Toward the oxidative deconstruction of lignin: oxidation of β-1 and β-5 linkages.

There have been numerous reports on methods for the oxidative cleavage of β-O-4 linkages in lignin model compounds, but relatively few reports of how those methods affect other linkages that are present in lignin. We have investigated the effect of several of these oxidation methods on the β-1 and the β-5 lignin linkages, using four β-1 and β-5 model compounds. We observed that direct oxidative cleavage of C-C bonds occurs in metal-catalyzed TEMPO oxidation systems and with iron porphyrin oxidations, neither of which had we observed in similar oxidations on β-O-4 models. The β-5 linkage proved to be largely resistant to all of these oxidative systems, but the dihydrofuran ring in the β-5 model 3 was opened when treated with KMnO4 at elevated temperature. Most promising was the oxidation of 2 with DDQ, which produced the benzylic ketone in high yield (84%), as it does in reactions with β-O-4 models. This reaction exhibits selectivity for the benzylic position as well as compatibility with phenols, characteristics that are highly desirable for a two-step, benzylic oxidation/Baeyer-Villiger route for cleavage of lignin.