Phosphonium ylides relative reactivities

The HWE reaction can be carried out on a ketone, but often the stereoselectivity is not as good as the reaction of a substituted phosphonate carbanion with the corresponding aldehyde. Because of the greater reactivity of the phosphonate reagent relative to the phosphonium carbanion, the HWE reaction has proven to be effective with hindered ketones that were unreactive toward classical Wittig ylides. 

An ylide is a neutral species whose Lewis structure contains opposite charges on adjacent atoms. The atoms involved are carbon and an element from either group 15 (VA) or 16 (VIA) of the periodic table, such as N, P, or S. The Wittig reaction uses phosphorus ylides, which are obtained by deprotonation of a phosphonium salt with a strong base. Phosphorus ylides are relatively stable, but reactive species, for which the following resonance structures may be written the phosphorus atom can exceed an octet by accommodating electron donation into its 3d orbitals. 

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