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.