Chiral configurations of spermine-bridged cyclotriphosphazatrienes

Abstract

Spermine-bridged gem-disubstituted cyclotriphosphazatrienes have been synthesised by two routes. In one route a spermine-bridged cyclophosphazene 2 reacted with monofunctional nucleophiles known to favour gem disubstitution in cyclophosphazene rings (e.g.tert-butylamine) to give 3b, or to favour formation of spiro-derivatives with difunctional nucleophiles (e.g. 1,3-propanediol) to give 3c. In the other route a gem-disubstituted cyclophosphazene (i.e. diphenyl, N₃P₃Ph₂Cl₄, compound 4) reacted with spermine to give 3a. The >P(N-spiro) group in each cyclophosphazene ring of 3a–c is stereogenic and homotopic, and so it is expected, and confirmed, that the spermine-bridged gem-disubstituted cyclotriphosphazatrienes (3a–c) should be chiral and exist in meso and racemic forms. The proton-decoupled ³¹P NMR spectroscopy of 3a–c gave rise to two sets of signals in a 1 ∶ 1 ratio consistent with formation of meso and racemic forms. The meso and racemic forms of 3a (gem-diphenyl derivative) were separated by column chromatography and characterised by X-ray crystallography; this enabled the unequivocal assignment of the two sets of ³¹P NMR signals to the meso and racemic forms of 3a. This is the first time that the existence of chiral configurational isomers has been elucidated in the field of spermine-bridged cyclophosphazene compounds, and only the second time in cyclophosphazene chemistry.