Substitution at Tetracoordinate Phosphorus

Of particular interest among numerous transformations of this type are the hydrolysis reactions of the phosphoric acid esters which occupy an important place in the series of enzymic processes [179]. Belonging to the second row 

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Bimolecular nucleophilic substitution at tetracoordinate phosphorus (Eq. (8)) may proceed by either direct (Sjq2) substitution or by an addition-elimination mechanism In the former, 16 represents a transition state, while in the... 

Figure 7 Pathways for nucieophiiic substitution at tetracoordinated phosphorus and siiicon. Path 3 is postuiated for siiicon oniy ...

Pentacoordinate hydroxyphosphoranes are likely intermediates or transition states in substitution reactions at tetracoordinate phosphorus (1). Recently, stable hydroxyphosphoranes (2, 3) and their conjugate bases - metal phosphoranoxides (J, 5) have been isolated. Spectroscopic evidence (J - 7) for equilibria between P(IV) compounds and hydroxyphosphoranes have been reported. Observation (8) and isolation (9) of P(IV) TBP phosphoranide species have also been announced. All these phosphoranes are stabilized by several features dominated by their spirobicyclic nature. 

In very general terms, the importance of the P=Z bond, particularly for Z = 0, lies in its strength, and the fact that its formation acts as the driving force for many of the reactions of organic phosphorus(V) compounds, but also and as far as the present chapter is concerned, its polarizability, which allows the setting up of pentacoordinated phosphorus intermediates this in turn affords an explanation for many of the substitution reactions which can occur at tetracoordinated phosphorus. 

Although unsynunetrically substituted amines are chiral, the configuration is not stable because of rapid inversion at nitrogen. The activation energy for pyramidal inversion at phosphorus is much higher than at nitrogen, and many optically active phosphines have been prepared. The barrier to inversion is usually in the range of 30-3S kcal/mol so that enantiomerically pure phosphines are stable at room temperature but racemize by inversion at elevated tempeiatuies. Asymmetrically substituted tetracoordinate phosphorus compounds such as phosphonium salts and phosphine oxides are also chiral. Scheme 2.1 includes some examples of chiral phosphorus compounds. 

On the other hand, evidence against an intermediate hypervalent tetracoordinated phosphorus anion in nucleophilic substitutions at phosphorus in tertiary phosphines was put forward by Kyba the fact that the substitution reaction 3 occurs with complete inversion of configuration at phosphorus was interpreted to mean that it proceeds without even one pseudorotation of 9, which makes the passage through such an intermediate unlikely ( ). 

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