carboxylic acids and
acid hydrazides [34,35].
RESULTS AND DISCUSSION
In the past years, several synthetic methods
have been reported for the preparation of (N-
isocyanimino)triphenylphosphorane (CNNPPh3) 2
(Scheme 1) [4,5]. There are several reports for the
use of (N-isocyanimino)triphenylphosphorane 2 in
the synthesis of metal complexes [4,5]. However,
application of 2 in the synthesis of organic com-
pounds has not been reported. As a part of our on-
going program to develop efficient and robust meth-
ods for the preparation of heterocyclic compounds
[13–28], we sought to develop a convenient prepa-
ration of 2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazole 3
from (E)-3-aryl-2-propenoic acid derivatives 1 and
(N-isocyanimino)triphenylphosphorane 2 in fairly
good yields in neutral conditions. The reaction is
completely stereoselective (Scheme 1).
The (E)-3-aryl-2-propenoic acid derivatives 1
and (N-isocyanimino)triphenylphosphorane 2 in
dry CH2Cl2 react together in a 1:1 ratio at room
temperature to produce 2-[(E)-2-aryl-1-ethenyl]-
1,3,4-oxadiazole 3 and triphenylphosphine oxide 4
(Scheme 1 and Table 1). The reaction proceeds
smoothly and cleanly under mild conditions.
use
of
(N-isocyanimino)triphenylphosphorane
2
in the preparation of metal complexes
[4,5] (Scheme 1). However, the role of (N-
isocyanimino)triphenylphosphorane in or-
2
ganic chemistry remains almost unexplored [4,5].
The (N-isocyanimino)triphenylphosphorane 2 is
expected to have unique synthetic potential be-
cause it provides a reaction system in which the
iminophosphorane group can react with a reagent
having a carbonyl functionality [4,5]. In recent
years, we have established a one-pot method for
the synthesis of organophosphorus compounds
[6–12]. As a part of our ongoing program to
develop efficient and robust methods for the
preparation of heterocyclic compounds [13–28],
we sought to develop a convenient preparation
of 2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazole 3 from
(E)-3-aryl-2-propenoic acid derivatives 1 and (N-
isocyanimino)triphenylphosphorane
2
in fairly
good yield in neutral conditions. The reaction is
completely stereoselective (Scheme 1).
1,3,4-Oxadiazoles have attracted interest in
medicinal chemistry as surrogates of carboxylic
acids, esters, and carboxamides. They are an im-
portant class of heterocyclic compounds that have
a wide range of pharmaceutical and biological ac-
tivities including antimicrobial, antifungal, antiin-
flammatory, and antihypertensive [29–31]. Several
methods have been reported in the literature for
the synthesis of 1,3,4-oxadiazoles. These protocols
are multistep in nature [32,33]. The most general
method involves the cyclization of diacylhydrazides
TABLE
1 Synthesis of 2-[(E)-2-Aryl-1-ethenyl]-1,3,4-
oxadiazole Derivatives 3 (See Scheme 1)
Compound
X
R
Yield (%)a
3a
3b
3c
3d
3e
3f
H
4-CH3
4-Cl
3-Cl
3-MeO
H
H
H
H
H
H
77
81
80
77
76
83
Me
aIsolated yields
Heteroatom Chemistry DOI 10.1002/hc

614 Ahankar et al.
The structures of the products were deduced
acid, benzylic acid, 3,5-dinitrosalicylic acid, (2-
hydroxyphenyl)acetic acid, 4-hydroxybenzoic
from their IR, 1H NMR and 13C NMR spectra,
and mass spectrometry (see the Experimental
section). The mass spectra of compound 3 displayed
acid, 3-hydroxybenzoic acid, salicylic acid, 4-
aminobenzenesulfonic acid, 5-sulfosalicylic acid,
nicotinic acid, 3-methoxyphenylacetic acid, 4-
methoxyphenylacetic acid, diphenylacetic acid,
4-fluorobenzeneacetic acid, 3-phenylpropanoic
acid, 4-chlorobenzeneacetic acid, benzenesulfonic
acid, acetylenedicarboxylic acid, and methyl red) in
this reaction, but no products were observed after 24
h. TLC indicated that the solution contained starting
materials and some minor colored products.
The mechanism of the reaction between the
(E)-3-aryl-2-propenoic acid derivatives 1 and (N-
isocyanimino)triphenylphosphorane 2 has not been
established experimentally. However, a possible ex-
planation is proposed in Scheme 2. On the basis of
the well-established chemistry of isocyanides [36],
it is reasonable to assume that protonation of 2 by
(E)-3- aryl-2-propenoic acid 1 followed by quenching
of the cationic center by the conjugate base of car-
boxylic acid (5) can generate the iminophosphorane
6 [3]. The intramolecular aza-Wittig [3] reaction of
the iminophosphorane 6 would lead to formation of
2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazole3 and triph-
enylphosphine oxide 4 (Scheme 2).
1
molecular ion peak at the m/z 172. The H NMR
spectrum of 3 compound exhibited five signals
readily recognized as arising from two vinylic
3
groups (δ = 7.09, d, 1 H, JHH = 16.5 Hz and δ =
3
7.62, d, 1 H, JHH = 16.5 Hz), aromatic moiety (δ =
7.41–7.45 ppm, m and δ = 7.58, d, 2 H, 3 JHH = 7.0 Hz,
arom) and a CH of oxadiazole ring (δ = 8.38, s,
1 H). Coupling constant (3 JHH) between the two
vinylic groups is ca. 16.5 Hz that is completely in
agreement with trans geometry around the carbon–
carbon double bond (C=C) of the molecule 3 [3].
1
The H decoupled 13C NMR spectrum of 3 showed
eight distinct resonances (δ = 109.53 (CH, vinylic);
δ = 127.57, 129.03, and 130.14 (CH, arom); δ =
134.53 (C, arom); δ = 139.85 (CH, vinylic); δ = 152.00
(CH, oxadiazole) and δ = 164.28 (C, oxadiazole)) in
agreement with the 3 formula. A partial assignment
of these resonances is given in the spectral analysis
1
section (see the Experimental section). The H and
13C NMR spectra of compounds 3b–3f were similar
to those of 3a, except for the aromatic or heteroaro-
matic moieties, which exhibited characteristic
signals with appropriate chemical shifts. We have
also used ((E)-3-(3-hydroxyphenyl)-2-propenoic
CONCLUSION
acid,
acid,
(E)-3-(3,4-dihydroxyphenyl)-2-propenoic
(E)-3-(4-hydroxyl-3-methoxyphenyl)-2-
In summary, we have found
for the stereoselective
a new method
propenoic acid, acetic acid, phthalic acid,
maleic acid, chloroacetic acid, trichloroacetic
preparation
of
2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazole
3
from
O
CH2Cl2
OH
C
N
N
PPh3
+
X
rt
R
2
1
Ph3P
O
N
O
N
O
O
H
HC
N
N
PPh3
X
X
R
R
5
6
N
N
H
O
Ph3PO
+
X
R
3
4
SCHEME 2 Proposed mechanism for the formation of 2-[(E)-2-aryl-1-ethenyl]-1,3,4-oxadiazole derivatives 3.
Heteroatom Chemistry DOI 10.1002/hc

Reaction of (N-Isocyanimino)triphenylphosphorane with (E)-3-Aryl-2-propenoic Acid Derivatives 615
3
1H NMR (250 MHz, CDCl3): δH 7.09 (d, 1H, JHH
(E)-3-aryl-2-propenoic acid derivatives 1 and (N-
isocyanimino)triphenylphosphorane in fairly
2
= 16.5 Hz, vinylic); 7.45–7.41 (m, 3H, arom); 7.58
3
3
moderate yield in neutral conditions. The reaction
is completely stereoselective (Scheme 1). We be-
lieve that the reported method offers a mild and
simple route for the preparation of 2-substituted
1,3,4-oxadiazole derivatives. Its ease of work-up and
fairly mild reaction conditions makes it a useful
addition to modern synthetic methodologies. Other
aspects of this process are under investigation.
(d, 2H, JHH = 7.0 Hz, arom); 7.62 (d, 1H, JHH
=
16.5 Hz, vinylic); 8.38 (s, 1H, oxadiazole); 13C NMR
(62.5 MHz, CDCl3): δC 109.53 (CH, vinylic); 127.57,
129.03 and 130.14 (CH, arom); 134.53 (C, arom);
139.85 (CH, vinylic); 152.00 (CH, oxadiazole); 164.28
(C, oxadiazole); C10H8N2O (172.06) MS (EI) m/z,%:
172 (M+, 100); 115 (22); 89 (9); 77 (7); 63 (8) and
51(8).
