Boron trifluoride allylstannane reactions with aldehydes

The allylation of aldehydes with allylstannanes is effected with Lewis acids such as boron trifluoride etherate or titanium(IV) chloride (eq (104)) [99], in which diastereo-selectivity is dependent on the catalyst employed the most striking feature is that. n77-homoallyl alcohols are stereoselectively produced using boron trifluoride etherate. Although this allylation occurs without a catalyst, high reaction temperature must be used. 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

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