Production of phosphorus ylides

The acidity of the proton on the a-carbon atom determines how strong a base is required to deprotonate a phosphonium salt. The range of bases used extends from aqueous potassium carbonate solution to organometallic compounds. Phosphonium salts that are substituted by strong electron acceptors on the a-carbon atom can be deprotonated by bases as weak as dilute aqueous alkali, even in two-phase systems, to give stable ylides. Even amines such as triethylamine can be used as proton acceptors in such cases. On the other hand, to deprotonate 

Phosphonium salts that bear substituted vinyl groups on the a-carbon atom are the usual precursors of semi-stabilized ylides in carotenoid synthesis. Alkali metal alkoxides, generally as a solution in the corresponding alcohol, are frequently the bases of choice. A two-phase system is also sometimes employed, e.g. dichloromethane/aqueous NaOH solution (see Section F). 

Stabilized and semi-stabilized ylides can also be produced under virtually neutral conditions by using oxiranes as proton acceptors [18]. This has a precondition that the anion of the phosphonium salt is a halide, since epoxide and phosphonium halide are in equilibrium with ylide and the corresponding halohydrin. The technique is particularly advantageous if base-labile functionalities are present. 

Usually, the semi-stabilized ylides are produced in the presence of the carbonyl component and are immediately scavenged by this to form the olefin. 

In addition to the acidity of the phosphonium salt, other factors also determine the choice of base, e.g. the reactivity, (i.e. the electrophilicity) of the carbonyl component. The presence of sensitive functional groups in the phosphonium salt also has an influence on the suitability of certain bases. If the phosphonium salt contains a carbonyl group, for example, there is the risk that this may be subject to nucleophilic attack by alkyl metals. In addition, the base, in particular its cation, intervenes to a considerable extent in the stereochemical course of the Wittig reaction (see Section C). 

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