Reactions

The Diels-Alder reaction is concerted, i.e. both new σ bonds form simultaneously through a single 6-electrons pericyclic transition state. The diene and the dienophile approach each other in a parallel, face-to-face arrangement. The HOMO of the diene overlaps with the LUMO of the dienophile in a suprafacial/suprafacial topology. The C1=C2 and C3=C4 𝜋 bonds of the diene break simultaneously with the C5=C6 𝜋 bond of the dienophile, forming two new σ bonds (C1-C6 and C4-C5) and generating the C2=C3 𝜋 bond of the cyclohexene product. It is important to note that (in the common cases) the Diels-Alder reaction requires an electron-rich diene (EDG) and an electron-poor dienophile (EWG), so that the HOMO-LUMO energy gap is minimized.

The Diels-Alder reaction has an exceptionally predictable stereochemistry. During the suprafacial/suprafacial approach, both new σ bonds form on the same face of the dienophile. As a consequence, substituents that are cis on the dienophile remain cis in the product (same for trans configuration). When the dienophile bears a substituent that can interact with the diene 𝜋 system in the transition state, two diastereomeric transition states are possible: endo and exo. In the endo transition state, the dienophile substituents (EWG) point toward the 𝜋 system, allowing secondary orbital interactions that stabilize this transition state. In the exo transition state, the dienophile substituents point away from the diene 𝜋 system, with no secondary orbital interactions. Under kinetic control (low temperature or Lewis acid catalysis), the endo product predominates. Under thermodynamic control (high temperature), the exo product may predominates as it is less sterically constrained.
Finally, when both the diene and dienophile are monosubstituted, two regioisomeric products are possible. The ortho/para rule predicts the major product based on FMO theory (Frontier Orbital Theory, i.e. largest HOMO-LUMO coefficient matching). For a 1-substituted diene reacting with a monosubstituted dienophile, the 1,2-product (ortho) predominates. For a 2-substituted diene reacting with a monosubstituted dienophile, the 1,4-product (para) predominates.