Loading VerChem
Preparing your chemistry tools...
CycloadditionIntermediateNobel Prize 1928
Diels-Alder Reaction
Also known as: [4+2] Cycloaddition
Discovered by Otto Diels, Kurt Alder (1928)
General Equation
Diene + Dienophile → Cyclohexene
Description
A concerted [4+2] pericyclic cycloaddition: a conjugated diene (4π) reacts with a dienophile (2π) to form a new 6-membered ring with one C=C. The diene must be in s-cis conformation. Suprafacial on both components. Nobel Prize 1950.
Reaction Scheme
Reactants:
- • Conjugated diene (4π electrons)
- • Dienophile (2π electrons)
Products:
- • Cyclohexene derivative (6-membered ring)
Reagents:
- • None (or Lewis acid catalyst)
Conditions:
- • Heat or Lewis acid (AlCl₃)
Mechanism (1 steps)
1
The diene (in s-cis conformation) and dienophile approach each other. Six electrons reorganize simultaneously: 3 π bonds break and 2 σ bonds + 1 π bond form in a concerted, synchronous process.
Arrow: curved
Concerted — all bonds form/break at once. No intermediate.
Key Points to Remember
- ✓Concerted pericyclic reaction — no intermediates, no catalysts needed
- ✓Diene MUST adopt s-cis conformation (locked s-trans dienes don't react)
- ✓Electron-rich diene + electron-poor dienophile = fastest (normal demand)
- ✓Endo rule: endo product kinetically favored (secondary orbital interactions)
- ✓Stereospecific: cis-dienophile → cis substituents in product (syn addition)
- ✓Retro-Diels-Alder: reverse at high temperature
- ✓Otto Diels & Kurt Alder — Nobel Prize 1950
Examples
Reactant:
1,3-Butadiene + Ethylene
Reagent:
Heat (200°C)
Product:
Cyclohexene
Yield: 70%
Reactant:
Cyclopentadiene + Maleic anhydride
Reagent:
RT (very reactive pair)
Product:
Norbornene derivative (endo)
Yield: 95%
Reactant:
1,3-Butadiene + Acrolein
Reagent:
AlCl₃ catalyst
Product:
2-Cyclohexene-1-carboxaldehyde
Yield: 90%
Synthetic Utility
Arguably the most powerful reaction in organic synthesis. Forms 2 C-C bonds and up to 4 stereocenters in one step. Essential for building 6-membered rings in natural product synthesis.
Limitations
- ⚠Diene must be able to adopt s-cis conformation
- ⚠Electron-poor dienes react poorly (inverse electron demand needed)
- ⚠Some reactions require very high temperatures
- ⚠Regiochemistry can be complex with unsymmetrical partners
#cycloaddition#pericyclic#4+2#ring formation#Nobel#concerted