Tributylphosphine-catalyzed stereoselective O-vinylation of 2-hydroxybenzaldehyde derivatives

Article abstract of DOI:10.1080/10426500902922941

Protonation of the highly reactive 1:1 intermediates, produced in the reaction between tributylphosphine and alkyl acetylenecarboxylates by 2-hydroxybenzaldehyde derivatives, leads to vinyltributylphosphonium salts, which undergo a Michael addition reaction with conjugate base to produce corresponding phosphorus ylides. The phosphorus ylides convert to electron-poor O-vinyl ether derivatives under reaction conditions.

Full text of DOI:10.1080/10426500902922941

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Tributylphosphine-Catalyzed
Stereoselective O-Vinylation of 2-
Hydroxybenzaldehyde Derivatives
Ali Ramazani a , Vahid Azizkhani a & Farideh Gouranlou b
a Chemistry Department , Payam Noor University (PNU) , Abhar,
Zanjan, Iran
b Chemistry Department , Malek Ashtar University of Technology ,
Tehran, Iran
Published online: 23 Mar 2010.
To cite this article: Ali Ramazani , Vahid Azizkhani & Farideh Gouranlou (2010) Tributylphosphine-
Catalyzed Stereoselective O-Vinylation of 2-Hydroxybenzaldehyde Derivatives, Phosphorus, Sulfur, and
Silicon and the Related Elements, 185:4, 719-724, DOI: 10.1080/10426500902922941
To link to this article: http://dx.doi.org/10.1080/10426500902922941
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Phosphorus, Sulfur, and Silicon, 185:719–724, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902922941
TRIBUTYLPHOSPHINE-CATALYZED STEREOSELECTIVE
O-VINYLATION OF 2-HYDROXYBENZALDEHYDE
DERIVATIVES
Ali Ramazani,1 Vahid Azizkhani,1 and Farideh Gouranlou2
1Chemistry Department, Payam Noor University (PNU), Abhar, Zanjan, Iran
2Chemistry Department, Malek Ashtar University of Technology, Tehran, Iran
Protonation of the highly reactive 1:1 intermediates, produced in the reaction between
tributylphosphine and alkyl acetylenecarboxylates by 2-hydroxybenzaldehyde derivatives,
leads to vinyltributylphosphonium salts, which undergo a Michael addition reaction with
conjugate base to produce corresponding phosphorus ylides. The phosphorus ylides convert
to electron-poor O-vinyl ether derivatives under reaction conditions.
Keywords Acetylenic ester; electron-poor O-vinyl ether; 2-hydroxybenzaldehyde; phosphorus
ylide; tributylphosphine; vinyltributylphosphonium salt
INTRODUCTION
The development of the modern chemistry of natural and physiologically active com-
pounds would have been impossible without the phosphorus ylides.1–16 The phosphorus
ylides represent an outstanding achievement of the chemistry of the 20th century.1–16 They
have found use in a wide variety of reactions of interest to synthetic chemists.1–16 Phos-
phorus ylides are important reagents in synthetic organic chemistry,1–16 especially in the
synthesis of naturally occurring products, and compounds with biological and pharmaco-
logical activity.6 These compounds have attained great significance as widely used reagents
for linking synthetic building blocks with the formation of carbon–carbon double bonds,
and this has aroused much interest in the study of the synthesis, structure, and properties
of P-ylides and their derivatives.1–6
Several methods have been developed for preparation of phosphorus ylides.7–10 These
ylides are most often prepared by the treatment of a phosphonium salt with a base. Most
of the phosphonium salts are usually made from phosphine and an alkyl halide,10–16 and
they are also obtained by the Michael addition of phosphorus nucleophiles to activated
olefins.10–16 β-Additions of nucleophiles to the vinyl group of vinylic phosphonium salts
leading to the formation of new alkylidenephosphoranes have attracted much attention as
a very convenient and synthetically useful method in organic synthesis.17–30 Phosphorus
Received 7 February 2009; accepted 24 March 2009.
This work was supported by the Payam Noor University (PNU), Abhar, Zanjan, Iran.
Address correspondence to Chemistry Department, Payam Noor University (PNU), P O Box 97, Abhar,
Zanjan, Iran. E-mail: aliramazani@gmail.com
719

