General silaindene synthesis based on intramolecular reductive cyclization toward new fluorescent silicon-containing π-electron materials

Article abstract of DOI:10.1021/ol0486932

(Chemical Equation Presented) The reaction of (o-silylphenyl)acetylene derivatives with lithium naphthalenide undergoes intramolecular reductive cyclization to produce various silaindene derivatives. On the basis of this methodology, a series of silaindene-containing π-electron systems are synthesized that show intense blue to greenish-blue fluorescence.

Full text of DOI:10.1021/ol0486932

ORGANIC
LETTERS
2004
Vol. 6, No. 21
3707-3710
General Silaindene Synthesis Based on
Intramolecular Reductive Cyclization
toward New Fluorescent
Silicon-Containing
π-Electron Materials
Caihong Xu,† Atsushi Wakamiya,‡ and Shigehiro Yamaguchi*,†,‡
Department of Chemistry, Graduate School of Science, Nagoya UniVersity,
and PRESTO, Japan Science and Technology Agency (JST),
Chikusa, Nagoya 464-8602, Japan
yamaguchi@chem.nagoya-u.ac.jp
Received July 8, 2004
ABSTRACT
The reaction of (o-silylphenyl)acetylene derivatives with lithium naphthalenide undergoes intramolecular reductive cyclization to produce various
silaindene derivatives. On the basis of this methodology, a series of silaindene-containing
intense blue to greenish-blue fluorescence.
π-electron systems are synthesized that show
Heteroindenes are fundamental and important skeletons in
organic chemistry. A typical synthetic route to heteroindenes
is the heteroannulation of arylacetylenes bearing nucleophilic
element substituents such as -OH, -NHR, and -SR at the
ortho position, as shown in Scheme 1a,1-3 that proceeds
through the metalation of the ortho substituents or through
the activation of the acetylene moiety with electrophilic
agents. This mode of cyclization is indeed useful for the
synthesis of group 15 and 16 element-containing heteroin-
denes such as indoles,1 benzofurans,2 and benzothiophenes;3
however, it is less effective for the synthesis of heteroindenes
with other group elements such as group 14 silicon. Only a
few examples related to this mode of cyclization have been
reported for the synthesis of silaindenes,4,5 which include
the intramolecular hydrosilation of (o-silylphenyl)acetylenes
catalyzed by a Pt complex6 or a Lewis acid.7 As a comple-
ment of this mode of cyclization, we now disclose a new
intramolecular reductive cyclization of (o-silylphenyl)acety-
lene derivatives that produces various functionalized silain-
(2) For recent benzo[b]furan preparations, see for instance: (a) Anderson,
S.; Taylor, P. N.; Verschoor, G. L. B. Chem.sEur. J. 2004, 10, 518-527.
(b) Dai, W.-M.; Lai, K. W. Tetrahedron Lett. 2002, 43, 9377-9380. (c)
Hu, Y.; Zhang, Y.; Yang, Z.; Fathi, R. J. Org. Chem. 2002, 67, 2365-
2368. (d) Bates, C. G.; Saejueng, P.; Murphy, J. M.; Venkataraman, D.
Org. Lett. 2002, 4, 4727-4729. (e) Hiroya, K.; Suzuki, N.; Yasuhara, A.;
Egawa, Y.; Kasano, A.; Sakamoto, T. J. Chem. Soc., Perkin Trans. 1. 2000,
4339-4346. (f) Arcadi, A.; Cacchi, S.; Fabrizi, G.; Moro, L. Eur. J. Org.
Chem. 1999, 1137-1141 and references therein.
(3) For recent benzo[b]thiophene preparations, see for instance: (a)
Hessian, K. O.; Flynn, B. L. Org. Lett. 2003, 5, 4377-4380. (b) Flynn, B.
L.; Verdier-Pinard, P.; Hamel, E. Org. Lett. 2001, 3, 651-654. (c) Larock,
R. C.; Yue, D. Tetrahedron Lett. 2001, 42, 6011-6013 and references
therein. (d) Kitamura, T.; Takachi, T.; Miyaji, M.; Kawasato, H.; Taniguchi,
H. J. Chem. Soc., Perkin Trans. 1 1994, 1907-1911.
† PRESTO, JST.
‡ Nagoya University.
(1) For recent indole preparations, see for instance: (a) Barluenga, J.;
Trincado, M.; Rubio, E.; Gonza´lez, J. M. Angew. Chem., Int. Ed. 2003, 42,
2406-2409. (b) Kamijo, S.; Yamamoto, Y. J. Org. Chem. 2003, 68, 4764-
4771. (c) Cacchi, S.; Fabrizi, G.; Parisi, L. M. Org. Lett. 2003, 5, 3843-
3846. (d) Hiroya, K.; Itoh, S.; Ozawa, M.; Kanamori, Y.; Sakamoto, T.
Tetrahedron Lett. 2002, 43, 1277-1280. (e) Koradin, C.; Dohle, W.;
Rodriguez, A. L.; Schmid, B.; Knochel, P. Tetrahedron 2003, 59, 1571-
1587. (f) Rodriguez, A. L.; Koradin, C.; Dohle, W.; Knochel, P. Angew.
Chem., Int. Ed. 2000, 39, 2488-2490 and references therein.
10.1021/ol0486932 CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/23/2004

