(N-Isocyanimino)triphenylphosphorane as an Efficient reagent for the synthesis of 1,3,4-Oxadiazoles from 3-substituted benzoic acid derivatives

Article abstract of DOI:10.1080/10426500802705537

The reaction of 3-substituted benzoic acid derivatives with (N-isocyanimino) triphenylphosphorane proceeds smoothly at room temperature to afford corresponding 1,3,4-oxadiazoles via an intramolecular aza-Wittig reaction in excellent yields under neutral c

Full text of DOI:10.1080/10426500802705537

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(N-
Isocyanimino)triphenylphosphorane
as an Efficient Reagent for the
Synthesis of 1,3,4-Oxadiazoles
from 3-Substituted Benzoic
Acid Derivatives
Ali Ramazani a & Ali Souldozi b
a Chemistry Department , Zanjan University ,
Zanjan, Iran
b Chemistry Department , Islamic Azad
University–Urmia Branch , Urmia, Iran
Published online: 18 Nov 2009.
To cite this article: Ali Ramazani & Ali Souldozi (2009) (N-
Isocyanimino)triphenylphosphorane as an Efficient Reagent for the Synthesis
of 1,3,4-Oxadiazoles from 3-Substituted Benzoic Acid Derivatives, Phosphorus,
Sulfur, and Silicon and the Related Elements, 184:12, 3191-3198, DOI:
10.1080/10426500802705537
To link to this article: http://dx.doi.org/10.1080/10426500802705537
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Phosphorus, Sulfur, and Silicon, 184:3191–3198, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802705537
(N-Isocyanimino)triphenylphosphorane as an Efficient
Reagent for the Synthesis of 1,3,4-Oxadiazoles from
3-Substituted Benzoic Acid Derivatives
Ali Ramazani1 and Ali Souldozi2
1Chemistry Department, Zanjan University, Zanjan, Iran
2Chemistry Department, Islamic Azad University–Urmia Branch,
Urmia, Iran
The reaction of 3-substituted benzoic acid derivatives with (N-isocyanimino)
triphenylphosphorane proceeds smoothly at room temperature to afford correspond-
ing 1,3,4-oxadiazoles via an intramolecular aza-Wittig reaction in excellent yields
under neutral conditions.
Keywords Benzoic acid derivatives; iminophosphorane; intramolecular aza-Wittig re-
action; isocyanide; (N-isocyanimino)triphenylphosphorane; 1,3,4-oxadiazole
INTRODUCTION
Organophosphorus compounds1–8 have been extensively employed in
organic synthesis as useful reagents, as well as ligands, in a number of
transition metal catalysts.3 Iminophosphoranes are a class of a special
type of zwitterions that bear a strongly nucleophilic electron rich nitro-
gen.6–8 The electron distribution around the P+–N− bond and its con-
sequent chemical implications have been probed and assessed through
theoretical, spectroscopic, and crystallographic investigations.6–8 The
proton affinity of these iminophosphoranes can be used as a molecu-
lar guide to assess their utility as synthetic reagents and their func-
tion as ligands in coordination and organometallic chemistry.6–8 The
intramolecular version of the aza-Wittig–type reaction has attracted
considerable attention recently because of its high potential for the
synthesis of a wide variety of nitrogen heterocycles, which can be at-
tributed, in good measure, to the rapid progress in the preparation
Received 25 October 2008; accepted 17 December 2008.
This work was supported by the Iran National Science Foundation (INSF) via the
research project number 86053.20.
Address correspondence to Ali Ramazani, Chemistry Department, Zanjan University,
P O Box 45195-313, Zanjan, Iran. E-mail: aliramazani@gmail.com
3191

3192
A. Ramazani and A. Souldozi
of functionalized iminophosphoranes.6 Several interesting heterocy-
clization reactions involving iminophosphoranes have been reviewed.6
These compounds can easily be converted through aza-Wittig reaction
with isocyanates, carbon dioxide, or carbon disulfide into functionalized
heterocumulenes, which exhibit a rich chemistry of unusual synthetic
promise.6 The nucleophilicity at the nitrogen is a factor of essential
mechanistic importance in the use of these iminophosphoranes as aza-
Wittig reagents.6
Isocyanide-based reactions have been known for about 80 years, with
the first described in 1921 and named after its founder, Passerini.9–14
The chemistry of the isocyanides began in 1859 when Lieke prepared
allyl isocyanide as the first isocyanide.14 Lieke, like many chemists to-
day, was immediately struck by their strange repulsive odor, one of the
only negatives of this branch of chemistry.14 The classical syntheses
of isocyanides were developed in 1867 by Gautier.14 For the following
century, only 12 isocyanides were known, and rather few types of re-
actions had been described.14 Thus for a whole century, from 1859 to
1958, isocyanides were not readily available, and their chemistry re-
mained an underinvestigated part of organic chemistry.9–14 In 1921,
Passerini pioneered the use of isocyanides and successfully developed
a three-component synthesis of α-acyloxycarboxamide by reaction of a
carboxylic acid, an aldehyde, and an isocyanide.14 Today most IMCR
chemistry relates to the classical reactions of Passerini and Ugi. In-
deed, the large number of different scaffolds now available mostly
builds on these two IMCRs and their combination with other types
of reactions.9–14 Passerini reactions involve an oxo-component, an iso-
cyanide, and a nucleophile.9–14 Ugi reactions are defined as the reaction
of a Schiff base or an enamine with a nucleophile and an isocyanide,
followed by a (Mumm) rearrangement reaction.14 Passerini reactions
are beginning to find utility in the drug discovery process and in total
syntheses of biologically relevant natural products.14
1,3,4-Oxadiazoles have attracted interest in medicinal chemistry
as surrogates of carboxylic acids, esters, and carboxamides.15 They
are an important class of heterocyclic compounds that have a wide
range of pharmaceutical and biological activities including antimicro-
bial, antifungal, anti-inflammatory, and antihypertensive.15 Several
methods have been reported in the literature for the synthesis of 1,3,4-
oxadiazoles. These protocols are multistep in nature.15–30 The most
general method involves the cyclization of diacylhydrazides with a va-
riety of reagents, such as thionyl chloride, phosphorous oxychloride, or
sulfuric acid, usually under harsh reaction conditions.29 Few reliable
and operationally simple examples have been reported for the one-step

Synthesis of 1,3,4-Oxadiazoles
3193
synthesis of 1,3,4-oxadiazoles, especially from readily available car-
boxylic acids and acid hydrazides.30
Iminophosphoranes are important reagents in synthetic or-
ganic chemistry, especially in the synthesis of naturally oc-
curring products—compounds with biological and pharma-
cological activity.6 However, the organic chemistry of (N-
isocyanimino)triphenylphosphorane 2 remains almost unexplored.7,8
(N-isocyanimino)triphenylphosphorane 2 is expected to have syn-
thetic potential because it provides a reaction system in which the
iminophosphorane group can react with a reagent having a carbonyl
functionality.9–11 In recent years, we have established a one-pot
method for the synthesis of organophosphorus compounds.31–41 As
part of our ongoing program to develop efficient and robust methods
for the preparation of organic compounds,31–41 we sought to develop
a convenient preparation of 1,3,4-oxadiazoles 5 from 3-substituted
benzoic acid derivatives 1 and (N-isocyanimino)triphenylphosphorane
2 in excellent yields under neutral conditions (Scheme 1).
SCHEME 1 Preparation of 1,3,4-oxadiazole derivatives 5 from 3-substituted
benzoic acid derivatives 1.

3194
A. Ramazani and A. Souldozi
RESULTS AND DISCUSSION
In the last few years, several synthetic methods have been re-
ported for the preparation of (N-isocyanimino)triphenylphosphorane
(CNNPPh3) 2 (Scheme 1).7,8 There are several reports for the use of
(N-isocyanimino)triphenylphosphorane 2 in the synthesis of metal
complexes.7,8 However, application of 2 in the synthesis of organic
compounds has been reported fairly rarely.9–11 As part of our ongoing
program to develop efficient and robust methods for the preparation of
heterocyclic compounds,2 we sought to develop a convenient prepara-
tion of 1,3,4-oxadiazoles 5 from 3-substituted benzoic acid derivatives 1
and (N-isocyanimino)triphenylphosphorane 2 in excellent yields under
neutral conditions (Scheme 1). The 3-substituted benzoic acid deriva-
tives 1 and (N-isocyanimino)triphenylphosphorane 2 in dry solvent
react together in a 1:1 ratio at room temperature to produce 1,3,4-
oxadiazoles 5 and triphenylphosphine oxide 6 (Scheme 1). The reaction
proceeds smoothly and cleanly under mild conditions. The mechanism
of the reaction between the 3-substituted benzoic acid derivatives 1
and (N-isocyanimino)triphenylphosphorane 2 has not been established
experimentally. However, a possible explanation is proposed in Scheme
1. On the basis of the well established chemistry of isocyanides,9–14
it is reasonable to assume that protonation of 2 by the 3-substituted
benzoic acid derivatives 1 followed by a quenching of the cationic
center by the conjugate base of the carboxylic acid can generate the
iminophosphorane 4.6 An intramolecular aza-Wittig6 reaction of the
iminophosphorane 4 would lead to formation of the 1,3,4-oxadiazoles
5 and triphenylphosphine oxide 6 (Scheme 1). The structures of the
1
products 5a–e were deduced from their IR, H NMR, and 13C NMR
spectra. The mass spectra of these compounds displayed molecular ion
peaks at the appropriate m/z values. The IR spectrum of 5a showed
strong absorptions at 3069 and 2923 (CH, aromatic); 1569 and 1462
(C=C, aromatic); 1231 (C−F) and 1100, 877, 808 and 739 (oxadiazole
and aromatic parts) cm−1, indicating the presence of the mentioned
functionalities in its structure. The 1H NMR spectrum of compound 5a
exhibited five signals readily recognized as arising from aromatic moi-
ety [δ = 7.24–7.31 (m, 1 H, arom), 7.48–7.56 (m, 1 H, arom), 7.78–7.81
(m, 1 H, arom), and 7.88–7.91 (m, 1 H, arom)] and a CH of oxadiazole
1
ring (δ = 8.50, s, 1 H). The H decoupled 13C NMR spectrum of 5a
showed eight distinct resonances [δ = 114.18 (d, 1 CH, 2JCF = 24.4 Hz,
arom), 119.13 (d, 1 CH, 2JCF = 21.4 Hz, arom), 122.89 (d, 1 CH, 4JCF
=
3.2 Hz, arom), 125.19 (d, 1C, 3JCF = 8.50 Hz, arom), 131.03 (d, 1 CH, 3JCF
= 8.2 Hz, arom), 152.81 (1CH, oxadiazole), 162.34 (d, 1C, 1JCF = 183.8
Hz, arom), and 164.79 (1C, oxadiazole)] that are in agreement with the

Synthesis of 1,3,4-Oxadiazoles
3195
formula and structure of 5a. Partial assignment of these resonances is
given in the spectral analysis section (see the Experimental section).
CONCLUSION
In summary, we have found a new method for the preparation of 1,3,4-
oxadiazoles 5 from 3-substituted benzoic acid derivatives 1 and (N-
isocyanimino)triphenylphosphorane 2 in excellent yields under neutral
conditions. We believe that the reported method offers a mild and sim-
ple route for the preparation of these derivatives. Its ease of workup
and reaction conditions make it a useful addition to modern synthetic
methodologies. Other aspects of this process are under investigation.
EXPERIMENTAL
Starting materials and solvents were obtained from Merck (Germany)
and Fluka (Switzerland) and were used without further purification.
Flash chromatography columns were prepared from Merck silica gel
powder. IR spectra were measured on a Shimadzu IR-460 spectrom-
1
eter. H and 13C NMR spectra were measured (CDCl3 solution) with
a Bruker DRX-250 Avance spectrometer at 250.0 and 62.5 MHz, re-
spectively. The methods were used to follow the reactions were TLC
and NMR. TLC and NMR indicated that there is no side product.
Melting points were measured on an Electrothermal 9100 apparatus
and are uncorrected. Mass spectra were recorded on a Finnigan-Matt
8430 mass spectrometer operating at an ionization potential of 20 eV.
(N-Isocyanimino)triphenylphosphorane 2 was prepared based on a re-
ported procedure.7,8
General Procedure for the Preparation of Compounds 4 and 5
To
a
magnetically
stirred
solution
of
(N-isocyanimino)
triphenylphosphorane7,8 2 (0.302 g, 1 mmol) in dry CHCl3 (4
mL), a solution of 3-substituted benzoic acid derivatives 1 (1 mmol) in
dry CHCl3 (4 mL) was added dropwise over 15 min. The mixture was
stirred for 12 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 (10:2)]. The
solvent was removed under reduced pressure, and the product 5 was
obtained. The characterization data of the compounds are given below:

