Enhancement of the Lewis acidity by substitution of sulfur-containing hetero aromatics in triarylboranes

Article abstract of DOI:10.1016/j.tetlet.2009.02.196

Triarylboranes bearing two thiophene, dimethylthiophene, or benzothiophene units were synthesized. X-ray crystallographic analysis revealed the planar structure around the boron atom and non-bonding S-S interaction of the thiophene. They exhibited several

Full text of DOI:10.1016/j.tetlet.2009.02.196

Tetrahedron Letters 50 (2009) 3467–3469
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Tetrahedron Letters
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Enhancement of the Lewis acidity by substitution of sulfur-containing
hetero aromatics in triarylboranes
*
*
Shinji Miyasaka, Junji Kobayashi , Takayuki Kawashima
Department 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:
Triarylboranes bearing two thiophene, dimethylthiophene, or benzothiophene units were synthesized. X-
ray crystallographic analysis revealed the planar structure around the boron atom and non-bonding S–S
interaction of the thiophene. They exhibited several intense absorption bands in the UV region. They were
stable in the air for months and showed extremely strong Lewis acidities.
Received 14 January 2009
Revised 25 February 2009
Accepted 26 February 2009
Available online 4 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Fluoride ion recognition and sensing have received considerable
attention in recent years because it is a key ionic species in envi-
ronmental problems and diseases such as caries or osteoporosis.
Pyrrole1, urea2, and borane units are often used as fluoride recog-
nition sites of synthetic chemosensors. Among them, p-conjugated
triarylboranes are an excellent motif due to drastic changes of UV–
Scheme 1. Synthesis of dithienylborane 1.
vis, or fluorescence spectra upon complexation with fluoride ion. It
leads to a drastic change of the frontier orbitals of
p-conjugation
systems, thereby dramatically altering the photophysical proper-
ties. A lot of triarylboranes have been developed and investigated
for fluoride ion sensing by taking advantage of the strong B–F bond
formation.3 Among them, monothienylboranes showed good fluo-
ride ion recognition ability with the high selectivity, because of a
relatively electron-deficient property of thiophene ring.4 Introduc-
tion of more than two thiophene rings is expected to improve the
recognition ability, and dithienyl- and trithienylboranes were in-
deed reported.5 However, the fluoride ion affinities of these com-
pounds were not examined due to their lack of stability against
air and moisture. Herein we report the syntheses, structures, opti-
cal properties, and fluoride recognition abilities of stable boranes
bearing two thiophene, dimethylthiophene, or benzothiophene
units on a boron atom.
Figure 1. Structural formulas of compounds 2 and 3.
signals of 1 (dB 54.9), 2 (dB 58.2), and 3 (dB 60.4) appeared in the
usual region for triarylboranes.
Single crystals of 1, 2, and 3 were obtained by recrystallization
from CH3CN, CHCl3/hexane, and Et2O solutions, respectively, and
X-ray crystallographic analyses were carried out. The crystal struc-
tures of 1 and 2 are shown in Figure 2. 1 had two independent mol-
ecules in a unit cell and only one molecule is shown in Figure 2
since they were almost identical. Boron centers of 1, 2, and 3
adopted the ideal trigonal planar geometries (RB = 360°). Dihedral
angles between the boron plane of 1 and two thiophene rings ran-
ged from 1° to 9°, meaning the highly planar structure and the
Dithienylborane 1 was synthesized by the reaction of 2-lithio-
thiophene, generated by direct lithiation of thiophene, with Mes-
B(OMe)2 in Et2O (Scheme 1).
1 was purified by HPLC and
obtained as a colorless solid (86%), being stable to air and moisture
both in the solid state and solution. Usually triarylborane requires
more than two bulky substituents for the kinetic stabilization.
