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 (see example 2) 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.
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Phosphonate carbanions are more nucleophilic than the corresponding phosphorus ylides, allowing them to react readily with a wide range of aldehydes and ketones under milder conditions. Hindered ketones that are typically unreactive in Wittig reactions can react readily in HWE olefinations. The preparation of starting alkyl phosphonates is possible using the Arbuzov reaction. See Tetrahedron Lett. 2018, 59, 568 for more details.
Reaction mechanism of Horner-Wadsworth-Emmons reaction
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.
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Examples and experimental procedures of Horner-Wadsworth-Emmons reactions
Example 3: J. Am. Chem. Soc. 2022, 144, 5253.
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To a solution of anhydrous LiBr (1.20 eq.) in THF (100 mL) was added a solution of the crude substrate (44.2 mmol, 1.0 eq.) in THF (100 mL) via cannula at room temperature. After 10 min of stirring, Et3N (1.69 eq.) was added and the stirring was continued for 1 h. The mixture was cooled to 0 ºC and (E)-2-butenal (2.96 eq.) was added dropwise. After 6 h of stirring at room temperature, the mixture was quenched with sat. aq. NH4Cl and diluted with hexane/Et2O (1:1). The mixture was washed with brine, and the aqueous layer was extracted with hexane/Et2O (1:1). The extracts were combined, dried (Na2SO4), and concentrated in vacuo. The residue was purified by SiO2 flash column chromatography.
Example 2: J. Am. Chem. Soc. 2021, 143, 8261.
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To a well stirred solution of 18-crown-6 (5.0 eq) in THF (16 mL) at –78 °C was added 0.5M solution of KHMDS in PhMe (1.5 eq) and the reaction was stirred for 20 minutes. After addition of the substrate (1.03 mmol, 1.0 eq) and stirring for 3 hour the reaction was quenched with sat. aq. NH4Cl, extracted with diethyl ether, dried over Na2SO4 and concentrated in vacuo. Purification by flash column chromatography yielded the product (78% yield).
Example 1: J. Org. Chem. 2021, 86, 3074.
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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.