An efficient synthesis of 2,6-disubstituted benzobisoxazoles: New building blocks for organic semiconductors

Article abstract of DOI:10.1021/ol802011y

(Chemical Equation Presented) 2,6-Disubstituted benzobisoxazoles have been synthesized by a highly efficient reaction of diaminobenzene diols with various orthoesters. The scope of this new reaction for the synthesis of substituted benzobisoxazoles has been investigated using four different orthoesters. The utility of these compounds as building blocks for the synthesis of conjugated polymers is demonstrated.

Full text of DOI:10.1021/ol802011y

ORGANIC
LETTERS
2008
Vol. 10, No. 21
4915-4918
An Efficient Synthesis of
2,6-Disubstituted Benzobisoxazoles:
New Building Blocks for Organic
Semiconductors
Jared F. Mike, Andrew J. Makowski, and Malika Jeffries-EL*
Department of Chemistry, Iowa State UniVersity, 3655 Gilman Hall, Ames, Iowa 50010
malikaj@iastate.edu
Received August 27, 2008
ABSTRACT
2,6-Disubstituted benzobisoxazoles have been synthesized by a highly efficient reaction of diaminobenzene diols with various orthoesters.
The scope of this new reaction for the synthesis of substituted benzobisoxazoles has been investigated using four different orthoesters. The
utility of these compounds as building blocks for the synthesis of conjugated polymers is demonstrated.
Conjugated polymers and oligomers are of interest for use
as organic semiconductors in applications such as thin-film
transistors (TFTs),1 light-emitting diodes (OLEDs),2 and
photovoltaic cells (PVCs).3 While there are a large number
of π-conjugated small molecules, oligomers, and polymers
which have been reported in the literature, the design and
synthesis of new π-conjugated organic materials remains an
important area of research. Of particular interest is the
creation of materials with good electron transport properties
(n-type), which are less abundant in the literature than those
with hole transport (p-type) properties.4 n-type materials with
efficient electron transport are essential for improving the
performance of TFTs, OLEDs, and PVCs and to enable other
cost-effective applications such as complementary circuits.5
We are interested in developing benzobisoxazoles for their
use as building blocks for novel organic semiconductors
because conjugated small molecules and polymers based on
benzobisoxazoles are well suited for use in organic semi-
conducting applications. These materials combine efficient
electron transport, photoluminescence, and third-order non-
linear optical properties6 with excellent mechanical strength
and thermal stability.7 Unfortunately, the fused benzobisox-
azole ring system, the rodlike conformation of the resulting
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106, 6129.
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Neher, D. Macromolecules 2006, 39, 4018. (c) Alam, M. M.; Jenekhe, S. A.
Chem. Mater. 2004, 16, 4647.
10.1021/ol802011y CCC: $40.75
Published on Web 10/03/2008
2008 American Chemical Society

