Unveiling the migration reactivity of bicyclic diaziridines: enantioselective synthesis of chiral pyrazolines

Ring-opening/cyclization represents a classic reaction of bicyclic diaziridines. In this study, an unprecedented ring-opening/migration cascade process was discovered in the reaction between bicyclic diaziridines and donor-acceptor (D-A) cyclopropanes. By employing a chiral <i>N</i>,<i>N</i>'-dioxide/scandium(iii) complex as the catalyst, a diverse array of chiral dihydro-1<i>H</i>-pyrazoles with a stereocenter in the side chain was efficiently synthesized featuring excellent ee values. Control experiments indicated that the substitution on the D-A cyclopropane is of critical importance in determining the cyclization or migration process. When combined with DFT calculations, a plausible reaction mechanism was proposed, which involves a key transition state. This work presents a novel method for accessing pyrazolines and broadens the scope of diaziridine chemistry.