Nomenclature and Preparation of Phosphonium Ylides

P-Ylide PI13P—CH Alkyl PI13P—CHAryl PI13P—CH— C02R 

Ylide type non-stabilized ylide semi-stabilized ylide stabilized ylide 

2 disubstituted alkenes typically result3). ..with 90% c/.s-selectivity. .. as cis-, trans-mixture. .. with 90% trans-selectivity 

2) So-called high-temperature Wittig reaction, which takes place via the equilibrium fraction of the ylide. 

All P ylides for Wittig reactions are obtained by deprotonation of phosphonium salts. Depending on whether one wants to prepare a nonstabilized, a semistabilized, or a stabilized ylide, certain bases are especially suitable (see Table 11.1 an unusual, i.e., base-free, generation of ylides is described in Side Note 11.1). In stereogenic Wittig reactions with aldehydes, P ylides exhibit characteristic stereoselectivities. These depend mainly on whether the ylide involved is nonstabilized, semistabilized, or stabilized. This can also be seen in Table 11.1. 

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