One-pot synthesis of 4,8-dibromobenzo[1,2-c;4,5-c′]bis[1,2,5] thiadiazole

Article abstract of DOI:10.1021/ol100989v

(Equation Presented). A one-step synthesis of 4,8-dibromobenzo[1,2-c;4,5- c′]bis[1,2,5]thiadiazole with use of 1,2,4,5-tetraaminobenzene tetrahydrobromide and thionyl bromide in good yield is reported. This unit can then be used in the synthesis of low bandgap materials via palladium-catalyzed coupling reactions. The approach offers a quick and easy way to prepare low bandgap materials as compared to the current literature methods.

Full text of DOI:10.1021/ol100989v

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3340-3343
One-Pot Synthesis of 4,8-Dibromobenzo-
[1,2-c;4,5-c′]bis[1,2,5]thiadiazole
Teck Lip Tam, Hairong Li, Fengxia Wei, Ke Jie Tan, Christian Kloc,
Yeng Ming Lam, Subodh G. Mhaisalkar, and Andrew C. Grimsdale*
School of Materials Science and Engineering, Nanyang Technological UniVersity,
50 Nanyang AVenue, Singapore 639798
acgrimsdale@ntu.edu.sg
Received May 13, 2010
ABSTRACT
A one-step synthesis of 4,8-dibromobenzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole with use of 1,2,4,5-tetraaminobenzene tetrahydrobromide and thionyl
bromide in good yield is reported. This unit can then be used in the synthesis of low bandgap materials via palladium-catalyzed coupling
reactions. The approach offers a quick and easy way to prepare low bandgap materials as compared to the current literature methods.
The benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBT) unit (1)
is a strong electron accepting unit that has been used to make
low band gap donor-acceptor materials. This has potential
applications in light-emitting devices (LED), ambipolar
organic field-effect transistors (OFETs), and solar cells.
As a sign of the effectiveness of BBT in promoting low
bandgaps, even the small oligomers 2 and 3 are reported
Figure 1. Structures of BBT (1) and some low bandgap 4,8-
to possess bandgaps of only 1.5 eV (Figure 1).1-3
Oligomer 3 has been used as emissive material in an LED
displaying efficient emission in the near-infrared (NIR).1
Polymeric films obtained by electropolymerization of 2 and
3 display bandgaps of about 0.5 eV.2,3 The small bandgaps
observed in these materials can be attributed to the high
electron affinity and large quinoid contribution of the BBT
unit.2-9
disubstituted BBT derivatives.
Oligomer 2 has also been used as the starting material for
synthesis of soluble polymers 4 and 5 with bandgaps of
0.65-0.67 eV,10 and of oligomer 6, which has been used to
make a NIR-emitting LED (Figure 2).5 Another NIR-emitter
(5) Qian, G.; Dai, B.; Luo, M.; Yu, D.; Zhan, J.; Zhang, Z.; Ma, D.;
(1) Yang, Y.; Farley, R. T.; Steckler, T. T.; Eom, S.-H.; Reynolds, J. R.;
Wang, Z. Y. Chem. Mater. 2008, 20 (19), 6208–6216.
Schanze, K. S.; Xue, J. J. Appl. Phys. 2009, 106, 044509
(2) Steckler, T. T.; Abboud, K. A.; Craps, M.; Rinzler, A. G.; Reynolds,
J. R. Chem. Commun. 2007, 46, 4904–4906
(3) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am. Chem.
Soc. 1995, 117 (25), 6791–6792
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2008, 10 (17), 3785–3787.
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(7) Wu, W. C.; Chen, W. C. J. Polym. Res. 2006, 13 (6), 441–449.
(8) Pai, C. L.; Liu, C. L.; Chen, W. C.; Jenekhe, S. A. Polymer 2006,
47 (2), 699–708.
(9) Ono, K.; Tanaka, S.; Yamashita, Y. Angew. Chem., Int. Ed. Engl.
1994, 33 (19), 1977–1979.
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(4) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8 (2),
570–578
.
10.1021/ol100989v  2010 American Chemical Society
Published on Web 07/08/2010

