A study on the Z-selective Horner-Wadsworth-Emmons (HWE) reaction of methyl diarylphosphonoacetates

Experimental and theoretical studies were conducted to explore the Z-selectivities in the Horner-Wadsworth-Emmons (HWE) reaction employing several methyl diarylphosphonoacetates (3, 4, 5 and 6) and aldehydes. The Z-selectivity depended upon the reaction conditions such as the bases, reaction temperature, and the aromatic substituents on the phosphorus atoms but the almost phosphonoacetates used in the present study showed Z-selectivity in the reactions with both aromatic and aliphatic aldehydes. While the phosphonoacetate (3) with bis(2,4-difluorophenyl)phosphono group showed the highest Z-selectivity in all reaction conditions employed, decrease of the selectivity was observed in the case of some phosphonoacetates with diarylphosphono groups. These experimental results and the theoretical studies calculated by AM1 or 3-21G* ab initio methods suggested that the steric effect in the transition states of the addition steps was important rather than the electronic effect. A different aspect of the reaction courses between the Wittig and HWE reactions in the present computational chemistry was also described.