Sulfone extrusion — alkene synthesis
Ramberg-Bäcklund reaction
How the Ramberg-Bäcklund reaction 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: α-Halo sulfone, strong base (KOH/t-BuOK or CCl₄/KOH–Al₂O₃). Conditions: t-BuOH or CH₂Cl₂, 25–80 °C. Bonds formed: C–C (episulfone); C=C; SO₂; bonds broken: α C–H; C–Cl; C–S (×2). Stereochemistry: Alkene geometry set by episulfone; Z often favoured with weak bases. Rate and selectivity: Ring closure usually rate-limiting; SO₂ loss fast.
At a glance
Step 1: Base deprotonates the α-carbon
Base deprotonates a carbon alpha to a sulfone bearing a halogen on the adjacent alpha carbon, forming a carbanion stabilized by the sulfone.
Step 2: Intramolecular SN2: episulfone
That carbanion performs an intramolecular SN2 attack, displacing the halide and closing a strained three-membered ring — an episulfone (a cyclic sulfone).
Step 3: Cheletropic SO₂ extrusion
The episulfone is thermally unstable and spontaneously extrudes SO₂ in a retro-cheletropic process, forming a new C=C bond directly between the two former sulfone-flanking carbons — a reliable way to build an alkene exactly where a sulfone group used to be.
Abbreviations and terms
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- KOtBu
- Potassium tert-butoxide. Strong, bulky alkoxide base – favours elimination over substitution.
- KOH
- Potassium hydroxide. Strong aqueous/alcoholic base.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
- Δ
- Heat. Reaction is run at elevated temperature.
How it compares with related reactions
| Ramberg-Bäcklund reaction | Julia-Kocienski olefination | Julia-Lythgoe olefination | |
|---|---|---|---|
| Reagents | α-Halo sulfone, strong base (KOH/t-BuOK or CCl₄/KOH–Al₂O₃) | 1-phenyl-1H-tetrazol-5-yl (PT) sulfone, KHMDS (or NaHMDS, LiHMDS), aldehyde | 1) PhSO₂CH₂R, n-BuLi; R′CHO 2) Ac₂O 3) Na(Hg), MeOH |
| Conditions | t-BuOH or CH₂Cl₂, 25–80 °C | THF or DME, –78 °C → 25 °C | THF, –78 °C; then Na/Hg, –20 °C |
| Stereochemistry | Alkene geometry set by episulfone; Z often favoured with weak bases | Highly E-selective (anti-adduct, then anti elimination) | E-alkene regardless of sulfone diastereomer (radical/anion equilibration) |
| Rate / selectivity | Ring closure usually rate-limiting; SO₂ loss fast | One-pot; Smiles shift and elimination are fast after addition | Single-electron transfer to the sulfone is key |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Ramberg-Bäcklund reaction?
α-Halo sulfone, strong base (KOH/t-BuOK or CCl₄/KOH–Al₂O₃). Conditions: t-BuOH or CH₂Cl₂, 25–80 °C.
Which bonds are formed and which are broken?
Formed: C–C (episulfone); C=C; SO₂. Broken: α C–H; C–Cl; C–S (×2).
What is the stereochemical outcome?
Alkene geometry set by episulfone; Z often favoured with weak bases
What controls the rate or the selectivity?
Ring closure usually rate-limiting; SO₂ loss fast
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go