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Solvent‐Promoted Oxidation of Aromatic Alcohols/Aldehydes to Carboxylic Acids by a Laccase‐TEMPO System: Efficient Access to 2,5‐Furandicarboxylic Acid and 5‐Methyl‐2‐Pyrazinecarboxylic Acid

Laccase coupled with 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO) is a well‐known catalytic system for the oxidation of alcohols to the carbonyl compounds. In this work, a simple yet effective solvent engineering strategy is developed to enable aromatic alcohols/aldehydes to be efficiently oxidized to carboxylic acids by a laccase‐TEMPO system. Citrate buffer (100–300 mm, pH 6) rather than the widely used acetate buffer (pH 4–5) proves to be optimal for this purpose. The roles of citrate are discussed in laccase‐TEMPO‐catalyzed synthesis of carboxylic acids. 5‐Hydroxymethylfurfural (HMF) of 200 mm is oxidized to 2,5‐furandicarboxylic acid (FDCA), a top‐value biobased chemical in the polymer industry, in 28 h with a yield of up to 97%. Of the 18 substrates examined, 11 alcohols are converted to target carboxylic acids with >90% yields by this method, with 2 alcohols giving >60% yields of carboxylic acids. Gram‐scale preparation of FDCA and 5‐methyl‐2‐pyrazinecarboxylic acid, an important drug intermediate, is demonstrated with total turnover numbers up to 110 000 for laccase and space‐time yields up to 1.0 g/(L•h). This chemoenzymatic approach may be a promising alternative to traditional chemical oxidations for the synthesis of carboxylic acids.