An unexpected phosphine-mediated olefination of salicylaldehydes with α-methyl allenoate

Article abstract of DOI:10.1080/10426500701690889

An unexpected phosphine-mediated olefination of salicylaldehydes with α-methyl allenoate is described. Thus, under the mediation of PPh3, six salicylaldehydes react with ethyl α-methyl butadienoate forming the corresponding (1E, 3E)-1-(2-hydroxyphenyl)-3-carboethoxy-1,3-pentadienes in fair to excellent yields. A plausible mechanism for this reaction is also discussed. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500701690889

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Phosphorus, Sulfur, and Silicon
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An Unexpected Phosphine-
Mediated Olefination of
Salicylaldehydes with α-Methyl
Allenoate
Zhengrong He a , Xiaofang Tang b & Zhengjie He b
a Department of Chemistry , Xiaogan University ,
Xiaogan, Hubei, China
b State Key Laboratory of Elemento-Organic
Chemistry , Nankai University , Tianjin, China
Published online: 04 Jun 2008.
To cite this article: Zhengrong He , Xiaofang Tang & Zhengjie He (2008) An
Unexpected Phosphine-Mediated Olefination of Salicylaldehydes with α-Methyl
Allenoate, Phosphorus, Sulfur, and Silicon and the Related Elements, 183:6,
1518-1525, DOI: 10.1080/10426500701690889
To link to this article: http://dx.doi.org/10.1080/10426500701690889
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Phosphorus, Sulfur, and Silicon, 183:1518–1525, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701690889
An Unexpected Phosphine-Mediated Olefination
of Salicylaldehydes with α-Methyl Allenoate
Zhengrong He,1 Xiaofang Tang,2 and Zhengjie He2
1Department of Chemistry, Xiaogan University, Xiaogan, Hubei, China
2State Key Laboratory of Elemento-Organic Chemistry, Nankai
University, Tianjin, China
An unexpected phosphine-mediated olefination of salicylaldehydes with α-methyl
allenoate is described. Thus, under the mediation of PPh , six salicylaldehydes
3
react with ethyl α-methyl butadienoate forming the corresponding (1E, 3E)-1-(2-
hydroxyphenyl)-3-carboethoxy-1,3-pentadienes in fair to excellent yields. A plausi-
ble mechanism for this reaction is also discussed.
Keywords Ethyl 2,3-butadienoate; olefination; salicylaldehyde; triphenylphosphine
INTRODUCTION
In recent years, the nucleophilic catalysis by phosphine has at-
tracted much attention within the chemistry community. Many new
phosphine-catalyzed reactions with high synthetic potentials have
been discovered.1 A class of substrate commonly used in these reac-
tions is allene, which generally is activated by electron-withdrawing
groups and has proven to be increasingly important and versatile
synthetic intermediate by renewed research efforts. The phosphine-
catalyzed reactions of activated allenes mainly fall into four categories:
isomerization,2 α-addition,3 γ -addition,4 and cycloaddition,5 in which
the phosphine acts as a nucleophilic organocatalyst. In addition to
its pronounced nucleophilicity, the tertiary phosphine also possesses a
strong deoxygenating function, which is believed to be a primary driv-
ing force for the famous Wittig and Mitsunobu reactions.6 Most recently,
Received 8 August 2007; accepted 12 August 2007.
Financial support from National Natural Science Foundation of China (Grant No.
20672057; 20421202), Scientific Research Foundation of State Education Ministry for the
Returned Overseas Chinese Scholars, and Research Starting Fund of Nankai University
is gratefully acknowledged.
Address correspondence to Zhengjie He, 94 Weijin Road, Department of Chemistry,
Nankai University, Tianjin 300071, China. E-mail: zhengjiehe@nankai.edu.cn
1518

