Ambidextrous catalytic access to dithieno[3,2- b:2′,3′- d ]thiophene (DTT) derivatives by both palladium-catalyzed C-S and oxidative dehydro C-H coupling

Article abstract of DOI:10.1021/ol501752f

A modular two-step synthesis of dithieno[3,2-b:2′,3′-d] thiophene (DTT) derivatives by C-S cross-coupling and oxidative dehydro C-H coupling is herein described. Dibenzo[d,d′]thieno[3,2-b;4,5-b′] dithiophene (DBTDT) and associated two donor (anisyl) and a

Full text of DOI:10.1021/ol501752f

Letter
pubs.acs.org/OrgLett
Ambidextrous Catalytic Access to Dithieno[3,2‑b:2′,3′‑d]thiophene
(
DTT) Derivatives by Both Palladium-Catalyzed C−S and Oxidative
Dehydro C−H Coupling
Peter Oechsle and Jan Paradies*
Karlsruhe Institute of Technology (KIT), Institute for Organic Chemistry, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
*
S Supporting Information
ABSTRACT: A modular two-step synthesis of dithieno[3,2-
b:2′,3′-d]thiophene (DTT) derivatives by C−S cross-coupling
and oxidative dehydro C−H coupling is herein described.
Dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (DBTDT) and
associated two donor (anisyl) and acceptor (acetyl) substituted DTT derivatives were synthesized by palladium-catalyzed cross-
coupling sequences in 17% to 71% yield over two steps. The 5,5′-disubstituted DTT derivatives were characterized in terms of
their photophysical (UV and fluorescence spectroscopy) and electrophysical (cyclovoltammography) properties.
hiophene-based materials are widely used in organic
Scheme 1. Synthetic Strategy for the Modular Construction
of the Dithieno[3,2-b:2′,3′-d]thiophene Core
1
2
T
electronics, e.g. dye-sensitized solar cells or semi-
3
conductors in organic field-effect transistors (OFET). The
fabrication of such electronic devices on flexible substrates is
4
5
particularly interesting for displays or sensors. Thienoacenes
have attracted considerable attention due to their increased
optical transparency of visible light as well as thermal and
6
atmospheric stability in comparison to the most widely studied
7
all-carbon-based semiconductor pentacene. Fused sulfur-
containing heteroacenes, e.g. dithieno[3,2-b:2′,3′-d]thiophene
(
DTT, 1, Figure 1) derivatives, are useful as an active layer in
8
thin film transistors, and dibenzo[d,d′]thieno[3,2-b;4,5-b′]-
dithiophene (DBTDT, 2, Figure 1) has been found to be a
promising candidate as a semiconductor in organic plastics.
Scheme 2. Modular Synthesis of DBTDT (2)
9
Notable efforts have been made for the efficient construction
of dithieno[3,2-b:2′,3′-d]thiophene derivatives using condensa-
1
0
6,9a,c,11
tion reactions or reductive couplings.
Despite the
number of metal-catalyzed oxidative dehydro C−H couplings
12
of thiophenes or benzothiophenes, the application of this
methodology has not been recognized for the synthesis of DTT
or DBTDT derivatives or higher homologues until now. Herein
we report the modular synthesis of DBTDT and two DTT
derivatives by a two-step reaction sequence using C−S cross-
coupling (A) followed by oxidative dehydro C−H coupling (B)
methodologies (Scheme 1, pathway I). The order of the two
Figure 1. Dithieno[3,2-b:2′,3′-d]thiophene (DTT, 1) dibenzo-
Received: June 17, 2014
Published: August 4, 2014
[
d,d′]thieno[3,2-b;4,5-b′]dithiophene (DBTDT, 2).
©
2014 American Chemical Society
4086
dx.doi.org/10.1021/ol501752f | Org. Lett. 2014, 16, 4086−4089

