Free radical addition to branched allylsilanes: Stereoselective formation and elimination of β bromosilanes

Article abstract of DOI:10.1016/S0040-4039(00)00902-3

Lewis acid-promoted free radical addition to 3-silyl-1-butenes occurs in good yield to give predominantly the cis-substituted alkene. The transfer reaction occurs by atom-transfer addition of the alkyl bromide precursor, an addition that occurs by Felkin-

Full text of DOI:10.1016/S0040-4039(00)00902-3

Tetrahedron Letters 41 (2000) 5773±5777
Free radical addition to branched allylsilanes: stereoselective
formation and elimination of b bromosilanes
Ned A. Porter,a,b,* Guiru Zhanga,b and Aimee D. Reedb
aDepartment of Chemistry, Vanderbilt University, Nashville, TN 37235, USA
bDepartment of Chemistry, Duke University, Durham, NC 27708, USA
Received 25 May 2000; accepted 30 May 2000
Abstract
Lewis acid-promoted free radical addition to 3-silyl-1-butenes occurs in good yield to give predominantly
the cis-substituted alkene. The transfer reaction occurs by atom-transfer addition of the alkyl bromide
precursor, an addition that occurs by Felkin±Anh control. Stereoselective anti elimination of the inter-
mediate b-silylhalide gives the cis ole®n. # 2000 Published by Elsevier Science Ltd.
Free radical reactions have received renewed interest because of recent demonstrations that
control of the stereochemistry in these transformations is possible. Diastereoselective reactions
involving substrate or chiral auxiliary-controlled processes are now well understood1 and exam-
ples of enantioselective processes have also been reported.2 The reaction of prostereogenic radi-
cals with allylstannane radical traps has been a common transformation in which control of
stereochemistry is at issue (Eqs. (1) and (2)). Trimethylallylsilanes and tris-trimethylsilylallylsilanes
have also been investigated in radical reactions, some of which involve achiral Lewis acid-controlled
transformations.3 Lewis acids can be used to control the facial bias in reactions of bidentate
radicals and diastereoselectivity can be enhanced or even reversed by selection of an appropriate
Lewis acid.4
ꢀ1†
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)00902-3

5774
ꢀ2†
The atom transfer sequence with simple allylsilanes proceeds through an unstable b-bromosilane
intermediate that then undergoes a non-radical elimination of bromosilane to give the alkene
product. When R0=H, the resultant product is formally the same as in allyl transfer. A more
interesting case for an atom transfer reaction is when R0 is an alkyl group. Then, not only is a new
stereogenic center created at the a-center, but also the alkene stereochemistry is at issue. In this
report, we explore the diastereoselectivity of the alkene products formed in atom transfer reac-
tions with branched allylsilanes.
Several methods for branched allylsilane synthesis were explored. The allyl Grignard method
gave 2d contaminated with 2±8% of the corresponding crotyl silane, an allylic transposition iso-
mer, while the Wittig method provided only 2b;5 therefore it was the preferred method for the
preparation of this silane (Scheme 1). The higher order silylcuprate6 reacted in an SN20 fashion
with crotyl chloride to give 2a and 2c according to literature precedent.
Scheme 1.
The a bromoimide 3a reacts with branched allylsilanes7 in a Lewis acid promoted transformation
to give a mixture of alkenes, 5Z and 5E (Scheme 2).8 The reaction does not proceed without
Lewis acid and the product distribution, yield, and reaction rate are highly solvent, temperature
Scheme 2.

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and Lewis acid dependent, see Table 1. In general, the best yields and selectivities were observed
for reactions carried out in THF with Yb(OTf)3 catalyst.9 The major product formed in every
example studied was the Z alkene. The selectivities for formation of this less stable diastereomer
were as high as 94:6.
Table 1
Lewis acid-promoted atom transfer
The phenyl selenide 3b gives products with higher selectivities than the bromide at room
temperature does, but reaction of this substrate does not proceed at lower temperatures.
Selectivity appears to be somewhat higher for bulkier silanes than for the smaller TMS derivative,
but the allyl-TMS derivative, 2d, is more reactive than the other silanes with nearly quantitative
yields being obtained with short reaction times and small amounts of initiator.
Reaction of phenylselenide 3b with silane 2a provided an isolable intermediate 4, that was a 9:1
mixture of two diastereomers.10 The threo and erythro isomers of 4 can be separated successfully
by ¯ash column chromatography on deactivated silica gel.11 The major isomer was then subjected
to oxidation by hydrogen peroxide at 0ꢁC to give alkene 6 as the only isomer.
NOE di€erence experiments proved the con®guration of the alkene 6 to be E. Assuming a syn
elimination of selenol, the con®guration of the major stereoisomer of 4 is thus threo (Scheme 3).
A threo product suggests a Felkin±Anh transition state for the atom transfer reaction, consistent
Scheme 3.

