Alkene synthesis — silicon-based
Peterson olefination
How the Peterson 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: Me₃SiCH₂⁻ (from Me₃SiCH₂Cl, Mg or n-BuLi), ketone/aldehyde. Conditions: THF, –78 → 25 °C; base (KH) or acid (H₂SO₄, BF₃) for elimination. Bonds formed: C–C; C=C; Si–O; bonds broken: C=O π; C–Si; C–O. Stereochemistry: Basic: syn elimination; acid: anti elimination (opposite alkene geometry). Rate and selectivity: Alkoxide addition fast; elimination driven by Si–O bond (≈ 110 kcal/mol).
At a glance
Step 1: α-Silyl carbanion adds to C=O
An alpha-silyl carbanion (stabilized by adjacent silicon) attacks a ketone's carbonyl carbon, forming a β-silyl alkoxide — the silicon analog of the Wittig's betaine intermediate.
Step 2: Syn elimination of Me₃SiO⁻
Under basic conditions, this alkoxide undergoes a syn-periplanar elimination, with oxygen and the adjacent silyl group leaving together as the new C=C π bond forms.
Step 3: Si–O bond formation drives it
The driving force is the exceptionally strong Si–O bond formed in the trimethylsilanolate byproduct — this reaction gives clean alkene geometry that's controllable by choosing acidic vs. basic elimination conditions.
Abbreviations and terms
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- n-BuLi
- n-Butyllithium. Very strong base/nucleophile (pKa of butane ≈ 50); deprotonates alkynes and does lithium–halogen exchange.
- KH
- Potassium hydride. Stronger/faster relative of NaH.
- BF3
- Boron trifluoride. Lewis acid (usually as the Et₂O complex).
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Peterson olefination | Julia-Kocienski olefination | Horner-Wadsworth-Emmons (HWE) reaction | |
|---|---|---|---|
| Reagents | Me₃SiCH₂⁻ (from Me₃SiCH₂Cl, Mg or n-BuLi), ketone/aldehyde | 1-phenyl-1H-tetrazol-5-yl (PT) sulfone, KHMDS (or NaHMDS, LiHMDS), aldehyde | Phosphonate ester (EtO)₂P(O)CH₂CO₂Et, NaH or NaOMe, aldehyde |
| Conditions | THF, –78 → 25 °C; base (KH) or acid (H₂SO₄, BF₃) for elimination | THF or DME, –78 °C → 25 °C | THF or DME, 0–25 °C |
| Stereochemistry | Basic: syn elimination; acid: anti elimination (opposite alkene geometry) | Highly E-selective (anti-adduct, then anti elimination) | Strong E-selectivity (trans) |
| Rate / selectivity | Alkoxide addition fast; elimination driven by Si–O bond (≈ 110 kcal/mol) | One-pot; Smiles shift and elimination are fast after addition | Aldehyde addition rate-limiting; water-soluble phosphate easily removed |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Peterson olefination?
Me₃SiCH₂⁻ (from Me₃SiCH₂Cl, Mg or n-BuLi), ketone/aldehyde. Conditions: THF, –78 → 25 °C; base (KH) or acid (H₂SO₄, BF₃) for elimination.
Which bonds are formed and which are broken?
Formed: C–C; C=C; Si–O. Broken: C=O π; C–Si; C–O.
What is the stereochemical outcome?
Basic: syn elimination; acid: anti elimination (opposite alkene geometry)
What controls the rate or the selectivity?
Alkoxide addition fast; elimination driven by Si–O bond (≈ 110 kcal/mol)
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go