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

• Published in: Journal of the American Chemical Society Vol. 147; no. 2; pp. 2061 - 2076
• Main Authors: Yang, Qiaomu, Song, Ellen, Wu, Yu, Li, Chenshuai, Gau, Michael R., Anna, Jessica M., Schelter, Eric J., Walsh, Patrick J.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.01.2025
• Subjects: blue light; carbon-hydrogen bond activation; cerium; chlorides; irradiation; methanol; photocatalysis; photocatalysts; redox reactions
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.4c15627

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.