Metal-Free Sonophotocatalytic C─C Coupling of Acetovanillone Using a Synthesis-Frequency-Tuned Chitosan-Lignin Hybrid...

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.