Phosphine-Mediated Olefination of Salicylaldehydes
1519
many novel phosphine-mediated heterocyclic constructions from the ac-
tivated allenes have been reported, in which both nucleophilicity and
deoxygenating ability of phosphine take effects.1c,7 These new findings
all further strengthen the versatility of phosphine in the organic syn-
thesis.
In most cases of the phosphine-mediated reactions of allenes, the new
bond formation in the product occurs at α- or/and γ -carbons of allenic
esters. However, subtle structural changes, such as the replacement
of hydrogen with methyl at α position in the allenoates, can signifi-
cantly alter the reactivity trend. For example, under the catalysis of a
nucleophilic phosphine, a series of α-alkyl substituted allenoates with
N-tosyl imines undergo the [4 + 2] annulation, forming highly func-
tionalized tetrahydropyridines in high yields,8 rather than the usual
[3 + 2] cycloaddition for α-unsubstituted analogous allenes under sim-
ilar conditions.5d,9 As part of our continuous efforts on the phosphine-
mediated carbon-carbon bond forming reactions,10 we are interested in
the reaction of allenic esters with substituted salicylaldehydes, which
possess both nucleophilic phenolic hydroxyl and electrophilic formyl.
Herein, we report an unexpected triphenylphosphine-mediated olefi-
nation of salicylaldehyde with α-methyl allenoate.
RESULTS AND DISCUSSION
Under the participation of one equivalent of triphenylphosphine, a se-
ries of substituted salicylaldehydes readily reacted with ethyl α-methyl
butadienoate, giving the corresponding (1E, 3E)-1-(2-hydroxyphenyl)-

1520
Z. He et al.
3-carboethoxy-1,3-pentadienes 1 in fair to excellent yields [Eq. (1)]. Pre-
liminary optimization on reaction conditions disclosed that the reaction
run in the methylene chloride at low temperature (0◦C) gave better re-
sult. It is surprising that the new double bond formation in the major
product takes place at the α-methyl group, other than the olefinic α- and
γ -carbons of the allenic ester. The structures of product 1 were identi-
1
1
fied by their microanalysis, H, and 13C NMR spectra. Their H NMR
data provide a diagnostic evidence for the E-configuration assignment
to the double bond at C1: relatively greater coupling constants (ca. 16
Hz) between the olefinic protons at C1 and C2 were observed for prod-
ucts 1 except 1c, whose olefinic proton signals severely overlap with the
aromatic proton signals. In the 1H NMR spectra of products 1, a broad
singlet peak in the range of 5.32–6.70 ppm also supports the assignment
of the phenolic hydroxyl. For all products 1, the 13C NMR data clearly
show there are 11 sp2-hybridized carbons in a single molecule. Also, the
X-ray crystallography analysis of product 1c (CCDC 653761) provides
unambiguous evidence for the structure determination (Figure 1).11
In contrast with the results obtained in this study, a cyclization
reaction of salicylic aldehydes with α-methyl allenic ester catalyzed
by either organic Lewis base or inorganic base was reported by Shi12
[Eq. (2)]. The annulation reaction constitutes an efficient synthesis of
2H-1-chromene derivatives. Although the authors did not rule out the
FIGURE 1 The X-ray structure of Compound 1c.

Phosphine-Mediated Olefination of Salicylaldehydes
1521
possibility that the Lewis base acts as a nucleophilic catalyst in a plau-
sible mechanism, it is believed that the basicity of the employed catalyst
plays a critical role in the catalysis. It is well known that triphenylphos-
phine has a weaker basicity compared to trialkylphosphines or tertiary
amines. This may be the reason for its ineffectiveness in the cataly-
sis of the above cyclization reaction.12a However, triphenylphosphine
still possesses a decent nucleophilicity due to the better polarization
ability of phosphorus atom, and therefore is the most often used nu-
cleophilic organocatalyst in recent years. Also, as the characteristics of
tertiary phosphine, it has a good deoxygenating ability. Probably, both
its nucleophilicity and strong affinity for oxygen play critical roles in
the olefination of salicylic aldehydes in this study.
The exact mechanism for this phosphine-mediated olefination re-
mains unclear. Based on experimental results in this study, a ratio-
nalized mechanism may be proposed, as shown in Scheme 1. The
olefination of salicylic aldehyde with α-methyl allenoate presumably
proceeds via two steps: the phosphine-catalyzed isomerization and the
SCHEME 1 A plausible mechanism.

