Selective hydrodeoxygenation of lignin β-O-4 model compounds and aromatic ketones promoted by palladium chloride with acidic CO2/MeOH system

Selective hydrogenolysis is a crucial challenge for lignin valorization. A PdCl2-catalyzed hydrogenolysis of COH bond of various lignin β-O-4 alcohol model compounds was developed for preparation of corresponding phenethoxybenzene in 78–98% isolated yield. Notably, the introduction of low-pressure CO2 (1 MPa) remarkably improves the reaction efficiency and selectivity of phenethoxybenzene. Neither over-reduction of aromatic ring nor further debenzylation was detected. This is understandable that the in situ methylcarbonic acid generated from CO2 and methanol acts as an acidic catalyst and enhances the leaving ability of hydroxyl group of β-O-4 alcohols. Besides, the present PdCl2/CO2/MeOH catalyst system proved to be effective for reductive cleavage of CO bonds of β-O-4 ketones, producing ethylbenzenes accompanied with phenols in high yields. Furthermore, this protocol could also be extended to selective hydrodeoxygenation of the carbonyl group to methylene in aromatic ketones. As a sound reaction medium, such in situ CO2/MeOH acidic system could be inherently neutralized by depressurizing CO2, providing feature advantages for simple post-processing and none waste disposal.