Metal-catalyzed rearrangement of homoallylic ethers to silylmethyl allylic silanes in the presence of a di-tert-butylsilylene source

Article abstract of DOI:10.1021/ol052456x

(Chemical Equation Presented) In examining the scope of the di-rert-butylsilylene transfer to gem-disubstituted alkenes to form silacyclopropanes, we discovered an unprecedented reaction of homoallylic ethers. When silylene transfer was performed at room

Full text of DOI:10.1021/ol052456x

ORGANIC
LETTERS
2005
Vol. 7, No. 24
5531-5533
Metal-Catalyzed Rearrangement of
Homoallylic Ethers to Silylmethyl Allylic
Silanes in the Presence of a
Di-tert-butylsilylene Source
Pamela A. Cleary and K. A. Woerpel*
Department of Chemistry, UniVersity of California, IrVine, California 92697-2025
kwoerpel@uci.edu
Received October 10, 2005
ABSTRACT
In examining the scope of the di-tert-butylsilylene transfer to gem-disubstituted alkenes to form silacyclopropanes, we discovered an
unprecedented reaction of homoallylic ethers. When silylene transfer was performed at room temperature or above, two di-tert-butylsilylene
units were incorporated into the molecule, and complete rearrangement of the carbon backbone occurred. This report describes the scope of
this unique reaction as well as the mechanistic studies conducted that led to a proposed mechanism.
Studies of silylenes1-3 and metal-silylene complexes4 have
emerged as important areas of organosilicon chemistry.
Among the reactions of silylenes, some have exhibited
unexpected reactivity, including rearrangements.5 Our labora-
tory has been interested in applying the reactions of silylenes
to organic synthesis. We have shown that metal-catalyzed
di-tert-butylsilylene transfer reactions to alkenes and alkynes
are efficient methods for the synthesis of silacyclopropanes
and silacyclopropenes.6-8 For example, silacyclopropanes can
be formed from homoallylic esters at low temperature in
good yield (Scheme 1).7
Scheme 1. Di-tert-butylsilylene Transfer to Alkenes
(1) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617-618,
209-223.
(2) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165-4183.
(3) Kira, M. J. Organomet. Chem. 2004, 689, 4475-4488.
(4) Ogino, H. Chem. Rec. 2002, 2, 291-306.
In examining the scope of the transformation shown in
Scheme 1, we discovered an unprecedented reaction of
homoallylic ethers. When silylene transfer was performed
at room temperature or above, two di-tert-butylsilylene units
were incorporated into the molecule and complete rearrange-
ment of the carbon backbone occurred (Scheme 2).9 While
allylic ethers are known to undergo sigmatropic rearrange-
(5) For recent examples, see: (a) Lee, M. E.; Cho, H. M.; Kim, C. H.;
Ando, W. Organometallics 2001, 20, 1472-1475. (b) Belzner, J.; Ihmels,
H.; Pauletto, L.; Noltemeyer, M. J. Org. Chem. 1996, 61, 3315-3319. (c)
Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Organometallics 1997, 16,
4861-4864.
(6) CÄ irakovic´, J.; Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2002,
124, 9370-9371.
(7) CÄ irakovic´, J.; Driver, T. G.; Woerpel, K. A. J. Org. Chem. 2004, 69,
4007-4012.
(8) Clark, T. B.; Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9522-
9523.
(9) The reaction of 4 with only 1 equiv of silacylopropane 2 yielded
50% of 6 and recovered starting material.
10.1021/ol052456x CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/28/2005

constructed from homoallylic ethers with substitution at the
homoallylic position. The rearrangement appears to be
specific to homoallylic ethers and esters: di-tert-butylsilylene
transfer to bishomoallylic ethers provided only the sila-
cyclopropane (Scheme 4).
Scheme 2. Rearrangement of Homoallylic Ethers
Scheme 4. Silacyclopropanation of Bishomoallylic Ethers
ments in the presence of silylenes,10-13 rearrangements of
homoallylic ethers have not been reported. We felt that this
transformation merited further study because silylmethyl
allylic silanes have been used in the synthesis of natural
products,14,15 but their syntheses have proven to be difficult.16
The rearrangement of alcohol derivatives is general for a
variety of substrates. In addition to gem-disubstituted homo-
allylic ethers, monosubstituted homoallylic ethers rearranged,
although higher temperatures were required (50 °C, Scheme
3). Homoallylic pivaloate esters, which form silacyclopro-
Because this reactivity of silylenes has not been observed,
experiments were designed to probe its mechanism. A
deuterium labeling/crossover experiment established the
connectivity of the rearrangement and demonstrated that it
is intramolecular. Subjecting a mixture of d2-homoallylic
benzyl ether 22 and homoallylic decyl ether 9 to the reaction
conditions resulted in no crossover products (Scheme 5).18,19
Scheme 3. Generality of the Rearrangement
Scheme 5. Deuterium Labeling/Crossover Experiment
Examination of the 1H and 2H NMR spectra of allylic silane
24 revealed the fate of the deuterium atoms as well as the
reorganization of the carbon backbone. One of the deuterium
atoms migrated to the terminal silicon atom, and the alkene
walked two atoms down the chain. To confirm that an
intermolecular reaction does not take place, 2,2-dimethyl-
silacylcopropane 23 was subjected to AgO2CCF3 and diethyl
ether. No reaction was observed, even at elevated temper-
atures (Scheme 6).
Scheme 6. Demonstration of Intramolecularity of
panes at low temperature (Scheme 1), rearranged at room
temperature. Substitution at the allylic position was also
tolerated.17 â-Substituted silylmethyl allylic silanes can be
Rearrangement
(10) Tzeng, D.; Weber, W. P. J. Org. Chem. 1981, 46, 693-696.
(11) Tortorelli, V. J.; Jones, M., Jr. J. Chem. Soc., Chem. Commun. 1980,
785-786.
(12) Ishikawa, M.; Katayama, S.; Kumada, M. J. Organomet. Chem.
1983, 248, 251-260.
Silacyclopropanes appear to be intermediates along the
reaction pathway. When isolated silacyclopropane 25 (pre-
pared by thermal silylene transfer20) was subjected to AgO2-
(13) Wang, D.; Chan, T.-H. J. Chem. Soc., Chem. Commun. 1984, 1273-
1274.
(14) For a review describing the reactions of allylic silanes, see: Chabaud,
L.; James, P.; Landais, Y. Eur. J. Org. Chem. 2004, 3173-3199.
(15) Peng, Z.-H.; Woerpel, K. A. Org. Lett. 2001, 3, 675-678.
(16) Smitrovich, J. H.; Woerpel, K. A. Synthesis 2002, 18, 2778-2785.
(17) X-ray crystallography confirmed the structure of 15. See the
Supporting Information for details.
5532
Org. Lett., Vol. 7, No. 24, 2005

