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

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 produced dehydrated ketone 8-13C2, 3,5-dimethoxybenzoic acid, and formic acid-13C1, suggesting that 7 is further oxidized under the catalytic conditions. In contrast, oxidation of β-O-4 model 5-13C2 using CuCl/TEMPO affords 3,5-dimethoxybenzaldehyde (43%), 3,5-dimethoxybenzoic acid (13%), 2-methoxyphenol (7%), formic acid-13C1 (7%), ketone-7 13C2 (1%), dehydrated ketone 8-13C2 (2%), and a number of higher molecular weight products, as determined by 1H and 13C NMR, GC-MS, and LC-MS. Attempted oxidation of ketone 7 using CuCl/TEMPO yielded primarily dehydrated ketone 8, indicating that the ketone is not an intermediate in the formation of the aldehyde product. The reactivities of the copper and vanadium catalysts in the oxidation of lignin model compounds 2 and 5 are discussed. Remarkably different selectivities were observed for the vanadium and copper catalyzed reactions, suggesting the potential of homogeneous catalysts for controlling selectivity in the aerobic oxidation of lignin.