Efficient Ruthenium-Catalyzed Allylation of Technical Lignins with Allyl Alcohol and Diallyl Ether: New Clean Strategies for Unsaturated Lignin

A ruthenium-catalyzed lignin allylation reaction has been developed using allyl alcohol or diallyl ether as reagents. This reaction, performed under mild conditions and without side products, provides lignins with a reactive terminal C═C bond. The protocol was successfully applied to the transformation of Kraft, Organosolv, and Soda lignins, which were used without prior purification. Allylation succeeded on a proportion of the phenols in lignin but was particularly efficient for aliphatic hydroxyl groups. To maximize allyl content, phenolic hydroxyl groups were selectively converted in a two-step approach. This strategy involves a ring-opening reaction with glycerol carbonate to generate lignin with a high content of aliphatic hydroxyl groups, followed by a ruthenium-catalyzed allylation protocol. Technical and modified lignins were characterized by NMR, IR, and SEC, and thermal properties were determined by DSC. Polarity and glass transition temperatures were significantly reduced, allowing shaping and blending of polymers.