3196
A. Ramazani and A. Souldozi
2-(3-Fluorophenyl)-1,3,4-oxadiazole 5a
White crystals; mp: 135.6◦C; Yield: 82.2%. IR (KBr) (υmax, cm−1):
3069 and 2923 (CH, aromatic); 1569 and 1462 (C=C, aromatic); 1231
(C−F) and 1100, 877, 808 and 739 (oxadiazole and aromatic parts). 1H
NMR (CDCl3) δH: 7.24–7.31 (m, 1 H, arom), 7.48–7.56 (m, 1 H, arom),
7.78–7.81 (m, 1 H, arom) and 7.88–7.91 (m, 1 H, arom); 8.50 (s, 1 H, CH
2
of oxadiazole ring). 13C NMR (CDCl3) δC: 114.18 (d, 1 CH, JCF = 24.4
2
Hz, arom), 119.13 (d, 1 CH, JCF = 21.4 Hz, arom), 122.89 (d, 1 CH,
3
4JCF = 3.2 Hz, arom), 125.19 (d, 1C, JCF = 8.50 Hz, arom), 131.03 (d,
1 CH, 3JCF = 8.2 Hz, arom), 152.81 (s, 1CH, oxadiazole), 162.34 (d, 1C,
1JCF = 183.8 Hz, arom) and 164.79 (s, 1C, oxadiazole). MS: m/z (%);
164 (M+, 15), 152 (49), 123 (45), 77 (100) and 51 (63).
2-(3-Methoxyphenyl)-1,3,4-oxadiazole 5b
White crystals; mp: 108.8◦C; Yield: 87.5%. IR (KBr) (υmax, cm−1):
3123 and 3008 (CH, aromatic); 2954 (CH, OMe); 1502 and 1485 (C=C,
1
aromatic) and 1231 (C-O). H NMR (CDCl3) δH: 3.88 (s, 3 H, OCH3),
3
7.07–7.11 (m, 1H, arom); 7.42 (t, 1H, JHH = 8.0 Hz, arom); 7.61–7.66
(m, 2H, arom) and 8.47 (s, 1H, oxadiazole). 13C NMR (CDCl3) δC: 55.50
(1C, OCH3); 111.68, 118.47, 119.43 and 130.27 (4CH, arom); 124.54
(C, arom); 152.64 (1CH, oxadiazole); 159.95 (C-O, arom); 164.71 (1C,
oxadiazole). MS: m/z (%); 176 (M+, 100), 135 (85), 107 (33), 92 (13), 77
(21), 63 (12) and 51 (10).
3-(1,3,4-Oxadiazol-2-yl)aniline 5c
Yellow crystals; mp: 142.7◦C; Yield: 80.1%. IR (KBr) (υmax, cm−1):
3346 and 3231 (NH2); 3010 (CH, aromatic); 1615 and 1339 (C=C,
1
aromatic); 1108 (C-N) and 739(C=C, aromatic). H NMR (CDCl3) δH:
4.01 (s, 2H, NH2); 6.79–6.83 (m, 1H, arom); 7.20–7.25 (m, 1H, arom);
7.32–7.33 (m, 1H, arom); 7.42–7.43 (m, 1H, arom) and 8.41 (s, 1H, oxa-
diazole). 13C NMR (CDCl3) δC: 112.91, 116.88, 118.49 and 130.04 (4CH,
arom); 124.13 (C, arom); 147.24 (C-N, arom); 152.58 (1CH, oxadiazole)
and 164.98 (1C, oxadiazole). MS: m/z (%); 161 (M+, 100), 120 (66), 92
(47), 77 (13), 65 (26), 51 (20) and 41 (12).
2-(3-Phenoxy-phenyl)-[1,3,4]oxadiazole 5d
White crystals; mp: 64.3–64.5◦C; Yield: 8.0%. IR (KBr) (υmax, cm−1):
3154 (CH, aromatic); 1592, 1554, 1446 and 1323 (C=C, aromatic); 1246
1
3
and 1108 (2 C-O). H NMR (CDCl3) δH: 7.09 (d, 2H, JHH = 8.0 Hz,
arom); 7.17–7.24 (m, 2H, arom); 7.37–7.54 (m, 3H, arom); 7.72–7.73
(m, 1H, arom); 7.82–7.85 (m, 1H, arom); 8.48 (s, 1H, oxadiazole). 13C
NMR (CDCl3) δC: 116.73, 119.44, 121.65, 122.04, 124.15, 130.04 and

Synthesis of 1,3,4-Oxadiazoles
3197
130.65 (9CH, arom); 124.96 (C, arom); 152.70 (1CH, oxadiazole); 156.22
and 158.18 (2C-O, arom) and 164.30 (1C, oxadiazole). MS: m/z (%); 238
(M+, 100), 237 (67), 221 (28), 197 (45), 169 (15), 141 (28), 115 (23), 77
(34), 63 (19) and 51 (44).
3-(1,3,4-Oxadiazol-2-yl)phenyl Acetate 5e
White crystals; mp: 91.6◦C; Yield: 84.7%. IR (KBr) (υmax, cm−1): 3085
1
(CH, aromatic); 1769 (C=O, ester) and 1207 (C-O). H NMR (CDCl3)
3
δH: 2.34 (s, 3H, CH3); 7.29 (d, 1H, JHH = 8.2 Hz, arom); 7.54 (t, 1H,
3
3JHH = 8.2 Hz, arom), 7.82 (s, 1H, arom); 7.96 (d, 1H, JHH = 8.2 Hz,
arom) (4H, arom), and 8.48 (s, 1H, oxadiazole). 13C NMR (CDCl3) δC:
21.06 (CH3), 120.45, 124.47, 125.37 and 130.34 (4CH, arom); 124.79
(C, arom); 151.08 (C-O, arom); 152.75 (1CH, oxadiazole); 163.98 (1c,
oxadiazole) and 169.08 (C=O). MS: m/z (%); 204 (M+, 16), 162 (100),
121 (57), 105 (7), 93 (8), 78 (8), 63 (8) and 43 (62).
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