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

Abstract

This study presents the direct observation of α-imino radical intermediates involved in the photoredox catalytic activation of enamines using the ultrawide single sideband phase-sensitive detection time-resolved electron paramagnetic reso-nance (U-PSD TREPR) technique. Enamines have been exten-sively used in singly occupied molecular orbital (SOMO) catal-ysis. The redox properties of enamines allow for facile oxida-tion, forming α-imino radical species that participate in vari-ous coupling reactions. Despite the synthetic value of these transformations, direct observation of the key radical inter-mediates has been limited. The U-PSD TREPR technique ena-bled direct in-situ observation of transient enamine radical cation and α-imino radical intermediates for the first time. Spectroscopic characterization and kinetic analysis provided insights into the structure and reactivity of these intermedi-ates. The results demonstrated the involvement of enamine radical cations as the key intermediates in the photoredox transformations by primary aminocatalysis.