Electro-Oxidation of Lignin Model Compounds and Synthetic Lignin with Transition-Metal Complexes (Manganese and Iron Complexes)

Transition-metal complexes Mn(acac)2, Mn(bpy)3, Fe(acac)2, and Fe(bpy)3 were investigated as mediators in lignin oxidation with an electrolytic mediator system (EMS). Fe(bpy)3 was screened among them as a mediator in the bulk electrolysis of a lignin dimer model compound (4-ethoxy3-methoxyphenylglycerol-beta-guaiacyl ether: 3G) and guaiacyl-type dehydrogenation polymer (G-DHP) by experiments using the lignin monomer model compound 1-(4-ethoxy-3-methoxyphenyl)ethanol. In Fe(bpy)3-mediated electro-oxidations of compound 3G, both C alpha-C beta cleavage and C alpha-carbonylation proceeded efficiently in 0.1 M LiClO4/70%-CH3CN/H2O, whereas C alpha- carbonylation predominated in 10%-dioxane/0.1 M acetate buffer (pH 4). Fe(bpy)3-mediated electro-oxidation of G-DHP also proceeded efficiently via both C alpha-C beta cleavage and C alpha-carbonylation in 0.1 M LiClO4/70%-CH3CN/ H2O. The oxidation was promoted by the addition of 2,6-lutidine as a base. By contrast, the Fe(bpy)3-mediated electro-oxidations of G-DHP did not proceed efficiently in 10%-dioxane/0.1 M acetate buffer (pH 4) with or without 2,6-lutidine under the present conditions. It was reconfirmed that electrolyte was also one of the important factors in lignin oxidation with EMS. Consequently, Fe(bpy)3 was found to be a promising mediator of lignin EMS for lignin modification and degradation.