Theoretical Mechanistic Study of C(sp3)-C(sp2) Cross-Coupling by Photoredox-Mediated Iridium(III)/Nickel(II)/Quinuclidine Triple Catalysis

The photoredox-mediated iridium(III)/nickel(II)/3-acetoxyquinuclidine triple-synergistic catalysis was comprehensively investigated by taking a C(sp(3))-C(sp(2)) bond cross-coupling as a reaction model using density functional theory (DFT) calculations. The synergistic mechanism of the triple catalytic system includes a reductive quenching cycle (Ir-III-*Ir-III-Ir-II-Ir-III), an organocatalytic cycle, and a nickel catalytic cycle (Ni-II-Ni-I-Ni-III-[Ni-III](circle minus)-Ni-II). Electronic process analysis shows that 3-acetoxyquinuclidine acts as a hydrogen atom transfer (HAT) catalyst to regioselectively provide alpha-carbon centered radical. Due to more favorable oxidative addition of C-Br to Ni(I) than HAT to avoid the formation of stable Ni(II) species, the generated alpha-carbon centered radical prefers to be captured by oxidative addition product Ni(III) to form an unusual [Ni-III]C-circle minus(circle plus) species when 3-acetoxyquinuclidine was employed. These theoretical insights not only provide deep electronic process understanding of the photoredox-mediated iridium(III)/nickel(II) synergistic catalysis, but also clarify the electron-withdrawing group effect of quinuclidine, which has a potential guiding role for further development of new cross-coupling reactions.