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

Functional group migrations are one of the most powerful strategies in complex molecular constructions. [1,3]-migrations, especially the 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, the narrow substrates tolerance, the basic additives or high temperature are unavoidable. Herein, the 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 radical cyclization of N-allyl enamines facilitate the reductive RPC process, which trigged 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 3-azabicyclo[3.1.0]hex-2-enes could be achieved in high selectivity mediated by acridine photosensitizer and Co(II) additive.