One-Pot Catalytic Cascade for the Depolymerization of the Lignin ß-O-4 Motif to Non-phenolic Dealkylated Aromatics.

ABSTRACT

Aromatic monomers obtained by selective depolymerization of the lignin ß-O-4 motif are typically phenolic and contain (oxygenated) alkyl substitutions. This work reveals the potential of a one-pot catalytic lignin ß-O-4 depolymerization cascade strategy that yields a uniform set of methoxylated aromatics without alkyl side-chains. This cascade consists of selective acceptorless dehydrogenation of the γ-hydroxy group, subsequent retro-aldol reaction cleaving the Cα-Cß bond followed by in situ acceptorless decarbonylation of the formed aldehydes. This three-step cascade reaction, catalyzed by an iridium(I)-BINAP complex, resulted in 75% 1,2-dimethoxybenzene from G-type lignin dimers alongside syngas (COH2 ≈ 1.41). Applying this method to a synthetic G-type polymer, 11 wt% 1,2-dimethoxybenzene was obtained. This versatile compound can be easily transformed into 3,4-dimethoxyphenol, a valuable precursor for pharmaceutical synthesis, through enzymatic catalytic approach. Moreover, the hydrodeoxygenation potential of 1,2-dimethoxybenzene offers a pathway to produce valuable cyclohexane or benzene derivatives, presenting enticing opportunities for sustainable chemical transformations without the necessity for phenolic mixture upgrading via dealkylation.