Ground-State Electron Transfer as an Initiation Mechanism for Asymmetric Hydroalkylations in Radical Biocatalysis

Stereoselective bond-forming reactions are essential tools in modern organic synthesis. However, catalytic strategies for controlling the stereochemical outcome of radical-mediated C–C bond formation remain underdeveloped. Here, we report an ‘ene’-reductase catalyzed asymmetric hydroalkylation of olefins using α-bromoketones as radical precursors. In these reactions, radical initiation occurs via ground-state electron transfer from the flavin cofactor located within the enzyme active site, representing a mechanistic departure from previous photoenzymatic hydroalkylations. Four rounds of site saturation mutagenesis based on wild-type nicotinamide-dependent cyclohexanone reductase (NCR) were deployed to access a variant capable of catalyzing a cyclization to furnish β-chiral cyclopentanones with high levels of enantioselectivity. Additionally, the wild-type NCR was identified that could catalyze the intermolecular coupling with precise stereochemical control over the radical termination step. This report demonstrates this enzyme family’s catalytic versatility and highlights the opportunity for protein engineering to address reactivity and selectivity challenges in radical biocatalysis.