Stereoselective synthesis of trans N/O dispirocyclic cyclotriphosphazenes based on the steric demands of the constrained 2-pyridyl group

Monospiro- and dispirocyclic cyclotriphosphazenes, 2a and 2b, were synthesized by the reaction of hexachlorocyclotriphosphazene, N3P3Cl6 (1), with 2-(2-pyridylamino)phenol in the presence of base. The reactions of phosphazenes 2a and 2b with sodium (4-methyl-2-pyridyl)oxide afforded tetrakis- and bis(4-methyl-2-pyridyloxy)cyclotriphosphazenes, 3a and 3b, respectively. 31P NMR spectroscopy of 2b and 3b using d-camphorsulfonic acid as a chiral solvating agent indicated that they were racemates of the trans isomers. The trans configuration of trans-3b could also be elucidated by X-ray crystal structural analysis, which suggested that the steric demand of the 2-pyridyl group was responsible for the stereoselective formation of trans-2b. The steric demand of the 2-pyridyl group was supported by the reaction of N3P3Cl6 (1) with N,N′-bis(2-pyridyl)benzene-1,2-diamine, which only gave monospirocyclic cyclotriphosphazene 4a.