polymers, and efficient π-stacking between chains render
fully conjugated poly(benzobisoxazoles) (PBOs) insoluble
in aprotic solvents. As a result, PBOs are typically processed
from acidic solvents, such as Lewis acid/nitromethane,
methanesulfonic acid, trifluoromethanesulfonic acid, and
sulfuric acid,8 which are impractical for use in device
manufacturing.
Generally, the solubility of π-conjugated materials can be
improved through structural modifcation. In the case of small
molecules and polymers containing the benzobisoxazole
moiety, the synthesis is achieved by the high-temperature
condensation of bis-o-aminophenols and aromatic diacids in
the melt, in polyphosphoric acid (PPA), in phosphorus
pentoxide/methanesulfonic acid, or in trimethylsilyl poly-
phosphate (PPSE)/o-dichlorobenzene.9 These conditions are
rather harsh, thereby limiting the types of substituents that
can be incorporated into the benzobisoxazole moiety.
In this contribution, we report the synthesis of optoelec-
tronic building blocks based on benzo[1,2-d;4,5-d′] bisox-
azole (trans-BBO) (3a-e) and benzo[1,2-d;5,4-d′] bisoxazole
(cis-BBO) (5a-e) by the reaction of various orthoesters
(2a-e) with 2,5-diaminohydroquinone (DAHQ) (1) and 4,6-
diaminoresorcinol (DAR) (4), respectively (Scheme 1). In
of orthoester, which serves as both a reagent and the solvent,
a catalytic amount of acid, and temperatures of 130 °C. Using
the reaction of triethyl orthoformate and DAHQ as our model
system, we first explored catalyst-free conditions, relying on
the acid coordinated with the diamino diol to catalyze the
reaction (entry 1). This yielded trans-BBO 3a in 61% yield.
When the reaction was performed under traditional condi-
tions, using catalytic amounts of H2SO4, the target compound
3a was obtained in 65% yield (entry 3). While the yields
were moderate in both cases, the product was contaminated
with a significant amount of dark red oxidation products,
complicating purification.
Due to the tendency of DAHQ 1 and DAR 4 to decompose
at higher temperatures, we needed to reduce the reaction
temperature. Furthermore, the use of such a large excess of
orthoester is undesirable since the substituted orthoesters are
costly. To reduce the need for excess orthoester, we used DMSO
as a cosolvent since it can dissolve both DAHQ and DAR. We
then explored the use of rare earth metal triflates as catalysts
since they have been demonstrated to reduce reaction times and
increase yields when used instead of traditional Lewis acid
catalysts.11 In the case of our model reaction, we found several
effective catalysts as shown in Table 1. Interestingly, the Lewis
Table 1. Investigation of the Effect of Catalyst on the Reaction
of DAHQ (1) with Triethyl Orthoformate 2aa
Scheme 1. Synthesis of 2,6-Disubstituted Benzobisoxazoles
catalyst/
cosolvent
entry solvent
temp (°C) time (h) yieldb (%)
1
2
3
4
5
6
7
8
none
DMSO none
none
DMSO H2SO4
DMSO PTSA
DMSO Bi(OTf)3
DMSO Hf(OTf)3
DMSO Eu(OTf)3
DMSO Sc(OTf)3
DMSO Y(OTf)3
none
130
60
130
130
110
60
60
60
60
60
5
2.5
2.5
1
4
4
4
4
4
4
61
64
65
64
71
0
36
82
82
81
0
H2SO4
all cases, the target compounds have been obtained cleanly
and in high yield. Using our strategy, we have synthesized
several building blocks suitable for the synthesis of benzo-
bisoxazole monomers.
We rationalized that orthoesters could be used for the
synthesis of benzobisoxazoles since they have been used for
the synthesis of benzimidazoles, benzoxazoles, and ben-
zothiazoles.10 The standard reaction conditions use 10 equiv
9
10
11
12
13
14
none
Yb(OTf)3
60
60
60
45
4
4
1
1
DMSO Yb(OTf)3
DMSO Y(OTf)3/Py
DMSO pyridine
75
92c
78c
a Standard reaction conditions: substrate 1 M in DMSO, 10 equiv of
orthoester, 5 mol % of catalyst, or py 2 equiv. b Isolated yields. c 3 equiv
of orthoester.
(6) (a) Osaheni, J. A.; Jenekhe, S. A. Macromolecules 1994, 27, 739.
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acid cataysts do not work in the absence of the DMSO solvent.
With the exception of Bi(OTf)3 and Hf(OTf)3, all of the metal
triflates gave improved yields when compared to the traditional
acid catalysts. The low reactivity of Bi(III) and Hf(III) may be
attributed to their larger ionic radii.11
(10) (a) Jenkins, G. L.; Knevel, A. M.; Davis, C. S. J. Org. Chem. 1961,
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1365.
4916
Org. Lett., Vol. 10, No. 21, 2008

