Application of the sila-Friedel-Crafts reaction to the synthesis of π-extended silole derivatives and their properties

Article abstract of DOI:10.1039/c0dt00136h

The intramolecular sila-Friedel-Crafts reaction was developed as a new method for the construction of a dibenzosilole skeleton. This reaction proceeds under mild conditions to afford the target in relatively good yield, indicating its availability as a versatile synthetic method. This reaction can be applied to the synthesis of π-extended silole derivatives such as ladder-type silafluorene 8 and spiro-type silabifluorene 9. Furthermore, the synthesis of two-dimensionally extended silole derivatives utilizing the sila-Friedel-Crafts reaction as the multiple intramolecular cyclization was achieved, including the first synthesis of trisilasumanene 18. The X-ray crystallographic analysis of trisilasumanene 18 demonstrated the planarity in the main π-framework, in contrast to sumanene and its sulfur analogue, trithiasumanene, bearing the bowl-shaped structures. In the UV-vis absorption spectra, the absorption bands of triphenylenosiloles 18 and 19 were slightly red-shifted compared to that of hexabutoxytriphenylene 22. The weak absorption bands were also observed in the longer-wavelength region in 18 and 19, which is derived from σ*-π* conjugation of the silole skeletons. In addition, 18 and 19 showed the blue fluorescence in dichloromethane and in the solid state. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0dt00136h

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www.rsc.org/dalton | Dalton Transactions
Application of the sila-Friedel–Crafts reaction to the synthesis of p-extended
silole derivatives and their properties†
Shunsuke Furukawa, Junji Kobayashi* and Takayuki Kawashima*‡
Received 15th March 2010, Accepted 22nd July 2010
DOI: 10.1039/c0dt00136h
The intramolecular sila-Friedel–Crafts reaction was developed as a new method for the construction of
a dibenzosilole skeleton. This reaction proceeds under mild conditions to afford the target in relatively
good yield, indicating its availability as a versatile synthetic method. This reaction can be applied to the
synthesis of p-extended silole derivatives such as ladder-type silafluorene 8 and spiro-type silabifluorene
9. Furthermore, the synthesis of two-dimensionally extended silole derivatives utilizing the
sila-Friedel–Crafts reaction as the multiple intramolecular cyclization was achieved, including the first
synthesis of trisilasumanene 18. The X-ray crystallographic analysis of trisilasumanene 18
demonstrated the planarity in the main p-framework, in contrast to sumanene and its sulfur analogue,
trithiasumanene, bearing the bowl-shaped structures. In the UV-vis absorption spectra, the absorption
bands of triphenylenosiloles 18 and 19 were slightly red-shifted compared to that of
hexabutoxytriphenylene 22. The weak absorption bands were also observed in the longer-wavelength
region in 18 and 19, which is derived from s*–p* conjugation of the silole skeletons. In addition, 18 and
19 showed the blue fluorescence in dichloromethane and in the solid state.
such p-extended silole derivatives has not been established so far.
So the development of a new type of synthetic methodology will be
a keystone for further progression to two-dimensionally extended
derivatives. As a new promising method, we propose the direct
silylation of an aromatic ring.
Introduction
A silole, a silicon-containing cyclopentadiene, has a significant
low-lying LUMO that is derived from s*–p* conjugation between
the s* orbital of the exocyclic two Si–C bonds with p* orbital of the
butadiene moiety.1 Because of this property, silole derivatives are
widely used as organic photoelectronic devices.2 The p-extended
silole derivatives such as ladder-type silafluorenes3 and spiro-type
silabifluorenes4 have desirable properties for application to the
organic devices, such as high luminescence efficiencies, high glass-
transition temperatures and so forth. Because of these advantages,
further extension or modification of the silole framework is
intriguing. However, these compounds are considered difficult
to synthesize using conventional methods. Therefore, a new
synthetic methodology of dibenzosiloles is required, and the
development of transition metal-catalyzed [2+2+2] cyclization,3a
the addition of a silyl group to an alkyne,2a,5 and the cross-coupling
reaction of silicon-bridged biaryls have been reported recently.6
Despite the recent remarkable progress in this field, synthesizable
silicon-containing p-conjugated skeletons are limited to mainly
benzo- or dibenzo-fused siloles, silicon-bridged stilbenes,5a,e and at
most one-dimensionally extended ladder type silafluorenes.3 Two-
dimensionally extended compounds are expected to have some
advantages for the application to the organic devices because of
their improved stability upon redox by the charge delocalization
and induction of the intermolecular interaction such as p–p
stacking in the crystalline state. However, a synthetic method for
The Friedel–Crafts reaction is a well-known method for the
direct substitution of an aromatic ring. On the other hand, Friedel–
Crafts-type silylation, a sila-Friedel–Crafts reaction, involving a
silicenium ion7,8 as an intermediate, only occurs with electron-
rich aromatic rings such as ferrocene and pyrrole.9 However, in
the case of a non-activated aromatic ring such as benzene, the
conversion of the reaction markedly decreased.10 Thus, the sila-
Friedel–Crafts reaction has not been used as a versatile synthetic
method to date. Herein, we describe the development of a sila-
Friedel–Crafts reaction that is applicable to the synthesis of
dibenzosiloles. Furthermore, we have succeeded in synthesizing
the p-extended silole derivatives including a silicon analogue of
sumanene,11 trisilasumanene, by application of the sila-Friedel–
Crafts reaction. Parts of this work have been communicated.12
Strategy for the development of the sila-Friedel–Crafts
reaction
It is known that the Friedel–Crafts reaction can introduce a
substituent into the substrate after the formation of a s-arene
complex. In the case of the introduction of a silyl group, however,
an intermediate, a silylated arenium ion has a p-complex character
significantly,13 and hence the reverse reaction rate (i.e., rate of
protodesilylation of the target arylsilane) is faster than that of
silylation.14 Because of these disadvantages, the sila-Friedel–Crafts
reaction has been considered difficult to achieve. To overcome
these problems, we proposed mainly three improvements as a
strategy. The basic strategy is shown in Scheme 1. (1) For the
generation of a silicenium cation that is well-known to be a reactive
Department of Chemistry, Graduate School of Science, The University
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033, Tokyo, Japan. E-mail:
takayuki@chem.s.u-tokyo.ac.jp
† CCDC reference numbers 768893 for 9 and 770156 for 19. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00136h
‡ Present address: Faculty of Science, Gakushuin University, 1-5-1 Mejiro,
Toshima-ku, Tokyo 171-8588, Japan.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9329–9336 | 9329
©

Scheme 1 The basic strategy for the development of the sila-Friedel–Crafts reaction.
intermediate, the reaction of a hydrosilane with a trityl cation
To elucidate the substituent effect on a silicon atom, hydrosilane
5 bearing two methyl groups on the silicon atom was used as a
starting material for the sila-Friedel–Crafts reaction (Scheme 2).
