Stereoselective synthesis of vinylsilanes by a gold(I)-catalyzed acetylenic sila-cope rearrangement

Article abstract of DOI:10.1021/ja0636800

Cationic tri-tert-butylphosphinegold(I) serves as a catalyst in the sila-Cope rearrangement of acetylenic allylsilanes. When phenol is employed as a nucleophile, the reaction allows for the stereoselective synthesis of vinylsilanes. Alternatively, use of methanol as a nucleophile leads to cyclic vinylsilanes, which can be viewed as latent vinylsilanes that are revealed on treatment with a mild Lewis acid. Thus, both of these reagents serve as useful reagents for stereoselective synthesis of trisubstituted olefins through transition-metal-catalyzed cross-coupling reactions. Copyright

Full text of DOI:10.1021/ja0636800

Published on Web 08/15/2006
Stereoselective Synthesis of Vinylsilanes by a Gold(I)-Catalyzed Acetylenic
Sila-Cope Rearrangement
Yoshikazu Horino, Michael R. Luzung, and F. Dean Toste*
Department of Chemistry, UniVersity of California, Berkeley, California 94720
Received May 25, 2006; E-mail: fdtoste@uclink.berkeley.edu
Table 1. Catalyst Optimization
Recent advances have dramatically increased the utility of
organosilanes and silanolates as reagents for metal-catalyzed cross-
coupling reactions.1 The application of these reactions to stereo-
selective olefin synthesis is contingent on efficient and selective
methods for the construction of vinylsilanes. Metal-mediated
stereoselective addition to alkynes is commonly employed for this
purpose.2 For example, trans-allylsilylation3 of alkynes and cis-
allylmetalation of silylacetylenes4 allow for the stereoselective
construction of 1,4-dienylsilanes in which the allyl group is trans
to silicon. On the other hand, application of allylmetalation reactions
to the stereoselective synthesis of olefins substituted with the allyl
group cis to silicon is rare.5 The gold(I)-catalyzed rearrangement
of propargyl vinyl ethers, which is hypothesized to proceed through
a cyclization-induced rearrangement, was recently reported.6 We
envisioned that gold(I)-induced 6-endo-dig cyclization of acetylenic
allylsilanes7 would initiate a related rearrangement and provide a
method for the stereoselective synthesis of cis-1,4-dienylsilanes.8
To this end, treatment of dimethylsilane 1a with catalytic amounts
of cationic tri-tert-butylphosphinegold(I), in the presence of 3 equiv
of methanol, furnished 1,4-diene 2a in 78% yield after 1 h at room
temperature (eq 1).9 While this reaction provided a general route
to 2,2-disubstituted-1,4-dienes, it failed to afford the desired
vinylsilane. We surmised that 2 was being formed through the
desired rearrangement followed by rapid protodesilation of the
resulting vinylsilane.
entry
AgX
time
R
4
5
2a
1
2
3
4
5
6
7
8
AgSbF6
20 min
30 min
2 h
50 min
1 h
12 h
12 h
Me (a)
52%
35%
0%
78%
91%
84%
44%
trace
48%
45%
0%
12%
0%
trace
0%
77%
trace
3%
78%
AgBF4
Bn (b)
i-Pr (c)
t-Bu (d)
Ph (e)
12 h
Table 2. Scope of Gold(I)-Catalyzed Sila-Cope Rearrangementa
In accord with this hypothesis, gold(I)-catalyzed rearrangement
of more robust diphenylsilane 3 furnished a mixture of silacycle
4a and vinylsilane 5a which slowly converted into 1,4-diene 2a
(Table 1, entries 1-3).10 Changing the silver additive from AgSbF6
to AgBF4 resulted in improved selectivity for the formation of
silacycle 4a without deterioration of reaction efficiency (entry 4).
While changing the sterics of the alcohol did not divert the reaction
pathway from silacycle formation (entries 5-7), the use of phenol
as a nucleophile completely reversed the selectivity in favor of
vinylsilane 5e (entry 8).
With these conditions in hand, the substrate scope of the gold-
(I)-catalyzed rearrangements was probed (Table 2). We were pleased
to find that substitution at the acetylenic position (R1) of allylsilane
6 was well tolerated and that the selectivity for cyclic and acyclic
products is generally retained. For example, the gold(I)-catalyzed
reaction of propargyl alcohol 6f produced silacycle 7k in 76% yield
in the presence of 3 equiv of methanol (entry 11). When the
nucleophile was changed to phenol, the reaction course was
completely diverted to generate vinylsilane 8l in 68% yield (entry
12). Notably, rearrangement of 1,5-enyne 6g resulted in exclusive
a Reaction conditions: 0.2 M 6 in CH2Cl2, rt. b Isolated yield after
chromatography. c 7-8:1 mixture of olefin isomers. d 1:1 mixture of
diastereomers. e 2:1 mixture of crotyl olefin isomers. f Run at 50 °C.
