Mechanism of the Reaction between Alkyl or Aryl Grignard Reagents and Hexachlorocyclotriphosphazene: An Explanation of Bi(cyclophosphazene) Formation^1,2

An understanding has been obtained of the complex mechanisms that are followed when alkyl or aryl Grignard reagents react with (NPCl₂)₃ (2) in tetrahydrofuran. The main products are monoalkylcyclotriphosphazenes (3) and bi-(cyclotriphosphazenes) (4). The predominance of one product or the other depends on the reaction temperature and on the organic functionality of the Grignard reagent. The structural characterization of the bi(cyclotriphosphazenes) is described together with the reaction pathways that lead to bi(cyclotriphosphazene) formation. Two competitive pathways exist. Nucleophilic substitution on 2 yields the monoalkylcyclotriphosphazenes (3), while metal-halogen exchange on 2, followed by chlorine replacement, generates the metallophosphazene intermediate (6). Species 3 and 6 react to form the bi(cyclophosphazenes). Compounds 3 can also result from metal-halogen exchange between 6 and 2. Steric effects play a powerful role in directing the course of the reaction. The possible application of these results to macromolecular synthesis is discussed.