Theoretical investigation of the mechanisms of the biphenyl formation in Ni-catalyzed reductive cross-coupling system

The mechanisms of formation of biphenyl in Ni-catalyzed reductive cross-coupling system have been studied using Density Functional Theory calculations. Our calculated results showed that the triplet Ni0 mechanism is more favored than the singlet Ni0 mechanism and the NiI mechanism. The overall catalytic cycle of the favored Ni0 mechanism includes the following basic steps: 1. First oxidative addition (Ni0 → NiII); 2. Reduction (NiII → NiI); 3. Second oxidative addition (NiI → NiIII); 4. Reductive elimination (NiIII → NiI); 5. Catalyst regeneration (NiI → Ni0). The rate-limiting step for the whole catalytic cycle is the second oxidative addition step (NiI → NiIII), where the electronic energy barrier ΔE is 11.91 kcal/mol in the gas phase and the Gibbs free energy in solvent CH3CN ΔGsol is 14.38 kcal/mol computed using the C-PCM method. Our calculated results also indicated that different functional groups in bipyridine style ligands have little effect on the homocoupling mechanisms.