Synthesis of tris(4-amino-2,6-dimethylphenyl)borane and facile extension of its π-conjugated system by utilizing the reactivity of the amino groups

Article abstract of DOI:10.1016/j.tetlet.2013.03.080

Tris(4-amino-2,6-dimethylphenyl)borane (1), a triarylborane bearing an unsubstituted amino group on all of its aryl substituents, and related triarylboranes were synthesized via Ullmann condensation. The facile dehydration reactions of 1 with benzaldehyde and nitrosobenzene gave tris[4- (benzylidenamino)-2,6-dimethylphenyl]borane and tris[2,6-dimethyl-4-(phenylazo) phenyl]borane, respectively. These triarylboranes bear an extended π-conjugated system bridged by a nitrogen-containing π-linker on each of their aryl groups. UV-vis absorption spectra and theoretical calculations revealed that the π-conjugated system of the triarylborane was effectively extended by utilizing the reactivity of the amino groups.

Full text of DOI:10.1016/j.tetlet.2013.03.080

Tetrahedron Letters 54 (2013) 2817–2820
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of tris(4-amino-2,6-dimethylphenyl)borane and facile extension
of its
p-conjugated system by utilizing the reactivity of the amino groups
⇑
⇑
Junro Yoshino , Yuri Nakamura, Shizuka Kunitomo, Naoto Hayashi, Hiroyuki Higuchi
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama, Toyama 930-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Tris(4-amino-2,6-dimethylphenyl)borane (1), a triarylborane bearing an unsubstituted amino group on
all of its aryl substituents, and related triarylboranes were synthesized via Ullmann condensation. The
facile dehydration reactions of 1 with benzaldehyde and nitrosobenzene gave tris[4-(benzylidenami-
no)-2,6-dimethylphenyl]borane and tris[2,6-dimethyl-4-(phenylazo)phenyl]borane, respectively. These
Received 12 February 2013
Revised 14 March 2013
Accepted 18 March 2013
Available online 25 March 2013
triarylboranes bear an extended
of their aryl groups. UV–vis absorption spectra and theoretical calculations revealed that the
p
-conjugated system bridged by a nitrogen-containing
p
-linker on each
-conju-
p
Keywords:
Triarylborane
Extended p-conjugated system
gated system of the triarylborane was effectively extended by utilizing the reactivity of the amino groups.
Ó 2013 Elsevier Ltd. All rights reserved.
Ullmann condensation
Dehydration
UV–vis spectrum
Theoretical calculation
Organoboron compounds are known as Lewis base sensors, a
function rendered by the Lewis acidity of the boron atom. In
particular, triarylboranes are highly proficient Lewis-base sensors
because they have strong Lewis acidity, and their extended
systems with CH@N3 or N@N4
pared to date. However, all of them have only one aryl group con-
nected to a nitrogen-containing -linker per boron atom, and no
triarylboranes bearing nitrogen-containing double bonds at all of
their aryl groups have been reported. Trigonally expanded -con-
p-linking moieties have been pre-
p
p-conjugated systems, connected by the vacant p orbital of the
p
boron atom, confer dramatic changes in their physical properties
upon complexation with a Lewis base. Such strong responses result
jugated systems are very appealing owing to their potential as
near-infrared dyes,5 two-photon absorption materials,6 and non-
from structural changes to the
p
-conjugated system.1 Reported
linear optical materials.7,8 Therefore, the extension of the
p-conju-
examples include fluoride- and cyanide-sensing compounds,
which can recognize fluoride and/or cyanide ions, they are toxic,
and authenticate their existence by converting recognition interac-
tions into physical phenomena, such as color and fluorescence
changes.1,2
gated system of a triarylborane at all three of its aryl groups using
nitrogen-containing double bonds is expected to be a facile method
for accessing sophisticated triarylborane-based chemosensors
bearing trigonal
ties. Here we report the synthesis of tris(4-amino-2,6-dimethyl-
phenyl)borane (1), triarylborane bearing an unsubstituted
p-conjugated systems and for tuning their proper-
The structure of the
p-conjugated system on the aryl groups of
a
triarylboranes is a key factor regulating their physical properties
and output behavior as chemosensors, such as color and/or fluores-
cence changes. Although many triarylboranes bearing various
extended
p-conjugated systems have been synthesized to tune
their properties, the extension of the
p-conjugated systems of
triarylboranes is not easily accomplished. Utilizing the simple
and reliable reactivity of an amino group to afford a nitrogen-
containing double bond is one possible approach to readily extend
the p-conjugated systems of triarylboranes. Using this approach, a
few examples of triarylboranes bearing extended
p-conjugated
⇑
Corresponding authors. Tel.: +81 76 445 6618; fax: +81 76 445 6549 (J.Y.); tel.:
+81 76 445 6616; fax: +81 76 445 6549 (H.H.).
E-mail addresses: yoshino@sci.u-toyama.ac.jp (J. Yoshino), higuchi@sci.u-toya-
ma.ac.jp (H. Higuchi).
Scheme 1. Compound 1 and extension of its p-conjugated system.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.080

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J. Yoshino et al. / Tetrahedron Letters 54 (2013) 2817–2820
Scheme 2. Synthesis of amine-substituted triarylboranes 1 and 3–6.
amino group on all of its aryl substituents, and the facile extension
of its -conjugated system by utilizing the reactivity of the amino
p
groups (Scheme 1).
BocNH-substituted triarylboranes 3–5 were synthesized by Ull-
mann condensation between bromo-substituted triarylborane 29
and tert-butyl carbamate (Scheme 2). The number of BocNH groups
introduced can be controlled by changing the coupling conditions.
Deprotection of the Boc-protecting groups of 3 and 5 gave NH2-
substituted triarylboranes 1 and 6, respectively, in good yields
(Scheme 2). Simple dehydrative condensation of compound 1 with
benzaldehyde and nitrosobenzene afforded compounds 7 and 8,
respectively (Scheme 3). Structures of compounds 1 and 3–8 were
confirmed by 1H, 11B, and 13C NMR and high-resolution MS.
In their 1H NMR spectra, symmetrically substituted
triaryboranes 1, 3, 7, and 8 showed one singlet which was assigned
to the methyl group, indicating their structures have D3 symmetry
at the tris(dimethylphenyl)borane core in CDCl3. In contrast,
methyl group signals in unsymmetrically substituted triarylbor-
anes 4–6 showed one sharp and two broad singlets with the inte-
gral ratio of 6:6:6. These spectral behaviors are consistent with
structures having C2 symmetry with the suppression of rotation
about the B–C bonds. To determine the rotation barrier about the
B–C bonds, VT-1H NMR spectra of compound 6 were acquired
(Fig. 1). The Gibbs energy of activation of rotation about the B–C
bonds of compound 6 was estimated to be 68 1 kJ/mol on the
basis of the temperature of coalescence of the methyl signals
(45 °C). The rotations about the B–C bonds in compounds 1, 3–5,
7, and 8 are considered to be suppressed to a similar degree as that
in compound 6.
Figure 1. VT-1H NMR spectra of compound 6.
Figure 2. UV–vis absorption spectra of compounds 1–3, 7, and 8 in chloroform.
Table 1
Absorption wavelengths (kabs) in chloroform, and calculated energy levels (E) of the
HOMO and LUMO of triarylboranes 1–3 and 7–9
The UV–vis spectra of trigonally substituted triarylboranes 1–3,
7, and 8 showed absorption maxima at 334–372 nm in chloroform,
which were assigned to the
p–
p⁄ transition (Fig. 2 and Table 1).
Compound 8 has an additional absorption maximum at 442 nm,
which was assigned to the n–p⁄ transition.10 In compounds 7
Compound
R
kabs (nm)
EHOMO (eV)
À5.01
ELUMO (eV)
À1.09
and 8, by connecting the
using the N-containing
tion maxima in comparison with those of compounds 1–3, bearing
the smaller -conjugated systems, is observed. These spectral
behaviors suggest that the -conjugated systems of compounds 7
and 8 are effectively extended by introducing the N-containing
-linkers. The longer
p⁄ absorption maxima of compound 8, in
comparison with that of 7, is considered to result from the
planarity of the azo group-connecting -conjugated systems.
p
-conjugated moieties to the aryl groups
p⁄ absorp-
1
2
3
7
8
9
NH2
Br
NHBoc
N@CHPh
N@NPh
H
362
334
354
367
p
-linkers, a red shift of their
p–
a
a
—
—
À5.48
À5.64
À6.00
À6.17
À1.48
À1.96
À2.58
À1.70
p
p
372, 442
b
—
p
p–
a
Not calculated.
Not measured.
b
p
Scheme 3. Synthesis of compounds 7 and 8.