However, only one mesityl group on the boron atom was enough
for kinetic stabilization of 1. Dimethylthiophene and benzothio-
phene were converted to the corresponding boranes 2 (45%) and
3 (80%), respectively, by the similar method, which were also sta-
ble in the solid state and solution (Fig. 1).6 In CDCl3, the 11B NMR
effective p-conjugation in 1. 3 also had a planar structure, in which
the corresponding dihedral angles ranged from 1° to 11°. On the
other hand, the mesityl ring was almost perpendicular to the boron
plane. In contrast, such a co-planarity between the boron plane and
thiophene rings was not observed in 2 (dihedral angles were about
30°), because of steric repulsion between a mesityl group and
methyl groups of dimethythienyl groups. Furthermore, it is inter-
* Corresponding authors. Tel.: +81 3 5841 4338; fax: +81 3 5800 6899 (T.K.).
E-mail address: takayuki@chem.s.u-tokyo.ac.jp (T. Kawashima).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.196

3468
S. Miyasaka et al. / Tetrahedron Letters 50 (2009) 3467–3469
Figure 4. UV–vis spectra of 3 (10 lM) in CH2Cl2 with 1 equiv TBAF and in MeOH.
240 nm for 3 arose with the isosbestic point in each spectrum,
which indicated the complexation with fluoride ion to form flu-
oroborates. For a typical example, the spectral changes of 1 in
CH3CN upon addition of TBAF are shown in Figure 3. The associ-
ation constants of 1, 2, and 3 in CH3CN were too large to be
determined in detail by titration method using UV–vis spectra
Figure 2. ORTEP drawing of 1 and 2 (50% probability): (a) top view of 1; (b) side
view of 1; (c) top view of 2; (d) side view of 2. Hydrogen atoms are omitted for
clarity.
and were approximately estimated to be more than 108 MÀ1
.
However, the association constants could be determined in
CH2Cl2, which has the larger Acceptor Number than CH3CN
(CH2Cl2: 20.4, CH3CN: 18.9).8 The association constants (Ka) of
1 was 2.3 Â 106 MÀ1, which was much larger than those of neu-
tral compounds previously reported.3a,b,d,4 Compound 2 showed
esting to note that the crystal structure of 1 showed an intramolec-
ular S–S non-bonding interaction with 3.26 Å and 3.23 Å, which
were shorter than the sum of the van der Waals radii of sulfur
atoms (3.6 Å). Such interaction was also observed in the crystal
structure of 3 (SÁÁÁS 3.15 Å).7
smaller association constant, 4.4 Â 105 MÀ1
, due to electron-
donating methyl groups. Benzothiophene derivative 3 had the
All compounds 1, 2, and 3 exhibited several intense absorption
bands including the longest ones at 326 nm, 342 nm, and 361 nm,
respectively, in CH3CN, and 328 nm, 343 nm, and 362 nm in
CH2Cl2, indicating no significant solvent effects. 2 and 3 showed
bathochromic shifts of the longest absorption band compared with
that of 1, due to the increase in HOMO energy level by electron-
enhanced fluoride ion affinity (Ka = 6.2 Â 107 MÀ1) because LUMO
energy level of 3 decreased by effective elongation of
p-conjuga-
tion.9 Interestingly, a spectrum of 3 in MeOH was similar to that
in CH2Cl2 after addition of 1 equiv TBAF (Fig. 4). This result sug-
gests the formation of tetracoordinated boron-MeOH complex,
revealing that 3 has the strong Lewis acidity.
donating methyl groups for 2 and extended
zo-fused framework for 3.
p-conjugation by ben-
In conclusion, mesitylboranes containing two thiophene,
dimethylthiophene, and benzothiophene units were synthesized
and characterized. Dithienyl borane 1 turned out to have a planar
structure and sulfur-sulfur non-bonding interaction determined by
X-ray crystallographic analysis. UV–vis spectra of these boranes
To clarify the sensing ability of fluoride ion, we monitored the
absorbance changes of 1, 2, and 3 associated with the addition of
n-Bu4NF (TBAF). The longer absorption bands of 1, 2, and 3 de-
creased and finally disappeared and a new shoulder band at
235 nm for 1, 240 nm for 2, and new bands at 277 and
showed that they had extended
p-conjugation systems through a
vacant p-orbital of the central boron atom. Introduction of thienyl
groups to a boron atom was revealed to raise the Lewis acidity of a
borane. Further studies on boranes bearing more
no rings are in progress.
p-extended thie-
Supplementary data
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center, CCDC
715341 for 1, CCDC 715342 for 2, and CCDC 715343 for 3. Copies of
these informations may be obtained free of charge from The Direc-
tor, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk or www:http://
ccdc.cam.ac.uk).
