Enantioselective Alkene Azidooxygenation by Direct Visible-Light Excitation of an Engineered Ene-Reductase

While radical azidooxygenation provides a direct route to valuable β-oxy azides, an asymmetric catalytic version remains elusive. Distinct from the previous biocatalytic strategies introducing azide moiety, here, we report a net-oxidative photoenzymatic strategy that leverages direct visible-light excitation of ene-reductases (EREDs) to achieve the highly enantioselective azidooxygenation of alkenes. This system utilizes the photoexcited flavin cofactor as a potent oxidant that, in concert with a TEMPO⁺-azide complex, initiates the generation of azide radicals. The ensuing N₃ radical and O-centered radicals are then orchestrated within the active site of the engineered enzyme to furnish enantioenriched β-oxy azides in excellent yields (up to 89%) and enantioselectivity (up to 99.5:0.5 e.r.). Mechanistic studies suggest the multifunctional role of 4-AcNH-TEMPO, which facilitates azide radical generation, stereoselectively traps the prochiral carbon-centered radical, and completes the catalysis as the terminal oxidant. Computational simulations further elucidate the origin of the stereocontrol.