potentially provides a quick access to 5,10-disubstituted
pyrazino[2,3-g]quinoxalines. Thus a convenient synthesis of
2 and 8 has great potential for assisting the preparation of a
range of low bandgap materials.
A shorter approach to 2 has been demonstrated by using
nucleophilic addition of 2-thienyllithium to the BBT-4,8-
dione 15 followed by reduction (Scheme 2).13 Even though
Scheme 2. Synthesis of 2 from p-Chloranil 12
Figure 2. Structures of some BBT related low bandgap polymers
and oligomers, and the synthetic precursor 4,8-dibromo-BBT (8).
7 was made from the 4,8-dibromo-BBT 8.5 As 4,8-dibromo-
BBT 8 is an obvious synthetic precursor to 2, it is thus an
important intermediate for the synthesis of low bandgap
oligomers and polymers.
There is, however, no short, efficient synthesis of compounds
2 or 8 available in the literature. The standard synthesis of these
compounds starting from commercially available benzothia-
diazole (BT, 9) uses the 4,7-dibromo-5,6-dinitrobenzothiadiazole
10 as a common intermediate.4 As the intermediate has to be
made in two steps with harsh conditions from 9, it can readily
be seen that this is not a route suitable for commercially viable
production of BBT-based materials (Scheme 1). Reduction of
this approach cuts down the total number of steps to 3, the
overall yield is still only about 25%. Also, the reagent S4N4
used to make 15 directly from p-chloranil (14)14 is a shock
sensitive explosive, which prohibits the possibility of large-
scale synthesis (there is a safer two-step alternative synthesis
of 15 from 14 but this is less efficient15). As a result this
approach has not been widely used.
We have now discovered a one-pot method for the efficient
synthesis of 8. In our first experiment we treated com-
mercially available 1,2,4,5-tetraaminobenzene (TAB) tet-
rahydrochloride (17) with thionyl chloride, expecting to
obtain 1. To our surprise the product showed no signals in
1
Scheme 1. Synthesis of 2, 8, and 13 According to the Literature
the H NMR spectrum, and was identified by the presence
of ions at m/z 261, 263, and 265 in the MALDI-TOF mass
spectrum as the dichloride 16, which was obtained in 50%
yield. We were unable to obtain satisfactory microanalytical
data for 16, but were able to perform X-ray diffraction on a
crystal of 16. The crystallographic data obtained were very
similar to those previously reported for the dibromide 8.9
10 in acetic acid with zinc produces the tetraaminobenzene 12,
which on addition of 1,2-diketone yields 5,10-bis(2-thie-
nyl)pyrazino[2,3-g]quinoxaline derivatives 13, which have also
been used to synthesize low band polymers.4,11,12 Reduction
of 2 should also generate the intermediate 12 and thus
Figure 3. Crystal structure of 16. Monoclinic, P21/n, a ) 3.8426
Å, b ) 7.4348 Å, c ) 15.2226 Å, ꢀ ) 90.339°, V ) 434.88(6) Å3,
Z ) 2, pcalcd ) 2.01 g/cm3.
(10) Bundgaard, E.; Krebs, F. C. Macromolecules 2006, 39 (8), 2823–
2831.
(11) Zoombelt, A. P.; Fonrodona, M.; Turbiez, M. G. R.; Wienk, M. M.;
Janssen, R. A. J. J. Mater. Chem. 2009, 19, 5336–5342
.
As shown in Figure 3, 16 forms a monoclinic crystal
(12) Zhang, F.; Bijleveld, J.; Perzon, E.; Tvingstedt, K.; Barrau, S.;
Inganas, O.; Andersson, M. R. J. Mater. Chem. 2008, 18, 5468–5474
Org. Lett., Vol. 12, No. 15, 2010
.
containing ribbon-like columns of molecules with an in-
3341

