Synthesis of dibenzochalcogenaborins and systematic comparisons of their optical properties by changing a bridging chalcogen atom

Article abstract of DOI:10.1002/asia.200800302

Novel hetero-π-conjugated compounds (dibenzochalcogenaborins) with the same molecular framework, bearing a boron atom as an acceptor and chalcogen atoms as a donor, were synthesized, and systematic comparisons among these molecules were performed. X-ray c

Full text of DOI:10.1002/asia.200800302

FULL PAPERS
DOI: 10.1002/asia.200800302
Synthesis of Dibenzochalcogenaborins and Systematic Comparisons of Their
Optical Properties by Changing a Bridging Chalcogen Atom
Junji Kobayashi, Keiko Kato, Tomohiro Agou, and Takayuki Kawashima*[a]
42
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2009, 4, 42 – 49

Abstract: Novel hetero-p-conjugated
compounds (dibenzochalcogenaborins)
with the same molecular framework,
bearing a boron atom as an acceptor
and chalcogen atoms as a donor, were
synthesized, and systematic compari-
sons among these molecules were per-
formed. X-ray crystallographic analysis
of these molecules showed similar
structures with high planarity. UV/Vis
spectroscopy and theoretical calcula-
tions revealed that the absorption
maxima and the HOMO–LUMO gap
changed by systematically changing the
bridging chalcogen atom. Dibenzooxa-
borin and dibenzothiaborin showed
fluorescence emission both in solution
and in the solid state with a small
Stokes shift, indicating the high rigidity
of these compounds. On the other
hand, dibenzoselenaborin exhibited a
very weak fluorescence as a result of
the heavy atom effect.
Keywords: conjugation
functional calculations
·
·
density
fluores-
cence spectroscopy · heterocycles ·
UV/Vis spectroscopy
Introduction
We have recently focused our interests on dibenzohetera-
borins, which are 9,10-dihydroanthracene derivatives con-
taining a boron atom and another main group element. The
rigid and planar structures of dibenzoheteraborins are ex-
pected to provide synergistic effects between p-orbitals and
the boron atom and/or the other bridging main group ele-
ment. Despite such interesting features, only a few examples
of dibenzoheteraborins, (e.g., dibenzoazaborine,[4] dibenzo-
diborin,[5] dibenzooxaborin,[6] and dibenzothiaborin[7]) have
been reported. A detailed study on the optical properties of
these compounds was performed only on dibenzoazaborine.
Actually, dibenzoazaborines showed acridine-like optical
properties with moderately strong fluorescence emission.
We have synthesized dibenzophosphaborin,[8] the heavier
analog of dibenzoazaborine, and revealed its UV/Vis ab-
sorption to be similar to that of dibenzoazaborine, while the
photoluminescent properties of dibenzophosphaborin are
quite different from those of dibenzoazaborine owing to the
structural difference between them. Moreover, the optical
properties of dibenzophosphaborin, as well as their reactivi-
ty, are varied by a modification on the phosphorus atom.
We have also reported the dibenzoazaborines and dibenzo-
thiaborins extend in a ladder-type fashion and showed that
their optical properties and reactivity are strongly influenced
by the bridging main group elements.[9] From these results,
the optical properties and reactivity of dibenzoheteraborins
can be tuned by changing the bridging main group element.
In the course of our study, we attempt to construct a library
p-Conjugated molecules bearing main group elements have
attracted much attention in the past decade. Electronic com-
munication between p-orbitals and main group elements
can decrease the HOMO–LUMO energy gap by raising the
HOMO energy level and/or lowering the LUMO energy
level, which leads to excellent photoelectronic properties
like photoluminescence. These molecules are utilized as or-
ganic light-emitting devices (OLED), organic field-effect
transistors (OFET), and so on, in various fields.[1] These
properties are strongly influenced by the properties of the
main group elements (i.e., electron-donating properties or
electron-withdrawing properties, caused by the lone pair,
the vacant orbital, or the lower-lying s* orbital). In other
words, change of the main group elements can tune the elec-
tronic and optical properties of these molecules. From such
a viewpoint, various hetero-p-conjugated molecules contain-
ing a main group element, not only electron donors like
oxygen, nitrogen, and sulfur, but also electron acceptors like
boron, silicon, and phosphorus, have been reported.[2]
Among these compounds, boron-containing hetero-p-conju-
gated molecules are very attractive because their vacant 2p
orbital can strongly interact with p-orbitals. Moreover, the
vacant 2p orbital is responsive to external stimuli (i.e.,
Lewis bases). Thus, tricoordinate boron compounds are uti-
lized as optical devices, molecular switches, and molecular
sensors.[3]
[a] Dr. J. Kobayashi, K. Kato, Dr. T. Agou, Prof. Dr. T. Kawashima
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5800-6899
E-mail: takayuki@chem.s.u-tokyo.ac.jp
Chem. Asian J. 2009, 4, 42 – 49
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43

