Stereoisomer discrimination through π-stacking interactions in spirocyclic phosphazenes bearing 2,2′-dioxybiphenyl units

The molecular structure of the 2,2'-dioxybiphenyl spirocyclic tetrachlorocyclotriphosphazene [N3P3Cl4(O2C12H8)] (3) was determined by X-ray crystallography, The structure and conformation of 3 in the crystal are compared and discussed with respect to those of its analogues trispiro and dispiro derivatives, [N(3)p(3)(O2C12H8)(3)] (1) and [N3P3Cl2(O2Cl2H8)(2)] (2), respectively. Both the (R) and (S) stereoisomers are present in the lattice of 3 as arranged in pairs (R)...(S) held together through sandwich-type pi-stacking interactions of the (C7-C12)/(C7-C12)' phenyl rings of the 2,2'-dioxybiphenyl groups. The polymer-like assembly of 2 was formed by interactions between the (R) and (S) portions of two meso molecules [in the sequence (...)(R)(...)(S)(...)(R)(...)(S)(...)(R)(...)(S)(...)]. In this case, the pertinent phenyl rings involved in the noncovalent pi-interactions assume a parallel-displaced configuration. To the best of our knowledge, no chiral discrimination in cyclic phosphazenes has been hitherto described, and the present findings provide the first evidence for the discrimination of stereoisomers in spirophosphazenes and the noncovalent forces that govern the chiral recognition in these systems. The energy of the pi-stacking interaction increases with increasing number of 2,2'-dioxybiphenyl groups; i.e., on passing from 3 to 2. In the case of molecule 1, the discrimination of enantiomers does not take place at all because the n-stacking interaction becomes impossible because of steric hindrance. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 6945t Weinheim, Germany, 2003)