Question

Wittig Reaction:

Under what conditions does the Wittig reaction favor the cis isomer?  Under what conditions does the Wittig reaction favor the trans isomer? _____________________________________________________________

Answer 1

E-Selective Wittig Reaction

The E-selective Wittig reaction requires stabilised ylids whereby the substituents on the carbon are conjugating or anion stabilising, such as a carbonyl group. 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.

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. 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.