Visible-Light-Induced Oxidative Lignin C-C Bond Cleavage to Aldehydes Using Vanadium Catalysts

Lignin is the largest carrier of aromatics on earth, and its depolymerization can afford value-added aromatic products. Direct cleavage of the C-C bonds in lignin linkages is significant, but it is challenging to obtain low-molecular-weight aromatic monomers. Herein, using vanadium catalysts under visible light, we selectively cleave the C-C bonds in beta-1 and beta-O-4 interlinkages occluded in lignin models and extracts by an oxidative protocol. Visible light irradiation triggered single electron transfer between the substrate and the catalyst, which further induced the selective C-alpha-C-beta bond cleavage and generated the final aromatic products through radical intermediates. Using this photocatalytic chemistry, the reactivity of lignin models and the selectivity of C-alpha-C-beta bond cleavage were significantly improved. More importantly, this protocol affords aromatic monomers through the fragmentation of organosolv lignins even at room temperature, indicating the potential of photocatalytic C-C bond cleavage of lignin linkages under ambient conditions.