produced by the two-electron reduction of the acetylene
moiety. We envisioned that the idea of this reaction can be
applied to the synthesis of the silaindene skeleton by re-
placing the starting material with mono(o-silyl)-substituted
diphenylacetylene 1. In fact, we found that when compound
1 having hydrogen as a leaving group on the silicon atom
was treated with 2 mol amounts of LiNaph followed by
quenching with water, the desired phenylsilaindene 2 was
obtained in 17% yield, together with its dimeric product 3
in 74% yield (Scheme 2).11 This result demonstrates that the
Scheme 1. Two Complementary Types of Cyclizations for the
Heteroindene Synthesis
Scheme 2. Cyclization of Mono(o-silyl)-diphenylacetylene
denes via a two-electron reduction of the acetylene moiety,
as shown in Scheme 1b. The development of this methodol-
ogy has led to the synthesis of a new family of highly
fluorescent silicon-containing π-electron compounds, which
are potential blue-emitting materials for organic light-emitting
diodes (LEDs).8,9 Here, we report the details of the cycliza-
tion as well as the fundamental photophysical properties of
the produced π-electron compounds.
We have recently reported that the reaction of bis(o-silyl)-
substituted diphenylacetylene with lithium naphthalenide
(LiNaph) undergoes an intramolecular reductive cyclization
to produce a disila-indeno[2,1-a]indene skeleton.10 The
postulated intermediate of this reaction is a dianion species
present cyclization indeed proceeds even using the mono-
(o-silyl)-substituted derivatives, while the formation of the
latter dimeric product suggests that the two-electron reduction
proceeds in a stepwise fashion.
Notably, the present reductive cyclization more efficiently
proceeds when π-extended starting materials are employed
instead of the monoacetylenic compound, as shown in
Scheme 3. Thus, the reaction of bis(phenylethynyl)benzene
4, having two silyl groups at the terminal phenyl rings, with
4 mol amounts of LiNaph followed by quenching with water
afforded bis(silaindenyl)benzene 5a in 52% yield, while the
use of compound 6, which is analogous but has two silyl
groups at the central benzene moiety, led to a totally new
π-electron compound 7 consisting of a disila-s-indacene
skeleton in 90% yield. In addition, the present cyclization
also proceeds in the case of bis(o-silylphenyl)diacetylene 8
to give bis(silaindene) 9, although the yield is moderate.
Noteworthy is the fact that these are the first examples of
the silaindene-based extended π-electron systems.
Compared with the conventional synthesis of silaindenes,4,5
the present cyclization has a great advantage in terms of facile
functionalization. Thus, the present reductive cyclization pro-
duces the anionic intermediates that can be easily transformed
into various functionalized derivatives by treatment with the
appropriate electrophiles. For instance, in the reaction of 4
with 4 mol amounts of LiNaph followed by treatments with
(4) Conventional silaindene synthesis based on the reaction of dilithiosty-
renes with halosilanes: (a) Korneev, S. M.; Kaufmann, D. E. Synthesis
2002, 491-496. (b) Ura, Y.; Li, Y.; Tsai, F.-Y.; Nakajima, K.; Kotora,
M.; Takahashi, T. Heterocycles 2000, 52, 1171-1189. (c) Xi, C.; Huo, S.;
Afifi, T. H.; Hara, R.; Takahashi, T. Tetrahedron Lett. 1997, 38, 4099-
4102. (d) Choi, S.-B.; Boudjouk, P.; Wei, P. J. Am. Chem. Soc. 1998, 120,
5814-5815. (e) Rausch, M. D.; Klemann, L. P. J. Am. Chem. Soc. 1967,
89, 5732-5733. For other silaindene synthesis, see: (f) Kunai, A.; Yuzuriha,
Y.; Naka, A.; Ishikawa, M. J. Organomet. Chem. 1993, 455, 77-81. (g)
Barton, T. J.; Groh, B. L. Organometallics 1985, 4, 575-580.
(5) Versatile synthesis of heteroindenes by the reaction of dilithiostyrenes
with metal halides: Kurita, J.; Ishii, M.; Yasuike, S.; Tsuchiya, T. J. Chem.
Soc., Chem. Commun. 1993, 1309-1310.
(6) Ma¨rkl, G.; Berr, K.-P. Tetrahedron Lett. 1992, 33, 1601-1604.
(7) Sudo, T.; Asao, N.; Yamamoto, Y. J. Org. Chem. 2000, 65, 8919-
8923.
(8) For silicon-containing π-electron compounds applicable to organic
LEDs, see for instance: (a) Yamaguchi, S.; Tamao, K. The Chemistry of
Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley
& Sons: Chichester, 2001; pp 641-694 and references therein. (b) Tamao,
K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem.
Soc. 1996, 118, 11974-11975. (c) Murata, H.; Kafafi, Z. H.; Uchida, M.
Appl. Phys. Lett. 2002, 80, 189-191. (d) Ohshita, J.; Kai, H.; Sumida, T.;
Kunai, A.; Adachi, A.; Sakamaki, K.; Okita, K. J. Organomet. Chem. 2002,
642, 137. (e) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.;
Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun.
2001, 1740-1741.
(10) Yamaguchi, S.; Xu, C.; Tamao, K. J. Am. Chem. Soc. 2003, 125,
13662-13663.
(11) Besides the diphenylacetylene derivative 1, we found that (trimeth-
ylsilyl)(o-dimethylsilylphenyl)acetylene also underwent the present reduc-
tive cyclization to give the corresponding cyclized product in a good yield.
(9) Use of silaindene-based materials for organic LEDs has been reported
in a patent; see: Kohama, T.; Tominaga, T.; Murase, S. JP2002198176.
3708
Org. Lett., Vol. 6, No. 21, 2004

