Horner-Wadsworth-Emmons Reaction
The Horner-Wadsworth-Emmons Reaction allows the stereoselective olefination of aldehydes and ketones using stabilized phosphonate carbanions. The reaction favors the formation of (E)-alkenes.
The Still-Gennari Conditions allow the preparation of (Z)-olefins with excellent stereoselectivity by the reaction between aldehydes and phosphonates bearing electron-withdrawing groups (trifluoromethyl). The use of electron-withdrawing phosphonates accelerates the elimination of the oxaphosphetane intermediates, boosting the production of (Z)-alkenes.
Horner-Wadsworth-Emmons Reaction
Reaction Mechanism
1. Deprotonation of the phosphonate. 2. Nucleophilic addition of the carbanion onto the aldehyde. 3. Formation of an oxaphosphetane intermediate. 4. Final elimination delivers predominantly the (E)-olefin, or the (Z)-olefin when the Still-Gennari conditions are employed instead.
Example
Experimental Procedure
The substrate (1 mmol, 1.0 eq) was dissolved in 1:1 THF/H2O (20 mL, 0.05 M), and potassium carbonate (4.5 eq) was added. The mixture was stirred at room temperature for 2 h and was then diluted with saturated NH4Cl solution and ethyl acetate. The phases were separated, and the aqueous layer was extracted with EtOAc. The combined organic extracts were washed with brine, dried over anhydrous Na2SO4, and concentrated under reduced pressure to give a crude oil. Purification of the residue by flash chromatography afforded the desired alkene.
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