Unfortunately, we found that even with the addition of
the DMSO the reaction yields significantly decreased when
less than 10 equiv of orthoester was used. Moreover, we
found that when the optimium reaction conditions, which
were established for our model reactions, were used with
ethyl orthobromoacetate12a (2c) and trimethylsilyl ethyl
orthopropiolate12b (2d), the yields of the target BBOs
decreased. We rationalized that the substituted orthoesters
were unstable to the current reaction conditions and set out
to further improve our reaction conditions. We explored the
use of pyridine as a cosolvent with DMSO since it has been
reported that it improved the yields of orthoester reactions.13
Table 2. Reaction of 1 and 4 with Various Orthoesters 2b-e
This modification was fortuitious, and we were able to reduce
the reaction temperatures and times while increasing the reaction
yields. While it was previously reported that the addition of
pyridine stabilizes the DMSO, we believe that this improvement
is a result of the removal of the acid coordinated with the DAHQ
and DAR, increasing their reactivity and preventing protic acid
catalyzed decomposition of the orthoesters. Using yttrium
triflate, the pure product 3a was obtained in 91% yield after
the addition of water to the reaction mixture and simple filtration
(entry 13). In the absence of the catalyst, the product was
obtained in 78% yield (entry 14). The advantages of this method
are the fact that the reaction occurred at a lower temperature,
preventing the formation of side products, and the desired
product can be precipitated out from the reaction mixture,
allowing for easy purification.
On the basis of the model reactions, we have determined the
optimum reaction conditions to be: substrate 1 M in DMSO, 2
equiv of pyridine, 3 equiv of orthoester, and 5% Y(OTf)3 for
DAHQ or La(OTf)3 for DAR. Using these conditions, we set
out to investigate the scope of the reaction with respect to the
orthoester (Table 2). We found that triethyl orthoacetate (2b)
and trimethylsilyl ethyl orthopropiolate12b (2d) could be em-
ployed to yield the corresponding BBOs in good yield.
However, the reaction of triethyl orthobromoacetate (2c) only
worked under traditional conditions, in the case of the trans-
derivative, and failed to yield product in the case of the cis-
derivative. The inability to synthesize benzobisoxazole 3c using
2c and our modified reaction conditions is most likely due to
the competing side reaction of the alkyl bromide of the
orthoester with DMSO.14 Thus, we explored the use of triethyl
orthochloroacetate (2e),12c which is less prone to side reactions
and gives good results using our reaction conditions. In all cases,
the products can be purified by simple recrystallization, and
this method can easily be scaled up to multigram quantities.
Thus, by utilizing the corresponding orthoesters, we can
incorporate methyl, bromomethyl, chloromethyl, and alkynyl
groups into the BBO’s. Compounds containing the latter three
a Isolated yields. b Synthesized using standard conditions.
groups cannot be prepared using traditional approaches. More-
over, direct bromination of 3b or 5b also fails to yield 3c and
5c.
To demonstrate the utility of the new BBOs as building
blocks for the synthesis of conjugated polymers, we first
attempted the synthesis of monomer 3f, by the oxidation of
compound 3b (Scheme 2). The reaction yields were very
Scheme 2. Synthesis of Benzobisoxazole Monomers
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16, 1831.
Org. Lett., Vol. 10, No. 21, 2008
4917

Scheme 3. Synthesis of Benzobisoxazole Polymer 7
low (15-20%). Thus, we explored the synthesis of diphos-
phonate ester 3g. Monomer 3g can be synthesized by the
Arbuzov reaction of 3c or 3e and triethylphosphite. The
polymerization of monomer 3g with 2,5-didodecyloxyben-
zaldehyde 615 by a Horner-Wadsworth-Emmons (HWE)
olefination reaction produced a red polymer in 86% yield
(Scheme 3). The resultant polymer was soluble in THF and
chloroform, and the structure of the polymer was verified
by 1H NMR spectroscopy. Gel permeation chromatography
(GPC) of the polymer showed monomodal distributions with
number average molecular weight (Mn) of 5672, PDI ) 1.77,
which corresponds to a number averaged degree of polym-
erization of approximately 8. We measured the emission and
absorption spectra of the polymer in solution and found that
the absorption maximum is 476 nm. The polymer exibits
green fluorescence with an emission maximum at 515 nm,
which is blue-shifted from the photoluminescence of MEH-
PPV (ca. 600 nm).16 This represents the first synthesis of a
PBO derivative which is soluble in aprotic organic solvents.
We are currently working to optimize the reaction conditions
to obtain a higher molecular weight polymer.
In summary, the reaction of orthoesters and DAHQ or
DAR affords new 2,6-disubstituted benzobisoxazoles in good
yields. Subsequent reactions can transform these compounds
into monomers for the synthesis of novel polymers containing
the electron-deficient benzobisoxazole moiety. Future efforts
will focus on the synthesis of other monomers based on these
materials, as well as device construction and evaluation of
the electronic and optical properties of oligomers and
polymers containing the benzobisoxazole moiety.
Acknowledgment. M.J.E. would like to thank the 3M
Foundation for a nontenured faculty grant. J.F.M. thanks the
Department of Education for a GAANN fellowship. We
thank Dr. Kamel Harrata and the Mass Spectroscopy
Laboratory of Iowa State University (ISU) for analysis of
our compounds and Dr. Tanay Kesharwani (ISU) and
Professor Gordon Miller (ISU) for helpful discussions
regarding this work.
Supporting Information Available: Experimental pro-
1
cedures and H and 13C NMR spectra for new compounds.
(14) (a) Nace, H.; Monagle, J. J. Org. Chem. 1959, 24, 1792. (b)
Kornblum, N.; Jones, W. J.; Anderson, G. J. J. Am. Chem. Soc. 1959, 81,
4113.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(15) Liao, J.; Wang, Q. Macromolecules 2004, 37, 7061.
(16) Chen, S. H.; Su, A. C.; Chou, H. L.; Peng, K. Y.; Chen, S. A.
Macromolecules 2004, 37, 167.
OL802011Y
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Org. Lett., Vol. 10, No. 21, 2008