DFT-HSAB Prediction of Regioselectivity in 1,3-Dipolar Cycloadditions: Behaviour of (4-Substituted)benzonitrile Oxides towards Methyl Propiolate

The regioselectivity of 1,3-dipolar cycloadditions between (4-substituted)benzonitrile oxides and methyl propiolate cannot be rationalized on the basis of the electron demand of the reactants or frontier molecular-orbital theory. To this problem, we have applied a quantitative formulation of the hard-soft acid-base principle developed within the density functional theory. Global and local reactivity indices were computed at B3LYP/6-311+G(d,p) level. The details of charge transfer upon the reactive encounter have been elucidated, and the computed regioselectivity has been shown to be in good agreement with experimental data.