Coordination-induced intramolecular double cyclization: Synthesis of boron-bridged dipyridylvinylenes and dithiazolylvinylenes

Article abstract of DOI:10.1055/s-0028-1083271

Di(heteroaryl)acetylenes having a pyridyl or thiazolyl as the heteroaryl group underwent a reaction with bromodiben-zoborole to produce coordination complexes, which were further treated with water to promote the intramolecular cascade double cyclization

Full text of DOI:10.1055/s-0028-1083271

PAPER
127
Coordination-Induced Intramolecular Double Cyclization: Synthesis of
Boron-Bridged Dipyridylvinylenes and Dithiazolylvinylenes
S
ynthesis of Boron
i
-Bridg
a
ed Dipyridy
n
lvinylenes
a
g
nd
D
ithiazolylviny
Z
lenes hao, Hongyu Zhang, Atsushi Wakamiya, Shigehiro Yamaguchi*
Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
Fax +81(52)7895947; E-mail: yamaguchi@chem.nagoya-u.ac.jp
Received 22 October 2008
ious bridging moieties, such as methylene,10 SiR2,5 S and
Abstract: Di(heteroaryl)acetylenes having a pyridyl or thiazolyl as
the heteroaryl group underwent a reaction with bromodiben-
zoborole to produce coordination complexes, which were further
treated with water to promote the intramolecular cascade double cy-
–
+ 8
Se,6,7,11,12 BR2 and PR2 , and P(=O)R9 (Scheme 1). All
these reactions require the synthesis of the appropriate
precursors having the requisite functional groups at the
clization to produce the dibenzoborolyl-bridged ladder di(heteroar- o,o¢-positions of the diphenylacetylene skeleton, prior to
yl)vinylenes. For the thiazolyl derivative, further derivatization at
conducting the IDC reactions. In contrast, we now present
the terminal positions produced a bis(dimesitylboryl)-substituted
a new strategy for the IDC reaction, that is, the pre-coor-
derivative. The ladder molecules prepared showed an intense emis-
dination of the N-heteroaryl ring to the boron atom13,14 for
sion in the fluorescence spectra as well as reversible reduction
the construction of the requisite o,o¢-disubstituted diaryl-
waves at low reduction potentials in the cyclic voltammetry.
acetylene precursor. In this article, we disclose the coordi-
Key words: boron, acetylene, N-heteroaryl, cascade cyclization, p-
nation-induced IDC reaction starting from the di(N-
electron systems
heteroaryl)acetylenes and haloboranes. This reaction pro-
duces a series of ladder p-electron systems with the in-
tramolecular B–N coordination, which would be
Ladder p-electron systems, having fully fused polycyclic
promising materials with highly electron-accepting prop-
skeletons, are promising materials in organic electronics,
erties for application in the organic electronic devices.
since their rigid and flat skeletons without any conforma-
Therefore, their fundamental photophysical and electro-
tional disorder give rise to a set of intriguing properties,
chemical properties have also been investigated.
such as intense fluorescence, high carrier mobility, and
high thermal stability. A number of fascinating ladder ma-
(a)
Ar1 Ar1
terials have been synthesized and used in organic light
emitting diodes as well as organic thin film transistors.1 In
B
N
the design of new ladder systems, one crucial issue is how
Ar
Ar
to modify the electronic structure in order to provide the
required property to the p skeleton. In this regard, we re-
cently reported that the incorporation of boryl groups into
a N-heteroaryl-based p-conjugated skeleton so as to form
the intramolecular B–N coordination is a useful strategy
for endowing the electron-accepting character to the re-
sulting ladder p skeleton (Scheme 1a).2,3 Indeed, the pre-
pared B–N-coordinated thienylthiazole p-electron
systems showed high performances as an electron-trans-
porting material.2 The other issue in the development of
new ladder systems is how to achieve their efficient syn-
thesis. A facile and general synthetic method that can pro-
duce a series of ladder systems is particularly demanded.
In this regard, the intramolecular double cyclization (IDC)
of o,o¢-disubstituted diphenylacetylene precursors is a
new powerful tool to synthesize ladder oligo(p-phenyl-
enevinylene) (LOPV) skeletons (Scheme 1b).4 Several
types of the IDC reactions, proceeding via the acetylene
reduction,5 nucleophilic cascade cyclization,6–9 or syn-
chronous double radical cyclization,10–12 have allowed us
to synthesize a series of LOPV derivatives containing var-
(b)
E1
double
E1
cyclization
E2
E2
E1,E2 = CR2, NR, SiR2,
BR2–, PR2+, S, Se, etc.
