Propargyl rearrangement — contrasting regiochemistry
Rupe rearrangement
How the Rupe 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: Acid (H₂SO₄, HCO₂H, TsOH). Conditions: Heat, aqueous or neat acid. Bonds formed: C=C (enyne); C=O (after hydration); bonds broken: C–OH; α C–H; C≡C π. Stereochemistry: No new stereocenters. Rate and selectivity: Ionization/E1 loss of H⁺ is rate-limiting; hydration is slower.
At a glance
Step 1: Ionization to the propargyl cation
A tertiary propargyl alcohol with an alpha hydrogen available ionizes under acid exactly as in the Meyer-Schuster, forming the same delocalized propargyl/allenyl cation.
Step 2: E1 loss of an α-proton
But here, instead of water re-adding to the far carbon, loss of a proton from the carbon adjacent to the original carbinol carbon forms a conjugated enyne, without any net oxygen transposition.
Step 3: Markovnikov hydration, tautomerization
Acid-catalyzed Markovnikov hydration of that enyne's triple bond, followed by tautomerization, gives a different regiochemical outcome than Meyer-Schuster — an enone with the carbonyl positioned differently — showing how the same cationic intermediate can diverge to different products depending on which proton is lost versus which nucleophile re-adds.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TsOH
- p-Toluenesulfonic acid. Strong organic acid catalyst, soluble in organic solvents.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- E1
- Unimolecular elimination. Carbocation forms first, then a base removes a β-H.
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
How it compares with related reactions
| Rupe rearrangement | Meyer-Schuster rearrangement | Bamberger rearrangement | |
|---|---|---|---|
| Reagents | Acid (H₂SO₄, HCO₂H, TsOH) | Brønsted acid (H₂SO₄, TsOH) or Lewis acid; water | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) |
| Conditions | Heat, aqueous or neat acid | Aqueous acid, heat (milder with Au or V catalysts) | H₂O, 25–60 °C |
| Stereochemistry | No new stereocenters | (E)/(Z) mixture possible at the new alkene | None; para-selective |
| Rate / selectivity | Ionization/E1 loss of H⁺ is rate-limiting; hydration is slower | Rate-limiting 1,3-shift of OH via the stabilized cation | Loss of water to give the nitrenium ion is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Rupe rearrangement?
Acid (H₂SO₄, HCO₂H, TsOH). Conditions: Heat, aqueous or neat acid.
Which bonds are formed and which are broken?
Formed: C=C (enyne); C=O (after hydration). Broken: C–OH; α C–H; C≡C π.
What is the stereochemical outcome?
No new stereocenters
What controls the rate or the selectivity?
Ionization/E1 loss of H⁺ is rate-limiting; hydration is slower
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti