Boron — 1,2-migration/elimination
Zweifel olefination
How the Zweifel 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: alkenyl–B(OR)₂, R′Li, I₂, NaOMe (or NaOH). Conditions: THF, −78 → 25 °C. Bonds formed: C–C (R′ to alkene carbon), new C=C; bonds broken: C–B, C–I. Stereochemistry: Net inversion of alkene geometry (E-boronate → Z-alkene). Rate and selectivity: Iodonium formation fast; anti-elimination fast with base.
At a glance
Step 1: Alkenyl boronate ate complex
An alkenylboronate reacts with an organolithium reagent to form an ate complex, exactly as in the Matteson homologation's first step — boron's empty orbital accepting a new carbanion ligand.
Step 2: Iodonium, then 1,2-migration
Electrophilic activation (commonly with I₂) of the alkene triggers the same kind of 1,2-migration seen in the Matteson reaction: an alkyl group migrates from boron to the adjacent, now-activated alkene carbon, displacing iodide as the migration terminus.
Step 3: Anti-elimination gives the alkene
A base-mediated anti-periplanar elimination of boron and the remaining halide then forms the new alkene directly — the two-step migration-then-elimination sequence lets this reaction convert a vinylboronate into a fully substituted alkene with predictable geometry.
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.
- NaOMe
- Sodium methoxide. Alkoxide base matched to methyl esters.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
How it compares with related reactions
| Zweifel olefination | Matteson homologation | CBS reduction (Corey-Bakshi-Shibata) | |
|---|---|---|---|
| Reagents | alkenyl–B(OR)₂, R′Li, I₂, NaOMe (or NaOH) | R–B(OR′)₂, LiCHCl₂ (from CH₂Cl₂ + LDA or n-BuLi), then ZnCl₂ (cat.); R″MgX for next step | (S)- or (R)-Me-CBS oxazaborolidine (5–10 mol%), BH₃·THF or BH₃·SMe₂ |
| Conditions | THF, −78 → 25 °C | THF, −100 → 25 °C | THF, –20 to 25 °C, anhydrous |
| Stereochemistry | Net inversion of alkene geometry (E-boronate → Z-alkene) | R migrates with retention; the new CHCl carbon is set with inversion at Cl displacement | Predictable absolute configuration: hydride to the face set by the chiral boron-N framework |
| Rate / selectivity | Iodonium formation fast; anti-elimination fast with base | ZnCl₂ accelerates 1,2-shift; chiral diols (pinanediol) give >99:1 d.r. | Hydride transfer is rate-limiting; catalyst turnover fast |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Zweifel olefination?
alkenyl–B(OR)₂, R′Li, I₂, NaOMe (or NaOH). Conditions: THF, −78 → 25 °C.
Which bonds are formed and which are broken?
Formed: C–C (R′ to alkene carbon), new C=C. Broken: C–B, C–I.
What is the stereochemical outcome?
Net inversion of alkene geometry (E-boronate → Z-alkene)
What controls the rate or the selectivity?
Iodonium formation fast; anti-elimination fast with base
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam