Photoredox catalysis with pyridinium and structurally related onium salts: sustainable synthesis via reactive oxygen species generation and electron transfer

Pyridinium and its derivatives, which incorporate both redox centers and Lewis acid sites, exhibit high structural tunability. Their excited states can generate singlet oxygen via energy transfer (EnT) or undergo electron transfer (ET) with electron-rich substrates to yield pyridinium radicals. These radicals subsequently engage in single-electron transfer (SET) with molecular oxygen, regenerating ground-state pyridinium and producing superoxide anions—a process that sustains redox cycling and oxygen activation. Additionally, the cationic nitrogen-containing group, as a Lewis acid site, can polarize and activate electron-rich substrates via electron-withdrawing effects. Building on these characteristics, this review provides a comprehensive overview of the photocatalytic systems based on pyridinium and structurally related onium salts, which demonstrate dual reactivity: direct oxidation, such as alcohols to aldehydes or carboxylic acids, and tandem reactions, including acetalization, esterification, and C–N bond formation. Beyond these transformations, pyridinium catalysts mediate diverse oxidations and bond cleavages, including C–H and C–C activation via SET or proton-coupled electron transfer (PCET). Representative examples include the oxidation of unactivated C–H bonds and thioethers, as well as the selective C_α–C_β bond cleavage in lignin model compounds. Unlike traditional photocatalysts, pyridinium compounds uniquely integrate tunable redox properties, reversible ET capability, and Lewis acidity, enabling their application as sustainable catalysts under mild, noble metal- and additive-free conditions. While existing researches have predominantly focused on oxidative pathways via oxygen activation, the reductive potential of these systems remains largely underexplored. The quenching of pyridinium radicals can release electrons, offering opportunities for photocatalytic reduction. Harnessing this bidirectional redox behavior could pave the way for the design of novel redox processes or tandem oxidative-reductive transformations, thereby expanding the synthetic versatility of pyridinium-based catalytic systems.