T. Kawashima et al.
FULL PAPERS
of dibenzoheteraborins with various bridging main group el-
ements.
are required to isolate the tricoordinate boron compounds.
In the case of dibenzoheteraborins, however, only one mesi-
tyl group is enough to protect the boron center as a result of
the conformation, in which the mesityl group is almost per-
pendicular to the chalcogenaborin rings. 11BNMR of diben-
zochalcogenaborins 1a–c showed signals at 52 ppm, 54 ppm,
and 57 ppm, respectively. These values are within the typical
range of tricoordinate boron compounds. The chemical
shifts are slightly shifted up-field on going up the periodic
table for the choice of the bridging chalcogen atoms. Such
up-field shifts may come from the strength of the electronic
donation (1a>1b>1c). This would mean that the differ-
ence in electronic donation is not significant since the differ-
ence in the chemical shifts are very small.
In this paper we report the synthesis of dibenzochalcoge-
naborins containing group 16 elements (oxygen, sulfur, and
selenium) with the same framework. We also make system-
atic comparisons on the structures and optical properties of
dibenzochalcogenaborins, which can provide information
about the influences of bridging main group elements on the
optical properties of dibenzoheteraborins.
Results and Discussion
Synthesis
Synthesis of dibenzochalcogenaborins was achieved using a
method similar to that of the dibenzoazaborines and diben-
zophosphaborins shown in Scheme 1. Dilithiation of bis(2-
Structures
Single crystals of 1a–c were obtained by slow evaporation
from saturated solutions of CH2Cl2/MeCN for 1a, THF for
1b, and CHCl3/MeOH for 1c, respectively. ORTEP draw-
ings of 1a–c are shown in Figure 1. In the cases of 1a and
1b, there are two independent molecules in the unit cell,
however, only one of them is shown because they have
quite similar structures. Their bond lengths and angles are
summarized in Table 1. The dibenzochalcogenaborins
ꢀ
showed similar structures. The B C bond lengths in diben-
zochalcogenaborin ring of 1a–c did not show a significant
Scheme 1. Synthesis of dibenzochalcogenaborins 1a–1c. a) i) tBuLi
(4.4 equiv), ꢀ788C; ii) MesB
A
change in spite of varying the bridging chalcogen atom. On
ꢀ ꢀ
the other hand, a slight widening of the C B C bond angle
ꢀ
ꢀ
bromo-4,5-dimethoxyphenyl) ether 2a, bis(2-bromo-4,5-di-
methoxyphenyl) sulfide 2b, and bis(2-bromo-4,5-dimethoxy-
phenyl) selenide 2c[10] was achieved using 4.4 molar equiva-
lents of tBuLi at ꢀ788C. The generated dilithio derivatives
and a narrowing of the C Ch C angle are observed. The di-
were allowed to react with MesBACHTUNRGTNEUNG(OMe)2 to give the desired
dibenzochalcogenaborins as pale yellow solids in moderate
yields. The dibenzochalcogenaborins are stable in air and
moisture. Usually, tricoordinate boron compounds are
known to be unstable to oxidation when the steric protec-
tion around the boron center is not sufficient and more than
two steric protecting groups, the mesityl group for example,
Abstract in Japanese:
Figure 1. ORTEP drawings of chalcogenaborins 1a–c (50% probability).
Hydrogen atoms are omitted for clarity. Mesityl and methoxy groups are
not shown in side views. a) 1a, top view. b) 1a, side view. c) 1b, top view.
d) 1b, side view. e) 1c, top view. f) 1c, side view.
44
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Chem. Asian J. 2009, 4, 42 – 49

Synthesis and Optical Properties of Dibenzochalcogenaborins
Table 1. Selected bond lengths (ꢁ) and angles (deg) of chalcogenaborins
1a–c.
