Kinetics and mechanism of the reaction between oligosiloxanes and P-trichloro-N-dichlorophosphoryl monophosphazene (Cl3P=N-POCl2)

摘要

P-31 NMR investigation has been made of the action of Cl3P=N-POCl2 (I) first on hexamethyldisiloxane (Me3Si)(2)O and then on oligosiloxanes Me3Si-(OSiMe2)(n)-OSiMe3 n = 2 and n = 3. The reactions were carried out in bulk or in solution with molar ratios siloxane/(1) varying from 1 to 5. It was demonstrated that only the monosubstitution of a chlorine atom by the -(OSiMe2)(n)-OSiMe3 species n = 0, 2, 3 with elimination of trimethylchlorosilane occurred leading to the derivatives Cl2OP-N=PCl2O-(SiMe2-O)(n)SiMe3 (II). For n = 2, 3 the siloxane redistribution reactions were observed by Si-29 NMR analysis. A two steps mechanism is proposed, consisting in a nucleophilic substitution, involving a tricoordinate phosphazenium intermediate, followed by the formation of an active ionic centre probably an oxonium ion, arising from the solvatation by the siloxane of this phosphazenium ion and /or of (II) leading to the redistribution reactions. The influences of the solvent, of trimethylchlorosilane, of the temperature, and of the addition of a protonated species (MDH) were investigated. [References: 25]