Modular synthesis of functionalized benzosiloles by tin-mediated cyclization of (o-alkynylphenyl)silane

Article abstract of DOI:10.1021/ja800636g

Trimethylstannyllithium promotes cyclization of (o-alkynylphenyl)silane into a 3-stannylbenzosilole, via addition to the triple bond followed by intramolecular cyclization in a cascade fashion. This intermediate can be functionalized with either electrophiles or nucleophiles to allow modular synthesis of 2,3-disubstituted benzosiloles. One of these compounds, a phenylene-bis(benzosilole) compound, shows electron drift mobility as high as 6 × 10^-4 cm²/Vs in an amorphous film, making this class of compounds promising electronic materials for organic light emitting devices and organic photovoltaic cells. Copyright

Full text of DOI:10.1021/ja800636g

Published on Web 03/11/2008
Modular Synthesis of Functionalized Benzosiloles by Tin-Mediated
Cyclization of (o-Alkynylphenyl)silane
Laurean Ilies,† Hayato Tsuji,*,† Yoshiharu Sato,‡ and Eiichi Nakamura*,†,‡
Department of Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan,
and Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency, Department of
Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received January 25, 2008; E-mail: tsuji@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
Scheme 1
Efficiency and diversity are central concepts in modern organic
synthesis. It is in this context that modular synthesis and molecular
libraries have attracted much interest in the design of functional
materials. We have been exploring for some time the potential of
this approach1 and have discovered a new class of hole-transporting
materials in our studies on 3-metallobenzofuran modules.2 We report
herein the synthesis of a variety of silole derivatives, based on a
new cyclization reaction of (o-alkynylphenyl)silane 1 and deriva-
tization of the resulting 3-stannylbenzosilole 3 (Scheme 1). This
stannane derivative 3 serves either as a useful nucleophilic unit by
itself or as an electrophilic unit after conversion into the corre-
sponding iodide 10 (eq 2). We have synthesized a variety of
benzosilole derivatives, including a new phenylene-bis(benzosilole)
9 (Table 2), which exhibits the highest electron drift mobility3
among known silole derivatives4 or commonly used electron
transporting materials (ETM).
Table 1. Metalative Cyclization of 1 into Benzosilole 3a
entry
reagent
conditionsb
3 (%)c
1 (%)d
1
2
3
4
Me3SnLi
Bu3SnLi
PhMe2SiLi
(Et2N)Ph2SiLi
Et2O, rt, 3 h
93
2
13
60
0
Et2O, 0 °C, 21 h
THF, 0 °C, 1 h
THF, 0 °C, 3 h
64
13d
87d
Despite the rich history of the synthesis of siloles,5-7 the concept
of diversity synthesis has been lacking in this field, and we
envisioned that an organometallic unit, such as 3, would become a
useful synthetic tool. In analogy to our recent benzoheterole
synthesis,1a,b we considered that anionic or cationic activation of
the silane moiety in the (o-alkynylphenyl)silane 1 would form the
desired silole ring. However, all attempts to achieve such cyclization
failed. Among our final attempts, we tried the reaction shown in
Scheme 1 and found that a high yield of the desired product 3
occurs when the reaction is carried out in diethyl ether. This was
a surprise for two reasons. First, there was a report that Et3SnLi
in THF adds to diphenylacetylene only in 2% yield.8 Second,
while the successful cyclization implies the formation of an
intermediate B instead of the less hindered one A, the same reaction
in hexane suggested the intermediacy of A. Thus, the reaction
in hexane instead of diethyl ether gave 2 after quenching with
aqueous NH4Cl. More interestingly, the reaction in hexane also
produced 3, when diethyl ether or N,N,N′,N′-tetramethylethylene-
diamine (TMEDA) was added before aqueous quenching.
Clearly, the reaction is highly sensitive to the solvent.9 The results
suggest that A and B interconvert with each other under certain
reaction conditions. Note that the displacement of a hydride by a
vinyllithium that occurs on the way from B to 3 is a known
reaction.10
a Details of the reaction conditions are described in the Supporting
Information. The reaction can be applicable to the synthesis of 2-arylben-
siloles other than the 2-phenyl compound 3 (data not shown). b Optimized
