Reagent: Phosphorus ylide (Ph3P=CR2), prepared from: Ph3P + R-CH2-X → Ph3P+-CH2R X- (phosphonium salt) → BuLi/NaH → Ph3P=CHR (ylide)

Reaction: Ph3P=CHR + R'2C=O → R'2C=CHR + Ph3P=O (triphenylphosphine oxide)

Advantages: Forms alkene with defined position of the double bond (no ambiguity about where C=C goes, unlike elimination which can give multiple alkenes). Extremely useful in synthesis of complex natural products.

Stereochemistry: Unstabilized ylides → Z (cis) alkene preferentially. Stabilized ylides (adjacent to EWG) → E (trans) alkene (HWE modification using phosphonate esters).