On the choice of the perturbed state for PMO prediction of selectivities in cycloaddition reactions

A model which makes use of the complete perturbative treatment of Salem and Devaquet is proposed for the interpretation of regio-, sito- and stereo-selectivity of cycloaddition reactions. The van der Waals intermolecular interaction potential-energy surface minima relevant to the hypothesized alternative reaction paths are suggested as model structures (perturbed states) for the corresponding weakly interacting complexes in the early stages of the reaction. It is shown how results are affected by the increase in the degrees of freedom for the approach of the two interacting molecules, by the choice of a particular atom-atom potential and by the two-partner internal-geometry relaxation which occurs at the perturbed state. The results obtained for some sample reactions (butadiene and ethylene, penta-1,3-diene and acrolein, penta-1,3 -diene and 2,5-dimethyl-1,4-benzoquinone) corroborate our previous choice of a Lennard-Jones function and of an undistorted reagent geometry to localize the non-bonding minima. The inclusion of a greater number of intermolecular variables and their simultaneous optimization during the non-bonding minima localization is a marked improvement for the definition of the perturbed state. The results obtained for the reactions of mono- and di-substituted dienes with unsymmetrically substituted quinones confirm the predictive ability of our improved procedure. © 1990.