'Click' the different stages to view 3D models of the Z- and E-stereoselective examples of the Wittig reaction mechanism:

Z-Selective Wittig Reaction

Stage 1:Unstabilised Ylid Formation

The nature of the selectivity of the Wittig reaction is chosen by the type of phosphonium ylid used. The Z-selective reaction is favoured with an unstabilised phosphonium ylid as in the example below.

Stage 2: Z-Selective Wittig Reaction

The Z-stereoselective Wittig reaction consists of a stereoselective first step forming the syn oxaphosphetane. This is then followed by a stereospecific elimination of this intermediate to form the Z alkene.

E-Selective Wittig Reaction

Stage 1: The E-selective Wittig reaction requires stabilised ylids whereby the substituents on the carbon are conjugating or anion stabilising, such as a carbonyl group. The mechanism for the formation of this ylid has been omitted here as these molecules are stable enough to be considered starting materials.

Stage 2: E-Selective Wittig Reaction

Due to the stabilisation that the conjugating or anion stabilising group supplies to the phosphonium ylid the intermediate produced is the anti oxaphosphetane. This then undergoes a stereospecific second step in order to produce the E alkene.

Animations based upon Raphaël Robiette, Jeffery Richardson,, Varinder K. Aggarwal, and, Jeremy N. Harvey, J.Am.Chem.Soc. 2006, 128 (7), 2394-2409