Chirality induction from a chiral guest to the hydrogen bonding network of its hexameric resorcinarene host capsule

The hexameric capsule of resorcin[4]arene 1 is capable of encapsulating tertiary amines, which has recently been used in the application of [( 1 ) 6 (H 2 O) 8 ] as (co-)catalyst in various asymmetric reactions. However, not much is known about the highly asymmetric but conformationally very dynamic structure of the capsule after uptake of chiral molecules. Therefore, in this contribution, we utilize electronic circular dichroism and vibrational circular dichroism spectroscopy to investigate how several chiral guest molecules affect the structural preferences of the capsule [( 1 ) 6 (H 2 O) 8 ]. In particular, we show that one small chiral amine encapsulated in [( 1 ) 6 (H 2 O) 8 ] is sufficient to control and dictate the stereochemical preferences of the entire capsule. Furthermore, neither strong π–π interactions nor a significant steric bulk are required for this induction. The observation of such a chiral imprint of the guest's stereochemistry onto its host molecule is expected to have implications also for other supramolecular capsules

The clusters-in-a-liquid approach for solvation: New insights from the conformer specific gas phase spectroscopy and vibrational optical activity spectroscopy

Vibrational optical activity spectroscopies, namely vibrational circular dichroism (VCD) and Raman optical activity (ROA), have been emerged in the past decade as a powerful spectroscopic tool for stereochemical information of a wide range of chiral compounds in solution directly. More recently, their applications in unveiling solvent effects, especially those associated with water solvent, have been explored. In this review article, we first select a few examples to demonstrate the unique sensitivity of VCD spectral signatures to both bulk solvent effects and explicit hydrogen-bonding interactions in solution. Second, we discuss the induced solvent chirality, or chiral transfer, VCD spectral features observed at the water bending band region in detail. From these chirality transfer spectral data, the related conformer specific gas phase spectroscopic studies of small chiral hydration clusters, and the associated matrix isolation VCD experiments of hydrogen-bonded complexes in cold rare gas matrices, a general picture of solvation in aqueous solution emerges. In such an aqueous solution, some small chiral hydration clusters, rather than the chiral solutes themselves, are the dominant species and are the ones who contribute mainly to the experimentally observed VCD features. We then review a series of VCD studies of amino acids and their derivatives in aqueous solution under different pHs to emphasize the importance of the inclusion of the bulk solvent effects. These experimental data and the associated theoretical analyses are the foundation for the proposed clusters-in-a-liquid approach to account for solvent effects effectively. We present several approaches to identify and build such representative chiral hydration clusters. Recent studies which applied molecular dynamics simulations and the subsequent snapshot averaging approach to generate the ROA, electronic CD, and optical rotatory dispersion spectra are also reviewed. Challenges associated with the molecular dynamics snapshot approach are discussed and the successes of the seemingly random ad hoc explicit solvation reported before are also explained. To further test and improve the clusters-in-a-liquid model in practice, future work in terms of conformer specific gas phase spectroscopy of sequential solvation of a chiral solute, matrix isolation VCD measurements of small chiral hydration clusters, and more sophisticated models for the bulk solvent effects woul