Direct Observation of Enamine-Derived Radicals with Time-Resolved Electron Paramagnetic Resonance.

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 is 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 α-imino radical intermediates, were directly observed for the first time, both spectroscopically and kinetically. When reacting with styrene, enamine radical cation was found to be faster than α-imino radical by one order of magnitude. This reveals the subtle role of deprotonation associated with secondary enamine radical cation in the photoredox transformations by primary aminocatalysis.