Transmetalation as a Route to Novel Styryllithium Reagents

Article abstract of DOI:10.1021/ol035193k

(Equation presented) Transmetalation of β-tributyl(styryl)stannanes with n-BuLi gives the functional equivalents of the corresponding styryllithium intermediates. Reaction of the intermediates with chlorotrimethylsilane, iodomethane, or dimethyl sulfate gives the substituted styryl products in moderate to good yields. In all cases, the configuration about the double bond was retained in the products.

Full text of DOI:10.1021/ol035193k

ORGANIC
LETTERS
2003
Vol. 5, No. 19
3443-3445
Transmetalation as a Route to Novel
Styryllithium Reagents
Chinwon Rim and David Y. Son*
Department of Chemistry, Southern Methodist UniVersity, P.O. Box 750314,
Dallas, Texas 75275-0314
dson@mail.smu.edu
Received June 26, 2003
ABSTRACT
Transmetalation of â-tributyl(styryl)stannanes with n-BuLi gives the functional equivalents of the corresponding styryllithium intermediates.
Reaction of the intermediates with chlorotrimethylsilane, iodomethane, or dimethyl sulfate gives the substituted styryl products in moderate
to good yields. In all cases, the configuration about the double bond was retained in the products.
In recent years, considerable effort has focused on the
synthesis of conjugated oligomers and polymers for applica-
tion as novel optoelectronic materials.1 As part of our own
research program, we have focused on the synthesis of
oligomers and polymers with alternating π-conjugated and
silylene units.2 Polymers such as these are known to possess
what is commonly called “σ-π conjugation” along the
polymer backbone.3 As part of our continuing efforts in this
area, we sought to develop new reagents for the construction
of these materials. To this end, we describe herein the
synthesis of new lithiated styrene reagents prepared by
transmetalation of the corresponding tributyl(styryl)stannanes.
In addition to their expected value in the synthesis of new
σ-π conjugated organosilicon systems, we believe these
reagents will also be useful in the construction of new organic
conjugated polymers via conventional metal-catalyzed cou-
pling methods.
of the reported successful preparation of (E)-â-styryllithium
by transmetalation of (E)-â-trimethyl(styryl)stannane or (E)-
â-triphenyl(styryl)stannane with methyllithium or phenyl-
lithium.4 We initially attempted the synthesis of (E)-â-
styryllithium by the transmetalation of (E)-â-tributyl(styryl)-
stannane (itself prepared in high yield by the AIBN-catalyzed
hydrostannation of phenylacetylene with tributyltin hydride)
with n-BuLi. As expected, the lithium reagent was formed
as evidenced by the isolation of the trimethylsilyl-substituted
product in 87% yield when the dark purple intermediate was
treated with chlorotrimethylsilane (Scheme 1). As in the
Scheme 1
Early on, we decided to synthesize these reagents by tin-
lithium exchange (transmetalation) reactions of styrylstan-
nanes, which could be prepared by the hydrostannation of
the corresponding ethynylbenzene compounds with readily
available tributyltin hydride. We chose this route because
(1) (a) Yamamoto, T. Macromol. Rapid Commun. 2002, 23, 583. (b)
Tour, J. M. Acc. Chem. Res. 2000, 33, 791.
(2) (a) Yao, J.; Son, D. Y. Organometallics 1999, 18, 1736. (b) Xiao,
Y.; Wong, R. A.; Son, D. Y. Macromolecules 2000, 33, 7232.
(3) Ohshita, J.; Kunai, A. Acta Polym. 1998, 49, 379.
earlier report,4 the (E)-â-isomer was formed exclusively with
tetrabutyltin as the only byproduct.
10.1021/ol035193k CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/21/2003

We then turned our attention to the synthesis of the
previously unknown dilithiated reagents. Free-radical hy-
drostannation of m-diethynylbenzene with tributyltin hydride
gave exclusively 1,3-bis((E)-2-(tributylstannyl)vinyl)benzene,
novel trilithium reagent, or at least its functional equivalent
(Scheme 3).
1
1, in 90% yield. The structure of 1 was confirmed by H
Scheme 3
NMR spectroscopy. Addition of n-BuLi to a THF solution
of 1 at -15 °C gave a dark purple intermediate, which when
quenched with either chlorotrimethylsilane or iodomethane
gave the corresponding substituted styryl compounds 1-Si
and 1-Me in 56 and 68% yields, respectively (Scheme 2).
Scheme 2
In all of the cases described above, the (E)-â compounds
were formed exclusively. In view of recent successes in
regioselective hydrostannation of acetylenes to form (Z)-â-
vinylstannanes,6 we examined the possibility of extending
this transmetalation chemistry to the synthesis of (Z)-â-
styryllithiums. Accordingly, reaction of phenylacetylene with
tributyltin hydride in the presence of ZrCl4 gave the known
(Z)-â-tributyl(styryl)stannane, 4, in 92% yield. Treatment of
4 with n-BuLi in the usual manner followed by addition of
chlorotrimethylsilane or dimethyl sulfate gave the corre-
sponding substituted styryl compounds 4-Si and 4-Me in 47
and 75% yields, respectively (Scheme 4). In both cases, the
The pure products could be separated from tetrabutyltin by
flash chromatography. The (E)-â configuration in the prod-
ucts was retained, as determined by 1H NMR spectroscopy.
Application of this method to p-diethynylbenzene was not
completely successful. Hydrostannation of p-diethynylben-
zene by free-radical methods was unsuccessful and gave rise
to an unidentified mixture of products. We eventually utilized
a cyanomethyltributylstannyl cuprate reagent5 and obtained
1,4-bis((E)-2-(tributylstannyl)vinyl)benzene, 2, in 40% NMR
yield. This product could not be purified (chromatography
resulted in decomposition) but was used as is for the
transmetalation step. Addition of n-BuLi to a THF solution
of 2 followed by addition of chlorotrimethylsilane or
iodomethane resulted in the formation of the corresponding
disubstituted compounds 2-Si and 2-Me in 43 and 85%
yields, respectively (Scheme 2). As in the case of 1, the
products retained the (E)-â configuration.
Scheme 4
We then attempted to synthesize the 1,3,5-trilithium
reagent using the methods described above. AIBN-catalyzed
hydrostannation of 1,3,5-triethynylbenzene with tributyltin
hydride was successful and gave exclusively 1,3,5-tris((E)-
2-(tributylstannyl)vinyl)benzene, 3, in 82% yield. Treatment
of 3 with n-BuLi in THF at -15 °C followed by quenching
with chlorotrimethylsilane or iodomethane gave the corre-
sponding trisubstituted compounds 3-Si and 3-Me in 52 and
78% yields, respectively, evidence of the formation of the
(Z)-â configuration was retained, as determined by 1H NMR
spectroscopy. These results are in marked contrast to the
earlier referenced report in which transmetalation of (Z)-â-
(4) Seyferth, D.; Vaughan, L. G.; Suzuki, R. J. Organomet. Chem. 1964,
1, 437.
(5) (a) Barero, A.; Cuadrado, P.; Fleming, I.; Gonzalez, A. M.; Pulido,
F. J.; Rubio, R. J. Chem. Soc., Perkin Trans. 1 1993, 1657. (b) Barero, A.;
Cuadrado, P.; Fleming, I.; Gonzalez, A. M.; Pulido, F. J.; Rubio, R. Chem.
Commun. 1992, 351. (c) Lipshutz, B. H.; Kozlowski, J. A.; Wilhem, R. S.
J. Org. Chem. 1984, 49, 3943.
(6) (a) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. Chem. Commun.
1995, 2405. (b) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Org.
Chem. 1996, 61, 4568.
3444
Org. Lett., Vol. 5, No. 19, 2003

trimethyl(styryl)stannane with methyllithium gave only
polymeric products when the transmetalated intermediate was
treated with chlorotrimethylsilane.4 Thus far, extension of
this chemistry to synthesize (Z)-â-di- and trilithium reagents
has been hampered by our inability to obtain exclusively (Z)-
â-tributyl(styryl)stannanes consistently using the ZrCl4-
catalyzed hydrostannation method.
of utility in the synthesis of extended organic conjugated
networks via Stille8 and Negishi9 coupling protocols. These
results will be reported in due course.
Acknowledgment. The authors thank the Welch Founda-
tion (N-1375) and the National Science Foundation (CHE-
0092495) for support of this work.
In conclusion, we have successfully demonstrated that
lithiated styrenes can be synthesized by transmetalation
reactions.7 The tributyl(styryl)stannane reagents can be easily
prepared by hydrostannation of ethynylbenzenes. We are
currently examining the application of these reagents in the
synthesis of new linear and branched organosilicon polymeric
systems. Furthermore, we believe these reagents should be
Supporting Information Available: Complete experi-
mental details and analytical data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL035193K
(7) It should be noted that the existence of the discrete di- and trilithium
reagents in solution was not proven (the reactions could in theory proceed
in a stepwise fashion, for example), but at the very least these reagents are
functional equivalents of the corresponding lithium compounds. We thank
a reviewer for pointing this out.
(8) Farina, V.; Krishnamurthy, V.; Scott, W. J. The Stille Reaction;
Wiley: New York, 1998.
(9) Negishi, E. I.; Liu, F. In Metal-Catalyzed Cross-Coupling Reactions;
Diederich, F., Stang, P., Eds.; Wiley: New York, 1998; p 1.
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