Reaction of Wittig

The reaction of Wittig is a particular case of the nucleophilic additions. It is a question of the addition of a ylide of phosphorus (phosphorane) on a carbonyl Composé (Aldéhyde S and Cétone S) to form compounds etylenic (ethylene substituted).

Reaction

This reaction thus is very much used in organic synthesis to form little subsititués ethylene derivatives, difficult to form by reactions of elimination S, which form the most substituted compounds mainly.

(images on the substituted ethylene derivatives, less substituted +équations assessments)

It has moreover one very good output, which one can thermodynamically explain by the formation of oxide of triphénylphophine which has a very strong connection P=O (about 535 KJ^-1)

One will note the " part thereafter; triphénylphosphine" - P (C₆H₅) ₃ for this molecule as for all those which has this part.

This reaction allows, moreover, the formation - then impossible or giving place to parasitic olefinic hydrocarbon reactions, from made up where the double connection CO is combined with connections DC doubles or triple, or belonged to a functional group of ester type.

Implementation

Preparation of the phosphonium salt

This salt, ABCHP⁺ (C₆H₅) ₃X^-, is obtained by nucleophilic Substitution of the Triphénylphosphine on a halogenous Dérivé.

This reaction is rather easy thanks to the strong nucleophilicity of phosphorus.

Preparation of phosphorus ylide

It is acted in fact of the elimination of hydrogen related to carbon in alpha of the atom of phosphorus thanks to a base (strong).

The formation of ylide often requires the use of a very strong base, of type Organolithien, ion amidure R-NH₂^- or Hydrure H^-. However, if the carbonaceous chain has a group having an attractile mésomère effect (ex: =O), stabilizing ylide by allowing the delocalization of the doublet carried by carbon, a less strong base (OH^- or RO^- alcoholate) can be used.

Nucleophilic addition

This addition has a rather complex mechanism in 3 stages. For reasons of facility, we will leave the shape mésomère of ylide of phosphorus or double liason C=P is transformed into polarized simple laison

The first stage of this addition, also stage kinetically determining (limiting), is the nucleophilic addition of phosphorus ylide on the laison double C=O, which produces a compound called phosphorus betain.

This phosphorus betain cyclise then to give a compound, called oxaphosphéthane, which again will be rearranged to form olefinic hydrocarbon and oxide of triphénylphosphine.

However this mechanism is disputed for the " Wittig not stabilisée". The other mechanism proposed is the attack concerted of double connection C=P on double connection C=O (double connections approaching one the other with angle of right, orbital pi directed one towards the other). This mechanism makes it possible to explain why the stabilized ylides give the olefin E and not-stabilized olefin Z gives.

Remarks

Georg Wittig received in 1979 the Nobel Prize of Chemistry, for the discovery of this new method of olefinic hydrocarbon synthesis.

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