Enzymatic Baeyer–Villiger Oxidation of Benzaldehydes

Abstract

AbstractThe selectivity of the chemical Baeyer–Villiger oxidation of benzaldehydes depends on steric and electronic factors, the type of oxidizing agent and the reaction conditions. Here we report on the enzymatic Baeyer–Villiger oxidation of fluorobenzaldehydes as catalyzed by the flavoprotein 4‐hydroxyacetophenone monooxygenase (HAPMO). HAPMO was most active with 4‐amino‐ and 4‐hydroxybenzaldehydes. With these compounds significant substrate inhibition occurred. Monofluoro‐ and difluorobenzaldehydes were readily oxidized by HAPMO without substrate inhibition. 19F NMR analysis revealed that 4‐fluoro‐, 2,6‐difluoro‐, 3,4‐difluoro‐, 2‐fluoro‐4‐hydroxy‐ and 3‐fluoro‐4‐hydroxybenzaldehyde were quantitatively converted by HAPMO to the corresponding fluorophenyl formates. These products spontaneously hydrolyzed to fluorophenols. The HAPMO‐mediated conversion of 2‐fluoro‐, 3‐fluoro‐, 2,3‐difluoro‐ and 2,4‐difluorobenzaldehyde yielded, besides fluorophenols, also minor amounts of fluorobenzoic acids. The high preference of HAPMO for the production of fluorophenols is in disagreement with the rule derived from chemical studies that electron‐poor benzaldehydes form mainly benzoic acids. This suggests that interactions of the benzaldehyde substrates with amino acids and/or the flavin cofactor in the enzyme active site influence the selection of the migratory group in favor of the phenyl ring.