UV/Vis Spectra
1a[a]
1b[b]
1c
UV/Vis spectra of dibenzochalcogenaborins 1a–c in cyclo-
hexane solution (10ꢀ4 m) at room temperature are shown in
Figure 3 and the data are presented in Table 2. In each spec-
ꢀ
B C (in plane)
1.529(2)
1.531(3)
1.530(3)
1.532(5)
1.537(4)
1.533(5)
1.536(4)
1.739(3)
1.733(3)
1.743(3)
1.746(3)
119.6(3)
119.7(3)
106.12(14)
106.32(15)
1.550(3)
1.534(4)
ꢀ
C Ch
1.366(2)
1.371(2)
1.374(2)
1.893(2)
1.888(2)
ꢀ ꢀ
C B C (in plane)
114.3(2)
121.4(2)
114.40(15)
121.27(19)
121.20(13)
ꢀ
ꢀ
C Ch C
102.05(10)
[a] Two independent molecules are contained in the unit cell, and one of
the molecules has a Cs symmetry. [b] Two independent molecules are
contained in the unit cell.
benzochalcogenaborin ring of 1b is slightly twisted and that
of 1c is slightly puckered. The boron center in 1a–c takes a
completely planar structure. The high planarity found in the
dibenzochalcogenaborin rings and around the boron center
resulted in the efficient interaction between p-orbitals and
molecular orbitals of the boron and chalcogen atoms, lead-
ing to good optical properties.
Figure 3. UV/Vis spectra of chalcogenaborins 1a–c in cyclohexane at
298 K.
Packing structures of 1a and 1b were similar to each
other, in which two dibenzochalcogenaborin rings are
stacked by a p–p interaction (Figure 2). However, arising
Table 2. Optical data for chalcogenaborins 1a–c in cyclohexane at 298 K.
[d]
compound
lmax [nm][a]
e[b]
lem [nm][c]
DE [cmꢀ1
]
1a
347(sh)
320
382
326
392
11000
22000
7100
15000
7100
377 (0.30)
2.3ꢂ103
1b
1c
412 (0.08)
1.9ꢂ103
2.1ꢂ103
427 (0.0007)
331
16000
[a] Absorption maxima in UV/Vis spectra. [b] Absorption coefficient.
[c] Emission maxima of fluorescence spectra and quantum yield, deter-
mined by using anthracene in cyclohexane as a standard, in parenthesis.
[d] DE=1/labꢀ1/lem
.
trum, there are two absorption bands. One at 347 nm for 1a,
382 nm for 1b, and 392 nm for 1c, and the other is observed
at 320 nm for 1a, 326 nm for 1b, and 331 nm for 1c. The
longer wavelength absorptions were attributed to intramo-
lecular charge transfer (ICT) from chalcogen atoms to the
boron center and red-shifted on going from 1a to 1c. This is
because of the smaller HOMO–LUMO gap caused by high
energy levels of the chalcogen lone pairs in 1b and 1c with
heavier chalcogen atoms (vide infra). The shorter wave-
length bands are also red-shifted slightly in the order of 1a,
1b, and 1c. These absorption bands are attributed to the
transition from HOMO-1 (p-orbital on benzene rings) to
LUMO. The energy levels of HOMO-1 of each compound
do not change so much (vide infra), and the latter red-shift
is attributable to the difference of the LUMO energy levels
of each compound.
Figure 2. Packing diagram of chalcogenaborins 1a–c (50% probability).
Hydrogen atoms are omitted for clarity. a) 1a, b) 1b, c) 1c, methoxy
groups are omitted for clarity. Shortest contacts between two molecules
are indicated by dashed lines.
from the bulkiness of the Mes group, the overlap of each di-
benzochalcogenaborin ring was small. On the other hand,
1c had the herringbone packing structure, in which the dis-
tance between the selenium atom and the centroid of the
fused benzene ring of the neighboring molecule is 3.373 ꢁ,
indicating the existence of the interaction between them.
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T. Kawashima et al.
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Fluorescence Spectra
The steady state fluorescence spectra of dibenzochalcogena-
borins in cyclohexane solution (10ꢀ4 m) at room temperature
are shown in Figure 4 and the photophysical data are listed
Figure 5. Solid-state fluorescence spectra of chalcogenaborins 1a–c at
298 K (powder sample)
Table 3. Optical data of chalcogenaborins 1a and 1b in the solid state.