CCF3 and silacyclopropane 2, rearrangement provided sil-
ylmethyl allylic silane 16 (Scheme 7).21 Control experiments
becomes nucleophilic. Attack by electrophilic silver silyle-
noid complex 27 affords silyl anion 29.26,27 Intramolecular
deprotonation and elimination then provides the silylmethyl
allylic silane.
With this new, simple synthesis of silylmethyl allylic
silanes, we felt it was important to show that these
compounds would react as allylic silanes. Treatment of allylic
silane 10 with N-chlorosulfonyl isocyanate provided lactone
31 in good yield and diastereoselectivity (Scheme 9).15
Scheme 7. Silacyclopropanes as Intermediates along the
Reaction Pathway
Scheme 9. Synthetic Utility of Rearrangement
Because the C-Si bonds can be oxidized to form C-O
bonds,28,29 these silylmethyl allylic silanes should find
application in organic synthesis.
In conclusion, homoallylic ethers undergo rearrangement
when treated with a metal-salt catalyst and a di-tert-
butylsilylene source to provide silylmethyl allylic silanes.
Because the allylic silanes participate in annulation reactions,
they should find utility in organic synthesis.
demonstrate that silacyclopropane 25 cannot liberate di-tert-
butylsilylene to convert another molecule of 25 to allylic
silane 16. No reaction was observed when silacyclopropane
25 was treated with AgO2CCF3 in the absence of silacyclo-
propane 2. In addition, silacyclopropane 25 does not transfer
di-tert-butylsilylene to an exogenous alkene under these
conditions.
Acknowledgment. This research was supported by the
National Institute of General Medical Sciences of the
National Institutes of Health (GM-54909). K.A.W. thanks
Amgen, Johnson & Johnson, and Merck Research Labora-
tories for awards to support research. We thank Dr. Phil
Dennison for assistance with NMR spectrometry, Dr. Joseph
Ziller for X-ray crystallography, and Dr. John Greaves for
mass spectrometry.
Scheme 8. Proposed Mechanism
Supporting Information Available: Experimental pro-
cedures; spectroscopic, analytical, and X-ray data for the
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL052456X
The mechanism shown in Scheme 8 is consistent with our
mechanistic experiments (Schemes 5-7). Upon formation
of silacyclopropane 26,22 the ether oxygen atom can com-
plex23,24 to the Lewis acidic silicon atom.25 The lengthened
apical Si-C bond25 of the resulting pentacoordinate siliconate
(23) Intramolecular coordination of heteroatoms to silicon atoms is
known: Belzner, J.; Dehnert, U.; Ihmels, H.; Hu¨bner, M.; Mu¨ller, P.; Uso´n,
I. Chem. Eur. J. 1998, 4, 852-863.
(24) tert-Butyldimethylsilyl ethers, whose basicities are comparable to
that of analogous dialkyl ethers, should be capable of complexation: Blake,
J. F.; Jorgensen, W. L. J. Org. Chem. 1991, 56, 6052-6059.
(25) Damrauer, R.; Crowell, A. J.; Craig, C. F. J. Am. Chem. Soc. 2003,
125, 10759-10766.
(18) 1H NMR spectroscopic experiments show that 9 and 22 rearrange
at comparable rates, indicating no significant kinetic isotope effect for the
rearrangement.
(19) We have observed that the reactivity of silacyclopropane 23 is
comparable to that of silacyclopropane 2.
(26) DePuy, C. H.; Damrauer, R.; Bowie, J. H.; Sheldon, J. C. Acc. Chem.
Res. 1987, 20, 127-133.
(27) We have observed that rearrangement of some substrates occurs in
the absence of a metal catalyst at temperatures above 100 °C.
(28) For reviews on the oxidation of silyl groups, see: (a) Tamao, K.
AdVances in Silicon Chemistry; JAI: Greenwich, CT, 1996; Vol. 3, pp 1-62.
(b) Jones, G. R.; Landais, Y. Tetrahedron 1996, 52, 7599-7662. (c)
Fleming, I. Chemtracts-Org. Chem. 1996, 9, 1-64.
(20) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2003, 125, 10659-
10663.
(21) The silacyclopropane recovered from this reaction was a single
1
diastereomer as determined by H NMR spectroscopy.
(22) Driver, T. G.; Woerpel, K. A. J. Am. Chem. Soc. 2004, 126, 9993-
10002.
(29) Smitrovich, J. H.; Woerpel, K. A. J. Org. Chem. 1996, 61, 6044-
6046.
Org. Lett., Vol. 7, No. 24, 2005
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