Enhanced Reactivity of Acridinium Perchlorate: Harnessing Redox Mediators for Trace Chloride Activation in Hydrogen Atom Transfer Photocatalysis

Acridinium perchlorates and tetrafluoroborates are commonly employed photocatalysts with similar photophysical characteristics. Nevertheless, perchlorate salts consistently exhibit superior catalytic performance relative to their tetrafluoroborate counterparts. In a serendipitous discovery, we unveiled the latent potential of trace chloride anions originating from acridinium perchlorate salts. When coupled with redox mediators like diphenyl ether or biphenyl, the trace chloride anions displayed remarkable hydrogen atom transfer (HAT) catalysis capabilities. This innovative approach enabled the efficient activation of a diverse range of unactivated C–H bonds, especially primary C(sp³)–H bonds, via Giese additions and Minisci-type cross-dehydrogenative coupling reactions. Our system stands as one of the most efficient HAT catalytic systems for C–H bond activation, showcasing remarkable substrate diversity. Additionally, the use of a stop-flow microtubing (SFMT) reactor enhanced reaction efficiency, enabling gram-scale synthesis and a seamless transition to continuous flow processes for practical-scale production.