Allylsilanes
Allylsilanes
Allylsilanes
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Scheme 2 Diastereoselective Reactions of E- and Z-But-2-enylsilanes [36]<br />
(E)-4<br />
SiMe 3<br />
SiMe3 (Z)-4<br />
+ Bu t CHO<br />
TiCl4<br />
(syn-5/anti-5) >99:1<br />
OH<br />
t Bu<br />
+<br />
TiCl4 + ButCHO syn-5<br />
(syn-5/anti-5) 65:35<br />
Titanium(IV) chloride has also been used widelyas the Lewis acid of choice to promote the<br />
reactions of allylsilanes with other electrophiles including epoxides, acetals, iminium<br />
ions, and acid chlorides.<br />
An allylsilane with silicon-centered chiralityshows a low level of chiralitytransfer in<br />
its Lewis acid mediated reactions with aldehydes and acetals. [37] However, when a stereogenic<br />
center is in one of the substituents on the silyl group, good asymmetric induction<br />
has been observed, for example, the formation of 6 in 47% ee (Scheme 3). [38]<br />
Scheme 3 Asymmetric Induction in a Lewis Acid Mediated Allylation Reaction [38]<br />
MeO 2C<br />
N<br />
SiMe 2<br />
+ PrCHO<br />
TiCl 4<br />
OH<br />
6 47% ee<br />
Besides titanium(IV) chloride, other catalysts such as tin(IV) chloride, the boron trifluoride±diethyl<br />
ether complex, ethylaluminum dichloride, trimethylsilyl trifluoromethanesulfonate,<br />
iron(III) chloride, bismuth(III) bromide, [39] Amberlyst 15, and so forth have also<br />
found use in the reactions of allylsilanes.<br />
The allylation reaction of aldehydes with allylsilanes generally requires stoichiometric<br />
or greater amounts of Lewis acid because of tight binding of the product homoallyl alcohol<br />
to the Lewis acid catalyst, as well as poor efficiency of silicon transfer to oxygen. A<br />
catalytic version of this reaction is possible if 2±10 mol% of scandium(III) triflate is used;<br />
thus, if a stoichiometric amount of acetic anhydride is used in the allylation reaction,<br />
homoallylic acetates can be prepared in an essentially one-pot reaction, for example, the<br />
formation of 7 (Scheme 4). [40]<br />
Scheme 4 Catalytic Allylation of Aldehydes with a Scandium(III) Triflate Catalyst [40]<br />
1<br />
SiMe3<br />
+ PhCHO<br />
FOR PERSONAL USE ONLY<br />
838 Science of Synthesis 4.4 Silicon Compounds<br />
Sc(OTf) 3 (cat.)<br />
Ac2O (2 equiv)<br />
74%<br />
Allylation reactions of allylsilanes can also be carried out under nucleophilic conditions<br />
(TBAF, KF, or NaOMe). Fluoride ion induced reaction of carbonyl compounds with allylsilanes<br />
generallytakes place readilyat room temperature. With á,â-unsaturated esters,<br />
nitriles, and amides as substrates, reactions of allyltrimethylsilane in the presence of fluoride<br />
ion occur exclusively in a 1,4-fashion, while Lewis acid catalyzed processes yield no<br />
addition products at all. [20]<br />
Allyltris(trimethylsilyl)silanes react by a free-radical chain mechanism with organic<br />
halides, therebyeffecting a net replacement of the halide bya propenyl group if the polar<br />
Ph<br />
OAc<br />
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG<br />
7<br />
Bu t<br />
OH<br />
anti-5