The conversion to target dibenzosilole 6 was relatively low (25%)
under the optimized conditions for the synthesis of diphenyl-
substituted silane 1. The reason may be owing to the instability of
intermediary generated silicenium ion, which should be sterically
protected by bulky substituents, and stabilized by p-conjugation
of phenyl groups.
was employed because it has been acknowledged as a superior
method for the generation of the silicenium ions under gentle
reaction conditions.13a,15 (2) For easy progression of the reaction,
an intramolecular reaction was carried out in anticipation of rate-
acceleration by the entropy effect. (3) For prevention of the reverse
reaction from occurring, a base was used to trap a released proton.
Results and discussion
Development of the intramolecular sila-Friedel–Crafts reaction
According to the basic strategy described in Scheme 1, hydrosilane
1 was prepared as a starting material.16 The reaction of hydrosilane
1 with trityl perchlorate and 2,6-lutidine in dichloromethane at
room temperature gave dibenzosilole 2 in a 52% yield. Silanol 3
and disiloxane 4 were also formed as side products (Table 1, entry
1).17 This moderate conversion may be caused by the deactivation
of the intermediate because of the coordination of a perchlorate
anion to the silicenium ion. By changing the counteranion to
tetrakis(pentafluorophenyl)borate, the conversion yield of 2 was
significantly increased to 84% (entry 2). This reaction was very
sensitive to the choice of a base, that is, no bases other than
2,6-lutidine worked well (entries 3–6). The reasons for the high
efficiency of 2,6-lutidine are still unclear, but the bulkiness and
basicity of 2,6-lutidine should be important factors in this reaction.
This reaction was also found to be sensitive to the choice of
solvents, judging from the fact that when MeCN or MeNO2 was
used as a solvent, the desired reaction did not proceed at all (entry
7–8).
Scheme 2 Sila-Friedel–Crafts reaction of a dimethyl-substituted hydrosi-
lane 5.
Application of the intramolecular sila-Friedel–Crafts reaction
As described above, the intramolecular sila-Friedel–Crafts reac-
tion afforded a dibenzosilole in good yield under mild conditions
(Table 1, entry 2). Therefore, we applied this reaction to the syn-
thesis of p-extended dibenzosilole derivatives. The intramolecular
sila-Friedel–Crafts reaction of 2,2¢¢-bis(diphenylsilyl)-1,1¢:4¢,1¢¢-
terphenyl 7 afforded the corresponding ladder-type silafluorene
8 (Scheme 3).12 This result suggests that the sila-Friedel–Crafts
reaction is applicable for intramolecular double cyclization. Fur-
themore, we have considered that spirobisilafluorene 9 can be
synthesized when twice sila-Friedel–Crafts reactions were carried
out on the same silicon atom. Thus we employed dihydrosilane
Table 1 Intramolecular sila-Friedel–Crafts reaction
Result (yield/%)a
Entry
Oxidant
Base
Solvent
1
2
3
4
1
2
3
4
5
6d
7
8
Ph3CClO4
2,6-lutidine
2,6-lutidine
proton-sponge
pyridine
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
MeNO2
0
0
100
—
100
50
100
100
52
84
0
23
12
0
25
4
0
Ph3CB(C6F5)4
Ph3CB(C6F5)4
Ph3CB(C6F5)4
Ph3CB(C6F5)4
Ph3CB(C6F5)4
Ph3CB(C6F5)4
Ph3CB(C6F5)4
b
DTBMPc
0
0
0
0
0
0
0
0
0
0
0
0
(i-Pr)2NEt
2,6-lutidine
2,6-lutidine
a Estimated by 1H NMR. b Complex mixture was obtained. c DTBMP represents 2,6-di-t-butyl-4-methylpypridine. d An unidentified compound was
obtained.
9330 | Dalton Trans., 2010, 39, 9329–9336
This journal is
The Royal Society of Chemistry 2010
©

Scheme 3 Synthesis of a ladder-type silafluorene 8 by the sila-Friedel–Crafts reaction.
10 as a starting material. Indeed, when the sila-Friedel–Crafts
addition, the weak absorption band derived from the HOMO–
LUMO transition was observed at the longer wavelength region
(331 nm). In the case of spirobisilafluorene 9, the spectral shape
was similar to that of 2, but the molar absorption coefficient was
approximately twice that of 2. The longest-wavelength absorption
maximum was slightly red-shifted to 336 nm. It is probably because
of the elevation of the energy level of HOMO by the spiro
conjugation and stabilization of LUMO by s*–p* conjugation
with two biphenyl units.18 These considerations were supported
by the theoretical calculation of a model compound, 9,9¢-spiro-
9-silabifluorene (9¢), optimized at B3LYP/6-31+G(d) levels of
theory (Fig. 3). On the other hand, the absorption bands of
ladder-type silafluorene 8 (370 nm) were significantly red-shifted
compared with that of 2. The theoretical calculation of the model
compound 8¢ suggested the extension of p-conjugation through
s*–p* conjugation with two silicon atoms. This p-extension
induced the HOMO–LUMO gap narrowing of 8, is responsible
for the red-shifted longer-wavelength absorption.
reaction was applied to 10, and the target 9 was produced,
although it was in low yield (Scheme 4). This result indicates
that the sila-Friedel–Crafts reaction is available as a new method
for multistep C–Si bond formation on the same silicon atom.
The structural determination of 9 was finally achieved by X-ray
crystallographic analysis (Fig. 1).
Scheme 4 Synthesis of spirobisilafluorene 9 by the sila-Friedel–Crafts
reaction.