Scheme 3. Synthesis of a Series of Silaindene-Containing
π-Electron Systems
Figure 1. ORTEP drawings of compounds (a) 5a and (b) 7a (50%
probability for thermal ellipsoids).
All the produced silaindene-based π-electron compounds
show intense blue to greenish-blue photoluminescence. In
Table 1, the UV-visible absorption and fluorescence spectral
Table 1. Photophysical Properties of Silaindene Derivatives
and Related Compounds.
a Reagents and conditions: (1) LiNaph (4 mol amt.), THF, rt,
5 min; (2) electrophiles such as H2O, (MeO)2SO2, HMe2SiCl,
i-PrOBpin, Br2, or C6F6. pin ) pinacolato.
UV-vis absorptiona
fluorescencea
d
cmpd
λmax (nm)b
log ꢀ
λmax (nm)c
ΦF
τS (ns)e
5a
7a
9
376
398
364
424
4.59
4.51
4.52
4.43
413, 437
445, 467
396, 417
473, 499
0.86
0.73
0.49
0.50
1.9
2.6
1.7
3.5
various electrophiles, a series of the substituted bis(silain-
denyl)benzenes 5b-f having Me, SiMe2H, B(pin), Br, and
C6F5 groups were obtained, as shown in Scheme 3, although
the yields of 5e and 5f were rather low due to undesirable
side-reactions such as protonation. Similarly, 3,7-C6F5-
substituted disilaindacene 7f was also synthesized. Among
these groups, B(pin) and Br functionalities may be particu-
larly useful for further transformation through the well-
established cross-coupling methodologies. These results
demonstrate the great utility of the present cyclization as the
first general synthetic route to the functionalized silaindene-
based π-electron systems.
Among the produced new π-electron systems, the struc-
tures of compounds 5a and 7a have been determined by
X-ray crystallography, as shown in Figure 1.12 Notably, both
compounds have highly coplanar π-conjugated frameworks.
The dihedral angles between the central benzene plane and
the outer benzene planes are 26.8 and 27.9° for 5a and 6.60°
for 7a, respectively. These results suggest that both the
silaindene and disila-s-indacene skeletons are effective
building units for the π-electron systems with well-extended
π-conjugation.
10f
a In THF. b Only the absorption maxima with the highest intensity are
listed. c Emission maxima upon excitation at the absorption maximum
wavelengths. d Determined with 9,10-diphenylanthracene for 5a, anthracene
for 9, and perylene for 7a and 10 as a standard, respectively. The ΦF is the
average of values of repeated measurements within (5% error. e Fluores-
cence lifetimes within ( 0.5 ns error. f Ref 10.
data for the selected compounds, 5a, 7a, and 9, are sum-
marized, together with those of the previously reported
(12) Crystallographic data. 5a: C26H26Si2, FW ) 394.65, crystal
dimensions 0.40 × 0.40 × 0.40 mm3, monoclinic, space group P21/n,
a ) 9.9095(5) Å, b ) 9.850(5) Å, c ) 24.523(5) Å, â ) 92.510(5)°,
V ) 2194.8(17) Å3, Z ) 4, Dc ) 1.194 Mg m-3, µ(Mo KR) ) 0.170 mm-1
,
T ) 100(2) K, F(000) ) 840, 2θmax ) 50°. 10 740 reflections mea-
sured, of which 3856 were unique (Rint ) 0.0284). Final R1 ) 0.0451
with wR2 ) 0.1092 for 3856 observed reflections with I > 2σ(I). 7a:
C26H26Si2, FW ) 394.65, crystal dimensions 0.20 × 0.20 × 0.20
mm3, orthorhombic, space group Pbca, a ) 6.750(4) Å, b ) 16.542(10) Å,
c ) 20.635(12) Å, V ) 2304(2) Å3, Z ) 4, Dc ) 1.138 Mg m-3
,
µ(Mo KR) ) 0.162 mm-1, T ) 173(2) K, F(000) ) 840, 2θmax ) 55°.
16 155 reflections measured, of which 2622 were unique (Rint ) 0.0407).
Final R1 ) 0.0574 with wR2 ) 0.1533 for 2622 observed reflections with
I > 2σ(I).
Org. Lett., Vol. 6, No. 21, 2004
3709