for each reagent. c Isolated yield. d Determined by 1H NMR using 1,1,2,2-
tetrachloroethane as an internal standard.
in THF also produced the desired 3-silylbenzosilole, and an
aminosilyllithium12 gave a much higher yield of the product (entries
3 and 4).
First we examined the nucleophilic reactivity of 3 under Kosugi-
Migita-Stille cross-coupling conditions. The reaction with iodo-
benzene in the presence of Pd2dba3‚CHCl3, P(t-Bu)3, and CsF in
1,2-dimethoxyethane13 gave 2,3-diphenylbenzosilole 5 in 63% yield
(Table 2, entry 2 in the parentheses). When the tin atom was first
transmetalated to lithium14 and then to zinc, the efficiency of the
cross-coupling improved considerably (Table 2, eq 1). Thus, 3 was
first quantitatively converted to 3-lithiobenzosilole upon treatment
with BuLi at 0 °C in THF, as proved by the ammonium chloride
quenching (entry 1). Subsequent transmetalation with zinc chloride,
followed by Negishi cross-coupling with iodobenzene in the
presence of Pd2dba3‚CHCl3 and dicyclohexyl(2′,6′-dimethoxybi-
phen-2-yl)phosphine L15 in THF/NMP gave the desired 2,3-
difunctionalized benzosilole 5 in 87% isolated yield (entry 2). Styryl
bromide and 2-pyridyl bromide also reacted smoothly and gave
the corresponding benzosiloles 6 and 7 in 90% and 91% yield,
respectively (entries 3 and 4). 2-Thienyl bromide gave the corre-
sponding benzosilole 8 in 53% yield (entry 5). The efficiency of
the modular approach is illustrated by the one-pot synthesis of the
phenylene-bridged bis(benzosilole) 9, which was achieved in 84%
yield (entry 6).
The cyclization reaction took place not only with stannyllithium
reagents but also with silyllithium reagents as summarized in
Table 1. The Me3SnLi-promoted reaction on a 1 mmol scale
proceeded in 93% yield (Table 1, entry 1) and, on a 20 mmol scale,
gave 90% yield to afford 7.15 g of 3. Bu3SnLi gave a lower yield
of the cyclized product (entry 2). A triorganosilyllithium reagent11
We can also use the benzosilole unit in 3 as an electrophile (eq
2). We thus treated 3 first with iodine to obtain the iodide 10 and
then with a 3-zinciobenzofuran that was obtained by cyclization
† The University of Tokyo.
‡ Nakamura Functional Carbon Cluster Project.
9
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10.1021/ja800636g CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Functionalization of 3 with Various Electrophilesa
inferred from the absorption spectrum of the film. The electron
drift mobility was found to be very high, 6 × 10-4 cm2/Vs, which
is higher than that of all previously reported silole-based materials
or that of the most widely used ETM, tris(8-hydroxyquinolinolato)-
aluminum(III) (Alq3).4a This mobility measurement was performed
by the time-of-flight technique using an amorphous film made by
vacuum deposition (thickness: 3.59 µm) at room temperature in
an electric field of 4.5 × 105 V/cm. The amorphous character of
the film was confirmed by XRD.16
In conclusion, we discovered a mechanistically intriguing stan-
nylative or silylative cyclization reaction of (o-alkynylphenyl)silane
1 that gives a stable 3-stannyl or 3-silylbenzosilole. The stannyl
compound 3 and the corresponding iodo compound 10 can then be
used as useful modules for quick construction of a variety of
previously inaccessible benzosilole derivatives including the electron-
transporting bis(benzosilole) 9. The versatility of this modular
approach will allow us to obtain a variety of silole derivatives, and
we expect that useful organic semiconductors and materials for
electroluminescent devices or solar cells can be discovered using
such a molecular library.
Acknowledgment. We thank MEXT (KAKENHI for E.N., No.
18105004) and the Global COE Program for Chemistry Innovation.
L.I. thanks the Research Fellowship of the Japan Society for the
Promotion of Science for Young Scientists (No. 19‚10183).
Supporting Information Available: Detailed experimental pro-
cedures and properties of the compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
a Reaction conditions are summarized in eq 1.16 b Isolated yield based
on the halide. c The reaction mixture was quenched with aqueous ammonium
chloride after destannylation. d The reaction was carried out under Kosugi-
Migita-Stille cross-coupling conditions.
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