The first general method for Z-selective olefination of acylsilanes via ynolate anions providing multisubstituted alkenes

Article abstract of DOI:10.1021/ja026275r

We have developed the first general method for a stereoselective olefination of acylsilanes via ynolate anions to produce (Z)-ss-silyl-α,β-unsaturated ester, which leads to tri- and tetrasubstituted alkenes. Copyright

Full text of DOI:10.1021/ja026275r

Published on Web 05/22/2002
The First General Method for Z-Selective Olefination of Acylsilanes via
Ynolate Anions Providing Multisubstituted Alkenes
Mitsuru Shindo,*,†,‡ Kenji Matsumoto,‡ Seiji Mori,§ and Kozo Shishido‡
Institute for Medicinal Resources, UniVersity of Tokushima, Sho-machi 1, Tokushima 770-8505, Japan, PRESTO,
Japan Science and Technology Corporation, and Department of EnVironmental Sciences, Ibaraki UniVersity, Mito,
Ibaraki 310-8512, Japan
Received March 21, 2002
Scheme 1
Silylalkenes (vinylsilanes) have frequently been used as a
powerful synthetic tool in synthetic organic chemistry,1 since their
silyl groups can potentially be converted into other substituents2
while also affording unique electronic effects such as R-anion
stabilization and â-cation stabilization.3 Although olefination of
acylsilanes4 is expected to become a useful method for the
preparation of silylalkenes, there have been few reports on this
process because conventional reagents, such as phosphorus ylides,
have low reactivity toward acylsilanes and afford poor selectivity.5
Stereoselective synthesis of tri- and tetrasubstituted alkenes is indeed
Scheme 2
a continuing issue in synthetic organic chemistry,6 since these types
of alkenes not only serve as synthetic intermediates but are also
important units in medicinal chemistry and material science.7
Herein, we describe the first general and efficient method for highly
Z-selective olefination of acylsilanes via ynolates leading to â-silyl-
R,â-unsaturated esters in high yields.
Since developing a novel synthetic method of ynolate anions,8
we have continued our investigation of ynolate chemistry.9 In the
course of this study, we found that ynolate anions react with ketones
Table 1. Olefination of Acylsilanes via Ynolate Anionsa
at room temperature to afford R,â-unsaturated carboxylic acids with
good to moderate stereoselectivities (Scheme 1).10
We therefore screened a variety of carbonyl compounds as
substrates for the olefination via ynolates and found that acylsilanes
show remarkably high efficiency in the olefination. To a THF
solution of the ynolate anion (2, R3 ) Me), generated from the
R,R-dibromoester with t-BuLi or lithium naphthalenide, was added
acylsilane (1, R1 ) PhCH2, R2 ) Me) at ambient temperature. After
1.5 h, the starting ketone disappeared, and iodomethane was added
for esterification of the expected carboxylate. Finally, we isolated
the desired â-silyl-R,â-unsaturated ester (3) in quantitative yield
with excellent Z-selectivity (Scheme 2).
To establish the generality of this olefination, we examined
reactions using a variety of acylsilanes and ynolates. As shown in
Table 1, the desired â-silyl-R,â-unsaturated esters were obtained
in high yields with perfect Z-selectivities.11 tert-Butyldimethyl-
acylsilanes also afforded the desired esters (entries 9, 13, 14, and
a The ynolates were prepared by lithium-halogen exchange with tert-
butyllithium, otherwise noted. b The ynolate was prepared by the method
of reductive lithiation with lithium metal and catalytic naphthalene (see
Supporting Information).
15). Functionalized alkylacylsilanes and benzoylsilane also gave
the desired esters with no geometric isomers (entries 11-16). A
TMS-substituted ynolate anion produced R,â-disilyl-R,â-unsaturated
ester, which could be the synthetic equivalents of di-, tri-, and
tetrasubstituted alkenes (entry 10). Although Sonderquist has
reported high selectivity in Wittig reaction5b,c and in Peterson
olefination5d of acylsilanes giving disubstituted silylalkenes, there
have been no reports on the highly stereoselective synthesis of
trisubstituted silylalkenes. An attempted Horner-Emmons reaction
with acylsilane (1) resulted in a complex mixture. Peterson
olefination of acetyl trimethylsilane was reported to give a 3:1
mixture of trisubstituted silylalkenes.5d Therefore ynolate anions
are superior to the conventional reagents for olefination of acyl-
silanes to afford â-silyl-R,â-unsaturated esters.
* Corresponding author. E-mail: shindo@ph2.tokushima-u.ac.jp.
‡ University of Tokushima.
† PRESTO, Japan Science and Technology Corporation.
§ Ibaraki University.
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J. AM. CHEM. SOC. 2002, 124, 6840-6841
10.1021/ja026275r CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
of Multi-Element Cyclic Molecules” from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and a research
grant from the Faculty of Pharmaceutical Sciences, the University
of Tokushima. Generous allotment of computational time from the
Institute for Molecular Science, Okazaki, Japan, is deeply acknowl-
edged.
Figure 1. An orbital interaction of inward TS in the ring opening of a
â-lactone enolate.
Supporting Information Available: Experimental and computa-
tional procedures and spectral data of selected compounds (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
Scheme 3a
References
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To probe the reaction path, we attempted to trap the intermediate
â-lactone enolate. The reaction of the acylsilane (1, R1 ) PhCH2,
R2 ) Me) with the ynolate (2, R3 ) Me) at -78 °C produced
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â-lactone (4) in good yield (IR, 1811 cm-1 12
thus revealing that
)
the â-lactone enolates are the intermediates in this olefination.
The stereochemistry would be controlled in the electrocyclic ring-
opening step10f of the â-lactone enolate (Figure 1). When tert-butyl
methyl ketone was employed for comparison sake, the Z:E
selectivity was only 4:1.13 The marked difference in the selectivities
could be explained by the stereoelectronic effect of silicon. With
the aid of B3LYP/6-31G* calculations (R ) H in Figure 1), the
transition state in the ring opening with inward rotation of the silanyl
group was found to be more stable in Gibbs energy than the outward
one by 17.5 kJ/mol. A localized molecular orbital analysis of the
TS of the inward ring opening can account for the torquoselectivity
as the interaction between a breaking C-O σ orbital and Si vacant
orbitals (i.e. 3d or 4sp orbitals, Figure 1).14
The silyl group of the silylalkenes can be transformed to other
functionalities (Scheme 3). After reduction of the ester, the resulting
silylalkene (5) was desilylated to give the E-alkene (6). The
silylalkene (5) was also coupled with allyl bromide2c to provide
the skipped diene (7). The conversion of silyl group into the iodide2d
was employed to afford the corresponding Z-iodoalkene (8), which
underwent a Heck reaction with ethyl acrylate and a Stille coupling
with vinyl tin to provide the dienes 9 and 10, respectively. Thus,
the efficient stereoselective synthesis of tetrasubstituted alkenes was
achieved.
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(13) Shindo, M.; Sato, Y.; Yoshikawa, T.; Shishido. K. Unpublished results.
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In conclusion, we have developed a stereoselective olefination
of acylsilanes via ynolate anions to produce (Z)-â-silyl-R,â-
unsaturated esters, which leads to tri- and tetrasubstituted alkenes.
This process is the first general method for producing tetrasubsti-
tuted alkenes by olefination of acylsilanes. Finally, it demonstrates
the synthetic utility of the multifunctional carbanions of ynolate
anions.
Acknowledgment. This work was supported in part by a Grant-
in-Aid for Scientific Research on Priority Areas (A) “Exploitation
JA026275R
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