Reactivity of phosphorus ylides

The method most frequently used to produce phosphorus ylides is deprotonation of alkyl-triphenylphosphonium halides by suitable bases [3]. 

The resonance form 5a accounts for the nucleophilic character of the ylide carbon atom. The reaction between ylide and carbonyl component can be regarded as nucleophilic attack of the ylide carbon atom on the carbonyl group (see Section D for a discussion of the reaction mechanism of the Wittig reaction). The reactivity, Le. the nucleophilicity of the ylide, on the one hand is influenced to a considerable extent by the character of the substituents and R, but, on the other hand, it is also influenced by the so-called stationary substituents on the phosphorus. 

Depending on their reactivity towards carbonyl components, the phosphorus ylides can be classified into three groups [3,5]. When R and/or R is an electron donor, e.g. an alkyl group, the nucleophilicity of the ylide, and thus its reactivity towards carbonyl groups, are increased. These ylides are termed reactive or non-stabilized . Strongly electron-withdrawing groups such as esters, nitriles, and ketones, on the other hand, stabilize the carbanion and thus reduce the nucleophilic reactivity of the ylide. These are so-called stable ylides . 

Reactive ylides must be produced in the absence of oxygen and moisture, whereas stable ylides can be isolated as solid substances which are relatively insensitive to hydrolysis. Ylides which are stabilized by a carbonyl group generally only react with aldehydes if need be, they can be made to react with ketones under relatively drastic conditions [3]. In these cases, the Horner-Emmons reaction is the method of choice because of the greater nucleophilicity of the phosphonate carbanions in comparison to phosphorus ylides [6] (see Section E). 

Intermediate between the reactive and the stable ylides, there is the group of so-called moderate or semi-stable ylides. In this case, the ylide carbon atom bears a vinyl, aryl, alkynyl or halogen substituent. The nucleophilicity of the semi-stabilized ylide is further decreased or increased by electron acceptors or donors on the vinyl or aryl group. 

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The second chapter (M. Taillefer and H. J. Cristau) is dedicated to new trends in ylide chemistry. The preparation and the reactivity of phosphorus ylides, C-substituted by heteroatoms is presented, ylides being substituted by groups 1 and 2 elements, by transition metals or by elements of groups 13 to 16. A rich and versatile chemistry is thus reported. 

The reactivity of phosphorus ylides towards carbonyl compounds such as ketones, esters, carbonates and amides has also been explored13. The mechanism shown in... 

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