720
A. RAMAZANI ET AL.
ylides are a class of 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 investigations.30 The proton affinity of these ylides can be used as a
molecular guide to assess their utility as synthetic reagents and their function as ligands in
coordination and organometallic chemistry.17,30 In the past we have established a conve-
nient, one-pot method for preparing stabilized phosphorus ylides utilizing an in situ genera-
tion of the phosphonium salts.18–28 Stabilized phosphorus ylides, versatile intermediates in
synthetic organic chemistry, can be prepared by the novel reaction of dialkyl acetylenedicar-
boxylates (DAAD), triphenylphosphine (TPP), and acids such as phenols, imides, amides,
enols, oximes, and alcohols.18–28 The reaction31 involves an intermediate formed by the
1:1 conjugate addition reaction of the TPP to DAAD, and concomitant protonation of the
intermediate by an acid leads to vinyltriphenylphosphonium salts.18–28 The salts are un-
stable intermediates and converted to stabilized phosphorus ylides via a Michael addition
reaction.18–28 The stabilized phosphorus ylides are able to take part in the intramolecular
Wittig reactions, but they are not generally able to participate in the intermolecular Wittig
reactions.18–28 The intermolecular Wittig reactions of the ylides are observed only with
highly electron-poor carbonyl groups such as indane-1,2,3-trione.31 The ylides are con-
verted to electron-poor alkenes via elimination of TPP in solvent-free conditions.31 Almost
all of the final products are valuable families of compounds.31 In this article, we wish
to describe a simple, one-pot method for the preparation of electron-poor O-vinyl ethers
from tributylphosphine 1, dialkyl acetylenedicarboxylates 2, and 2-hydroxybenzaldehyde
derivatives 3 (Scheme 1).
Scheme 1
RESULTS AND DISCUSSION
Tributylphosphine 1, dialkyl acetylenedicarboxylates 2, and 2-hydroxybenzaldehyde
derivatives 3 were reacted in a 1:1:1 ratio in dichloromethane at room temperature to give

STEREOSELECTIVE O-VINYLATION OF 2-HYDROXYBENZALDEHYDE
721
electron-poor O-vinyl ethers 8 (Scheme 1). TLC indicated the formation of products. The
reaction proceeded smoothly and cleanly under mild conditions, and no side reactions were
observed. In the reaction, tributylphosphine 1 acts as catalyst (Scheme 1). Reactions are
known in which an α,β -unsaturated carbonyl compound is produced from a phosphorane
and a carbonyl compound such as an aldehyde or ketone.19–21 Thus, compounds 8 may
be regarded as the product of an addition–elimination reaction. Such addition–elimination
products may result from an initial addition of tributylphosphine 1 to the acetylenic es-
ter 2 and concomitant protonation of the 1:1 adduct, followed by attack of the anion
of 2-hydroxybenzaldehyde derivative 3 on the β-carbon atom of the vinylphosphonium
cation 5 to form intermediates 6 and 7. Elimination of tributylphosphine 1 from inter-
mediate 7 would lead to stereoselective formation of electron-poor O-vinyl ethers (8) in
fairly high yields (Scheme 2). However, in the presence of triphenylphosphine, chromene
derivatives have been reported previously.31 The mechanism of the reaction outlined above
has not been established experimentally. However, a possible explanation19 is proposed in
Scheme 2.
Scheme 2
The structure of products 8 was proved by their IR, 1H NMR, and 13C NMR spectral
data(see the Experimental section). TheNMRspectra indicatedthat solutions of compounds
8a, 8b, and 8d (CDCl3 as solvent) contain the Z isomer as a major product19 that may result
from the easy formation of the Z isomer to E isomer. However, in the case of 8c, E isomer
as a major product was observed. Increasing of steric strain in the Et group in comparison
with Me group may be a plausible factor for the formation of E isomer, as a major product
in the case of 8c. Because of the closing of spot distances of the stereoisomers in TLC
and even using simple flash column chromatography, separations of stereoisomers are not
possible. The use of HPLC techniques may be suitable for the separation of them. The ratio
of the stereoisomers was determined from their 1H NMR spectra. The 1H NMR spectrum of

