Sulfone-based alkene synthesis — classic
Julia-Lythgoe olefination
How the Julia-Lythgoe olefination 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) PhSO₂CH₂R, n-BuLi; R′CHO 2) Ac₂O 3) Na(Hg), MeOH. Conditions: THF, –78 °C; then Na/Hg, –20 °C. Bonds formed: C–C; C=C; PhSO₂⁻; AcO⁻; bonds broken: C=O; C–S; C–O. Stereochemistry: E-alkene regardless of sulfone diastereomer (radical/anion equilibration). Rate and selectivity: Single-electron transfer to the sulfone is key.
At a glance
Step 1: Sulfonyl carbanion adds to the aldehyde
A metalated phenyl sulfone's carbanion attacks an aldehyde's carbonyl carbon in an ordinary nucleophilic addition, forming a β-alkoxysulfone.
Step 2: Acylate; electron transfer cleaves C–S
The alkoxide is converted to a good leaving group (commonly as an acetate ester), and reduction with sodium amalgam or a similar single-electron reductant triggers a radical-anion elimination — electrons add to the sulfone, which fragments, expelling both the sulfinate and the acetate leaving group together.
Step 3: β-Elimination of acetate gives the E-alkene
This reductive elimination forms the new alkene directly, favoring the E-isomer — the classic Julia olefination predates the milder Julia-Kocienski variant covered earlier, which was developed specifically to avoid this reaction's harsh reductive elimination step.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- MeOH
- Methanol. Polar protic solvent (also a nucleophile in solvolysis).
- n-BuLi
- n-Butyllithium. Very strong base/nucleophile (pKa of butane ≈ 50); deprotonates alkynes and does lithium–halogen exchange.
- Ac2O
- Acetic anhydride. Acetylating agent.
- E1cB
- Elimination, unimolecular conjugate base. Deprotonation first to a carbanion, then loss of the leaving group.
How it compares with related reactions
| Julia-Lythgoe olefination | Julia-Kocienski olefination | Peterson olefination | |
|---|---|---|---|
| Reagents | 1) PhSO₂CH₂R, n-BuLi; R′CHO 2) Ac₂O 3) Na(Hg), MeOH | 1-phenyl-1H-tetrazol-5-yl (PT) sulfone, KHMDS (or NaHMDS, LiHMDS), aldehyde | Me₃SiCH₂⁻ (from Me₃SiCH₂Cl, Mg or n-BuLi), ketone/aldehyde |
| Conditions | THF, –78 °C; then Na/Hg, –20 °C | THF or DME, –78 °C → 25 °C | THF, –78 → 25 °C; base (KH) or acid (H₂SO₄, BF₃) for elimination |
| Stereochemistry | E-alkene regardless of sulfone diastereomer (radical/anion equilibration) | Highly E-selective (anti-adduct, then anti elimination) | Basic: syn elimination; acid: anti elimination (opposite alkene geometry) |
| Rate / selectivity | Single-electron transfer to the sulfone is key | One-pot; Smiles shift and elimination are fast after addition | Alkoxide addition fast; elimination driven by Si–O bond (≈ 110 kcal/mol) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Julia-Lythgoe olefination?
1) PhSO₂CH₂R, n-BuLi; R′CHO 2) Ac₂O 3) Na(Hg), MeOH. Conditions: THF, –78 °C; then Na/Hg, –20 °C.
Which bonds are formed and which are broken?
Formed: C–C; C=C; PhSO₂⁻; AcO⁻. Broken: C=O; C–S; C–O.
What is the stereochemical outcome?
E-alkene regardless of sulfone diastereomer (radical/anion equilibration)
What controls the rate or the selectivity?
Single-electron transfer to the sulfone is key
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