Heterophosphonium salts

Alkoxyphosphonium salts are a special case, in that they show thermal instability, due to the thermodynamic stability of the P=0 bond formed (reaction 114). This enables them to be isolated only in favourable cases (reaction 114). 

Various structural factors are generally favourable for the stability of these salts, as follows  

An important steric hindrance of the carbon in the a-position to the oxygen (or a bicyclic structure) reduces or even inhibits formation of the phosphoryl group by substitution reaction on this carbon. It must be pointed out that phosphoryl formation occurs, generally, in Arbuzov-type reactions, by a substitution process, but can take place also by a -elimination which is favoured by major steric hindrance. 

A weak nucleophilicity of the X- anion (SbF-, PF-, BF, CIO-, CF3SO ) causes the same favourable stabilization effect. 

Electron-donating R groups, which decrease the positive charge on phosphorus, inhibit also phosphoryl formation, but also have another stabilizing effect, because they decrease the solvolysis or ligand-exchange reactions occurring by nucleophilic attack on the phosphorus. 

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Accordingly, heterophosphonium salts are generally more difficult to prepare, to handle and to characterize than phosphonium salts themselves. The salt stability increases... 

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). 

Finally, heterophosphonium salts are reduced with cleavage of the phosphorus-heteroatom bond optically active aminophosphonium compounds are usefully reduced to phosphines with retention of configuration747. Methyltrineopentoxyphosphonium trifluoromethanesulphonate gave the quantitative formation of dineopentylmethyl phos-phonite, which is indicative of P—O bond cleavage748 (reaction 223)... 

In the formation of halophosphonium salts, the chlorophosphine alkylation occurs easily with diverse alkyl halides40 5fl-c. Generally, the reaction proceeds without any solvent or in CH2C12, with a Lewis acid such as A1C13, especially in the case of PC13 or RPC12 (reaction 143). Finally in heterophosphonium synthesis by the alkylation of... 

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