Double elimination protocol for access to unsymmetrical di(phenylethynyl)benzenes

Article abstract of DOI:10.1246/cl.2003.104

For facile access to phenylene-ethynylenes, a new methodology for construction of unsymmetrically substituted di(phenylethynyl)benzenes has been established by taking advantage of double elimination protocol of α-sulfonyl aldolates.

Full text of DOI:10.1246/cl.2003.104

104
Chemistry Letters Vol.32, No.1 (2003)
Double Elimination Protocol for Access to
Unsymmetrical Di(phenylethynyl)benzenes
Akihiro Orita, Fangguo Ye, Atsushi Doumoto, and Junzo Oteraꢀ
Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 700-0005
(Received October 24, 2002; CL-020904)
For facile access to phenylene-ethynylenes, a new metho-
phenylene-ethynylenes 1 (Scheme ). In this process, key inter-
dology for construction of unsymmetrically substituted di(phe-
nylethynyl)benzenes has been established by taking advantage of
double elimination protocol of ꢀ-sulfonyl aldolates.
mediate 3 would be easily separated from the remaining starting
aldehyde and/or a possible co-product divinylsulfone, if any,
because of high polarity of the phenylsulfonyl group. This is
indeed the case, and the consecutive Peterson olefination and
double elimination give rise to the formation of the desired
unsymmetrically substituted phenylene-ethynylenes 1.
Phenylene-ethynylenes have received great attention as
organic materials like liquid crystal1 and photo-2 and electro-
luminecent compounds.3 Although it is well recognized that the
organic liquid crystal is composed of rigid and flexible parts, it is
greatly fascinating that the phenylene-ethynylene serves as a rigid
An anion of ꢀ-trimethylsilylbenzylsulfone (2a) prepared in
situ reacted with terephthalaldehyde (1.0 equiv.) at ꢁ78 ꢂC to
furnish a formylvinylsulfone 3a (X = H) in 75% yield as a 39 : 61
mixture of geometrical isomers. Notably, discrimination procee-
ded quite efficiently to provide 3 without protection-deprotection
technology.10 In sharp contrast, it has been described that
discriminative C–C bond formation by use of the Sonogashira
coupling is difficult even if an excess of dihaloarene is used.11
Consecutive addition of anion of p-methoxybenzylphenylsul-
fone, chlorophosphate and t-BuOK to 3a gave the desired
unsymmetrically substituted di(phenylethynyl)benzene 1a (X =
H, Y = MeO) in 91% yield (Table 1, entry 1). According to this
two-step procedure, several diynes were obtained successfully as
shown in Table 1.12 The unsymmetrically substituted diynes are
accessed irrespective of addition order of the sulfone (entries 1
and 2, 3 and 4, 5 and 6, 7 and 8), yet initial addition of sulfones
bearing a more electron-withdrawing group resulted in better
yields than those of the reversed order except the combination
shown in entries 7 and 8.
moiety in some cases4 and as a flexible one in the other.5
A
number of preparative methodologies for acetylenes have been
developed so far.6 Although the Sonogashira coupling between
aryl halides and acetylenes have been utilized routinely for access
to phenylene-ethynylenes,7 this methodology requires either
separate or in situ protection-deprotection operations for
construction of unsymmetrically substituted phenylene-
ethynylenes.8 On the other hand, we reported a double elimina-
tion protocol by use of sulfone and aldehyde as starting
compounds (Scheme 1).9
Table 1. Preparation of vinylsulfones 3 and di(phenylethynyl)-
