Highly Selective Oxidation and Depolymerization of α,γ-Diol-Protected Lignin

Lignin oxidation offers a potential sustainable pathway to oxygenated aromatic molecules. However, current methods that use real lignin tend to have low selectivity and a yield that is limited by lignin degradation during its extraction. We developed stoichiometric and catalytic oxidation methods using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as oxidant/catalyst to selectively deprotect the acetal and oxidize the alpha-OH into a ketone. The oxidized lignin was then depolymerized using a formic acid/sodium formate system to produce aromatic monomers with a 36 mol% (in the case of stoichiometric oxidation) and 31 mol% (in the case of catalytic oxidation) yield (based on the original Klason lignin). The selectivity to a single product reached 80% (syringyl propane dione, and 10-13% to guaiacyl propane dione). These high yields of monomers and unprecedented selectivity are attributed to the preservation of the lignin structure by the acetal.