Synthesis of silafluorenes by iridium-catalyzed [2 + 2 + 2] cycloaddition of silicon-bridged diynes with alkynes

Article abstract of DOI:10.1021/ol062732n

(Chemical Equation Presented) Silicon-bridged 1,6-diynes underwent [2 + 2 + 2] cycloaddition with alkynes in the presence of an iridium(I)-phosphine catalyst to afford densely substituted silafluorene derivatives. Extended silafluorene skeletons were cons

Full text of DOI:10.1021/ol062732n

ORGANIC
LETTERS
2007
Vol. 9, No. 1
133-136
Synthesis of Silafluorenes by
Iridium-Catalyzed [2 2]
Cycloaddition of Silicon-Bridged Diynes
with Alkynes
+
2 +
Takanori Matsuda, Sho Kadowaki, Tsuyoshi Goya, and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto UniVersity,
Katsura, Kyoto 615-8510, Japan
murakami@sbchem.kyoto-u.ac.jp
Received November 8, 2006
ABSTRACT
Silicon-bridged 1,6-diynes underwent [2
+
2
+
2] cycloaddition with alkynes in the presence of an iridium(I)
−phosphine catalyst to afford
densely substituted silafluorene derivatives. Extended silafluorene skeletons were constructed by the [2
+
2 + 2] cycloaddition of tetraynes.
Siloles (silacyclopentadienes) are members of an intriguing
class of silicon-based π-conjugated molecules that possess
unique photophysical and electronic properties owing to their
low-lying LUMO associated with σ*-π* conjugation.1 For
example, high electron-transporting performances have been
reported for some of the siloles.2 9-Silafluorenes (diben-
zosiloles), which embody a silicon-bridged biphenyl frame-
work, are also considered to be a promising candidate for
materials of such properties.3 There are several methods
reported for the syntheses of silafluorenes.4 However, starting
substances are limited to biaryl derivatives and the cyclization
step requires strongly basic conditions and/or reducing
conditions which require special precautions (Scheme 1). It
is, therefore, highly desired to develop a new method for
the synthesis of silafluorenes. Transition-metal-catalyzed [2
+ 2 + 2] cycloaddition reactions of alkynes (cyclotrimer-
ization) proceed under barely basic conditions and have been
widely used for constructing benzene structures.5 Herein, we
report a new synthetic method for silafluorene derivatives
by the iridium(I)-catalyzed [2 + 2 + 2] cycloaddition of
silicon-bridged 1,6-diynes with alkynes.6
Dimethyl(phenylethynyl)[2-(phenylethynyl)phenyl]-si-
lane (1a) was easily prepared by the chemoselective pal-
Scheme 1
(1) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 3693.
(2) (a) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi,
S. J. Am. Chem. Soc. 1996, 118, 11974. (b) Palilis, L. C.; Murata, H.;
Uchida, M.; Kafafi, Z. H. Org. Electron. 2003, 4, 113. (c) Kulkarni, A. P.;
Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556.
(3) (a) Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B. J.
Am. Chem. Soc. 2005, 127, 7662. (b) Mo, Y.; Tian, R.; Shi, W.; Cao, Y.
Chem. Commun. 2005, 4925. (c) Usta, H.; Lu, G.; Facchetti, A.; Marks, T.
J. J. Am. Chem. Soc. 2006, 128, 9034.
10.1021/ol062732n CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/05/2006