722
A. RAMAZANI ET AL.
the major (Z) stereoisomer of 8a exhibited five signals readily recognized as arising from
two OMe groups [δ = 3.71 and 3.76 (6 H, 2 s, 2 OCH3)], CH [δ = 6.75 (1 H, s, CH)],
aromatic moieties [δ = 6.80–8.00 (4 H, m, arom.)], and [δ = 10.54 (1 H, s, aldehyde)]. The
13C NMR spectrum of the major (Z) stereoisomer of 8a showed 13 distinct resonances, as
expected. Partial assignment of these resonances is given in the Experimental section. The
1H and 13C NMR signals of the minor stereoisomer (E) of 8a are similar to those of the
major stereoisomer (Z) of 8a (see the Spectral Analysis section).28,32
CONCLUSION
In summary, we have found a new and efficient method for the stereoselective syn-
thesis of electron-poor O-vinyl ethers (8) from dialkyl acetylenedicarboxylates (2) and
2-hydroxybenzaldehyde derivatives (3) in the presence of tributyphosphine (1) as catalyst
(Scheme 1 and Scheme 2). We believe the reported method offers a simple and efficient route
for the preparation of the electron-poor O-vinyl ethers 8 (Scheme 1). Its ease of workup
and high yields make it a useful method.33–36 Possible extensions are under investigation.
EXPERIMENTAL
IR spectra were recorded on a Mattson 1000 FT-IR spectrometer. 1H and 13C NMR
spectra were measured with a Bruker Spectrospin spectrometer at 250.0 and 62.5 MHz,
respectively. The ratio of the stereoisomers was determined from their 1H NMR spectra.
General Procedure for the Preparation of Electron-Poor O-Vinyl Ethers
8a–d
To a magnetically stirred solution of tributylphosphine 1 (0.260 mL, 1.00 mmol) and
2-hydroxybenzaldehyde derivative 3 (1.0 mmol) in CH2Cl2 (10 mL), a mixture of dialkyl
acetylenedicarboxylate 2 (0.13 mL, 1.0 mmol) in CH2Cl2 (5 mL) was added dropwise at
–10 ◦C over 15 min. The mixture was allowed to warm up to room temperature and stirred
for 6 h. 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 to give products 8a–d.
Dimethyl 2-(2-Formyl phenoxy)-2-butenedioate (8a)
Light yellow oil; yield: 82%.%Z = 75%; 1H NMR (CDCl3) δH: 3.71 and 3.76 (6 H,
2 s, 2 OCH3); 6.75 (1 H, s, CH); 6.80–8.00 (4 H, m, arom.); 10.54 (1 H, s, aldehyde).
13C NMR (CDCl3) δC: 52.22 and 53.32 (2 OCH3); 115.32, 116.75, 123.75, 128.55 and
135.54 (5 CH); 125.64, 149.06 and 158.86 (3 C); 162.11 and 163.46 (2 C O of 2 CO2Me);
1
189.07 (C O of aldehyde).%E = 25%; H NMR (CDCl3) δH: 3.67 and 3.92 (6 H, 2 s,
2 OCH3); 5.22 (1 H, s, CH); 6.80–8.00 (4 H, m, arom.); 10.28 (1 H, s, aldehyde). 13C
NMR (CDCl3) δC: 52.02 and 53.12 (2 OCH3); 101.63, 121.61, 126.92, 127.72 and 136.06
(5 CH); 129.06, 155.11 and 159.86 (3 C); 162.27 and 165.21 (2 C O of 2 CO2Me); 187.77
(C O of aldehyde).

