Radical-triggered base-free 1,3-C → C migrations: chemodivergent synthesis of cyclic imines from N-allyl enamines

Abstract

Functional group migrations are one of the most powerful strategies in the construction of complex molecules. [1,3]-Migrations, especially 1,3-C → C migrations are more challenging for the difficulty in the formation of highly strained four-membered ring intermediates. In the limited reports, narrow substrate tolerance, basic additives or high temperature are unavoidable. Herein, visible-light-mediated novel 1,3-C → C migrations of N-allyl enamines under base-free conditions are researched. Radical-triggered 1,3-C → C acyl and cyano migrations were achieved, initiated by a reductive radical–polar crossover (RPC) process. A variety of biologically active cyclic imines were synthesized in good-to-excellent yields under very mild reaction conditions. DFT calculations support our proposal that the radical cyclization of N-allyl enamines facilitates the reductive RPC process, which triggers the formation of strained four-membered ring intermediates for 1,3-migrations. Thus, a tandem radical cyclization/RPC/1,3-C → C migration process has been developed under base-free conditions. Furthermore, from the same N-allyl enamines, different cyclic imines, i.e., (3-azabicyclo[3.1.0]hex-2-enes), could be achieved in high selectivity mediated by the acridine photosensitizer and Co(II) additive.