[c]
compound
lem [nm][a]
lex [nm][b]
DE [cmꢀ1
]
1a
1b
413
461
378
430
2.2ꢂ103
1.6ꢂ103
Figure 4. Steady state fluorescence spectra of chalcogenaborins 1a–c in
cyclohexane at 298 K.
[a] Emission maxima of fluorescence spectra in the solid state. [b] Excita-
tion spectral maxima in the solid state. [c] DE=1/lexꢀ1/lem
.
in Table 2. 1a and 1b exhibited a fluorescence emission
around 400 nm, while 1c showed very weak emission proba-
bly on account of the heavy atom effect. Reflecting the red-
shift of the UV/Vis absorption maxima, the emission
maxima of dibenzochalcogenaborins were also red-shifted
on going from 1a to 1c. In each case, Stokes shifts were
very small (DE<2300 cmꢀ1), indicating the high rigidity of
framework of dibenzochalcogenaborins. The concentration
effect upon the emission maxima of 1a–c was not observed.
On the other hand, the polarity of the solvent slightly affect-
ed the emission maxima. The emission maxima of 1a–c in
dichloromethane (e=8.93) are red-shifted compared to
those in cyclohexane (e=2.02) (lem in dichloromethane; 1a
385 nm, 1b 416 nm, 1c 430 nm), although absorption
maxima of 1a–c did not shift upon changing the solvent to
dichloromethane. These results indicate that the excited
states of 1a–c are slightly polar.
The solid state fluorescence spectra of powder samples of
dibenzochalcogenaborins were also measured. 1a and 1b
showed intense photo-luminescence in the solid state as well
as in solution, while 1c was almost non-emissive in the solid
state. The spectra of 1a and 1b are shown in Figure 5. Their
photophysical data are summarized in Table 3. Intense fluo-
rescence of 1a and 1b is observed in the solid state, proba-
bly because the sterically bulky Mes group prohibits aggre-
gation, which results in degradation from the excited state
through a non-radiative process. The solid-state emissions
are red-shifted compared to those in solution, as observed in
azaborine, thiaborin, and their ladder-type congeners.[9]
These phenomena indicate the existence of dipole–dipole in-
teractions in the excited states. Although direct p–p stacking
is not significant, as observed in the packing structure of 1a
and 1b, it is likely that the long-range interaction between
excited dipoles may influence the fluorescence emission in
the solid state.
Theoretical Calculations
To evaluate the energy levels of molecular orbitals of diben-
zochalcogenaborins, theoretical calculations were performed
using density functional theory[11] with the model com-
pounds 1a’–c’, in which the mesityl group is replaced by a
phenyl group to reduce the computational time, as well as
the methylene bridged analog 1d’ for comparison. The ge-
ometry optimizations at B3LYP/6-31+G(d) level repro-
duced X-ray crystallographic structures of 1a, 1b and 1c.
The energy levels of HOMO, HOMO-1, LUMO, and the
HOMO–LUMO energy gaps, obtained at B3LYP/6-311+G-
AHCTUNGTREG(NNNU d,p)//B3LYP/6-31+G(d) level, are summarized in Table 4.
Orbital plots for the HOMO, HOMO-1, and LUMO of each
Table 4. Frontier molecular orbitals of dibenzochalcogenaborins 1a’–c’
and related compound 1d’.[a]
compound HOMO [eV] LUMO [eV] HOMO-1 [eV] DE [eV][b]
1a’
1b’
1c’
1d’
ꢀ5.65
ꢀ5.45
ꢀ5.38
ꢀ5.61
ꢀ1.51
ꢀ1.63
ꢀ1.69
ꢀ1.59
ꢀ5.80
ꢀ5.86
ꢀ5.86
ꢀ5.87
4.14
3.82
3.69
4.29
[a] Calculation level: B3LYP/6-311+G
ACHTUNGTRNE(NUNG d,p)//B3LYP/6-31+G(d). [b] The
energy gap between HOMO and LUMO.
