Catalytic Asymmetric Umpolung (n+1) Cyclization of Bifunctional Hydroxylamines: Entry to Chiral Spirooxindole-Fused Azacycles.

Herein we report a general catalytic asymmetric umpolung (n+1) (n = 4, 5) cyclization using bifunctional hydroxylamines as bis-electrophiles, affording five- and six-membered azaspirooxindoles in good yields with excellent enantioselectivities (up to 98% ee) under mild phase-transfer catalytic conditions. Mechanistic studies reveal cooperative halogen-bonding, together with ion-pairing interactions, between the catalyst and substrate as key to reactivity and stereocontrol. Such discovery will inspire the design of new bifunctional phase-transfer catalysts with halogen-bonding donors in the future.