Enantio- and Diastereotopos Differentiation in the Palladium(II)-Catalyzed Hydrosilylation of Bicyclo[2.2.1]alkene Scaffolds with Silicon-Stereogenic Silanes

The palladium(II)‐catalyzed hydrosilylation of meso‐configured bicyclo[2.2.1]alkene scaffolds proved to be an invaluable model reaction for the development of reagent‐controlled asymmetric transformations based on silicon‐stereogenic silanes as stereoinducers. In the present investigation, the subtle structural requirements of the silane substitution pattern in enantiotopos‐differentiating single hydrosilylations of a norbornene‐type substrate are disclosed. Extension of this chemistry to a double hydrosilylation of norbornadiene entails a significant increase in stereochemical complexity. Although differentiation of enantiotopic positions by the chiral reagent is demanded in the first hydrosilylation, the same reagent must then differentiate diastereotopic positions in the second. Remarkably high stereocontrol was found in this double hydrosilylation with several silanes first used in the hydrosilylations of the norbornene‐type system. Depending on the enantiomeric purity of the silane, C2‐ and Cs‐symmetric adducts, respectively, were obtained. The identity of the key quaternary silanes was revealed by crystallographic analysis. By this method, the relative and absolute configurations were also assigned, which, in turn, imply that all enantiospecific substitutions at silicon proceed with stereoretention. On the basis of these solid‐state structures, we also discuss the structural implications of silane substitution for the diastereoselectivity‐determining step of this palladium(II)‐catalyzed hydrosilylation reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) Chirality at silicon induces high site selectivity in the hydrosilylation of bicyclic alkenes. Pronounced reagent control by silicon‐stereogenic silanes in the reaction of enantiopure reagents with norbornadiene exclusively furnishes C2 isomers with preserved excesses, whereas racemic mixtures provide almost equal amounts of C2 and Cs isomers.