2-[(E)-2-(4-Methylphenyl)-1-ethenyl]-1,3,4-oxadi-
azole (3b). White crystals; mp: 83.2–84.1◦C; yield:
81%; IR (KBr): (υmax, cm−1): 2926 (m), 1643 (s),
1533 (m), 1100 (s), and 972 (m); 1H NMR (250
MHz, CDCl3): δH 2.39 (s, 3H, CH3); 7.02 (d, 1H,
3 JHH = 16.5 Hz, vinylic); 7.22 (d, 2H, 3 JHH = 7.75 Hz,
EXPERIMENTAL
Starting materials and solvents were obtained from
Merck (Darmstadt, Buchs, Germany) and Fluka
(Switzerland) and were used without further pu-
rification. The methods used to follow the reac-
tions were TLC and NMR. TLC and NMR indi-
cated that there is no side product. Melting points
were measured on an Electrothermal 9100 appara-
tus (LABEQUIP LTD., Markham, Ontario, Canada)
and are uncorrected. IR spectra were measured on
a Jasco 6300 FTIR spectrometer (JASCO Ltd., Eas-
3
arom); 7.46 (d, 2H, JHH = 7.75 Hz, arom); 7.57 (d,
1H, 3 JHH = 16.5 Hz, vinylic); 8.36 (s, 1H, oxadiazole);
13C NMR (62.5 MHz, CDCl3): δC 21.45 (CH3); 108.46
(CH, vinylic); 127.5 and 129.75 (CH, arom); 134.53
and 140.57 (C, arom); 139.82 (CH, vinylic); 151.91
(CH, oxadiazole); 164.47 (C, oxadiazole); Anal Calcd
for C11H10N2O (186.21): C, 70.95; H, 5.41; N, 15.04%;
Found: C, 70.99; H, 5.44; N, 15.01.
1
ton, MD). H and 13C NMR spectra were measured
(CDCl3 solution) with a Bruker DRX-250 Avance
spectrometer (Bruker Ltd., Ettlingen, Germany) at
250.0 and 62.5 MHz, respectively. Elemental analy-
ses were performed using a Heraeus CHN-O-Rapid
analyzer (Heraeus Ltd., Vienna, Austria). Mass spec-
tra were recorded on a Finnigan-Matt 8430 mass
spectrometer operating at an ionization potential
of 20 eV (Thermo Scientific Ltd., Waltham, MA).
Flash chromatography columns were prepared from
Merck silica gel powder.
2-[(E)-2-(4-Chlorophenyl)-1-ethenyl]-1,3,4-oxadi-
azole (3c). White crystals; mp: 95.4–96.3◦C; yield:
80%; IR (KBr): (υmax, cm−1): 2933 (m), 1646 (s), 1510
(m), 1088 (s), and 980 (m); 1H NMR (250 MHz,
CDCl3): δH 7.04 (d, 1H, 3 JHH = 16.5 Hz, vinylic); 7.39
3
3
(d, 2H, JHH = 8.50 Hz, arom); 7.49 (d, 2H, JHH
=
8.50 Hz, arom); 7.55 (d, 1H, 3 JHH = 16.5 Hz, vinylic);
8.38 (s, 1H, oxadiazole); 13C NMR (62.5 MHz,
CDCl3): δC 110.07 (CH, vinylic); 128.72 and 129.31
(CH, arom); 134.62 and 136.57 (C, arom); 138.41
(CH, vinylic); 152.11 (CH, oxadiazole); 164.51 (C,
oxadiazole); Anal Calcd for C10H7ClN2O (206.63): C,
58.13; H, 3.41; N, 13.56%; Found: C, 58.15; H, 3.44;
N, 13.52.
General Procedure for the Preparation
of Compounds 3 and 6
To
a
magnetically stirred solution of (N-
isocyanimino)triphenylphosphorane [7,8] 2 (0.302
g, 1 mmol) in dry CH2Cl2 (4 mL), a solution of
(E)-3-aryl-2-propenoic acid 1 (1 mmol) was added
dropwise in dry CH2Cl2(4 ml) over 15 min. The
mixture was stirred for 3 h at room temperature.
The solvent was removed under reduced pressure,
and the viscous residue was purified by flash column
chromatography (silica gel; petroleum ether-ethyl
acetate (10:1)). The solvent was removed under
reduced pressure, and the product 3 was obtained.
