Studies of phosphazenes. Part 14. The tautomerism of oxocyclotriphosphazadienes

Abstract

The tautomeric behaviour of ‘monohydroxycyclotriphosphazatrienes’ has been investigated by ³¹P n.m.r. spectroscopy. These derivatives exist as oxocyclotriphosphazadiene tautomers in which the hydrogen atom is attached to a ring nitrogen atom α to the phosphoryl group. Three types of prototropic behaviour are observed : (a) no exchange is detected and only one tautomer is present [e.g. N₃HP₃(NHBu^t)₂R₃O (R = OMe or OEt)]; (b) exchange takes place between two equivalent sites and only one tautomer is observed [e.g. N₃HP₃R₅O (R = OMe or OPh); N₃HP₃Ph₄RO (R = OMe or OEt)]; and (c) exchange occurs between two non-equivalent sites and two tautomers are present [e.g. N₃HP₃Ph₂R₃O (R = OMe, OEt, or OPrⁿ)]. It is shown that basicity calculations using substituent constants have predictive value since they are in good agreement with the spectroscopic observations.