The Wittig-olefination reaction is commonly used to convert an aldehyde or a ketone to an alkene through the use of a triphenyl phosphonium ylide (Wittig reagent).

A ylide is a neutral molecule with positive and negative charges on adjoining atoms.  Triphenyl phosphonium ylide is the significant resonance contributor of the phosphorane form:

Simple ylides tend to yield Z-alkenes.  However, stabilized ylides such as Ph₃P=CH−COOR and Ph₃P=CH-Ph, contain atoms that stabilize the negative charge of the carbon atom.  These stable ylides are less reactive and usually fail to react with ketones.  Also, instead of producing the Z-alkene, these stabilized compounds typically produce the E-alkene.
In the reaction, the carbanion of the triphenyl phosphonium ylide acts as a nucleophile that attacks the carbonyl to form a betaine (3).  A betaine consists of a positively charged phosphonium ion or ammonium ion and a negatively charged carboxyl group.  The betaine rotates to so that the charges align, (4) to form the oxaphosphatane, (5).  Elimination then produces the Z-alkene and triphenylphosphine oxide.

Schlosser modification of the Wittig-olefination: To produce the E-alkene,  threo betaine is used.  Threo betaine is a diastereomer of the erythro betaine utlized in typical Wittig-olefination reactions.  Threo betaine is produced by adding phenyllithium to the erythro betaine at low temperature to create a betaine ylide.  Afterwards, hydrochloric acid is added to create threo betaine.