The synthesis of an olefin from the reaction between a carbonyl compound (aldehyde or ketone) and a phosphonium ylide, via either a betaine and/or oxaphosphetane intermediate is generally known as the Wittig reaction. Occasionally, it is also referred to as the Wittig alkenylation or Wittig process. The extension of this reaction by the application of a phosphine oxide carbanion, instead of a phosphonium ylide is known as the Horner reaction or Wittig Horner reaction, while the preparation of olefin from a phosphonate carbanion is commonly referred to as the Horner–Wadsworth–Emmons olefination. Several other extensions of this reaction have been addressed. All the reaction steps of the Wittig reaction have been discussed. The reversible process to give the carbonyl compound and phosphorane is also possible, and such reversible reaction is known as the
retro
‐Wittig reaction. The most important feature of the Wittig reaction is probably the easy control of stereochemistry of olefins (i.e., either
Z
‐ or
E
‐configuration). It has been observed that ultimate configuration of olefins depends on many factors, such as the structure of phosphoranes, the base and the solvent used, the concentration of ylide, and the reaction temperature. In particular, the
Z
‐selectivity from nonstabilized ylides can be further enhanced by replacement of the phenyl group in triphenylphosphane moiety with either a 2‐pyridyl or 2‐furyl group. It has been reported that even the replacement of one phenyl ring by a pyridyl group can apparently increase the
Z
‐selectivity without affecting the yields of olefins. It has been reported that
E
‐olefin is the major product in the protic solvent, whereas
Z
‐olefin predominates in the Wittig reactions when carried out in aprotic media. Other facets of the Wittig reaction have also been discussed, which include, application of the catalysts and the enhancement of reactivity in organic solvent by the application of a phase‐transfer catalyst or promotion of the reaction by photochemical or microwave irradiation. This reaction has very wide applications in organic synthesis, such as transformation of unprotected carbohydrates and preparation of artificial carbon‐linked (1,6)‐oligosaccharides.
