Stereoselective synthesis of multisubstituted butadienes through directed Mizoroki-Heck reaction and homocoupling reaction of vinyl(2-pyridyl)silane

Article abstract of DOI:10.1021/ol048620i

(Chemical Equation Presented) We have developed the homocoupling reaction of alkenyl(2-pyridyl)silanes mediated by Cul and CsF, in which the strong directing effect of the 2-pyridyl group was observed. The homocoupling reaction was successfully integrated with the Mizoroki-Heck reaction of vinyl(2-pyridyl)-silane with aryl halides, enabling a rapid and stereoselective synthesis of multisubstituted butadienes. From a relatively small compound library, it was possible to detect a number of fluorescent butadienes with a wide range of fluorescence color variations (blue to reddish-orange).

Full text of DOI:10.1021/ol048620i

ORGANIC
LETTERS
2004
Vol. 6, No. 21
3695-3698
Stereoselective Synthesis of
Multisubstituted Butadienes through
Directed Mizoroki−Heck Reaction and
Homocoupling Reaction of
Vinyl(2-pyridyl)silane
Kenichiro Itami,* Yousuke Ushiogi, Toshiki Nokami, Youichi Ohashi, and
Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan
itami@sbchem.kyoto-u.ac.jp; yoshida@sbchem.kyoto-u.ac.jp
Received July 16, 2004
ABSTRACT
We have developed the homocoupling reaction of alkenyl(2-pyridyl)silanes mediated by CuI and CsF, in which the strong directing effect of
the 2-pyridyl group was observed. The homocoupling reaction was successfully integrated with the Mizoroki Heck reaction of vinyl(2-pyridyl)-
−
silane with aryl halides, enabling a rapid and stereoselective synthesis of multisubstituted butadienes. From a relatively small compound
library, it was possible to detect a number of fluorescent butadienes with a wide range of fluorescence color variations (blue to reddish-
orange).
As exemplified by 1,1,4,4-tetraphenyl-1,3-butadiene, which
is known as a useful blue-emitting material in organic
electroluminescent devices,1 an extended π-system based on
a multisubstituted butadiene structure should be an interesting
target as a functional material. However, the paucity of
general methods for the stereoselective synthesis of multi-
substituted butadienes hampers the rapid construction and
property-evaluation of such extended π-systems.2 We herein
report a rapid and stereoselective synthesis of multisubstituted
butadienes through Pd- and Cu-mediated sequential assembly
of π-systems onto the CdC core of vinyl(2-pyridyl)silane.
Recently, we reported that the otherwise difficult
Mizoroki-Heck reaction of vinylsilanes was efficiently
promoted by appending a catalyst-directing 2-pyridyl group
on silicon (Scheme 1).3 Because of the strong directing effect
of the 2-pyridyl group, a hard-to-achieve double Mizoroki-
Heck reaction is also feasible (Scheme 1).4 We also found
that the thus-obtained alkenyl(2-pyridyl)silanes (2 and 3)
undergo efficient Hiyama-type silicon-based cross-coupling
reactions with aryl halides, producing di- and triarylethenes
(1) Selected examples: (a) Kido, J.; Shionoya, H.; Nagai, K. Appl. Phys.
Lett. 1995, 67, 2281. (b) Kim, J.-H.; Noh, S.; Kim, K.; Lim, S.-T.; Shin,
D.-M. Synth. Met. 2001, 117, 227.
(2) For previous synthesis of tetraphenylbutadiene, see: (a) Wittig, G.;
von Lupin, F. Chem. Ber. 1928, 61, 1627. (b) Simes, B. E.; Rickborn, R.;
Flournoy, J. M.; Berlman, I. B. J. Org. Chem. 1988, 53, 4613. (c) Feit,
B.-A.; Buzhansky, L. J. Chem. Soc., Perkin Trans. 1 2000, 1777.
(3) Itami, K.; Mitsudo, K.; Kamei, T.; Koike, T.; Nokami, T.; Yoshida,
J. J. Am. Chem. Soc. 2000, 122, 12013.
(4) Itami, K.; Nokami, T.; Ishimura, Y.; Mitsudo, K.; Kamei, T.; Yoshida,
J. J. Am. Chem. Soc. 2001, 123, 11577.
10.1021/ol048620i CCC: $27.50
© 2004 American Chemical Society
Published on Web 09/24/2004

