Mechanism Homer-Wadsworth-Emmons reaction

The mechanism of Homer-Wadsworth-Emmons reactions has not been definitively established. A contemporary rationalization is shown in Figure 11.13 for the reaction between an... 

Yastrebova in 1970, the use of dialkyl l-(alkoxycarbonyl)methylphosphonates in Homer-Wadsworth-Emmons reactions has carefully been covered in several exhaustive and excellent reviews.All the factors governing the reaction (nature of the carbanion and carbonyl group, reaction conditions, mechanism, and stereochemistry) have been studied in depth. We invite the reader to refer to these papers. We discuss here the synthetic applications resulting directly from the use of dialkyl l-(alkoxycarbonyl)methylphosphonates, which is the pathway of choice for the preparation of a,P-unsaturated alcohols and related compounds via the a,P-unsaturated esters. 

The commonly accepted mechanism for the Homer-Wadsworth-Emmons reaction is as depicted in Scheme 1.6. Here, reaction of the phosphonate stabilized carbanion with an aldehyde forms the oxyanion intermediates 4 under reversible conditions. Rapid decomposition of 4, via the four-centered intermediates 5, then affords alkenes 6. 

For a discussion of the Wittig reaction and a list of references, including the mechanism and modifications, see Experiment [19]. The role of the phase-transfer catalyst in the Homer-Wadsworth-Emmons modification of the Wttig reaction is also discussed in some detail in that experiment. 

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