Electro-oxidation of lignin dimer model compounds bearing β−O−4, β−5, and β−β linkages and synthetic lignin (guaiacyl-type dehydrogenation polymer [G-DHP]) in the presence of different mediators previously used in the laccase mediator system (LMS), that is, promazine hydrochloride (PZH), N-hydroxyphthalimide (NHPI), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), was investigated. The characteristics of the electro-oxidations of the lignin dimer model compounds were reflected in the electro-oxidation of G-DHP. The three mediators effectively mediated the electro-oxidation of G-DHP, although they showed different reaction selectivities toward the three major lignin linkages of G-DHP. Specifically, PZH preferentially mediated the oxidation of β−O−4 and β−β linkages over β−5 linkages, NHPI preferred β−O−4 linkages over β−5 and β−β linkages, and ABTS preferred β−O−4 and β−β linkages over β−5 linkages under the present conditions. NHPI and ABTS tended to mediate Cα−Cβ cleavage in contrast to PZH, and the three mediators all mediated the conversion of coniferyl alcohol end-units.Consequently, PZH, NHPI, and ABTS were found to be promising mediators for the electrolytic mediator system of lignin.

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 Cαcarbonylation 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.