Stepwise synthesis and properties of a 9,10-dihydro-9,10-diboraanthracene

Article abstract of DOI:10.1016/j.tetlet.2010.07.068

A stepwise synthetic method for a 9,10-dihydro-9,10-diboraanthracene was developed, and the optical and electrochemical properties as well as the molecular structure of the 9,10-dihydro-9,10-diboraanthracene were elucidated. The 9,10-dihydro-9,10-diboraanthracene exhibited high complexation ability against fluoride ion, and the complexation was accompanied by the enhancement of photoluminescence emission, probably due to intramolecular charge transfer between the two boron atoms.

Full text of DOI:10.1016/j.tetlet.2010.07.068

Tetrahedron Letters 51 (2010) 5013–5015
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stepwise synthesis and properties of a 9,10-dihydro-9,10-diboraanthracene
Tomohiro Agou a,b, Masaki Sekine c, Takayuki Kawashima c,
*
a Kyoto University Pioneering Research Unit for Next Generation, Kyoto University, Gokasho, Uji 611-0011, Japan
b Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Japan
c Departmetnt of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 April 2010
Revised 9 July 2010
Accepted 14 July 2010
Available online 16 July 2010
A stepwise synthetic method for a 9,10-dihydro-9,10-diboraanthracene was developed, and the optical
and electrochemical properties as well as the molecular structure of the 9,10-dihydro-9,10-diboraanthra-
cene were elucidated. The 9,10-dihydro-9,10-diboraanthracene exhibited high complexation ability
against fluoride ion, and the complexation was accompanied by the enhancement of photoluminescence
emission, probably due to intramolecular charge transfer between the two boron atoms.
Ó 2010 Elsevier Ltd. All rights reserved.
p
-Conjugated molecules bearing boron atoms as electron- or Le-
Synthesis of 9,10-dihydro-9,10-diboraanthracene 3 is shown in
Scheme 2.9 1-Bromo-2-iodobenzene (1) was converted to the cor-
responding Grignard reagent using i-PrMgCl and treated with
B(OMe)3 to give an intermediate borinic ester. This borinic ester
was reacted with MesMgBr to afford triarylborane 2 in a moderate
yield. In the second step, 2 was treated with t-BuLi, B(OMe)3 and
MesMgBr successively to give 9,10-dihydro-9,10-diboraanthracene
wis base acceptors have been investigated extensively as multi-
functional organic materials for organic semi-conductors, photon-
ics, and chemosensors,1 and the accumulation of boron atoms in
conjugated systems is expected to improve these characteristics
effectively.2 A 1,4-diborin has two boron atoms in a cyclohexadiene
ring, and thus the strong electronic interaction between the boron
atoms through the p-orbitals is anticipated. Several dibenzo-fused
1,4-diborin derivatives, 9,10-dihydro-9,10-diboraanthracenes were
synthesized, and their properties were investigated including
electron acceptability,3 charge transfer complex formation with
electron donors (e.g., TTF),3b,c activation of olefin polymerization
catalyst,4 photo-luminescene,5 and complexation behavior to Lewis
bases6 or transition metals.7 Owing to these attractive features,
9,10-dihydro-9,10-diboraanthracenes will be applied to molecular
devices by the incorporation of proper functional groups, like elec-
tron-donating substituents.
One drawback in the development of functionalized 9,10-dihy-
dro-9,10-diboraanthracenes is the lack of a general synthetic
method. As shown in Scheme 1, previous synthetic methods em-
ployed transmetallation of silicon or tin atoms to boron atoms
using BX3, quite a strong Lewis acid.8 Acid-sensitive functional
groups can not be tolerated under such harsh conditions.
Previous method
R2
B
X
ER1
ER1
B
3
3
R2M
BX3
B
X
B
R2
E = Si, Sn
X = Cl, Br
Alternative method (this work)
1) t-BuLi
R2
B
OMe
Br
B
R2M
2) B(OMe)3
2 BR1
B
R1
B
R1
In this Letter, we describe a new and simple synthetic method-
ology for a 9,10-dihydro-9,10-diboraanthracene framework under
ambient conditions where various functional groups can resist
(Scheme 1, bottom). In addition, the molecular structure, optical
properties, electrochemical properties, and complexation abilities
of a 9,10-dihydro-9,10-diboraanthracene are also reported.
