Comparison of Copper and Vanadium Homogeneous Catalysts for Aerobic Oxidation of Lignin Models

Abstract

The reactivity of copper and vanadium catalysts toward the aerobic oxidation of lignin models has been explored. Both (dipic)VV(O)(OiPr) (3) (dipic = dipicolinate) and CuCl/TEMPO (TEMPO = tetramethylpiperidine N-oxide) catalyzed the aerobic oxidation of the lignin model compound 1,2-diphenyl-2-methoxyethanol (2). The vanadium catalyst 3 produced benzoic acid (85%) and methyl benzoate (84%) as the major products via the intermediate ketone benzoin methyl ether (4). The copper catalyzed reaction afforded benzaldehyde (84%) and methylbenzoate (88%) directly, with no intermediate formation of 4. The more complex lignin model system 1-(3,5-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol-[2,3-13C2] (5-13C2) was oxidized under air by vanadium catalyst 3, affording ketone 7-13C2 (65%), dehydrated ketone 8-13C2 (5%), alkene product 9-13C2 (14%), 3,5-dimethoxybenzoic acid (11%), 3,5-dimethoxybenzaldehyde (2%), 2-methoxyphenol, and formic acid-13C1 (4%). Aerobic oxidation of ketone 7-13C2 using catalyst 3 prod...