Syntheses and properties of donor - Acceptor-type 2,5-diarylthiophene and 2,5-diarylthiazole

Article abstract of DOI:10.1021/ol049386z

Equation presented. Syntheses of differently substituted 2,5-diarylthiophenes and 2,5-diarylthiazoles are carried out with the palladium-catalyzed C-H substitution reaction using AgF as an activator. Remarkably strong photoluminescence is observed in 2,5-

Full text of DOI:10.1021/ol049386z

ORGANIC
LETTERS
2
004
Vol. 6, No. 12
011-2014
Syntheses and Properties of
Donor−Acceptor-Type
2
2
,5-Diarylthiophene and
2,5-Diarylthiazole
Kentaro Masui, Atsunori Mori,* Kunihiko Okano, Kenji Takamura,
Motoi Kinoshita, and Tomiki Ikeda
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Yokohama 226-8503, Japan
amori@res.titech.ac.jp
Received April 5, 2004
ABSTRACT
Syntheses of differently substituted 2,5-diarylthiophenes and 2,5-diarylthiazoles are carried out with the palladium-catalyzed C−H substitution
reaction using AgF as an activator. Remarkably strong photoluminescence is observed in 2,5-diarylthiophene. Differences between the physical
properties of liquid crystalline characteristics and cyclic voltammograms of thiophene and thiazole are also studied.
Five-membered heteroaromatics bearing substituents at the
- and 5-positions have attracted much attention as light-
been studied on the characteristics of these molecules because
of the lack of practical synthetic methodology.
2
1
emitting and liquid crystalline materials. These compounds
are potentially applied for single-layer electroluminescent
devices involving hole-transporting, light-emitting, and elec-
tron-transporting characteristics in the same molecule. In
particular, unsymmetrical donor-acceptor-type 2,5-diarylated
thiophenes that possess both electron-donating and -with-
We have recently reported that the C-H bond at the
2-position of thiazole is efficiently substituted by various
aryl groups with a palladium/copper catalyst system and
tetrabutylammonium fluoride (TBAF) as an activator to
afford the 2-arylated product under extremely mild reaction
1
a
conditions. By contrast to the mild substitution at such an
electron-deficient C-H bond at a lower temperature, C-H
substitution at the electron-enriched carbon atom still requires
drastic conditions to achieve the carbon-carbon bond
2
drawing groups are of much interest. However, little has
(
1) (a) Mori, A.; Sekiguchi, A.; Masui, K.; Shimada, T.; Horie, M.;
Osakada, K.; Kawamoto, M.; Ikeda, T. J. Am. Chem. Soc. 2003, 125, 1700.
b) Mochizuki, H.; Hasui, T.; Kawamoto, M.; Shiono, T.; Ikeda, T.; Adachi,
3
(
formation. However, our recent studies on C-H homocou-
C.; Taniguchi, Y.; Shirota, T. Chem. Commun. 2000, 1923. (c) D o¨ lling,
K.; Zaschke, H.; Schubert, H. J. Prakt. Chem. 1979, 321, 643.
4
pling of heteroaromatic compounds suggest that cross
coupling at an electron-enriched C-H of thiophene may also
take place at a lower temperature and would be applicable
to the diarylthiophene synthesis. We herein report a facile
synthesis of 2,5-diarylated thiophene (1a) under mild condi-
(2) (a) Briehn, C. A.; Schiedel, M.-S.; Bonsen, E. M.; Schuhmann, W.;
B a¨ uerle, P. Angew. Chem., Int. Ed. 2001, 40, 4680. (b) Facchetti, A.; Yoon,
M.-H.; Stern, C. L.; Katz, H. E.; Marks, T. J. Angew. Chem., Int. Ed. 2003,
4
2, 3900. (c) Yamamoto, T.; Arai, M.; Kokubo, H.; Sasaki, S. Macromol-
ecules 2003, 36, 7986.
1
0.1021/ol049386z CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/14/2004

