Notably, we also proposed a Ni(I)H-involved mechanistic variant. However, the high energy barrier (IN7'→TS7', 31.0 kcal/mol; see Supplementary Figure ) for β-H elimination on a stable six-membered nickelacycle made this variant relatively less probable. In addition, DFT calculations were also carried out to explain the formation of the unexpected linear product 5fa at low reaction temperature (Supplementary Figure ).