Visible-Light Induced Thiol–Ene Reaction on Natural Lignin

Abstract

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)3Cl2, 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)3Cl2 was the most efficient, resulting in conversions of 97% with 2.5 mol % catalyst loading. The Ru(bpy)3Cl2 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 p...