Facile synthesis of [1]benzothieno[3,2-b]benzothiophene from o-dihalostilbenes

Article abstract of DOI:10.1016/j.tetlet.2010.07.152

A convenient one-pot synthesis of [1]benzothieno[3,2-b]benzothiophene from readily available o-dihalostilbenes using combined reagents of sodium sulfide nonahydrate (Na₂S·9H₂O) or sodium hydrosulfide hydrate (NaSH·nH₂O) and sulfur is described along with plausible reaction paths in this intriguing reaction.

Full text of DOI:10.1016/j.tetlet.2010.07.152

Tetrahedron Letters 51 (2010) 5277–5280
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Facile synthesis of [1]benzothieno[3,2-b]benzothiophene
from o-dihalostilbenes
Masahiko Saito a, Tatsuya Yamamoto a, Itaru Osaka a, Eigo Miyazaki a, Kazuo Takimiya a,b,
,
*
Hirokazu Kuwabara c, Masaaki Ikeda c
a Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
b Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
c Functional Chemicals Research Laboratories, Nippon Kayaku Co. Ltd, 3-31-12 Shimo, Kita-ku, Tokyo 115-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient one-pot synthesis of [1]benzothieno[3,2-b]benzothiophene from readily available o-dihalo-
stilbenes using combined reagents of sodium sulfide nonahydrate (Na2Sꢀ9H2O) or sodium hydrosulfide
hydrate (NaSHꢀnH2O) and sulfur is described along with plausible reaction paths in this intriguing
reaction.
Received 16 June 2010
Revised 26 July 2010
Accepted 29 July 2010
Available online 10 August 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Benzo[b]thiophene (BT) is an important structural component in
the development of organic optoelectronic materials, including or-
ganic photovoltaics1 and field-effect transistors.2 For these applica-
dichlorostilbene substrate took place, but the amount of sulfur
source was not sufficient and/or that the intramolecular SNAr reac-
tion favorably occurs in combination with the isomerization from
trans- to cis-form to afford 1.
tions, the BT moiety is often incorporated into further p-extended
molecular systems, which can be classified into several types. One
of these includes compounds where two or more thiophene rings
are fused onto a benzene or a naphthalene core, for example, ben-
zo[1,2-b:4,5-b0]dithiophene (BDT),3 benzo[1,2-b:3,4-b0:5,6-b00]tri-
thiophene (BTT),4 or naphtho[1,2-b:5,6-b0]dithiophene (NDT)5
(Class I in Fig. 1). Another class is compounds with two BT units fused
at the thiophene b-bonds, for example, [1]benzothieno[3,2-b]ben-
zothiophene (BTBT)6 and dinaphtho[2,3-b:20,30-f]thieno[3,2-b]thio-
phene (DNTT)7 (Class II in Fig. 1).
With these experimental results and considerations, we then
attempted a similar reaction by using a mixed reagent of an equi-
molar amount of Na2Sꢀ9H2O and elemental sulfur in NMP at 180 °C
(entry 2 in Table 1).11 The isolated products after purification with
S
Class I
S
S
For the synthesis of the compounds in Class I, we recently
developed a convenient one-pot procedure from readily available
o-halo-substituted ethynylbenzenes (Scheme 1).8 The procedure
only requires heating of the substrate with sodium sulfide nonahy-
drate (Na2Sꢀ9H2O) in N-methyl-2-pyrrolidone (NMP) at 180 °C and
is applicable to the synthesis of not only simple BT derivatives but
also the BDT, BTT,8 and NDT derivatives.5
S
S
S
S
BDT
BTT
NDT
Class II
S
S
With this successful development of the facile synthetic proce-
dure for the Class I compounds, we preliminarily tested a similar
reaction for the synthesis of BTBT using Na2Sꢀ9H2O as a reagent
and o-dichlorostilbene as a substrate, the latter of which is easily
available via a low-valent titanium-mediated coupling reaction of
o-chlorobenzaldehyde (Scheme 2, entry 1 in Table 1).9 However,
the major product of this reaction was not the desired compound,
BTBT, but dibenzo[b,f]thiepin (1),10 with the sulfur-containing se-
ven-membered ring, in 44% isolated yield. This result indicates that
the aromatic nucleophilic substitution (SNAr) reaction on the
S
BTBT
S
DNTT
p-extended BT structures.
Figure 1. Two classes of
R
Na2S·9H2O
NMP
R
S
Br
* Corresponding author. Tel.: +81 82 424 7734; fax: +81 82 424 5494.
E-mail address: ktakimi@hiroshima-u.ac.jp (K. Takimiya).
Scheme 1. One-pot synthesis of BT derivatives from o-halo-substituted
ethynylbenzenes.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.152

