Isotope Effects and Mechanism of the Asymmetric BOROX Bronsted Acid Catalyzed Aziridination Reaction

The mechanism of the chiral VANOL-BOROX Bronsted acid catalyzed aziridination reaction of imines and ethyldiazoacetate has been studied using a combination of experimental kinetic isotope effects and theoretical calculations. A stepwise mechanism where reversible formation of a diazonium ion intermediate precedes rate-limiting ring closure to form the cis-aziridine is implicated. A revised model for the origin of enantio- and diastereoselectivity is proposed based on relative energies of the ring-closing transition structures.