Sigmatropic Rearrangements Phospha-Cope

A sigmatropic rearrangement is defined as a migration of a tt bond adjacent to one or more a bonds, to a new position in a molecule, with the double bond n system becoming reorganised in the process as, for example, in the diene (3.51). 

In the special case of the original compound being symmetrical the product is identical and an equilibrium is established. This is sometimes known as a degenerate Cope rearranganent (3.52). 

Phospha-Cope rearrangements occur with some tetraphosphahexadienes (3.53). 

In the case of diphosphahexadienes, the two forms are not equivalent and such sigmatropic rearrangements appear to take place only in the direction which replaces XW P with P (3.54). 

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