Formation and degradation of strongly reducing cyanoarene-based radical anions towards efficient radical anion-mediated photoredox catalysis

Abstract

Cyanoarene-based photocatalysts are widely used due to their catalytic performance, but the formation of the active species and its potential degradation pathways are poorly understood. Here, the authors investigate these pathways under commonly-used photoredox-mediated reaction conditions. Cyanoarene-based photocatalysts (PCs) have attracted significant interest owing to their superior catalytic performance for radical anion mediated photoredox catalysis. However, the factors affecting the formation and degradation of cyanoarene-based PC radical anion (PC center dot-) are still insufficiently understood. Herein, we therefore investigate the formation and degradation of cyanoarene-based PC center dot- under widely-used photoredox-mediated reaction conditions. By screening various cyanoarene-based PCs, we elucidate strategies to efficiently generate PC center dot- with adequate excited-state reduction potentials (E-red*) via supra-efficient generation of long-lived triplet excited states (T-1). To thoroughly investigate the behavior of PC center dot- in actual photoredox-mediated reactions, a reductive dehalogenation is carried out as a model reaction and identified the dominant photodegradation pathways of the PC center dot-. Dehalogenation and photodegradation of PC center dot- are coexistent depending on the rate of electron transfer (ET) to the substrate and the photodegradation strongly depends on the electronic and steric properties of the PCs. Based on the understanding of both the formation and photodegradation of PC center dot-, we demonstrate that the efficient generation of highly reducing PC center dot- allows for the highly efficient photoredox catalyzed dehalogenation of aryl/alkyl halides at a PC loading as low as 0.001 mol% with a high oxygen tolerance. The present work provides new insights into the reactions of cyanoarene-based PC center dot- in photoredox-mediated reactions.