ladium-catalyzed coupling reaction of 1-bromo-2-iodoben-
zene with phenylacetylene and the following installation of
alkynylsilane moieties.7 Diyne 1a and 1,4-dimethoxybut-2-
yne (2a, 2 equiv) in dibutyl ether were heated at 110 °C for
24 h in the presence of [IrCl(cod)]2 (2.5 mol %; cod )
cycloocta-1,5-diene) and triphenylphosphine (10 mol %). A
cross [2 + 2 + 2] cycloaddition between 1a and 2a
selectively took place to give 2,3-bis(methoxymethyl)-1,4-
diphenylsilafluorene 3aa in 86% isolated yield (Scheme 2).
Table 1. Iridium-Catalyzed [2 + 2 + 2] Cycloaddition of
Diynes 1 with Monoynes 2a
entry
1 (R1, R2)
1a (Ph, Ph)
2 (R3)
3 (% yieldb)
1
2b (CH2OBn) 3ab (93)
Scheme 2. Iridium-Catalyzed [2 + 2 + 2] Cycloaddition of
Silicon-Bridged 1,6-Diyne 1a with Monoyne 2a
2
1a (Ph, Ph)
1a (Ph, Ph)
1a (Ph, Ph)
1b (4-(CH2dCH)C6H4, Ph)
1c (4-MeOC6H4, 4-MeOC6H4) 2a
1d (Ph, n-C5H11
1e (n-C5H11, n-C5H11
1f (Ph, H)
2c (CH2OH)
2d (Pr)
2e (CO2Me)
2a
3ac (42)
3ad (75)
3ae (7)
3ba (79)
3ca (77)
3da (81)
3ea (69)
3fa (25)
3c
4d
5
6
7
8d
9c
)
2a
2a
2a
)
a Unless otherwise noted, diyne 1 and monoyne 2 (2 equiv) were heated
in Bu2O at 110 °C in the presence of [IrCl(cod)]2 (2.5 mol %) and PPh3
(10 mol %) for 24 h. b Isolated yield. c [IrCl(cod)]2 (5 mol %) and PPh3
(20 mol %) were used. d 145 °C.
Iridium acts as a template to assemble the three carbon-
carbon triple bonds into the six-membered aromatic ring on
it. Dibutyl ether afforded a better yield of 3aa than other
solvents (toluene, 110 °C, 64%; 1,4-dioxane, 100 °C, 52%).
The use of a similar rhodium catalyst (2.5 mol % of [RhCl-
(CH2dCH2)2]2 and 10 mol % of PPh3) under identical
conditions gave 73% yield of 3aa.
Various diynes 1 and monoynes 2 were subjected to the
iridium(I)-catalyzed [2 + 2 + 2] cycloaddition reaction
(Table 1). Diyne 1a reacted with 1,4-dibenzyloxybut-2-yne
(2b) to provide silafluorene 3ab in 93% yield (entry 1).
Unprotected but-2-yne-1,4-diol also participated in the cy-
cloaddition (entry 2). Oct-4-yne (2d) required 10 mol % of
the iridium catalyst to get a good yield (entry 3). The reaction
of 1a with dimethyl acetylenedicarboxylate (2e) was sluggish
even at 145 °C giving only 7% yield of product 3ae (entry
4). Diphenylacetylene failed to react with 1a. Diynes (1b
and 1c) having functionalized aromatic rings on the alkyne
termini gave the corresponding silafluorenes (3ba and 3ca)
in good yields (entries 5 and 6). The functionalized silafluo-
renes obtained are potential candidates for the monomer for
the synthesis of silafluorene-containing polymers.3 Alkyl
groups on the alkyne termini of the diynes were also tolerated
(entries 7 and 8). Diyne 1f possessing an unsubstituted
terminal alkyne moiety afforded 25% yield of silafluorene
3fa (entry 9).
Structural modification of the o-phenylene linker was also
studied. A diyne having two methoxy groups on the
o-phenylene tether gave hexasubstituted silafluorene 3ga in
76% yield. Pyridine-fused product 3ha was obtained by the
reaction of the corresponding pyridine-tethered diyne with
a higher catalyst loading (5 mol % of [IrCl(cod)]2 and 20
mol % of PPh3) at a higher reaction temperature (145 °C).
(4) (a) Gilman, H.; Gorsich, R. D. J. Am. Chem. Soc. 1955, 77, 6380.
(b) Ishikawa, M.; Tabohashi, T.; Sugisawa, H.; Nishimura, K.; Kumada,
M. J. Organomet. Chem. 1983, 250, 109. (c) van Klink, G. P. M.; de Boer,
H. J. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Spek, A. L.
Organometallics 2002, 21, 2119. (d) Liu, Y.; Stringfellow, T. C.; Ballweg,
D.; Guzei, I. A.; West, R. J. Am. Chem. Soc. 2002, 124, 49. (e) Wang, Z.;
Fang, H.; Xi, Z. Tetrahedron Lett. 2005, 46, 499. (f) Chen, R.-F.; Fan,
Q.-L.; Zheng, C.; Huang, W. Org. Lett. 2006, 8, 203. (g) Hudrlik, P. F.;
Dai, D.; Hudrlik, A. M. J. Organomet. Chem. 2006, 691, 1257.
(5) For recent examples of [2 + 2 + 2] cycloaddition forming benzene
derivatives, see: (a) Miljanic´, O. Sˇ.; Holmes, D.; Vollhardt, K. P. C. Org.
Lett. 2005, 7, 4001. (b) Yamamoto, Y.; Ishii, J.; Nishiyama, H.; Itoh, K.
J. Am. Chem. Soc. 2005, 127, 9625. (c) Shibata, T.; Tsuchikama, K.; Otsuka,
M. Tetrahedron: Asymmetry 2006, 17, 614. (d) Kezuka, S.; Tanaka, S.;
Ohe, T.; Nakaya, Y.; Takeuchi, R. J. Org. Chem. 2006, 71, 543. (e) Tanaka,
K.; Takeishi, K.; Noguchi, K. J. Am. Chem. Soc. 2006, 128, 4586. (f) Tracey,
M. R.; Oppenheimer, J.; Hsung, R. P. J. Org. Chem. 2006, 71, 8629. For
reviews, see: (g) Saito, S.; Yamamoto, Y. Chem. ReV. 2000, 100, 2901.
(h) Gandon, V.; Aubert, C.; Malacria, M. Chem. Commun. 2006, 2209.
(6) Synthesis of carbazoles by an analogous rhodium-catalyzed [2 + 2
+ 2] cycloaddition of nitrogen-bridged diynes and alkynes has been reported.
Witulski, B.; Alayrac, C. Angew. Chem., Int. Ed. 2002, 41, 3281.
(7) See Supporting Information for details.
The core skeleton required for the present [2 + 2 + 2]
cycloaddition reaction is an o-phenylene-tethered siladiyne
unit. The core unit can be multiply embodied in the starting
substances. This advantageous feature rendered it possible
to synthesize various types of arrayed silafluorenes.
Ladder-type π-conjugated molecules have received con-
siderable attention due to effective conjugation by the rigid
coplanar structures.8 Tetrayne 4 was readily prepared starting
(8) (a) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (b) Martin, R. E.;
Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350. (c) Wong, K.-T.;
Chi, L.-C.; Huang, S.-C.; Liao, Y.-L.; Liu, Y.-H.; Wang, Y. Org. Lett. 2006,
8, 5029.
134
Org. Lett., Vol. 9, No. 1, 2007

