Interconversion of Mono and Bicyclic Phosphoranes

Provided pseudorotation is possible (13.115), interconversion of the various isomers in Table 13.5 can be effected without bond-breaking being involved. This is in contrast to the interconversion of isomers involving tetrahedral carbon atoms, as, for example, in the classical case of mutarotation of glucose where a bond must be broken (10.5). 

Isomers of type (13.124a) can become asymmetric and optically active when different substituents on the phospholane ring render the ring C atoms centres of chirality. The 12 possible arrangements are indicated in Table 13.11. The positional isomers and their enantiomers can be divided into 

2 sets of 6, because the carbon atoms are not equivalent, one being closer to an equatorial O atom and the other closer to an axial O atom. 

The three forms of Equation 13.125 are indicated for compounds such as the anion in Equation 6.482 in Equation 13.126a, which shows that three isomers could also arise if a tetragonal pyramidal configuration were adopted. The enantiomorphic pair (13.125ab) are interconvertible by BPR, which is probably why in most cases optically active forms have not been isolated in compounds of this type. 

If slight asymmetry is introduced into the biphenylene rings by substitution of a p-methyl group, pseudorotation of one isomer does not produce its mirror image and racemisation is not possible. However, both forms are asymmetric and such a mixture might be expected to show some resultant optical activity (13.126b). 

---