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M. Saito et al. / Tetrahedron Letters 51 (2010) 5277–5280
Table 2
Cl
Reaction of o-dichlorostilbene with NaSHꢀnH2O + S (2:1) at various temperaturesa
Na2S·9H2O
NMP
Cl
NaSH·nH2O + S
(2:1)
S
1
Cl
+
+
4
BTBT
3
NMP
X
44%
Cl
isomerization
Entry
Temp (°C)
Time (h)
Yield (%)
+
intramolecular
SNAr reaction
S
BTBT
3
4
1b
2
3
100
120
120
16
16
48
—
23
22
—
—
25
—
42
19
Scheme 2. Reaction of o-dichlorostilbene with Na2Sꢀ9H2O.
a
All the reactions were carried out using 1.0 mmol o-dichlorostilbene under
gel-permeation chromatography (JAI-GEL 1H, 2H column assem-
bly) included desired BTBT (16% yield, Rv = 250 mL), 1 (12% yield,
Rv = 240 mL), and 2-pheneylbenzo[b]thiophene (2, 13% yield,
Rv = 245 mL). Together with these well-characterized products,
substantial amount of ill-characterized byproducts with relatively
high molecular weight judged from EI-MS spectra (m/z = 450) and
the retention volume of gel-permeation chromatography
(Rv = 205 mL) was obtained and thus the material balance includ-
ing the byproduct seemed to be relatively good (>70%). When the
same reaction conditions were applied to trans-o-dibromostil-
bene12 as the substrate, BTBT was isolated in 30% yield (entry 3)
indicating higher reactivity of the bromo compound than that of
the chloro compound. These experimental results indicate that
the sodium sulfide-based reagent is useful for the synthesis of
BTBT from o-dihalostilbene via the direct SNAr-type reaction, cycli-
zation, and dehydrogenation to form the central thieno[3,2-b]thio-
phene moiety.
We then tested sodium hydrosulfide hydrate (NaSHꢀnH2O) in-
stead of Na2Sꢀ9H2O in a similar cyclization reaction from o-dichloro-
stilbene (Table 1, entries 4–8). Without the addition of elemental
sulfur (entry 4), only small amount of BTBT was obtained (3%)
together with dihydro-BT derivative (3, 41%) as the major product.
Addition of elemental sulfur (entry 4)11 altered the product distribu-
tion: BTBT (17%), 3 (13%), and other minor products with large
amounts of the ill-defined byproduct. In this reaction, one half of
elemental sulfur added was recovered after purification and then
the amount of sulfur was reduced to half of NaSHꢀnH2O (entry 6),
which gave almost the same yield of BTBT. Interestingly, when a
reaction with the same reagents was carried out under ambient air
conditions, the yield of BTBT was improved significantly up to 33%
ambient air conditions.
b
The substrate was recovered (84%).
Cl
Cl
NaSH·nH2O + S
(2:1)
BTBT
24%
+
NMP
180 ºC, 16 h
S
SH
4
3
20% (conversion)
(conversion)
Scheme 3. Reaction of 4 with NaSHꢀnH2O + S (2:1).
(entry 7). Under the identical reaction conditions, o-dibromostil-
bene as the substrate gave BTBT in 44% isolated yield (entry 8),
which is fairly better than the previously reported yield of BTBT from
(dichloromethyl)benzene (33%).13 It should also be noted that the
reactions of NaSHꢀnH2O-based reagents (entries 4–8) did not afford
dibenzothiepin (1), which means that the intramolecular SNAr-type
reaction does not occur with these reagents.
The experimental results summarizedin Table 1 clearly indicated
that the combined reagents of Na2Sꢀ9H2O/S or NaSHꢀnH2O/S in situ
generating the sodium oligosulfide reagents (Na2Sn) can be a good
nucleophile in the SNAr-type-reaction on o-dihalostilbene to afford
BTBT in moderate yields (up to 44%). The formation of BTBT must in-
volve the initial SNAr reaction of the sodium oligosulfide nucleo-
phile, subsequent cyclization onto the central double bond, and
final aromatization. To investigate the reaction path to BTBT from
o-dichlorostilbene, we then carried out the reactions at a lower
Table 1
Reaction of trans-o-dihalostilbene with various reagentsa
X
X
X
reagents
+
+
+
BTBT
+
S
S
S
NMP
SH
X
1
2
3
4
Entry
X
Reagent(s)
Temp (°C)
Time (h)
Yield (%)
BTBTc
1c
2c
3c
4c
1
2
3
4
5
6
7
8
Cl
Cl
Br
Cl
Cl
Cl
Cl
Br
Na2Sꢀ9H2O
180
180
180
180
180
180
180
180
16
16
16
16
16
16
4
—
44
12
6
—
—
—
—
—
—
13
7
3
18
—
—
41
13
10
10
6
—
—
6
16
Trace
9
Na2Sꢀ9H2O + S (1:1)
Na2Sꢀ9H2O + S (1:1)
NaSHꢀnH2O
16
30
3
17
18
33
44
NaSHꢀnH2O + S (1:1)
NaSHꢀnH2O + S (2:1)
NaSHꢀnH2O + S (2:1)b
NaSHꢀnH2O + S (2:1)b
6
Trace
9
Trace
4
Trace
Trace
a
All the reactions were carried out using 1.0 mmol trans-o-dihalostilbene under nitrogen atmosphere unless otherwise noted. All the products were isolated by gel-
permeation chromatography on a LC-9204 (Japan Analytical Industry Co. Ltd) with a JAI-GEL 1H, 2H column assembly eluted with chloroform.
b
The reaction was carried out under ambient conditions.
BTBT, 1, and 2 were characterized by comparing the authentic samples. Compounds 3 and 4 were fully characterized by means of spectroscopic analyses.
c

