Solvent effect as the result of frontier molecular orbital interaction. V. Diels-Alder with heterodienophiles: a unified approach to the solvent effect of the Diels-Alder reactions

Abstract

The solvent effect of Diels-Alder (D.A.) reactions with heterodienophiles was measured in several solvents. The rate of the reaction between 2,3-dimethylbutadiene (DMB) and diethylazodicarboxylate increases with the increases in the Acceptor Number of the solvent which behaves as an electrophile. The rate of the reaction between DMB and p. bromonitrosobenzene shows a small solvent effect which correlates with the cohesive pressure (δH2) of the solvent. The rate of the reactions between DMB and tetrachloro- or tetrabromo-o.benzoquinones give an inverse linear relationship with Dπ basicity parameters, thus rate decreases with increases in the nucleophilic character of the solvent..p]These monoparametric correlations (alternatively obtained with electrophilic, nucleophilic or cohesive pressure parameters of the solvent) rationalize the solvent effect of D.A. reactions reported in the literature, which fit one of the above reported three classes. The same result is obtained if the Kamlet-Taft multiparametric equation is applied to the same set of reactions. The main contribution remains that outlined with the monoparametric approach and some secondary effects or some borderline cases are usefully focussed