STEREOSELECTIVE O-VINYLATION OF 2-HYDROXYBENZALDEHYDE
723
Dimethyl 2-(2-Formyl-6-methoxyphenoxy)-2-butenedioate (8b)
Light brown oil; yield: 81%. IR (υmax, cm−1): 2928 (CH); 2853 (CH); 1723 (C O,
carbonyl); 1276 (C-O).%Z = 71%; 1H NMR (CDCl3) δH: 3.65, 3.77 and 3.80 (9 H, 3 s, 3
OCH3); 6.33 (1 H, s, CH); 7.10–7.55 (3 H, m, arom.); 10.49 (1 H, s, aldehyde). 13C NMR
(CDCl3) δC: 51.82, 52.99 and 56.51 (3 OCH3); 108.57 ( CH); 118.43, 124.42 and 127.61
(3 CH, arom.); 148.07, 127.86, 149.55 and 152.05 (4 C); 162.35 and 164.02 (2 C O of
2 CO2Me); 189.37 (C O of aldehyde).%E = 29%; 1H NMR (CDCl3) δH: 3.68, 3.91 and
3.95 (9 H, 3 s, 3 OCH3); 5.04 (1 H, s, CH); 7.10–7.55 (3 H, m, arom.); 10.25 (1 H, s,
aldehyde). 13C NMR (CDCl3) δC: 51.90, 53.27 and 56.45 (3 OCH3); 98.91 ( CH); 188.02
(C O of aldehyde).
Diethyl 2-(2-Formyl-6-methoxyphenoxy)-2-butenedioate (8c)
Light brown oil; yield: 86%. IR (υmax, cm−1): 2928 (CH); 2853 (CH); 1730 (C O,
carbonyl); 1276 (C O).%Z = 36%; 1H NMR (CDCl3) δH: 1.00–1.50 (6 H, m, 2 Me); 3.76
(3 H, s, OCH3); 4.10–4.45 (4 H, m. OCH2); 6.31 (1 H, s, CH); 7.10–7.53 (3 H, m, arom.);
10.49 (1 H, s, aldehyde). 13C NMR (CDCl3) δC: 13.90 and 14.07 (2 Me of 2 Et); 56.50
(OCH3); 60.74 and 62.17 (2 OCH2); 108.56 ( CH); 118.36, 124.26 and 129.26 (3 CH,
arom.); 127.90, 148.25, 149.74 and 152.26 (4 C); 161.78 and 163.67 (2 C O of 2 CO2Et);
189.43 (C O of aldehyde).%E = 64%; 1H NMR (CDCl3) δH: 1.00–1.50 (6 H, m, 2 Me);
3.84 (3 H, s, OCH3); 4.10–4.45 (4 H, m. OCH2); 5.03 (1 H, s, CH); 7.10–7.53 (3 H, m,
arom.); 10.26 (1 H, s, aldehyde). 13C NMR (CDCl3) δC: 13.88 and 14.05 (2 Me of 2 Et);
56.42 (OCH3); 60.80 and 62.56 (2 OCH2); 99.18 ( CH); 118.43, 119.58 and 127.49 (3
CH, arom.); 127.84, 143.66, 151.57 and 160.14 (4 C); 162.17 and 164.96 (2 C O of 2
CO2Et); 188.08 (C O of aldehyde).
Diethyl 2-(2-Formyl-5-methoxyphenoxy)-2-butenedioate (8d)
Brown oil; yield: 83%.%Z = 58%; 1H NMR (CDCl3) δH: 0.80–1.42 (6 H, m, 2 Me);
3.73 (3 H, s, OCH3); 3.78–3.95 (4 H, m. OCH2); 6.28 (1 H, s, CH); 6.60–7.90 (3 H, m,
arom.); 10.39 (1 H, s, aldehyde). 13C NMR (CDCl3) δC: 11.10 and 13.89 (2 Me of 2 Et);
52.24 (OCH3); 55.79 and 57.32 (2 OCH2); 101.26, 109.00, 119.55 and 129.44 (4 CH);
128.59, 148.85, 160.47 and 161.17 (4 C); 163.78 and 165.67 (2 C O of 2 CO2Et); 187.73
(C O of aldehyde).%E = 42%; 1H NMR (CDCl3) δH: 0.80–1.42 (6 H, m, 2 Me); 3.70 (3
H, s, OCH3); 3.78–3.95 (4 H, m. OCH2); 5.27 (1 H, s, CH); 6.60–7.90 (3 H, m, arom.);
10.11 (1 H, s, aldehyde). 13C NMR (CDCl3) δC: 11.10 and 13.89 (2 Me of 2 Et); 52.02
(OCH3); 55.70 and 57.39 (2 OCH2); 101.55, 106.44, 116.96 and 129.14 (4 CH); 125.95,
157.19, 160.15 and 161.20 (4 C); 163.11 and 165.87 (2 C O of 2 CO2Et); 188.08 (C O
of aldehyde).
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