Mechanisms and Synthetic Applications of Cyclic, Nonstabilized Isodiazenes: Nitrogen-Atom Insertion into Pyrrolidines and Related Rearrangements

Reactive intermediates that can promote nonintuitive bond disconnections underpin advancements in skeletal editing methodologies. Accordingly, a detailed understanding of their reactivity and its underlying mechanisms is central to progress in this space. Herein, we catalog and study the reactivity of nonstabilized cyclic isodiazene intermediates generated via the reaction of cyclic secondary amines with an anomeric amide reagent. Depending on the amine structure, distinct and predictable product classes can be accessed: cyclic hydrazones are formed from pyrrolidines, N-amino indoles from indolines, orthoquinodimethane intermediates from isoindolines, cyclopropanes from azetidines, and cyclic tetrazines from piperidines. Mechanistic experiments and density functional theory calculations suggest that many of these transformations proceed through an azomethine imine intermediate. In most cases, this reactive species subsequently rearranges to a cyclic hydrazone by an unusual self-catalysis mechanism proceeding through a dimeric tetrazine. This oxidative nitrogen insertion was leveraged in several subsequent synthetic applications. Redox diversification of the cyclic hydrazones enables access to pyridazines and cyclic hydrazines, including the synthesis of an orthogonally protected l-piperazic acid from the readily available chiral pool l-prolinol.