Scheme 1
Scheme 2
bridged dimers) in moderate to high yields (Scheme 2).
Although satisfactory X-ray crystal structure analysis was
not possible with these complexes, we succeeded in the X-ray
crystal structure analysis of the p-tolyl analogue 8, which
unambiguously ascertained the vinyl-N chelation to copper
(Figure 1). Moreover, we found that the further treatment
of 7 with CsF did afford 4a in reasonable yield (Scheme 2;
see also eq 1 and ref 8).
in a regio- and stereoselective manner (Scheme 1).2,5 We
envisaged that if we could develop a procedure for the
homocoupling reaction of alkenyl(2-pyridyl)silanes (2 and
3), a new synthetic avenue toward multisubstituted butadienes
(4 and 5) would be established (Scheme 1). In addition, such
a transformation should also be useful as a detagging protocol
for the 2-pyridylsilyl group in the phase tag strategy using
this group.6
Although there are a number of reports on the homo-
coupling reaction of organosilanes,7 the applications to
alkenyl(2-pyridyl)silanes resulted in disappointing outcomes
in many cases, presumably because activating groups such
as fluoride are necessary for these methods. During further
investigations, we found that the use of CuI in combination
with CsF resulted in efficient homocoupling of (E)-styryl-
(2-pyridyl)silane (2a) in CH3CN to afford (E,E)-1,4-diphenyl-
1,3-butadiene (4a) quantitatively (eq 1).8 Interestingly, the
corresponding phenyl analogue 6 remained unaffected under
otherwise identical conditions. Therefore, a promoting effect
of the 2-pyridyl group in homocoupling was obvious at this
point.
Figure 1. Synthesis and X-ray structure of 8.
Currently, we assume that the homocoupling occurs
through Si-to-Cu transmetalation to generate alkenylcopper
species, followed by their homocoupling. The suitably
positioned pyridyl group should help the CdC bond to
coordinate to copper through precoordination and/or vinyl-N
chelation, thereby allowing subsequent transmetalation very
efficiently. As expected from the results in eq 1 and Scheme
2, the treatment of CuX with 6 did not produce such a Cu
complex at all, which clearly supports the directing effect
of the 2-pyridyl group in the formation of key Cu-olefin
complexes.
Subsequently we found that the stoichiometric reactions
of 2a with CuX produced Cu complexes 7 (as halogen-
(5) Itami, K.; Nokami, T.; Yoshida, J. J. Am. Chem. Soc. 2001, 123,
5600.
(6) Reviews: (a) Yoshida, J.; Itami, K. Chem. ReV. 2002, 102, 3693.
(b) Yoshida, J.; Itami, K. J. Synth. Org. Chem. Jpn. 2001, 59, 1086.
(7) Pd-mediated methods: (a) Weber, W. P.; Felix, R. A.; Willard, A.
K.; Koenig, K. E. Tetrahedron Lett. 1971, 4701. (b) Yoshida, J.; Tamao,
K.; Yamamoto, H.; Kakui, T.; Uchida, T.; Kumada, M. Organometallics
1982, 1, 542. (c) Yamaguchi, S.; Ohno, S.; Tamao, K. Synlett 1997, 1199.
(d) Yoshida, H.; Yamaryo, Y.; Ohshita, J.; Kunai, A. Chem. Commun. 2003,
1510. Cu-mediated methods: (e) Yoshida, J.; Tamao, K.; Kakui, T.;
Kumada, M. Tetrahedron Lett. 1979, 1141. (f) Kang, S.-K.; Kim, T.-H.;
Pyun, S.-J. J. Chem. Soc., Perkin Trans. 1 1997, 797. (g) Nishihara, Y.;
Ikegashira, K.; Toriyama, F.; Mori, A.; Hiyama, T. Bull. Chem. Soc. Jpn.
2000, 73, 985. Ag-mediated methods: (h) Mueller, R.; Dressler, M.; Dathe,
C. J. Prakt. Chem. 1970, 312, 150. (i) Tamao, K.; Matsumoto, H.; Kakui,
T.; Kumada, M. Tetrahedron Lett. 1979, 1137.
With an efficient procedure established, we next embarked
on the homocoupling of various alkenyl(2-pyridyl)silanes (2
and 3), which can be prepared stereoselectively by the
Mizoroki-Heck reaction of vinyl(2-pyridyl)silane (1) with
(8) Other Cu and Ag complexes (with CsF co-promoter) also mediated
the homocoupling of 2a: CuCl (59%), CuBr (59%), [Cu(CH3CN)4]PF6
(87%), AgBF4 (65%).
3696
Org. Lett., Vol. 6, No. 21, 2004

