Phosphorus stereochemistry : Mechanistic implications of the observed stereochemistry of bond forming and breaking processes at phosphorus in some 5- and 6-membered cyclic phosphorus esters

Abstract

The stereochemistry of endocyclic and exocyclic bond forming and breaking processes in 5- and 6-membered cyclic phosphorus esters is summarised and comparisons are made with analogous reactions in acyclic phosphorus esters. The factors that determine which bonds are broken and whether reactions occur with inversion or retention of configuration at phosphorus are complex and usually have more obvious effects for reactions in cyclic than in acyclic phosphorus esters ; in particular conformational effects may be important. The stereochemistry of migration of phosphorus ester groups across 1,3-diols is also described. It is suggested that nucleophilic substitutions at phosphorus are inherently stereospecific in the sense that trigonal bipyramidal reaction intermediates break down either directly or following a single Berry Pseudorotation or Turnstile rotation process. Multiple Turnstile rotations which would lead to racemisation, and which apparently do occur in stable phosphoranes, are insignificant for reactions involving trigonal bipyramidal intermediates.