Visible-Light Induced Thiol-Ene Reaction on Natural Lignin

The current use of lignin as a raw material is very limited and focused only on cheap and poorly defined nonfunctional materials mainly due to challenges in synthetic modification of lignin. Herein, we report a new low energy and environmentally friendly lignin modification method induced by visible blue light. The key modification reaction is a photoredox catalyzed thiol-ene reaction. The lignin was modified to possess alkenes for the thiol-ene reaction. Three photochemical reagents-Ru(bpy)(3)Cl-2., Eosin Y, and 2,2-dimethoxy-2-phenylacetophenone-were tested to determine the best thiol-ene modification method. The thiol-ene reaction between lignin-alkene and 1-decanethiol revealed that Ru(bpy)(3)Cl-2 was the most efficient, resulting in conversions of 97% with 2.5 mol % catalyst loading. The Ru(bpy)(3)Cl-2 was further investigated with diverse thiol compounds. All tested thiol-ene reactions showed excellent efficiencies, with conversions of 93-97% under low-energy 3W blue LED light. In particular, thiol terminal poly(ethylene glycol) also displayed 94% conversion after 80 min of irradiation. The developed photoredox catalyzed thiol-ene modification of lignin was very conveniently controlled by simply turning the light source on and off. Excellent conversion, 95%, of lignin thiol-ene modification was achieved even by natural sunlight after 4 h of irradiation.