Triphenylphosphine-catalyzed preparation of sterically congested, electron-poor N-vinylimidazole derivatives from acetylenic esters and imidazole-containing NH-acids

Article abstract of DOI:10.1080/10426501003713080

Protonation of the highly reactive 1:1 intermediates, produced in the reaction between triphenylphosphine and alkyl acetylenecarboxylates (or ethyl 3-phenyl-2-propynoate), by azathioprine or imidazole leads to vinyltriphenylphosphonium salts, which undergo a Michael addition reaction with a conjugate base to produce phosphorus ylides. Dipotassium hydrogen phosphate, potassium dihydrogen phosphate, and potassium iodide were found to catalyze the conversion of the phosphorus ylides to electron-poor N-vinyl imidazoles in solvent-free conditions under thermal (90 °C, 1 h) conditions. The structural analysis of the products indicated that the reaction is regio- and stereoselective. Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426501003713080

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Phosphorus, Sulfur, and Silicon and the
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Triphenylphosphine-Catalyzed
Preparation of Sterically Congested,
Electron-Poor N-Vinylimidazole
Derivatives from Acetylenic Esters and
Imidazole-Containing NH-Acids
Azam Farshadi a , Ali Ramazani a , Amir Tofangchi Mahyari a , Mehdi
Khoobi a , Yavar Ahmadi b & Fatemeh Zeinali Nasrabadi a
a Chemistry Department , Zanjan University , Zanjan, Iran
b Member of Young Researchers Club , Islamic Azad University ,
Zanjan Branch, Zanjan, Iran
Published online: 19 Nov 2010.
To cite this article: Azam Farshadi , Ali Ramazani , Amir Tofangchi Mahyari , Mehdi Khoobi , Yavar
Ahmadi & Fatemeh Zeinali Nasrabadi (2010) Triphenylphosphine-Catalyzed Preparation of Sterically
Congested, Electron-Poor N-Vinylimidazole Derivatives from Acetylenic Esters and Imidazole-
Containing NH-Acids, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:12, 2489-2495,
DOI: 10.1080/10426501003713080
To link to this article: http://dx.doi.org/10.1080/10426501003713080
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Phosphorus, Sulfur, and Silicon, 185:2489–2495, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003713080
TRIPHENYLPHOSPHINE-CATALYZED PREPARATION OF
STERICALLY CONGESTED, ELECTRON-POOR
N-VINYLIMIDAZOLE DERIVATIVES FROM ACETYLENIC
ESTERS AND IMIDAZOLE-CONTAINING NH-ACIDS
Azam Farshadi,1 Ali Ramazani,1 Amir Tofangchi Mahyari,1
Mehdi Khoobi,1 Yavar Ahmadi,2
and Fatemeh Zeinali Nasrabadi1
1Chemistry Department, Zanjan University, Zanjan, Iran
2Member of Young Researchers Club, Islamic Azad University, Zanjan Branch,
Zanjan, Iran
Protonation of the highly reactive 1:1 intermediates, produced in the reaction between triph-
enylphosphine and alkyl acetylenecarboxylates (or ethyl 3-phenyl-2-propynoate), by aza-
thioprine or imidazole leads to vinyltriphenylphosphonium salts, which undergo a Michael
addition reaction with a conjugate base to produce phosphorus ylides. Dipotassium hydrogen
phosphate, potassium dihydrogen phosphate, and potassium iodide were found to catalyze
the conversion of the phosphorus ylides to electron-poor N-vinyl imidazoles in solvent-free
conditions under thermal (90◦C, 1 h) conditions. The structural analysis of the products
indicated that the reaction is regio- and stereoselective.
Keywords Acetylenic esters; dipotassium hydrogen phosphate; electron-poor N-vinyl imi-
dazoles; potassium dihydrogen phosphate; potassium iodide; regioselective; stereoselective;
vinyltriphenylphosphonium salt
INTRODUCTION
Imidazole chemistry currently attracts considerable attention, where the imidazole
derivatives are widely applied as N-ligand coordinating transition metals.1,2 The application
of imidazoles in medicinal chemistry3 or chemistry of natural products/alkaloids4,5 or of
1,3-disubstituted imidazole salts as ionic liquids6,7 are also well known.
