A Simple Route to N-Arylated 2-Aminothiophenes as a New Class of Amorphous Glass Forming Molecules

Article abstract of DOI:10.1021/ol015875c

(matrix presented) By thermal decarboxylation of N-arylated 2-aminothiophene-5-cacrboxylates, a versatile, heavy-metal free method for preparing the title compounds as new class of highly reactive and easily oxidable, amorphous glass forming molecules has been elaborated.

Full text of DOI:10.1021/ol015875c

ORGANIC
LETTERS
2001
Vol. 3, No. 11
1673-1675
A Simple Route to N-Arylated
2-Aminothiophenes as a New Class of
Amorphous Glass Forming Molecules
Horst Hartmann,* Peter Gerstner, and Dirk Rohde
UniVersity of Applied Sciences, Geusaer Strasse, D-06217 Merseburg, Germany
horst hartmann@cui.fh-merseburg.de
Received March 22, 2001
ABSTRACT
By thermal decarboxylation of N-arylated 2-aminothiophene-5-cacrboxylates, a versatile, heavy-metal free method for preparing the title compounds
as new class of highly reactive and easily oxidable, amorphous glass forming molecules has been elaborated.
As pointed out recently,1 2-dialkylaminothiophenes 1 (Scheme
1) are among the most interesting developments in dye
chemistry of the past few years. When these compounds are
substituted with strong electron acceptors at the 5-position,
they exhibit enhanced nonlinear optical (NLO) properties
which significantly exceed those of the corresponding
dialkylaniline derivatives.2 Moreover, 5-(dimethylamino)-5′-
nitro-2,2′-bithiophene is among the best indicators for
studying the solvent polarity due to its outstanding positive
solvatochromism.3
disubstituted 2-aminothiophene compounds can be signifi-
cantly increased if their amino moiety is aryl-substituted.5
Therefore, N,N-diaryl-substituted 2-aminothiophenes 2 gained
prominence as building blocks for preparing the above-
mentioned dyes. Moreover, such compounds received, as
exemplified by a few authors recently,6,7 some attention as
building blocks for new materials, exhibiting a high tendency
to form stable radical cations and a high tendency to form
amorphous glasses in which the radical cations formed by
oxidation possess a strong mobility. These materials can be
used for constructing special optical and/or electronic devices,
such as organic luminescence diodes8 or organic field-effect
transistors.9
Scheme 1
Currently, N,N-diaryl-substituted 2-aminothiophenes 2 are,
in contrast to their N,N-dialkyl-substituted analogues 1, nearly
(2) Wolff, J. J.; Wortmann, R. AdV. Phys. Org. Chem. 1999, 32, 121.
Jen, A. K.-Y.; Rao, V. P.; Wong, K. Y.; Drost, K. J. Chem. Commun. 1990,
90.
(3) Effenberger, F.; Wuerthner, F. Angew. Chem., Int. Ed. Engl. 1993,
32, 719.
(4) Bosshard, C.; Sutter, K.; Pretre, P.; Hulliger, J.; Floersheimer, M.;
Kaatz, P.; Guenther, P. Organic Nonlinear Optical Materials; Gordon and
Braech: Basel, 1995.
(5) Jen, A. K.-Y.; Cai, Y.; Bedworth, P. V.; Marder, S. R. AdV. Mater.
1997, 9, 132. Moylan, C. R.; Twieg, R. J.; Lee, V. Y.; Miller, R. D.;
Volksen, W.; Thackare, J. I. And Walsh, C. A. SPIE 1994, 2285, 17. Twieg,
R. J.; Burland, D. M.; Hedrick, J.; Lee, V. Y.; Miller, R. D.; Moylan, C.
R.; Seymour, C. M.; Volksen, W.; Walsh, C. A. SPIE 1994, 2143, 2.
(6) Ueta, E.; Nakano, H.; Shirota, Y. Chem. Lett. 1994, 2397.
(7) Wu, I.-Y.; Lin, J. T.; Tao, Y.-T.; Balasubramaniam, E. AdV. Mater.
2000, 12, 668.
Despite these advantages, the mentioned 2-(dialkylamino)-
thiophene derivatives have a disadvantage. They exhibit a
relatively low thermal stability, which restricts, e.g., their
use in NLO applications. Here a pooling procedure at
elevated temperatures is an essential precondition for their
practical use in this field.4 As exemplified with some NLO-
active azo or methine dyes, the thermal stability of N,N-
(1) Wuerthner, F.; Thalacker, C.; Matschiner, R.; Kukaszuk, K.; Wort-
mann, R, Chem. Commun. 1998, 1739.
10.1021/ol015875c CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/09/2001

