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 PdCl 2 -catalyzed hydrogenolysis of C OH 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 CO 2 (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 CO 2 and methanol acts as an acidic catalyst and enhances the leaving ability of hydroxyl group of β-O-4 alcohols. Besides, the present PdCl 2 /CO 2 /MeOH catalyst system proved to be effective for reductive cleavage of C O 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 CO 2 /MeOH acidic system could be inherently neutralized by depressurizing CO 2 , providing feature advantages for simple post-processing and none waste disposal.