Organic Letters
Letter
1
2f
Scheme 3. Modular Synthesis of 2,6-Substituted DTT
Derivatives
applying Moris conditions provided the product 2 in 29%
yield (17% yield over two steps). However, reversal of the
cross-coupling sequence improved the overall yield to 53% over
two steps (pathway II). The oxidative dimerization of 3 to
bis(3-bromobenzo[b]thiophene) (5) was achieved in excellent
yield (90%). Subsequent reaction of 5 with potassium
thioacetate in the presence of 10 mol % Pd(dba) /dppf
2
furnished DBTDT (2) in 59% yield. Pathway II is attractive not
only in light of the higher yields for both individual steps but
also because the purification is facilitated. The bis-benzo[b]-
thiophene derivative 5 and DBTDT (2) were both purified by
extraction and crystallization. Our synthetic approach benefits
from two catalytic transformations avoiding air or moisture
sensitive reagents combining high yield with minimal synthetic
effort, in comparison to literature-reported syntheses by
19
Schroth (22% over two steps) or Yamaguchi (56% over
20
four steps).
To demonstrate the feasibility of both pathways for the
synthesis of 2,6-substituted DTT derivatives we conducted
both reaction sequences with donor- and acceptor-substituted
21
3
-bromo-5-(4-anisyl)thiophene (6a) and commercially avail-
able 3-bromo-5- acetyl-thiophene (6b, Scheme 3). In general
the unknown anisyl- and the acetyl-substituted DTT derivatives
catalytic coupling reactions can be reversed (first B then A)
permitting for the highest modularity while starting from the
identical 3-bromothiophene as a precursor. This strategy both
allows for choice of the sequence with the highest yield and also
the ability to select the sequence with the most facile
purification. Additionally, both reactions, the C−S and the
oxidative dehydro C−H couplings, have well-documented
functional group tolerance, which makes our strategy
particularly attractive for acceptor-substituted DTT derivatives.
Consequently this approach combines crucial synthetic aspects
with multiple practical advantages. We selected C5-substituted
7
a and 7b were accessible by both pathways (yields over two
steps: pathway I: 23% (7a); 35% (7b); pathway II: 71% (7a);
6% (7b)). Pathway II is very well suited for the synthesis of 7a
3
while the acetyl-substituted derivative 7b was obtained in
almost identical yields according to both reaction sequences.
Such carbonyl-functionalized DTT derivatives are not acces-
sible by the commonly applied reductive coupling since
metalation of the 2-position by strong lithium bases is
6,9a,c,11a,b
required.
Such reaction conditions are incompatible
with carbonyl-containing functional groups. Our novel
approach provides an efficient access to such acceptor-
substituted DTT derivatives.
The photophysical and electrophysical properties of 2, 7a,
and 7b were investigated by UV-absorption, fluorescence
3-bromothiophene derivatives as substrates for the C−S/C−C
cross-coupling sequence, since this position generally allows for
electronic modification of DTT derivatives. 5-Substitued 3-
bromothiophenes are readily accessible from commercial
sources or by selective cross-coupling of 2,4-dibromothio-
1
3
(
Figure 2), and cyclovoltametric measurements (summarized in
phene. To test our hypothesis we selected 3-bromobenzo-
b]thiophene (3) as starting material for the two-step synthesis
of DBTDT (2). First we investigated the palladium-catalyzed
Table 1; see Supporting Information for details). The
photophysical and electrochemical data for 2 are in excellent
agreement with literature reported values. The PMP-substituted
DTT derivative 7a shows a red shift in the absorption and
emission spectra compared to DTT (10) and DPDTT (11)
and a relatively high lying HOMO with an energy gap of −5.64
eV. This can be attributed to the electron-donating nature of
the PMP group and significant overlap of the aromatic systems.
Its optical HOMO/LUMO gap of 2.83 eV is the smallest for all
investigated DTT derivatives. The electron-withdrawing nature
of the acyl group in 7b results in a significant decrease of the
HOMO and LUMO level with a separation of 3.19 eV.
[
1
4
C−S coupling reaction of 3 using different H S-surrogates,
2
1
5
16
17
18
e.g. TIPS-SH, Na S, thiourea, or thioacetate. Generally
2
these reactions were very efficient for the synthesis of the bis(3-
benzothiophene)thioether (4), but only the thioacetate
1
8b
method gave synthetically useful yields for both of the C−
S bond forming reactions according to pathways I and II
(
thioether and thiophene synthesis, Scheme 2). The palladium-
catalyzed C−S cross-coupling was achieved in 60% yield
starting from 3-bromobenzo[b]thiophene (3, Scheme 2,
pathway 1). The final oxidative dehydro C−H coupling
Figure 2. Normalized UV absorption and fluorescence spectra of (a) DBTDT (2), (b) 7a, and (c) 7b in tetrahydrofuran.
4
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dx.doi.org/10.1021/ol501752f | Org. Lett. 2014, 16, 4086−4089

Organic Letters
Letter
Table 1. Photophysical and Electrophysical Data for 2, 7a, and 7b
a
b
c
d
e
compound
DBTDT
λmax [nm]
λonset [nm]
λem [nm]
ΔEopt [eV]
eE1/2 Fc/Fc+ [eV]
EHOMO [eV]
ELUMO [eV]
328
274
357
353
439
390
369
416
398
3.52
2.83
3.19
0.91
0.54
−6.01
−5.64
−6.48
−2.49
−2.81
−3.29
7
7
a
f
b
1.38
data of literature reported compounds
2
2
9c
22
9c
9c
DBTDT
327
291
369
359
368
3.46
0.8
−6.08
−5.60
−5.71
−2.62
−1.72
−2.65
d
g
23
23
23
23
24
DTT (10)
319
343
3.88
0.98
8
a
8a
8a
25
DPDTT (11)
406
426
3.06
0.61
a
b
c
Calculated from the cross point of the absorption onset with baseline. Maximum of emission. Calculated from E = h·c/λonset. EHOMO
=
+
e
f
g
−
(eE1/2(substrate) + 5.1 eV) referenced to Fc/Fc couple. E
= EHOMO + ΔEopt. Onset of oxidation peak. Electronic spectra were reported for
LUMO
CH Cl ; DPDTT = 2,6-diphenyldithieno[3,2-b:2′,3′-d]thiophene.
2
2
In summary, we exemplified the two-step synthesis of 2,6-
donor- and acceptor-substituted dithieno[3,2-b:2′,3′-d]-
thiophenes by two reaction sequences applying palladium-
catalyzed C−S cross-coupling and oxidative dehydro C−H
coupling. The new DTT derivatives were characterized by
photophysical and electrophysical methods. The presented
methodology generally allows the efficient synthesis of DTT
derivatives by two alternative reaction sequences, which enable
the optimization of the chemical yield and/or the purification
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AUTHOR INFORMATION
Corresponding Author
Kalfagiannis, N.; Logothetidis, S.; Yassar, A.; Miozzo, L.; Nosal, A.;
Gazicki-Lipman, M.; Jung, J.; Ulanski, J. Opt. Mater. 2012, 34, 1660.
■
*
(b) Miyata, Y.; Yoshikawa, E.; Minari, T.; Tsukagoshi, K.; Yamaguchi,
E-mail: jan.paradies@kit.edu.
S. J. Mater. Chem. 2012, 22, 7715. (c) Gao, J. H.; Li, R. J.; Li, L. Q.;
Notes
Meng, Q.; Jiang, H.; Li, H. X.; Hu, W. P. Adv. Mater. 2007, 19, 3008.
(10) (a) Frey, J.; Proemmel, S.; Armitage, M. A.; Holmes, A. B. Org.
The authors declare no competing financial interest.
Synth. 2006, 83, 209. (b) Oyaizu, K.; Iwasaki, T.; Tsukahara, Y.;
Tsuchida, E. Macromolecules 2004, 37, 1257. (c) Frey, J.; Bond, A. D.;
Holmes, A. B. Chem. Commun. 2002, 2424. (d) Kwon, T. H.; Armel,
V.; Nattestad, A.; MacFarlane, D. R.; Bach, U.; Lind, S. J.; Gordon, K.
C.; Tang, W. H.; Jones, D. J.; Holmes, A. B. J. Org. Chem. 2011, 76,
4088.
ACKNOWLEDGMENTS
The German Science Foundation is gratefully acknowledged for
a Heisenberg-fellowship to J.P. and for financial support PA
■
1562/4-1.
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S. J.; Friend, R. H. J. Am. Chem. Soc. 1998, 120, 2206. (c) San Miguel,
L.; Porter, W. W.; Matzger, A. J. Org. Lett. 2007, 9, 1005.
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