The characterization data of the compound are
given below:
2-[(E)-2-(3-Chlorophenyl)-1-ethenyl]-1,3,4-oxadi-
azole (3d). White crystals; mp: 81.4–82.2◦C; yield:
77%. IR (KBr) (υmax, cm−1): 2933 (m), 1650 (s),
1509 (m), 1101 (s), and 971 (m); 1H NMR (250
3
MHz, CDCl3): δH 7.08 (d, 1H, JHH = 16.5 Hz,
vinylic); 7.28–7.57 (m, 4H, arom); 7.54 (d, 1H,
3 JHH = 16.5 Hz, vinylic); 8.39 (s, 1H, oxadiazole); 13
C
NMR (62.5 MHz, CDCl3): δC 110.94 (CH, vinylic);
125.64, 127.45, 130.03, and 130.27 (CH, arom);
134.64 and 136.60 (C, arom); 138.25 (CH, vinylic);
152.19 (CH, oxadiazole); 164.53 (C, oxadiazole);
Anal Calcd for C10H7ClN2O (206.63): C, 58.13; H,
3.41; N, 13.56%; Found: C, 58.14; H, 3.42; N, 13.54.
2-[(E)-2-Phenyl-1-ethenyl]-1,3,4-oxadiazole (3a).
Light yellow oil, yield: 77%; IR (neat): (υmax, cm−1):
2930 (m), 1639 (s), 1523 (m), 1100 (s), and 977 (m);
Heteroatom Chemistry DOI 10.1002/hc

616 Ahankar et al.
[9] Tofangchi Mahyari, A.; Shajari, N.; Kazemizadeh,
A. R.; ’Slepokura, K.; Lis, T,; Ramazani, A. Z Natur-
forsch b, 2007, 62(6), 829.
[10] Yavari, I.; Ramazani, A. Synth Commun 1997, 27(8),
1385.
[11] Yavari, I.; Ramazani, A. Synth Commun 1997, 27(8),
1449.
2-[(E)-2-(3-Methoxyphenyl)-1-ethenyl]-1,3,4-oxa-
diazole (3e). Light yellow oil, Yield: 76%; IR (neat):
(υmax, cm−1): 2932 (m), 1649 (s), 1599 (m), 1509
1
(m), 1240 (s), 1173 (s), 1099 (s), and 970 (m); H
NMR (250 MHz, CDCl3): δH 3.85 (s, 3H, OCH3),
6.93–7.36 (m, 4H, arom); 7.00 (d, 1H, 3 JHH = 16.5 Hz,
3
[12] Ramazani, A.; Noshiranzadeh, N.; Ghamkhari, A.;
vinylic); 7.57 (d, 1H, JHH = 16.5 Hz, vinylic); 8.37
´
Slepokura, K.; Lis, T. Helv Chim Acta 2008, 91,
(s, 1H, oxadiazole); 13C NMR (62.5 MHz, CDCl3):
δC 55.33 (OCH3), 109.81 (CH, vinylic); 112.58,
115.90, 120.25, and 130.03 (CH, arom); 134.59 and
160.59 (C, arom); 139.75 (CH, vinylic); 152.03 (CH,
oxadiazole); 163.82 (C, oxadiazole); Anal Calcd for
C11H10N2O2 (202.21): C, 65.34; H, 4.98; N, 13.85%.
Found: C, 65.36; H, 4.99; N, 13.83.
2252.
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2007, 37, 3181.
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2-[(E)-1-Methyl-2-phenyl-1-ethenyl]-1,3,4-oxadia
zole (3f). Light yellow oil, yield: 83%; IR (neat):
(υmax, cm−1): 2994 (m), 1630 (s), 1529 (m), 1096 (s),
and 980 (m); 1H NMR (250 MHz, CDCl3): δH 2.42 (s,
3H, CH3); 7.34–7.57 (m, 6H, arom and vinylic); 8.38
(s, 1H, oxadiazole); 13C NMR (62.5 MHz, CDCl3):
δC 14.68 (CH3); 121.12 (C, vinylic); 128.44, 128.52,
and 129.56 (CH, arom); 135.04 (CH, vinylic); 135.35
(C, arom); 152.35 (CH, oxadiazole); 166.82 (C,
oxadiazole); Anal Calcd for C11H10N2O (186.21): C,
70.95; H, 5.41; N, 15.04%; Found: C, 70.97; H, 5.43;
N, 15.06.
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Heteroatom Chemistry DOI 10.1002/hc