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Chem. Asian J. 2009, 4, 42 – 49

Synthesis and Optical Properties of Dibenzochalcogenaborins
Table 5. Time-dependent density functional theory calculations of diben-
compound are shown in Figure 6. The HOMO of each com-
pound consists of the lone pair orbital of the chalcogen
atoms and the p-orbitals on benzene rings. In the case of
zochalcogenaborins 1a’–c’.[a]
compound
E [eV][b]
l [nm][c]
f[d]
transition
1a’
3.63
3.87
3.33
3.17
3.20
3.70
341.1
320.6
372.1
329.0
387.6
335.4
0.0696
0.3672
0.0744
0.2306
0.0791
0.2213
HOMO!LUMO
HOMO-1!LUMO
HOMO!LUMO
HOMO-1!LUMO
HOMO!LUMO
HOMO-1!LUMO
1b’
1c’
[a] Calculation level: B3LYP/6-311+GACTHNUGRTNEUNG(d,p)//B3LYP/6-31+G(d). [b] Exci-
tation energy. [c] Wavelength. [d] Oscillator strength.
lowest excited state is attributed to the transition form
HOMO-1 to LUMO, and does not show a large difference
among chalcogen congeners.
Conclusions
In this study, we synthesized dibenzochalcogenaborins and
compared their structural and optical properties systemati-
cally. These molecules were shown to have a rigid and
planar framework, as determined from X-ray crystallograph-
ic analysis and theoretical calculations. UV/Vis and fluores-
cence spectroscopy as well as theoretical calculations,
proved that the nature of chalcogen atoms at the bridging
position strongly affects their optical and electronic proper-
ties. Dibenzooxaborin and dibenzothiaborin emitted intense
fluorescence both in solution and in the solid state because
of the rigid framework and inhibition of strong intermolecu-
lar interactions like p–p stacking in the solid state, while di-
benzoselenaborin showed only weak fluorescence in solu-
tion. The HOMO–LUMO energy gap was found to decrease
on going down the periodic table for the choice of bridging
chalcogen atoms.
Figure 6. Orbital plots of LUMO, HOMO, and HOMO-1 of dibenzochal-
cogenaborins 1a’-c’ and the reference compound 1d’.
1d’, this orbital corresponded to HOMO-1. The energy level
of this orbital (HOMO for 1a’, 1b’, and 1c’; HOMO-1 for
1d’) is elevated in the order of 1d’, 1a’, 1b’ and 1c’. The ele-
vation of the HOMO energy level in the dibenzochalcoge-
naborins is consistent with the high energy level of the lone
pair orbital of heavier chalcogen atoms. On the other hand,
the LUMO energy level decreases from 1a’ to 1c’ and the
LUMO energy level of 1b’ and 1c’ is even lower than that
of 1d’. The HOMO-1 consists mainly of p-orbitals on the
benzene rings and is rarely affected by changing the bridg-
ing chalcogen atoms. A similar behavior, as found for the
HOMO and HOMO-1 energy levels of dibenzochalcogena-
borins, was observed for the difference of the HOMO and
HOMO-1 energy levels of dibenzochalcogenophenes (diben-
zofuran, dibenzothiophene, dibenzoselenophene, and diben-
zotellurophene).[12]
Experimental Section
General Procedure
General chemicals were used as received. Cyclohexane and THF, spec-
trochemical or fluorometric grade (Dojindo), were used for optical mea-
surement. All manipulations were carried out using modified Schlenk
technique under an argon atmosphere. Solvents were purified by
MBRAUN MB-SPS system. Wet column chromatography (WCC) was
performed using Kanto Silica Gel 60N. Gel permeation liquid chromatog-
raphy (GPC) was performed using LC-918 with JAIGEL 1H+2H col-
umns (Japan Analytical Industry) using chloroform as solvent. NMR
Time-dependent density functional calculations were also
performed to elucidate the transition observed in the UV/
Vis spectra. TD-DFT calculations were carried out at
spectra were recorded by
a
Bruker DRX-500 spectrometer (1H,
JEOL AL-400 spectrometer (11B,
500 MHz; 13C, 126 MHz) and
a
B3LYP/6-311+GACHTUNGTRENNUNG(d,p)//B3LYP/6-31+G(d) level and the re-
128 MHz). Chemical shifts d are reported in ppm. 1H NMR spectra are
referenced to residual protons in the deuterated solvent; 13C NMR spec-
tra are referenced to carbon-13 in the deuterated solvent; 11B NMR spec-
tra are referenced to an external standard of BF3·Et2O. UV/Vis spectra
were recorded on a JASCO V-530 spectrophotometer. Fluorescence spec-
tra were recorded on a JASCO F-6500 fluorescence spectrophotometer.