Scheme 1. Comparison of two synthetic methods of 9,10-dihydro-9,10-
diboraanthracenes.
* Corresponding author at Present address: Faculty of Science, Gakushuin
University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan. Tel.: +81 3 3986
0221.
Scheme 2. Synthesis of 9,10-dihydro-9,10-diboraanthracene 3. Reagents and
conditions: (a) i-PrMgCl, THF, ꢀ78 °C; (b) B(OMe)3, ꢀ78 °C to rt; (c) MesMgBr,
reflux, 55% over three steps; (d) t-BuLi, ꢀ78 °C, Et2O; (e) B(OMe)3, ꢀ78 °C to rt; (f)
MesMgBr, THF, reflux; 47% over three steps.
E-mail address: Takayuki.Kawashima@gakushuin.ac.jp (T. Kawashima).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.068

5014
T. Agou et al. / Tetrahedron Letters 51 (2010) 5013–5015
3. Compounds 2 and 3 were obtained as colorless and pale yellow
solids, respectively, and both boranes can be handled and stored
under air.
red-shift of the absorption tail in THF may reflect the nature of
the intramolecular charge transfer absorption.
In cyclohexane, 3 exhibited a fluorescence emission centered at
Single crystals of 3 were obtained by slow evaporation of a sat-
urated Et2O solution, and its molecular structure was elucidated by
X-ray crystallographic analysis (Fig. 1).10 The fused benzene rings
did not show large bond alternation (1.39–1.43 Å), and the B–C
bond lengths are in the range of typical B(sp2)-C(aryl) bond lengths
(cf. 1.57–1.59 Å in BPh3).11 These structure parameters are compa-
rable to those of a 9,10-dihydro-9,10-diboraanthracene bearing
methyl groups on the boron atoms3c,7b and indicate that the contri-
413 nm (U 0.02). Small Stokes shift (<10 nm) indicates the molec-
ular rigidity of the optically excited 3. Changing the solvent from
cyclohexane to THF resulted in a red shift of the emission maxi-
mum by 70 nm and a broadening of the band shape (kem 483 nm,
U
0.05). These results suggest that the emissive state of 3 is highly
polarized. As described above, the lowest excited state of 3 is an
intramolecular charge transfer state, which is effectively stabilized
in more polar THF, resulting in the observed solvatochromic shift.
Because the radiative de-activation from this state is symmetri-
cally forbidden (f = 0), the fluorescence quantum yields were small.
The electron acceptability of 3 was investigated by using cyclic
voltammetry. The measurements were performed in THF contain-
ing (n-Bu)4N ClO4 (0.1 M) as a supporting electrite at 298 K with a
scan rate of 0.1 V sꢀ1 (Fig. 3). Compound 3 exhibited two reversible
one-electron reduction waves at E1/2 = ꢀ1.82 and ꢀ2.78 V (vs
Cp2Fe/Cp2Fe+), indicating the generation of radical anion and dian-
ion, respectively. The shape of the voltammogram did not change
after five cycles, suggesting the durability of the anionic species.
Because the first reduction occurred at a much lower potential
compared with that of Mes3B (E1/2 = ꢀ2.79 V), 3 should work as a
good electron acceptor. Meanwhile, the enhanced electron accept-
ability of 3 suggested its high Lewis acidity, and thus we next
investigated the complexation ability of 3 against fluoride ion, a
typical Lewis base against boron Lewis acids.
