synthesis, thermal rearrangement and silylation reactions of (alkoxy) (aryloxy) cyclotriphospazenes

Abstract

The details of the synthesis of chloro(p?cresoxy)- and (alkoxy)(p?cresoxy)cyclotriphosphazenes, thermal rearrangement of (alkoxy)(p?cresoxy)cyclotriphosphazenes, and silylation reactions of a few alkoxy? and (alkoxy)(p?cresoxy)cyclotriphosphazenes are presented in this thesis. In addition, the results of an X?ray crystallographic study of an oxocyclotriphosphazane obtained in the thermal rearrangement study is also discussed. A systematic study of the reaction of N?P?Cl? with sodium p?cresoxide has been carried out and a series of chloro(p?cresoxy)cyclotriphosphazenes — N?P?Cl???(OC?H?CH??p)? (n = 1–6) — is obtained. Dimethylamino derivatives of the chloro(p?cresoxy)cyclotriphosphazenes have been prepared and their ¹H NMR spectra recorded to deduce the isomeric compositions at various stages of substitution (n). By far, the non?geminal mode of substitution predominates and only a small quantity of geminal isomers (< 5%) is formed at the bis and tris stages. A 2p–3d “through?space” interaction has been proposed to account for the formation of the cis isomer in greater yield compared to that of the trans isomer at the bis stage of chlorine replacement. Statistical factors largely determine the relative yields of isomers at the subsequent stages of chlorine replacement. The chloro(p?cresoxy)cyclotriphosphazenes have been used to prepare the alkoxy derivatives N?P?(OC?H?CH??p)???(OR)? (n = 1–6; R = Me, Et, CH?CF?, CH?Ph). The positional isomers of the (methoxy)(p?cresoxy) derivatives have been separated by chromatography and characterised. The (alkoxy)(p?cresoxy) derivatives, N?P?(OC?H?CH??p)???(OR)? (n = 1–6; R = Me, Et, CH?Ph), have been subjected to thermal rearrangement to obtain oxocyclophosphazanes. The thermal rearrangement reactions have been carried out in the temperature range 150–285°C, under low pressure (0.1–1.0 mm Hg). The mono(p?cresoxy) derivative yields a mixture of two isomeric oxocyclophosphazanes. In all other cases, a single product is observed. An X?ray crystallographic investigation of an oxocyclotriphosphazane, N?Me?P?O?(OC?H?CH??p)?, obtained from the thermal rearrangement of trans?N?P?(OC?H?CH??p)?(OMe)?, shows that the configuration of the starting material (trans) is retained in the oxocyclotriphosphazane — N?Me?P?O?(OC?H?CH??p)?. The rearrangement thus appears to be stereospecific. Based on crystallographic and spectroscopic evidence, it is proposed that all the trans?oxocyclotriphosphazanes adopt a twist?boat conformation for the N?P? ring, whereas the ring in cis derivatives assumes a chair conformation. The involvement of both inter? and intramolecular mechanisms in these reactions has been inferred by carrying out the thermal rearrangement reaction of a mixture of alkoxy derivatives, N?P?(OC?H?CH??p)?(OMe)? and N?P?(OC?H?CH??p)?(OCH?Ph)?. The conventional procedure for the silylation reactions of (alkoxy)cyclophosphazenes, which requires high temperatures (> 150°C) and prolonged reaction periods (~48 h), has been modified in the present study by using iodosilanes generated in situ by the treatment of chlorosilanes with NaI/KI. The silylation of (alkoxy)cyclophosphazenes is complete within a few hours at ambient temperature. The resultant siloxy products are highly moisture?sensitive and undergo hydrolysis rapidly, leading to the formation of (hydroxy)cyclophosphazenes. Even bulky substituents and polymeric structures do not improve the hydrolytic stability. However, this method affords a relatively simple method for the controlled hydrolysis of (alkoxy)cyclophosphazenes. A series of (hydroxy)cyclotriphosphazenes has been prepared using the silylation method and characterised. Spectroscopic data suggest that all the hydroxy phosphazenes exist in the oxophosphazane tautomeric form.