Horner-Wadsworth-Emmons Reaction of Phosphonate Anions

4 Horner-Wadsworth-Emmons Reaction of Phosphonate Anions 

The Horner-Wadsworth-Emmons modification of the Wittig reaction continues to find wide application in contemporary organic synthesis. Two reviews, looking at different aspects of the stereoselectivity of this procedure, have been published. One considers approaches to improve the Z-selectivity of the Horner-Wadsworth-Emmons reaction, and the other the use of heteroatoms to improve the stereoselectivity of the Horner-Wadsworth-Emmons reaction. A combined experimental and computational study of the selectivity of the Horner-Wadsworth-Emmons reaction of a series of phosphonoacetates (172) has 

Horner-Wadsworth-Emmons procedures have been used to prepare a variety of compounds including chiral tetrahydrofuran- and tetrahydropyran-deriva-tives/ novel organo-sulfur electron donors containing dithiole and azulene 

Tandem reactions have emerged as promising procedures for the synthesis of highly functionalised compounds in a single step. Horner-Wadsworth-Emmons reactions have been coupled with 1,4-Michael additions to prepare 6-substituted a,p-unsaturated carboxylic acids derivatives, and with Diels-Alder reactions, which utilised sugar-derived phosphonate (176) to prepare enantiomerically pure bicyclo[4.3.0]nonanes.  

New phosphonates that have been prepared specifically for Horner-Wadsworth-Emmons processes include a new series of a-substituted phosphonates (177), and (178), designed for the synthesis of JE-bromoacrylates.  

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Horner-Wadsworth-Emmons Reactions of Phosphonate Anions. - As with the Horner modification of the Wittig reaction, the principal focus of papers that mention the Horner-Wadsworth-Emmons reaction relate to synthetic applications. The use of pressure to induce the synthesis of P-amino esters, p-thioesters and P-thionitriles via tandem Horner-Wadsworth-Emmons and Michael reactions has been reported. The reagent (l-tritylimidazol-4-yl)methylphosphonate (99) has been prepared and, when treated with aldehydes and ketones, affords (E)-vinylimidazoles in high yields. ... 

The format of this chapter is similar to that used in previous volumes. The first section deals with methylene phosphoranes and their Wittig reactions, the second looks at the Horner-Wadsworth-Emmons reaction of phosphonate anions and the third the structure and reaction of lithiated phosphine oxides - an area which continues to receive particular attention. The majority of reports concerning ylides relate to their use in synthesis and in the final section some of these applications are reviewed. 

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