emission maxima from 5a to 10 reach 48 and 60 nm,
respectively. (4) Accompanied with these differences, the
fluorescence quantum yields (ΦF) tend to decrease from 5a
to 7a to 10, while the fluorescence lifetime increases in this
order. These observations clearly demonstrate not only the
significant contribution of the silicon bridges to the photo-
physical properties and thus to the electronic structures but
also the strong dependence of the properties on the position
of the silicon-bridges. In a view of the application as a blue-
emitting material, the bis-silicon-bridged derivatives may be
promising because of their high fluorescence quantum yields
as well as the color purity.
Figure 2. Fluorescence spectra of compounds 5a, 7a, 9, and 10
in THF.
In summary, we have developed a new intramolecular
reductive cyclization that is a complement of the conventional
heteroannulation for the synthesis of heteroindenes. This
cyclization methodology allows us to synthesize a series of
fluorescent silaindene- and disilaindacene-containing π-elec-
tron systems and to disclose the significant effect of the
silicon bridges on their photophysical properties. The readily
accessible functionalization and structural modification are
other notable advantages of the present methodology. On
the basis of this synthesis, further structural optimization
toward more promising silicon-containing π-electron materi-
als is now in progress in our laboratory.
tetrakis-silicon-bridged bis(styryl)benezene 10,10 for com-
parison, and Figure 2 shows their fluorescence spectra. Since
compounds 5a, 7a, and 10 have identical π-conjugation
lengths but have a different number of silicon bridges at the
different positions, the comparison among these compounds
provides some crucial information about the effect of the
silicon moieties: (1) Compounds 5a, 7a, and 10 have their
absorption maxima at 376-424 nm and emission maxima
at 413-473 nm. Notably, these values are rather longer than
those of the nonbridged parent 1,4-bis(styryl)benezene
(λabs,max ) 350 nm and λem,max ) 405 nm).13 (2) Compound
7a, bearing the two silicon-bridges on the internal benzene
ring, has longer absorption and emission maxima by 22 and
32 nm, respectively, compared to compound 5a, which has
the two silicon-bridges on the external benzene rings. (3)
From the bis-silicon-bridged 5a and 7a to the tetrakis-silicon-
bridged 10, the absorption and emission maxima further shifts
to longer wavelengths. The red shifts in the absorption and
Acknowledgment. This work was supported by a Grants-
in-Aid (No. 12CE2005 for Elements Science and No.
15205014) from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan, and PRESTO, Japan
Science and Technology Agency (JST).
Supporting Information Available: Spectral and ana-
lytical data for silaindene derivatives and CIF data for 5a
and 7a. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) Renak, M. L.; Bartholomew, G. P.; Wang, S.; Ricatto, P. J.;
Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 7787.
OL0486932
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