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.
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Org. Lett., Vol. 5, No. 19, 2003