Self-organization by chiral recognition based on ad hoc chiral pockets in cyclotriphosphazenes with binaphthoxy and biphenoxy substituents: an X-ray, NMR and computational study

The chiral cyclic triphosphazenes (R)-(-)-[N(3)P(3)(O(2)C(20)H(12))-(O(2)C(12)H(8))C(12)] (I) and (R, R)-(-) [N(3)P(3)(O(2)C(20)H(12))(2)(O(2)C(12)H(8))] (II), prepared by sequential substitution of [N(3)P(3)Cl(6)] using (R)-2,2'-dihydroxy-1,1'-binaphthyl, 2,2'-dihydroxy-1,1'-biphenyl and Cs(2)CO(3) in acetone, have been investigated by single-crystal X-ray diffraction, variable-temperature NMR spectroscopy in solution and MM calculations under periodic boundary conditions in the gas state. In both compounds only the diastereomer corresponding to the (R)-biphenoxy configuration is present in the solid state. In spite of the existence of quite large voids potentially occupied by solvents, the configuration adopted by the biphenoxy substituents is solely determined by packing interactions consisting of definite preferential supramolecular interactions between a biphenoxy group of one molecule and the chiral pocket created between the biphenoxy and binaphthoxy groups of another molecule. A network of T-shaped non-covalent interactions ensures an efficient chiral discrimination. The in silico simulations confirmed this hypothesis. Both the concentration-dependence and temperature-variable (1)H NMR spectra in solution confirmed this behaviour showing that on lowering the temperature one of the two diasteroisomers vanished after conversion into the more stable atropisomer as a consequence of chiral induction of the (R)-binaphthyl system on the (R)-biphenyl configuration. In further agreement with the X-ray structure as well as with the energy distribution resulting from the in silico calculations, the (1)H NMR spectra show the dependence of the chemical shift on concentration, which suggests the presence of aggregates in solution. Therefore molecules I and II are capable of a special type of self-organization by chiral recognition based on the ad hoc generation of chiral pockets. RI Punzo, Francesco/A-4921-2011; Lombardo, Giuseppe/H-8227-2012 OI Punzo, Francesco/0000-0003-4212-8064;