How Racemic Secondary Alkyl Electrophiles Proceed to Enantioselective Products in Negishi Cross-Coupling Reactions

Two mechanisms, namely, the Ni(0)-Ni(II) and Ni(I) -Ni(III) mechanisms, for nickel-bis(oxazolinyl)pyridine complex catalyzed Negishi cross-coupling reaction were investigated with density functional calculations. The Ni(I)-Ni(III) mechanism, containing sequential steps of transmetalation-oxidative addition-reductive elimination, is more favorable than the Ni(0)-Ni(II) mechanism, based on the energetic span model. The enantioselectivity of the coupled product from a racemic secondary alkyl electrophile was calculated by the relative reaction rate (r(s)/r(R)) of the reductive elimination step that forms the coupled product in the S-enantiomer over that leading to the R-enantiomer. The r(s)/r(R) can be calculated from the relative free energy of the transition states for these two reductive elimination pathways in the Ni(I)-Ni(III) mechanism. The calculated enantioselectivity for the model reaction is consistent with the experimental report. The influence of the asymmetric steric hindrance of the catalyst ligand on the reductive elimination step is also discussed.