Fig. 1 ORTEP drawing of spirobisilafluorene 9 (50% probability).
To elucidate the optical property of silole derivatives 8 and 9,
the UV-vis absorption spectra were measured in dichloromethane.
Dibenzosilole 2 was also studied as a reference compound. The
absorption spectrum of dibenzosilole 2 showed the relatively
intense absorptions at around 230 and 297 nm (Fig. 2). In
Fig. 3 Relative HOMO and LUMO energy levels and profiles for model
compounds 2¢, 8¢, and 9¢ obtained at B3LYP/6-31+G(d) level of theory.
The results of the emission spectra were summarized in Table 2.
Dibenzosilole 2 (lem = 362, UF = 0.18) and spirobisilafluorene 9
(lem = 370, UF = 0.14) showed the emission at the UV region
in dichloromethane, and similar emissions with slightly enhanced
Table 2 Emission properties of 2, 8, and 9 in dichloromethane and in the
solid state
Compound
2
8
9
l
em/nm (UF)a
362 (0.18)
361 (0.24)
400, 410 (0.43)
419 (0.66)
370 (0.14)
370 (0.20)
(in CH2Cl2)b
em/nm (UF)a
(in the solid state)
l
a Absolute fluorescence quantum yield estimated by the integrating sphere
system. b Measured at 3.6 ¥ 10-5 for 2, 2.3 ¥ 10-5 for 8, and 1.4 ¥ 10-5 M for
9 in CH2Cl2.
Fig. 2 UV-vis absorption spectra of 2 (dotted line), 8 (solid line), and 9
(dashed line) in CH2Cl2 (3.6 ¥ 10-5 for 2, 2.3 ¥ 10-5 for 8, and 1.4 ¥ 10-5
for 9).
M
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Dalton Trans., 2010, 39, 9329–9336 | 9331
©

fluorescence quantum yields were obtained in the solid state (lem
=
in the main framework has also been reported recently.21 For
background, a silicon analogue of sumanene, trisilasumanene has
attracted much attention as both a sumanene analogue and a novel
non-linearly expanded silole derivative. However, trisilasumanene
was considered difficult to synthesize using conventional methods.
Thus, we applied the intramolecular sila-Friedel–Crafts reaction
to the synthesis of trisilasumanene.12
361, UF = 0.24, for 2 and lem = 370, UF = 0.20 for 9). On the other
hand, ladder-type silafluorene 8 showed an intense blue emission
in dichloromethane (lem = 400, 410, UF = 0.43), and this blue
emission was also observed in the solid state (lem = 419, UF = 0.66)
(Fig. 4). Because of these properties, 8 is expected to be applied to
the organic blue light emitting material.
Starting from an isomeric mixture of tribromotriphenylene
15,22 double-cyclized monobromide 16 was synthesized in two
steps including a dual sila-Friedel–Crafts reaction (Scheme 6).
Transformation to the precursor 17 was achieved by lithiation of 16
followed by the addition of diphenylsilane in a 51% yield. Finally,
the intramolecular sila-Friedel–Crafts reaction was applied again
to 17, and trisilasumanene 18 was obtained in an 18% yield as a
colorless solid. A desilylated compound, triphenylenodisilole 19,
was also obtained as a by-product.
Fig. 4 Emission spectra of 8 in CH2Cl2 (solid line) and in the solid state
(dashed line), and a photograph of 8 in CH2Cl2 under UV irradiation.
As described above, the intramolecular sila-Friedel–Crafts
reaction contributed to the development of a novel synthetic
method of dibenzosilole derivatives under mild conditions. To this
advantage, this modified reaction was applied to the intermolecular
silylation of an aromatic ring. The reaction of triphenylsilane
with Ph3CB(C6F5)4, 2,6-lutidine in benzene was carried out at
room temperature (Scheme 5). Under these reaction conditions, a
target silylated compound, tetraphenylsilane (11), was isolated in
31% yield as a major product in sharp contrast to conventional
methods requiring severe conditions such as use of moisture-
sensitive silanes as the starting materials or employing electron-
rich aromatic substances, and stirring at high temperature for
a long time.9,10 Thus, the improvement of intermolecular sila-
Friedel–Crafts reaction was also achieved.
Scheme 6 Synthesis of trisilasumanene 18. Reaction conditions: (a) (i)
n-BuLi (2.0 eq), THF, -78 ◦C, (ii) Ph2SiCl2, r.t., (iii) LiAlH4, reflux;
(b) Ph3CB(C6F5)4, 2,6-lutidine, CH2Cl2, r.t.; (c) (i) t-BuLi (2.0 eq), Et2O,
-78 ◦C, (ii) Ph2SiH2, reflux; (d) Ph3CB(C6F5)4, 2,6-lutidine, CH2Cl2, r.t.
The structure of trisilasumanene 18 was established by X-ray
crystallographic analysis. The unit cell contains two crystallo-
graphically independent molecules that have almost the same
structure;12 one is shown in Fig. 5. It is known that sumanene
12 and trithiasumanene 13 have a bowl-shaped structure with
23,20
˚
bowl depths of 1.11 and 0.65 A, respectively.
In contrast,
Scheme 5 The intermolecular sila-Friedel–Crafts reaction.
X-ray structural analysis of trisilasumanene 18 indicated that the
main framework was almost planar. These results are consistent
with the theoretical calculations reported by Priyakumar and
Sastry,24 and such a structural difference is mainly attributed to
the larger covalent radii of the main group elements such as sulfur
and silicon. Unfortunately, further consideration of the detailed
structure of 18 was unsuccessful because the structural refinement
Two-dimensionally extended silole derivatives
One of the partial fragment of C60, sumanene (12), has attracted
attention because of its bowl-shaped structure and novel chemical
and physical properties (Chart 1).11,19 Its heteroatom analogues,
heterasumanenes, are also very interesting research targets, be-
cause these compounds have both two-dimensional extended p-
systems and various properties derived from the introduced main
group elements. As the first example for the heterasumanene,
the synthesis of trithiasumanene (13) was reported by Otsubo.20
A heterasumanene 14 that contains three different heteroatoms
Fig. 5 (a) Top view and (b) side view of the crystal structure of
trisilasumanene 18. In (b), the six butoxy groups have been omitted for
clarity.
