Convenient and stereoselective synthesis of symmetrical (E)-stilbenes via homocoupling of 1,3-dibenzylbenzotriazolium bromides

Article abstract of DOI:10.1055/s-0030-1260814

Using NaH as the base and DMSO as the solvent, a series of symmetric (E)-stilbenes were prepared in good yields via the -homocoupling of 1,3-dibenzylbenzotriazolium bromides at room temperature. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0030-1260814

LETTER
1731
Convenient and Stereoselective Synthesis of Symmetrical (E)-Stilbenes via
Homocoupling of 1,3-Dibenzylbenzotriazolium Bromides
S
ynthesis of Symm
i
etrical
a
(
E
)-Stilben
o
e
s
hui Xiao,*a Daqin Lin,a Shuitian Tong,a Hong Luo,b Yinfeng He,a Hailan Moa
a
Department of Chemistry, Zhejiang Normal University, 688 Yingbing Road, Jinhua, Zhejiang 321004, P. R. of China
Fax +86(579)82282610; E-mail: xxh@zjnu.cn
b
College of Geography and Environmental Science, Zhejiang Normal University,
688 Yingbing Road, Jinhua, Zhejiang 321004, P. R. of China
Received 9 April 2011
NaH as the base and DMSO as the solvent (entry 6), an ex-
cellent yield of 2a (91%) was obtained. However, the re-
actions in MeCN, THF, or t-BuOH produced 2a in low
yields due to the poor solubility of 1a in these solvents.
Using K2CO3 or Et3N as the base, compound 2a was not
obtained. This illustrated that the basicities of K2CO3 and
Et3N were not strong enough to generate benzotriazolium
ylide at room temperature.
Abstract: Using NaH as the base and DMSO as the solvent, a series
of symmetric (E)-stilbenes were prepared in good yields via the
homocoupling of 1,3-dibenzylbenzotriazolium bromides at room
temperature.
Key words: stilbenes, 1,3-dibenzylbenzotriazolium bromides,
ylides, coupling, elimination
Benzotriazole is an inexpensive, stable, and versatile
compound. It can act as a leaving group, proton activator,
cation stabilizer, radical precursor, and anion precursor.
Therefore, benzotriazole-based intermediates have
proved to be important and efficient reagents in organic
synthesis.1 Benzotriazolium salts have also been prepared
and reported.2–5 However, most of the reported benzotria-
zolium salts were investigated as cyclophanes2 or ionic
liquids.3 Although some publications have discussed the
preparation and properties of benzotriazolium ylides,4 but
very few have reported the reactions of benzotriazolium
ylides.5 Pardo and co-workers obtained new stable betains
from the reaction of 3-dicyanomethyl-1-ethylbenzotriazo-
lium ylide with acetylenic esters.5a It was interpreted that
the new betains were formed by the [2+3] cycloaddition
of 3-dicyanomethyl-1-ethylbenzotriazolium ylide with
acetylenic esters followed by a subsequent ring-opening
reaction. Gandasegui and co-worker investigated the reac-
tions of 3-methyl-1-phenacylbenzotriazolium ylide with
isocyanates or isothiocyanates, and they also obtained
new betains.5b More interestingly, it was reported that the
dihydrofuran derivatives could be prepared from the reac-
tion of 3-methyl-1-phenacylbenzotriazolium ylide with
aldehyde.5c
Table 1 Homocoupling of 1,3-Dibenzylbenzotriazolium Bromide:
Effects of Solvent and Basea
Br–
N
N
N
N
Ph
Ph
N
N
base
r.t.
Ph
+
2
2
Ph
Ph
1a
2a
3a
Entry
Base
Solvent
DMSO
DMF
Yield (%)b
1
2
3
4
5
6
7
8
9
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
KOt-Bu
NaH
59
28
39
25
10
91
0
MeCN
THF
t-BuOH
DMSO
DMSO
DMSO
THF
K2CO3
Et3N
0
n-BuLi
19
a Reaction conditions: 1,3-dibenzylbenzotriazolium bromide (1a, 3
mmol), base (6 mmol), solvent (6 mL), atmosphere, r.t.
Considering 1-substituted benzotriazoles are good leaving
groups and benzotriazolium ylides are good nucleophiles, b Isolated yields.
we tried to prepare the stilbenes from the homocoupling
of 1,3-disubstituted benzotriazolium salts in the presence
Using NaH as the base and DMSO as the solvent, various
of base. As expected, 1,2-diphenylethene (2a) was isolat-
ed in good yield (Table 1, entry 1) from the reaction of
1,3-dibenzylbenzotriazolium bromide 1a with KOt-Bu. In
order to improve the yield of 2a, the effect of the solvent
and base was investigated. As shown in Table 1, using
1,3-disubstituted benzotriazolium salts were used in the
preparation of 1,2-diphenylethene (2a, Table 2). It was
shown that the homocoupling of 1-benzyl-3-ethylbenzo-
triazolium bromide also produced 2a in high yield
(Table 2, entry 4), but both 1-benzyl-3-methylbenzotriaz-
olium bromide (Table 2, entry 2), and 1-benzyl-3-methyl-
benzotriazolium iodide (Table 2, entry 3) produced 2a in
lower yield. Styrene was found in the reaction mixtures by
SYNLETT 2011, No. 12, pp 1731–1734
x
x
.x
x
.2
0
1
1
Advanced online publication: 05.07.2011
DOI: 10.1055/s-0030-1260814; Art ID: W08911ST
© Georg Thieme Verlag Stuttgart · New York

