EXTENSION OF A PERTURBATION MOLECULAR-ORBITALS MODEL TO THE STUDY OF REGIOSELECTIVITY IN 1,3-DIPOLAR CYCLOADDITIONS - REACTIONS OF 3,5-DICHLORO-2,4,6-TRIMETHYLBENZONITRILE OXIDE WITH HETEROAROMATIC SYSTEMS

A model which makes use of the complete perturbation treatment of Salem and Devaquet has already been applied to Diels-Alder reactions and is now proposed for the interpretation of regioselectivity in the 1,3-dipolar cycloadditions of 3,5-dichloro-2,4,6-trimethylbenzonitrile oxide with some heteroaromatic systems. The van der Waals minima relevant to the hypothesized alternative reaction paths have been suggested as model structures (perturbed states) for the corresponding weakly interacting complexes in the early stages of the reaction. To evaluate the atom-atom non-bonded interactions between polar partners an explicit Coulombic term is added to the Lennard-Jones (6-12) function previously used. Moreover, because of the dominant contribution of non-interacting atoms in determining the van der Waals minima, the minimization procedure is constrained to the region of the non-bonding energy hypersurface where the 2p-pi-orbitals involved in the new bond formation overlap significantly. The obtained regioselectivities are in good agreement with experimental ones.