J. Yoshino et al. / Tetrahedron Letters 54 (2013) 2817–2820
2819
Figure 3. Molecular orbital diagrams of compounds 7 and 8. (a and b) HOMO of 7 (degenerated), (c) LUMO of 7, (d and e) HOMO of 8 (degenerated), and (f) LUMO of 8.
resulting in the longer wavelength of the
maximum.
p–
p⁄ absorption
As a preliminary experiment, complexation of triarylborane 8
with fluoride ion was investigated. Addition of tetrabutylammo-
nium fluoride to a THF-d8 solution of compound 8 gave the solu-
tion color change from orange to yellow as a result of formation
of fluoroborate, which was confirmed by 1H and 11B NMR. This
fluoroborate gave back compound 8 upon hydrolysis.
In conclusion, triarylborane 1 bearing an unsubstituted amino
group on all of its aryl substituents was synthesized via Ullmann
condensation. Additional
to each of the aryl groups of 1 by facile dehydration reactions to
provide triarylboranes 7 and 8, bearing N-containing -linkers.
UV–vis absorption spectra and theoretical calculations revealed
that the -conjugated system of the triarylborane was effectively
extended by utilizing the reactivity of the amino groups. The meth-
od developed in this study to readily expand -conjugated systems
of triarylboranes in a trigonal manner may provide new sophisti-
cated Lewis base sensors. The further extension of -conjugated
p-conjugated moieties were connected
p
p
Figure 4. Energy levels of the frontier molecular orbitals of compounds 1, 3, and 7–
9. The upper and lower lines denote the energy levels of the LUMO and HOMO,
respectively.
p
p
systems of triarylboranes based on this study and evaluation of
the chemosensing ability of the triarylboranes bearing N-contain-
ing p-linkers are currently underway.
To evaluate the effectiveness of the N-containing
p-linkers on
the extension of the -conjugated system of the triarylborane,
p
molecular orbitals of 1, 3, 7, and 8 were calculated by a DFT meth-
od at the B3LYP/6-31G(d) level of theory. Those of tris(2,6-dimeth-
ylphenyl)borane (9) were also calculated. In the calculations, C3
symmetry was imposed on the structures of the triarylboranes.
In 7 and 8, their HOMOs and LUMOs are delocalized over the entire
Acknowledgments
We gratefully acknowledge the financial support provided by
Tamura Science and Technology Foundation and the Hokuriku
Bank, Ltd.
molecule, indicating that the
p-conjugated system of the triarylbo-
rane is effectively extended by incorporating N-containing
p
-link-
ers at each aryl group (Fig. 3). The calculation of the HOMO/LUMO
energy levels for the triarylboranes shows that the energy levels of
the LUMO of compounds 7 (À1.96 eV) and 8 (À2.58 eV) are low-
ered more than those of compounds 1, 3, and 9 (À1.09, À1.48,
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
03.080.
and À1.70 eV) by the extension of the
results in the red shift of the
UV–vis spectra for 7 and 8 (Fig. 4 and Table 1). The more planar
-linker moiety in compound 8 (dihedral angle (Ph)N@N–C–C in
p
-conjugated system, which
p–
p⁄ absorption maxima in the
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