Mechanochemical Oxidation and Cleavage of Lignin β-O-4 Model Compounds and Lignin

Abstract

A mechanochemical oxidation and cleavage reaction in lignin β-O-4 model compounds and lignin catalyzed by HO–TEMPO/KBr/Oxone (TEMPO is 2,2,6,6-tetramethyl-1-piperidinyloxy) has been developed under milling conditions. The studies on nonphenolic lignin β-O-4 model compounds led to selective oxidations of the benzylic hydroxyl groups. Subjecting phenolic lignin model compounds to the oxidative conditions in a ball mill initiated aryl–Cα bond cleavage reactions leading to the formation of the corresponding quinones and phenol derivatives. Transferring the mechanochemical protocol to lignin resulted in the simultaneous oxidation and cleavage of bonds with varied selectivity for monomeric products. Finally, a scale-up approach of the oxidative procedure by using vibrating disc mill technology enabled the mechanochemical protocol to be applied in gram-scale batch reactions under reduced milling time, while affording a similar extent of oxidation.