Origin of exo selectivity in norbornene. An ab initio MO study

The geometries of the reactants, the reactant π‐complex, the transition state and the product for exo and endo addition of BH3 to norbornene were optimized at the HF/3–21G level, and their energies were calculated at the MP2/6–31G level. The analysis of activation energy and model calculations singled out the endo deformability, a major portion of the norbornene deformation energy, as the origin of exo selectivity; norbornene, with the olefinic hydrogens already bent toward endo in the equilibrium geometry, requires less deformation to reach the exo than the endo transition state. The reason why olefinic hydrogens are bent in norbornene has been elucidated. An analysis indicates that the difference between norbornene exo side and bicyclo [2.2.1] hex‐2‐ene is also dictated by the endo deformability.