Photoredox Catalysis with Pyridinium and Structurally Related Onium Salts: Sustainable Synthesis via Reactive Oxygen Species and Electron Transfer

Abstract

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 or undergo electron transfer with electron-rich substrates to yield pyridinium radicals. These radicals subsequently engage single-electron transfer (SET) with molecular oxygen, regenerating ground-state pyridinium and producing superoxide anion__a process that sustains redox cycling and oxygen activation. Additionally, the cationic nitrogen-containing group as Lewis acid sites, can polarize and activate electron-rich substrates via electron-withdrawing effect. Building on these characteristics, this review provides a comprehensive overview of 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 oxidation of unactivated C-H bonds and thioethers, as well as the selective Cα-Cβ bond cleavage in lignin model compounds. Unlike traditional photocatalysts, pyridinium compounds uniquely integrate tunable redox properties, reversible electron transfer capability, and Lewis acidity, permitting their application as sustainable catalysts under mild, noble metal- and additive-free conditions. While existing researches has predominantly focused on oxidative pathways via oxygen activation, the reductive potential of these systems remains largely underexplored. 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.