Silicon and phosphorus centres

Another centre which can be attacked by nucleophiles to react via an addition-elimination pathway is silicon. This has been the subject of several studies in the last few years, and reactions between various anions and substituted silanes have been investigated in detail. 

The results of studies concerning reactions between nucleophiles and trimethyl phosphate and trimethyl phosphite should be mentioned briefly. In both esters the nucleophiles may attack the phosphorus centre to react in this way via pentacoordinate and tetrahedral type species, respectively, to products. Various nucleophiles, such as F-, CD30 , DNO , OH and NH2 have been allowed to react with trimethyl phosphate, but only OH and NH have been found to attack the phosphorus centre to a minor extent. The most prominent process is nucleophilic attack on one of the carbon atoms to generate by an SN2 reaction the phosphate diester anion as 

Based upon observations made for the CD30-/(CH30)3P system, the tetracoordinate phosphorus anion would be an intermediate rather than a transition state. Finally, it must be noted that an attack on one of the carbon atoms of trimethyl phosphite via an SN2 mechanism can only become competitive with the nucleophilic attack on the phosphorus centre, if displacement of methoxide by the latter is sufficiently endothermic (Anderson et al., 1984). 

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