Insights into lignin depolymerization through palladium-catalyzed hydrogenolysis of α-O-4 model compounds

Lignin is an abundant renewable resource with high potential for the production of aromatic bulk and fine chemicals via hydrogenolysis. However, its efficient depolymerization remains challenging due to its structural complexity. In this work, the influence of functional groups on the palladium-catalyzed hydrogenolysis of benzyl aryl α-O-4 ethers serving as lignin model compounds, which have been considerably less studied in the literature than the commonly investigated β-O-4 compounds, was systematically investigated under mild reaction conditions. To broaden the substrate scope, new α-O-4 lignin model compounds were synthesized and analytically characterized. Kinetic studies revealed that both para -methoxy and especially para -hydroxy substituents on the benzyl moiety strongly decrease ether cleavage rates, providing insight into the limited cleavability of lignin, which contains substantial amounts of phenolic hydroxy groups. In contrast, ortho -methoxy substituents and para -propyl groups on the phenyl unit enhanced hydrogenolysis reactivity, while increasing methoxy substitution generally reduced reaction rates. Studies on more complex trilignols, including dibenzodioxocins, demonstrated selective α-O-4 bond cleavage, whereas β-O-4 linkages largely remained intact. Formation of phenolic para -hydroxy groups suppressed further depolymerization, highlighting the potential of controlled partial depolymerization combined with simultaneous lignin functionalization for targeted applications.