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
作者:Boris S. El'yanov、Svetlana K. Shakhova、Boris D. Polkovnikov、Lev F. Rar
DOI:10.1039/p29850000011
日期:——
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 transitionstate in Diels–Alderreactions. 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
The reactions of (E)-1-methoxy-1,3-butadiene (1) and 1,1-dimethoxy-1,3-butadiene (2) with a series of dienophiles of increasing electrophilicity are described Stereochemical studies reveal that the cycloadditions of 1 are concerted processes, even for the most electron-deficient olefins dimethyl dicyanofumarate and dimethyl dicyanomaleate. 1,1-Dimethoxy-1,3-butadiene reacts under our conditions (dilute solutions and temperatures less than or equal to 60 degrees C) only with those dienophiles which can give zwitterions out of the antiperiplanar conformation of the diene. Zwitterionic intermediates can be trapped by methanol. In the case of tetracyanoethene the kinetics of decay of an intermediate, interpreted as the zwitterion, can be followed by stopped flow techniques: E(a) = 14.8 +/- 0.2 kcal mol(-1), log A = 11.9 +/- 0.1, Delta H double dagger = 10.8 +/- 0.1 kcal mol(-1), Delta S double dagger = -6.2 +/- 0.1 cal mol(-1) K-1, and Delta G double dagger = 11.40 +/- 0.03 kcal mol(-1).