Angewandte
Chemie
DOI: 10.1002/anie.201204367
Boron Heterocycles
Photochemical Synthesis of a Ladder Diborole: A New Boron-
Containing Conjugate Material**
Juan F. Araneda, Benedikt Neue, Warren E. Piers,* and Masood Parvez
Incorporation of main-group elements into organic frame-
works has emerged in the last decade as a promising route to
develop new materials.[1] For instance B,[2] Si,[3] P[4] or S[5]
impart interesting features to conjugated hydrocarbon frame-
works such as effective orbital interactions, change in dipole
moments, and diversity in coordination number. In particular,
“ladder” compounds[6] are attractive because, as acene
analogs, they are planar molecules and possess an extended
p-conjugated system, leading to highly desirable photophys-
ical and electronic properties like intense luminescence[7,8]
and high carrier mobility.[9] In 2003 Tamao and co-workers
reported the synthesis of the bis-silicon-bridged stilbene I by
an intramolecular reductive cyclization reaction.[10] Com-
pared to its carbon analogue, this compound has an intense
blue emission, which is red-shifted by about 60 nm.[10] Several
other examples, such as phosphonium borates[11] and bis-
phosphorus-bridged stilbenes[12] have also been reported.
carbenes,[16] unlike anthracene itself, absorbs in the visible
region because the HOMO–LUMO gap is strongly decreased
compared to its all-carbon analogue. The same effect was
found in the more extended boratetracene and borapenta-
cenes analogues.[17]
Apart from the promising materials that can be prepared
with boron-containing molecules, boron heterocycles are of
fundamental interest and have been used for decades to probe
the basic concepts of aromaticity and antiaromaticity.[18]
A
multitude of interesting borole derivatives have been
reported in the past few years,[19] which include metal-
containing,[20] tetrathienyl-substituted,[21] and the highly
Lewis-acidic perfluoropentaphenyl boroles.[22] In this context,
recent studies by Yamaguchi et al. on heteroarene-fused
boroles have provided more insight into the effect incorpo-
ration of the borole ring into extended p systems has on the
antiaromaticity of boroles.[5a,7] Despite the fact that a lower
antiaromatic character and higher stability was predicted for
compound III based on the electron-donating capability of
the thiophene unit, the fused borole showed a surprisingly
high antiaromatic character and air and moisture sensitivity.[7]
This is in contrast to the observations made for the
dibenzoborole derivative IV,[23] where the presence of the
benzene rings decreased the antiaromaticity of the borole
core.
With the remarkable properties of ladder, main group
element-containing p-systems and boroles in mind, we
focused our attention on the synthesis of hitherto unknown
bis-boron-bridged stilbene derivatives. Transmetalation of
Group 14 element-based heterocycles with RnBX3ꢀn is
a reliable method for making boracycles.[24] We thus first
attempted to synthesize the desired bis-boron-bridged stil-
bene core by treatment of Tamaoꢀs silole derivative I with
neat BBr3 (1008C, 48 h), but this approach failed. Therefore,
reductive routes akin to those pioneered by Tamao[10] were
employed in an effort to assemble the desired ladder diborole
framework. It was envisioned that reduction of the halobor-
ane-functionalized diaryl acetylene 1 would result in a cycli-
zation reaction to yield the targeted diborole.
Within the context of heteroacene chemistry, those based
on boron are an emerging class of materials that have been
used to prepare functional p-conjugated molecules for use as
organic light emitting diodes,[13] field effect transistors,[14] and
photovoltaics.[15] Arguably the most important characteristic
of a three-coordinate boron center situated in a p-conjugated
framework is the empty p-orbital, which allows for effective
delocalization of electrons throughout the whole system. The
transposition of carbon for the more electropositive boron
atom can also alter the HOMO–LUMO characteristics of
a compound significantly. For example, we found that 9-
boraanthracene derivatives II, stabilized by N-heterocyclic
The route to the precursor 1 is depicted in Scheme 1 and
relies on two consecutive boron–metal exchange reactions.
First, 2,2’-di-bromophenyl acetylene was lithiated at low
temperature with tBuLi then quenched with Me3SnCl. The
bis-dichloroborane was synthesized as an orange solid in 85%
yield by treating the distannane with an excess of BCl3.
Incorporation of the bulky aryl group was accomplished by
selective transmetalation of one chloro substituent per boron
with the 2,4,6-triisopropylphenyl (tipp) copper(I) reagent.[25]
Compound 1 was isolated as a colorless solid in 70% yield and
is a moisture-sensitive compound whose 11B NMR spectrum
[*] J. F. Araneda, B. Neue, Prof. W. E. Piers, M. Parvez
Department of Chemistry, University of Calgary
2500 University Drive N.W., Calgary, Alberta, T2N 1N4 (Canada)
E-mail: wpiers@ucalgary.ca
wpiers/
[**] Funding for this work was provided by NSERC of Canada in the form
of a Discovery Grant to W.E.P. B.N. thanks the Deutsche
Forschungsgemeinschaft for financial support in the form of
a Postdoctoral Fellowship.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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