What is the reason for the difference in Diels-Alder reactivity of isomeric substituted dienes? A case involving 1,3-dienylboronates

Semi-empirical and ab initio calculations were run to account for the differences in reactivity of 1,3-dienylboronates 1, (E)-2 and (Z)-2 in the Diels-Alder cycloaddition with methyl acrylate. The results show that the s-cis conformation is accessed from the most stable s-trans, as easily for the unreactive (Z)-2 isomer as it is for the reactive (E)-2 isomer and for 1. Furthermore, the diene geometries and pi molecular orbitals in the s-cis conformation are not sufficiently affected by (Z) versus (E) substitution to explain the experimental observations. These results lead to the conclusion that variations in diene reactivity are most probably governed by steric repulsions in the Diels-Alder transition state in the case of (Z)-2.