tramolecular spacing of 3.43 Å. This close packing indicates
there are strong intermolecular interactions between the BBT
units and helps explain the poor solubility of 16, 8, and 2
which precluded our obtaining 13C NMR data for them.
Treatment of TAB·4HCl with thionyl bromide gave impure
dibromide 8, which was found by MALDI-TOF analysis to be
contaminated with the dichloride 16 and the mixed halide 18.
To obtain pure 8 it was found necessary to convert the
tetrachloride salt to the tetrabromide salt 19. Reaction of the
resulting crude tetrabromide then gave the desired dibromide 8
in 56% yield. This was improved to 87% yield when 19
obtained from a commercial source (Hangzhou Trylead Chemi-
cal Technology Co., Ltd., 95% purity) was reacted with thionyl
bromide. In view of the difficulty and expense of obtaining the
bromide commercially, conversion of the cheaper and more
readily available chloride is an attractive option, especially if
further optimization of the halide exchange can improve the
yield. Stille coupling of 8 with a thienyltin compound gave 2
in 82% yield (71% yield from 19 to 2) (Scheme 3).
by thionyl chloride via electrophilic attack of the chlorine
has been reported elsewhere,17 and it is likely that the chloro
compound 22 arises from such an electrophilic attack to form
23 followed by dehydrogenation.
The importance of the anion on TAB in the current
work, however, suggested that halogenation does not occur
via electrophilic substitution but via a nucleophilic process.
A direct nucleophilic attack on BBT 1, while electronically
favorable, we consider unlikely as the low solubility of
BBT and its derivatives would most probably result in a
mixture of mono- and dihalogenated products. Also this
route would require the intermediate to be oxidized to form
the product, and while thionyl chloride is known to oxidize
hydroquinones to quinones,14 we are not aware of any
precedent for it being able to perform the oxidation that
would be necessary here. We propose that, as shown in
Scheme 5, there is an initial electrophilic attack of the
Scheme 5
.
Proposed Mechanism for the Formation of 8 via
TAB
Scheme 3. Synthesis of 4,8-Disubstituted BBT via TAB
There has also been reported one example of a chlorination
during synthesis of a benzothiadiazole from a diamine with
thionyl chloride.16 Treatment of the diamino-BT 20 with
refluxing neat thionyl chloride gave a mixture, which by mass
spectrometric analysis contained not only the desired ben-
zobis(thiadiazole) 21 but also the monochloride 22 and the
chlorinated bis-NSO compound 23 (Scheme 4). However,
excessive thionyl bromide (which is complexed with
pyridine) on the intermediate 24 generating the intermedi-
ate 25, followed by nucleophilic displacement of the SOPy
group by bromide. The four amine groups in TAB make
it extremely electron rich, and even the less activated
sulfinyl intermediate 24 should still be susceptible to
electrophilic attack. Reaction of arenes with thionyl halides
is used to prepare diaryl sulfoxides18 with structures such
as 25 being plausible intermediates in such processes in
the presence of pyridine. (Similar sulfinyl-pyridine com-
plexes have been accepted as intermediates in other
processes involving thionyl halides and pyridine.19,20) The
nucleophilic attack on intermediate 25 is proposed as it
accounts for the dichloride 16 and the mixed halide 18 being
formed upon treatment of the TAB hydrochloride with
thionyl bromide. Such a nucleophilic aromatic substitution
would be assisted by the strong electron withdrawing sulfinyl
Scheme 4
.
Previously Reported Chlorination during a BT
Synthesis
when thionyl chloride and pyridine was used only compound
21 was obtained. Chlorination of aromatic N-sulfinylamines
(16) Komin, A. P.; Carmack, M. J. Heterocycl. Chem. 1975, 12 (5),
829–833.
(13) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.;
Meijer, E. W. Mater. Sci. Eng., R 2001, 32, 1.
(17) Ottmann, G.; Hooks, H. J. Org. Chem. 1965, 30 (3), 952–954.
(18) Smith, M. B.; March, J. In March’s AdVanced Organic Chemistry,
6th ed.; John Wiley and Sons: Hoboken, NJ , 2007; pp 697-698.
(14) Shi, S.; Katz, T. J.; Yang, B. V.; Liu, L. J. Org. Chem. 1995, 60
(5), 1285–1297.
(19) Cram, D. J. J. Am. Chem. Soc. 1953, 75, 332–338
.
(15) Anzenbacher, P., Jr.; Palacios, M. A.; Jursikova, K.; Marquez, M.
Org. Lett. 2005, 7 (22), 5027–5030.
(20) Smith, M. B.; March, J. In March’s AdVanced Organic Chemistry,
6th ed.; John Wiley and Sons: Hoboken, NJ, 2007; pp 468-469.
3342
Org. Lett., Vol. 12, No. 15, 2010

group on the aromatic ring. Repetition of this process,
followed by dehydration during reflux, yields 8. A similar
mechanism is proposed for chlorination with thionyl chloride.
In conclusion, the one-step synthesis of dibromo BBT with
TAB and thionyl bromide is an expeditious method to
generate the important intermediates 2 and 8 for synthesis
of low bandgap materials. Though the exact mechanism of
this halogenation reaction is not yet elucidated, it is probable
that halogenation proceeds via nucleophilic rather than
electrophilic substitution.
The main advantages of this new method are that the
desired dibromide 8 can be obtained in a much higher
yield, by a much shorter route and can be carried out on
a larger synthetic scale than by existing procedures, thus
greatly increasing the synthetic accessibility of oligomers
and polymers containing the BBT unit, and potentially
also other electron accepting units. We are currently
investigating the reduction of 2 as a route to pyrazino-
quinoxaline units, the results of which will be published
separately. The main synthetic novelty of this new process
is that by changing the counterion on the starting material
we are able to selectively obtain either chlorinated or
brominated BBT.
Acknowledgment. This work was supported by Robert
Bosch (SEA) Pte Ltd.
Supporting Information Available: Detailed descriptions
of experimental procedures for compounds 2, 8, and 16, plus
mass spectra, NMR spectra, and crystallographic data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL100989V
Org. Lett., Vol. 12, No. 15, 2010
3343