Acknowledgments
This work was supported in part by Global COE Program (Chem-
istry Innovation through Cooperation of Science and Engineering),
MEXT, Japan. We also thank Tosoh Finechem Corp. for the generous
gifts of alkyllithiums.
Figure 3. Changes in UV–vis absorption of 1 (20 lM) in CH3CN upon the addition of
TBAF.

S. Miyasaka et al. / Tetrahedron Letters 50 (2009) 3467–3469
3469
2H); 13C NMR (126 MHz, CDCl3) d 14.89 (q), 14.97 (q), 21.30 (q), 22.42 (q),
References and notes
127.39 (d), 131.85 (d), 137.29 (s), 137.86 (s), 140.38 (s), 140.82 (s), 143.18 (s),
149.11 (s); 11B NMR (160 MHz, CDCl3) d 58.2; UV–vis (CH3CN) k/nm (log
e): 342
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6. General procedure for the synthesis of dithienylboranes. To an Et2O (3 mL) solution
of thiophene (2.2 mmol) was added n-BuLi (2.2 mmol) dropwise at room
temperature. The reaction mixture was stirred at room temperature for 3 h.
Then MesB(OMe)2 (1.0 mmol) was added at room temperature and the reaction
mixture was stirred for 10 h. The resulting precipitates were filtered and washed
with Et2O. The crude product was purified by HPLC to afford products. Spectral
data for selected products: Mesityldi(2-thienyl)borane (1): colorless solid. Mp 99–
100 °C. 1H NMR (500 MHz, CDCl3) d 2.05 (s, 6H), 2.34 (s, 3H), 6.85 (s, 2H), 7.27–
7.29 (m, 2H), 7.81 (d, J = 3.5 Hz, 2H), 7.91 (d, J = 4.6, 2H); 13C NMR (126 MHz,
CDCl3) d 21.29 (q), 22.62 (q), 126.90 (d), 129.21 (d), 137.06 (s), 137.72 (d),
138.35 (s), 141.56 (s), 142.03 (d), 143.92 (s); 11B NMR (160 MHz, CDCl3) d 54.9;
(4.34), 279 (3.87). Anal. Calcd for C21H25BS2: C, 71.58; H, 7.15. Found: C, 71.32;
H, 7.21. Bis(benzo[b]thien-2-yl)mesitylborane (3): colorless solid. Mp 120 °C
decomp. 1H NMR (500 MHz, CDCl3) d 2.11 (s, 6H), 2.39 (s, 3H), 6.92 (s, 2H), 7.35–
7.45 (m, 4H), 7.90–7.95 (m, 4H), 8.11 (s, 2H); 13C NMR (126 MHz, CDCl3) d 21.37
(q), 22.73 (q), 122.71 (d), 124.50 (d), 125.72 (d), 126.74 (d), 127.08 (d), 137.61
(s), 138.46 (s), 140.38 (d), 140.60 (s), 141.33 (s), 144.36 (s), 146.79 (s); 11B NMR
(160 MHz, CDCl3) d 60.4; UV–vis (CH3CN) k/nm (loge): 361 (sh) (4.48), 346
(4.52), 299 (4.06), 263 (4.24). Anal. Calcd for C25H21BS2: C, 75.75; H, 5.34. Found:
C, 75.51; H, 5.41.
7. Crystal structure of 3 had the disorder on benzothiophene moiety. Two sulfur
atoms faced to each other in one disordered structure and one sulfur atom was
directed to mesityl group in the other structure.
