Diels Alder

Alkene Reactions: Diels-Alder Reaction

Alkene Reactions: Diels–Alder Cycloaddition (thermal [4+2])

The Diels–Alder reaction is the prototypical concerted, pericyclic [4+2] cycloaddition joining a conjugated diene (4 π electrons) with a dienophile (2 π electrons from an alkene or alkyne) to form a cyclohexene (or cyclohexadiene). Both partners react suprafacially in a single cyclic transition state, so stereochemistry is transferred intact and π-substituted dienophiles typically deliver the endo adduct under kinetic control. Electron-rich dienes and electron-poor dienophiles (normal electron demand) maximise the HOMO/LUMO interaction, while the diene must adopt an s-cis conformation to react.

Test mechanisms in the Mechanism Solver, check outcomes with the Reaction Solver, and name products confidently using the IUPAC Namer.


Quick Summary

  • Mechanism class: Concerted, thermal pericyclic [4+2]; one cyclic arrow set, no radicals or carbocations.
  • Key requirements: Diene accessible in the s-cis conformation; dienophile usually electron-poor (e.g., COOR, COR, CN, NO₂) for normal electron demand. Lewis acids further activate.
  • Stereochemistry: Stereospecific. Dienophile cis/trans relationships remain intact; diene substitution patterns map directly; endo product favoured under kinetic control with π substituents.
  • Regioselectivity: Predict via FMO analysis (largest coefficients) or the ortho/para rule; inverse electron demand flips polarity.
  • Conditions: Moderate heat or Lewis acid catalysis (AlCl₃, BF₃·OEt₂, TiCl₄, SnCl₄). Elevated temperature promotes retro-Diels–Alder and exo equilibration.
  • Safety note: Lewis acids and strong electrophiles demand dry glassware, inert handling, and acidic waste neutralisation.


Mechanism (2 Didactic Frames)

Class: Concerted suprafacial–suprafacial pericyclic process. The two panels below are teaching frames of one continuous transition state (no real intermediates).

Butadiene approaching the alkene dienophile in an endo alignment for the Diels–Alder step.
Step 1 — Concerted [4+2] alignment: the s-cis diene overlaps with the dienophile, positioning secondary π interactions for endo control.
Cyclohexene adduct after the concerted Diels–Alder reaction retaining endo orientation.
Step 2 — Product release: two new σ bonds form simultaneously, delivering the endo cyclohexene adduct with dienophile substituents intact.

Both panels come from the same concerted transition state; the first highlights alignment and secondary orbital overlap, while the second shows the cyclohexene adduct with stereochemistry locked in.


Mechanistic Checklist (Exam Focus)

  • Draw a single six-electron arrow loop—no intermediates or radicals.
  • Confirm the diene is depicted s-cis before the transition state.
  • Dienophile cis/trans geometry and diene substitution relationships transfer directly.
  • Endo product dominates for π-substituted dienophiles under kinetic control; exo increases with heat or sterics.
  • Regioselectivity follows FMO coefficients—align largest lobes (ortho/para rule).
  • Alkynes act as dienophiles to yield cyclohexadienes; hetero-Diels–Alder variants deliver heterocycles (pyrans, piperidines).
  • Lewis acids lower the dienophile LUMO and reduce Pauli repulsion, enhancing rate and selectivity.
  • Safety: Lewis acids and strong electrophiles require gloves, eye protection, fume hood, and acidic waste neutralisation.


Worked Examples

Example A — Normal electron demand, endo selectivity

Example A substrate: conjugated diene approaching an electron-poor dienophile
Substrate — Conjugated diene engaging an activated alkene primed for normal electron demand.
Example A mechanism step showing butadiene approaching the dienophile in an endo alignment
Mechanism — Solver output: the butadiene reagent aligns suprafacially, setting up secondary orbital overlap for the endo approach.
Example A product: 4-methylpent-2-en-2-ol rendered from the mechanism solver output
Product — Solver output mapped to the IUPAC name 4-methylpent-2-en-2-ol; dienophile stereochemistry carries through the endo adduct.

Teaching point: Matching an electron-rich diene with an electron-poor alkene retains the dienophile’s geometry and funnels the reaction through the endo transition state highlighted in Step 1. The IUPAC Namer tool confirms the product identity as 4-methylpent-2-en-2-ol.

Example B — Conjugated dienophile with carbonyl activation

Example B substrate: diene approaching a conjugated alkene bearing an aldehyde
Substrate — Conjugated dienophile bearing a carbonyl substituent to accentuate endo bias.
Example B mechanism step showing the dienophile aligning with the diene for the endo pathway
Mechanism — Solver output highlights the endo trajectory as the dienophile approaches the reagent diene.
Example B product: 3-methylbuta-1,3-dien-1-ol rendered from the mechanism solver output
Product — Solver output mapped to the IUPAC name 3-methylbuta-1,3-dien-1-ol; regioselectivity aligns with the frontier-orbital prediction.

Teaching point: Carbonyl activation locks the dienophile’s π system into the endo channel while preserving conjugation in the resulting cyclohexene. The IUPAC Namer tool provides the systematic name 3-methylbuta-1,3-dien-1-ol for the product.


Practical Tips & Pitfalls

  • Conformation control: Use preformed s-cis dienes (cyclopentadiene) or protect/lock conformations to accelerate.
  • Activation: Lewis acids or pressure lower activation barriers; monitor carefully and quench with aqueous base.
  • Temperature: Stay moderate for endo selectivity; deliberate heating drives exo and retro pathways.
  • Stereochemical checks: Track cis/trans relationships meticulously—draw the dienophile explicitly before mapping onto the product.
  • Scope notes: Alkynes → cyclohexadienes; hetero-Diels–Alder gives oxygen/nitrogen heterocycles; photochemical variants are outside this guide.
  • Waste handling: Lewis acid residues and halogenated solvents require segregated waste streams.


Exam-Style Summary

Thermal [4+2] cycloaddition between an s-cis diene and a dienophile proceeds through a single suprafacial–suprafacial transition state, preserving stereochemistry and typically delivering the endo regioisomer predicted by FMO analysis. Lewis acids accelerate and improve selectivity; heating can reverse the process (retro-Diels–Alder).


Interactive Toolbox

  • Reaction Solver — predict major Diels–Alder adducts from custom reactants.
  • Mechanism Solver — visualise concerted [4+2] pathways with step-by-step SVG output.
  • IUPAC Namer — confirm systematic names for your cycloaddition products.


FAQ / Exam Notes

Why is the endo product usually favoured? Secondary orbital interactions between the dienophile π substituent and the diene interior stabilize the endo transition state under kinetic control.
What shuts the reaction down? A diene forced s-trans (e.g., E,E-hexadiene in a rigid framework) cannot align orbitals and remains unreactive.
How can I tune selectivity? Lower temperatures and Lewis acids reinforce endo/regio control; higher temperatures or bulky substituents promote exo outcomes.
Are there intermediates? No. The Diels–Alder is fully concerted—avoid drawing carbocations or radicals.
Which arrow notation is correct? Use closed-shell curved arrows forming a six-membered loop; never fishhook arrows.

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