Metal-free radical catalysis: an sp2 C–H activation strategy for efficient synthesis of lignin–biphenyl dimers†

As the only natural non-petroleum resource capable of generating renewable aromatic compounds, lignin holds considerable potential for reducing dependence on fossil fuels. However, its practical applications are constrained by its structural rigidity, molecular complexity, and heterogeneity, necessitating chemical modification. Additionally, the synthesis of biphenyl compounds through C–C coupling of functional group-substituted phenols is hampered by severe reaction conditions, high substrate and energy consumption, and significant environmental concerns. Here, we have developed a novel metal-free catalytic strategy for lignin coupling reactions to produce lignin-derived biphenyl dimers. This approach focuses on unconventional linkages formed via DDQ-mediated inverse oxidative aldol condensation, followed by radical coupling pathways. Utilizing advanced 2D-NMR analysis based on the guaiacol sp² C–H coupling synthesis hypothesis, we identified and characterized novel structural motifs in S-unit and G-unit eugenol dimers (S–S) and guaiacol dimers (G–G). These model compounds exhibit distinct signatures of inverse aldol and radical-mediated coupling mechanisms, offering valuable insights into lignin reactivity under mild catalytic conditions.