bution of cyclic
ring is marginal, and thus the electronic structure of the diborin
ring of 3 can be classified as a non-aromatic rather than a 4
p-electron delocalization over the central diborin
p
anti-aromatic. A NICS(1) value calculated on model compound 30
(+3.5) also indicated that the diborin ring of 3 has only weak
anti-aromaticity.12 In contrast, 1,2-diborins were reported to have
substantial anti-aromatic character, and thus the position of the
boron atoms in the six-membered ring systems should affect the
electronic structure substantially.13
UV–vis and fluorescence spectra of 3 were recorded in cyclo-
hexane and THF (Fig. 2). Compound 3 showed a strong absorption
band at 349 nm bearing a small shoulder in both the solvents.14
*
This strong absorption was attributed to the
p–p excitations of
the 9,10-dihydro-9,10-diboraanthracene, on the basis of TD–DFT
calculations (kcalcd 339 nm, f 0.1253).15 The calculations also indi-
cated that 3 has several forbidden excitations at lower energy lev-
els (kcalcd 442, 422, 409, 408, 377 nm), which can be described as an
intramolecular charge transfer from the electron-rich mesityl
groups to the electron-accepting 9,10-dihydro-9,10-diboraanthra-
cene ring. The observed shoulder-like absorption around 400 nm
is thought to correspond to these forbidden transitions. Small
In THF, 3 (5.2 ꢁ 10ꢀ5 M) was mixed with a THF solution of (n-
Bu)4NF, and the reaction was monitored with UV–vis and fluores-
cence spectroscopy (Fig. 4a). The addition of fluoride ion (up to
1 equiv vs 3) resulted in the decay of the absorption band around
349 nm and in the development of a new band centered at
280 nm with a set of isosbestic points. The new band was assigned
to an intramolecular charge transfer absorption between the fluo-
roborate moiety and the trivalent boron atom in [3-F]ꢀ, ꢀjudging
from TD to DFT calculation on model compound [30-F] (kcalcd
378 nm). The increase in the absorption ceased, when just 1 equiv
of fluoride ion was added, revealing that the formation of [3-F]ꢀ
was quantitative in this condition which was also shown by the
large complexation constant (K = 2(1) ꢁ 108 Mꢀ1).16 Further addi-
tion of fluoride ion resulted in the decrease in the absorption of
[3-F]ꢀ, but this change did not finish even after the addition of
3 equiv of fluoride ion. Another isosbestic point was observed at
246 nm, suggesting that this spectral change is corresponding to
the conversion of [3-F]ꢀ to another species rather than the decom-
position of [3-F]ꢀ.17
The complexation could also be monitored with fluorescence
spectroscopy (Fig. 4b). Corresponding to the blue shift of the UV–
vis absorption, the emission maximum also showed hypsochromic
shift from 483 to 400 nm. Although the absorbance at the excita-
tion light (kex 348 nm) decreased steeply (Fig. 4a), apparent fluo-
Figure 1. Thermal ellipsoid plot of 3 (50% probability level). Hydrogen atoms are
omitted. Selected bond lengths (Å) and angles (°): B–C(Ar), 1.560(2), 1.571(2); B–
C(Mes), 1.589(2); C(Ar)–B–C(Ar), 118.33(12); C(Ar)–B–C(Mes), 120.08(12),
121.59(13).
Figure 2. UV–vis and fluorescence spectra of 3. Black line: in cyclohexane. Gray
line: in THF.
Figure 3. Cyclic voltammogram of 3.

T. Agou et al. / Tetrahedron Letters 51 (2010) 5013–5015
5015
tor, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk or http://ccdc.cam.ac.uk).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.07.068.
References and notes
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Figure 4. UV–vis and fluorescence spectral change of 3 ([3] = 5.2 ꢁ 10ꢀ5 M) upon
addition of (n-Bu)4NF in THF at 298 K. (a) UV–vis spectra. (b) Fluorescence spectra.
The X marks indicate scattered excitation light (348 nm).
8. (a) Bieller, S.; Zhang, F.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Wagner, M.
Organometallics 2004, 23, 2107; (b) Müller, P.; Huck, S.; Köppel, H.; Pritzkow,
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rescence intensity did not change very much. Therefore, the fluo-
rescence quantum yield itself should improve by the formation
of [3-F]ꢀ. In fact, the quantum yield was estimated to be 0.15,
when 1 equiv of fluoride ion was added.18 Interestingly, addition
of excess fluoride ion enhanced the emission intensity (Fig. 4b,
middle), and the fluorescence quantum yield reached 0.24 or
0.37 upon addition of 1.4 equiv or 2.0 equiv of fluoride ion, respec-
tively. Because the formation of [3-F]ꢀ was completed by addition
of 1 equiv of fluoride ion, the emission enhancement should not
originate from the progress of the formation of [3-F]ꢀ. The origin
of this strange behavior is still unclear.
