Alkaline hydrolysis with migration SN P mig mechanism

TABLE 21. Alkaline hydrolysis of a-halomethyl-phosphonium salts658 

Schlosser670 proposed an equivalent mechanism at the phosphorane 64 stage, formed from the corresponding ylide and not from the phosphonium salt the driving force for the migration comes from the carbenoid nature of the carbon in the a-position to the phosphorus. This is consistent with the anionotropic nature of the migration. The mechanism is corroborated by a similar migration from the ylide 65670 (reaction 198). 

In the case of phosphonium salts, some examples of decomposition by Hofmann elimination reactions are known. The phosphonium salts which normally undergo such a decomposition possess an activated hydrogen / to the phosphorus the salts 70, with a strong electron-withdrawing Z group on the carbon / to the phosphorus (reaction 202) and the a-hydroxyalkylphosphonium salts 71 (reaction 203) 

It is noteworthy that structures like such as 71 are generally more favourable to elimination reactions, because of the particularly high acidity of the hydroxylic hydrogen. Indeed, for the mixed salt 72, the hydroxymethyl group is preferentially eliminated692 (reaction 204). 

Studies on the influence of molar ratios between the base and the phosphonium salt have shown696,697 that the substitution reaction is the main reaction when an excess of base is used however, in contrast, the elimination reaction is clearly favoured when an insufficient amount of base is used. These results are not surprising if it is assumed that the substitution mechanism is second order with respect to the hydroxide ion concentration696,697, whereas the elimination reaction EHp is only first order. 

---