Transylidation phosphonium salt deprotonation

Deprotonation of a phosphonium salt by an ylide is a transylidation reaction,- which is of importance especially in those cases where ylides are reacted with electrophiles (see Section 1.6.1.3), but may also be applied to isolated phosphonium salts. For an unequivocal reaction the two involved ylides must differ sufficiently with respect to their base strength (as for example in equation 14). 

If the reaction provides a salt whose acidity is greater than that of the starting phosphorane precursor (for example in the case of X = acyl), a second mole of the starting ylide reacts with the intermediate phosphonium salt in a transylidation reaction cf equation 14) giving rise to the formation of an a-substituted ylide and the conjugated salt of the original ylide. If the proton of the phosphonium salt generated in the first step is not sufficiently acidic for this proton transfer to occur, the phosphonium salt formed in the first step can be isolated and has to be deprotonated by an external base. 

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