Scheme 1 Design toward a new ladder p skeleton: (a) intramolecu-
lar B–N coordination for effective electronic modulation, and (b)
intramolecular double cyclization (IDC) as a useful synthetic tool
Coordination-Induced Intramolecular Double
Cyclization
We first employed di(2-pyridyl)acetylene (1a) as the
starting material (Scheme 2). When compound 1a was re-
acted with bromodibenzoborole in THF at room tempera-
ture, the bis(borylpyridine) complex 2a was immediately
produced as a yellow precipitate. Whereas this complex
can be isolated by recrystallization as a stable compound,
SYNTHESIS 2009, No. 1, pp 0127–0132
0
2
.
0
1
.2
0
0
9
Advanced online publication: 12.12.2008
DOI: 10.1055/s-0028-1083271; Art ID: C05208SS
© Georg Thieme Verlag Stuttgart · New York

128
Q. Zhao et al.
PAPER
bromodibenzoborole may play an important role in pro-
moting this reaction.
B
This IDC reaction has an advantage that the synthesis of
complex precursor molecules is not necessary and should
be applicable to other N-heteroaryl derivatives. Indeed,
we found that the reaction also proceeded when we em-
ployed di(2-thiazolyl)acetylene (1b) as the starting mate-
rial and obtained the mono-cyclized product 3b and
double-cyclized product 4b in 26 and 23% yields, respec-
tively, as shown in Scheme 2.
Br
N
N
1)
B
Ar
Ar
Br
THF, r.t.
Ar
Ar
N
N
Br
B
1a Ar(N) =
1b Ar(N) =
N
S
The double-cyclized products 4a and 4b have good solu-
bilities in the common organic solvents (for example, 39.2
mg/mL in THF for 4a and 16.8 mg/mL in THF for 4b), de-
spite their rigid bis(spiro) frameworks. These high solu-
bilities are probably due to their unsymmetrical structures.
2
N
B
O
B
O
N
Ar
2) H2O
Ar
N
Ar
N
Utility as a Building Unit for More Extended p-Elec-
tron Materials
+
N
Ar
B
3a 30%
3b 26%
As the building unit for more complex p-electron materi-
als, the thiazolyl derivative 4b has a great advantage such
as its facile functionalization at the terminal positions (5-
position of the thiazole rings). To demonstrate the utility
of 4b, we conducted the incorporation of the boryl groups
onto the terminal positions, since this is an effective way
to enhance the electron-accepting ability.16 Thus, the lithi-
ation with LDA in the presence of dimesitylboron fluoride
produced the monoboryl derivative 5 and diboryl deriva-
tive 6 in 29 and 43% yields, respectively (Scheme 3).
These derivatives were isolated by silica gel column chro-
matography as stable compounds. Although the formation
of the monoborylated product was simply due to the in-
complete dilithiation, the regioselectivity of the mono-
lithiation may have some implications. Thus, this result
suggests the possible synthesis of unsymmetrically di-
functionalized derivatives, which would be the basis for
further precise syntheses, such as the synthesis of regio-
regular polymeric materials or donor-acceptor-type mole-
cules. The regioselectivity of the monolithiation is proba-
bly due to the difference in the electronic effect of the
bridging moieties. The oxy(dibenzoborolyl) bridge may
be slightly more electron-withdrawing than the dibenzo-
borolyl group and thus may more stabilize the produced
anion on the adjacent thiazole ring.
4a 20%
4b 23%
Scheme 2 Coordination-induced double cyclization
we directly used it without purification for the subsequent
reaction. Thus, the treatment of 2a with a stoichiometric
amount of water under refluxing conditions followed by
silica gel column chromatography produced two products,
the mono-cyclized 3a and double-cyclized 4a in 30 and
20% yield, respectively. The structures of these products
were verified by H, 13C, and 11B NMR spectroscopies,
1
mass spectrometry, and finally by X-ray crystallography
for 4a (vide infra).15 The mechanism of this reaction is un-
clear at this stage, but is presumably that after formation
of the hydroxyborole, the nucleophilic cascade cycliza-
tion occurs to produce the doubly cyclized product 4a. In
order to retard the formation of the mono-cyclized 3a, we
tried the addition of several bases, such as Ag2O, howev-
er, this resulted in no improvement. For the present reac-
tion, the use of bromodibenzoborole is essential. The use
of BF3·OEt2 or dimesitylboron fluoride, instead of the bro-
modibenzoborole, only resulted in no reaction. Presum-
ably, the high Lewis acidity and high reactivity of the
B
O
B
O
LDA (2.4 equiv)
Mes2BF (3.0 equiv)
S
S
N
N
+
BMes2
4b
N
THF, –78 °C to r.t.
