The mechanism of the Horner−Wadsworth−Emmons (HWE) reaction has been investigated using high level quantum mechanical calculations on a realistic model system. The solvation contribution has been evaluated using the PCM/DIR method. In the free, anionic system, the rate-determining step was found to be the ring closure of an oxyanion to an oxaphosphetane. Solvation was found to have a drastic influence on the reaction path. In most solvated systems, the bimolecular formation of the oxyanion intermediate may be rate limiting. Previously postulated isomerization side paths were investigated. Several effects that could rationalize experimentally observed trends for ( E )/( Z )-selectivities have been identified.
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Brandt et al. (1998) studied this question.
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