Synthesis of Bicyclic N-Heterocycles via Photoredox Cycloaddition of Imino-Alkynes and Imino-Alkenes

Cycloaddition reactions offer great advantages regarding atom and step economy for the construction of various carbocycles and heterocycles. While the recent development based on sensitized visible light photocatalysis allowed the synthesis of azetidines via imine-alkene [2 + 2] cycloaddition, imine-alkyne [2 + 2] cycloaddition under visible light photocatalysis has not been reported. In this regard, we report the synthesis of pyrrolizidinones based on intramolecular imine-alkyne [2 + 2] cycloaddition under visible light photocatalysis. This redox-neutral reaction involves formal imine-alkyne metathesis followed by redox-mediated annulation with concomitant rearrangement. In contrast, the use of imino-alkenes provides dihyro-1,4-oxazines via an alternative [4 + 2] cycloaddition pathway. The proposed reaction mechanisms were supported by control experiments and DFT calculations.

Automated summary

By utilizing visible light photocatalysis, we successfully reported the synthesis of pyrrolizidinones through intramolecular imine-alkyne [2 + 2] cycloaddition. This reaction involves imine-alkyne metathesis and subsequent rearrangement, with the proposed reaction mechanisms supported by both control experiments and DFT calculations.