The RHF– and UHF–AM1 potential-energy surfaces for the diels–alder reaction of substituted butadienes with substituted ethenes. Part 58.—Determination of reactivity by MO theory

Abstract

The potential-energy surfaces for the Diels–Alder reactions between substituted butadienes and ethenes have been investigated by intensive RHF- and UHF-grid searches using the AM1 method. The RHF/AM1 potential surface show a concerted, synchronous path, but the UHF/AM1 potential surface has four transition states, two intermediates and two maxima with two stepwise, diradical reaction paths. The diagonal, concerted path in the UHF/AM1 potential surface vanishes owing to the coalescence of two maxima induced by the two stable intermediates on the two-dimensional contour diagram. Substituent effects on the activation barriers are of electronic and steric origin, and only the UHF/AM1 results agreed with experiments.