Biocatalytic Metal Hydrogen Atom Transfer for Asymmetric Olefin-Olefin Coupling.

Metal-mediated hydrogen atom transfer (MHAT) enables versatile radical hydrofunctionalization reactions, yet asymmetric variants, especially for olefin-olefin coupling, remain unknown with small-molecule catalysts. Here we report an enzymatic solution using engineered protoglobins, establishing a biocatalytic platform for asymmetric MHAT-triggered intramolecular cyclization. Directed evolution of a protoglobin furnished a variant that catalyzes 5-exo-trig radical cyclizations to generate 3,4-substituted pyrrolidines in high yields and up to 98:2 e.r. The reaction tolerates diverse donor and acceptor substituents, including esters, amides, nitriles, nitro groups, imines, and ketones. Mechanistic experiments and DFT calculations support a radical pathway with an ultrafast enantiodetermining radical cyclization. These results show that engineered heme proteins can support new-to-nature MHAT chemistry and exert stereocontrol over rapid radical steps, providing a powerful system for asymmetric MHAT reactions.