1732
X. Xiao et al.
LETTER
GC-MS determination. However, it was still unclear why
there were only small amounts of 2a obtained in the ho-
mocoupling of 1-benzyl-3-phenylbenzotriazolium bro-
mide (Table 2, entry 5).
Table 2 Preparation of 1,2-Diphenylethene 2a from Various 1,3-
Disubstituted Benzotriazolium Saltsa
X–
N
R
N
N
N
Ph
N
N
NaH, DMSO
r.t.
Ph
Under the optimized conditions (Table 1, entry 6), a series
of 1,3-dibenzylbenzotriazolium bromides were subjected
to this reaction, which are listed in Table 3. In all these
cases the desired stilbenes were obtained in good yields.
It was found that both electron-donating and electron-
withdrawing substituents on the aromatic ring of the ben-
zyl group decreased the yield of stilbenes. In addition,
ortho, meta, and para substituents on the aromatic ring of
the benzyl group gave similar yields of stilbenes. More in-
triguingly, all reactions displayed high stereoselectivity
and most of them only produced (E)-stillbenes (entries 1–
5 and 10).
2
+
2
Ph
R
2a
Entry
R
X
Yield (%)b
1
2
3
4
5
Bn
Me
Me
Et
Br
Br
I
91
56
68
Br
Br
83
Ph
trace
a Reaction conditions: 1,3-disubstituted benzotriazolium salts (3
mmol), NaH (6 mmol), DMSO (6 mL), atmosphere, r.t.
b Isolated yields.
Table 3 Preparation of Stilbenes from Various 1,3-Dibenzylbenzotriazolium Bromidesa
Br–
N
N
N
N
Ar
Ar
N
N
NaH, DMSO
r.t.
Ar
+
2
2
Ar
Ar
1
2
3
Entry
1
1,3-Dibenzylbenzotriazolium bromides 1 Product 2
E/Zb
Yield (%)c
91
N
N
Br–
N
100:0
2a
1a
N
N
Br–
N
N
N
N
2
3
4
100:0
100:0
100:0
72
70
73
2b
2c
1b
1c
Br–
Br–
N
N
N
2d
1d
MeO
Br–
N
N
N
5
100:0
64
OMe
OMe
OMe
1e
2e
Synlett 2011, No. 12, 1731–1734 © Thieme Stuttgart · New York

LETTER
Synthesis of Symmetrical (E)-Stilbenes
1733
Table 3 Preparation of Stilbenes from Various 1,3-Dibenzylbenzotriazolium Bromidesa (continued)
Br–
N
N
N
N
Ar
Ar
N
N
NaH, DMSO
r.t.
Ar
+
2
2
Ar
Ar
1
2
3
Entry
6
1,3-Dibenzylbenzotriazolium bromides 1 Product 2
E/Zb
Yield (%)c
Cl
Cl
Br–
N
N
Cl
N
90:10
59
Cl
1f
2f
Cl
Br–
N
N
N
7
8
86:14
98:2
57
61
64
65
Cl
Cl
Cl
1g
2g
Cl
N
Br–
N
N
Cl
Cl
Cl
2h
1h
Br–
N
N
N
Br
Br
9
95:5
Br
Br
2i
1i
Br–
N
N
N
F
F
10
100:0
F
F
2j
1j
a Reaction conditions: 1,3-dibenzylbenzotriazolium bromide 1 (3 mmol), NaH (6 mmol), DMSO (6 mL), atmosphere, r.t.
b E/Z ratio was determined by GC-MS.
c Isolated yields.
A possible mechanism is presented in Scheme 1, which is reactions,8 and homocoupling of arylmethyl phosphoni-
similar to the reported mechanism of homocoupling of um salts6,9 or aryl aldehyde tosylhydrazones,10 most of the
arylmethyl phosphonium salts.6 First, 1,3-dibenzylbenzo- reported synthetic methods suffer from the use of expen-
triazolium ylide A is formed by the reaction of 1,3-diben- sive and toxic materials, such as transition metals, phos-
zylbenzotriazolium bromide with NaH. And then it reacts phonium salts, and hydrazines. In this process, the starting
with another equivalent 1,3-dibenzylbenzotriazolium bro- materials 1,3-dibenzylbenzotriazolium bromides 1 are
mide to form the intermediate B. Finally, intermediate B nontoxic, cheap, and easily available from the reaction of
undergoes a Hoffmann elimination to give stilbene 2. The 1-benzylbenzotriazole with benzyl bromide.2d
byproduct 1-benzylbenzotriazole 3 can be easily recov-
ered by column chromatography.
In conclusion, we have developed a novel, convenient,
and efficient synthesis of symmetrical (E)-stilbenes by
Stilbenes are widely used as dyes, brighteners, whiteners homocoupling of 1,3-dibenzylbenzotriazolium bromides.
of paper and textiles, and photobleachers due to their out- The advantages of this method are the use of cheap, non-
standing optical properties. Although they can be pre- toxic, easily available materials, mild reaction conditions,
pared by Wittig-type reactions,7 metal-catalyzed coupling simple operations, and high stereoselectivity.
Synlett 2011, No. 12, 1731–1734 © Thieme Stuttgart · New York

1734
X. Xiao et al.
LETTER
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–
N
N
N
N
N
N
Ar
Ar
Ar
Ar
NaH
1
A
Br–
N
N
N
Ar
Ar
SN2
Ar
Br–
H
N
N
N
Ar
Ar
Ar
NaH
Ar
Hoffmann elimination
2
B
Scheme 1 Proposed mechanism of the homocoupling reaction
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
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