Propargyl rearrangement — [1,3]-shift
Meyer-Schuster rearrangement
How the Meyer-Schuster rearrangement 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: Brønsted acid (H₂SO₄, TsOH) or Lewis acid; water. Conditions: Aqueous acid, heat (milder with Au or V catalysts). Bonds formed: C=O (enone/enal); C=C; bonds broken: C–OH (propargylic); C≡C π. Stereochemistry: (E)/(Z) mixture possible at the new alkene. Rate and selectivity: Rate-limiting 1,3-shift of OH via the stabilized cation.
At a glance
Step 1: Protonation, loss of water
Acid protonates a propargyl alcohol's hydroxyl, and loss of water generates a resonance-stabilized propargyl/allenyl cation, with positive character delocalized across all three carbons of the former triple bond.
Step 2: Water attacks the far (γ) carbon
Water attacks this delocalized cation specifically at the far (γ) carbon rather than where the original OH left — an overall [1,3]-shift of the oxygen function along the three-carbon unit.
Step 3: Enol tautomerization
Tautomerization of the resulting enol gives an α,β-unsaturated carbonyl (an enone) — this transposes an alcohol from a propargylic position directly into a conjugated enone, using only acid catalysis.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- TsOH
- p-Toluenesulfonic acid. Strong organic acid catalyst, soluble in organic solvents.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
How it compares with related reactions
| Meyer-Schuster rearrangement | Rupe rearrangement | Meisenheimer rearrangement | |
|---|---|---|---|
| Reagents | Brønsted acid (H₂SO₄, TsOH) or Lewis acid; water | Acid (H₂SO₄, HCO₂H, TsOH) | Allylic tertiary amine N-oxide |
| Conditions | Aqueous acid, heat (milder with Au or V catalysts) | Heat, aqueous or neat acid | Heat, 25–100 °C (neutral) |
| Stereochemistry | (E)/(Z) mixture possible at the new alkene | No new stereocenters | Suprafacial [2,3]; chirality transfer from N-oxide to carbon |
| Rate / selectivity | Rate-limiting 1,3-shift of OH via the stabilized cation | Ionization/E1 loss of H⁺ is rate-limiting; hydration is slower | Concerted, 5-membered envelope TS; unimolecular |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Meyer-Schuster rearrangement?
Brønsted acid (H₂SO₄, TsOH) or Lewis acid; water. Conditions: Aqueous acid, heat (milder with Au or V catalysts).
Which bonds are formed and which are broken?
Formed: C=O (enone/enal); C=C. Broken: C–OH (propargylic); C≡C π.
What is the stereochemical outcome?
(E)/(Z) mixture possible at the new alkene
What controls the rate or the selectivity?
Rate-limiting 1,3-shift of OH via the stabilized cation
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti