Wittig Reaction

The Wittig Reaction

            It is a reaction in which organophosphorus ylides react with aldehydes or ketones to give substituted alkenes. George Wittig was awarded the Nobel prize for this work in 1979.  The most important advantage of alkene synthesis by this reaction is that the location of the double bond is absolutely fixed, as compare to the mixtures of alkenes produced by alcohol dehydration.

The Mechanism of the Wittig Reaction

The Wittig reaction mechanism proceeds via three steps. These steps are:

Step 1: 

The negatively charged carbon of  the ylide act as nucleophile and  attack on the carbonyl carbon of the aldehyde or ketone. This leads to the formation of a charge separated (and dipolar) intermediate called a betaine shown as follows:

 

The betaine intermediate transformed in to a new oxygen phosphorus bond, yielding another intermediate which has a four-membered ring structure. This step is illustrated below:

 

Step 3: 

In the four-membered ring intermediate, the carbon-oxygen bond and the carbon-phosphorus bonds are cleaved. The oxygen takes both the bonding electrons and forms a new double bond with the phosphorus which lost the bonding pair of electrons to the carbon atom. A new carbon-carbon double bond is formed with this electron pair as well, yielding the required alkene product. This step is illustrated below.