Investigation of a spiro to ansa rearrangement with di-functional alcohols in cyclotriphosphazene derivatives

Abstract

A spiro to ansa rearrangement has been investigated by the reactions of cyclotriphosphazene derivatives having five-membered spiro rings, N3P3Cl4[O(CH2)2NH], (1) and N3P3Cl4[O(CH2)2O], (2) with disodium salts of 1,3-propanediol and 2,2,3,3-tetrafluoro-1,4-butanediol. Compounds 1 and 2 with the strong nucleophile, 1,3-propapanediol, give rearrangement reactions to form stable ansa–ansa compounds, whereas reaction of compounds 1 and 2 with the fluorinated alkoxide results in normal nucleophilic substitution at the PCl2 groups and no rearrangement takes place with the weaker nucleophile. The synthesised compounds (3a, 3b, 4b, 5a–d, 6c and 6d), have been fully characterised by elemental and mass (MS) analysis, and by 1H and 31P NMR spectroscopy. Crystal structures of 3b, 4b, 5b–d and 6c have been characterised by X-ray crystallography. It is found that 1,3-propapanediol is a sufficiently strong nucleophile to cause a spiro to ansa rearrangement in the mono-spiro cyclotriphosphazenes compounds 1 and 2, even though crystallographic evidence shows that formation of the seven- and eight-membered ansa–ansa rings in compound 3b results in distortion and compression of the cyclophosphazene ring. A possible mechanism, which depends on attack of the anionic oxygen nucleophile at spiro-bearing phosphorus atom, was used to help understand the spiro-to-ansa rearrangement reaction.