Enantioselective Formal [3 + 1 + 1] Cycloaddition Reaction by Ru(II)/Iminium Cocatalysis for Construction of Multisubstituted Pyrrolidines.

A Ru(II)/iminium cocatalyzed asymmetric formal [3 + 1 + 1] cycloaddition reaction of diazoacetophenones, anilines, and enals is disclosed to construct multisubstituted pyrrolidines in one step with excellent diastereoselectivity and enantioselectivity. The reaction mechanism was postulated as a successful trapping of Ru(II)-associated ammonium ylides via a selective 1,4-addition to chiral amine activated enals followed by a tandem aza-aldol process. The control experiments and theoretical density functional theory investigation revealed that the reversible NaOAc-facilitated aza-aldol process led to the diastereomeric conversion to provide a stable product.