Absolute Configuration Assignment of Chiral Resorcin[4]arenes from ECD Spectra

Racemates of five chiral resorcin[4]arenes, four tetra-O-substituted and one hepta-O-substituted, have been resolved by enantioselective HPLC, and their ECD spectra have been recorded online by stopped-flow measurements. The absolute configuration has been assigned by comparison of the experimental ECD spectra with DFT and semiempirical calculations. For the four tetra-O-substituted resorcin[4]arenes, the ECD exciton couplet at longer wavelength depends on the chirality induced in the arene scaffold by the substituents rather than on the precise nature of the substituents themselves. Accordingly, the exciton chirality model with excitons localized on the arene scaffold, here generalized to C_n symmetry, accurately describes the relationship between stereochemistry and chiroptical properties for this couplet, while its application at shorter wavelengths is unsafe. For the significantly larger hepta-O-substituted system the assignment particularly benefits from the use of the semiempirical ZINDO method.