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with results obtained for analogous radical transformations.12 Chromatography of threo 4 on
13
silica gives only the Z stereoisomer of 5, suggesting an anti elimination of X-SiR3 (Fig. 1).
Figure 1. Stereochemistry of atom transfer and elimination
The use of branched allylsilanes expands the scope of diastereoselective atom transfer reactions.
The transformation resembles the Peterson ole®nation but avoids the use of strongly basic con-
ditions, providing Z alkenes under relatively mild reaction conditions. The scope and limitations
of this reaction are currently under investigation.
Acknowledgements
This research is supported by grants from NIH (HL17921) and the NSF (CHE999168).
References
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1995, 117, 11029. (c) Sibi, M. P.; Ji, J.; Wu, J. H.; Gurtler, S.; Porter, N. A. J. Am. Chem. Soc. 1996, 118, 9200. (c)
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5. The allylic (crotyl) isomer is formed in less than 2% by the use of the Wittig methodology.
6. Ager, D. J.; Fleming, I.; Patel, S. K. J. Chem. Soc., Perkin Trans. 1 1981, 2520.
7. In those cases where the starting allylsilane was contaminated with the allylic isomer, the crotyl silane reacted
much more slowly than 2 did. Use of excess silane gave products containing insigni®cant amounts of the allylic
isomer of 5.
8. The con®guration of the product alkenes was determined by 1H and 13C NMR and by accurate mass
determination and independent synthesis (HRMS, calculated 183.0810; measured 183.0819). The cis- and trans-
isomers were separated by chromatography on silver ion impregnated silica gel. The cis-isomer was later eluting.
The vinylic vicinal proton±proton coupling constant for the cis-isomer was substantially less than that of the trans
and the viniylic and allylic carbons displayed an up®eld shift in the 13C spectrum from that of the trans-isomer.
9. In a typical reaction, bromide 3a (50 mg, 0.24 mmol) and Yb(OTf)3 (164 mg, 0.26 mmol) were stirred in 12 mL of
THF at rt for 1 h. The solution was cooled to ^78ꢁC and silane (1.2 mmol) was added via syringe. Dry air was
blown on the surface of the solution continuously. BEt3 (1.0 M, 0.24 mmol) was added 2 min later and the
reaction stirred for 20 min. This was repeated until no starting bromide 3a could be detected by GC. For the

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reactive silane 2d, the reaction was complete in 5 min. The solution was quenched with 3 mL water and the
mixture was then extracted with methylene chloride, and dried over MgSO4. The products were puri®ed by ¯ash
column chromatography on silica gel (85:15 hexanes:EtOAc) to give the products as a mixture of cis:trans alkenes.
10. The two isomers can be distinguished by signals in the 1H NMR due to the (-SiMe2Ph) methyl protons at ꢀ
(CDCl3)=major 0.29 (3H, s), 0.32 (3H, s) and minor 0.35 (3H, s), 0.42 (3H, s); (-CH3) protons at major 1.10 (3H,
d, J=7.5 Hz) and minor 1.13 (3H, d, J=7.4 Hz; (-CH(CH3)SiMe2Ph) methine proton at major 1.52 (1H, dq,
J1=2.4 Hz, J2=7.5 Hz) and minor 1.48 (1H, dq, J1=4.3 Hz, J2=7.4 Hz); (-CH2-CH(SePh)-) methylene protons at
major 1.72±1.91 (2H, m) and minor 1.95±2.04 (1H, m), 2.13±2.22 (1H, m); (-CH2CO-); methylene protons at major
3.00±3.15 (2H, m) and minor 2.71±2.79 (1H, m), 2.98±3.06 (1H, m); (-CHSePh) methine proton at major 3.6 (dt,
J1=11.6 Hz, J2=2.6 Hz) and minor 3.3 (dt, J1=4.3 Hz, J2=7.2 Hz).
11. The phenylselinide product 4b is converted to the alkene 5 on silica that has not been deactivated.
12. See, for example: Giese, B.; Damm, W.; Dickhaut, J.; Wetterich, F.; Sun, S.; Curran, D. P. Tetrahedron Lett. 1991,
32, 6097.
13. Jarvis, A. W. P.; Holt, P.; Tompson, J. J. Chem. Soc. (B) 1970, 746.