Facile synthesis of 2,6-disubstituted benzobisthiazoles: Functional monomers for the design of organic semiconductors

Article abstract of DOI:10.1021/jo9023864

(Chemical Equation Presented) The synthesis of several synthetically useful 2,6-disubstituted benzobisthiazoles is described. The method is based on the Lewis acid-catalyzed ring-closing reaction between substituted orthoesters and diamino benzene dithiol. The resulting benzobisthiazoles are obtained cleanly and in good yields. These materials are of interest for the development of new organic semiconductors.

Full text of DOI:10.1021/jo9023864

pubs.acs.org/joc
thermal stability,3,4 electron affinity,5 and interesting non-
Facile Synthesis of 2,6-Disubstituted
Benzobisthiazoles: Functional Monomers for the
Design of Organic Semiconductors
linear optical properties.2,6,7 Typical synthesis of benzo-
bisthiazoles requires strong acids or oxidants8 and high
temperatures.9 These harsh reaction conditions restrict the
types of substituents that can be incorporated onto these
moieties, hindering the exploration of materials containing
benzobisthiazoles.
Jared F. Mike, Jeremy J. Inteman, Arkady Ellern, and
Malika Jeffries-EL*
Recently we reported the synthesis of 2,6-disubstituted
benzobisoxazoles using substituted orthoesters and rare
earth metal triflates as catalysts.10 These reaction conditions
facilitated the synthesis of novel benzobisoxazoles bearing a
variety of substituents cleanly and in high yield. Inspired by
these promising results, we set out to develop a mild, low-
temperature method for the synthesis of the analogous 2,6-
disubstituted benzobisthiazoles. Although benzo[1,2-d;4,
5-d0]bisoxazole (trans-BBO) and trans-BBZT are structurally
similar, the sulfur atom is less electronegative than the
oxygen atom, and has similar electronegativity to the carbon
atom. Thus the electron density is more equally shared
between sulfur and carbon in trans-BBZT than between
oxygen and carbon in trans-BBO and the π-orbitals will be
more delocalized. Additionally the empty d orbitals of the
sulfur atom can contribute to the molecular π-orbitals
decreasing the energy of the π-π* transition.7,11 These
changes can be beneficial for the development of new organic
semiconducting materials. Herein, we report the successful
synthesis of several new benzobisthiazoles. We also demon-
strate that functionality can be increased by a simple reaction
following ring formation.
Department of Chemistry, Iowa State University,
1605 Gilman Hall, Ames, Iowa 50010
malikaj@iastate.edu
Received November 7, 2009
The synthesis of several synthetically useful 2,6-disubsti-
tuted benzobisthiazoles is described. The method is based
on the Lewis acid-catalyzed ring-closing reaction between
substituted orthoesters and diamino benzene dithiol. The
resulting benzobisthiazoles are obtained cleanly and in
good yields. These materials are of interest for the devel-
opment of new organic semiconductors.
The general synthetic route for the benzobisazoles is
shown in Scheme 1. Previously, we found that best reaction
conditions for the synthesis of benzobisoxazoles are DMSO
as a solvent, pyridine as a cosolvent, and rare metal triflates
as catalysts.10 The use of the pyridine as a cosolvent is bene-
ficial since it scavenges the hydrochloride salts that coordi-
nate with the diamino diol. Removing the acids prevents the
decomposition of the substituted orthoesters, which is cata-
lyzed by protic acids.10 Using the reaction between 2,5-
diamino-1,4-benzene dithiol (DABDT) (1) and triethy-
lorthoacetate (2b) as a model, we explored these conditions.
Unfortunately, when DABDT was mixed with pyridine in
DMSO, an insoluble green precipitate was formed. This was
most likely caused by the formation of disulfide linkages,
although the insolubility of the material prevented its char-
acterization (entry 1). We then attempted to perform the
The design and synthesis of new organic semiconducting
materials is of current interest due to the important roles
these materials play in the development of plastic electro-
nics.1 Although many π-conjugated materials are known,
most of them are electron rich and exhibit electron-donating
and hole-transporting (p-type) electronic properties. Thus
the synthesis of electron-deficient π-conjugated materials,
which can exhibit electron-accepting and electron-transport-
ing properties (n-type), remains an important problem in the
field.
The electron-deficient benzo[1,2-d;4,5-d0]bisthiazole moi-
ety (trans-BBZT) is a promising building block for the
development of new organic semiconductors because mate-
rials containing benzobisthiazoles exhibit high fluorescence,2
(7) Reinhardt, B. A.; Unroe, M. R.; Evers, R. C. Chem. Mater. 1991, 3,
864.