1-Vinylimidazole is employed as a copolymer in the production of cationic poly-
mers for various uses. Alkylimidazoles are used as hardeners for epoxy resins and for
polyurethanes.8 Less important uses for alkyl- and arylimidazoles include photography and
dyes.8
β-Additions of nucleophiles to the vinyl group of vinylic phosphonium salts lead-
ing to the formation of new alkylidenephosphoranes have attracted much attention as a
Received 13 January 2010; accepted 17 February 2010.
This work was supported by the Zanjan University.
Address correspondence to Ali Ramazani, Chemistry Department, Zanjan University, PO Box 45195-313,
Zanjan, Iran. E-mail: aliramazani@gmail.com
2489

2490
A. FARSHADI ET AL.
very convenient and synthetically useful method in organic synthesis.9 Organophosphorus
compounds have been extensively used in organic synthesis as useful reagents as well
as ligands of a number of transition metal catalysts.10 Phosphorus ylides are a class of
a special type of zwitterions, which bear strongly nucleophilic electron-rich carbanions.
The electron distribution around the P+–C− bond and its consequent chemical implications
had been probed and assessed through theoretical, spectroscopic, and crystallographic in-
vestigations.9 The nucleophilicity at the ylidic carbon is a factor of essential mechanistic
importance in the use of these ylides as Wittig reagents. Phosphorus ylides are important
reagents in synthetic organic chemistry, especially in the synthesis of naturally occurring
products, and compounds with biological and pharmacological activity.11 These ylides are
usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are
usually obtained from a phosphine and an alkyl halide. Phosphonium salts are also prepared
by Michael addition of phosphorus nucleophiles to activated olefins and in other ways.12
The phosphonium salts are most often converted to the ylide by treatment with a strong
base, though weaker bases can be used if the salt is acidic enough. In recent years, we have
established a one-pot method for the synthesis of stabilized phosphorus ylides.13–35 In this
article, we describe the preparation of sterically congested, electron-poor N-vinyl imida-
zoles from alkyl acetylenecarboxylates (or ethyl 3-phenyl-2-propynoate) and azathioprine
or imidazole in the presence of triphenylphosphine in fairly good yields.
RESULTS AND DISCUSSION
The zwitterionic intermediate 6 may result from the initial addition of triphenylphos-
phine 1 to the ethyl acetylenecarboxylate 2 and concomitant protonation of the 1:1 adduct
3, followed by attack of the NH-acid anion on the vinyltriphenylphosphonium cation
(Scheme 1). TLC indicated the formation of zwitterionic intermediate 6 in CH2Cl2. Dipotas-
sium hydrogen phosphate, potassium dihydrogen phosphate, and potassium iodide powders
were found to catalyze conversion of the zwitterionic intermediates 6 to electron-poor imi-
dazole 7 in solvent-free conditions36 under thermal (90◦C, 1 h) conditions. In the absence of
the K2HPO4, KH2PO4, and KI powders, the conversion of the zwitterionic intermediates 6
to electron-poor N-vinyl imidazole 7 were not observed, and decomposition of the starting
materials were observed (Scheme 1). The reaction proceeds smoothly and cleanly under
mild conditions, and no side reactions were observed. The mechanism of the reaction has
not been established experimentally. However, a possible explanation37,38 is proposed in
Scheme 1. We have also used MgO, K2CO3, Al2O3, SiO2, and Na2CO3 in this reaction,
but no products were observed (or in the some cases the conversion percentages were very
low), and in all cases decomposition was observed.
We have also used methyl acetylenecarboxylate (or ethyl 3-phenyl-2-propynoate) 8
(Scheme 2 ) instead of ethyl acetylenecarboxylate 2 in this reaction, but the regiochemistry
of the products was completely different from the ethyl ester derivative. Differences in the
steric effects of Me and Et groups may be plausible factors in the formation of different
regioisomers. The zwitterionic intermediate 13 may result from the initial addition of triph-
enylphosphine 1 to the methyl acetylenecarboxylate (or ethyl 3-phenyl-2-propynoate) 8
and concomitant protonation of the 1:1 adduct 9, followed by attack of the NH-acid anion
on the vinyltriphenylphosphonium cation to form the phosphorane 12 (Scheme 1). That
fact that phosphorane 12 undergoes intramolecular proton transfer37,38 leads to formation
of zwitterionic intermediate 13 in CH2Cl2. Dipotassium hydrogen phosphate, potassium

ELECTRON-POOR N-VINYLIMIDAZOLE DERIVATIVES
2491
N
N
N
N
N
H
S
N
C
O
CH2Cl2
OEt
H
4
O2N
C
Ph3P:
1
+
H
C
C
C
-10 0
C
PPh3
O
OEt
2
3
N
N
N
N
N
N
H
N
+
N
Ph3P
C
+
CH
CH
Ph3P
S
N
S
N
N
O
CH
OEt
N
O
O2N
O2N
OEt
5
6
N
S
N
K2HPO4
N
N
H
N
Ph3P:
1
90 0C
+
H
60 min
O
N
OEt
O2N
7
Scheme 1 Synthesis of N-vinyl azathioprine 7 from ethyl acetylenecarboxylate 2 and azathioprine 4 in the
presence of triphenylphosphine 1.
