Synthesis of Linear and Cyclic Carbophosphazenes via an Oxidative Chlorination Strategy

The use of a mild, oxidative chlorination route for the synthesis of linear and cyclic carbophosphazenes is described.
For example, chlorination of the linear PNCN chain Ph2P−NC(Ph)−N(SiMe3)2 (1) with C2Cl6 led to the clean
formation of the previously known 8- and 6-membered rings [Ph2PNC(Ph)N]2 (2) and [Ph2PNC(Ph)NP(Ph)2N] (3),
respectively. In a similar fashion, the N-alkyl-substituted PNCN derivatives, Ph2P−NC(Ph)−N(tBu)SiMe3 (4) and
Ph2P−NC(Ph)−NiPr2 (7) were readily converted by C2Cl6 into the halogenated derivatives ClPh2PN−C(Ph)NtBu (5) and [ClPh2PNC(Ph)−NiPr2]Cl (8), respectively. Protonation of 5 was accomplished using HCl and gave
the carbophosphazenium salt [ClPh2PN−C(Ph)N(tBu)H]Cl (6). In addition, the isolation of a rare 8-membered
P2N4C2 heterocycle [(Cl3PN)ClPNC(Ph)NP(Cl)2NC(Ph)N] (9) from the reaction of PCl5 and Li[PhC(NSiMe3)2] is
reported. Treatment of 9 with one equivalent of GaCl3 led to the discovery of an unusual Lewis acid-induced ring
contraction reaction whereby the (PNCN)2 ring in 9 is converted into the novel 6-membered P2N3C heterocyclic
adduct [(Cl3PN)ClPNP(Cl)2NC(Ph)N]·GaCl3 (10) with concomitant release of PhCN. Structural characterization of
compounds 1, 5, 6, and 8−10 by single-crystal X-ray diffraction is also provided.