Phosphazene bond formation

Phosphoranimines can react as electrophiles in electrocyclic reactions with arynes. Specifically, P-vinyl-X -phosphazenes with varying substituents have been found to undergo electrocyclic ring formation with benzyne precursors. Curiously, they do not exhibit straightforward [2-1-2] or [2-1-4] addition involving the C=C double bond, rather they insert the benzyne into the phosphazene bond, Scheme 5, yielding a heteroatomic bicyclic ring system. 

Thus far, the survey of phosphazene elastomers has been based on the formation and modification of poly(dlchlorophosphazene). Although a large variety of polymers can be prepared by this approach, there are limitations In the preparation of polyphosphazenes with phosphorus-carbon bonds. The reaction of poly(dlchlorophosphazene) with organometalllc agents, such as RMgX or KLl, results mainly In decomposition and not the desired polymers [NPR.]. There are three possible approaches to the preparation ox polyphosphazenes with phosphorus-carbon bonds polymerization of substituted trlmers, poly(dlfluorophosphazene), and thermolysis of small linear molecules. These three approaches will be discussed In turn. 

Benzynes, generated from 2-(trimethylsilyl)phenyl triflates, react with P-alkenyl-X(5)-phosphazenes via a formal r-insertion into the PN bond. A subsequent retro 2 - -2-cycloaddition/6 r electrocyclization-protonation cascade rationalizes the formation of the resulting 1,4-benzazaphosphorinium triflates (Scheme 21). 

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