Solvent-Activated Valence-Adaptive Ruthenium Catalysis Strategy for Integrated Cleavage–Acetalization of Lignin Ethers to Aromatic Acetals

The study focuses on efficiently converting lignin to aromatic acetals, valuable in pharmaceuticals, by solving the challenges of concurrent Cγ–OH dehydroxymethylation and Cβ–O cleavage with minimal condensation. The solution involves a specialized ruthenium catalyst created by activating RuCl3 (0.03–0.05 mmol) in ethylene glycol at 150 °C. This catalyst uses a synergistic approach with Run+/Cl– interactions, enabling selective β-O-4 bond breaking, in situ acetalization through Cl–-derived Brønsted acids, and Run+-mediated Cγ–OH cleavage. The process achieves an 82.7 wt % yield of aromatic acetal from β-O-4 models, significantly outperforming catalysts using only Run+ or Cl–. The system’s effectiveness is improved by optimizing Ru speciation, loading, temperature, and reaction time. Its versatility is demonstrated with versatile complex substituted β-O-4 substrates, yielding acetals of between 17.1 and 34.3 wt %. Mechanistic studies reveal that dual active species, Run+ cations and H+ from anion hydrolysis, enable coordinated reactions that confirm valence-adaptive catalysis as a novel method for lignin valorization.