1522
Z. He et al.
olefination via in situ formed triphenylphosphonium ylide. It is well
documented that the nucleophilic phosphine can catalyze the isomer-
ization of electron-deficient alkynes or allenes into conjugated dienes2.
Thus, the α-methyl allenoate is first isomerized into 1,3-diene 4 via the
phosphine nucleophilic attack at the β-carbon of the allenoate, followed
by hydrogen shift and elimination of the phosphine. Subsequently, the
Michael addition of triphenylphosphine to the 1,3-diene 4, followed by
proton transfer, leads to the in situ formation of triphenylphosphonium
ylide 5, which then reacts with salicylic aldehyde forming a new dou-
ble bond with E-configuration. The in situ formation of phosphonium
ylide from electron-deficient alkene or alkyne has been reported since
the early times,13 which also has been developed as an effective way to
the olefination of aldehydes via the Wittig reaction. In this study, all at-
tempts to employ other aromatic aldehydes without phenolic hydroxyl
instead of salicylic aldehydes failed in giving the similar products. This
result clearly shows the phenolic hydroxyl plays a critical role in the
olefination, possibly by the way of facilitating the proton transfer as
shown in the above-proposed mechanism (Scheme 1).
In summary, a novel phosphine-mediated olefination between the
salicylic aldehydes and α-methyl allenoate is described. To the best of
our knowledge, it is the first example of its kind for the substrate allene.
It also furnishes a convenient synthesis of highly functionalized 1,3-
dienes. Triphenylphosphine acts as both nucleophilic organocatalyst
and deoxygenating agent in the reaction.
EXPERIMENTAL
Unless otherwise noted, all reactions were carried out in the ambi-
ent atmosphere. Commercially available chemicals were used as re-
ceived. Ethyl α-methyl butadienoate was prepared as described in the
literature.14
Nuclear magnetic resonances (1H, 13C) were recorded on Bruker 300
MHz and Variant 400 MHz NMR spectrometers using TMS as internal
standard (1H, 13C), and CDCl3 as solvent. Elemental analyses were
performed on a Yanaco CHN Corder MT-3 automatic analyzer. ESI mass
spectra were recorded with ThermoFinnigan LCQ Advantage LC-MS.
X-ray crystal diffraction data were collected on a Nonius Kappa CCD
˚
diffractometer with Mo Kα radiation (λ = 0.7107 A).
Phosphine-Mediated Olefination of Salicylaldehydes with
α-Methyl Allenoate (General Procedure)
To a magnetically stirred mixture of triphenylphosphine (0.26 g, 1.0
mmol) and salicylic aldehyde (1.0 mmol) in dichloromethane (2 mL) was

Phosphine-Mediated Olefination of Salicylaldehydes
1523
dropwise added a solution of ethyl 2-methyl-2,3-butadienoate (0.15 g,
1.2 mmol) in dichloromethane (1 mL) at 0◦C over 10 min. The reaction
was then allowed to warm to room temperature and stirred for 12–24 h
until the aldehyde was consumed (TLC monitoring). After the removal
of solvent and column chromatography on silica gel (200–300 mesh)
eluted with a mixture of petroleum ether and ethyl acetate (20:1, V/V),
a pure product 1 was obtained in 32–92% isolated yield (based on the
substrate aldehyde).
1a (R = H)
Reaction time 24 h; solidified on standing as a white solid, m.p. 42–
44◦C; yield 92%; 1H NMR: δ 7.46 (m, 1H), 7.28 (d, J = 16.4 Hz, 1H), 7.10
(m, 1H), 6.92 (d, J = 16.4 Hz, 1H), 6.91-6.84 (m, 3H), 6.70 (br s, 1H),
4.28 (q, J = 7.2 Hz, 2H), 2.00 (d, J = 7.2 Hz, 3H), 1.32 (t, J = 7.2 Hz,
3H); 13C NMR: δ 167.9, 153.6, 138.2, 131.4, 128.7, 128.4, 126.8, 124.8,
121.5, 120.5, 116.0, 60.9, 14.8, 14.1 ppm; microanalysis calculated for
C14H16O3 (MW: 232.1): C 72.39, H 6.94; Found: C 72.30, H 7.11; ESI-MS
(positive mode): m/z 233.3 [M + H]+.
1b (R = 3-Me)
Reaction time 24 h; solidified on standing as a white solid, m.p. 75–
1
77◦C; yield 60%; H NMR: δ 7.30 (m, 1H), 7.25 (d, J = 16.2 Hz, 1H),
7.15 (m, 1H), 6.90–6.81 (m, 3H), 5.32 (br s, 1H), 4.25 (q, J = 7.2 Hz,
2H), 2.26 (s, 3H), 1.93 (d, J = 7.2 Hz, 3H), 1.32 (t, J = 7.2 Hz, 3H);
13C NMR: δ 167.3, 151.5, 138.2, 131.3, 130.2, 128.2, 124.9, 124.5, 123.9,
122.6, 120.4, 60.7, 15.9, 14.7, 14.2 ppm; microanalysis calculated for
C15H18O3: C 73.15, H 7.37; Found: C 73.11, H 7.54.
1c (R = 5-Me)
Reaction time 12 h; yield 70%; 1H NMR: δ 7.24 (m, 2H), 6.92–6.84 (m,
3H), 6.71 (m, 1H), 5.98 (br s, 1H), 4.26 (q, J = 7.2 Hz, 2H), 2.26 (s, 3H),
2.00 (d, J = 7.2 Hz, 3H), 1.32 (t, J = 7.2 Hz, 3H); 13C NMR: δ 164.9, 151.3,
138.0, 131.5, 129.8, 129.4, 128.3, 127.3, 124.4, 121.6, 115.9, 60.8, 20.5,
14.8, 14.2 ppm; microanalysis calculated for C15H18O3 (MW: 246.3): C
73.15, H 7.37; Found: C 73.17, H 7.42; ESI-MS (positive mode): m/z
246.4 [M]+; slow evaporation of its solution in a mixture of petroleum
ether/ethyl acetate (10:1, V/V) afforded colorless crystals suitable for
X-ray diffraction, m.p. 84–85◦C.
1d (R = 4-OMe)
Reaction time 24 h; solidified on standing as a white solid, m.p. 39–
42◦C; yield 79%; 1H NMR: δ 7.29 (d, J = 8.4 Hz, 1H), 7.18 (d, J = 16.5
Hz, 1H), 6.80 (q, J = 7.5 Hz, 1H), 6.78 (d, J = 16.5 Hz, 1H), 6.46 (dd, J =

