KINETIC ISOTOPE EFFECT PROBE FOR THE PI-FACIAL STEREOSELECTIVITY IN NUCLEOPHILIC ADDITIONS TO CYCLOHEXANONE - A THEORETICAL-STUDY

The origin of the sc-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 BH, attacks at the axial side whereas MeLi attacks at the equatorial side, Calculated kinetic acid 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.