Comparison of a series of laccase mediators in the electro-oxidation reactions of non-phenolic lignin model compounds

The electro-oxidations of non-phenolic lignin model compounds in an electrolytic mediator system (EMS) have been investigated with several laccase mediators, including N-hydroxyphthalimide (NHPI), 1-hydroxybenzotriazole (HBT), violuric acid (VLA), 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS), under the same reaction conditions. In the bulk electrolyses of the monomeric model compound [1-(4′-ethoxy-3′-methoxyphenyl)ethanol (1G)], oxidation with NHPI gave the corresponding CαO product (2G) in high yield, whereas the oxidations with HBT, VLA, and TEMPO afforded 2G in moderate yields. The highest reaction selectivity for the guaiacyl-units was found in the oxidation conducted in the presence of ABTS, although the yield was low. In the bulk electrolyses of the dimeric model compound [4-ethoxy-3-methoxyphenylglycerol-β-guaiacyl ether (3G)], the oxidation with NHPI gave the corresponding CαO product (4G) in high yield, whereas the oxidations with HBT, VLA, and TEMPO gave 4G in low yields. In contrast, the oxidation with ABTS gave a Cα–Cβ cleavage product (5G) in 5.5% yield. The selectivity of the mediators in the EMS reaction effectively reflected the mechanisms of their reactions, as reported for the laccase mediator system. NHPI was confirmed to be the best mediator in the present system for the selective Cα-carbonylation of the non-phenolic β-O-4 structures in lignin.