Cascade cyclization of aryldiynes using iodine: Synthesis of iodo-substituted benzo[b]naphtho[2,1-d]thiophene derivatives for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2012.02.007

A facile, efficient, and general synthetic method for iodo-substituted benzo[b]naphtho[2,1-d]thiophenes has been developed via a cascade cyclization of thioanisole-substituted aryldiynes using iodine. A new donor-π linker-acceptor (D-π-A) organic dye, G1,

Full text of DOI:10.1016/j.tetlet.2012.02.007

Tetrahedron Letters 53 (2012) 1946–1950
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Tetrahedron Letters
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Cascade cyclization of aryldiynes using iodine: synthesis of iodo-substituted
benzo[b]naphtho[2,1-d]thiophene derivatives for dye-sensitized solar cells
Giovanni Ferrara a,b, Tienan Jin a, , Md. Akhtaruzzaman b, Ashraful Islam c, Liyuan Han c, Hua Jiang a,d
,
⇑
Yoshinori Yamamoto a,
⇑
a WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
b Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
c Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
d State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile, efficient, and general synthetic method for iodo-substituted benzo[b]naphtho[2,1-d]thiophenes
Received 10 January 2012
Revised 31 January 2012
Accepted 2 February 2012
Available online 9 February 2012
has been developed via a cascade cyclization of thioanisole-substituted aryldiynes using iodine. A new
donor–
p linker–acceptor (D–p–A) organic dye, G1, with the benzo[b]naphtho[2,1-d]thiophene moiety
as an electron donor has been synthesized, and the performance of dye-sensitized solar cell based on
G1 has been investigated.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Cascade cyclization
Iodine
Thioanisole-substituted aryldiynes
Iodine-substituted benzo[b]naphtho[2,1-
d]thiophenes dye-sensitized solar cell
Thiophene-fused polyheteroaromatic compounds have at-
tracted increasing interest as organic semiconductors for various
electronic device applications, such as organic field-effect transis-
tors (OFETs), organic photovoltaic devices (OPVs), and organic light
emitting diodes (OLEDs).1 In contrast to the widely studied benzo-
dithiophene, naphthodithiophene, and anthradithiophene deriva-
tives as organic electronics,1a–c the scarcity of convenient and
practical synthetic methods2 restricts the applications of benzo-
naphthothiophene derivatives in the field of materials science.
Recently, we and other groups proved that the cascade cyclization
of aryldiynes was an efficient method to synthesize hetero-
aryl[a]annulated carbazoles.3 Moreover, it is well-known that the
electrophilic iodocyclization of alkyne bound substrates is one of
the alternatively powerful methods for the efficient synthesis of a
variety of functionalized carbocycles and heterocycles having a
mono- or di-iodo-substituent under very mild conditions.4–6
Furthermore, the corresponding iodine-containing products can
be readily converted to structurally interesting and elaborated
compounds regioselectively through transition metal-catalyzed
coupling reactions. Herein, we report a facile, efficient, and general
method for the synthesis of iodo-substituted benzo[b]naphtho
[2,1-d]thiophenes using iodine-mediated cascade cyclization of
thioanisole-substituted aryldiynes. Moreover, in the continuation
of our interest in the development of dye-sensitized solar cells
(DSCs) based on new organic dyes,7 a new donor–
p
linker–acceptor
(D–p–A) molecule, G1, with benzo[b]naphtho[2,1-d]thiophene
moiety as an electron donor has been synthesized and applied in
DSCs as a new organic dye (Eq. 1).
R
I
R
I2 (2 equiv)
CH3NO2, rt
S
R = Ph
SMe
1
2
Ph
S
CO2H
S
S
NC
G1
ð1Þ
First we investigated the effect of solvents on the iodine-medi-
ated cascade cyclization of the thioanisole-substituted aryldiyne
1a for the formation of the 5-iodo-6-phenylbenzo[b]naphtho[2,1-
d]thiophene 2a as shown in Table 1. The use of polar solvents, such
⇑
Corresponding authors. Tel.: +81 22 217 6177; fax: +81 22 217 6165 (T.J.);
tel.: +81 22 217 6164; fax: +81 22 217 5979 (Y.Y.).
