Enantioselective olefin epoxidation using axially chiral biaryl azepinium salts as Catalysts. Rapid in-situ Screening and Origin of the Stereocontrol

To unravel the origin of the stereocontrol in epoxidation reactions of unfunctionalized alkenes by diastereomeric biaryl oxaziridinium salts, two series of novel iminium cations were prepared. These moieties combine (Ra)-dimethylbiphenyl or (Ra)-5,5,6,6,7,7,8,8-octahydrobinaphthyl cores with chiral exocyclic appendages derived from commercially available (S)- or (R)-3,3-dimethylbutan-2-amine and (S)- or (R)-1-phenylpropan-1-amine. Under biphasic enantioselective olefin epoxidation conditions, in-situ generated bromide salts of these derivatives have displayed similar or better asymmetric efficiency than the classical binaphthyl derivatives. A structural analysis was performed in search of a correlation between the origin of the stereocontrol/level of enantioselectivity in the products, and dihedral angles around the biaryl twist of the catalysts