Investigating Competing Inner- and Outer-Sphere Electron-Transfer Pathways in Copper Photoredox-Catalyzed Atom-Transfer Radical Additions: Closing the Cycle

This integrated computational and experimental study comprehensively examines the viability of competing inner-sphere electron transfer (ISET) and outer-sphere electron transfer (OSET) processes in [Cu(dap)₂]⁺-mediated atom-transfer radical additions (ATRA) of olefins and CF₃SO₂Cl that can deliver both R−SO₂Cl and R−Cl products. Five sterically- and electronically-varied representative alkenes were selected from which to explore and reconcile a range of experimentally observed outcomes. Findings are consistent with photoexcited [Cu(dap)₂]⁺ initiating photoelectron transfer via ISET and the subsequent regeneration of the oxidized catalyst via ISET in the ground state to close the catalytic cycle and liberate products. R−SO₂Cl/R−Cl product ratios appear to be primarily governed by the relative rates of direct catalyst regeneration {i.e., [Cu(dap)₂SO₂Cl]⋅⁺+R⋅} and ligand exchange {i.e., [Cu(dap)₂SO₂Cl]⋅⁺+Cl⁻ }. Through this work, a more consistent and more complete conceptual framework has been developed to better understand this chemistry and how catalyst regeneration occurs. It is this important ground state process, which closes the catalytic cycle, and ultimately controls the enantioselectivity of ATRA reactions employing chiral copper photocatalysts.