Z-selective alkene synthesis
Still-Gennari modification (Z-selective HWE)
How the Still-Gennari modification (Z-selective HWE) 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: (CF₃CH₂O)₂P(O)CH₂CO₂Me, KHMDS, 18-crown-6, aldehyde. Conditions: THF, –78 °C. Bonds formed: C=C (Z); bonds broken: α C–H; C=O π; P–C. Stereochemistry: Z-selective (kinetic). Rate and selectivity: Fast, irreversible addition/elimination at low temperature.
At a glance
Step 1: Deprotonation of the electron-poor phosphonate
A phosphonate bearing electron-poor trifluoroethyl ester groups (instead of the ordinary HWE's ethyl esters) is deprotonated, forming a carbanion that's less stabilized and more reactive than a standard HWE phosphonate anion.
Step 2: Carbanion adds to the aldehyde
This carbanion attacks an aldehyde's carbonyl carbon and the resulting oxaphosphetane-forming step proceeds through a different, kinetically favored transition-state geometry than the ordinary HWE — the electron-poor trifluoroethyl groups change which diastereomer of the intermediate forms preferentially.
Step 3: Oxaphosphetane collapses to the Z-alkene
Collapse of that intermediate delivers the alkene with high Z-selectivity — the exact opposite geometric outcome from the standard, strongly E-selective Horner-Wadsworth-Emmons reaction covered earlier, achieved purely by changing the phosphonate's ester groups.
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.
- KHMDS
- Potassium hexamethyldisilazide. Strong, bulky base that favours more reactive (“naked”) enolates.
How it compares with related reactions
| Still-Gennari modification (Z-selective HWE) | Bamford-Stevens reaction | Horner-Wadsworth-Emmons (HWE) reaction | |
|---|---|---|---|
| Reagents | (CF₃CH₂O)₂P(O)CH₂CO₂Me, KHMDS, 18-crown-6, aldehyde | Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt) | Phosphonate ester (EtO)₂P(O)CH₂CO₂Et, NaH or NaOMe, aldehyde |
| Conditions | THF, –78 °C | Aprotic solvent (diglyme, toluene), 100–200 °C | THF or DME, 0–25 °C |
| Stereochemistry | Z-selective (kinetic) | Mixture of E/Z; thermodynamic (more substituted) alkene favoured | Strong E-selectivity (trans) |
| Rate / selectivity | Fast, irreversible addition/elimination at low temperature | Diazo decomposition to carbene is rate-limiting | 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 Still-Gennari modification (Z-selective HWE)?
(CF₃CH₂O)₂P(O)CH₂CO₂Me, KHMDS, 18-crown-6, aldehyde. Conditions: THF, –78 °C.
Which bonds are formed and which are broken?
Formed: C=C (Z). Broken: α C–H; C=O π; P–C.
What is the stereochemical outcome?
Z-selective (kinetic)
What controls the rate or the selectivity?
Fast, irreversible addition/elimination at low temperature
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