Phosphonates Wittig reaction variants

Important and widely used variants of the Wittig reaction are based on carbanionic organophosphorus reagents, and are known as the Wadsworth-Emmons reaction, Wittig-Horner reaction or Horner-Wadsworth-Emmons reaction. As first reported by Horner, carbanionic phosphine oxides can be used today carbanions from alkyl phosphonates 13 are most often used. The latter are easily prepared by application of the Arbuzov reaction. The reactive carbanionic species—e.g. 14 —is generated by treatment of the appropriate phosphonate with base, e.g. with sodium hydride ... 

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). 

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. 

---