Diels–Alder reaction: practice and mechanism
Diene and dienophile close a six-membered ring in one step: two new C–C bonds, and the new double bond lies in the middle of the former diene.
Example
A diene and a dienophile (ideally electron-poor, e.g. next to a C=O) react in a [4+2] cycloaddition. Two new C–C bonds and a cyclohexene ring form in one step; the new double bond lies between the central carbons of the diene.
Mechanism step by step
- 1
The diene must be in the s-cis conformation (both double bonds on the same side).
- 2
Six π electrons shift at once in a cyclic transition state (pericyclic, [4+2]).
- 3
Two new σ bonds form; the new double bond lies between C2 and C3 of the former diene.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Only one bond was formed here (open chain). The Diels–Alder reaction forms two bonds at once and closes a ring.
Often confused with: Catalytic hydrogenation · Addition of bromine
Frequently asked questions
Why an electron-poor dienophile?
Electron-withdrawing groups (C=O, CN) lower the dienophile LUMO, which fits the diene HOMO better and speeds up the reaction.
What is the endo rule?
With cyclic dienes the dienophile substituent preferably points under the diene (endo), due to secondary orbital interactions.