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

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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-ethoxy-3-methoxyphenylglycerol-β-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 Cα–Cβ cleavage and Cα-carbonylation proceeded efficiently in 0.1 M LiClO 4 /70%-CH 3 CN/H 2 O, whereas Cα-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 Cα–Cβ cleavage and Cα-carbonylation in 0.1 M LiClO 4 /70%-CH 3 CN/H 2 O. 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.