Chart 1 Structures of sumanene and its main-group analogues.
9332 | Dalton Trans., 2010, 39, 9329–9336
This journal is
The Royal Society of Chemistry 2010
©

of 18 was not good enough owing to the severe disorder of the
butoxy groups and solvent molecules.
The structure of triphenylenodisilole 19 obtained as a major
product (Scheme 6) was also investigated. X-ray crystallographic
analysis of 19 indicated that the main p-framework of 19 is slightly
twisted owing to the steric repulsion of two hydrogen atoms at
the bay area of the triphenylene framework (Fig. 6). The C9–
C10–C11 bond angle (124.7(2)◦) and the C8–C11 interatomic
˚
distance (3.084 A) of 19◦are larger and longer than those of
triphenylene (20)25 (121.5 , 2.943 A) (Fig. 7). Compared with
˚
diphenyldibenzosilole 21,26 the little longer Si–C bonds (Si1–C5:
˚
˚
1.890(3) A, Si1–C2: 1.889(3) A) were observed in 19, although
there was only a small difference between the C14–Si2–C17 bond
angle (91.87(10)◦) and that of 21 (91.8◦) in the five-membered
Fig. 8 UV-vis absorption spectra of 18 (solid line), 19 (dashed line), and
22 (dotted line) in CH2Cl2 (1.0 ¥ 10-5 M for 22, 1.6 ¥ 10-5 M for 19 and
1.7 ¥ 10-5 M for 18).
˚
˚
ring. The C2–C5 (2.727 A) and C14–C17 (2.708 A) interatomic
distances were only a little longer than that of 21. These results
suggest that triphenylenodisilole 19 also has both structural
characters of triphenylene and dibenzosilole, but the dibenzosilole
character is more preferred.
the HOMO–LUMO transitions, suggesting the existence of s*–
p* conjugation in the silole skeletons of 18 and 19 as described
above (Fig. 3). For further consideration of the electronic struc-
tures of triphenylenosiloles, theoretical calculations using density
functional theory (B3LYP/6-31+G(d)) were performed. Fig. 9
shows the frontier orbitals and their energy levels of the model
compounds 18¢ for 18, and 19¢ for 19, in which phenyl groups
are replaced by methyl groups and the peripheral butoxy groups
are replaced by hydrogen atoms, as well as a reference compound
20. The character of HOMOs of 18¢ and 19¢ reflects in that of
the parent framework 20. On the other hand, LUMOs of 18¢ and
19¢ showed the s*–p* conjugations of the silole skeletons, and the
energy levels decreased from -1.30 eV to -1.51 eV as the number of
the silicon atom increased. As a result, the HOMO–LUMO gaps
narrowed, and these results were consistent with the red-shifted
absorption bands of triphenylenosiloles 18 and 19 described above
(Fig. 8).
Fig. 6 (a) Top view and (b) side view of the crystal structure of
triphenylenodisilole 19. In (b), the six butoxy groups and four phenyl
groups have been omitted for clarity.
Fig. 9 Graphical drawings and HOMOs and LUMOs energy levels of
model compounds 18¢, 19¢, and 20 obtained at B3LYP/6-31+G(d).
◦
˚
Fig. 7 The selected bond lengths [A] and angles [ ] of the crystal structure
of 19–21.
The emission spectra of 18, 19, and 22 were measured in
dichloromethane at room temperature (Fig. 10). In the emission
maximum of the reference compound 22 was observed at 386 nm.
In contrast, the triphenylenosilole derivatives showed the slightly
red-shifted blue emissions (19, lmax = 406, 426 nm; 18, lmax = 427,
451 nm), respectively. In addition, these compounds also showed
blue emissions in the solid state at room temperature (Fig. 11). The
fluorescence quantum yields are summarized in Table 3. There was
only a small difference between the quantum yields in the solution
and the solid states for these compounds, indicating the existence
of a nonradiative decay process to some extent. It is known
To elucidate the electronic structure, the UV-vis absorp-
tion spectra of triphenylenosiloles 18 and 19 were measured
in dichloromethane, and compared with that of 2,3,6,7,10,11-
hexabutoxytriphenylene (22) (Fig. 8). The intense absorption
bands of 19 (lmax = 289 nm, log e = 4.81) and 18 (lmax = 299 nm,
log e = 4.67) were slightly red-shifted from that of 22 (lmax
=
280 nm, log e = 5.09) and sumanene 12 (lmax = 278 nm).19d In
addition, weak absorption bands were observed in the longer-
wavelength region (>350 nm) in triphenylenosiloles 18 and 19.
These longer-wavelength absorptions were mainly attributed to
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9329–9336 | 9333
©

Table 3 Emission properties of 18, 19, and 22 in dichloromethane and in
two-dimensionally extended silole derivatives utilizing the sila-
Friedel–Crafts reaction as the multiple intramolecular cyclization
was achieved, including the first synthesis of trisilasumanene
18. The X-ray crystallographic analysis of 18 demonstrated the
planarity in the main p-framework, in contrast to sumanene and
trithiasumanene bearing the bowl-shaped structures. In the UV-vis
absorption spectra, the absorption bands of 18 and 19 were slightly
red-shifted compared to that of hexabutoxytriphenylene 22. The
weak absorption bands were observed in the longer-wavelength
region in 18 and 19 derived from s*–p* conjugation of the silole
skeletons. In addition, 18 and 19 showed the blue fluorescence both
in dichloromethane and in the solid state at room temperature.
the solid state
Compound
22
19
406, 426 (0.19) 427, 451 (0.05)
431, 447 (0.07)
18
l
em/nm (UF)a
386 (0.10)
(in CH2Cl2)b
em/nm (UF)a
(in the solid state)
l
386, 408 (0.09) 420 (0.17)
a Absolute fluorescence quantum yield estimated by the integrating sphere
system. b Measured at 1.0 ¥ 10-5 for 22, 1.6 ¥ 10-5 for 19, and 1.7 ¥ 10-5
for 18 in CH2Cl2.
