Photocatalytic Oxidation and Subsequent Hydrogenolysis of Lignin beta-O-4 Models to Aromatics Promoted by In Situ Carbonic Acid

The cleavage of C-O bond in lignin beta-O-4 model compounds to form aromatics has been achieved via a two-step process, comprising visible-light photocatalytic oxidation and in situ carbonic acid-facilitated hydrogenolysis. In the first step, with readily available persulfate as radical initiator and cheap copper as catalyst, the secondary alcohol in the beta-O-4 alkyl-aryl ether linkage is selectively oxidized to the corresponding ketone in up to 99% yield under visible light irradiation. The second step features the C-O bond cleavage of lignin beta-O-4 ketones promoted by in situ acidic EtOH/H2O/CO2 system in the presence of zinc powder, producing acetophenones and phenols in high yield. This protocol provides a novel alternative to selective fragmentation of beta-O-4 linkage to aromatic monomers under mild reaction conditions.