(8) Pang, H.; Vilela, F.; Skabara, P. J.; McDouall, J. J. W.; Crouch, D. J.;
Anthopoulos, T. D.; Bradley, D. D. C.; de Leeuw, D. M.; Horton, P. N.;
Hursthouse, M. B. Adv. Mater. 2007, 19, 4438.
(9) (a) Kricheldorf,H.R.;Domschke, A.Polymer1994, 35, 198.(b) Osman,
A. M.; Mohamed, S. A. U.A.R.J. Chem. 1971, 14, 475. (c) Osman, A. M.;
Mohamed, S. A. Indian J. Chem. 1973, 11, 868. (d) Imai, Y.; Itoya, K.;
Kakimoto, M.-A. Macromol. Chem. Phys. 2000, 201, 2251. (e) Imai, Y.;
Taoka, I.; Uno, K.; Iwakura, Y. Makromol. Chem. 1965, 83, 167.
(10) Mike, J. F.; Makowski, A. J.; Jeffries-EL, M. Org. Lett. 2008, 10,
4915.
(11) Zhao, M.; Samoc, M.; Prasad, P. N.; Reinhardt, B. A.; Unroe, M. R.;
Prazak, M.; Evers, R. C.; Kane, J. J.; Jariwala, C.; Sinsky, M. Chem. Mater.
2002, 2, 670.
(1) (a) Singh, T. B.; Sariciftci, N. S. Annu. Rev. Mater. Res. 2006, 36, 199.
(b) Forrest, S. R. Nature 2004, 428, 911.
(2) Osaheni, J. A.; Jenekhe, S. A. Macromolecules 1993, 26, 4726.
(3) (a) Evers, R. C.; Dotrong, M. Mater. Res. Soc. Symp. Proc. 1989, 134,
141. (b) Wolfe, J. F. In Encyclopedia of Polymer Science and Engineering;
John Wiley and Sons: New York, 1988; Vol. 11, p 601.
(4) Wolfe, J. F.; Loo, B. H.; Arnold, F. E. Macromolecules 1981, 14, 915.
(5) (a) Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2002, 14, 4775.
(b) Babel, A.; Jenekhe, S. A. J. Phys. Chem. B 2002, 106, 6129.
(6) (a) Jenekhe, S. A.; Osaheni, J. A.; Meth, J. S.; Vanherzeele, H. Chem.
Mater. 1992, 4, 683. (b) Lee, S.-H.; Otomo, A.; Nakahama, T.; Yamada, T.;
Kamikado, T.; Yokoyama, S.; Mashiko, S. J. Mater. Chem. 2002, 12, 2187.
(c) Jenekhe, S. A.; Osaheni, J. A. Chem. Mater. 1994, 6, 1906. (d) Osaheni,
J. A.; Jenekhe, S. A. Chem. Mater. 1995, 7, 672.
DOI: 10.1021/jo9023864
r
Published on Web 12/18/2009
J. Org. Chem. 2010, 75, 495–497 495
2009 American Chemical Society

Mike et al.
JOCNote
SCHEME 2. Synthesis of Monomer 3f
SCHEME 1. Synthesis of 2,6-Disubstituted Benzobisiazoles
TABLE 1. Optimization Reactions with DABDT 1 and Triethyl
Orthoacetate 2ba
TABLE 2. Reaction of 1 with Various Orthoesters 2a and 2c-eb
entry
solvent
catalyst
temp (°C)
time (h)
yieldb (%)
1
2
3
4
5
6
7
DMSO
none
none
DMAc
DMAc
DMAc
DMAc
DMAc
Y(OTf)3
nonec
H2SO4
Eu(OTf)3
La(OTf)3
Sc(OTf)3
Y(OTf)3
Yb(OTf)3
50
90
90
50
50
50
50
50
n/a
6
3
1.5
1.5
1.5
1.5
1.5
0
51
45
77
72
75
69
77
c
aStandard reaction conditions: substrate 1 M in solvent, 3 equiv of
ortho ester, 5 mol % of catalyst, and 2 equiv of pyridine. bIsolated yields.
c10 equiv of orthoester used without pyridine.
reaction without any additional solvents, using 10 equiv of
the triethylorthoformate and the coordinated hydrochloride
salts as an acid catalyst (entry 2). These conditions produced
2,6-dimethylbenzobisthiazole 3b in a 51% yield. With use of
the same reaction conditions and sulfuric acid as a catalyst,
similar results were obtained (entry 3). In both cases the
resulting products were formed along with some dark im-
purities, which could be removed after careful recrystalli-
zation.