N
S
B
Mes2B
S
B
Mes2B
5 29%
6 43%
Scheme 3 Borylation of thiazolyl derivative 4b
Synthesis 2009, No. 1, 127–132 © Thieme Stuttgart · New York

PAPER
Synthesis of Boron-Bridged Dipyridylvinylenes and Dithiazolylvinylenes
129
Crystal Structures of the Ladder Skeleton Having
Intramolecular B–N Coordination
Among the produced compounds, the crystal structures of
4a and 4b were determined by X-ray crystallography,15
whose ORTEP drawings are shown in Figure 1. In both
crystal structures, two crystallographically independent
molecules are included in the crystal lattice. In all these
structures, the B–O-containing six-membered ring takes
the half-chair conformation, in which the boron atom is
deviated from the plane by 0.35–0.41 Å in 4a and 0.48–
0.50 Å in 4b. In the dipyridyl derivative 4a, except for the
boron atom in the six-membered ring, the other part of the
ladder p-conjugated skeleton maintains a rather coplanar
structure. The dihedral angles between the two terminal
pyridine rings are 5.52° and 11.85°. In contrast, the dithi-
azolyl derivative 4b has more distorted p-conjugated
frameworks. The dihedral angles between the two outer
thiazole rings are 7.47° and 23.36°, suggesting a more se-
vere ring strain in this ladder skeleton. As for the intramo-
lecular B–N distance, in both compounds, no significant
difference is observed between that in the B–O-containing
six-membered ring [1.607(3)–1.623(4) Å] and that in the
five-membered ring [1.596(2)–1.626(5) Å].
Figure 2 Photophysical properties of 3–6: (a) UV/vis absorption
spectra; and (b) fluorescence spectra in CH2Cl2
Table 1 Photophysical Data of Compounds 3–6 in CH2Cl2
UV/vis absorptiona
max (nm)
Fluorescenceb
max (nm)
Compound
l
log e
4.25
4.17
4.23
4.19
4.50
4.60
l
Ffc
3a
4a
3b
4b
5
398
428
402
443
471
495
485
510
510
541
570
598
0.22
0.85
0.06
0.42
0.65
0.28
6
a Only the longest absorption maxima are given.
b Excited at the longest absorption maxima.
c Absolute fluorescence quantum yield determined by a calibrated in-
tegral sphere system.
In the absorption spectra, the doubly cyclized compounds
4a and 4b have an absorption maximum at 428 and 443
nm, respectively. These absorption bands can be assigned
to the p-p* transition of the ladder p skeletons. These lmax
values are about 30–40 nm longer than those of the re-
spective mono-cyclized 3a and 3b, demonstrating the ex-
tent of the electronic effect of the second boryl bridges.
The DFT calculations of 4a and 4b at the B3LYP-6-
31G(d) level suggested that the incorporation of the sec-
ond boryl bridges decreases the p* (LUMO) level by
about –0.3 eV, while it does not affect the energy level of
the p orbital (HOMO-2). In both compounds, the HOMO
and HOMO-1 are mainly localized on the dibenzoborole
moieties.
Figure 1 ORTEP drawings of (a) 4a and (b) 4b (50% probability
for thermal elipsoids); hydrogen atoms are omitted for clarity
In the fluorescence spectra, 4a and 4b show intense green
and yellow fluorescences with the maximum wavelengths
at 510 and 540 nm, respectively. These values are again
longer by 25–30 nm compared to the respective mono-
cyclized derivatives 3a and 3b. Notably, the fluorescence
quantum yields of 4a and 4b are much higher that those of
3a and 3b, probably due to the rigid ladder structure in 4a
and 4b.
Photophysical and Electrochemical Properties
The UV/vis absorption and fluorescence spectra of com-
pounds 3–6 are shown in Figure 2, the data of which are
summarized in Table 1.
