16.18:
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
The Diels–Alder reaction is a thermally allowed [4+2] cycloaddition between a 4π conjugated diene and a 2π dienophile to form a six-membered cyclic product.
In this process, two weaker π bonds are converted to two stronger σ bonds.
The reaction involves a concerted movement of six π electrons via a cyclic transition state.
For the two new σ bonds to form simultaneously, the frontier molecular orbitals of the two π systems must align such that the interacting lobes are in phase.
Under thermal conditions, a constructive overlap is possible when the ground state HOMO of the diene and the ground state LUMO of the dienophile interact in a suprafacial manner.
The same is true for the alternative HOMO-LUMO pairing. However, the former pairing is preferred since a smaller HOMO-LUMO energy gap leads to a better overlap.
So, the diene is generally the electron-rich component and the dienophile electron-deficient.
Lastly, the Diels–Alder reaction is syn stereospecific, meaning that the stereochemistry of the dienophile is retained in the product.
16.18:
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
From a molecular orbital perspective, the interacting lobes of the two π systems must be in phase to permit the formation of new σ bonds in a synchronous manner. For molecules in the ground state, the interaction between HOMO (diene) and LUMO (dienophile) or HOMO (dienophile) and LUMO (diene) satisfies the orbital symmetry requirements. The two π components interact suprafacially in each case, making Diels–Alder a thermally allowed [4+2] cycloaddition reaction.
