The Wittig and Related Reactions Phosphorus Ylides

Thephosphoms ylide starting material is synthesized from the corresponding phosphonium ion, which is deprotonated by a strong base such as n-butyllithium  

The strained four-membered ring falls apart under the reaction conditions, yielding an alkene and PhjPO as the final products. 

Once again, formation of Ph3PO is the driving force for the last part of the mechanism. Note the ambiguity in the stereochemistry of the alkene product (as indicated by the squiggly bonds), which results from the fact that the betaine intermediate equilibrates to some extent with both the starting materials and the heterocyclobutane intermediate. With sterically unhindered ylides, a Z (or cis) stereochemistry is generally the preferred stereochemical outcome. 

A major deficiency of the Wittig reaction is the difficulty of separating the desired alkene product from the triphenylphosphine oxide byproduct. The fact that triphenylphosphine is used as a stoichiometric reagent also leads to a poor atom economy, which is defined as  

Molecular mass of desired product Molecular mass of all reactants 

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