Kinetic isotope effect probe for the π‐facial stereoselectivity in nucleophilic additions to cyclohexanone: A theoretical study

Abstract

AbstractThe origin of the π‐facial selectivity in nucleophilic additions to cyclohexanone was analysed by means of ab initio MO calculations (MP2/6–31G*//HF/6–31G*). The calculations showed that BH3 attacks at the axial side whereas MeLi attacks at the equatorial side. Calculated kinetic and equilibrium deuterium isotope effects and the structures of the transition states (TSs) demonstrated that four factors, stabilization of the axial TS by antiperiplanar allylic bonds, destabilization of the equatorial TS by torsional strain, destabilization of the axial TS by steric hindrance between a nucleophile and the 3,5‐axial hydrogens and destabilization due to the conformational deformation of the ring in both the equatorial and axial TSs, operate in the same direction and can bring about the apparent selectivity changeover. The magnitudes of anti‐D KIEs for the MeLi and the EtLi additions to acetone were consistent with the Anh‐Eisenstein model rather than the Cieplak model.