Interpretation theorique de la reaction de diels-alder dans la serie des dienes fonctionnels dissymetriquement substitues-II : Etude des mecanismes de reaction par calcul mindo des etats de transition

Abstract

The different behaviour of two 1- or 2-ethoxy butadiene isomers with respect to acrolein in the Diels-Alder reaction is explained on the basis of reaction mechanisms. Using MINDO/2 calculations, various models of the transition state are investigated and geometric parameters for each reaction path coordinate are optimized. The choice of a model describing the transition state is given by the corresponding potential barrier height, which characterizes the approach of reagents and the site of the functional of the diene. Its lowest value corresponds to a concerted reaction mechanism with acrolein; a synchronous mechanism is found for 2-ethoxy butadiene, and a two-stage one for 1-ethoxy butadiene. The variation in barrier height in the two systems is well correlated with the corresponding change in the observed reaction rate.