tions utilizing palladium-catalyzed C-H substitution in the
presence of AgF. We also apply the method for the
preparation of the related thiazole derivative (1b), and
spectroscopic and thermal properties of these compounds are
compared.
silver ion would allow the reaction of the electron-enriched
heteroaromatic compound to proceed under mild conditions.
Encouraged by the above results, synthesis of differently
substituted 2,5-diarylthiophene 1a was performed as shown
in Scheme 2. The reaction of thiophene (4a) with 3 in the
Scheme 2
Figure 1.
We first examined the reaction of 2-phenylthiophene (2),
whose carbon atom at the 5-position was electron-enriched,
with 4-methoxy-1-iodobenzene (3) under several conditions.
The reaction did not proceed at 60 °C for 5 h, with 10 mol
%
of Pd(OAc)
2 3
and 20 mol % of PPh in the presence of
Cs
2
CO , which were the reported conditions that proceeded
3
3a,5
at 140 °C. By contrast, the C-H substitution reaction was
found to take place when AgF was employed as an activator
using 3 mol % of PdCl
2
(dppb) as a catalyst (Scheme 1).
presence of AgF, in which 4 equiv of 4a toward 3 was
employed, afforded the 2-arylated product 5a in 43% yield
6
along with 3% of 2,5-diarylated 6. Synthesis of 5a was
Scheme 1
7
alternatively carried out with Suzuki-Miyaura coupling of
thiophene-2-boronic acid (7) with 3. The palladium-catalyzed
reaction of 5a with ethyl 4-iodobenzoate (8) in the presence
of AgF also underwent C-H substitution to afford the 2,5-
diarylthiophene 1a in 65% yield.
Synthesis of 2,5-diarylthiazole (1b) was carried out under
mild conditions as shown in Scheme 3 by the combination
Scheme 3
Although the mechanism of the C-H substitution is not
clear, specific interaction of the sulfur atom of 2 with the
(3) (a) Pivsa-Art, S.; Satoh, T.; Kawamura, Y.; Miura, M.; Nomura, M.
Bull. Chem. Soc. Jpn. 1998, 71, 467. (b) Yokooji, A.; Okazawa, T.; Satoh,
T.; Miura, M.; Nomura, M. Tetrahedron 2003, 59, 5685. (c) Kondo, Y.;
Komine, T. Sakamoto, T. Org. Lett. 2000, 2, 3111. (d) Sezen, B.; Sames,
D. J. Am. Chem. Soc. 2003, 125, 5274. (e) Sezen, B.; Sames, D. J. Am.
Chem. Soc. 2003, 125, 10580. (f) Sezen, B.; Sames, D. Org. Lett. 2003, 5,
3
607. (g) Lewis, J. C.; Wiedemann, S. H.; Bergman, R. G.; Ellman, J. A.
Org. Lett. 2004, 6, 35. (h) Kamigata, N. Yoshikawa, M. Shimizu, T. J.
Fluorine Chem. 1998, 87, 91. For a review; (i) Miura, M.; Nomura, M.
Top. Curr. Chem. 2002, 219, 211.
of Pd/Cu/TBAF and Pd/AgF protocols. Substitution at the
2-position of thiazole (4b) with 3 proceeded under the
conditions as reported previously to give 2-arylated thiazole
(
4) Masui, K.; Ikegami, H.; Mori, A. J. Am. Chem. Soc. 2004, 126, 5074.
(5) Reactions of thiophene derivatives with organic halides: (a) Ohta,
A.; Akita, Y.; Ohkkuma, T.; Chiba, M.; Fukunaga, R.; Miyafuji, A.; Nakata,
T.; Tani, N.; Aoyagi, Y. Heterocycles 1990, 31, 1951. (b) Aoyagi, Y.; Inoue,
A.; Koizumi, A.; Hashimoto, T.; Tokunaga, K.; Gohma, K.; Komatsu, J.;
Sekine, K.; Miyafuji, A.; Kunoh, J.; Honma, R.; Akita, Y.; Ohta, A.
Heterocycles 1992, 33, 257. (c) Gozzi, C.; Lavenot, L.; Ilg, K.; Penalva,
V.; Lemaire, M. Tetrahedron Lett. 1997, 38, 8867. (d) Lavenot, L.; Gozzi,
C.; Ilg, K.; Orlova, I.; Penalva, V.; Lemaire, M. J. Organomet. Chem. 1998,
(6) Diarylated thiophene 6 (37%) and monoarylated 5a (37%) were
obtained when the reaction was carried out with 4 equiv of 3 and 2 equiv
of AgF (toward 4a).
(7) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Suzuki,
A. J. Organomet. Chem. 1999, 576, 147.
5
67, 49.
2012
Org. Lett., Vol. 6, No. 12, 2004

