A spiro to ansa rearrangement in cyclotriphosphazene derivatives

Abstract

Nucleophilic substitution reactions of cyclotriphosphazene derivatives having five-membered spiro rings, N₃P₃Cl₄[O(CH₂)₂X] (X = NH or O) with alkoxides (of tetraethylene glycol and some mono-functional alcohols) give unexpected rearrangements to form stable seven-membered ring ansa compounds, even though crystallographic evidence shows ring distortion and compression of the cyclophosphazene ring. With weaker nucleophiles such as sodium phenoxide and pyrrolidine substitution at a PCl₂ group is preferred and no rearrangement takes place. In contrast, reactions of the analogous phosphazenes containing six-membered spiro rings, N₃P₃Cl₄[O(CH₂)₃X], with all of the above reagents give only normal substitution reactions at the PCl₂ moieties and no rearrangement products. The spiro to ansa rearrangements in cyclophosphazenes are remarkable as the reported primary reaction products with the same difunctional reagents HO(CH₂)₂XH are predominantly spiro, with some dangling and bridging derivatives, but no ansa compounds.