M
Experimental
General considerations
Dichloromethane was shaken with conc. H2SO4, washed with
water, and distilled from CaH2 for use in the sila-Friedel–Crafts
reaction. THF, Et2O, and benzene were purified by MBRAUN
MB-SPS system equipped with activated Al2O3 and molecular
sieves columns. The other solvents were purified before use by
the reported methods. All reactions were carried out under an
argon atmosphere. NMR spectra were recorded by a Bruker DRX-
500 spectrometer (1H NMR, 500 MHz; 13C NMR, 126 MHz;
29Si NMR, 99 MHz) and a JEOL AL-400 spectrometer (1H
NMR, 400 MHz; 13C NMR, 100 MHz; 29Si NMR, 79 MHz).
Fig. 10 Emission spectra of 18, 19, and 22 in CH2Cl2 (solid line, 1.0 ¥
10-5 M for 22, 1.6 ¥ 10-5 M for 19 and 1.7 ¥ 10-5 M for 18) and in the solid
states (dotted line) at room temperature.
that triphenylene derivatives, which have the parent skeleton
of triphenylenosiloles, show phosphorescence emission under
the cryogenic temperature.27 In fact, upon measuring emission
1
Tetramethylsilane was used as an external standard for the H
spectrum of 19 at 77 K, a bluish green phosphorescence (lonset
=
1
1
NMR, 13C{ H}, and 29Si{ H} NMR spectra. High resolution and
low resolution mass spectra were obtained with a JEOL JMS-
700P (FAB) and a Shimadzu GCMS-QP2010 gas chromatograph
mass spectrometer. Melting points were measured with a Yanaco
MP-S3 instrument. All melting points are uncorrected. Silica gel
column chromatography was performed using Kanto Silica Gel
469 nm) was observed. These results indicate that the nonradiative
decay process of 19 can be attributed to the spin-forbidden
relaxation to triplet excited states from the singlet excited state.
The T1 transition energy estimated from the emission maximum
was 2.64 eV. Compared to the T1 energy of 2,3,6,7,10,11-
hexakis(hexyloxy)triphenylene (23) (lonset = 441 nm, 2.81 eV),28
the T1 state of 19 is considered to be stabilized to a lower energy
level.
˚
60 N and MP Biomedicals MP silica DCC 60 A. Preparative
thin layer chromatography was performed using Merck Silica gel
60 PF254. Gel permeation liquid chromatography was performed
using LC-918 and LC-908/C-60 with JAIGEL 1H+2H columns
(Japan Analytical Industry) using chloroform as a solvent. UV-vis
spectra and fluorescence spectra were measured with a JASCO V-
670 spectrometer and a JASCO FP6500 fluorescence spectropho-
tometer, respectively, in spectral grade dichloromethane. Absolute
fluorescence quantum yields were determined with a Hamamatsu
C9920-02 calibrated integrating sphere system. Elemental analyses
were performed by the Microanalytical Laboratory of Department
of Chemistry, Faculty of Science, The University of Tokyo.
Conclusions
The intramolecular sila-Friedel–Crafts reaction was developed as
a new synthetic method of dibenzosilole derivatives. This reaction
proceeds under mild conditions to afford the target in relatively
good yield, indicating its availability as a versatile synthetic
method. The p-extended silole derivatives such as a ladder-type
silafluorene 8 and a spiro-type silabifluorene 9 were synthesized
by applying the present reaction. Furthermore, the synthesis of
Fig. 11 Solid state light emission of 22 (left), and triphenylenosiloles 19 (middle), 18 (right) excited at 254 nm at room temperature.
9334 | Dalton Trans., 2010, 39, 9329–9336 This journal is The Royal Society of Chemistry 2010
©

Synthesis of compounds 1–4, 7–8, 16–18, and crystal data for
18 have been reported in ref. 12
2H), 7.28 (d, J = 8.0 Hz, 4H), 7.41 (dd, J = 8.0 Hz, 2.0 Hz, 2H),
7.46 (d, J = 2.0 Hz, 2H); 13C NMR (126 MHz, CDCl3) d 31.34
(q), 31.38 (q), 34.44 (s), 34.46 (s), 124.74 (d), 126.66 (d), 128.66
(d), 129.05 (d), 130.81 (s), 134.33 (d), 140.15 (s), 146.45 (s), 148.45
(s), 149.52 (s); LRMS (FAB): m/z 561 ([M+H]+); Anal Calcd for
C40H52Si: C, 85.65; H, 9.34. Found: C, 85.42; H, 9.42.
Preparation of (4,4¢-di-tert-butylbiphenyl-2-yl)dimethylsilane (5)
To a solution of 2-bromo-4,4¢-di-tert-butylbiphenyl (3.00 g,
8.69 mmol) in Et2O (20 mL) was added a hexane solution of
n-BuLi (1.6 M, 5.4 mL, 8.69 mmol) dropwise at -78 ◦C. After
stirring for 30 min, the resulting solution was slowly added to
chlorodimethylsilane (980 mL, 9.00 mmol) in Et2O (10 mL) at
room temperature. The resulting mixture was stirred for 2 days.
After the addition of aq. NaHCO3 solution (1.0 M), the mixture
was extracted with Et2O, and the extracts were dried over MgSO4.
After removal of the solvent, the residue was purified by GPC to
give 5 (2.54 g, 7.83 mmol) in 90% yield as a colorless oil. 1H NMR
(400 MHz, CDCl3) d 0.05 (d, J = 3.6 Hz, 6H), 1.35 (s, 9H), 1.36
(s, 9H), 4.36 (sept, J = 3.6 Hz, 1H), 7.22–7.28 (m, 3H), 7.38 (d,
J = 8.4 Hz, 2H), 7.42 (dd, J = 8.0 Hz, 2.0 Hz, 1H), 7.65 (d, J =
2.0 Hz, 1H); 13C NMR (126 MHz, CDCl3) d -2.87 (q), 31.44 (q),
34.53 (s), 34.56 (s), 124.68 (d), 126.17 (d), 128.87 (d), 128.98 (d),
132.01 (d), 135.36 (s), 140.66 (s), 146.37 (s), 148.50 (s), 149.89 (s)
(One signal in aliphatic region was overlapped.); LRMS (FAB):
m/z 324 (M+). Anal. Calcd for C22H32Si: C, 81.41; H, 9.94. Found:
C, 81.48; H, 9.91.
