Stereo-selectivity in a cyclotriphosphazene derivative bearing an exocyclic P–O moiety

Abstract

Nucleophilic substitution reactions of N₃P₃Cl₄[O(CH₂)₂NCH₃], (1) with the sodium salts of mono- and di-functional alcohols [methanol (2), phenol (3), tetraethyleneglycol (4) and 1,3-propanediol (5)] were carried out in order to investigate a possible directing effect of the spiro O-moiety on the formation of mono-substituted (2a, 3a), non-geminal di-substituted (2c, 3c) and ansa (4a, 5a) derivatives. Compounds isolated from the reactions were characterized by elemental analysis, mass spectrometry, ¹H and ³¹P NMR spectroscopy and X-ray crystallographic analysis showed that the substituent OR in compounds (2a, 3a and 2c, 3c) and the ansa-ring in compounds (4a, 5a) formed cis to the P–O moiety of the exocyclic [O(CH₂)₂NCH₃] spiro ring. The formation of products (2a–d, 3a–d, 4a, 5a and 5b) was quantified from the ³¹P NMR spectra of the reaction mixtures, which showed an overwhelming preference for derivatives (2a, 3a, 2c, 3c, 4a, 5a) with the substituent cis to the P–O moiety of the exocyclic spiro ring (2a, 3a, 2c, 3c, 4a, 5a), except for reaction with 1,3-propanediol where the six-membered ring spiro derivative (5b) was about three times more abundant than the eight-membered ring ansa-derivative (5a). Overwhelming formation of products with the substituent cis to the exocyclic P–O moiety is proof that the cation-assisted mechanism is responsible for the stereo-selectivity in the reactions with alkoxides.