Development of a sila-Friedel-Crafts reaction and its application to the synthesis of dibenzosilole derivatives

Article abstract of DOI:10.1021/ja906566r

(Chemical Equation Presented) An intramolecular sila-Friedel-Crafts reaction was developed and applied to the synthesis of dibenzosilole derivatives. This reaction proceeds under mild conditions to afford the target in relatively high yield, indicating it

Full text of DOI:10.1021/ja906566r

Published on Web 09/16/2009
Development of a Sila-Friedel-Crafts Reaction and Its Application to the
Synthesis of Dibenzosilole Derivatives
Shunsuke Furukawa, Junji Kobayashi,* and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, 113-0033 Tokyo, Japan
Received August 3, 2009; E-mail: jkoba@chem.s.u-tokyo.ac.jp; takayuki@chem.s.u-tokyo.ac.jp
Table 1. Intramolecular Sila-Friedel-Crafts Reaction
Silole-based π-electron systems are currently receiving much
attention as new organic optical materials because of their low-
lying LUMOs,1 and π-extended silole derivatives involving spiro-
type2 and ladder-type3 molecules have desirable properties, such
as high fluorescence quantum yields and high glass-transition
temperatures. 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 for siloles is
required, and the development of transition-metal-catalyzed [2 +
2 + 2] cyclization,3a the addition of a silyl group to an alkyne,4
and the cross-coupling reaction of silicon-bridged biaryls have been
reported recently.5 Despite the recent remarkable progress in this
field, synthesizable silicon-containing π-conjugated skeletons are
limited to benzo- or dibenzo-fused siloles, silicon-bridged stilbenes,
and at most one-dimensionally extended ladder-type silafluorenes.
The development of a new type of synthetic methodology will be
a keystone for the further progression to two-dimensionally extended
derivatives. As a new promising method, we propose the direct
silylation of an aromatic ring.
The Friedel-Crafts reaction is a well-known method for the
direct substitution of an aromatic ring. On the other hand, the sila-
Friedel-Crafts reaction, i.e., Friedel-Crafts-type silylation involv-
ing a silicenium ion6,7 as an intermediate, occurs only with electron-
rich aromatic rings such as a ferrocene and a pyrrole.8 However,
in the case of a nonactivated aromatic ring such as benzene, the
conversion of the reaction markedly decreases.9 Thus, the sila-
Friedel-Crafts reaction has not been used as a versatile synthetic
method to date. Herein, we describe the development of an
intramolecular sila-Friedel-Crafts reaction that is applicable to the
synthesis of dibenzosilole derivatives. Furthermore, we have
succeeded in synthesizing trisilasumanene (Chart 1), a silicon
analogue of sumanene,10 by application of this sila-Friedel-Crafts
reaction.
% yielda
entry
oxidant
base
1
2
3
4
1
2
3
4
Ph3CClO4
2,6-lutidine
0
0
100
52 23 25
84 12
Ph3CB(C6F5)4 2,6-lutidine
Ph3CB(C6F5)4 proton sponge
Ph3CB(C6F5)4 pyridine
4
0
0
-
0
b
5
Ph3CB(C6F5)4 DTBMPc
100
50
0
0
0
0
0
0
6d
Ph3CB(C6F5)4 diisopropylethylamine
a Estimated by 1H NMR. b A complex mixture was obtained.
c DTBMP 2,6-di-tert-butyl-4-methylpyridine. d An unidentified
)
compound was generated.
The reaction of hydrosilane 1 with trityl perchlorate and 2,6-
lutidine in dichloromethane at room temperature gave dibenzosilole
2 in 52% yield. Silanol 3 and disiloxane 4 were also formed as
side products (Table 1, entry 1).12 This moderate conversion may
be due to deactivation of the intermediate by coordination of a
perchlorate anion to the silicenium ion. Changing the counteranion
to tetrakis(pentafluorophenyl)borate significantly increased the
conversion yield of 2 to 84% (entry 2). This reaction was very
sensitive to the choice of base, and 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.
As described above, the intramolecular sila-Friedel-Crafts
reaction afforded a dibenzosilole in a good yield under mild
conditions. Therefore, we applied this reaction to the synthesis of
π-extended dibenzosilole derivatives. The intramolecular sila-
Friedel-Crafts reaction of 2,2′′-bis(diphenylsilyl)-1,1′:4′,1′′-ter-
phenyl 5 afforded the corresponding ladder-type silafluorene 6
(Scheme 2). This result suggests that the sila-Friedel-Crafts reaction
