A tt Molecular Orbital Analysis of the Diels-Alder Reaction

17 A TT Molecular Orbital Analysis of the Diels-Alder Reaction 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective. Chemical experience, such as the observation that substituents that increase the reactivity of a dienophile tend to be those that attract electrons, suggests that electrons flow from the diene to the dienophile during the reaction. Thus, the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile. As shown in Mechanism 10.4 for the case of ethylene and 1,3-butadiene, the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap in phase with each other. Cycloaddition of a diene and an alkene is said to be a symmetry-allowed reaction. 

Contrast the Diels-Alder reaction with a reaction that looks superficially similar, the cycloaddition of two ethylene molecules to give cyclobutane. 

Reactions of this type are rare and seem to proceed in a stepwise fashion rather than by way of a concerted mechanism involving a single transition state. 

---

A TT MOLECULAR ORBITAL ANALYSIS OF THE DIELS-ALDER REACTION... 

---