New mechanistic insight into the aerobic oxidation of methylaromatic compounds catalyzed by Co–Mn–Br and its applications

• The aromatic alcohols were found inert to direct oxidation to aromatic aldehydes via Co–Mn–Br catalysts.
• It was found that alcohols could efficiently hinder Co–Mn–Br catalytic oxidation.
• UV–vis spectra indicated that alcohols could interrupt the oxidation from Co(II) to Co(III) in the reaction.
• A new mechanism in the oxidation of methylaromatic compounds catalyzed by Co–Mn–Br was proposed.
• Aromatic alcohols and aldehydes were synthesized highly selectively via aerobic oxidation.

The mechanism for aerobic oxidation of methylaromatic compounds using Co(CH3COO)2–Mn(CH3COO)2–KBr was further studied. The experimental results suggested that the intermediates (aromatic alcohols) are unexpectedly inert to direct oxidation via Co–Mn–Br catalysts under moderate conditions. In addition, it is found that both aromatic and aliphatic alcohols can inhibit the oxidation of the substrates and other intermediates (aromatic esters, aromatic aldehydes). UV–vis spectra indicated that oxidation from Co(II) to Co(III) would be interrupted in the presence of a given amount of alcohol. A plausible mechanism was also proposed. The intermediates analyzed by LC–MS and control experiments showed that aromatic alcohols prefer to undergo esterification rather than being directly oxidized as described in previous studies, and aromatic acetates are subsequently oxidized to corresponding aldehydes. According to the new mechanism, a strategy of highly selectively synthesizing aromatic alcohols and aldehydes from corresponding methylaromatic compounds was designed. The selectivities of aromatic alcohols and aldehydes could also be tuned by the added amount of CH3COOK.

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