Wittig reaction is an important name reaction of organic chemistry. It is used in organic synthesis for the preparation of alkenes. It is a coupling reaction which is also known as Wittig olefination. It is different from Wittig rearrangement. The reaction is carried out by Wittig reagent which is a triphenyl phosphonium ylide. It is prepared by phosphonium salt and phosphonium salt is prepared by the reaction of triphenylphosphine with an alkyl halide. Typically, tetrahydrofuran (THF) or diethyl ether are used as solvent in the reaction. The Wittig reaction is named after German Chemist Georg Wittig who discovered it in 1954. He got the Nobel Prize in Chemistry in 1979 which he shared with Herbert C. Brown.
Wittig reaction is a chemical reaction in which a carbonyl compound (aldehyde or ketone) reacts with a triphenyl phosphonium ylide to give an alkene. Thus, it is a useful reaction to convert aldehydes or ketones into alkenes. In this reaction Wittig reagent reacts with carbonyl compound and gives alkenes and triphenylphosphine oxide as side product. The reaction is given below (General form)
Thus, we can say two main components of Wittig reaction are as follows –
Carbonyl compound (aldehyde or ketone)
Carbonyl compounds are those compounds which have -C=O group in them and an ylide is a species which has opposite formal charges (positive and negative) on adjacent atoms. In Wittig reaction phosphonium ylide is used.
As you can see phosphonium ylide has a nucleophilic carbon. This carbon attacks on the carbon of the carbonyl group and initiates the reaction.
Wittig reaction starts with the preparation of phosphonium ylide. Although ylides look like a difficult species, but their synthesis or preparation is quite easy and straightforward. Their preparation reactions simply follow a SN2 (bimolecular nucleophilic substitution) reaction mechanism.
Preparation of Wittig Reagent - Triphenyl phosphine reacts with alkyl halide and forms triphenyl phosphonium salt. Now this triphenyl phosphonium salt is made to react with a strong base (such as CH3-Li) to give triphenyl phosphonium ylide.
The Phosphate atom of triphenyl phosphine has a lone pair of electrons and act as an excellent nucleophile. This nucleophile attacks from the back on alkyl halide and displaces the leaving group (halide ion) which shows this reaction (preparation of ylide) follows SN2 reaction mechanism. Thus, formed phosphonium salt reacts with a strong base and goes through deprotonation and gives phosphonium ylide. Reaction mechanism of preparation of Wittig reagent is given below –
Step 1. Reaction of alkyl halide with triphenylphosphine -
Remember, in the above step, we use either primary or secondary alkyl halide. Tertiary halide cannot be used.
Step 2. Deprotonation
After preparation of ylide mechanism of the Wittig reaction takes place by following three steps –
Step 1. Attack of ylide carbon on carbonyl - Now the above prepared phosphonium ylide reacts with carbonyl compound (aldehyde or ketone). The Ylide carbon attacks on the carbonyl group due to the pi-electrons of the carbonyl group shifts towards the oxygen atom. Thus, betaine is formed.
Step 2. Attack of oxygen on phosphorus – Negatively charged oxygen atom attacks on positively charged phosphorus atom and forms oxaphosphetane.
Step 3. Reverse [2+2] cycloaddition – In this step, in oxaphosphetane [2+2] reverse cycloaddition takes place which give rise to the main product alkene and side product triphenylphosphine oxide.
In many cases, step 1 and step 2 takes place simultaneously.
Ylide reaction with cyclohexanone –
Use of Wittig reaction to form ring compound using 1-bromo-6- heptanone-
Importance of Wittig reaction can be understood by its popularity among the various methods of preparation of alkenes from aldehydes and ketones. Wittig reaction can be used for carbonyl compounds containing many functional groups. As Wittig reagent shows reactions with functional groups containing carbonyl compounds as well. It is a very effective method of preparation of alkenes. The geometry of the double bond can easily be predicted in the alkenes prepared by Wittig reaction, if the ylide’s nature is known. The components used in Wittig reaction are readily available or can be easily synthesized. It results in the formation of a new carbon – carbon double bond, which allows increase in carbon chain.
With many advantages, Wittig reaction has few limitations which are listed below –
The main limitation of Wittig reaction is that the reaction proceeds mainly through betaine intermediate, which leads to Z – alkene.
The ylides lacking electron withdrawing groups form both Z and E isomers. Although Z – isomer dominates.
When sterically hindered ketones are used in Wittig reaction, the rate of reaction decreases.
Some variations of Wittig reactions are also available such as Schlosser modification.
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