Characterization of a chiral stationary phase by HR/MAS NMR spectroscopy and investigation of enantioselective interaction with chiral ligates by transferred NOE

The surface chemistry of a chiral stationary phase (CSP) with a (tert-butyl carbamoyl) quinine selector immobilized on thiol-modified silica has been characterized by H-1 HR/MAS NMR and Si-29 Cp/MAS NMR spectroscopy. The mostly well-resolved H-1 signals could be assigned to stem from the surface-bound selector and the latter suggested a bi- and trifunctional silane linkage. Suspended-state NMR spectroscopy thus proved a well-characterized surface chemistry as proposed. To study chiral recognition phenomena in the presence of the CSP, H-1 HR/MAS 2D transfer NOESY investigations in methanol-d(4) have been undertaken with various solutes including N-3,5-dinitrobenzoyl derivatives of leucine (DNBLeu) and N-acetyl phenylalanine (Ac-Phe). Both (R)- and (S)-enantiomers of DNB-Leu and Ac-Phe interacted with the tBuCQN-CSP as indicated by negative cross-peaks in the trNOESY spectra, while the 2D NOESY of the dissolved solutes in absence of the chiral stationary phase showed positive cross-peaks. The intensities of the trNOE cross-peaks were much stronger for the (S)-enantiomers. This stereoselectivity paralleled the experimental chromatographic behavior, where the (S)-enantiomers revealed stronger binding and retention on the tBuCQN-CSP as well. Hence, we were able to correlate the retention behavior to the trNOE NMR spectroscopic data in a qualitative manner.