Enantioselective ring opening of epoxides with silicon tetrachloride in the presence of a chiral lewis base: Mechanism studies

The enantioselective ring opening of meso-epoxides (from both cyclic and acyclic olefins) with silicon tetrachloride under catalysis by chiral phosphoramides affords enantiomerically enriched chlorohydrins in excellent yields. Experiments designed to elucidate the mechanistic foundation and origins of enantioselectivity are described. From studies on the loading and stoichiometry of the reagent (SiCl₄) and the catalyst [(R)-1] it was established that only one chloride per SiCl₄ is delivered and that the nature of eactive species does not change over the course of the reaction. Kinetic studies together with asymmetric amplification experiments have suggested that more than one catalyst molecule may be bound to SiCl₄ in the stereochemistry-determining transition structure.