E-mail addresses: tjin@m.tohoku.ac.jp (T. Jin), yoshi@m.tohoku.ac.jp (Y. Yama
moto).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.007

G. Ferrara et al. / Tetrahedron Letters 53 (2012) 1946–1950
1947
as CH2Cl2, CH3CN, and CH3NO2 led to high yields of 2a (entries 1–
3). Among them, CH3NO2 was the best solvent, giving 2a in a 90%
isolated yield (entry 3).8 It is interesting to note that the use of less
corresponding product 2a0 in an 86% yield.10 The 1H and 13C NMR
spectra of 2a0 were completely consistent with the protonized
product as shown in Eq. 2.
n-BuLi, THF, -78 °C
then H2O
I
H
Ph
Ph
ð2Þ
95%
S
S
2a
2a'
Ph
Ph
Pd(PPh3)4 (10 mol%)
+
B(OH)2
S
K2CO3, DME/H2O
reflux, 12 h, 65%
Br
S
ð3Þ
5a
6a
7a
PtBr4 (5 mol%)
2a'
toluene, reflux, 8 h
86%
polar solvents such as THF and toluene, and protic solvent MeOH
produced the first cyclization product, 3-iodo-substituted benzo-
thiophene 3a, in good to high yields without the detection of 2a
(entries 4–6). These results indicated that the polar aprotic sol-
vents should be favorable for the iodine activation of alkyne moiety
in 3a during the second cyclization (Scheme 1).
The general design principle for a metal-free organic dye in DSCs
consists of three units such as a donor and acceptor bridged by a
conjugation linker (D– –A) to realize the efficient charge transfer
from a donor moiety to TiO2 conduction band through a -bridge
and acceptor moiety.11 Thiophene units are one of the best moieties
for the -conjugate bridge because of their excellent charge transfer
p-
p
p
p
The scope and limitations of the iodine-mediated cascade cycli-
zation of various aryldiynes 19 using CH3NO2 as a solvent were
summarized in Table 2. The reactions of substrates 1b–e bearing
an electron-donating and an electron-withdrawing aromatic group
at R produced the corresponding 5-iodo-substituted benzo[b]
naphtho[2,1-d]thiophenes 2b–e in high yields (entries 1–4). The
presence of heterocycle, such as 3-thienyl group at R in substrate
1f was also tolerated, giving the desired cascade cyclization prod-
uct 2f in an 88% yield (entry 5). It was noted that when R was
substituted by an alkyl group, such as cyclohexyl in 1g, a trace
amount of the corresponding product 2g was obtained (entry 6).
We carried out the following control experiments to gain the
information of the present cascade reaction mechanism
(Scheme 1). The reaction of 1a under the standard conditions at
0 °C for 7 min gave the expected 3-iodo-substituted benzothio-
phene derivative 3a in a 91% yield.5e Subsequently, treatment of
the isolated substrate 3a with the standard conditions at room
temperature for 10 min gave the desired product 2a in a 96% yield,
suggesting that the present cascade cyclization must proceed
through the formation of 3-iodo-substituted benzothiophene 3a
followed by the sequential 6-endo cyclization.
