Evidence for a racemisation reaction giving diastereoisomers in the aminolysis of a chiral tetra-aminocyclotriphosphazene

The reactions of racemic trans-1,3-bis(dibenzylamino)-1,3,5,5-tetrachlorocyclotriphosphazene (2) with an excess of pyrrolidine under progressively more forcing conditions gave successively derivatives with bis, tris and tetrakis-pyrrolidino substitution; viz. 5,5-bispyrrolidino-trans-1,3-bis(dibenzylamino)-1,3-dichlorocyclotriphosphazene (3), 1,5,5-trispyrrolidino-cis and 1,5,5-trispyrrolidino-trans-1,3-bis(dibenzylamino)-3-chlorocyclotriphosphazene (4) and 1,3,5,5-tetrakispyrrolidino-cis-1,3 and 1,3,5,5-tetrakispyrrolidino-trans-1,3-bis(dibenzylamino)cyclotriphosphazene (5). It was shown by 31P NMR spectroscopy that on addition of a chiral solvating agent the gem di-substituted derivative (3) is a racemate, that the tris-pyrrolidino derivative (4) exists as a pair of racemic diastereoisomers, and that the tetrakis-pyrrolidino derivative (5) is also diastereoisomeric, but exists as a racemic and meso pair of molecules. Compound (4) can only be formed as a result of a racemisation reaction, which may indicate an SN1 reaction in going from the tetrakis-amino substituted derivative (3) to the pentakis-amino derivative (4), and probably also to the hexakis-amino derivative (5).