benzenes 1 according to Scheme
Entry
X
Yield of 3/%
Y
Yield of 1/% Total yield/%a
Scheme 1.
1
2
3
4
5
6
7
8
H
MeO
F
H
Br
75
71
82
75
76
71
MeO
H
H
F
MeO
Br
91
75
89
80
87
69
67
86
68 (75 ꢃ 91)
53 (71 ꢃ 75)
73 (82 ꢃ 89)
60 (75 ꢃ 80)
66 (76 ꢃ 87)
49 (71 ꢃ 69)
45 (67 ꢃ 67)
61 (71 ꢃ 86)
In this process, a series of reactions such as aldol type C–C
bond formation, protection of the resulting aldolate and double
elimination of the b-substituted sulfone are integrated in one-pot.
We postulated that discriminative stepwise reactions with
dialdehydes would enable access to unsymmetrically substituted
MeO
C
MeO
F
67
71
MeO
CF3
3
aIsolated yield.
Subjection of 3 to the double elimination reaction with meta-
substituted sulfones afforded the corresponding meta-substituted
derivatives (Scheme 3). When isophthalaldehyde was utilized as
a starting compound, this two-step protocol served quite well to
afford the desired meta-substituted phenylene-ethynylene deri-
vatives (Scheme 4).
In summary, we have succeeded in the convenient syntheses
of unsymmetrically substituted phenylene-ethynylenes. Further
application of this protocol is underway in our laboratory.
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.1 (2003)
105
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Scheme 4.
10 Although a small amount of divinylsulfone (11% yield) was
also produced, its higher polarity enabled easy separation
from 3a by column chromatography.
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Financial support from New Energy and Industrial Technol-
ogy Development Organization (NEDO) of Japan for Industrial
Technology Research Grant Program (01B68006d) to A.O. is
gratefully acknowledged.
12 A typical procedure is as follows: To a THF solution (5 mL) of
benzylphenylsulfone (232 mg, 1.0 mmol) was added a hexane
solution of BuLi (1.60 M, 0.69 mL, 1.1 mmol) at ꢁ78 ꢂC, and
the mixture was stirred for 0.5 h. Trimethylsilyl chloride
(0.14 mL, 1.1 mmol) was added, and the mixture was stirred at
rt for 1 h. BuLi (1.60 M, 0.69 mL, 1.1 mmol) was added at
ꢁ78 ꢂC, and the mixture was stirred for 0.5 h. Terephthalal-
dehyde (134 mg, 1.0 mmol) was added and the mixture was
stirred for 2 h. After usual workup with ethyl acetate and
NH4Claq, drying over MgSO4 and evaporation, the residue
was subjected to column chromatography to furnish 3a as a
39 : 61 mixture of geometric isomers (261 mg, 75%). To a
THF solution (5 mL) of p-methoxybenzylphenylsulfone
(79 mg, 0.30 mmol) was added BuLi (1.60 M, 0.21 mL,
0.35 mmol), and the mixture was stirred for 0.5 h. A THF
solution (3 mL) of 3a (87 mg, 0.25 mmol) was added, and the
mixture was stirred for 1 h. At ꢁ78 ꢂC, ClP(O)(OEt)2
(0.05 mL, 0.30 mmol) was added, and the mixture was stirred
at rt for 2 h. t-BuOK (421 mg, 3.75 mmol) was added at
ꢁ78 ꢂC, and the mixture was stirred at ꢁ78 ꢂC for 1 h and at rt
for 2 h. After usual workup with ethyl acetate and NH4Claq,
drying over MgSO4 and evaporation, the residue was
subjected to column chromatography to furnish 1a (70 mg,
91%). 1a: mp 175–177 ꢂC; 1H NMR (CDCl3) ꢁ 3.83 (s, 3H),
6.89 (d, J ¼ 8:9 Hz, 2H), 7.34–7.37 (m, 3H), 7.46–7.54 (m,
8H); 13C NMR (CDCl3) ꢁ 55.3, 87.9, 89.2, 91.1, 91.3, 114.0,
115.1, 122.7, 123.1, 123.4, 128.4, 128.5, 131.3, 131.5, 131.6,
133.1, 159.8. Anal. Calcd for C23H16O: C, 89.58; H, 5.23.
Found: C, 89.53; H, 5.06.
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