19F NMR study on the biological Baeyer-Villiger oxidation of acetophenones

The biological Baeyer-Villiger oxidation of acetophenones was studied by 19F nuclear magnetic resonance (NMR). The 19F NMR method was used to characterise the time-dependent conversion of various fluorinated acetophenones in either whole cells of Pseudomonas fluorescens ACB or in incubations with purified 4'-hydroxyacetophenone monooxygenase (HAPMO). Whole cells of P. fluorescens ACB converted 4'-fluoroacetophenone to 4-fluorophenol and 4'-fluoro-2'-hydroxyacetophenone to 4-fluorocatechol without the accumulation of 4'-fluorophenyl acetates. In contrast to 4-fluorophenol, 4-fluorocatechol was further degraded as evidenced by the formation of stoichiometric amounts of fluoride anion. Purified HAPMO catalysed the strictly NADPH-dependent conversion of fluorinated acetophenones to fluorophenyl acetates. Incubations with HAPMO at pH 6 and 8 showed that the enzymatic Baeyer-Villiger oxidation occurred faster at pH 8 but that the phenyl acetates produced were better stabilised at pH 6. Quantum mechanical characteristics explained why 4'-fluoro-2'-hydroxyphenyl acetate was more sensitive to base-catalysed hydrolysis than 4'-fluorophenyl acetate. All together, 19F NMR proved to be a valid method to evaluate the biological conversion of ring-substituted acetophenones to the corresponding phenyl acetates, which can serve as valuable synthons for further production of industrially relevant chemicals.