Theoretical Study of the Effects of Structure and Substituents on Reactivity in Allylboration

To clarify the structural factors controlling the reactivity of allylboron derivatives in allylboration, we have carried out a theoretical analysis by applying the ab initio MO method. Electrophilicity of the boron center in several types of allylboron models has been estimated by projecting out the unoccupied reactive orbital that has the maximum amplitude on a boron p-type atomic orbital chosen specifically. Two important factors, the electron-accepting level and vacancy of the orbital of the reaction site, have been included for the purpose of comparing the relative reactivity of those molecules. The theoretically estimated electrophilicity of the boron center is mostly in agreement with experimental observations for allylborons with different chiral auxiliary structures, indicating that electron delocalization from the oxygen of an attacking aldehyde to the boron of an allylboron reagent dominates the reaction. The activation energies of the reaction have been calculated by locating the complex and the transition states on the potential surfaces to compare with the experimental results and the theoretically estimated electrophilicity.