The extension of the p-conjugation by the incorporation
of the boryl groups generally results in the bathochromic
Synthesis 2009, No. 1, 127–132 © Thieme Stuttgart · New York

130
Q. Zhao et al.
PAPER
shifts in the absorption and fluorescence spectra. Indeed, noting, since its facile functionalization would allow us to
the monoborylated 5 and diborylated 6 show the red-shift- synthesize a variety of functional p-electron materials.
ed absorption and fluorescence maxima compared to Further structural modification as well as the application
those of 4a and 4b. Notably, the fluorescence maximum of these materials in organic electronic devices are now in
of 6 reaches 598 nm with a reddish orange emission color. progress in our laboratory.
a
Table 2 Electrochemical Properties of Compounds 3–6 in CH2Cl2
Melting points were measured on a Yanaco MP-S3 instrument. 1H,
13C, and 11B NMR spectra were recorded on a Jeol AL-400 spec-
trometer (400 MHz for 1H, 100 MHz for 13C, and 128 MHz for 11B)
in CDCl3. Mass spectra were measured with a Jeol JMS700. UV/vis
absorption and fluorescence spectra were performed with a Shimad-
zu UV-3150 spectrometer and a Hitachi F-4500 spectrometer, re-
spectively. Quantum yields were determined with a Hamamatsu
C9920-01 calibrated integral sphere system. TLC was performed on
plates coated with 0.25 mm thickness of silica gel 60 F-254
(Merck). Column chromatography was performed using neutral sil-
ica gel PSQ 60B (Fuji Silysia Chemical). Di(2-pyridyl)acetylene
(1a) and di(2-thiazolyl)acetylene (1b) were prepared according to
the literature.17,18 B-Bromodibenzoborole was prepared according
to the literature.19 The other materials were purchased from com-
mon commercial sources and used without additional purification.
All reactions were performed under argon, unless otherwise stated.
X-ray crystal structure determination was performed on a Rigaku
Single Crystal CCD X-ray Diffractometer. Crystals of 4a and 4b
suitable for X-ray structural analysis were obtained by recrystalliza-
tion from a CHCl3–hexane mixed solvent. Theoretical calculations
were conducted using Gaussian 03 program.20
Compound Eox1 (V)b
Eox2 (V)b
Ered1 (V)
–2.29b
–2.05
–2.00
–1.82
–1.63
–1.36
Ered2 (V)
3a
4a
3b
4b
5
+1.05
+0.95
+1.12
+0.94
+0.99
+1.02
+1.08
+1.06
+1.10
+1.12
–2.41b
–2.18b
–1.71
6
a Measurements conditions: sample 1 mM with Bu4NPF6 (0.1 M) at a
scan rate of 100 mVs–1. All potentials versus a ferrocene/ferrocenium
couple.
b Peak potentials.
The electrochemical properties of compounds 3–6 were
also investigated by cyclic voltammetry in CH2Cl2, the
data of which are summarized in Table 2. All of the com-
pounds showed reversible reduction waves, except for the
mono-cyclized 3a, while the oxidation waves for these
Intramolecular Double Cyclization of Di(heteroaryl)acetylenes;
Compounds 3a and 4a; Typical Procedure
To a mixture of 1a (200 mg, 1.11 mmol) and bromodibenzoborole
compounds are irreversible processes. The difference in (674 mg, 2.77 mmol) was added anhyd THF (30 mL) at r.t. After
heating the mixture with stirring at 80 °C for 2 h, H2O (1.1 M soln
in THF; 1 mL, 1.1 mmol) was added to the resulting suspension.
The mixture was stirred at the same temperature for 24 h. After re-
moval of the solvents, purification by column chromatography on
the reversibility of the reduction process between 3a and
4a demonstrates the significance of the ladder structure on
the stabilization of the produced radical anion species. It
is also worth noting that the borylated compounds 5 and 6
silica gel (CH2Cl2–hexane) followed by recrystallization from
showed the first reversible reduction waves at rather low
reduction potentials. Moreover, the diborylated 6 exhibit-
ed a second reduction as a reversible process. The struc-
tural modification from 3b to 6 resulted in a decrease of
the reduction potential by 0.64 V. This value well demon-
strates the effectiveness of the p-p* conjugation between
the p* orbital of the ladder skeleton and the vacant p-
orbitals of the terminal boryl groups.
CH2Cl2–hexane gave 120 mg (0.33 mmol) of 3a and 115 mg (0.22
mmol) of 4a in 30% and 20% yield, respectively.