from 1,4-dibromo-2,5-diiodobenzene by sequential installa-
tion of two alkyne parts. On heating 4 in the presence of the
iridium catalyst, double [2 + 2 + 2] cycloaddition with 2a
occurred on both sides of 4 to furnish ladder-type silafluorene
59 in 58% yield (Scheme 3).
ethynyl)lithium. The iridium-catalyzed reaction of 8 with 2a
produced asymmetrically substituted spirosilabifluorene 9 in
good yield (Scheme 5).13
Scheme 5. Synthesis of Spirosilabifluorene 9
Scheme 3. Synthesis of Ladder-Type Silafluorene 5
Photophysical and thermal data for the produced silafluo-
renes are summarized in Table 2. The silafluorenes exhibited
Poly(p-phenylene)s (PPPs) have been the focus of much
study because they act as organic conductors upon doping
and as light-emitting materials.10 Tetrayne 6 was prepared
from 1,4-diethynylbenzene through the palladium-catalyzed
coupling reaction with 1-bromo-2-iodobenzene followed by
installation of the alkynylsilane moieties. Cycloaddition of
6 with 2a gave p-phenylenebis(silafluorene) 7 carrying a
p-quinquephenyl moiety, in which three phenyl rings and
two aromatic groups of silafluorene skeletons were alterna-
tively arrayed (Scheme 4).11
Table 2. Photophysical and Thermal Properties of
Silafluorenes
UV-visa
fluorescenceb
silafluorene
λabs (log ꢀ)
λem (ΦF)
Tg
Tm
3aa
3ea
5
323 nm (3.60) 354 nm (0.12) 37 °C
323 nm (3.75) 349 nm (0.29) -40 °C
151 °C
-
64 °C
296 °C
215 °C
362 nm (4.37) 396 nm (0.91)
-
7
9
323 nm (3.53) 356 nm (0.10) 74 °C
334 nm (3.72) 364 nm (0.16) 91 °C
a Measured in CHCl3. b Measured in hexane. Determined with reference
to quinine sulfate in 0.1 N H2SO4 and anthracene in EtOH (excited at 250
nm).
Scheme 4. Synthesis of p-Phenylenebis(silafluorene) 7
their absorption maxima at 323-362 nm and their fluores-
cence maxima at 349-396 nm. For comparison, a UV-vis
spectrum of 9,9-dimethyl-9-silafluorene was measured. It
showed a fluorescence maximum at 339 nm (10% quantum
yield). Introduction of the substituents at the 1, 2, 3, and 4
positions of the silafluorene cores caused a bathochromic
shift of 10-15 nm. Of particular note is that ladder-type 5
has a longer absorption wavelength and exceptionally high
fluorescence efficiency (91%). The two silafluorene cores
share the internal benzene ring. The extension of the
π-conjugation shifted the absorption and fluorescence maxima
to longer wavelengths, which still lay within the UV region.
The photophysical data of p-phenylenebis(silafluorene) 7
were very similar to those of 3aa. Spirosilabifluorene 9 has
longer absorption and fluorescence maxima by ca. 10 nm
compared to 3aa. As for the thermal properties, the glass
transition temperatures (Tg) and melting points (Tm) were
Spiro-linked molecules have many favorable properties
such as high glass transition temperatures and high morpho-
logical stabilities.12 Tetrayne 8 was prepared by the reaction
of dichlorobis[2-(phenylethynyl)phenyl]silane and (phenyl-
(9) For ladder-type siloles, see: (a) Yamaguchi, S.; Xu, C.; Tamao, K.
J. Am. Chem. Soc. 2003, 125, 13662. (b) Xu, C.; Wakamiya, A.; Yamaguchi,
S. J. Am. Chem. Soc. 2005, 127, 1638.
(12) Steuber, F.; Staudigel, J.; Sto¨ssel, M.; Simmerer, J.; Winnacker,
A.; Spreitzer, H.; Weisso¨rtel, F.; Salbeck, J. AdV. Mater. 2000, 12, 130.
(13) For spirosilabifluorenes, see: (a) Gilman, H.; Gorsich, R. D. J. Am.
Chem. Soc. 1958, 80, 1883. (b) Russell, A. G.; Spencer, N. S.; Philp, D.;
Kariuki, B. M.; Snaith, J. S. Organometallics 2003, 22, 5589. (c) Lee, S.
H.; Jang, B.-B.; Kafafi, Z. H. J. Am. Chem. Soc. 2005, 127, 9071.
(10) (a) Tour, J. M. AdV. Mater. 1994, 6, 190. (b) Berresheim, A. J.;
Mu¨ller, M.; Mu¨llen, K. Chem. ReV. 1999, 99, 1747.
(11) Synthesis of oligo-p-phenylenes by [2 + 2 + 2] cycloaddition has
been reported. McDonald, F. E.; Smolentsev, V. Org. Lett. 2002, 4, 745.
Org. Lett., Vol. 9, No. 1, 2007
135

investigated. Silafluorenes 7 and 9 melted above 200 °C,
and the Tg values were higher than 70 °C. Silafluorenes 3aa,
3ea, and 5 exhibited inferior thermal properties.
In summary, densely substituted silafluorenes have been
synthesized by the iridium-catalyzed [2 + 2 + 2] cycload-
dition of silicon-bridged 1,6-diynes with alkynes. Structurally
intriguing extended silafluorenes have also been prepared
by the cycloaddition.
Acknowledgment. S.K. thanks the Japan Society for the
Promotion of Science for fellowship support.
Supporting Information Available: Experimental pro-
cedures and characterization data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL062732N
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Org. Lett., Vol. 9, No. 1, 2007