Synthesis of 2,2¢,7,7¢-tetra-tert-butyl-9,9¢-spiro-9-silabifluorene
(9)29 by using a sila-Friedel–Crafts reaction
To a solution of 10 (300 mg, 0.535 mmol) and 2,6-lutidine (132 mL,
1.13 mmol) in CH2Cl2 (4 mL) was added a CH2Cl2 (4 mL)
solution of Ph3CB(C6F5)4 (1.04 g, 1.13mmol) at room temperature.
The solution was stirred for 7 h at the same temperature, and
then water was added. The mixture was extracted with CH2Cl2.
The organic layer was dried over MgSO4. After removal of the
solvent, the residue was purified by GPC and preparative thin-layer
chromatography (PTLC) to give 9 (36 mg, 0.065 mol) in 12% yield
as a colorless solid. mp 219–223 ◦C; 1H NMR (500 MHz, CDCl3) d
1.25 (s, 36H), 7.40 (d, J = 1.9 Hz, 4H), 7.51 (dd, J = 8.2 Hz, 1.9 Hz,
4H), 7.83 (d, J = 8.2 Hz, 4H); 13C NMR (126 MHz, CDCl3) d 31.35
(q), 34.74 (s), 120.22 (d), 128.19 (d), 131.20 (d), 133.27 (s), 147.43
(s), 150.26 (s); 29Si NMR (79 MHz, CDCl3) d -6.20; LRMS (FAB):
m/z 556 (M+); Anal Calcd for C40H48Si·0.5CH2Cl2: C, 81.16; H,
8.24. Found: C, 81.02; H, 8.22.
Synthesis of 2,7-di-tert-butyl-9,9-dimethyl-9-silafluorene (6) by
using the sila-Friedel–Crafts reaction
Synthesis of tetraphenylsilane (11) by using the intermolecular
sila-Friedel–Crafts reaction
To a solution of hydrosilane 5 (100 mg, 0.308 mmol) and 2,6-
lutidine (36 mL, 0.308 mmol) in CH2Cl2 (2 mL) was added a
CH2Cl2 (3 mL) solution of Ph3CB(C6F5)4 (284 mg, 0.308 mmol)
at room temperature. The solution was stirred for 12 h at the
same temperature, and then aq. NaHCO3 solution was added.
The mixture was extracted with CH2Cl2, and the extracts were
dried over MgSO4. After removal of the solvent, the residue was
purified by silica gel column chromatography to give 6 (25 mg,
0.078 mmol) in 25% yield as a colorless solid. mp 170–171 ◦C; 1H
NMR (400 MHz, CDCl3) d 0.41 (s, 6H), 1.35 (s, 18H), 7.41 (dd,
J = 8.0 Hz, 1.6 Hz, 2H), 7.60 (d, J = 1.6 Hz, 2H), 7.69 (d, J =
8.0 Hz, 2H); 13C NMR (126 MHz, CDCl3) d -2.90 (q), 31.47 (q),
34.66 (s), 120.12 (d), 127.22 (d), 129.21 (d), 138.60 (s), 145.22 (s),
149.56 (s); LRMS (FAB): m/z 322 (M+); Anal. Calcd for C22H30Si:
C, 81.92; H, 9.37. Found: C, 81.44; H, 9.52.
To triphenylsilane (10) (260 mg, 1.00 mmol) was added a benzene
(20 mL) solution of Ph3CB(C6F5)4 (920 mg, 1.00 mmol) and 2,6-
lutidine (232 mL, 2.00 mmol) at room temperature. The solution
was stirred overnight at the same temperature, and then water was
added. The mixture was extracted with Et2O, and the extracts were
dried over MgSO4. After removal of the solvent, the residue was
purified by silica gel column chromatography with hexane–CHCl3
eluent and GPC to give 11 (104 mg, 0.309 mmol) in 31% yield as
a colorless solid. The target 11 was identified by comparison of
spectral data with authentic data.30 1H NMR (400 MHz, CDCl3)
d 7.35 (t, J = 6.8 Hz, 8H), 7.39 (m, 4H), 7.53–7.56 (m, 8H); LRMS
(EI): m/z 336 (M+).
Synthesis of trisilasumanene 18 and triphenylenodisilole 19
To a solution of 17 (163 mg, 0.135 mmol) and 2,6-lutidine
(16 mL, 0.135 mmol) in CH2Cl2 (2 mL) was added a CH2Cl2
(2 mL) solution of Ph3CB(C6F5)4 (125 mg, 0.135 mmol) at room
temperature. After stirring for 25 h at the same temperature, aq.
NaHCO3 solution (1.0 M) was added. The mixture was extracted
with CH2Cl2, and the extracts were dried over MgSO4. After
removal of the solvent, the residue was purified by silica gel
column chromatography (hexane–CHCl3 = 1 : 1) to give 18 (30 mg,
0.025 mol) in 18% yield and 19 (43 mg, 0.042 mmol) in 31% yield.
Preparation of bis(4,4¢-di-tert-butylbiphenyl-2-yl)silane (10)
To a solution of 4,4¢-di-tert-butyl-2-bromobiphenyl (3.00 g,
8.69 mmol) in Et2O (30 mL) was added a hexane solution of n-BuLi
(1.6 M, 5.4 mL, 8.69 mmol) dropwise at -40 ◦C. The mixture was
allowed to warm to room temperature with stirring for 1 h 20 min.
