Heteroatomic ligand exchange in heterophosphonium salts

This synthetic route is based on heterophosphonium salt formation, as an intermediate reagent in which the labile heteroatomic Y group is easily exchanged in situ by an other heteroatomic Z group (reaction 124). 

Halophosphonium salt formation, immediately followed by its transformation to enoxyphosphonium, takes place in the reaction of tertiary phosphines with the positive halogen of some secondary a-haloketones. Indeed, the reaction, which can also lead to a 2-oxoalkylphosphonium salt by a classical SN2 reaction (route a), is oriented to the enoxyphosphonium salt (route b) by different factors459-461 (reactions 131) an increasing steric hindrance on the phosphine and/or the carbon linked to the halogen, or an increasing stability of the enolate and an increasing electrophilicity of the halogen. 

The enoxyphosphonium salts are relatively stable, even if they are very sensitive to solvolysis they have been considered as the intermediates in Perkow s reaction462. A very similar reaction leads to enaminophosphonium salts (or to iminophosphonium salts when there is a subsequent isomerization possibility of the double bond by prototropy)463 (reaction 132). 

Halophosphonium salts are the useful precursors for the preparation of heteropho-sphonium by ligand exchange, although some relatively stable alkoxyphosphonium salts 

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