The structures of the new 5-iodo-substituted benzo[b]naph-
tho[2,1-d]thiophene derivatives 2 were determined by 1H and 13C
NMR spectra and high resolution mass analysis. Moreover, the io-
dine substituent in 2a was further protonized by n-BuLi mediated
deiodination (Eq. 2). 1H NMR spectrum of the corresponding prod-
uct 2a0 clearly showed a new singlet peak at 6.67 ppm which was
assigned to the 5-CH proton signal. 2a0 was further determined by
the alternative synthetic way from the alkynyl-substituted benzo-
thiophene 7a (Eq. 3). Suzuki coupling of benzo[b]thiophen-2-ylbo-
ronic acid 5a and 1-bromo-2-(phenylethynyl)benzene 6a afforded
7a in a 65% yield.5f Treatment of 7a with Furstner’s Pt-catalyzed
selective 6-endo hydroarylation of alkyne methodology gave the
properties. In most cases a cyanoacrylic acid group is used as the
acceptor moiety which is the best anchoring group to the TiO2 semi-
conductor. In the continuation of our interest in the development of
new organic dyes based on DSCs,7 we synthesized a new D–
p–A or-
ganic dye G1 using benzo[b]naphtho[2,1-d]thiophene moiety as an
electron donor, cyanoacrylic acid as an acceptor and bithiophene as
a
p-linker (Scheme 2). Pd-catalyzed coupling of 5-iodo-6-phen-
ylbenzo[b]naphtho[2,1-d]thiophene 2a and 2,20-bithiophene-5-
carbaldehyde through the direct C–H bond activation gave the
corresponding aldehyde 4a in a 35% yield.12 Further condensation
of 4a with 2-cyanoacetic acid afforded the desired organic dye G1
in a 78% yield.13
Figure 1a showed the UV/vis spectra of organic dye G1 measured in
chloroform solution. The absorption spectrum displayed a prominent
band at 350–500 nm due to the p–
p⁄ transition of the conjugated mol-
ecule. Electrochemical propriety of G1 was also investigated by cyclic
voltammetry (CV) in dichloromethane containing 0.1 M tetrabutylam-
monium hexafluorophosphate (Fig. 1b). The calculated LUMO
(ꢀ2.54 eV) energy level of G1 is sufficiently more positive than the
conduction band of the nanocrystalline TiO2 (ꢀ4.2 eV), which is ener-
getically favorable for the efficient electron injection from the excited
state into the conduction band of TiO2 in DSCs. In addition HOMO en-
ergy level (ꢀ5.53 eV) is more negative than the redox potential of Iꢀ/I3ꢀ
(ꢀ5.20 eV), ensuring the favorable dye regeneration.
A double-layer TiO2 photoelectrode (10 + 5)
lm in thickness
with a 10 m thick nanoporous layer and a 5 m thick scattering
l
l
layer (area: 0.25 cm2) was prepared by screen printing on a conduct-
ing glass substrate.7 A dye solution of G1 with 3 ꢁ 10ꢀ4 M concen-
tration in acetonitrile/tert-butyl alcohol (1/1, v/v) was used to
uptake the dye onto the TiO2 film. Deoxycholic acid (DCA) as a co-
adsorbent was added into the dye solution to prevent the aggrega-
tion of the dye molecule. The TiO2 film was immersed into the dye
solution for 30 h at 25 °C. Photovoltaic measurements were

1948
G. Ferrara et al. / Tetrahedron Letters 53 (2012) 1946–1950
Table 1
Optimisation of solvents for the formation of 2aa
Ph
Ph
I
I
Ph
I2 (2 equiv)
solvent, rt, 20 min
+
S
S
SMe
1a
2a
3a
Entry
Solvent
2a, Yieldb (%)
3a, Yieldb (%)
1
2
3
4
5
6
CH2Cl2
CH3CN
CH3NO2
THF
Toluene
MeOH
84
86
90c
0
0
0
0
0
0
85d
65d
75d
a
b
c
Reaction conditions: 1a (0.2 mmol), I2 (0.6 mmol), anhydrous solvents (0.04 M), room temperature, 20 min.
1H NMR yield was determined by using CH2Br2 as an internal standard.
Isolated yield.
The reaction time was 5 h.
d
Ph
Ph
I
Ph
I2 (2 equiv)
I2 (2 equiv)
I
CH3NO2, 0 °C, 7 min
91%
CH3NO2, rt, 10 min
96%
S
S
SMe
1a
3a
2a
Ph
Ph
I+
I
I
I2
I2
Ph
I
I-
I+
-MeI
S+
S
SMe
Scheme 1. Control experiments for the cascade cyclisation pathway.
