Cobalt-Schiff Base Complex Catalyzed Oxidation of Para-Substituted Phenolics. Preparation of Benzoquinones

Article abstract of DOI:10.1021/jo00113a020

Para-substituted phenolics, serving as models for lignin (a renewable source of carbon), are oxidized to the corresponding benzoquinone with oxygen in the presence of catalytic amounts of Co-Schiff base complexes.The reaction products observed depend on the structure of the catalyst.The 5-coordinate catalysts (pyridine)cobalt <(pyr)Co(salen)> and amine>cobalt convert syringyl alcohol (3,5-dimethoxy-4-hydroxybenzyl alcohol) to 2,6-dimethoxybenzoquinone in high yield.In contrast, syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde) is unreactive toward these catalysts.However, the 4-coordinate Co(salen) converts syringaldehyde to 2,6-dimethoxybenzoquinone in 72percent isolated yield.Phenols bearing a single methoxy group on the ring are unreactive toward any catalyst in MeOH.However, vanillyl alcohol (3-methoxy-4-hydroxybenzyl alcohol) is converted to 2-methoxybenzoquinone with Co(N-Me salpr) and oxygen in 43percent yield in CH2Cl2, and 58percent yield in CH2Cl2 in the presence of 1percent CuCl2.The success of the oxidations appears to be related to the ease of removal of the phenolic hydrogen by the Co/O2 complex.Competitive deactivation of the catalyst occurs with substrates of lower reactivity.