Anomalous Stereoselectivity in the Wittig Reaction: The Role of Steric Interactions

Density functional theory calculations combined with a distortion/interaction energy analysis show that the anomalous Z selectivity observed in Wittig reactions of ortho-substituted benzaldehydes is not caused by phosphorus–heteroatom interactions in the addition transition state, as assumed in earlier work, but is predominantly steric in nature. The calculations reproduced correctly the stereoselectivity preferences for a wide range of reactant pairs as well as relative reactivities for different substituent types, providing deeper structural insight into the mechanism of Wittig olefination.