Wittig reaction polar aprotic solvents

The Homer-Wadsworth-Emmons reaction is an important variant of the Wittig reaction and involves using a phosphonate ester in place of a phosphonium salt. Like the phase-transfer Wittig reaction just discussed, these reactions may be easily performed in the undergraduate laboratory. In one of the procedures that follows, the phosphonate ester 12 is deprotonated with potassium tert-butoxide in the polar, aprotic solvent N,N-dimethylformamide, (CH3)2NCHO (DMF), to provide the resonance-stabilized, nucleophilic phosphonate anion 13 (Eq. 18.7). 

The Wittig reaction is regiospecific. The double bond forms between the carbonyl carbon atom and the carbon atom bonded to the phosphorus atom of the ylide. However, the reaction is not stereospecific. Thus, if geometric isomers are possible, both isomers form. The Wittig reaction is carried out in polar aprotic solvents such as diethyl ether, tetrahydrofuran, or dimethyl sulfoxide. The Wittig reaction can be carried out in the presence of alkene, alkyne, halogen, ether, or ester functional groups. 

As many parameters influence the ( /Zj-ratio, it is difficult to predict the stereochemical course of a Wittig reaction. Nevertheless, some basic rules have been worked out [6,19,20]. Reactive ylides in apolar solvents such as benzene or ether preferentially form the thermodynamically less stable (Zj-isomer, in particular under so-called salt-free conditions, i.e. when no soluble lithium salts are present in the reaction mixture. In the presence of LiX, the proportion of (Fj-isomer increases in the order X = Cl>Br>I>BPh4 [20]. In polar aprotic... 

This reaction is a variant of the Witfig reaction (Sects. 2.2.1-2.2.3). The phosphonate group stabilizes the carbanion and can be easily cleaved on the completed reaction as a water-soluble side product [24,25]. DMPU is an aprotic polar solvent that has replaced the toxic hexamethylphosphortriamide (HMPTA) formerly used in the laboratory and industry. The mechanism of this reaction is analogous to the Wittig reaction (Sect. 2.2.2), with enhanced thermodynamic control of the formation of the E isomer. 

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