Pentacoordination, phosphoms

Tri- and pentacoordinate phosphoms compounds often react by electron-pair mechanisms as demonstrated by the nucleophilic reactivity of the lone pair electrons in trivalent compounds, and the electrophilicity of the phosphoms atom in the pentavalent compounds. Some compounds also react by free-radical mechanisms. The theoretical and synthetic aspects of the chemistry of phosphoms compounds have been described (6—9). 

Holmes RR (1980) Pentacoordinated phosphoms, vol. 11. American Chemical Society, Washington, DC, p 87... 

The first example of a pentacoordinate 1,27, -azaphosphetidine is reported. The compound 36 may be regarded as potential intermediate in the reaction of a phosphoms... 

The hydrolysis of DNA proceeds as a two-step reaction. In the first step, one hydroxide ion attacks the phosphoms atom, forming a pentacoordinated intermediate. In the second step, the 5 -OH of 2 -deoxyribonucleotide is removed from the phosphoms atom through the scission of the P-0(2 ) bond (in non-enzymatic hydrolysis, the P-0(3 ) scission can also take place). 

Evans [211] and Akiba [212] reported in 1996 and 1997, respectively, that some pentacoordinated phosphorane-stabilized carbanions could undergo olefination with aldehydes, which we suggest be called the Evans-Akiba reaction. Similar to the HWE and Homer-Wittig reactions, this reaction has been proposed by Akiba et al. to proceed through diastereoselective carbonyl addition, cyclization, and elimination to yield an alkene (Scheme 45) [213-216]. It is noteworthy that the phosphoms atom is hexacoordinated in the four-center transition state. 

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