Metal‐Free Sonophotocatalytic C─C Coupling of Acetovanillone Using a Synthesis‐Frequency‐Tuned Chitosan–Lignin Hybrid Catalyst

ABSTRACT A metal‐free sonophotocatalytic system based on a biopolymer‐derived hybrid catalyst is reported for the selective C─C coupling of lignin‐derived acetovanillone to the pharmaceutically relevant dimer diapocynin. The system employs a frequency‐optimized chitosan–lignin (CL) sonophotocatalyst (Syn‐340) under synergistic 365 nm UV‐A irradiation (∼300 W m‐2) and high‐frequency ultrasound (500 kHz), affording diapocynin in 33% yield with >97% selectivity at ambient temperature and pressure after 15 h. The lower photon energy of UV‐A relative to UV‐C suppresses non‐selective substrate photolysis, thereby preserving the >97% C─C selectivity. Selective excitation suppresses over‐oxidation of phenoxyl radicals to phenoxenium ions, preventing ring opening and mineralization. The high primary amine density of Syn‐340 promotes single‐electron oxidation. The radical mechanism was confirmed by TEMPO inhibition, while scavenger tests identified •OH and O2• − as the dominant reactive species, with h + contributing least. A mechanistic investigation combining acoustic pressure mapping, time‐resolved mass spectrometry, and luminol imaging reveals a frequency‐dependent trade‐off between reactive species generation and cavitation stability. Although 500 kHz enables the reaction, 22 kHz provides greater stability and reproducibility for future optimization. This work demonstrates a metal‐free route to lignin‐derived acetovanillone and offers design insights for future lignin valorization using biopolymer‐based sonophotocatalysts.