Z-Selective Horner-Wadsworth-Emmons Reaction

The Horner-Wadsworth-Emmons modification of the Wittig reaction is a widely used method for the preparation of α, β-unsaturated esters. The phosphonate anions are strongly nucleophilic and react readily with carbonyl compounds under mild conditions to form olefins and water-soluble phosphate esters in good yields. Since this reaction preferentially gives more stable E-olefins in general, extensive efforts have been devoted to the stereoselective construction of Z -α, β-unsaturated esters. Several attempts have been made by the choice of cation, temperature, and solvent, but they are with a limited success. On the other hand, the special phosphonate reagents including (CF3CH2O) 2P (O) CH2CO2Me and (ArO) 2P (O) CH2CO2R gave highly selective routes to a wide range of Z-α, β-unsaturated esters. This review describes these efforts. The mechanism of the HWE reaction is also described briefly.