is applicable to intramolecular double cyclization.
The basic strategy of the sila-Friedel-Crafts reaction for the
construction of a dibenzosilole framework is shown in Scheme 1.
The reaction of a hydrosilane with a trityl cation is a useful method
for the generation of a silicenium ion, which should spontaneously
form the arene complex.11 The dibenzosilole would be obtained
by deprotonation from this species. The presence of a base is
necessary in this reaction to prevent the reverse reaction from
occurring.
Trisilasumanene has attracted much attention as both a sumanene
analogue13 and a novel π-extended silole derivative.14 However,
trisilasumanene is considered difficult to synthesize using conven-
tional methods. Thus, we applied the intramolecular sila-
Friedel-Crafts reaction to the synthesis of trisilasumanene. Starting
Scheme 1. Strategy for the Intramolecular Sila-Friedel-Crafts
Reaction
Scheme 2. Synthesis of Ladder-Type Silafluorene 6 Utilizing the
Intramolecular Sila-Friedel-Crafts Reaction
9
14192 J. AM. CHEM. SOC. 2009, 131, 14192–14193
10.1021/ja906566r CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Chart 1. Structures of Sumanene and Its Main-Group Analogues,
Trithiasumanene and Trisilasumanene
a weak absorption band was observed in the longer-wavelength
region (>350 nm) in 10. This longer-wavelength absorption was
mainly attributed to the HOMO-LUMO transition of 10 and
suggested the existence of σ*-π* conjugation on the silicon atoms.
In the emission spectrum (Figure S2 in the Supporting Information),
10 showed a blue fluorescence in dichloromethane solution (λmax
) 427 nm) and in the solid state (λmax ) 447 nm).
In conclusion, we have succeeded in developing an intramo-
lecular sila-Friedel-Crafts reaction as a novel synthetic method
for dibenzosilole derivatives. This reaction proceeds under mild
conditions to afford the target in a relatively high yield, indicating
its availability as a versatile synthetic method. Using the intramo-
lecular sila-Friedel-Crafts reaction, we have achieved the syntheses
of a ladder-type silafluorene and trisilasumanene, a new sumanene
analogue.
from an isomeric mixture of tribromotriphenylene 7, doubly cyclized
monobromide 8 was synthesized in two steps, including a dual sila-
Friedel-Crafts reaction (Scheme 3). Transformation to the precursor
9 was achieved by lithiation of 8 followed by the addition of
diphenylsilane in 51% yield. Finally, the intramolecular sila-
Friedel-Crafts reaction was applied again to 9, and trisilasumanene
10 was obtained in 18% yield as a colorless solid. We succeeded
in X-ray crystallographic analysis of 10. The unit cell consists of
two crystallographically independent molecules that have almost
the same structure; one is shown in Figure 1. It is known that
sumanene and its sulfur analogue, trithiasumanene, have a bowl-
shapedstructurewithbowldepthsof1.11and0.65Å,respectively.15,16
The difference between the bowl depths is attributed to the larger
covalent radius of the sulfur atom. X-ray structural analysis of
trisilasumanene 10 indicated that the main framework was almost
planar. This result is consistent with the theoretical calculations
reported by Priyakumar and Sastry.14
Acknowledgment. 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 Shin-etsu Chemical Co., Ltd., Tosoh Finechem Co.,
Ltd., and Central Glass Co., Ltd., for the generous gifts of silicon
reagents, alkyllithiums, and fluorine reagents, respectively.
Supporting Information Available: Experimental procedures,
spectroscopic data for new compounds, optical properties of 10 and
11, and a CIF file for 10. This material is available free of charge via
the Internet at http://pubs.acs.org.
To elucidate the electronic structure, the UV-vis absorption
spectrum of trisilasumanene 10 was measured in dichloromethane
(Figure S1 in the Supporting Information). An intense absorption
band of 10 (λmax ) 299 nm, log ε ) 4.67) was slightly red-shifted
from that of 2,3,6,7,10,11-hexabutoxytriphenylene 11 (λmax ) 280
nm, log ε ) 5.09) and sumanene (λmax ) 278 nm).17 In addition,
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Figure 1. (a) Top view and (b) side view of the crystal structure of
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