Quantifying Intermolecular Interactions in Asymmetric Peptide Organocatalysis as a Key toward Understanding Selectivity

The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs free energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this, we use advanced methods such as transverse relaxation (R-2), diffusion measurements, saturation transfer difference (STD), and chemical shift (delta) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results. The molecular recognition by the catalyst is approximately 3 kJ mol(-1) in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol(-1) is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions, the self-association of both the catalyst and the substrate in toluene was found to play an important role and thus needs to be taken into account in reaction screening.