dihydrogen phosphate, and potassium iodide powders were found to catalyze conversion
of the zwitterionic intermediate 13 to electron-poor imidazoles (14) in solvent-free con-
ditions36 under thermal (90◦C, 1 h) conditions. In the absence of the K2HPO4, KH2PO4,
and KI powders, the conversion of the zwitterionic intermediate 13 toelectron-poor N-vinyl
imidazoles (14) was not observed, and decomposition of the starting materials were ob-
served (Scheme 2). The reaction proceeds smoothly and cleanly under mild conditions and
no side reactions were observed. The mechanism of the reaction has not been established
experimentally. However, a possible explanation37,38 is proposed in Scheme 2. We have
also used MgO, K2CO3, Al2O3, SiO2, and Na2CO3 in this reaction, but no products were
observed (or in the some cases the conversion percentages were very low), and in all cases
decomposition were observed.
The structures 7 and 14 (a, b, and c) were deduced from their IR, 1H and 13C NMR
spectra. The 1H NMR spectra of compound 7 exhibited two doublet peaks at δ = 7.09 and
8.23 for the olefinic protons of the electron-poor double bond, which are very consistent
with their E (3JHH = 14.3 Hz) stereochemistry. The ethoxy group and the heterocyclic

2492
A. FARSHADI ET AL.
E1
HY
10
+
Ph3P
C
C
E1
CH2Cl2
Y
C
E2
CH
Ph3P:
1
+
E1
C
C
E2
-10 0
C
PPh3
E2
8
9
11
K2HPO4
90 0
C
E1
E1
+
Y
+
Ph3P CH
Ph3P:
1
60 min
E1
Ph3P
C
+
Y
CH
E2
Y
C
H-shift
E2
E2
H
12
14
14
13
a
b
c
N
N
N
S
N
N
N
N
Y
N
N
N
S
N
N
N
N
O2N
Ph
CO2Et
O2N
H
E1
E2
Ph
CO2Et
CO2Me
Scheme 2 Synthesis of N-vinyl imidazole derivatives 14 from acetylenic esters 8 and imidazoles 10 in the
presence of triphenylphosphine 1.
moiety give characteristic signals at the appropriate chemical shifts (see the Experimental
section). Further evidence was obtained from the 13C NMR spectra, which displayed a
1
signal for the carbonyl carbon at δ = 166.00 (see the Experimental section). The H
NMR spectra of compound 14a exhibited two doublet peaks at δ = 6.42 and δ = 6.75
(2JHH = 1.0 Hz) for the two diastereotopic protons of the olefinic methylene group.36 The
methoxy group and the heterocyclic moieties give characteristic signals at the appropriate
chemical shifts (see the Experimental section). Further evidence was obtained from the
13C NMR spectra, which displayed a signal for the methylene carbon at δ = 125.19 and
1
a signal for the carbonyl carbon at δ = 162.24 (see the Experimental section). The H
NMR spectra of compounds 14b exhibited a singlet peak at δ = 7.76 for the olefinic proton
of the electron-poor double bond, which is very consistent with its Z stereochemistry.23
The ethoxy group and the heterocyclic moiety give characteristic signals at the appropriate
chemical shifts (see the Experimental section). Further evidence for 14b was obtained from
the 13C NMR spectra, which displayed a signal for the carbonyl carbon at δ = 163.00 (see
the Experimental section), which is very consistent with its structure. The 1H and 13C NMR
spectra of compound 14c were similar to those of 14b, except for the heterocyclic moiety,
which exhibited characteristic signals with appropriate chemical shifts.
