Tandem cationic cyclization/rearrangement
Prins-pinacol tandem reaction
How the Prins-pinacol tandem 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: Homoallylic diol bearing a tertiary carbinol, CH₂O (or acetal), Lewis acid (SnCl₄, BF₃·OEt₂) or Brønsted acid. Conditions: CH₂Cl₂, –78 to 25 °C. Bonds formed: C–C (ring closure); C–C (shifted group); C=O (ketone); bonds broken: C=C π; C–CH₃ (migrating group); C–OH₂⁺. Stereochemistry: Chair-like cyclisation; migration with retention at the migrating carbon. Rate and selectivity: Cyclisation is rate-limiting; the shift is fast.
At a glance
Step 1: Aldehyde activation, oxocarbenium ion
An acid-activated aldehyde is attacked by a nearby homoallylic alcohol's alkene, forming a new carbocation through the same Prins cyclization mechanism covered earlier — except here, the alcohol carbon also bears an adjacent leaving group set up for what comes next.
Step 2: Alkene attacks the C=O⁺: ring closes
Rather than simply being trapped by the original oxygen (as in the standalone Prins reaction), this cation instead triggers a 1,2-alkyl or hydride shift from the adjacent carbinol carbon — the identical migration logic as the pinacol rearrangement, occurring immediately after, and directly triggered by, the Prins cyclization step.
Step 3: Pinacol-type shift, then proton loss
Loss of a proton from the resulting oxocarbenium gives a ring-expanded or substituent-migrated cyclic ketone — chaining these two mechanisms together in one pot builds molecular complexity (a new ring plus a rearranged skeleton) far beyond what either the Prins or pinacol mechanism could deliver alone.
Abbreviations and terms
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- BF3
- Boron trifluoride. Lewis acid (usually as the Et₂O complex).
How it compares with related reactions
| Prins-pinacol tandem reaction | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | Homoallylic diol bearing a tertiary carbinol, CH₂O (or acetal), Lewis acid (SnCl₄, BF₃·OEt₂) or Brønsted acid | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | CH₂Cl₂, –78 to 25 °C | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | Chair-like cyclisation; migration with retention at the migrating carbon | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | Cyclisation is rate-limiting; the shift is fast | Loss of water to give the nitrenium ion is rate-limiting | Migration is rate-limiting; leaving-group ability of N–OX matters |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Prins-pinacol tandem reaction?
Homoallylic diol bearing a tertiary carbinol, CH₂O (or acetal), Lewis acid (SnCl₄, BF₃·OEt₂) or Brønsted acid. Conditions: CH₂Cl₂, –78 to 25 °C.
Which bonds are formed and which are broken?
Formed: C–C (ring closure); C–C (shifted group); C=O (ketone). Broken: C=C π; C–CH₃ (migrating group); C–OH₂⁺.
What is the stereochemical outcome?
Chair-like cyclisation; migration with retention at the migrating carbon
What controls the rate or the selectivity?
Cyclisation is rate-limiting; the shift is fast
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti