Dipotassium hydrogen phosphate-catalyzed synthesis of dialkyl 2-(4-fluoro-phenoxy)-2-butendioates from stabilized phosphorus ylides in solvent-free conditions

Article abstract of DOI:10.1080/10426500600920007

Protonation of the highly reactive 1:1 intermediates, produced in a reaction between triphenylphosphine and dialkyl acetylenedicarboxylates, by 4-fluorophenol leads to vinyltriphenylphosphonium salts, which undergo a Michael addition reaction with a conjugate base to produce dialkyl 2-(4-fluoro-phenoxy) -3-(triphenylphosphoranylidene)butanedioates. Dipotassium hydrogen phosphate powder was found to catalyze the conversion of dialkyl 2-(4-fluoro-phenoxy)-3- (triphenylphosphoranylidene)butanedioates to dialkyl 2-(4-fluoro-phenoxy)-2- butenedioates in solvent-free conditions under microwave (0.5 KW, 3 min) and thermal (90°C, 60 min) conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500600920007

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Dipotassium Hydrogen
Phosphate-Catalyzed Synthesis
of Dialkyl 2-(4-Fluoro-
Phenoxy)-2-Butendioates From
Stabilized Phosphorus Ylides in
Solvent-Free Conditions
Ali Ramazani a , Mahshid Rahimifard a , Nader
Noshiranzadeh a & Ali Souldozi a
a Chemistry Department, Zanjan University, Zanjan,
Iran
Published online: 18 Feb 2007.
To cite this article: Ali Ramazani , Mahshid Rahimifard , Nader Noshiranzadeh & Ali
Souldozi (2007) Dipotassium Hydrogen Phosphate-Catalyzed Synthesis of Dialkyl 2-(4-
Fluoro-Phenoxy)-2-Butendioates From Stabilized Phosphorus Ylides in Solvent-Free
Conditions, Phosphorus, Sulfur, and Silicon and the Related Elements, 182:2, 413-417,
DOI: 10.1080/10426500600920007
To link to this article: http://dx.doi.org/10.1080/10426500600920007
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Phosphorus, Sulfur, and Silicon, 182:413–417, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600920007
Dipotassium Hydrogen Phosphate-Catalyzed Synthesis
of Dialkyl 2-(4-Fluoro-Phenoxy)-2-Butendioates From
Stabilized Phosphorus Ylides in Solvent-Free Conditions
Ali Ramazani
Mahshid Rahimifard
Nader Noshiranzadeh
Ali Souldozi
Chemistry Department, Zanjan University, Zanjan, Iran
Protonation of the highly reactive 1:1 intermediates, produced in a reaction be-
tween triphenylphosphine and dialkyl acetylenedicarboxylates, by 4-fluorophenol
leads to vinyltriphenylphosphonium salts, which undergo
a Michael addi-
tion reaction with a conjugate base to produce dialkyl 2-(4-fluoro-phenoxy)-
3-(triphenylphosphoranylidene)butanedioates. Dipotassium hydrogen phosphate
powder was found to catalyze the conversion of dialkyl 2-(4-fluoro-phenoxy)-
3-(triphenylphosphoranylidene)butanedioates to dialkyl 2-(4-fluoro-phenoxy)-2-
butenedioates in solvent-free conditions under microwave (0.5 KW, 3 min) and ther-
mal (90◦C, 60 min) conditions.
Keywords 4-fluorophenol; dipotassium hydrogen phosphate; Michael addition;
acetylenic esters; vinyltriphenylphosphonium salt; microwave irradiation
INTRODUCTION
β-additions of nucleophiles to the vinyl group of vinylic phospho-
nium salts leading to the formation of new alkylidenephosphoranes
have attracted much attention as a very convenient and syntheti-
cally useful method in organic synthesis.1−3 Organophosphorus com-
pounds have been extensively used in organic synthesis.2 Silica
gel as an additive promotes Wittig reactions of phosphorus ylides
with aldehydes, including sterically hindered aldehydes, to increase
the rate and yields of alkenes.4,5 In the past we have established
a convenient, one-pot method for preparing stabilized phosphorus
Received June 6, 2006; accepted July 11, 2006.
This work was supported by the Sandoogh Hemayate as Pajuoheshgharane Keshvare
Iran via research project number N 84114.
Address correspondence to Ali Ramazani, Chemistry Department, Zanjan University,
P.O. Box 45195-313, Zanjan, Iran. E-mail: aliramazani@yahoo.com
413

414
A. Ramazani et al.
OH
CH2Cl2
(C6H5)3P
+
RO2CC
CCO2R
+
0
-10 C
1
2
3
F
CO2R
P(C6H5)3
_
O
F
O
+
C
CHCO2R
(C6H5)3P
CO2R
H
CO2R
5
4
F
CO2R
K2HPO4 powder, 900C, 60 min
H
O
(or microwave irradiation, 0.5 KW, 3 min)
No solvent
(C6H5)3P
+
CO2R
1
6a: R=Me; %Z=59, %E=41
6b: R=Et; %Z=47, %E=53
F
SCHEME 1
ylides utilizing in situ generation of phosphonium salts.1,3 In this
article, we report on the catalytic role of dipotassium hydrogen
phosphate powder in the conversion of dialkyl 2-(4-fluoro-phenoxy)-
3-(triphenylphosphoranylidene)butanedioates to dialkyl 2-(4-fluoro-
phenoxy)-2-butenedioates in solvent-free conditions under microwave
(0.5 KW, 3 min) and thermal (90◦C, 60 min) conditions. (Scheme 1).
RESULTS AND DISCUSSION
The ylide (5) may result from an initial addition of triphenylphos-
phine 1 to acetylenic ester 2, and the concomitant protonation
of the 1:1 adduct by 4-fluorophenol leads to vinyltriphenylphos-
phonium salts 4, which undergo a Michael addition reaction
with a conjugate base to produce dialkyl 2-(4-fluoro-phenoxy)-3-
(triphenylphosphoranylidene)butanedioates (5). TLC indicated forma-
tion of ylides 5 in CH2Cl2. Dipotassium hydrogen phosphate powder
was found to catalyze the conversion of dialkyl 2-(4-fluoro-phenoxy)-3-
(triphenylphosphoranylidene)butanedioates (5) to dialkyl 2-(4-fluoro-
phenoxy)-2-butenedioates (6) in solvent-free conditions under mi-
crowave (0.5 KW, 3 min) and thermal (90◦C, 60 min.) conditions. We
have also used SiO2, MgO, MgSO4, NaH2PO4, Na2HPO4, NaHCO3,

Dialkyl 2-(4-Fluoro-Phenoxy)-2-Butendioates
415
NaHSO4KH2PO4, CuSO4, and K2CO3 in this reaction, but the yield
of product was very low, and in all cases decomposition and several
products were observed. In the absence of K2HPO4 powder, powdered
ylide 5 was not reacted under microwave irradiation at microwave
power 0.5 KW after 3 min or under thermal (90◦C, 60 min) conditions,
and decomposition of the starting materials was observed. Previously,
other phenols (such as 4-bromophenol, 1-naphthol, and 2-naphthol)
were used in a similar reaction, but in the all cases coumarins were
observed.1 High electronegativity of the fluorine atom may be a factor
in the reaction.
CONCLUSION
In summary, we have found that K2HPO4 powder is able to cat-
alyze stereoselective conversion of ylides 5 to compounds 6 in solvent-
free conditions6,7 (Scheme 1). Other aspects of this process are under
investigation.
EXPERIMENTAL
Commerical-oven Butane M245 was used for microwave irradiation. IR
spectra were recorded on a Shimadzu IR-460 spectrometer. Elemental
analyses were performed using a Heraeus CHN-O-Rapid analyzer. 1H,
19F, and 13C NMR spectra were measured with a BRUKER DRX-500
AVANCE spectrometer at 500, 470.6, and 125 MHz, respectively.
General Procedure for the Preparation of Ylides 5 and
Compounds 6a–b
To a magnetically stirred solution of triphenylphosphine 1 (0.262 g,
1 mmol) and 4-fluorophenol 3 (0.112 g, 1 mmol) in CH2Cl2 (4 mL) was
added dropwise a mixture of 2 (1 mmol) in CH2Cl2 (3 mL) at –10◦C
over 15 min. The mixture was allowed to warm up to r.t. 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 6a, 42%; yield for 6b, 29%) for 90 min at 60◦C (or ir-
radiated in a microwave oven for 3 min at microwave power 0.5 KW;
yield for 6a, 40.5%; yield for 6b, 27%) and then placed over a column of
silica-gel powder (12 g). The column chromatography was washed using
ethyl acetate-light petroleum ether (1:10) as an eluent. The solvent was
removed under reduced pressure, and products were obtained as vis-
cous yellow oils (6a–b). The relative population of E and Z isomers were

416
A. Ramazani et al.
determined via their 1H NMR spectra (Scheme 1). The characterization
data of new compounds (6a–b) follow.
Dimethyl 2-(4-fluoro-phenoxy)-2-butenedioate (6a)
Viscous yellow oil. IR(neat) (νmax, cm−1): 3084, 3015, 2961, 2853, 1734,
and 1649. Found: C, 56.98; H, 4.69. C12H11FO5 requires C, 56.70; H,
4.36%. 1H NMR (CDCl3) for Z isomer, δH: 3.73, and 3.75 (6H, 2s, 2OCH3),
3
6.59 (1H, s, vinylic), 6.95 (2H, m, arom.), 7.10 (2H, d, JHH = 6.5 Hz,
3
arom.), 7.10 (2H, d, JHH = 6.5 Hz, arom.). 13C NMR (CDCl3) for Z
isomer, δC: 52.01 and 53.02 (2OCH3), 115.06 (=CH, vinylic), 116.15 (1C,
2
d, 1 J = 23.5 Hz, C F), 117.53 (2C, d, JCF = 8.4 Hz, C3 and C5),
CF
149.86 (C2 and C6), 152.53 (C1, arom.), 159.76 (OC=, vinylic), 162.45
and 163.81 (2C=O, ester). 19F NMR (CDCl3) for Z isomer, δF: −120.54
1
(1F, s, CF). H NMR (CDCl3) for E isomer, δH: 3.68 and 3.93 (6H, 2s,
2OCH3), 5.12 (1H, s, vinylic), 6.95 (2H, m, arom.), 7.10 (2H, d, 3 JHH
=
6.5 Hz, arom.), 7.10 (2H, d, JHH = 6.5 Hz, arom.). 13C NMR (CDCl3)
for E isomer, δC: 51.78 and 53.11 (2OCH3), 98.99 (=CH, vinylic), 116.93
(1C, d, 1 JCF = 23.5 Hz, C F), 122.40 (2C, d, 2 JCF = 8.4 Hz, C3 and C5),
149.74 (C2 and C6), 157.84 (C1, arom.), 159.48 (OC =, vinylic), 163.12
and 165.74 (2C=O, ester). 19F NMR (CDCl3) for E isomer, δF: −115.84
(1F, s, CF).
3
Diethyl-2-(4-fluoro-phenoxy)-2-butenedioate (6b)
Viscous yellow oil; IR(neat) (νmax, cm−1): 3077, 2985, 2939, 2908, 1727
and 1643. Found: C, 59.85; H, 5.62. C12H11FO5 requires C, 59.57; H,
1
5.36%. H NMR (CDCl3) for Z isomer, δH: 1.18–1.24 (6H, m, 2CH3 of
2Et), 4.15–4.24 (4H, m, 2OCH2 of 2Et), 6.60 (1H, s, vinylic), 6.96 (2H, m,
C3 and C5, arom.), 7.09 (2H, d, 3 JHH = 6.2 Hz, C2 and C6, arom.). 13
C
NMR (CDCl3) for Z isomer, δC: 14.02 and 14.04 (2CH3 of 2Et), 60.67 and
1
62.45 (2OCH2), 115.31 (=CH, vinylic), 116.12 (1C, d, JCF = 23.5 Hz,
2
C F), 117.43 (2C, d, JCF = 8.6 Hz, C3 and C5), 149.89 (C2 and C6),
150.00 (C1, arom.), 159.71 (OC=, vinylic), 162.70 and 163.34 (2C=O,
ester). 19F NMR (CDCl3) for Z isomer, δF: −120.85 (1F, s, CF).
3
1H NMR (CDCl3) for E isomer, δH: 1.23 and 1.37 (6H, 2t, JHH
=
7.2 Hz, 2CH3 of 2Et), 4.17 (2H, m, OCH2), 4.38 (2H, q, 3 JHH = 7.1 Hz,
OCH2); 5.11 (1H, s, vinylic), 6.96 (2H, m, C3 and C5, arom.), 7.09 (2H,
d, 3 JHH = 6.2 Hz, C2 and C6, arom.). 13C NMR (CDCl3) for E isomer, δC:
13.85 and 14.10 (2CH3 of 2Et), 61.04 and 62.31 (2OCH2), 99.37 (=CH,
1
2
vinylic), 116.78 (1C, d, JCF = 23.6 Hz, C F), 122.47 (2C, d, JCF
=
8.7 Hz, C3 and C5), 149.55 (C2 and C6), 152.66 (C1, arom.), 159.46

Dialkyl 2-(4-Fluoro-Phenoxy)-2-Butendioates
417
(OC=, vinylic), 162.55 and 165.23 (2C=O, ester). 19F NMR (CDCl3) for
E isomer, δF: −116.24 (1F, s, CF).
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