Oxidative rearrangement approach for the ring contraction of N–H piperidines to pyrrolidines

We have developed a method for ring contraction of N–H piperidines using oxidative rearrangement with PhI(OAc)2. The reaction forms iminium ion intermediates that are effectively trapped by nucleophiles (e.g., NaBH4, H2O) yielding the corresponding pyrrolidine derivatives. Additionally, we propose an ionic mechanism supported by experiments and density‐functional theory calculations.