Further Structural Modification of Sulfur-Stabilized Silicon Cations with Binaphthyl Backbones

The synthesis and spectroscopic characterization of two novel cationic silicon-sulfur Lewis pairs with a chiral 4,4-disubstituted binaphthyl silepine backbone are described. Both Lewis acids induce significant enantioselectivity in the model Diels-Alder reaction of cyclohexa-1,3-diene and chalcone but additional substitution of the binaphthyl backbone exerts a minimal effect on enantioinduction compared to previously reported Lewis acids. Another silicon cation with a chiral spirocyclic backbone induces enantioselectivity in the same range but its synthesis is laborious.