TEMPO-mediated electro-oxidation reactions of non-phenolic β-O-4-type lignin model compounds

Oxidation reactions of a non-phenolic beta-O-4-type lignin model compound [4-ethoxy-3-methoxy-phenylglycerol-beta-guaiacyl ether (1)] were conducted in an electrolytic mediator system (EMS) using 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (TEMPO) or 4-acetoamido-TEMPO (AcNH-TEMPO) as mediators. The results revealed that the electrolytes strongly influenced the chemoselectivity between C alpha-carbonylation and C gamma-carboxylation of compound 1. The C alpha-carbonylation proceeded preferentially in the EMS oxidation conducted in the LiClO4/CH3CN-H2O system, although the yield of the resulting C alpha-carbonyl compound 2 was low (1.9%-11.1%). In contrast, C gamma-carbonylation was preferred, if the EMS oxidation reactions were conducted in the dioxane-carbonate buffer (pH 10.0), dioxane-borate buffer (pH 10.0), and dioxane-phosphate buffer (pH 7.0). Especially, the C gamma-carboxyl compound 3 was obtained in a high yield (72.0%-93.2%), when the dioxane-phosphate buffer served as the electrolyte. The catalytic efficiency of AcNH-TEMPO was higher than that of TEMPO for the EMS oxidation reactions. The conclusion is that AcNH-TEMPO-EMS oxidation reaction in dioxane-phosphate buffer could be used for the chemoselective C gamma-carboxylation of lignins.