All melting points were measured with a Yanaco MP-S3 and were uncor-
rected. Elemental analyses were performed by the Microanalytical Labo-
ratory of the Department of Chemistry, Faculty of Science, The Universi-
ty of Tokyo. Bis(2-bromo-4,5-dimethoxyphenyl) ether 2a, bis(2-bromo-
sults are summarized in Table 5. Predicted wavelengths
matched well with the absorption bands observed in the
UV/Vis spectra. The lowest excited state in each compound
is contributed by the transition from HOMO to LUMO,
that is, intramolecular charge transfer from the lone pair of
the chalcogen atom to the vacant orbital of the boron atom.
Transition energy is lowered from 1a’ to 1c’ in accordance
with the observation in the UV/Vis spectra. The second
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47

T. Kawashima et al.
FULL PAPERS
4,5-dimethoxyphenyl) sulfide 2b, and bis(2-bromo-4,5-dimethoxyphenyl)
selenide 2c were prepared according to the literature.[10]
reflections were collected, of which 7741 were independent (Rint =0.0451)
and employed for refinement: 721 parameters, 0 restraints, Goodness of
fit on F2 =0.969, R1 (I>2s(I))=0.0534, wR2 (all data)=0.1437. Crystallo-
graphic data for 1c: C25H27BSeO4·CHCl3, yellow block, monoclinic, space
group P21/n, a=17.2958(17) ꢁ, b=8.0957(5) ꢁ, c=20.0850(15) ꢁ, b=
Synthesis
1a: 9-Mesityl-9H-9-boraxanthene. tBuLi (2.2m pentane solution, 6.1 mL,
13.4 mmol) was added to a solution of bis(2-bromo-4,5-dimethoxyphenyl)
ether 2a (1.37 g, 3.06 mmol) in THF (30 mL) at ꢀ788C. After stirring for
98.1777(11)8, V=2783.7(4) ꢁ3, Z=4, F
ACTHNUTRGNE(NUG 000)=1224, crystal size 0.20ꢂ
0.20ꢂ0.20 mm3, 6.04ꢁ2Vꢁ50.00. In total, 16917 reflections were collect-
ed, of which 4829 were independent (Rint =0.0410) and employed for re-
finement: 360 parameters, 0 restraints, Goodness of fit on F2 =1.021, R1
(I>2s(I))=0.0369, wR2 (all data)=0.0951. The intensities of reflections
were collected at 120 K on a RIGAKU MSC Mercury CCD diffractome-
ter with graphite-monochromated MoKa radiation (l=0.71070 ꢁ) using
CrystalClear (Rigaku Corp.). The structure was solved by direct methods
(SHELXS) and expanded using Fourier techniques. The structure was re-
fined by full-matrix least-squares methods on F2 (SHELXL-97).[13] All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
assigned idealized positions and were included in structure factor calcula-
tions. CCDC 667887, CCDC 667888 and CCDC 667889 contain the sup-
plementary crystallographic data for 1a, 1b and 1c, respectively. These
data can be obtained free of charge from the Cambridge Crystallographic
Data Centre at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB21EZ, UK; fax: (+44) 1223–336–033; or deposit@ccdc.cam.ac.uk).
30 min at ꢀ788C, MesB
ACHTUNGTRENNUNG(OMe)2 (824.5 mg, 4.29 mmol) was added to the
reaction mixture at ꢀ788C. The reaction mixture was refluxed overnight
and the reaction was quenched with water. The organic layer was extract-
ed with CHCl3 and the extracts were combined and dried over anhydrous
MgSO4. After removal of the solvent under reduced pressure, the residue
was purified by reprecipitation from CHCl3/MeOH to give 1a as a pale
yellow solid (493.6 mg, 39%). m.p. 224–2258C; 1H NMR (500 MHz,
CDCl3, r.t.): d=7.25 (s, 2H), 6.94 (s, 2H), 6.91 (s, 2H), 4.01 (s, 6H), 3.77
(s, 6H), 2.38 (s, 3H), 2.00 ppm (s, 6H); 13C NMR (126 MHz, CDCl3, r.t.):
d=155.4, 154.6, 145.2, 138.8, 138.0, 136.5, 127.0, 117.4, 114.2, 99.6, 56.3,
56.1, 22.7, 21.3 ppm; 11B NMR (128 MHz, CDCl3, r.t.): d=52 ppm; UV/
Vis (cyclohexane): lmax (e)=347 (sh, 11000), 320 (22000); elemental
analysis calcd (%) for C25H27BO5: C 71.78, H 6.51; found: C 71.51,
H 6.65.
