Reaction Paths with Phosphoranes

Transformations of tetrahedral compounds involving either inversion or retention of configuration are believed to take place via an intermediate (usually unstable) trigonal bipyramidal compound. This is 

Inversion usually occurs in the alkaline hydrolysis of phosphonium salts when they contain a benzyl or allyl substituent group (13.132). Benzyl is much more apicophilic than the other substituent groups but when the apicophilicities of the leaving groups are more similar, varying degrees of inversion or retention can occur. 

When tertiary butyl groups are present, hydrolysis proceeds with retention (13.133). This is believed to be the result of steric hindrance from these groups, which results in attack of OH opposite to the Bu group and the process is apical-equatorial (Chapter 3.3). With tertiary butyl alkoxy-phosphonium salts, however, there is inversion due to the high apicophilic nature of the leaving OEt group (13.134) (apical-apical elimination). 

Elimination processes of this kind can be further complicated by pseudorotation if the lifetime of the intermediate trigonal bipyramid is of sufficient duration. This probably explains the observed racemisation on alcoholysis, of phosphonium salts, which is a slower process than alkaline hydrolysis (13.137). While half the molecules may invert by apical-apical elimination, pseudorotation of 50% of the trigonal bipyramidal intermediate could lead to equatorial elimination and hence retention of configuration of the remaining molecules. 

The high rate of acid hydrolysis of ethylene phosphate and its methyl ester is probably due mainly to release of ring strain on formation of a 5-coordinated intermediate with O/P/O = 90° (13.138). The fact that pseudorotation can occur is itself proof that trigonal bipyramidal structures are involved. 

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In mechanistic terms, the PMR states that the pathways for forward and reverse reactions at equilibrium are described by the same energy surface it does not state that the profile of such a surface must be symmetrical with respect to the reaction path 2 Application of the PMR to displacement reactions at phosphorus is aided by Fig. 9, which depicts all of the possible reaction pathways for degenerate ligand exchange at tetracoordinate phosphorus that proceed either via pentacoordinate transition states or via phosphorane intermediates capable of pseudorotation. The letters a and e in Fig. 9... 

Details of the mechanism of the reaction of chlorophosphazenes with ortho-dinucleophiles such as catechol or o-aminophenol have been discussed. Although it is not possible to isolate the substituted phosphazene (43a), good evidence is available for its initial formation from isolation of the spiro-phosphorane (44) and an imino-intermediate (45). Complete investigation of this system is thus difficult, but further aspects of the reaction paths were elucidated by studying the reaction of o-aminophenol with the tri-catechol derivative (43b). A non-geminal structure has been assigned, on the basis of P n.m.r. measurements, to the disubstituted... 

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