Depolymerization of Lignin via a Microscopic Reverse Biosynthesis Pathway

A standing challenge in biomass conversion is the valorization of lignin, an irregular polymer representing the largest renewable source of aromatic feedstock in nature. Lignin originates from phenylpropanoid building blocks, which polymerize through a sequence of oxidative radical coupling reactions. A lignin depolymerization strategy based on the microscopic reverse biosynthesis cleaves the polymer and converges the complex structural units to phenylpropanoid monomers at room temperature within a few hours. The phenylpropanoid monomers retain important functionality and can be readily isolated in high yields as essential ingredients in fragrances, pesticides, and pharmaceutical precursors. Mechanistic studies preclude the commonly proposed titanium-mediated C-O bond homolysis in alcohol activation. Instead, the reaction proceeds through stepwise carbocation formation, followed by reduction to generate a radical that proceeds to β-scission.