CONCLUSION
In conclusion, we have developed a convenient, one-pot, regio-, and stereoselective
method for preparing electron-poor imidazoles 7 and 14 (a, b, and c) utilizing in situ

ELECTRON-POOR N-VINYLIMIDAZOLE DERIVATIVES
2493
generation of the phosphonium salts (5 and 11) (Schemes 1 and 2). We believe that the
reported method offers a mild and simple route for the preparation of these derivatives.
Its ease of workup and reaction conditions make it a useful addition to modern synthetic
methodologies.39–49 Other aspects of this process are under investigation.
EXPERIMENTAL
Melting points were measured on an Electrothermal 9100 apparatus and are uncor-
1
rected. IR spectra were recorded on a Shimadzu IR-460 spectrometer. H and 13C NMR
spectra were measured with a Bruker Spectrospin spectrometer at 250 and 62.5 MHz,
respectively. Elemental analyses were performed using a Heraeus CHN-O-Rapid analyzer.
General Procedure for the Preparation of 7 and 14a
To a magnetically stirred solution of triphenylphosphine (0.262 g, 1 mmol) and
azathioprine (0.28 g, 1 mmol) in dichloromethane (5 mL), a mixture of ethyl acetylenecar-
boxylate 2 (or methyl acetylenecarboxylate 8a) (1 mmol) in dichloromethane (2 mL) at
–10◦C was added dropwise over 15 min. The mixture was allowed to warm up to room
temperature. Dipotassium hydrogen phosphate powder (1.5 g) was added, and the solvent
was evaporated. Dry dipotassium hydrogen phosphate and the residue were heated (yield
for 7, 66%; yield for 14a, 60%) for 60 min at 90◦C and then placed over a column of silica
gel powder (12 g). The flash column chromatography was washed using ethyl acetate/light
petroleum ether (5:10) as eluent. The solvent was removed under reduced pressure, and
products were obtained as viscous yellow oils (7, 14a) (Scheme 1 and Scheme 2). The
characterization data of the compounds (7, 14a) are given below.
Selected data for ethyl (E)-2-{6-[(1-methyl-4-nitro-1H-imidazol-5-yl)
sulfanil]-7H-purin-7-yl}acrylate (7). Viscous yellow oil; yield: 66%, IR (Neat) (υmax
,
=
1
cm−1): 2923, 2853, 1723, 1661 and 1192. H NMR (DMSO-d6) δH: 1.24 (t, 3 H, 3JHH
7.0 Hz, CH3 of OEt), 3.34 (s, 3 H, CH3), 4.19 (q, 2 H, 3JHH = 7.0 Hz, OCH2 of OEt),
3
7.09 and 8.23 (2 d, 2 H, JHH = 14.3 Hz, HC CH), 8.22 and 8.73 (2 s, 2 H, CH N of
imidazole), 8.91 (s, 1 H, CH N of pyrimidine). 13CNMR (DMSO-d6) δC: 14.52 (CH3 of
OEt) 33.43 (CH3), 61.02 (CH2 of OEt), 109.95 and 134.85 (2 CH of vinyl), 140.01 and
145.72 (2 CH of imidazole), 153.18 (CH of pyrimidine), 116.96, 131.81, 149.65, 150.02
and 156.95 (5 C), 166.00 (C O of ester). Anal. Calcd for C14H13N7O4S (375.36): C,
44.80; H, 3.49; N, 26.12%. found: C, 45.02; H, 3.44; N, 26.01%.
Selected data for methyl-2-{6-[(1-methyl-4-nitro-1H-imidazol-5-yl)
sulfanil]-7H-purin-7-yl} acrylate (14a). Viscous yellow oil; yield: 60%.
IR(neat)(υmax, cm−1): 3123, 2923, 1738, 1646 and 1192 . 1H NMR (CDCl3) δH:
3.76 (s, 3 H, CH3), 3.89 (s, 3 H, OCH3), 6.42 and 6.75 (2 d, 2H, 2JHH = 1.0 Hz, CH2),
7.76 and 8.24 (2 s, 2 H, CH N of imidazole), 8.59 (s, 1 H, CH N of pyrimidine). 13
C
NMR (CDCl3) δC: 33.21 (CH3), 53.36 (OCH3), 125.19 ( CH2), 138.03 and 143.94 (2 CH
of imidazole), 152.54 (CH of pyrimidine), 116.51, 130.57, 138.03, 150.35 and 157.04 (6
C), 162.24 (C O of ester). Anal. Calcd for C13H11N7O4S (361.34): C, 43.21; H, 3.07; N,
27.13%. found: C, 43.32; H, 3.03; N, 27.01%.
