Visible light-driven ligand-to-metal charge transfer-mediated selective cleavage of β-O-4 lignin model compounds: a greener route to lignin valorization

Lignin is the most abundant renewable source of aromatics in nature. The beta-O-4 bond is the most predominant linkage in lignin; therefore, methods for the selective cleavage of the beta-O-4 bond are of great importance in order to break down lignin and produce value-added aromatic compounds. Herein, we report a visible light-driven, ligand-to-metal charge transfer (LMCT)-mediated, two-step approach for cleaving C beta-O bonds in beta-O-4 alcohol model compounds using titania (TiO2) as a photocatalyst. In the first step, the alcohol forms a visible light-absorbing LMCT complex on the surface of titania, which enables oxidation to the corresponding ketone under green light. The LMCT-mediated oxidation afforded high conversion of beta-O-4 alcohol model compounds (79-97%) with high selectivity for beta-O-4 ketones (>95%). Our studies reveal that the superoxide radical anion likely plays a key role in the oxidation. In the second step, the LMCT-assisted reductive cleavage of beta-O-4 ketone is achieved by employing triethylammonium tetraphenylborate as a visible light sensitizer and proton donor. The LMCT-facilitated reductive cleavage of beta-O-4 ketones exhibits high selectivity (up to 100%) for target fragmentation products under blue light. Experiments including EPR analysis suggest that in situ formed Ti3+ is responsible for the reductive cleavage of beta-O-4 ketones. Moreover, a two-step, one-pot cleavage reaction was successfully carried out with good to high selectivity for C beta-O bond cleavage products with a single catalyst. Our work offers a promising solution for the selective cleavage of beta-O-4 bonds under mild conditions to promote lignin valorization. Furthermore, it provides potentially general strategies for enabling visible light-driven LMCT-mediated photocatalysis in related organic transformations.