Allylsilanes reactions

These results can be explained in terms of an interplay of stereoelectronic and steric factors. Steric factors are evidently more important in the reaction of bulky cuprate clusters than in the Sakurai reaction. Thus stereoelectronic factors predominate in the allylsilane reaction. Similar effects are observed in conjugate additions to substituted cyclohepten-ones. 

The second proposal by Speckamp and co-woikers used an allylsilane as an excellent nucleophile for an A -acyliminium cyclization reaction (Esch et al. 1987) (Scheme 7.13). The 3-effect of the silicon atom is a powerM determinant of the regiochemistry of allylsilane reactions with electrophiles, so the new carbon-carbon bond is formed at the vinyl caibon distal to silicon, that is, at the y-position (Hiemstra et al. 1984,1985). 

It is perhaps more simple to note that both the vinylsilane reaction 6.504 and the allylsilane reaction 6.507 are showing the normal pattern of stereochemistry for their reactions with electrophiles a preference for retention of configuration in the double bond geometry for a vinylsilane, and anti for an allylsilane, where anti refers to the side of the double bond to which the new bond is formed relative to the side on which the silyl group resides. In the product 6.509, the new C—C bond has formed to the lower surface of the left-hand double bond, while the silyl group was conjugated to the top surface in the allylsilane 6.508. 

A different set of mechanistic possibilities comes into play when nucleophilic catalysis is used in place of the electrophilic catalysis induced by Lewis acids. The most common form of this reaction is fluoride ion catalysis of allylsilane reactions, where the carbon nucleophile is probably a hypervalent allylsilane (12), with the fluoride ion coordinated to the silicon, making the allylsilane unit nucleophilic enough to react with aldehydes without acid catalysis. ... 

Dienylmethylstannanes react with aldehydes in the presence of Lewis acids equally at the y and the e position,in contrast to the allylsilane reactions in Schemes 9 and 14. 

Honda and his co-workers synthesised methyl (+)-nonactate 179, setting up the C-6 to C-8 relationship by a chelation controlled allylsilane reaction on the aldehyde 169, and the C-3 centre by hydrogenation of the dehydro intermediate 176 carrying two methoxycarbonyl groups (37) (Scheme 24). The thiolactone 175 and dimethyl diazomalonate gave the dehydro intermediate 176 in the presence of dirhodium tetraacetate, by way of a sulfur-ylid rearrangement developed by these... 

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