Chiroptical study of cryptophanes subjected to self-encapsulation

In 1,1,2,2-tetrachloroethane-d₂ , the ¹²⁹ Xe NMR spectrum of the Xe@cryptophane-223 complex bearing seven acetate groups (Xe@1 complex) shows an unusually broad signal compared with that of its congeners (Chapellet, LL. et al. J. Org. Chem. 2015;80:6143-6151). To interpret this unexpected behaviour, a ¹ H NMR analysis and a thorough study of the chiroptical properties of 1 as a function of the nature of the solvent have been performed. The ¹ H NMR spectra of 1 reveal that a self-encapsulation phenomenon takes place in DMSO-d₆ and 1,1,2,2-tetrachloroethane-d₂ solvents. Thanks to the separation of the two enantiomers of 1 by HPLC on chiral stationary phase, the two enantiomers of 1 have been studied in detail by polarimetry, electronic (ECD), and vibrational (VCD) circular dichroism spectroscopies. Except for ECD spectroscopy, these chiroptical techniques reveal spectroscopic changes as a function of the nature of the solvent. For instance, in DMSO and 1,1,2,2-tetrachloroethane, in which the self-encapsulation phenomenon takes place, the sign of the specific optical rotation of [CD(-)₂₅₄ ]-1 and [CD(+)₂₅₄ ]-1 is changed. These results have then been compared with those obtained with cryptophane-223 bearing only one acetate group on the propylenedioxy linker (compound 2) and with cryptophane-223 bearing six acetate groups (compound 3). A self-encapsulation phenomenon is also observed with compound 2. Finally, compounds 2 and 3 show different chiroptical properties compared with those obtained with the two enantiomers of compound 1.