Lewis Acid-Promoted Carbon−Carbon Bond Cleavage of Aziridines:  Divergent Cycloaddition Pathways of the Derived Ylides

Lewis acids are shown to cleave the carbon−carbon bond of activated aziridines at ambient temperature. The derived metal-coordinated azomethine ylides undergo cycloaddition reactions with electron-rich alkenes. Cyclic alkenes afford products that are formally [4+2] adducts most likely derived from a Mannich-type addition to the ylide, followed by intramolecular Friedel−Crafts alkylation. Alternatively, acyclic alkenes undergo [3+2] cycloaddition to give new pyrrolidine products.