M. Saito et al. / Tetrahedron Letters 51 (2010) 5277–5280
5279
X
ill-defined byproducts
further reaction
with reagents
S
X
X
3
NaSH·nH2O + S
(2:1)
NMP
HS
SH
X
180 ºC
4
BTBT
SH
5
Scheme 4. Plausible reaction paths from o-dihalostilbene with NaSHꢀnH2O and elemental sulfur.
temperature (Table 2). At 100 °C or at a much lower temperature no
reaction took place, and most of the substrate was recovered (entry
1), whereas the reaction at 120 °C gave 2-chloro-20-mercaptostil-
bene (4, 42% isolated yield) as the major product together with BTBT
(23% isolated yield) (entry 2). Since the former compound, which
was most likely formed through the initial SNAr reaction on one ben-
zene ring was thought to be an intermediate for BTBT, the longer
reaction time at the same temperature was examined. However,
the yield of BTBT was not increased instead the yield of 3 was in-
creased (25%) with decreased yield of 4 (entry 3) suggesting that
the intramolecular addition of the thiol moiety of 4 to the olefine
took place almost exclusively.
venient method for the synthesis of BTBT. Furthermore, since the
compounds with o-dihalostilbene substructure can be readily pre-
pared from the corresponding aldehyde, the present reaction will
be a useful method for the synthesis of the Class II compound in
Figure 1. In fact, preliminary reactions under similar conditions
using trans-1,2-bis(3-chloronaphth-2-yl)ethane as the substrate
afforded DNTT, a superior OFET material recently utilized in vari-
ous applications.16 Optimization of the DNTT synthesis as well as
the applications to the synthesis of other compounds is now ac-
tively conducted in our group.
Acknowledgments
By using isolated and purified 4 as the substrate, we then tested
the reactions under the identical conditions in entries 7 and 8 in
Table 1 (Scheme 3). Unexpectedly, nearly half of 4 was recovered
in this reaction and BTBT and 3 were obtained in 24% and 20% con-
version yields, respectively, together with the byproducts. These re-
sults indicate that 4 is indeed an important intermediate in the
formation of BTBT, although the second SNAr reaction on 4 with
the thiolate anion is relatively slow and the intramolecular addition
of the thiol moiety to the olefine part affording 3 is competing. Fur-
thermore, we tested the reaction of 3 under the identical conditions.
However, this reaction afforded only the ill-defined byproducts
described above. Thus, we concluded that BTBT does not form via 3.
Considering the results from all the reactions, we propose a
plausible reaction path from trans-o-dihalostilbene to BTBT as
shown in Scheme 4. The key intermediate 4 produced by the initial
SNAr-type reaction has two reaction paths: one is a further reaction
with the reagent to produce o-dimercaptostilbene (5, not isolated)
that smoothly cyclizes and aromatizes to BTBT.14 The other is the
intramolecular addition of the thiol moiety to the olefine part to
produce 3, further reaction of which finally gives the ill-defined
byproducts. From the proposed reaction path, it is clear that the
formation of o-dimercaptostilbene (5) from o-dihalostilbene via 4
is critical for the formation of BTBT. However, there are two reac-
tion paths from 4 as mentioned above and these two paths com-
pete under the present reaction condition, which would be the
major reason for the low to moderate yields of BTBT. This consid-
eration is supported by the fact that the reactive substrate in the
SNAr-type-reaction, o-dibromostilbene gave a better yield (44%
yield) than the chloro counterpart did (33% yield). In addition, an
improved yield of BTBT (39%) from o-dichlorostilbene when the
reaction was carried out in hexamethylphosphoramide (HMPA), a
solvent that accelerates the SNAr-type reaction,15 also supports this
consideration.
This work was partially supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan (No. 20350088) and Promoting R&D
program from the Japan Science and Technology Agency (JST) of Ja-
pan. The measurements of HRMS were made at the Natural Science
Center for Basic Research and Development (N-BARD), Hiroshima
University.
Supplementary data
Supplementary data (experimental details and spectroscopic
characterizations of new compounds (3 and 4, X = Cl)) associated
with this article can be found, in the online version, at doi:10.
1016/j.tetlet.2010.07.152.
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