anticipated to form a mixture of the regioisomers, which
limits a diversity of regio- and stereospecific silylborylated
olefins.7d
Scheme 1. Regio- and Stereoselective Silylborylation of 2
In this communication, we report a novel synthetic
protocol for the preparation of tetrasubstituted olefins,
especially for tetraarylated analogues with four different
aryl groups, using the palladium-catalyzed silylboryla-
tion of alkynylboronates,11 to yield 1-phenyl-1-silyl-2,2-
diborylated olefins with perfect regio- and stereoselectivities,
followed by chemoselective SuzukiꢀMiyaura coupling12
to deliver (Z)-1-silyl-2-borylstilbene motifs. Because the
reagents are readily available and the operations are
simple, this synthetic strategy proves more selective and
tolerant than those previously reported.
According to a report in 1999,7d in the presence of the
in situ generated palladium(0)ꢀisonitrile complex, the
reaction of the silylborane 1 with the alkynylboronate 2
took place in refluxing toluene (Scheme 1; Bpin is pina-
colatoboryl). A catalytic amount of Pd(OAc)2 (2 mol %)
and 1,1,3,3-tetramethylbutyl isonitrile (tOctNC) (30 mol %)
efficiently gave rise to the silylborylation product 3 in 60%
yield as a single isomer. However, the reaction did not
proceed smoothly at lower temperatures with decreased
yields being obtained (17% yield at 50 °C). Pd(OAc)2 in
conjunction with cyclohexyl isonitrile (CyNC) likewise gave
a poor yield (19%). In addition, phosphine and phosphite
ligands could not generate an efficient Pd catalyst for the
reaction.
The stereochemistry of the adduct 3 was confirmed
by X-ray crystallographic analysis.13,14 Thus, silylbor-
ylation was found to take place regio- and stereoselec-
tively with the silyl moiety geminal to the phenyl group.
Although the similar 1,1-diboryl-1-alkenes have been
prepared by gem-diborylation of 1,1-dibromo-1-al-
n
kenes with BuLi/bis(pinacolato)diboron15 or ketone
addition of tris(pinacolato)borylmethyllithium,16 to
the best of our knowledge, there are no known examples
of the silylated 1,1-diboryl-1-alkenes, and they would
be difficult to synthesize under basic conditions due to
desilylation.
A possible catalytic cycle forming 3 is proposed to
explain the observed regioselectivity. This proposed cycle
is presented in Scheme 2. We propose that silylborane 1
oxidatively adds to the Pd(0) catalyst to generate the Pd(II)
species A. Intermediate A then undergoes migratory inser-
tion, wherein alkynylboronate 2 inserts into the PdꢀB
bond (borylpalladation) to form B. Although the boryl-
palladation mechanism has been proposed as the result of
theoretical studies,8c,17 another possibility, silylpallada-
tion, cannot be neglected. This selectivity is opposite to
that observed for the analogous process with organozirco-
nium species.18 Finally, the adduct 3 is produced by
reductive elimination, regenerating the Pd catalyst.
Since functional materials, natural products, and bio-
active pharmaceuticals have all been synthesized with
1,1-diborylated olefins,19,20 to further test the utility of this
building block, compound 3 was successively subjected to
SuzukiꢀMiyaura coupling with an equimolar amount of
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(14) CCDC-643360 (3), -643361 (5a), and -643362 (8) contain the
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