Valorization of Refined Lignin Fractions as Green Promoters in Photochemical Atom Transfer Radical Addition Reactions

Herein, we report a novel green photochemical procedure for the direct functionalization of terminal olefins with radical precursors promoted by technical lignin architectures. Spruce Kraft lignin was initially subjected to solvent refining to achieve fractions presenting a controlled molecular weight and fully characterized structural features. The obtained fractions were then tested as photoinitiators for organic transformations under visible light irradiation. The results demonstrated that a low molecular weight refined lignin, namely, L-EtOAc, was able to successfully promote relevant photoinduced radical reactions, providing several densely functionalized alkyl halides (14 examples, up to 98% yield). Mechanistic investigations were also performed to understand the role of lignin architecture in the reaction pathway. This work proposes an innovative role for lignin in organic chemistry, laying the basis for a brand-new field for the valorization of cheap technical lignocellulosic byproducts, while opening new exciting scenarios concerning green photochemical processes.