Wittig reaction and Allylic Bromination – Dr. Mrs.Nikalje A. G. Department of Chemistry Y B Chavan College of pharmacy Aurangabad

Contents Introduction Reaction Mechanism Application References

Introduction Wittig reaction Def.n :- The formation of carbon carbon double bonds (olifins) from carbonyl compounds and phosphoranes(phosphorus ylides) is known as wittig reaction. In 1950, Wittig and Geissler investigated the use of phosporanes for the synthesis of olifins but they were not the first to report it,since Staudinger and Marvel had reported the synthesis of such compounds three decades before Wittig reaction is the most important and most effective method for the synthesis of Alkenes The active reagent in this transformation is the phosphorous ylide

Preparation of the phosporanes This can be prepared by following way

Types of ylides There are three types of ylides depending on the substituents attached to the phosphene 1)Stabilized Ylide :- In this the alkyl halide component has at least one strong electron withdrawing group which stabilize the negative charge on carbon atom eg.-COOR,SOOR etc 2)Semi-Stabilized Ylide:- The ylide have at least one aryl or alkenyl substituents as the side group,which are less stabilizing 3)Nonstabilised :- These have only alkyl substituents,which do not stabilize the negative charge on carbon atom

General Reaction

Features of reaction The phosphonium salts are usually prepared using the triphenylphosphine, and the phosphorous ylide are generated before the reaction or in situ The phosphorus ylide chemoselectively react with aldehydes (fast) and ketones (slow), other carbonyl compounds remain intact during the reaction The stereoselectivity of reaction i.e. E or Z selectivity is influnced by many factors as type of ylide,type of carbonyl compound,nature of solvent and the counterion for the ylide formation

Nonstabilised ylide under salt free condition in a dipolar aprotic solvent with aldehyde afford olifins with high Z- selectivity Stabilized ylide give predominantly E olefin with aldehyde under same salt free condition Semi stabilized ylide give alkenes with poorer stereoselectivity

Reaction mechanism

Modification in the wittig reaction Horner Wittig reaction takes place when the phosphorous ylides contain phosphine oxide in place of triarylphosphenes The use of stabilized alkyl phosphonate carbanion is known as Horner Wadsworth-Emmons reaction in which E-α,β-unsaturated ester are formed

Stereochemistry As we discussed earlier that the configuration of reaction is depend on the type of ylide taken, type of carbonyl compound ,nature of solvent and counterion for ylide formation

Applications of the reaction: 1)The total synthesis of amaryllidaceae alkaloid was achieved by Horner –Wittig reaction between a biaryl aldehyde and a metalated carbamate

2) This reaction is used for the formation or the addition of the exocyclic double bond at the ring Eg. a)

Eg.b)

3) The main use of reaction is for the double ring closure

4) It is found that the Wittig reaction is very useful in synthesis of many natural product

Advantages over the Reformatsky reaction :- The result obtained by this reaction is same as that of the Reformatsky reaction but is more general since no ester or other group is required tobe α to halogen In this the position of the double bond is always certain, in contrast to the result in Reformatsky reaction and in most of the base catalysed condensation

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