The solution was slowly added to a solution of tetrachlorosilane
(500 mL, 4.35 mmol) in Et2O. After stirring for 1 h, the resulting
mixture was added to LiAlH4 (660 mg, 17.4 mmol), and refluxed
for overnight. The suspension was poured over crushed ice, and the
mixture was extracted with Et2O, and the extracts were dried over
MgSO4. After removal of the solvent, the residue was purified
by WCC on silica gel with hexane eluent to give 10 (1.41 g,
2.51 mmol) in 58% yield as a colorless solid. mp 168–170 ◦C;
1H NMR (500 MHz, CDCl3) d 1.26 (s, 18H), 1.31 (s, 18H), 4.63 (t,
JSi–H = 101.5 Hz, 2H), 7.18 (d, J = 8.0 Hz, 4H), 7.24 (d, J = 8.0 Hz,
Data for 19. A colorless solid; mp 195–201 ◦C; 1H NMR
(500 MHz, CDCl3) d 0.68 (t, J = 7.4 Hz, 6H), 0.74 (t, J = 7.4 Hz,
6H), 1.01 (t, J = 7.4 Hz, 6H), 1.09–1.18 (m, 8H), 1.40 (quint, J =
7.7 Hz, 4H), 1.46 (quint, J = 7.7 Hz, 4H), 1.56 (sext, J = 7.4 Hz,
4H), 1.90 (quint, J = 7.7 Hz, 4H), 3.75–3.81 (m, 8H), 4.19 (t, J =
6.4 Hz, 4H), 7.31 (t, J = 7.5 Hz, 8H), 7.37 (tt, J = 7.5 Hz, 1.4 Hz,
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 9329–9336 | 9335
©

4H), 7.74 (s, 2H), 7.85 (dd, J = 7.5 Hz, 1.4 Hz, 8H); 13C NMR
(126 MHz, CDCl3) d 13.77 (q), 13.82 (q), 13.95 (q), 18.90 (t), 19.46
(t), 31.54 (t), 32.01 (t), 32.09 (t), 68.60 (t), 72.91 (t), 73.20 (t), 108.22
(d), 123.77 (s), 124.73 (s), 126.86 (s), 127.83 (d), 129.88 (d), 133.54
(s), 134.94 (s), 135.23 (s), 135.95 (d), 152.06 (s), 152.49 (s), 155.93
(s); LRMS (FAB): m/z 1021 (M+); Anal Calcd for C66H76O6Si2: C,
77.60; H, 7.50. Found: C, 77.48; H, 7.61.
Chin. Chem. Lett., 2007, 18, 1436; (e) C. Xu, H. Yamada, A. Wakamiya,
S. Yamaguchi and K. Tamao, Macromolecules, 2004, 37, 8978.
6 M. Shimizu, K. Mochida and T. Hiyama, Angew. Chem., Int. Ed., 2008,
47, 9760.
7 Recent reviews on silicenium ions: (a) V. Ya. Lee and A. Sekiguchi, Acc.
Chem. Res., 2007, 40, 410; (b) C. A. Reed, Acc. Chem. Res., 1998, 31,
325.
8 Reactions using silicenium ions: (a) D. J. Parks and W. E. Piers, J. Am.
Chem. Soc., 1996, 118, 9440; (b) J. M. Blackwell, K. L. Foster, V. H.
Beck and W. E. Piers, J. Org. Chem., 1999, 64, 4887; (c) D. J. Parks,
J. M. Blackwell and W. E. Piers, J. Org. Chem., 2000, 65, 3090.
9 (a) G. P. Sollott and W. R. Peterson, J. Am. Chem. Soc., 1967, 89, 5054;
(b) U. Frick and G. Simchen, Synthesis, 1984, 929.
X-ray experimental data
Crystal data for 9. C40H48Si, M = 556.87, monoclinic, space
10 G. A. Olah, T. Bach and G. K. S. Prakash, J. Org. Chem., 1989, 54,
3770.
11 H. Sakurai, T. Daiko and T. Hirao, Science, 2003, 301, 1878.
12 S. Furukawa, J. Kobayashi and T. Kawashima, J. Am. Chem. Soc., 2009,
131, 14192.
˚
group C2/c,◦a = 24.689(8), b = 15.373(5), c = 29.430(10) A, b =
3
-3
˚
107.1954(14) , V = 10671(6) A , Z = 12, Dc = 1.040 g cm , m =
0.090 mm-1, T = 120(2) K, 33325 reflections measured, 9294 unique
(Rint = 0.0354), final R1 (I > 2s(I)) = 0.0925, wR2 (all data) = 0.2819,
GOF = 1.068, The unidentified reflection were detected which may
be derived from solvents such as pentane or hexane.
13 (a) J. B. Lambert, S. Zhang, C. L. Stern and J. C. Huffman, Science,
1993, 260, 1917; (b) C. A. Reed and Z, Xie, Science, 1994, 263, 985;
(c) C. A. Reed, K.-C. Kim, E. S. Stoyanov, D. Stasko, F. S. Tham, L. J.
Mueller and P. D. W. Boyd, J. Am. Chem. Soc., 2003, 125, 1796.
14 (a) R. W. Bott, C. Eaborn and P. M. Greasley, J. Chem. Soc., 1964, 4804;
(b) I. Szele, Helv. Chim. Acta, 1981, 64, 2733 and references therein.
15 For example, see: (a) J. Y. Corey, J. Am. Chem. Soc., 1975, 97, 3237;
(b) J. B. Lambert, Y. Zhao and S. M. Zhang, J. Phys. Org. Chem., 2001,
14, 370; (c) S. Duttwyler, Y. Zhang, A. Linden, C. A. Reed, K. K.
Baldridge and J. S. Siegel, Angew. Chem., Int. Ed., 2009, 48, 3787.
16 See experimental section for the preparation of hydrosilane 1.
17 Formation of silanol 3 and disiloxane 4 may be derived from trace
amounts of water.
Crystal data for 19. C66H76O6Si2, M = 1021.45, triclinic, space
˚
¯
group P1, ◦a = 9.125(6), b =◦ 17.394(12), c =◦19.237(13) A, a =
3
˚
107.019(8) , b = 101.693(8) , g = 92.934(7) , V = 2839(3) A ,
Z = 2, Dc = 1.195 g cm-3, m = 0.114 mm-1, T = 120(2) K, 18332
reflections measured, 9677 unique (Rint = 0.0331), final R1 (I >
2s(I)) = 0.0505, wR2 (all data) = 0.1413, GOF = 1.066.
Acknowledgements
18 J. Ohshita, K.-H. Lee, D. Hamamoto, Y. Kunugi, J. Ikadai, Y.-W. Kwak
and A. Kunai, Chem. Lett., 2004, 33, 892; T. Katoh, K. Ogawa, Y.
Inagaki and R. Okazaki, Bull. Chem. Soc. Jpn., 1997, 70, 1109.
19 (a) Y.-T. Wu and J. S. Siegel, Chem. Rev., 2006, 106, 4843; (b) T. Kawase
and H. Kurata, Chem. Rev., 2006, 106, 5250; (c) M. A. Petrukhina,
Angew. Chem., Int. Ed., 2008, 47, 1550; (d) T. Amaya, K. Mori, H.-L.
