Diels Alder Reaction: MOs

This animation of a Diels-Alder reaction between 2,4-hexadiene (diene) and acrolein (dienophile) demonstrates how molecular orbital interactions guide reactivity and stereoselectivity.

1. The HOMO of the diene is displayed as the conjugated π system spanning all four carbons.
2. The LUMO of acrolein consists of the π* orbital of the alkene and the π* orbital of the carbonyl.
3. As the reactants approach, the diene HOMO overlaps with the dienophile LUMO, leading to concerted formation of both new σ bonds.
4. A secondary orbital interaction between the diene HOMO and the carbonyl π* provides additional stabilization in the endo transition state that would be absent in the exo TS.