Wittig-Horner reagents, reactivity

The reactivity of phospha-Wittig-Horner reagents towards ketenes has been adopted as a route to 1-phosphaallenes (Scheme 181). ... 

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 ... 

By far the main interest in the reaction of halo [ C] acetates with phosphorus nucleophiles is for the preparation of phosphoryl-stabilized carbanions for use in Wittig and related reactions. The presence of the additional electron-withdrawing ester group provides additional stabilization, significantly modifying the reactivity of the ylide species and the stereochemical course of its reactions. The two phosphorus reagents discussed here include the triphenylphosphonium salt type 158. precursors of Wittig methylenetriphe-nylphosphoranes, and the trialkylphosphonoacetate type 159. applied in the Horner-Wadsworth-Emmons family of reactions . ... 

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