The reaction of aldehydes or ketones with stabilized phosphorus ylides (phosphonate carbanions) leads to olefins with excellent E-selectivity.
Mechanism of the Wittig-Horner Reaction
The reaction mechanism is similar to the mechanism of the Wittig Reaction. The stereochemistry is set by steric approach control, where the antiperiplanar approach of the carbanion to the carbon of the carbonyl group is favored when the smaller aldehydic hydrogen eclipses the bulky phosphoranyl moiety. This places the ester group syn to the aldehyde R group, but the incipient alkene assumes an E-orientation of these groups after rotation to form the oxaphosphetane. As the lithium counterion does not interfere with oxaphosphetane formation, use of BuLi is possible, but NaH and NaOMe are also suitable bases for forming the ylide. The resulting phosphate byproduct is readily separated from the desired products by simply washing with water.
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