Mechanistic Investigation of the Ce(III) Chloride Photoredox Catalysis System: Understanding the Role of Alcohols as Additives

Photocatalytic C–H activation is an emerging area of research. While cerium chloride photocatalysts have been extensively studied, the role of alcohol additives in these systems remains a subject of ongoing discussion. It was demonstrated that the photocatalyst [NEt4]2[CeIVCl6] (1) produces •Cl and added alcohols exhibit zero-order kinetics. Prior studies by other researchers suggested that 1 and alcohols lead to cerium alkoxide [Ce–OR] and alkoxy radical intermediates. To understand these seemingly divergent mechanistic proposals, an expanded investigation comparing cerium(IV) catalyst 1 and cerium(III) complex [NEt4]3[CeIIICl6] (2), which exhibit markedly different reactivity and C–H selectivity, is disclosed. Our findings reveal that alcohol additives accelerate the conversion of cerium(III) to cerium(IV) catalysts, forming key intermediates such as [NEt4]2[CeIIICl5(HOCH3)] (5) and [NEt4]2[CeIVCl5(OCH3)] (6), driven by excited-state di-tert-butyl azodicarboxylate under blue light irradiation. The active complex 6 releases the •OCH3 radical, in sharp contrast to •Cl radicals initiated by cerium(IV) photoredox catalyst 1. These different reactivity and selectivity profiles can be understood in the context of complex 5 generation and in situ formation of base to afford complex 6. Experimental validation shows enhanced selectivity toward C–H bonds with different reactivity with catalyst 1 and methanol upon the addition of base and decreased selectivity with catalyst 2 and methanol upon the addition of acid. These findings unify the previously contrasting observations of cerium halide/alkoxide photocatalytic systems and provide a comprehensive understanding on the essential role of base/acid and alcohol in selectivity and reactivity.