3a
Yellow solid; mp 208–209 °C.
1H NMR (400 MHz, CDCl3): d = 6.90 (t, JHH = 6.4 Hz, 1 H), 7.09
(t, JHH = 7.2 Hz, 3 H), 7.26–7.34 (m, 4 H), 7.58 (d, JHH = 7.2 Hz, 2
H), 7.64 (d, JHH = 6.4 Hz, 4 H), 7.72 (t, JHH = 7.6 Hz, 1 H), 7.93 (d,
JHH = 8.0 Hz, 1 H), 8.65 (d, JHH = 4.4 Hz, 1 H).
13C NMR (100 MHz, CDCl3): d = 95.33, 119.17, 120.63, 121.76,
122.47, 124.56, 126.87, 128.31, 130.05, 136.75, 140.31, 141.11,
148.42, 148.78, 152.49, 152.71, 164.51.
Conclusion
11B NMR (128 MHz, CDCl3): d = 6.62.
We have developed a new type of intramolecular double
cyclization starting from the di(N-heteroaryl)acetylenes
with bromodibenzoborole, which proceeds through the
formation of the B–N coordination complex. This method
has a significant advantage such that a simple procedure
using simple precursors allows us to produce various het-
eroaryl-containing ladder p-conjugated skeletons. Thus
the prepared ladder p systems with the intramolecular B–
N coordination show intriguing photophysical and elec-
trochemical properties, such as intense emission and re-
versible reduction waves at low reduction potentials in the
cyclic voltammetry. The utility of the ladder di(2-thiaz-
olyl)vinylene derivative as a building unit is also worth
HRMS (EI): m/z calcd for C24H17BN2O: 360.1434; found:
360.1440.
4a
Yellow solid; mp >300 °C.
1H NMR (400 MHz, CDCl3): d = 6.56 (d, JHH = 8.4 Hz, 1 H), 6.80
(t, JHH = 6.4 Hz, 1 H), 7.12–7.21 (m, 7 H), 7.29–7.41 (m, 5 H), 7.66–
7.71 (m, 5 H), 7.83 (t, JHH = 7.6 Hz, 3 H), 7.89–7.96 (m, 2 H).
13C NMR (100 MHz, CDCl3): d = 119.06, 119.27, 119.56, 121.35,
122.49, 123.75, 126.72, 126.93, 127.63, 128.33, 130.00, 130.13,
140.71, 140.77, 141.45, 142.43, 148.39, 150.59, 152.26, 155.33,
159.05.
11B NMR (128 MHz, CDCl3): d = 1.32, 7.46.
Synthesis 2009, No. 1, 127–132 © Thieme Stuttgart · New York

PAPER
Synthesis of Boron-Bridged Dipyridylvinylenes and Dithiazolylvinylenes
131
HRMS (EI): m/z calcd for C36H24B2N2O: 522.2075; found:
129.62, 129.71, 139.44, 140.08, 140.48, 140.50, 144.51, 146.41,
522.2089.
148.25, 150.67, 157.59, 171.68, 174.83.
11B NMR (128 MHz, CDCl3): d = –0.01, 6.43, 65.95.
HRMS (APCI): m/z calcd for C68H63N2S2B4O [(M + H)+]:
3b
Yield: 26%; yellow solid; mp 183–184 °C.
1031.4754; found: 1031.4749.
1H NMR (400 MHz, CDCl3): d = 6.84 (d, JHH = 4.0 Hz, 1 H), 6.91
(d, JHH = 4.0 Hz, 1 H), 7.06 (s, 1 H), 7.10 (t, JHH = 8.0 Hz, 2 H), 7.30
(d, JHH = 7.6 Hz, 2 H), 7.48–7.52 (m, 3 H), 7.64 (d, JHH = 7.6 Hz, 2
H), 7.94 (d, JHH = 3.2 Hz, 1 H).
Acknowledgment
This work was partly supported by Grants-in-Aid (No. 19675001
and 17069011) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
13C NMR (100 MHz, CDCl3): d = 88.96, 114.82, 119.13, 123.03,
126.79, 128.45, 129.95, 135.11, 144.20, 148.22, 159.73, 164.46,
165.43.
11B NMR (128 MHz, CDCl3): d = 6.27.
References
HRMS (EI): m/z calcd for C20H13BN2OS2: 372.0562; found:
372.0580.
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4b
Yield: 23%; yellow solid; mp 290–292 °C.
