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.