Chiral Phosphoric Acid Catalyzed Asymmetric Ring Contraction of Isoxazoles under Dye-Free Photodriven Conditions.

Cooperative phosphoric acid/photocatalysis enables novel asymmetric methods, and prior systems are largely confined to radicals generated from photoredox, energy-transfer, or EDA-complex mechanisms. Reported herein is excitation of chiral phosphoric acid-isoxazole adduct to deliver enantioenriched 2H-azirines via ring contraction. This catalytic system proceeds under dye-free and operationally simple conditions with high enantioselectivity and efficiency. Mechanistic studies revealed that the isoxazole undergoes CPA-assisted photoexcitation and homolysis of the N-O bond to a triplet diradical. A subsequent C-C rotation prior to the C-N cyclization has been established as the enantio-determining step.