Table 1. CuI/CsF-Mediated Homocoupling Reaction of Alkenyl(2-pyridyl)silanes
Org. Lett., Vol. 6, No. 21, 2004
3697

aryl halides (Scheme 1).9,10 The results are summarized in
Table 1. Both 2 and 3 underwent homocoupling under the
influence of CuI/CsF to give structurally and electronically
diverse multisubstituted butadienes (4 and 5) in moderate to
high yields. In all cases examined, virtually complete
retention of stereochemistry during the homocoupling was
observed. Therefore, a set of stereoisomers such as 5be and
5eb can be prepared at will by simply changing the addition
order of aryl halides in the double Mizoroki-Heck reaction
step (Scheme 1).
The emissive behaviors of representative compounds upon
irradiation of light (365 nm) in CHCl3 are shown in Figure
2. As apparent from the picture, fluorescence color variations
Interestingly, most of the multisubstituted butadienes (4
and 5) prepared were found to be fluorescent. The photo-
physical properties of these compounds are listed in Table
2. As expected, the absorption maxima as well as emission
Table 2. Photophysical Properties of Multisubstituted
Butadienesa
UV-vis
λabs (nm)
fluorescence
4 or 5
Ar1
C6H5
C6H4Ac-4
2-thienyl
3-thienyl
C6H5
C6H4Ac-4
3-thienyl
C6H4OMe-4
C6H4F-4
Ar2
λem (nm)
4a
4b
4c
333
368
363
322
348
381
353
365
345
376
389
391
383
417
322
381
425
430
394
449
480
435
448
435
472
499
500
490
593
578
4d
Figure 2. Emissive behaviors of representative compounds.
5a a
5bb
5d d
5ee
5ff
5a b
5be
5eb
5eg
5eh
5ib
C6H5
C6H4Ac-4
3-thienyl
C6H4OMe-4
C6H4F-4
C6H4Ac-4
C6H4OMe-4
C6H4Ac-4
C6H4CN-4
C6H4NO2-4
over a wide range of wavelengths (blue to reddish-orange)
were realized. Since the parent tetraphenylbutadiene (5aa)
has been used as a material in organic electroluminescent
devices, these multisubstituted butadienes might be also
useful for such purpose.
In summary, we have developed the homocoupling reac-
tion of alkenyl(2-pyridyl)silanes mediated by CuI and CsF,
in which the strong directing effect of the 2-pyridyl group
was observed. The homocoupling reaction was successfully
integrated with the Mizoroki-Heck reaction of vinyl(2-
pyridyl)silane with aryl halides, enabling a rapid and
stereoselective synthesis of multisubstituted butadienes. The
discovery of a number of fluorescent materials with a wide
range of color variations speaks well for the potential of the
current synthetic strategy in the development of functional
organic materials. Investigations along this line are currently
ongoing in our laboratory.
C6H5
C6H4Ac-4
C6H4OMe-4
C6H4OMe-4
C6H4OMe-4
C6H4NMe2-4 C6H4Ac-4
a UV-vis absorption and fluorescence spectra were measured in CHCl3.
maxima were found to depend heavily on the nature of the
terminal aryl groups (Ar1 and Ar2). For example, the λabs
and λem values increase as the phenyl group is replaced by
the p-acetylphenyl group (4a f 4b; 5aa f 5ab f 5bb),
presumably reflecting effective extension of π-conjugation
with terminal carbonyl groups. More interestingly, when Ar1
and Ar2 groups are in a donor/acceptor (push/pull) relation-
ship, a significant bathochromic shift of absorption and
fluorescence is observed (5be, 5eb, 5eg, 5eh, and 5ib). Such
a donor/acceptor substitution can bring about the reddish-
orange emission (5ib) from otherwise blue-fluorescent mate-
rial 5aa.
Acknowledgment. This work was financially supported
by a Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science.
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds,
including data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
(9) See Supporting Information for the preparation of 2 and 3.
(10) Dimethyl(2-pyridyl)vinylsilane (1) is now commercially available
from Tokyo Kasei Kogyo Co., Ltd (TCI). Catalog Number: D2935.
OL048620I
3698
Org. Lett., Vol. 6, No. 21, 2004