Characterization and Monitoring of Transient Enamine Radical Intermediates in Photoredox/Chiral Primary Amine Synergistic Catalytic Cycle.

Abstract

Enamine-derived radicals are crucial intermediates in singly occupied molecular orbital (SOMO) catalysis. However, observing them directly is elusive and remains a long-standing challenge. Here, an advanced time-resolved electron paramagnetic resonance technique was employed to characterize and monitor the key intermediates in photoredox transformations by primary aminocatalysis on a microsecond timescale. The transient enamine radical cation, generated by single electron transfer (SET), and the deprotonated form of α-imino radical intermediates were directly observed for the first time, both spectroscopically and kinetically. In reactions with styrene, enamine radical cation was found to be faster than α-imino radical by one order of magnitude. This revealed the subtle role of deprotonation associated with secondary enamine radical cation in the photoredox transformations by primary aminocatalysis.