The asymmetric allylation and crotylation of aromatic aldehydes with allylic trichlorosilanes can be promoted by chiral phosphoramides in high yield and modest enantiomeric excess. The reaction likely proceeds via a hexacoordinate siliconate species.The asymmetric addition of allylmetal reagents to aldehydes has evolved into a powerfiil and selective tactic in modem organic synthesis.1 Among the most common strategies to accomplish asymmetric allylation is the use of reagents in which the metal is ligated by chiral modifiers. This approach has been extensively developed with excellent results for boron* 12 and titanium,3 but with more modest results for silicon4 5and tin.5,6 The reason for this dichotomy rests squarely in the mechanistic differences in these transformations; i.e., allylboranes and titanium reagents are type l7 reagents which react through associative cyclic transition structures while allylsilanes and -stannanes are type 2 reagents which react (under Lewis acid catalysis) through less rigid, open transition structures.8Recently, a number of laboratories have recorded a significant advance in asymmetric additions of allyl-silanes9 and -stannanes10 *by the use of chiral Lewis acid

The addition of allylic trichlorosilanes to benzaldehyde promoted by chiral phosphoramides to give the enantioenriched homoallylic alcohol has been investigated. In a survey of Lewis bases as activators for the addition of allyltrichlorosilane to benzaldehyde, phosphorus-based amides have been found to be the most effective promoters. To achieve asymmetric induction, chiral phosphoric triamides derived from chiral diamines have been developed and applied in the allylation reaction albeit with modest enantioselectivities. The addition of 2-butenylsilanes was highly diastereoselective, suggesting a closed, chair-like transition structure. A detailed mechanistic study has been carried out to probe into the origin of activation. From a combination of nonlinear effects and kinetics studies, the reaction was found to likely involve two phosphoramides in both the rate and stereochemistry determining steps. These studies provided the background for the development of highly selective and reactive catalysts.

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