General Procedure for the Preparation of 14b and 14c
To a magnetically stirred solution of triphenylphosphine (0.262 g, 1 mmol) and
azathioprine 10b (or imidazole 10c) (1 mmol) in dichloromethane (5 mL), a mixture

2494
A. FARSHADI ET AL.
of ethyl 3-phenyl-2-propynoate 8b (1 mmol) in dichloromethane (2 mL) at –10◦C was
added dropwise over 15 min. The mixture was allowed to warm up to room temperature.
Dipotassium hydrogen phosphate powder (1.5 g) was added, and the solvent was evaporated.
Dry dipotassium hydrogen phosphate and the residue were heated (yield for 14b, 71%;
yield for 14c, 97%) for 60 min at 90◦C and then placed over a column of silica gel
powder (12 g). The flash column chromatography was washed using ethyl acetate/light
petroleum ether (5:10) as eluent. The solvent was removed under reduced pressure, and
products were obtained as viscous yellow oil (14b) and white crystals (14c) (Scheme 2).
The characterization data of the compounds (14b and 14c) are given below.
Selected data for ethyl (Z)-2-{6-[(1-methyl-4-nitro-1H-imidazol-5-yl)
sulfanil]-7H-purin-7-yl}-3-phenyl-2-propenoate (14b). Viscous yellow oil; yield:
71%. IR (Neat) (υmax, cm−1): 3115, 2930, 1723, 1646, 1269 and 1207. 1H NMR (CDCl3)
δH: 1.31 (t, 3 H, 3JHH = 7.0 Hz, CH3 of OEt), 3.75 (s, 3 H, CH3), 4.33 (q, 2 H, 3JHH
=
3
7.0 Hz, OCH2 of OEt), 6.90 (d, 2 H, JHH = 7.3 Hz, arom), 7.22–7.37 (m, 5 H, arom),
7.76 (s, 1 H, CH), 7.90 and 8.16 (2 s, 2 H, CH N of imidazole), 8.53 (s, 1 H, CH N
of pyrimidine). 13C NMR (CDCl3) δC: 14.17 and 33.29 (2 CH3), 62.58 (OCH2 of OEt),
121.29 (CH of vinyl), 129.29, 129.80 and 130.83 (5 CH of Ph), 141.13 and 144.07 (2 CH
of imidazole), 152.76 (CH of pyrimidine), 116.87, 129.29, 131.49, 138.00, 150.35 and
157.01 (7 C), 163.00 (C O of ester). Anal. Calcd for C20H17N7O4S (451.46): C, 53.21;
H, 3.80; N, 21.72%. found: C, 53.42; H, 3.84; N, 21.59%.
Selected data for ethyl (Z)-2-(1H-imidazol-1-yl)-3-phenyl-2-propenoate
(14c). White crystals; yield: 97%, mp: 90–92◦C. IR (KBr) (υmax, cm−1): 2969, 1712,
1643, 1257 and 1211. 1H NMR (CDCl3) δH: 1.35 (t, 3 H, 3JHH = 7.0 Hz, CH3 of OEt), 4.33
(q, 2 H, 3JHH = 7.0 Hz, OCH2 of OEt), 6.89 and 7.29 (2 d, 2 H, CH CH of imidazole),
7.25–7.35 (m, 5 H, arom), 7.44 (s, 1 H, CH), 7.85 (s, 1 H, CH of imidazole). 13C NMR
(CDCl3) δC: 14.17 (CH3), 62.14 (OCH2 of OEt), 119.37, 130.06 and 138.15 (3 CH of
imidazole), 125.12 (CH of vinyl), 128.99, 130.06 and 130.92, (5 CH of Ph), 131.27 and
137 (2 C), 163.78 (C O of ester). Anal. Calcd for for C14H14N2O2 (242.27): C, 69.41; H,
2.82; N, 11.56%. found: C, 69.38; H, 2.85; N,11.58%.
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