Diels-Alder Reaction
Diels-Alder Reaction: A conjugated diene and a dienophile combine in a single concerted step: the diene's terminal carbons form two new σ bonds to the…
What happens
A conjugated diene and a dienophile combine in a single concerted step: the diene's terminal carbons form two new σ bonds to the dienophile while the diene's central bond becomes a new π bond, building a six-membered ring in one step.
Reaction summary
Buta-1,3-diene + ethylene → cyclohexene (via [4+2] cycloaddition)
Reagents and conditions
- Starting material: Buta-1,3-diene; Ethylene (dienophile)
- Conditions: Heat, or a Lewis acid catalyst for activated dienophiles
- Products: Cyclohexene
Mechanism
- Concerted [4+2] cycloaddition: All bond-forming/breaking happens in one transition state: two new C–C σ bonds form simultaneously while the π bonds shift position.
- Cyclohexene product: The result is a new six-membered ring with one remaining double bond, exactly where the diene's central bond was.
Selectivity
- Regioselectivity: With an unsymmetrical diene and dienophile, the 'ortho/para rule' predicts which regiochemistry dominates.
- Stereoselectivity: Stereospecific: substituents that are cis on the dienophile stay cis in the product (suprafacial-suprafacial addition).
Common mistakes
- Forgetting the diene must be able to reach the s-cis conformation, a diene locked s-trans (e.g. inside a rigid ring) cannot react.
- Mixing up which component is the diene (4π, nucleophile-like) and which is the dienophile (2π, usually electron-poor).
Memory trick
"4 + 2 = 6": 4 π electrons from the diene + 2 from the dienophile build one 6-membered ring in a single step.