Pericyclic — concerted cycloaddition
Diels-Alder: butadiene + ethylene → cyclohexene
How the Diels-Alder works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: 1,3-diene + dienophile (best with EWG). Conditions: Heat, 80–200 °C or Lewis acid. Bonds formed: two C–C σ bonds; bonds broken: two C=C π bonds. Stereochemistry: Suprafacial, stereospecific; endo preference. Rate and selectivity: Fast: electron-rich diene (HOMO) + electron-poor dienophile (LUMO).
At a glance
Step 1: s-cis diene meets dienophile
The diene's electron-rich π system (HOMO) lines up over the dienophile's electron-poor π bond (LUMO). No charges are involved — this is a purely orbital-driven reaction.
Step 2: Six electrons move in one loop
Three curved arrows move in a single closed loop, all at once: two new σ (C–C) bonds form as the π bonds shift position. There's no stepwise intermediate — it's concerted, which is why stereochemistry is fully preserved.
Step 3: Cyclohexene
The result is a new six-membered ring, cyclohexene, with one double bond left over exactly where the loop of arrows didn't close a bond.
Abbreviations and terms
- EWG
- Electron-withdrawing group. A substituent (C=O, NO₂, CN…) that pulls electron density away, stabilising anions and activating electrophiles.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- HOMO
- Highest occupied molecular orbital. The orbital that donates electrons in a reaction.
- LUMO
- Lowest unoccupied molecular orbital. The orbital that accepts electrons in a reaction.
How it compares with related reactions
| Diels-Alder | 1,3-dipolar cycloaddition | Danishefsky diene Diels-Alder | |
|---|---|---|---|
| Reagents | 1,3-diene + dienophile (best with EWG) | Nitrile oxide (from hydroximoyl chloride + Et₃N, or ArCH=NOH + NCS/base), alkene | Danishefsky diene (trans-1-methoxy-3-TMSO-butadiene), aldehyde, Lewis acid (ZnCl₂, Eu(fod)₃, BF₃·OEt₂) or heat |
| Conditions | Heat, 80–200 °C or Lewis acid | CH₂Cl₂ or toluene, 25–80 °C; no catalyst | THF or toluene, 25 °C; then dilute acid (TFA or HCl) |
| Stereochemistry | Suprafacial, stereospecific; endo preference | Suprafacial, stereospecific (alkene geometry retained); 5-substituted regioisomer favored | Suprafacial; Lewis-acid catalysis can be enantioselective |
| Rate / selectivity | Fast: electron-rich diene (HOMO) + electron-poor dienophile (LUMO) | Concerted, thermal; HOMO/LUMO of dipole and alkene both matter | Diene HOMO raised by OMe and OSiMe₃; fast even with simple aldehydes |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Diels-Alder?
1,3-diene + dienophile (best with EWG). Conditions: Heat, 80–200 °C or Lewis acid.
Which bonds are formed and which are broken?
Formed: two C–C σ bonds. Broken: two C=C π bonds.
What is the stereochemical outcome?
Suprafacial, stereospecific; endo preference
What controls the rate or the selectivity?
Fast: electron-rich diene (HOMO) + electron-poor dienophile (LUMO)
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti