1,3,4-oxadiazoles as dienes in Diels-Alder reactions studied with AM1 semiempirical and hybrid density functional methods. Are 1,3,4-oxadiazoles practical synthones for the preparation of valuable organic materials

AMI semiempirical and B3LYP/6-31G(d)/AM1 DFT computational studies were performed with the aim to determine which 1,3,4-oxadiazoles would participate in Diels-Alder reactions as a diene and under which conditions. Variously substituted 1,3,4-oxadiazoles were selected for this study. Their reactivity was compared with cyclopentadiene since it is a traditional diene for the Diels-Alder reaction. The reactivity was evaluated through the frontier molecular orbital (FMO) energy gap between the pair of reactants, the energy changes in the FMO necessary to bring reactants to the transition state, and through activation barriers for cycloaddition. The stability of the cycloadduct was investigated and in some cases, a second addition of a dienophile was explored. The results were compared with experimental observations, and in this way, the reliability of the chosen computational approach was tested. All of our computational conclusions were in agreement with experimental observations for the reactivity of 1,3,4-oxadiazoles. (C) 1998 Elsevier Science B.V. All rights reserved.