To reduce the amount of orthoester required for these
reactions, we performed a solubility test with DABDT to
find a cosolvent. We investigated the use of DMA as a
solvent, since DABDT dissolves in DMA at room tempera-
ture, and this solvent was not prone to the side reactions
experienced previously. The reaction of DABDT and tri-
ethylorthoformate occurrs rapidly, when performed slightly
above room temperature. The solid that forms is easily iso-
lated by precipitation into water and filtration. We then ex-
plored the use of several different rare earth metal triflates to
catalyze the reaction. The results are summarized in Table 1.
On the basis of the model reactions, we found that most
triflates gave similar results. Since all of these catalysts are
commercially available, our bias toward the use of Eu(OTf)3,
La(OTf)3, or Y(OTf)3 was due to their lower cost relative to
the other rare earth metal triflates.
aStandard reaction conditions: substrate 1 M in solvent, 3 equiv of
orthoester, 5 mol % of catalyst, and 2 equiv of pyridine, 1.5 h. bIsolated
yields. cReaction time was 18 h.
for the synthesis of benzobisoxazoles. Interestingly, signifi-
cantly lower yields were observed with DMSO as solvent.10
These lower yields were most likely due to the poor solubility
of diaminohydroquinone and diaminoresorcinol in this sol-
vent. We did not investigate the synthesis of benzo[1,2-d;5,
4-d0]bisthiazole (cis-BBZT) derivatives because of the inabi-
lity to synthesize 4,6-diamino-1,3-benzene dithiol. Although
the synthesis of this compound has been reported pre-
viously,12 we, like those before us, have struggled to obtain
pure material due to its instability.4
To increase the functionality of these BBZT derivatives,
we synthesized the diphosphonate ester 3f via the Arbuzov
reaction of 3c with triethylphosphite (Scheme 2). This reac-
tion occurred cleanly to produce the 3f in an 80% yield
(Table 2). Monomers of this type have been useful for the syn-
thesis of vinylene polymers via the Horner-Wadsworth-
Emmons reaction.
We were able to obtain X-ray quality crystals of 3a, 3b, 3c,
and 3f suitable for analyses to be performed by recrystalliza-
tion. Detailed crystallographic data can be found in the
Supporting Information, and a representative example is
shown in Figure 1. In addition to confirming the identity of
these new compounds, the X-ray analyses show that these
Upon determining the optimum reaction conditions, we
evaluated the scope of this reaction with respect to the
orthoester. To accomplish this we utilized the commercially
available triethyl orthoformate (2a) in addition to triethyl
orthochloroacetate (2c), triethyl orthopropiolate (2d), and
ethyl triethoxyacetate (2e), which were prepared in our labo-
ratories. In all cases the orthoesters reacted with DABDT
cleanly and in moderate yields. As a follow-up to our
previous work, we also explored the use of DMA as a solvent
(12) Wolf, R.; Okada, M.; Marvel, C. S. J. Polym. Sci., Part A: Polym.
Chem. 1968, 6, 1503.
496 J. Org. Chem. Vol. 75, No. 2, 2010

Mike et al.
JOCNote
compound 3a with 4 and triethyl orthochloroacetate 2c. The
product was obtained after recrystallization from heptanes as
pale yellow needles. The reaction was run on 15 mmol scale with a
1
2.54 g yield (59%). Melting point 219-220 °C. H NMR (400
MHz; DMSO-d6) δ 5.28 (4H, s), 8.82 (2H, s); 13C NMR (100
MHz; DMSO-d6) δ 42.1, 116.5, 134.7, 150.5, 168.8. HRMS (EI)
287.93549, C10H6Cl2N2S2 requires 287.93495, deviation 1.9 ppm.
2,6-Bis(trimethylsilylethynyl)benzo[1,2-d;4,5-d0]bisthiazole (3d).
Compound 3d was synthesized following the same protocol
as for compound 3a with 4 and triethyl orthopropiolate 2d.
The product was obtained after recrystallization from pentane
as pale yellow needles (38% yield). Melting point >260 °C
(pentane). 1H NMR (400 MHz; CDCl3) δ 0.331 (18H, s), 8.478
(2H, s); 13C NMR (100 MHz; CDCl3) δ -0.4, 96.8, 104.9, 115.9,
135.1, 150.0, 151.7. HRMS (EI) 384.06158, for C18H20N2S2Si2,
requires 384.06064, deviation 2.4 ppm.
Benzo[1,2-d;4,5-d0]bisthiazole Diethyl Ester (3e). Compound
3e was synthesized following the same protocol as for com-
pound 3a with 4 and ethyl triethoxyacetate 2e. The product was
obtained after recrystallization from pentane as pale yellow
needles (53% yield). Melting point 241-242 °C (chloroform/
ethanol). 1H NMR (400 MHz; CDCl3) δ 1.52 (6H, t, J=5.4 Hz),
4.59 (4H, q, J =5.4 Hz), 8.83 (2H, s); 13C NMR (100 MHz;
CDCl3) δ 14.5, 63.7, 118.9, 136.5, 152.6, 160.4, 161.3. HRMS
(EI), found 336.02464, C14H12N2O4S2 requires 336.02385,
deviation 2.3 ppm.
