C O MMU N I C A T I O N S
Table 1. Trans Alkyne Reduction by
Hydrosilylation-Protodesilylationa
mechanism of this activation of the vinylsilane remains elusive.
Because new synthetic applications may derive from its understand-
ing, probing the mechanism is the subject of our continuing studies.
Presently, together with the ruthenium-catalyzed trans hydrosily-
lation, the process represents an efficient chemoselective trans
reduction of all types of alkynes to permit selective synthesis of
trans- as well as cis-olefins.
A general procedure for the synthesis of methyl (E)-2-octenoate
(Table 1, entry e) follows. To methyl 2-octynoate (105 mg, 0.68
mmol) in CH2Cl2 (1.4 mL) under an Ar atmosphere was added
triethoxysilane (0.150 mL, 0.82 mmol). [Cp*Ru(MeCN)3]PF6 (1)
(3.4 mg, 0.0068 mmol) was added after cooling to 0 °C. The flask
was immediately allowed to warm to ambient temperature, where
it was stirred for 30 min. The solution was diluted with ether (5 mL),
filtered through a plug of florisil (1-2 cm), and washed with
additional ether (15 mL). The filtrate was concentrated under
reduced pressure and taken up in THF (3.4 mL) under Ar at ambient
temperature. To the solution was added CuI (13 mg, 0.068 mmol)
followed by dropwise addition of TBAF (1.3 mL, 1.3 mmol), and
the resulting orange slurry was stirred for 20 h. The mixture was
then filtered through a plug of silica gel, concentrated, and purified
by silica gel column to afford 89 mg (83%) of the desired methyl
(E)-2-octenoate.
Acknowledgment. We acknowledge the National Science
Foundation and the National Institutes of Health for their generous
support of our programs. Z.T.B. thanks Stanford University for a
Stanford Graduate Fellowship. T.J. acknowledges receipt of a
fellowship from the German Academic Exchange Service (DAAD).
Mass spectra were provided by the Mass Spectrometry Facility,
University of San Francisco, supported by the NIH Division of
Research Resources.
a Hydrosilylations performed at 0.5 M in CH2Cl2 under Ar and were
generally complete within 1 h. Vinylsilanes were not purified and in several
cases were formed as regiomeric mixtures. Desilylation was performed at
rt for 16 h under Ar with 2.0 equiv (cat. CuI) or 3.0 equiv (1.5 equiv CuI)
TBAF unless otherwise noted. b Isolated yield. c Desilylation performed at
35 °C. d Modest yield due at least in part to volatility of product olefin.
e Me2(EtO)SiH used as silane.
Supporting Information Available: Characterization data for
vinylsilanes 3 and 8 as well as for all olefin products (PDF). This
entry e afforded 65% of the desired trans-olefin when, cuprous
iodide and TBAF in THF were added directly to the hydrosilylation
reaction mixture.
References
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It has been noted that 1,1-disubstituted vinylsilanes are difficult
protodesilylation substrates, presumably because olefin protonation
requires formation of a primary carbocation.10 We became interested
to see if our cuprous iodide-mediated conditions would succeed
for this substrate class as well and were gratified to find that under
the same conditions utilized above, vinylsilane 8 was smoothly
cleaved to the terminal olefin 9 at ambient temperature (eq 2).
The cuprous iodide/TBAF system outlined above permits facile
protodesilylation of vinylsilanes under conditions much milder than
previously reported. The reaction proceeds with a catalytic amount
of CuI. For sensitive substrates, the use of stoichiometric CuI
moderates the reactivity of TBAF with respect to other nucleophilic
or basic processes and permits more selective reactivity. To the
extent that the hydrosilylation is regioselective, this method also
allows selective introduction of hydrogen isotopes. The intriguing
(8) Taguchi, H.; Ghoroku, K.; Tadaki, M.; Tsubouchi, A.; Takeda, T. Org.
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to a silane in the context of cross-coupling, see: Ito, H.; Sensui, H.;
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(10) Anderson, J. C.; Flaherty, A. J. Chem. Soc., Perkin Trans. 1 2000, 3025.
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