8. Mayer, U.; Gutmann, V.; Gerger, W. Monatsh. Chem. 1975, 106, 1235–1257.
9. Fluoroborates 1ÁTBAF, 2ÁTBAF, and 3ÁTBAF were obtained by the reactions of 1,
2, and 3 with TBAF, respectively. Their NMR spectral data are shown below.
1ÁTBAF: 1H NMR (500 MHz, CDCl3) d 0.92 (t, J = 7.3 Hz, 12H), 1.20–1.30 (m, 8H),
1.30–1.40 (m, 8H), 2.17–2.20 (m, 9H), 2.75–2.85 (m, 8H), 6.61 (br s, 2H), 6.89–
6.93 (m, 4H), 7.14 (d, J = 4.2 Hz, 2H); 13C NMR (126 MHz, CDCl3) d 13.70 (q),
19.43 (t), 20.94 (q), 23.75 (t), 24.72 (q), 24.77 (q), 57.71 (t), 123.19 (d), 126.27
(d), 127.69 (d), 127.73 (d), 128.37 (d), 132.00 (s), 141.77 (s), 151.23 (s), 163.35
(s); 11B NMR (160 MHz, CDCl3) d 2.3; 19F NMR (376 MHz, CDCl3) d À153.07.
2ÁTBAF: 1H NMR (500 MHz, CDCl3) d 0.93 (t, J = 7.3 Hz, 12 H), 1.22–1.31 (m, 8H),
1.36–1.45 (m, 8H), 1.94 (s, 6H), 2.07 (s, 6H), 2.09 (br s, 6H), 2.17 (s, 3H), 2.81–
2.88 (m, 8H), 6.57 (s, 2H), 6.65 (s, 2H); 13C NMR (126 MHz, CDCl3) d 13.70 (q),
14.42 (q), 15.32 (q), 19.51 (t), 20.99 (q), 23.92 (t), 24.48 (q), 24.52 (q), 58.05 (t),
117.76 (d), 128.18 (d), 131.80 (s), 135.51 (s), 137.82 (s), 142.37 (s), 151.16 (s),
157.28 (s); 11B NMR (160 MHz, CDCl3) d 2.0; 19F NMR (376 MHz, CDCl3) d
À165.52. 3ÁTBAF: 1H NMR (500 MHz, CDCl3) d 0.80 (t, J = 7.1 Hz, 12H), 0.98–1.12
(m, 16H), 2.21 (s, 3H), 2.27 (br s, 6H), 2.35–2.43 (m, 8H), 6.64 (s, 2H), 7.05–7.09
(m, 2H), 7.13 (s, 2H), 7.14–7.18 (m, 2H), 7.54 (d, J = 8.1 Hz, 2H), 7.69 (d,
J = 8.1 Hz, 2H); 13C NMR (126 MHz, CDCl3) d 13.57 (q), 19.29 (t), 20.95 (q), 23.39
(t), 24.77 (q), 24.83 (q), 57.36 (t), 121.16 (d), 121.64 (d), 121.85 (d), 122.51 (d),
123.98 (d), 124.02 (d), 128.48 (d), 132.63 (s), 141.83 (s), 142.26 (s), 142.39 (s),
149.68 (s), 165.52 (s); 11B NMR (160 MHz, CDCl3) d 2.5; 19F NMR (376 MHz,
CDCl3) d À158.14.
UV–vis (CH3CN) k/nm (loge): 326 (4.41), 276 (3.97), 261 (3.99). Anal. Calcd for
C17H17BS2: C, 68.87; H, 5.88. Found: C, 68.92; H, 5.78. Bis(3,4-dimethyl-2-
thienyl)mesitylborane (2): colorless solid. Mp 177–178 °C. 1H NMR (500 MHz,
CDCl3) d 1.95 (s, 6H), 2.00 (s, 6H), 2.17 (s, 6H), 2.29 (s, 3H), 6.78 (s, 2H), 7.42 (s,