Mild Redox-Neutral Depolymerization of Lignin with a Binuclear Rh Complex in Water

A mild redox-neutral lignin depolymerization system featuring a water-soluble binuclear Rh complex has been developed. The catalytic system could be successfully applied to the depolymerization of a lignin-like polymer, alkaline lignin, as well as raw lignocellulose samples to produce aromatic ketones, providing a homogeneous catalytic system for "lignin-first" biorefinery in water. Mechanistic studies on the model substrate suggest that the reaction proceeds via a metal-catalyzed dehydrogenation step to afford a carbonyl intermediate, followed by C–O bond cleavage to afford ketone and phenol products. Deuterium labeling study shows that the hydrogen used for cleavage of the C–O bond originates from the alcohol moiety in the substrate.