Low-valent Iridium Enables “Outer-Sphere” Radical–Radical Coupling in Photoinduced Asymmetric Allylic Alkylation

Photochemical catalysis enables challenging enantioselective alkyl–alkyl cross-couplings between allylic alcohols and readily available radical precursors, yet the operative bond-forming mechanism remains elusive. Here we present a comprehensive computational mechanistic study on photoexcited iridium-catalyzed asymmetric Csp3–Csp3 cross-couplings. Contrary to the prevailing “inner-sphere” proposal invoking radical capture by an Ir(II)(allyl) species followed by reductive elimination from an alkyl–Ir(III)–allyl intermediate, our calculations identify a distinct low-valent pathway governed by an Ir(I/III/II/I) catalytic cycle. Photoexcitation of the (η3-allyl)–Ir(III) complex triggers ligand-to-metal charge transfer to generate an Ir(II)–(allyl radical) species in situ. Subsequent single-electron transfer (SET) with the radical precursor produces a chiral Ir(I)–(allyl radical) intermediate together with a transient alkyl radical, which undergoes “outer-sphere” radical–radical coupling at the Ir(I)–bound allyl radical to deliver the enantioenriched product. Structural characterization and spin density analysis support the involvement of the Ir(II)– and Ir(I)–(allyl radical) intermediates and substantiate the radical–radical bond-forming event. These findings establish a mechanistic framework for photoexcited iridium catalysis, mapping an Ir(I/III/II/I) catalytic cycle and highlighting low-valent Ir(I)–(allyl radical) species as a strategic platform for enantioselective radical interception in asymmetric Csp3–Csp3 cross-couplings.