Asymmetric [3 + 2] Cycloaddition of Methyleneindolinones with N,N′‑Cyclic Azomethine Imines Catalyzed by a N,N′‑Dioxide–Mg(OTf)2 Complex

A highly efficient chiral N,N′-dioxide–Mg­(OTf)2 catalyst system has been developed for the asymmetric 1,3-dipolar cycloaddition between methyleneindolinones and N,N′-cyclic azomethine imines. The desired pyrazolidine products with contiguous quaternary–tertiary stereocenters were obtained in up to 99% yields with up to 99% ee and >19:1 dr under mild reaction conditions.