Photoinduced organocatalytic lignin C–C bond cleavage in mixed binary solvents

Lignin presents a renewable alternative of fossil feedstock to produce value-added aromatic chemicals. Oxidative cleavage of C−C bonds in lignin is of great importance, but obtaining aromatic monomers in high selectivity under mild conditions is challenging. Herein, we report a novel strategy of photoinduced oxidative C–C cleavage of lignin β–O–4 derivatives using photoredox organocatalysts, among which Mes-10-phenyl-Acr+-BF4– exhibited high yields of aromatic aldehydes and phenyl formate. Besides, mixed binary organic solvents were demonstrated to promote bond cleavage and inhibit over-oxidation side reactions. The absorbance, excitation, and interaction with the lignin model of the photocatalyst in different solvents were investigated by UV-Vis, PL, and CV tests. Furthermore, through mechanism studies, a C-centered radical intermediate was captured, providing evidence for the direct Cα−Cβ bond cleavage mechanism during the reaction. This work presents a new photocatalytic strategy and provides deep insights into the bond cleavage mechanism for lignin valorization.