FIGURE 1. Crystal structure of 3c along the a-crystallographic
axis.
monomers are all planar with a mean deviation from pla-
˚
narity of 0.0114 A. The flat nature of the benzobisthiazoles is
beneficial to promoting efficient π-stacking and improving
the charge transport of materials derived from them.
In summary, we have developed a new method for the
synthesis of novel 2,6-disubstituted benzobisthiazoles. The
benefits of this approach include the mild reaction condi-
tions and ease of purification without column chromatogra-
phy. Currently we are developing new organic semiconduc-
tors using these compounds.
2,6-Dimethylbenzo[1,2-d;4,5-d0]bisthiazolediethylphosphonate
Ester (3f). Triethylphosphite (1.12 g, 7.75 mmol) and 2,6-(bis-
chloromethyl)benzo[1,2-d;4,5-d0]benzobisthiazole 3c (650 mg,
2.25 mmol) were heated to 150 °C for 4 h. The reaction was
cooled to yield crude 3f. Recrystallization from chloroform/
heptanes afforded the product as a white solid (885 mg, 80%).
Melting point 203-204 °C. 1H NMR (400 MHz; CDCl3) δ 1.32
(12H, t, J = 6.0 Hz), 3.75 (4H, d, J = 15 Hz), 4.18 (8H, m), 8.44
(2H, s). 13C NMR (100 MHz; CDCl3) δ 16.6 (d, J=6.0 Hz), 33.1,
34.5, 63.1 (d, J=6.0 Hz), 115.3, 135.2, 148.8, 151.1, 162.6, 162.7.
HRMS (EI) 492.07227, C18H26N2O6P2S2 requires 492.07075,
deviation 3.0 ppm.
Experimental Section
Typical Procedure for the Synthesis of Benzobisthiazoles
(3a-e). Benzo[1,2-d;4,5-d0]bisthiazole (3a). In a round-bottomed
flask 2,5-diamino-1,4-benzene dithiol 1 (1.23 g, 5.0 mmol) and
pyridine (791 mg, 10.0 mmol) were dissolved in DMA (10 mL).
The resulting solution is added via a syringe to a mixture
of triethylorthoformate 2a (2.22 g, 15.0 mmol) and Y(OTf)3
(134 mg, 0.255 mmol) in a round-bottomed flask. The reaction is
stirred at 55 °C for 1 h and then cooled. The reaction is diluted
with water and the crude product collected by filtration. Re-
crystallization of the crude with dichloromethane/heptanes gave
1
white needles (0.65 g, 68% yield). Melting point >260 °C. H
NMR (400 MHz; DMSO-d6) δ 8.92 (2H, s), 9.49 (2H, s); 13C
NMR (100 MHz; DMSO-d6) δ 116.3, 132.7, 151.2, 157.9.
HRMS (EI) 191.98189, C8H4N2S2 requires 191.98159, devia-
tion 1.6 ppm.
Acknowledgment. We are grateful to the 3M Foundation
and the National Science Foundation (DMR-0846607) for
their generous support of this work. We thank Dr. Kamel
Harrata and the Mass Spectroscopy Laboratory of Iowa
State University (ISU) for analysis of our compounds. We
also thank Brian Tlach for help with the synthesis of 2e and
Dr. Jessie Waldo (ISU) for helpful discussions of this re-
search.
2,6-Dimethylbenzo[1,2-d;4,5-d0]bisthiazole (3b). Compound
3b was synthesized following the same protocol as for com-
pound 3a with 4 and triethylorthoacetate 2b. The product was
obtained after recrystallization from dichloromethane/heptanes
as white needles (0.85 g, 77% yield). Melting point 232-233 °C.
1H NMR (400 MHz; CDCl3) δ 2.86 (6H, s), 8.34 (2H, s); 13C
NMR (100 MHz; CDCl3) δ 20.5, 114.4, 134.5, 151.0, 167.9.
HRMS (EI) 220.01289, C10H8N2S2 requires 220.01330, devia-
tion 1.9 ppm.
Supporting Information Available: Crystallographic infor-
mation files (CIF) for 3a-c and 3e, experimental details for the
synthetic intermediates, and NMR spectra (1H and 13C) for
3a-e. This material is available free of charge via the Internet at
http://pubs.acs.org.
2,6-Bis(chloromethyl)benzo[1,2-d;4,5-d0]bisthiazole (3c). Com-
pound 3c was synthesized following the same protocol as for
J. Org. Chem. Vol. 75, No. 2, 2010 497