5
b in 80% yield.1a C-H substitution at the 5-position of
thiazole was also found to take place similarly to the case
of thiophene. The reaction of 5b with ethyl 4-iodobenzoate
(8) at 60 °C in the presence of PdCl
2
(dppb) (3 mol %) and
AgF (1 equiv) afforded 1b in 58%.
With differently substituted 2,5-diarylthiophene (1a) and
thiazole (1b) in hand, we then compared characteristics of
-
these compounds. Figure 2 shows UV-vis absorption and
Figure 2. UV-vis absorption (gray) and PL emission (bold)
spectra of 1a and 1b. The spectra were measured in a chloroform
-
5
-6
solution (1 × 10 and 1 × 10 M, respectively).
photoluminescent spectra. Both compounds showed similar
λ
max at ca. 350 nm, and the wavelength of emission was also
similar. However, quantum yield (φ) of photoluminescence
of 2,5-diarylthiophene 1a was 0.79, which is ca. 3-4 times
higher than that of the corresponding thiazole 1b (φ ) 0.24).
It should be pointed out that such a strong light emission
Figure 3. (a) DSC thermogram of 1a (i) and 1b (ii) on the second
cooling. (b) Liquid crystalline texture of 1a at 184 °C (i) and 1b at
177 °C (ii) with a polarizing microscope.
2
has not been observed in other thiophene derivatives.
A remarkable difference in thermotropic behaviors of 1a
and 1b was also observed. Figure 3a shows DSC thermogram
on the second cooling. The donor-acceptor-type 1a exhibited
two exothermic peaks at 61.3 and 187.2 °C. The former
was observed in the measurement of 1a, suggesting that 2,5-
diarylthiophene potentially involves electron-transporting
-
1
showed a smaller ∆H (0.5 KJ‚mol ), and the latter was a
-1
larger ∆H (24.5 KJ‚mol ). The behavior was not the typical
isotropic-nematic-crystal transition, which was observed in
the thermogram of symmetrical 2,5-di(4-methoxyphenyl)-
Table 1. Cyclic Voltammogram of 1a and 1ba
Epa (E1/2)/V
vs Ag+/Agb
Epc (E1/2)/V
8
thiophene (6) but suggested a higher order mesophase such
+
b
compound
vs Ag /Ag
as a smectic phase. On the other hand, the behavior of
donor-acceptor-type thiazole 1b bearing the same substit-
uents at both 2- and 5-positions of 1a showed a different
1
1
a
0.87 (0.84)
1
.22
b
1.13
-2.20 (2.16)
-1
thermogram, in which a large ∆H of 12.6 KJ‚mol at 107.1
a
Cyclic voltammogram was measured in an acetonitrile solution of
Et4NBF4 (0.1 M). Ag /Ag was used as reference electrode. Peak potentials
are shown: Epa ) peak anode potential; Epc ) peak cathode potential.
-
1
°
C and the smaller ∆H of 2.1 KJ‚mol at 177.0 °C were
+
b
shown, suggesting a nematic phase. In addition, observation
of liquid crystalline texture with a polarizing microscope was
also shown in Figure 3b. The results also support the nematic
phase of 1b (ii) and the smectic phase of 1a (i).9
,10
characteristics as well as light emission in EL and LED
Measurements of cyclic voltammogram of 1a and 1b were
also carried out. As shown in Table 1, an oxidation wave
devices. On the other hand, 1b was found to show oxidation
1
1
(
10) Further studies on the liquid crystalline characteristics and physical
(
8) Kishikawa, K.; Micheke, C. H.; Timothy, M. S. Chem. Mater. 1999,
1, 867.
9) Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W.,
properties of 2,5-diarylthiophenes are under investigation, and details will
be described in due course.
(11) Kaifer, A. E.; G o´ mez-Kaifer, M. Supramolecular Electrochemistry;
Wiley-VCH: Weinheim, 1999.
1
(
Spiess H.-W., Eds.; Wiley-VCH: Weinheim, 1998.
Org. Lett., Vol. 6, No. 12, 2004
2013

and reduction peaks, which suggest both hole and electron
transportations leading to a single-layer electroluminescent
device.
substituents on physical properties and introduction of these
heteroaromatic moieties into a side chain of a polymer for
the purpose of preparing thin film on an electrode are under
way.
In conclusion, 2,5-diarylthiazole and -thiophene were
synthesized using mild palladium-catalyzed C-H substitution
reactions. Since the preparation method is quite simple and
practical, it would be suitable for the synthesis of a wide
variety of related derivatives in a combinatorial manner.
Spectroscopic studies and thermal characterization revealed
that the structure of the heteroaromatic moiety largely
influences those properties. Further studies on the effect of
Supporting Information Available: Experimental details
and additional data on the characterization of 1a and 1b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL049386Z
2014
Org. Lett., Vol. 6, No. 12, 2004