Table 2
iodide (DMPII), 0.5 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine
(TBP) in acetonitrile. Photocurrent density–voltage (I–V) of the
sealed solar cell was measured under AM 1.5G simulated solar light
at a light intensity of 100 mW cmꢀ2 with a metal mask of 0.25 cm2
(Fig. 2a). The photovoltaic parameters, that is, short circuit current
(Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion
Iodine-mediated cascade cyclization of various thioanisole-substituted aryldiynes 1
for the formation of 2a
R
I
R
I2 (2 equiv)
CH3NO2, rt, 30 min
efficiency (
nation. The G1 dye showed a Jsc of 7.04 mA cmꢀ2, Voc of 0.653 V, FF of
0.728, and a moderate power conversion efficiency ( ) of 3.3%. How-
g), were estimated from I–V characteristic under illumi-
S
SMe
1
g
2
ever, the incident photon-to-current conversion efficiency (IPCE) for
DSC based on G1 showed a high sensitization of nanocrystalline TiO2
from 350 to 500 nm with a maximum value of 84%, the lower sensi-
tization in the infrared sensitive region resulted in the lower Jsc and
Entry
R
1
2
Yieldb (%)
1
2
3
4
5
6
4-CN-C6H4
4-F-C6H4
4-CH3-C6H4
1b
1c
1d
2b
2c
2d
2e
2f
87
85
90
85
the moderate power conversion efficiency (g
) (Fig. 2b).14
4-CH3O-C6H4
3-Thienyl
1e
1f
In conclusion, we have developed a facile, efficient, and general
method for the synthesis of 5-iodo-substituted benzo[b]naph-
tho[2,1-d]thiophenes through the iodine-mediated cascade cycliza-
tion of the thioanisole-substituted aryldiynes. The corresponding
iodo-substituted product 2a was converted to the new organic
dye G1 with the benzo[b]naphtho[2,1-d]thiophene moiety as an
electron donor, cyanoacrylic acid as an acceptor, and bithiophene
88
Cyclohexyl
1g
2g
Tracec
a
Reaction conditions: 1 (0.2 mmol), I2 (0.6 mmol) in CH3NO2 (0.04 M) at room
temperature for 30 min.
b
Isolated yields.
c
Complex mixture with a trace amount of 2g.
as a p-linker. The DSC based on G1 showed moderate overall conver-
performed in a sandwich type solar cell in conjunction with an elec-
trolyte consisting of a solution of 0.6 M dimethylpropylimidazolium
sion efficiency. Further molecular modifications of organic dyes
with the benzo[b]naphtho[2,1-d]thiophene derivatives to broaden

G. Ferrara et al. / Tetrahedron Letters 53 (2012) 1946–1950
1949
Ph
I
Ph
S
(a)
(b)
CHO
S
G1
S
S
2a
4a
Scheme 2. Synthesis of organic dye G1. (a) Pd(OAc)2 (4 mol %), PCy3ꢂHBF4 (8 mol %), pivalic acid (30 mol %), 2,20-bithiophene-5-carbaldehyde (3 equiv), K2CO3 (1.5 equiv),
toluene, 100 °C, 2 days, 35%; (b) NH4OAc, 2-cyanoacetic acid, AcOH, 100 °C, 6 h, 78%.
Figure 1. (a) UV–vis spectra of G1 in chloroform. (b) CV spectra of G1.
Figure 2. (a) Current–voltage characteristics of DSC based on G1. (b) Photocurrent action spectra (IPCE) of the nanocrystalline TiO2 film sensitized by G1.
Oishi, S.; Fujii, N.; Ohno, H. J. Org. Chem. 2011, 76, 1212–1227; (c) Chen, C.-C.;
Chin, L.-Y.; Yang, S.-C.; Wu, M.-J. Org. Lett. 2010, 12, 5652–5655; (d) Chen, C.-C.;
Yang, S.-C.; Wu, M.-J. J. Org. Chem. 2011, 76, 10269–10274; (e) Ferrara, G.; Jin,
the absorption spectra for enhancing the light harvesting ability in
the infrared region are in progress.
