Reactions of Pentachloro- and Pentafluoro(triphenylphosphazenyi)cyclotriphosphazenes with Sodium Methoxide. Mechanistic Aspects and Their Implications for Nucleophilic Displacement at a Tetrahedral Phosphorus(V) Center

Abstract

Reactions of N3P3(NPPh3)X5 [X = Cl (1), F (2)] with sodium methoxide afford the derivatives N3P3(NPPh3)(OCH3)nX5-n(n = 1-5; X = Cl, F) (3–20), whose structures have been elucidated by NMR (1H, 31P, and 19F) spectroscopy. The successive replacement of chlorine from 1 yields geometrical isomers in unequal proportions whereas the substitution of fluorine from 2 gives geometrical isomers in roughly equal proportions. The chlorine at the ≡P(NPPh3)Cl center is easily replaced whereas the fluorine at the ≡P(NPPh3)F site is not replaced until the last stage. These differences are rationalized in terms of a changeover from an Sn2(P) to an SN1(P) mechanism for the methoxylation of 1 at later stages of substitution and an SN2(P) mechanism persisting throughout for the fluoro system. Attack of the methoxide in the plane of the phosphazene ring is postulated to explain the stereochemical course found for the reaction of 2. © 1986, American Chemical Society. All rights reserved.