Wu, S. Ishida, J. Nakamura, K. Murata and T. Hirao, Chem. Commun.,
2007, 1902; (e) T. Amaya, H. Sakane, T. Muneishi and T. Hirao, Chem.
Commun., 2008, 765; (f) T. Amaya, H. Sakane and T. Hirao, Angew.
Chem., Int. Ed., 2007, 46, 8376; (g) T. Amaya, S. Seki, T. Moriuchi, K.
Nakamoto, T. Nakata, H. Sakane, A. Saeki, S. Tagawa and T. Hirao,
J. Am. Chem. Soc., 2009, 131, 408; (h) Y. Kameno, A. Ikeda, Y. Nakao,
H. Sato and S. Sakai, J. Phys. Chem. A, 2005, 109, 8055.
20 K. Imamura, K Takimiya, Y. Aso and T. Otsubo, Chem. Commun.,
1999, 1859.
We thank Mr. Hiroshi Kita and Mr. Shun Furukawa, KONICA
MINOLTA TECHNOLOGY CENTER, Inc., for their help in
the measurement of cryogenic phosphorescence spectra. This
work was supported by the Global COE Program for Chemistry
Innovation. Partial financial support from MEXT (Kakenhi,
20685005, 21108507) is gratefully acknowledged. We also thank
Shinetsu Chemical Co., Ltd., Tosoh Finechem Co., Ltd., and
Central Glass Co., Ltd. for the generous gifts of silicon reagents,
alkyllithium, and fluorine reagents, respectively.
21 M. Saito, T. Tanikawa, T. Tajima, J. D. Guo and S. Nagase, Tetrahedron
Notes and references
Lett., 2010, 51, 672.
1 (a) K. Tamao, M. Uchida, I. Izumizawa, K. Furukawa and S.
Yamaguchi, J. Am. Chem. Soc., 1996, 118, 11974; (b) S. Yamaguchi
and K. Tamao, Chem. Lett., 2005, 34, 2; (c) F. Wang, J. Luo, K. Yang,
J. Chen, F. Huang and Y. Cao, Macromolecules, 2005, 38, 2253.
2 (a) L. Ilies, H. Tsuji, Y. Sato and E. Nakamura, J. Am. Chem. Soc., 2008,
130, 4240; (b) M. Uchida, T. Izumizawa, T. Nakano, S. Yamaguchi, K.
Tamao and K. Furukawa, Chem. Mater., 2001, 13, 2680; (c) H. Murata,
G. G. Malliaras, M. Uchida, T. Shen and Z. H. Kafafi, Chem. Phys.
Lett., 2001, 339, 161; (d) H. Murata, Z. H. Kafafi and M. Uchida, Appl.
Phys. Lett., 2002, 80, 189.
3 (a) T. Matsuda, S. Kadowaki, T. Goya and M. Murakami, Org. Lett.,
2007, 9, 133; (b) L. Li, J. Xiang and C. Xu, Org. Lett., 2007, 9, 4877.
4 (a) S. H. Lee, B.-B. Jang and Z. H. Kafafi, J. Am. Chem. Soc., 2005,
127, 9071; (b) G. C. Yang, Z. M. Su and C. S. Qin, J. Phys. Chem. A,
2006, 110, 4817; (c) M. Sun, B. Niu and J. P. Zhang, Theor. Chem. Acc.,
2008, 119, 489; (d) M. Sun, B. Niu and J. Zhang, THEOCHEM, 2008,
862, 85; (e) H. Xiao, H. Shen, Y. Lin, J. Su and H. Tian, Dyes Pigm.,
2007, 73, 224.
22 (a) Z. Li, N. T. Lucas, Z. Wang and D. Zhu, J. Org. Chem., 2007, 72,
3917; (b) N. Boden, R. J. Bushby, A. N. Cammidge, S. Duckworth and
G. Headdock, J. Mater. Chem., 1997, 7, 601.
23 H. Sakurai, T. Daiko, H. Sakane, T. Amaya and T. Hirao, J. Am. Chem.
Soc., 2005, 127, 11580.
24 (a) U. D. Priyakumar and G. N. Sastry, J. Org. Chem., 2001, 66, 6523;
(b) U. D. Priyakumar, M. Punnagai, G. P. K. Mohan and G. N. Sastry,
Tetrahedron, 2004, 60, 3037.
25 J. C. Collings, K. P. Roscoe, R. L. Thomas, A. S. Batsanov, L. M.
Stimson, J. A. K. Howard and T. B. Marder, New J. Chem., 2001, 25,
1410.
26 V. K. Bel’skii and A. V. Dzyabchenko, Zh. Struct. Khim., 1985, 26, 94.
27 (a) K. F. Lang, M. Zander and E. A. Theiling, Chem. Ber., 1960, 93,
321; (b) N. Nishi, K. Matsui, M. Kinoshita and J. Higuchi, Mol. Phys.,
1979, 38, 1; (c) D. Baunsgaard, N. Harrit, M. El Balsami, F. Negri, G.
Orlandi, J. Frederiksen and R. Wilbrandt, J. Phys. Chem. A, 1998, 102,
10007.
28 D. Baunsgaard, M. Larsen, N. Harrit, J. Frederiksen, R. Wilbrandt
and H. Stapelfeldt, J. Chem. Soc., Faraday Trans., 1997, 93, 1893.
29 S. H. Lee and Z. H. Kafafi, Am. Chem. Soc. Org. Prepr., 2002, 224, 490.
30 Y. Yamanoi, T. Taira, J. Sato, I. Nakamula and H. Nishihara, Org.
Lett., 2007, 9, 4543.
5 (a) S. Yamaguchi, C. Xu and K. Tamao, J. Am. Chem. Soc., 2003,
125, 13662; (b) C. Xu, A. Wakamiya and S. Yamaguchi, Org. Lett.,
2004, 6, 3707; (c) S. Yamaguchi, C. Xu, H. Yamada and A. Wakamiya,
J. Organomet. Chem., 2005, 690, 5365; (d) C. Li, H. Qiu and C. Xu,
9336 | Dalton Trans., 2010, 39, 9329–9336
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The Royal Society of Chemistry 2010
©