In conclusion, the stepwise synthetic route to the 9,10-dihydro-
9,10-diboraanthracene was developed. This method can be utilized
for the synthesis of 9,10-dihydro-9,10-diboraanthracene deriva-
tives bearing acid-sensitive functional groups. The 9,10-dihydro-
9,10-diboraanthracene has a planar structure, and the central dibo-
rin ring is classified as a non-aromatic system on the basis of crystal-
lographic analysis and GIAO calculations. UV–vis and fluorescence
spectroscopy as well as theoretical calculations revealed that unique
photo-physical characteristics of the 9,10-dihydro-9,10-diboraan-
thracene, and electrochemical measurement showed the enhanced
electron-accepting property of this compound. The 9,10-dihydro-
9,10-diboraanthracene formed complexes with fluoride ions, result-
ing in the change of the absorption and fluorescence.
11. Zettler, F.; Hausen, H. D.; Hess, H. J. Organomet. Chem. 1974, 72, 157.
12. DFT calculations at B3LYP level of theory were performed on model
compounds 30 and [30-F]ꢀ bearing 2,6-dimethylphenyl groups instead of
mesityl groups in real molecules 3 and [3-F]ꢀ using GAUSSIAN 03 program
package, and 6-31G(d) and 6-31G+(d) level of theory were employed for
geometry optimization and single point calculations, respectively. Frisch, M. J.;
Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.;
Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega,
N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.;
Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo,
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C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz,
J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-
Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03,
Revision C.02; Gaussian: Wallingford, CT, 2004.
13. Wakamiya, A.; Mori, K.; Araki, T.; Yamaguchi, S. J. Am. Chem. Soc. 2009, 131,
10850.
14. 9,10-Dihydro-9,10-diboraanthracene derivatives bearing smaller substituents
on the boron atoms were reported to form complexes with THF, resulting in
blue-shift of the absorption maxima unlike 3. See Ref. 3b.
15. HOMO, HOMO-1, HOMO-2, and HOMO-3 of 3 were mixtures of occupied
orbitals of the mesityl groups, and the highest occupied -orbital on the 9,10-
dihydro-9,10-diboraanthracene skeleton was found as HOMO-4. On the other
p-
Acknowledgments
p
*
This work was partially supported by Grants-in-Aid for the Glo-
bal COE Program (Chemistry Innovation through Cooperation of
Science and Engineering) and for the Scientific Researches from
MEXT and JSPS. We also thank Tosoh Finechem Corp. for the gen-
erous gifts of alkyllithiums.
hand, LUMO and LUMO+1 of
diboraanthracene formed by the mixing of 2p (B) orbitals and
the benzene rings. See the Supplementary data for detail.
3
were
p
orbitals of 9,10-dihydro-9,10-
*
*
p orbitals of
16. Because the complexation constant was too large, the estimation quality was
not good.
17. The reactioin of 3 and excess amount of (n-Bu)4NF (6 equiv) in THF-d8 seemed
to afford only monofluoroborate [3-F]ꢀ, judging from 1H NMR spectrum, even
at low temperature (193 K). However, negative-mode FAB-MS of this mixture
showed ion peaks at m/z 693 and 451 that correspond to (n-Bu4N)+ or H+
adduct of difluoroborate [3ꢀ2F]2ꢀ, respectively. The presence of the isosbestic
point in the reaction of [3-F]ꢀ and more than 1 equiv of fluoride ion further
indicates the formation of the difluoroborate. If the formation of the
difluoroborate is supposed, its formation constant was calculated to be
Supplementary data
Syntheses and characterization data of 2 and 3; optimized
geometry and plots of molecular orbitals of 30 and [3-F]ꢀ. Crystal-
lographic data for the structural analysis have been deposited with
the Cambridge Crystallographic Data Center (CCDC-774409). Copy
of this information can be obtained free of charge from The Direc-
K = 1.4(8) ꢁ 106 Mꢀ1
.
18. The fluorescence quantum yields after the complexation were estimated by a
comparative method based on the absolute value for 3 in THF (0.05).