Nucleophilic substitution reactions of 10- and 11-membered fluorodioxy ansa cyclotriphosphazene derivatives

Abstract

The reactions of cyclophosphazenes with 10-membered ansa-{N₃P₃Cl₄[OCH₂(CF₂)₃CH₂O] (1a)} and 11-membered ansa-{N₃P₃Cl₄[OCH₂(CF₂)₄CH₂O] (1b)} rings with the sodium salts of methanol in a THF solution at different molar ratios were used to investigate the reaction pathways and mechanism of nucleophilic substitution at the PCl₂ and PCl(OR) phosphorus atoms. The reactions afforded eleven products, whose structures have been characterized by elemental analysis, mass spectrometry, ¹H, ¹⁹F and ³¹P NMR spectroscopy and X-ray crystallography; mono-methoxy derivatives (2a, 3a, 3b), di-methoxy derivatives (5a–7a, 5b), tri-methoxy derivatives (8a, 8b) and the tetra-methoxy derivatives (9a, 9b). The X-ray crystallographic studies of four compounds (6a–8a and 8b) demonstrated unambiguously that nucleophilic substitution reactions at the ansa-ring PCl(OR) phosphorus atoms of the cyclotriphosphazene compounds N₃P₃Cl₄[OCH₂(CF₂)_nCH₂O] n = 3 (1a) and 4 (1b) occurred with a retention of configuration for both the 10- and 11-membered fluorodioxy ansa rings, respectively. The results confirmed that the reactions with 1a containing the 10-membered ansa-ring occurred competitively at both the PCl₂ and P(OR)Cl moieties with an approximate 8 : 1 preference at the PCl₂ group, whereas reactions with 1b containing the 11-membered ansa-ring occurred exclusively at the PCl₂ group before the P(OR)Cl moiety. The results were mainly rationalized in terms of the P–Cl bond lengths of the reactants and the cation-assisted mechanism of reaction.