1b: 9-Mesityl-9H-9-borathioxanthene. tBuLi (2.2m pentane solution,
4.0 mL, 8.8 mmol) was added to a solution of bis(2-bromo-4,5-dimethoxy-
phenyl) sulfide 2b (0.93 g, 2.02 mmol) in Et2O (30 mL) at ꢀ788C. After
stirring for 30 min at ꢀ788C, MesB
ACHTUNGRTEN(NUNG OMe)2 (535.8 mg, 2.79 mmol) was
added to the reaction mixture at ꢀ788C. The reaction mixture was re-
fluxed overnight and the reaction was quenched with water. The organic
layer was extracted with CHCl3, and the extracts were combined and
dried over anhydrous MgSO4. After removal of the solvent under re-
duced pressure, the residue was subjected to silica gel column chromatog-
raphy (CHCl3/nHex=2:1) and GPC to give 1b as a pale yellow solid
(276.0 mg, 23%). m.p. 2298C; 1H NMR (500 MHz, CDCl3, RT): d=7.18
(s, 2H), 7.17 (s, 2H), 6.89 (s, 2H), 4.00 (s, 6H), 3.71 (s, 6H), 2.37 (s, 3H),
1.94 ppm (s, 6H); 13C NMR (126 MHz, CDCl3, RT): d=152.7, 146.9,
138.5, 138.0, 137.3, 136.3, 127.2, 126.9, 119.0, 106.5, 56.0, 56.0, 22.3,
21.3 ppm; 11B NMR (128 MHz, CDCl3, RT): d=54 ppm; UV/Vis (cyclo-
hexane): lmax (e)=382 (7100), 326 (15000); elemental analysis calcd (%)
for C25H27BO4S: C 69.13, H 6.27; found: C 68.88, H 6.28.
Acknowledgements
This work was supported in part by the Global COE Program (T.K.) for
Chemistry Innovation and Scientific Research (T.K.) from MEXT, Japan.
We also thank Tosoh Finechem Corp. for the generous gift of the alkyl-
lithiums.
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ACHTUNGRTEN(NUNG OMe)2 (370.7 mg, 1.93 mmol)
was added to the reaction mixture at ꢀ788C. The reaction mixture was
refluxed overnight and the reaction was quenched with water. The organ-
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dried over anhydrous MgSO4. After removal of the solvent under re-
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m.p. 1888C; 1H NMR (500 MHz, CD2Cl2, RT): d=7.28 (s, 2H), 7.21 (s,
2H), 6.90 (s, 2H), 3.95 (s, 6H), 3.62 (s, 6H), 2.36 (s, 3H), 1.91 ppm (s,
6H); 13C NMR (126 MHz, CDCl3, RT): d=152.6, 147.1, 141.4, 138.3,
137.4, 136.3, 128.9, 126.9, 121.1, 108.8, 56.0, 55.9, 22.6, 21.3 ppm;
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lmax (e)=392 (7100), 331 (16000); elemental analysis calcd (%) for
C25H27BO4Se: C 62.30, H 5.65; found: C 62.28, H 5.70.
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X-ray Crystallography
1a, 1b, and 1c: Crystallographic data for 1a: C25H27BO4·C2H3N, pale
yellow block, monoclinic, space group C2/c, a=36.365(13) ꢁ, b=
23.334(8) ꢁ, c=8.268(3) ꢁ, b=102.7605(14)8, V=6842(4) ꢁ3, Z=12, F-
A
21596 reflections were collected, of which 5975 were independent (Rint
=
0.0227) and employed for refinement: 450 parameters, 0 restraints, Good-
ness of fit on F2 =1.050, R1 (I>2s(I))=0.0518, wR2 (all data)=0.1588.
Crystallographic data for 1b: C25H27BSO4, pale yellow block, monoclinic,
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95.9878, b=90.598(7)8, g=103.874(8)8, V=2269(2) ꢁ3, Z=4, F
ACTHNUTRGNE(NUG 000)=
920, crystal size 0.20ꢂ0.20ꢂ0.20 mm3, 6.04ꢁ2Vꢁ50.00. In total, 14375
48
www.chemasianj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: August 5, 2008
Revised: September 13, 2008
Published online: November 19, 2008
Chem. Asian J. 2009, 4, 42 – 49
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
49