Studies of the transition-state structure by the method of volumetric steric effects. Part 4. Transition state in Diels–Alder reactions of (E)-1-alkyl(alkoxy)-buta-1,3-dienes with alkyl acrylates

Abstract

A method of studying transition-state structures by analysis of volumetric steric effects is proposed. The method is used to reveal features of the steric structure of the transition state in Diels–Alder reactions. The structure has been found to change from one approximating the structure of the prereactional complex to that resembling the half-chair adduct as the diene substituent changes from methyl to t-butyl. As this takes place, no intermediate half-boat adduct is formed. An increase in steric hindrance during the change in the substituent is regarded as the driving force responsible for the transition-state shift along the reaction co-ordinate. This is consistent with the Hammond postulate. The CO and CC groups of the dienophile in the endo-transition state are in the s-cis-conformation.