formation of desired adducts without significant competing enyne
cycloisomerization (entries 13 and 14).11,15c
Substitution on the allyl moiety is also tolerated but has
consequences on the product distribution. For example, C2-
substituted allylsilane 6h underwent the gold(I)-catalyzed cyclization
onto an alkynoate to afford silacycle 7o in 75% yield (entry 15)
and vinylsilane 8p in 46% yield (entry 16) in the presence of
methanol and phenol, respectively. It is noteworthy that, in contrast
to typical â-addition of nucleophiles to alkynoates,8c in this reaction,
allylsilane regioselectively adds to the R-position. On the other hand,
substituted allylsilanes 6i and 6j underwent addition of methanol
to provide mixtures of cyclic and acyclic products (entries 17 and
19); however, the gold(I)-catalyzed reaction of phenol with 6i and
6j afforded vinylsilanes 8r12 and 8t, respectively (entries 18 and
20). Furthermore, gold(I)-catalyzed rearrangement of an allylsilane
(6k) that is substituted adjacent to the silicon produced vinylsilane
9
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J. AM. CHEM. SOC. 2006, 128, 11364-11365
10.1021/ja0636800 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism of Au(I)-Catalyzed
Rearrangement
groups. Furthermore, depending on the choice of nucleophile, either
cyclic or acyclic vinylsilanes were produced. Both of these reagents
can be employed for stereoselective synthesis of trisubstituted18
olefins through transition-metal-catalyzed cross-coupling reactions.
Acknowledgment. We gratefully acknowledge the University of
California, Berkeley, NIHGMS (R01 GM073932-01), Merck Research
Laboratories, Bristol-Myers Squibb, Amgen Inc., DuPont, GlaxoSmith-
Kline, Eli Lilly & Co., Pfizer, AstraZeneca, Abbott, and Roche for
financial support. Y.H. thanks JSPS for a postdoctoral fellowship.
Supporting Information Available: Experimental procedures and
compound characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
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product 8u even when methanol was employed as a nucleophile
(entry 21).
A proposed mechanism involving SE2′ addition of the allylsilane
onto a gold(I)-complexed alkyne to generate a gold-stabilized cation
is outlined in Scheme 1.13 While â-silyl fragmentation is generally
faster than trapping of the cation,14 stabilization of 9 by back-
bonding from gold(I) (i.e., resonance structure 10)15 allows for
methanol addition to afford cyclic silane 11. On the other hand,
when the relative rate of nucleophilic trapping is decreased, selective
trapping occurs at silyl cation 12 to afford vinylsilane 13.
Presumably, this is the case when the less nucleophilic phenol is
used and when addition to the cation is sterically encumbered (Table
2, entries 17 and 19). Furthermore, an increase in the relative rate
of â-silyl fragmentation as a result of the steric clash of the methyl
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(9) Under identical conditions, (Ph3P)AuCl required 10 h to afford 72% yield
of 2a. On the other hand, no reaction was observed on treatment of 1
with AgSbF6, 10% AgSbF6/4% PPh3, PtCl2, or (PhCN)2PdCl2.
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rearrangement serve as useful reagents in a number of transforma-
tions. For example, palladium-catalyzed cross-coupling1 of vinyl-
silane 8h with ethyl 4-iodobenzoate produced trisubstituted olefin
14 in 60% yield (eq 2). Moreover, R-allylated aldehyde 15 was
prepared through a Tamao oxidation16 of 8h. Additionally, the
silacycles can be viewed as latent vinylsilanes that can be revealed
on treatment with a mild Lewis acid. This allowed for chemose-
lective cross-coupling reactions of arylbromide 16 to be performed,
while the vinyl silane remained protected as the silacycle (eq 3).
Reaction of 17 with 5 mol % of Sc(OTf)3 generated vinylsilane
18, which was subjected to a second cross-coupling reaction to
afford trisubstituted olefin 19.
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ate (see Supporting Information).
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In conclusion, a cationic gold(I) complex has been developed
as a catalyst for the first transition-metal-catalyzed acetylenic sila-
Cope rearrangement.17 The reaction allows for the stereoselective
synthesis of vinylsilanes substituted with a wide range of functional
JA0636800
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