unknown. The few known derivatives of this series have been
prepared by a heavy-metal catalyzed C-N coupling reaction
starting from diarylamines and 2-halothiophenes. For in-
stance, 2-diphenylaminothiophene 2a has been prepared
either by a copper-mediated Ullmann reaction10 or in the
course of a palladium-catalyzed coupling reaction11 from
2-bromothiophene and diphenylamine. Satisfactory yields of
products could only be obtained, however, by using special
reaction conditions. Thus, in the copper-catalyzed reaction,
stoichiometric amounts of copper iodide and a polar aprotic
solvent, such as HMPT, were necessary, and in the pal-
ladium-catalyzed reaction special phosphine ligands for
activating the palladium catalyst were necessary. The desired
products thereby obtained are usually contaminated with
heavy-metal impurities. Their separation is urgently necessary
for the practical use of the products, e.g, as electronic
material, resulting in a time- and cost-consuming procedure
which raises the price of such products significantly.
To overcome these difficulties, a simple synthesis of
N-arylated 2-aminothiophenes 2 has been elaborated. This
method follows a known route for preparing N-disubstituted
2-aminothiophenes12 and starts from N-acetylated diaryl-
amines 3 which were transformed, at first, by means of a
Vilsmeier-Haak-Arnold reaction via the 1-chlorovin-
amidinium salts 413 into the N-diaryl derivatives of alkyl
2-aminothiophene-5-carboxylates 5.14 Subsequently, these
alkyl thiophene carboxylates 5 were transformed by reaction
with aqueous bases into the corresponding free acids 6 from
which the carboxy group free thiophenes 2 can be obtained
by thermal decarboxylation (Scheme 2).
2a-2f have been prepared in yields between 44 and 92%
(Scheme 3). Their structures as well as the structures of their
precursors 3-6 were confirmed by analytical and spectro-
scopic data.
Scheme 3
As expected, the N-arylated 2-aminothiophenes 2a-2f
exhibit low oxidation potentials. Also, in contrast to their
N-alkylated derivatives, e.g., the 2-morpholino derivative 2g,
they exhibit a strong tendency to form, after cooling their
melts, stable amorphous glasses. Their Tgs could by measured
by DSC under nitrogen and with a heating rate of 5 K min-1
(see Table 1). This method, which was very recently applied
Scheme 2
Table 1. Thermal and Electrochemical Data of the
N-Diarylated 2-Aminothiophenes 2 and Their Precursors 6
a
Tm[A]
Tm[H]
Tg[H]
Td[H]
Eox
entry
[°C]
entry
[°C]
[°C]
[°C]
[V]
6a
6b
6c
6d
6e
6f
157
166
167
155
181
163
157
2a
2b
2c
2d
2e
2f
83
62
102
104
109
83
-29
-23
(0
+2
-21
-8
274
303
272
332
345
358
305
+0.86
+0.82
+0.87
+0.92
+1.17
+0.65b
+0.55
6gc
2gd
20
a Measured toward the SCE, irreversible potentials. b Reversible potential.
c 2-Morpholinothiophene-5-carboxylic acid.12 d 2-Morpholinothiophene.
to the estimation of Tgs of certain dyes derived from N,N-
disubstitued 2-aminothiophens,15 was also useful for estimat-
ing the melting points (Tm[A]) of the 2-aminothiophene-5-
The decarboxylation can be performed either by heating
the 2-aminothiophene-5-carboxylic acids 6 at their decom-
position temperature or by refluxing the acids 6 in acetic
acid until the evolution of carbon dioxide ceases. Thus, by
these methods the N-diarylated 2-aminothiophene derivatives
(10) Bedworth, P. V.; Cai, K.-Y.; Jen, A.; Marder, S. R. J. Org. Chem.
1996, 61, 2242.
(11) Watanabe, M.; Yamamoto, T.; Nishiyama, M. Chem. Commun.
2000, 133.
(12) Liebscher, J.; Hartmann, H. J. Prakt. Chem. 1971, 318, 731.
Liebscher, J.; Hartmann, H. Synthesis 1979, 241. Liebscher, J.; Abegaz,
B.; Areda, A. J. Prakt. Chem. 1983, 325, 168. Liebscher, J.; Feist, K.
Synthesis, 1985, 412. Hartmann, H.; Eckert, K.; Schro¨der, A. Angew. Chem.,
Int. Ed. 2000, 39, 556.
(8) Rothberg, L. J.; Lovinger, A. J. J. Mater. Res. 1996, 11, 3174.
(9) Brown, A. R.; Jarrett, C. P., deLeeuw, D. M.; Matters, M. Synth.
Methods 1997, 88, 37.
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Org. Lett., Vol. 3, No. 11, 2001

carboxylates 6 as well as the melting points (Tm[H]) and the
decomposition points (Td[H]) of the resulting 5H-substituted
N-arylated 2-aminothiophenes 2.
as reagent, according to the recently published method of
M. Periasamy et al.16 (Scheme 4).
To verify that the observed Tgs are really glass transition
temperatures and do not originate from other relaxation
processes, the corresponding transition energies were esti-
mated. They are generally in the range of 0.2-0.3 J g-1 K-1.
Furthermore, the measurements were repeated a second and
a third time. No significant changes were observed in course
of these runs.
Scheme 4
Owing to the free 5-position at the 2-aminothiophene
moieties, the electrochemical oxidation of the compounds
2a-2f was found to be irreversible, indicating that in course
of this process an oxidative dimerization occurs. For
compound 2a, the resulting dimer 7a was alternatively
synthesized by a controlled chemical oxidation using TiCl4
Acknowledgment. The authors thank the Deutsche For-
schungsgemeinschaft for financial support.
Supporting Information Available: Experimental pro-
cedures and full charaterization for compounds 2a-2f (1H
NMR spectral data). This material is available free of charge
via the Internet at http://pubs.acs.org.
(13) Arnold, Z. Collect. Czech. Chem. Commun. 1961, 26, 3051. Schroth,
W.; Jahn, U.; Stro¨hl, D. Chem. Ber. 1994, 127, 2013.
(14) Hartmann, H.; Heyde, C.; Zug, I. Eur. J. Org. Chem. 2000,
3273.
OL015875C
(15) Wuerthner, F.; Yao, S.; Schilling, J.; Wortmann, R.; Redi-Abshiro,
M.; Mecher, E.; Gallengo-Gomez, F.; Meerholz, K. J. Am. Chem. Soc. 2001,
123, 2810.
(16) Periasamy, M.; Jayakumar, K. N.; Bharathi, P. J. Org. Chem. 2000,
65, 3548.
Org. Lett., Vol. 3, No. 11, 2001
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