Pentacoordinated phosphorus nucleophilic substitution

The lack of direct evidence for TBP intermediates has allowed the proposal of alternative pentacoordinate intermediates and of hexacoordinate intermediates in nucleophilic substitution reactions at tetrahedral phosphorus. Pentacoordinate phosphorus species (spirocyclic phosphoranes) have been observed in which the most stable form adopts the square pyramidal (SP) geometry (Howard et ai, 1973). Such SP species have been postulated as intermediates in substitution reactions at tetrahedral phosphorus (Boudreau et ai, 1975). 

Although direct information has come from studies on phosphate esters, studies on other tetrahedral and pentacoordinate phosphorus species have been required to elucidate principles such as pseudorotation, apicophilicity and the trigonal bipyramidal transition state. Application of the guidelines for associative nucleophilic substitution at phosphorus, discussed in the previous section of this review, has led to a good understanding of many biological mechanisms involving substitution at phosphorus. 

Dialkyl H-phosphonates are hydrolytically unstable compounds. The hydrolysis of H-phos-phonate diesters and phosphate esters is believed to proceed via pentacoordinated intermediates and transition structures that are formed by nucleophilic attack of the tetracoordinated phosphorus atom [14-16]. These intermediates undergo further pseudorotation [17] and elimination of alcohol to form the final products of the nucleophilic substitution. 

The general conclusions on the mechanism of associative nucleophilic substitution at pentacoordinate phosphorus come down to two points... 

During this review period a lot more emphasis has been placed on research in which pentacoordinated species play a crucial role in the product distribution or have influence on the outcome of the process. This includes a degenerative nucleophilic substitution at phosphonium centres, non-enzymatic aminolysis of 2,4-dinitrophenyl ethyl phosphate promoted by methylamine in gas phase and aqueous solution, and the mechanism of a selective transfer of a phosphorus atom or DNA cleavage by bacteriophage X exonuclease. Moreover, the oxidative addition of diverse O-H and N-H-containing compounds to geometrically constrained tricoordinated phosphorus compounds has been described and their structures have been evaluated. 

The chlorine atom of 43 may be displaced readily by carbon-based nucleophiles with predominant overall retention of configuration at the phosphorus atom, similar to that observed in pentacoordinated analogues [37, 38], In some reactions, unselective substitution was observed [39], This approach was successfully applied for the... 

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