Pericyclic — [2,3]-sigmatropic
[2,3]-Wittig rearrangement
How the [2,3]-Wittig 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: n-BuLi (or LDA, t-BuLi), allyl ether. Conditions: THF, –78 °C → 0 °C; H₃O⁺ work-up. Bonds formed: C–C; C=O/C–OH (homoallylic alcohol); shifted C=C; bonds broken: C–O (allylic); α C–H. Stereochemistry: Concerted, suprafacial; erythro/threo and E/Z control at low T. Rate and selectivity: Faster than [1,2]-Wittig; favored at low temperature.
At a glance
Step 1: Base removes the α-H of the ether
Strong base deprotonates a carbon alpha to an ether oxygen that is itself allylic on the other side — setting up a carbanion positioned exactly five atoms from the alkene's far end.
Step 2: Five-membered [2,3] transition state
Rather than the direct 1,2-shift of the classic [1,2]-Wittig rearrangement covered earlier, this carbanion instead undergoes a concerted [2,3]-sigmatropic shift, moving through a five-membered cyclic transition state analogous to the Mislow-Evans rearrangement.
Step 3: Homoallylic alkoxide, then alcohol
The C–O bond breaks as a new C–C bond forms at the allylic terminus and the double bond shifts position — this pathway is typically faster and more stereospecific than the competing [1,2] pathway whenever an allylic system is available to react through.
Abbreviations and terms
- Bu
- Butyl. –(CH₂)₃CH₃.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- LDA
- Lithium diisopropylamide. Strong (pKa ≈ 36), bulky, non-nucleophilic base that makes kinetic enolates quantitatively.
- n-BuLi
- n-Butyllithium. Very strong base/nucleophile (pKa of butane ≈ 50); deprotonates alkynes and does lithium–halogen exchange.
- t-BuLi
- tert-Butyllithium. Extremely strong, pyrophoric organolithium.
How it compares with related reactions
| [2,3]-Wittig rearrangement | Claisen rearrangement | Cope rearrangement of 1,5-hexadiene | |
|---|---|---|---|
| Reagents | n-BuLi (or LDA, t-BuLi), allyl ether | Allyl vinyl ether (e.g. from allyl alcohol + enol ether, or Johnson–Claisen) | 1,5-Hexadiene (and substituted analogues) |
| Conditions | THF, –78 °C → 0 °C; H₃O⁺ work-up | Heat, 100–200 °C, no catalyst | Heat, ≈ 150–260 °C; or Pd(II) catalysis |
| Stereochemistry | Concerted, suprafacial; erythro/threo and E/Z control at low T | Chair TS; suprafacial; chirality transfer | Chair TS; suprafacial; E/Z-selective |
| Rate / selectivity | Faster than [1,2]-Wittig; favored at low temperature | Concerted [3,3]; accelerated by polar solvents; driven by strong C=O | Degenerate here; ΔG° = 0, driven by substituents in general |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the [2,3]-Wittig rearrangement?
n-BuLi (or LDA, t-BuLi), allyl ether. Conditions: THF, –78 °C → 0 °C; H₃O⁺ work-up.
Which bonds are formed and which are broken?
Formed: C–C; C=O/C–OH (homoallylic alcohol); shifted C=C. Broken: C–O (allylic); α C–H.
What is the stereochemical outcome?
Concerted, suprafacial; erythro/threo and E/Z control at low T
What controls the rate or the selectivity?
Faster than [1,2]-Wittig; favored at low temperature
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti