Inorganic — carbene methylenation
Tebbe olefination of an ester
How the Tebbe olefination of an ester 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: Tebbe reagent Cp₂Ti(μ-Cl)(μ-CH₂)AlMe₂; Lewis base (pyridine, THF, DMAP). Conditions: THF/toluene, –40 → 25 °C. Bonds formed: C=CH₂ (enol ether); Ti=O; bonds broken: C=O π; Ti–Cl–Al bridge. Stereochemistry: No new stereocentre; no epimerisation of α-stereocentres. Rate and selectivity: Ti=CH₂ release is fast; strong Ti=O bond (≈ 160 kcal/mol) drives the reaction.
At a glance
Step 1: Base frees the titanium methylidene
The Tebbe reagent, a Ti-Al bridged methylene complex, releases a titanium methylidene (Ti=CH₂) upon treatment with a mild base — a stable, isolable carbene surrogate.
Step 2: [2+2] onto the ester C=O
This titanium carbene reacts with the ester's carbonyl in a [2+2] cycloaddition, forming an oxatitanacyclobutane — the same ring-forming logic as Wittig's oxaphosphetane, just with titanium instead of phosphorus.
Step 3: Cycloreversion: alkene + Ti=O
The ring opens to release the alkene and a titanium oxo species — unlike the Wittig, Tebbe methylenation works directly on esters and amides, which ordinary phosphorus ylides can't touch.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Cp
- Cyclopentadienyl. A five-carbon aromatic ring ligand (C₅H₅⁻) that binds metals through all five carbons.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- DMAP
- 4-Dimethylaminopyridine. Hypernucleophilic acyl-transfer catalyst – forms a very reactive N-acylpyridinium.
How it compares with related reactions
| Tebbe olefination of an ester | Associative ligand substitution at a square-planar Pt(II) complex | Asymmetric phase-transfer catalysis | |
|---|---|---|---|
| Reagents | Tebbe reagent Cp₂Ti(μ-Cl)(μ-CH₂)AlMe₂; Lewis base (pyridine, THF, DMAP) | K₂[PtCl₄] + NH₃ (aq) | Chiral cinchona-derived Q*⁺Br⁻ (1–10 mol%), 50% aq. KOH, glycine imine tBu ester, BnBr |
| Conditions | THF/toluene, –40 → 25 °C | H₂O, 25 °C | Toluene/CH₂Cl₂, 0 °C, vigorous stirring |
| Stereochemistry | No new stereocentre; no epimerisation of α-stereocentres | Trigonal-bipyramidal intermediate; retention of geometry | High ee: chiral ion pair shields one enolate face |
| Rate / selectivity | Ti=CH₂ release is fast; strong Ti=O bond (≈ 160 kcal/mol) drives the reaction | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) | Alkylation of the ion pair is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Tebbe olefination of an ester?
Tebbe reagent Cp₂Ti(μ-Cl)(μ-CH₂)AlMe₂; Lewis base (pyridine, THF, DMAP). Conditions: THF/toluene, –40 → 25 °C.
Which bonds are formed and which are broken?
Formed: C=CH₂ (enol ether); Ti=O. Broken: C=O π; Ti–Cl–Al bridge.
What is the stereochemical outcome?
No new stereocentre; no epimerisation of α-stereocentres
What controls the rate or the selectivity?
Ti=CH₂ release is fast; strong Ti=O bond (≈ 160 kcal/mol) drives the reaction
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam