Efficient synthesis of benzo[1,2- b:6,5- b']dithiophene-4,5-dione (BDTD) and its chemical transformations into precursors for π-conjugated materials

Article abstract of DOI:10.1021/ol302704v

A straightforward synthesis of the fused-aromatic dione benzo[1,2-b:6,5-b']dithiophene-4,5-dione (BDTD) has been developed. This fused-aromatic dione was subjected to various chemical transformations to generate diverse molecules with potential use in π-conjugated materials for organic electronics.

Full text of DOI:10.1021/ol302704v

ORGANIC
LETTERS
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Vol. XX, No. XX
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Efficient Synthesis of
Benzo[1,2‑b:6,5‑b0]dithiophene-4,5-dione
(BDTD) and Its Chemical Transformations
into Precursors for π‑Conjugated Materials
Frank A. Arroyave,‡ Coralie A. Richard,†,‡ and John R. Reynolds*,†,‡
School of Chemistry and Biochemistry, School of Materials Science and Engineering,
Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, United States, and The George and Josephine Butler Polymer
Research Laboratory, Department of Chemistry, Center for Macromolecular Science
and Engineering, University of Florida, Gainesville, Florida 32611, United States
reynolds@chemistry.gatech.edu
Received September 30, 2012
ABSTRACT
A straightforward synthesis of the fused-aromatic dione benzo[1,2-b:6,5-b0]dithiophene-4,5-dione (BDTD) has been developed. This fused-
aromatic dione was subjected to various chemical transformations to generate diverse molecules with potential use in π-conjugated materials for
organic electronics.
The donorꢀacceptor approachꢀorbital mixing of alter-
nating donor and acceptor units in a π-conjugated copol-
ymer or oligomer to decrease the electronic band gap has
become one of the most powerful strategies for band gap
engineering in π-conjugated polymers;1,2 therefore, access
to new donor and acceptor molecules is highly convenient
since it can lead to new π-conjugated systems with assorted
band gaps and properties. Aromatic diones are versatile
starting materials that can generate a wide variety of
monomers for the synthesis of π-conjugated materials.
The ketone groups in these diones can be easily converted
into other organic functionalities leading to new molecules
with different properties and characteristics (i.e., electron
‡ University of Florida.
† Georgia Institute of Technology.
(1) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268.
(2) Van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.;
Meijer, E. W. Mater. Sci. Eng., R 2001, 32, 1.
r
10.1021/ol302704v
XXXX American Chemical Society

donors or electron acceptors), and most of these transfor-
mations can be carried out in one or two steps, which
demonstrates the usefulness of these intermediates. The
research described in this report focuses on the synthesis of
the fused-aromatic dione benzo[1,2-b:6,5-b0]dithiophene-
4,5-dione (BDTD) and its conversion to new π-conjugated
materials precursors by different synthetic approaches.
This work is a helpful contribution to the field since it
reports not only the synthesis of novel donor and acceptor
molecules based on BDTD but also a creative synthetic
approach to new molecules. This approach can be adapted
to other aromatic diones, giving access to novel donor
and acceptor molecules, which is key to generating new
π-conjugated materials with improved physical and elec-
tronic properties.
Molecules such as BDTD (shown in Scheme 1) offer
various features that make them attractive for π-conju-
gated materials. These molecules inherit the wide range of
electronic properties offered by thiophene-based materials3a
and also offer the possibility for postderivatization on the
thiophene rings (i.e., halogenation, borylation, stannyla-
tion, etc.) and on the carbonyl groups. Due to these
features, various syntheses and uses for BDTD have been
reported.3bꢀe
(Me3Si)2-BDTD was only achieved by dissolving the
(Me3Si)2-BDTD in concentrated sulfuric acid and immedi-
ate quenching with water. This approach was necessary
since the removal of the TMS groups could not be accom-
plished by the standard deprotection methods such as
using fluoride sources or dilute acids.
We report here an alternative synthetic route toward
BDTD, and this approach is presented in Scheme 2. This
synthetic path uses the inexpensive and readily available
3-bromothiophene as starting material, and it is highly
convenient, since the reaction can be carried out in only
two steps, in high yields, from inexpensive starting materi-
als, and requiring no column purification, which allows a
multigram synthesis.
The synthesis of the diketone 2, which is the key inter-
mediate, was carried out using a modified literature pro-
cedure for analogous systems.5 After 2 was isolated, it was
subjected to oxidative ring closing using iron trichloride or
iron tribromide in dichloromethane (DCM).
Scheme 2. Short Synthesis of BDTD
Initially, to make BDTD, we used the synthetic route
shown in Scheme 1. This synthetic route employs the
readily available 2-bromothiophene as the starting mate-
rial and was based on previously reported literature
procedures4 to generate the intermediate 1. Unfortunately,
this route contains a considerable number of synthetic
steps (>6), with a low overall yield (∼9%), and in some
cases requires column purification, which makes the synthe-
sis expensive and unsuitable for a large-scale synthesis of
BDTD. It is worthy to note that, as shown in the final step
in Scheme 1, the deprotection of the silyl compound
Various experiments were carried out to optimize the
oxidative ring-closing conditions in DCM, i.e. different
concentrations, reaction times, and temperatures (20, 35,
and 40 °C). Monitoring of the reaction by TLC for several
hours (1ꢀ24 h) lead to the conclusion that the reaction
must be carried out at room temperature, for at least 2 h,
and using 3 equiv or more of FeCl3 or FeBr3 to achieve
high conversion. If less than 2.5 equiv of iron chloride were
employed full reaction conversion was not observed, even
if the reaction was run for more than 24 h.
Scheme 1. Multi-step Route toward BDTD
We carried out various chemical transformations on
BDTD (Table 1). Most of these chemical transformations
are straightforward and can be done in one or two steps,
(3) (a) Skotheim, T. A., Reynolds, J. R., Eds. Conjugated Polymers:
Theory, Synthesis, Properties, and Characterization (Handbook of Con-
ducting Polymers), 3rd ed.; CRC Press LLC.: Boca Raton, FL, 2007. (b)
Wynberg, H.; Sinnige, H. J. M. Recl. Trav. Chim. Pays-Bas 1969, 88, 1244.
(c) Letizia, J. A.; Cronin, S.; Ortiz, R. P.; Facchetti, A.; Ratner, M. A.;
Marks, T. J. Chem.;Eur. J. 2010, 16, 1911. (d) Mondal, R.; Becerril,
H. A.; Verploegen, E.; Kim, D.; Norton, J. E.; Ko, S.; Miyaki, N.; Lee,
S.; Toney, M. F.; Bredas, J.-L.; McGehee, M. D.; Bao, Z. J. Mater.
Chem. 2010, 20, 5823. (e) Getmanenko, Y. A.; Risko, C.; Tongwa, P.;
Kim, E.-G.; Li, H.; Sandhu, B.; Timofeeva, T.; Bredas, J.-L.; Marder,
S. R. J. Org. Chem. 2010, 76, 2660.
(4) (a) Khor, E.; Ng, S. C.; Li, H. C.; Chai, S. Heterocycles 1991, 32,
1805. (b) Nicolas, Y.; Blanchard, P.; Roncali, J.; Allain, M.; Mercier, N.;
Deman, A.-L.; Tardy, J. Org. Lett. 2005, 7, 3513. (c) Usta, H.; Lu, G.;
Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 9034. (d) Hou,
J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008, 130,
16144.
(5) Babudri, F.; Fiandanese, V.; Marchese, G.; Punzi, A. Tetrahedron
Lett. 1995, 36, 7305.
B
Org. Lett., Vol. XX, No. XX, XXXX

and without requiring laborious purifications, hence de-
monstrating the versatility of the aromatic dione BDTD.
During the synthesis of the dioxime 3, the formation of
an additional product was observed, and in a further
investigation of this reaction, we found that this product
corresponded to the dithieno[30,20:3,4;200,300:5,6]benzo[1,2-
c]furazan (DTBF). Acceptable yields for the formation of
DTBF can be achieved if the reaction is carried out for
10 days under refluxing conditions or in 3 days when run in
a pressure vessel at 140 °C (Table 2, entry 1). Table 2
summarizes the results of this reaction and its application
to analogue molecules.
Table 1. Various Chemical Transformations for BDTD
Electrochemical experiments on DTBF showed HOMO
and LUMO energy values of ꢀ6.36 and ꢀ3.13 eV respec-
tively, and in the case of iso-DTBF these values were
HOMO = ꢀ6.65 eV and LUMO = ꢀ3.25 eV. Thus,
based on this and other preliminary data (see UVꢀvis and
electrochemistry in the Supporting Information), we pro-
pose that these molecules will perform as acceptor moieties
in π-conjugated materials. Therefore, the furazan deriva-
tives, along with those shown in Table 1, will be investi-
gated as donor and acceptor building blocks for π-
conjugated oligomers and polymers.
Table 2. Synthesis of Furazan-Based Acceptors
Treatment of BDTD with hydroxylamineꢀhydrochloride
yielded dioxime 3; use of other reagents, including
methanolic or aqueous hydroxylamine solutions and hy-
droxylamine hydrochlorideꢀbase did not yield 3. Com-
pound 3 was treated with palladium on carbon and
hydrazine to produce benzo[1,2-b:6,5-b0]dithiophene-4,5-
diamine, BDTDA, and as shown in Table 1 (entries 3ꢀ5),
this is an interesting derivative that can be further deriva-
tized to generate new molecules such as dithieno[30,20:
3,4;200,300:5,6]benzo[1,2-c][1,2,5]thiadiazole (DT-BTD),
2H-dithieno[30,20:3,4;200,300:5,6]benzo[1,2-d][1,2,3]triazole
(DTBT) and tetrathieno[3,2-a:20,30-c:300,200-h:2000,3000-j]phena-
zine (TTPA). It is noteworthy that DT-BTD can also be
synthesized by reacting the dioxime 3 and sulfur mono-
chloride in DMF,6 but this reaction condition leads to a
mixture of two products, the thiadiazole and the oxadia-
zole derivatives.
a Reaction was carried out in a glass pressure vessel.
A plausible mechanism for the formation of DTBF is
presented in Scheme 3. As was mentioned before, the
dioxime 3 only formed when hydroxylamine hydrochloride
was employed. As such, the acid plays an important role in
the mechanism, not only in the formation of the dioxime
but also in its tautomerization and further dehydration to
form DTBF. It can be assumed that the tautomerization is
favored due to aromatization of the central phenyl ring.
(6) Weinstock, L. M.; Davis, P.; Handelsman, B.; Tull, R. J. J. Org.
Chem. 1967, 32, 2823.
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C

Table 3. Bromination of BDTD, DT-BTD, and DTBF
Scheme 3. Proposed Mechanism for Formation of DTBF
The bromination conditions for various molecules pre-
sented in Table 1 are summarized in Table 3. The bromina-
tion of BDTD can be carried out in high yield by
employing NBS in DMF at 65 °C, and brominations for
DT-BDTD and DTBF were carried out in high yield using
bromine and chloroform.
Currently, we are pursuing the synthesis of a BDTD
analogue containing solubilizing alkyl chains, and also
applying the transformations presented in this communi-
cation to other fused-aromatic diones. In addition, we are
including these moieties in π-conjugated oligomers and
polymers.
new π-conjugated materials with improved physical and
electronic properties.
Acknowledgment. We gratefully acknowledge the DOE
(DE-FG-02-96ER14617) for the funding for this project.
Dr. Dan Patel and Dr. Chad Amb are also acknowledged
for their helpful scientific discussions.
In summary, we have shown a straightforward synthetic
path toward BDTD and how this aromatic dione can be
derivatized to produce various acceptor and donor mole-
cules by relatively simple synthetic methodologies. This
scientific contribution can help the continuous develop-
ment of the π-conjugated polymer field given that access to
novel donor and acceptor molecules is vital to generate
Supporting Information Available. Experimental pro-
cedures and spectral data for relevant compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The authors declare no competing financial interest.
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Org. Lett., Vol. XX, No. XX, XXXX