Engineering non-haem iron enzymes for enantioselective C(sp³)–F bond formation via radical fluorine transfer

In recent years there has been a surge in the development of methods for the synthesis of organofluorine compounds. However, enzymatic methods for C–F bond formation have been limited to nucleophilic fluoride substitution. Here we report the incorporation of iron-catalysed radical fluorine transfer, a reaction mechanism that is not used in naturally occurring enzymes, into enzymatic catalysis for the development of biocatalytic enantioselective C(sp³)–F bond formation. Using this strategy, we repurposed (S)-2-hydroxypropylphosphonate epoxidase from Streptomyces viridochromogenes (SvHppE) to catalyse an N-fluoroamide-directed C(sp³)–H fluorination. Directed evolution has enabled SvHppE to be optimized, forming diverse chiral benzylic fluoride products with turnover numbers of up to 180 and with excellent enantiocontrol (up to 94% enantiomeric excess). Mechanistic investigations showed that the N–F bond activation is the rate-determining step, and the strong preference for fluorination in the presence of excess NaN₃ can be attributed to the spatial proximity of the carbon-centred radical to the iron-bound fluoride.