1524
Z. He et al.
8.4, 2.1 Hz, 1H), 6.42 (d, J = 2.1 Hz, 1H), 4.26 (q, J = 6.9 Hz, 2H), 3.74 (s,
3H), 1.98 (d, J = 7.5 Hz, 3H), 1.33 (t, J = 6.9 Hz, 3H); 13C NMR: δ 167.9,
160.3, 154.7, 137.1, 131.6, 128.0, 127.8, 119.6, 117.9, 106.7, 101.8, 60.8,
55.2, 14.7, 14.2 ppm; microanalysis calculated for C15H18O4: C 68.68,
H 6.92; Found: C 68.87, H 7.02.
1e (R = 5-OMe)
Reaction time 24 h; yellowish viscous oil; yield 32%; 1H NMR: δ 7.22
(d, J = 16.8 Hz, 1H), 6.98 (s, 1H), 6.88-6.65 (m, 4H), 6.34 (br s, 1H), 4.25
(q, J = 7.2 Hz, 2H), 3.76 (s, 3H), 1.97 (d, J = 7.2 Hz, 3H), 1.31 (t, J =
7.2 Hz, 3H); 13C NMR: δ 167.8, 153.5, 147.9, 138.4, 131.4, 128.4, 125.5,
121.7, 116.9, 114.6, 111.6, 60.9, 55.8, 14.8, 14.2 ppm; microanalysis
calculated for C15H18O4: C 68.68, H 6.92; Found: C 69.01, H 6.93.
1f (R = 5-Cl)
Reaction time 24 h; yellowish semi-solid; yield 67%; 1H NMR: δ 7.36
(s, 1H), 7.18 (d, J = 16.4 Hz, 1H), 7.00-6.75 (m, 4H), 6.13 (br s, 1H),
4.24 (q, J = 7.2 Hz, 2H), 1.97 (d, J = 7.2 Hz, 3H), 1.32 (t, J = 7.2 Hz,
3H); 13C NMR: δ 167.7, 152.6, 138.9, 131.2, 128.3, 127.4, 126.4, 126.2,
125.1, 122.3, 117.5, 60.9, 14.8, 14.2 ppm; microanalysis calculated for
C14H15ClO3: C 63.04, H 5.67; Found: C 62.98, H 5.77.
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Phosphine-Mediated Olefination of Salicylaldehydes
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˚
orthorhombic, space group P212121, a = 8.371(16), b = 10.106(19), c = 17.26(3) A,
α = 90◦, β = 90◦, γ = 90◦, F(000) = 528, Z = 4, Dc = 1.120 g cm−3, U = 1460(5)
3
˚
˚
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int
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