1H NMR (400 MHz, CDCl3): d = 6.76 (d, JHH = 3.6 Hz, 1 H), 6.89
(d, JHH = 3.6 Hz, 1 H), 7.12–7.19 (m, 6 H), 7.28–7.38 (m, 6 H), 7.57
(d, JHH = 7.2 Hz, 2 H), 7.65 (d, JHH = 7.6 Hz, 2 H), 7.79 (d, JHH = 7.6
Hz, 2 H).
13C NMR (100 MHz, CDCl3): d = 116.99, 119.26, 119.57, 121.71,
126.60, 126.89, 127.85, 128.55, 129.91, 130.02, 134.89, 135.03,
148.25, 150.72, 156.44, 165.32, 168.83.
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Functionalization of Compound 4b
To a solution of 4b (50.0 mg, 0.094 mmol) and dimesitylboron
fluoride (75.3 mg, 0.28 mmol) in anhyd THF (10 mL) was added
LDA (0.83 M in THF, 0.28 mL, 0.23 mmol) dropwise at –78 °C.
The resulting mixture was stirred at the same temperature for 1 h
and then was allowed to warm to r.t. with stirring overnight. After
removal of the solvent, the residual sticky red oil was passed
through a silica gel column (CH2Cl2–hexane, 1:2) followed by re-
crystallization from a CHCl3–hexane mixed solvent to give pure
products 5 (21 mg, 0.027 mmol) and 6 (41 mg, 0.040 mmol).
5
Yield: 29%; orange solid; mp 208–210 °C.
1H NMR (400 MHz, CDCl3): d = 1.80 (s, 12 H), 2.19 (s, 6 H), 6.62
(s, 4 H), 7.07 (s, 1 H), 7.11–7.17 (m, 6 H), 7.28–7.35 (m, 6 H), 7.56
(d, JHH = 6.8 Hz, 2 H), 7.61 (d, JHH = 7.6 Hz, 2 H), 7.73 (d, JHH = 7.2
Hz, 2 H).
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Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc. 2006, 128,
3044.
13C NMR (100 MHz, CDCl3): d = 21.00, 23.13, 119.26, 119.65,
122.21, 126.52, 126.86, 127.78, 128.35, 128.50, 129.67, 129.90,
129.94, 134.90, 139.40, 140.49, 146.46, 148.23, 150.76, 157.71,
168.05, 171.99.
(13) Entwistle, C. D.; Batsanov, A. S.; Howard, J. A. K.; Fox, M.
A.; Marder, T. B. Chem. Commun. 2004, 702.
(14) Vagedes, D.; Kehr, G.; König, D.; Wedeking, K.; Fröhlich,
R.; Erker, G.; Mück-Lichtenfeld, C.; Grimme, S. Eur. J.
Inorg. Chem. 2002, 2015.
(15) Crystallographic data for 4a and 4b have been deposited
with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 706194 and 706195,
respectively. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: +44(1223)336033 or e-mail:
data_request@ccdc.cam.ac.UK].
(16) (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120, 9714.
(b) Noda, T.; Ogawa, H.; Shirota, Y. Adv. Mater. 1999, 11,
283.
11B NMR (128 MHz, CDCl3): d = –0.03, 6.97, 65.56.
HRMS (EI): m/z calcd for C50H41B3N2OS2: 782.2939; found:
782.2936.
6
Yield: 43%; red solid; mp 260–262 °C.
1H NMR (400 MHz, CDCl3): d = 1.79 (s, 12 H), 2.00 (s, 12 H), 2.19
(s, 6 H), 2.24 (s, 6 H), 6.62 (s, 4 H), 6.75 (s, 4 H), 7.08 (s, 1 H), 7.10–
7.16 (m, 6 H), 7.25–7.31 (m, 5 H), 7.43 (s, 1 H), 7.53 (d, JHH = 7.2
Hz, 2 H), 7.59 (d, JHH = 7.6 Hz, 2 H), 7.70 (d, JHH = 7.6 Hz, 2 H).
13C NMR (100 MHz, CDCl3): d = 21.00, 21.03, 23.14, 23.29,
119.18, 119.70, 126.49, 126.78, 127.73, 128.35, 128.43, 128.61,
(17) Zaman, M. B.; Smith, M. D.; Ciurtin, D. M.; Loye, H.-C. Z.
Inorg. Chem. 2002, 41, 4895.
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