Chiral configurations of cyclophosphazenes

Tetracoordinated phosphorus atoms in derivatives of cyclophosphazenes are p entavalent and are potentially chiral. Chirality does not seem to have been investigated nor discussed for any literature examples of crystal structur es of those cyclophosphazene compounds which are expected to be chiral. In cyclotriphosphazatriene compounds with ansa-substituted macrocyclic rings t he two phosphorus atoms attached to the macrocycle are chiral, although the cis-ansa cyclotriphosphazatriene-macrocycle 1 is the meso form. Reaction o f 1 with the di-secondary amine, piperazine, provides a convenient way to i nvestigate the chiral configurational properties of cyclophosphazene compou nds. It is found by X-ray crystallography that there are two configurationa l isomers (one meso and one racemate) of the singly bridged di(cyclophospha zene-macrocyclic) piperazine derivative 6i, and that there are two configur ational isomers (both meso) of the doubly bridged di(cyclophosphazene-macro cyclic) piperazine derivative 8i; in 8i one meso form has a center of symme try and the other a plane of symmetry. The chiral configurational propertie s of each stage of the reaction scheme for formation of 6i and 8i from 1 ha ve been confirmed by P-31 NMR spectroscopy and the results are consistent w ith inversion of configuration at phosphorus at each step of the reaction o f >P(OR)Cl groups with HNR'R" to form >P(OR)(NR'R") derivatives: viz. react ion of 1 with piperazine gives the monosubstituted compound 4i, which exist s as a racemate with the macrocyclic ring in the trans-configuration; react ion of compound 4i with 1 gives the two configurational forms (meso and rac emate) of the singly bridged derivative 6i with the macrocyclic rings in th e trans-trans configuration; reaction of 6i with piperazine gives the monos ubstituted singly bridged derivative 7i. which exists as two racemic mixtur es with the macrocyclic rings in cis-trans configurations; and intramolecul ar condensation of 7i gives the two configurational forms of the doubly bri dged derivative 8i with each of the macrocyclic rings in the cis-cis config uration. In this work the configurational properties of derivatives of cycl otriphosphazatriene rings with two (one N3P3 unit) or four (two N3P3 units) chiral centers have been rationalized. It is found that the chiral structu res of cyclotriphosphazatriene derivatives may be represented by 2D molecul ar diagrams, as long as the Fischer rules for chiral carbon compounds are f ollowed for phosphorus compounds.