T.; Oniwa, K.; Zhao, J.; Asiri, A. M.; Yamamoto, Y. Tetrahedron Lett. 2012, 53,
914–918.
Acknowledgment
4. For reviews, see: (a) Larock, R. C. In Acetylene Chemistry. Chemistry, Biology, and
Material Science; Diederich, F., Stang, P. J., Tykwinski, R. R., Eds.; Wiley: New
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This work was supported by the World Premier International
Research Center Initiative (WPI), MEXT, Japan.
Salerno, G.; Jenks, W. S.; Larock, R. C. J. Org. Chem. 2010, 75, 897–901; (b)
Barluenga, J.; Vázquez-Villa, H.; Ballesteros, A.; Gánzalez, J. M. J. Am. Chem. Soc.
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1950
G. Ferrara et al. / Tetrahedron Letters 53 (2012) 1946–1950
8. Representative experimental procedure for the synthesis of 2a (Table 1, entry
3): To a solution of aryldiyne 1a (0.2 mmol, 65 mg) in CH3NO2 (0.04 M, 5 mL)
was added I2 (2 equiv, 101.5 mg). The reaction mixture was stirred at room
temperature for 20 min. A saturated sodium thiosulfate was then added into
the reaction mixture. The organic layer was extracted with EtOAc, and dried
over Na2SO4. After concentration of the filtrate, the residue was purified by
chromatography on silica gel by column chromatography to afford the
corresponding product 2a in a 90% yield (78.5 mg) as a pale yellow solid; mp
146 °C; 1H NMR (400 MHz, CDCl3) d 8.49–8.46 (m, 1H), 8.14–8.12 (m, 1H), 7.87
(d, J = 8.4 Hz, 1H), 7.68–7.63 (m, 5H), 7.37–7.33 (m, 3H), 7.08 (t, J = 8.0 Hz, 1H),
6.46 (d, J = 8.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 104.5, 122.4, 124.1, 124.6,
124.7, 125.7, 127.2, 128.1, 128.2, 128.9, 129.3, 131.4, 132.3, 134.3, 136.0, 138.6,
138.9, 142.5, 145.2; UV–vis (CHCl3): 253.0, 261.0, 285.0, 306.0, 326.0,
357.0 nm; HRMS (EI+) calcd for C22H13SI: 435.9783, found: 435.9790.
9. Thioanisole-substituted aryldiynes 1 were prepared by the similar method to
the aniline-substituted aryldiynes as reported in Ref.3
10. Fürstner, A.; Davies, P. W. Angew. Chem. Int. Ed. 2007, 46, 3410–3449.
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12. Schipper, D. J.; Fagnou, K. Chem. Mater. 2011, 23, 1594–1600.
13. Analytic data of G1: red solid, mp 205 °C; 1H NMR (400 MHz, CDCl3) d 8.43 (s,
1H), 8.28 (d, J = 7.2 Hz, 1H), 8.13 (d, J = 7.2 Hz, 1H), 7.91 (d, J = 7.2 Hz, 1H),
7.76–7.72 (m, 4H), 7.50–7.44 (m, 8H), 7.41–7.34 (m, 2H), 6.39 (d, J = 8.4 Hz,
1H): 13C NMR (100 MHz, CDCl3) d 116.5, 123.1, 123.9, 124.2, 124.3, 124.7,
126.1, 126.4, 126.8, 127.1, 127.2, 127.4, 127.7, 127.9, 128.4, 129.5, 130.1, 130.9,
131.7, 133.8, 135.8, 135.9, 137.6, 138.1, 138.3, 138.5, 140.5, 141.0, 144.9, 145.9,
163.3, 171.8; UV–vis/kmax (CHCl3): 437 nm; HRMS (EI+) calcd for C33H19NS3
(M-CO2H): 525.0680, found: 525.0681.
14. Grätzel, M. Acc. Chem. Res. 2009, 42, 1788–1798.