612
Published on the web May 15, 2010
Synthesis, Structure, and Properties of a Dinaphthoazaborine
³
Tomohiro Agou, Hiroki Arai, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received March 1, 2010; CL-100187; E-mail: takayuki@chem.s.u-tokyo.ac.jp)
Table 1. Synthesis of dinaphthoazaborine 1
A 1,4-azaborine derivative fused by two naphthalene rings, a
1) n-BuLi, –78 °C
2) MesB(OMe)2
temp
dinaphthoazaborine was synthesized, and its molecular structure
was revealed by X-ray crystallographic analysis. UV-vis
absorption and fluorescence spectra of the dinaphthoazaborine
Mes
B
+
3
Et2O
N
Me
1
N
Me
7
indicated
a decrease in the HOMO-LUMO energy gap
compared with that of dibenzoazaborines. Electrochemical
measurements elucidated the enhanced electron-accepting char-
acteristics of the dinaphthoazaborine.
Entry
Temp
Product (yield/%)
1
2
-78 °C (3 h), then ¹78 °C to rt
-78 °C to rt
1 (56)
1 (26), 7 (36)
Boron-containing ³-conjugated molecules are an interesting
class of compounds because of their various optical, electronic,
or chemical properties, such as photoluminescence, electron-
accepting ability, and chemosensing of anions.1 Therefore,
recently the development of ³-extended boron compounds with
novel characteristics has been extensively investigated, and from
such a viewpoint we have also reported new functional
triarylboranes based on dibenzoazaborine 2 (Figure 1).2,3 Owing
to donor-acceptor interaction between the nitrogen and boron
atoms as well as a rigid framework, dibenzoazaborines and their
³-extended derivatives exhibit strong light absorption and
emission, indicating the potential of a 1,4-azaborine as a good
building unit for organic functional materials. The introduction
of larger aromatic systems, such as naphthalene or higher acenes
to the azaborine ring should further decrease the HOMO-LUMO
energy gap and improve the optoelectronic properties. In this
communication, we describe the synthesis and optical properties
of new azaborine derivative 1.4
Scheme 1 shows the preparation of the precursor 3 of 1. The
coupling reaction of known naphthalene derivatives 45 and 56
under standard palladium-catalyzed amination conditions gave
dinaphthylamine 6 in 77% yield. The nitrogen atom of 6 was
methylated under phase-transfer conditions to give precursor 3.
Dinaphthoazaborine 1 was synthesized in 56% yield by the
reaction of the dilithio derivative of 3 with MesB(OMe)2 at
low temperature (Table 1, Entry 1). When the temperature was
Figure 2. Molecular structure of 1. Thermal ellipsoid plots are
drawn at 50% probability level. Hydrogen atoms are omitted for
clarity. (a) Top view. (b) Side view. Mes and Me groups are omitted.
(c) Packing diagram. Short contacts (¯3.6 ¡) are indicated with dashed
lines.
raised soon after the addition of MesB(OMe)2, carbazole
derivative 7,7 a formal oxidative coupling product of the dilithio
derivative, was obtained as a major product (Entry 2).8 Under
these conditions, some boron-containing compounds would
work as oxidants to promote the C-C coupling reaction of the
dilithio derivative.
Dinaphthoazaborine 1 was obtained as air-stable red solid,
and its molecular structure was elucidated by X-ray diffraction
analysis of single crystals obtained from a saturated toluene/
hexane solution (Figure 2).9,10
The dihedral angle between the Mes group and the
azaborine ring of 1 (87°) is slightly larger than that of
dibenzoazaborine 2 (79°, an averaged value of two independent
molecules in the single crystals).3a As shown in Figure 2b, the
dinaphthoazaborine skeleton of 1 is butterfly-like, and the angles
between the two naphthalene rings is 15°, which is larger than
the bent angle between the two benzene rings of 2 (9°, an
averaged value of the two independent molecules). These
differences between the molecular structures of 1 and 2 may
come from the crystal packing, because DFT calculations
predicted almost the same geometries around the central
azaborine rings in the two compounds.11,12 In the crystal
structure of 1, two molecules of 1 formed a dimeric structure
through the short contacts between the peripheral parts of the
naphthalene rings. In addition, the dimeric motif was capped
Mes
Mes
Me
Me
B
B
N
N
Me
Me
1
2
Figure 1. Dinaphthoazaborine 1 and dibenzoazaborine 2.
Br
I
Br
Br
Br
i
ii
+
NH2
NH
NMe
3 (82%)
2
2
4
5
6 (77%)
Scheme 1. Reagents and conditions: (i) Pd(dba)2, DPPF, t-BuONa,
toluene, 100 °C; (ii) MeI, KOH, (n-Bu)4NI, THF, 55 °C.
Chem. Lett. 2010, 39, 612-613
© 2010 The Chemical Society of Japan