1,3-Dipolar cycloaddition reactions:: A DFT and HSAB principle theoretical model

The hard and soft acids and bases principle is used together with the condensed fukui function to analyze the regioselectivity and reactivity of two model 1,3-dipolar cycloaddition reactions. Results obtained for benzonitrile oxide with vinyl p-nitrobenzoate or 1-acetyl vinyl p-nitrobenzoate illustrate the value of these concepts to describe their inherent reactivity. The calculations of the interaction energy by density functional theory using a perturbative, orbital independent method suggest the specific direction of the electronic process at each of the reaction sites. The electrophilic nature of the 1,3-dipole and the nucleophilic nature of the two dipolarophiles was determined by this model. The partitioning of the interaction energy in a term resulting from the chemical potential equalization principle at constant external potential and a term resulting from the maximum hardness principle at constant chemical potential show that the former term, arising from the charge-transfer process, contributes to a lesser extent than the term arising from the reshuffling of the charge distribution. The use of density functional theory concepts and the hard and soft acids and bases principle is a promising alternative to frontier orbital theory.