Enantioselective Recognition of Racemic Amino Alcohols in Aqueous Solution by Chiral Metal-Oxide Keplerate {Mo132 } Cluster Capsules.

Robert W Pow,Nancy Watfa,Zoё L Sinclair,Leroy Cronin,De‐Liang Long,Yousef M Abul‐Haija,Nicola L Bell

doi:10.1002/chem.202100899

Abstract

Determining the relative configuration or enantiomeric excess of a substance may be achieved using NMR spectroscopy by employing chiral shift reagents (CSRs). Such reagents interact noncovalently with the chiral solute, resulting in each chiral form experiencing different magnetic anisotropy; this is then reflected in their NMR spectra. The Keplerate polyoxometalate (POM) is a molybdenum‐based, water‐soluble, discrete inorganic structure with a pore‐accessible inner cavity, decorated by differentiable ligands. Through ligand exchange from the self‐assembled nanostructure, a set of chiral Keplerate host molecules has been synthesised. By exploiting the interactions of analyte molecules at the surface pores, the relative configuration of chiral amino alcohol guests (phenylalaninol and 2‐amino‐1‐phenylethanol) in aqueous solvent was establish and their enantiomeric excess was determined by 1H NMR using shifts of ΔΔδ=0.06 ppm. The use of POMs as chiral shift reagents represents an application of a class that is yet to be well established and opens avenues into aqueous host‐guest chemistry with self‐assembled recognition agents.

Highlights

Chiral shift reagents (CSRs) or chiral solvating agents (CSAs) allow the relative configuration or enantiomeric excess of a mixture of chiral molecules to be determined by NMR through the formation of noncovalent diastereomers
We have subsequently demonstrated its noncovalent chiral recognition towards amino alcohol species in aqueous solutions, including the assignment of relative configurations of guest species and the determination of approximate enantiomeric excesses of scalemic mixtures
This molybdate framework can be regarded as an icosahedron in which twelve {(MoVI)MoVI5O21(H2O)6} pentagonal units are placed at the vertices and are linked by thirty {MoV2O4}2+ dinuclear linkers (Figure 1a, blue and red polyhedra, respectively)

Summary

Introduction

Systems may be soluble in water and, depending upon the countercation, in nonaqueous media. The molybdenum-based spherical {Mo132} Keplerate-type structure provides an interesting inorganic framework for use in the study of host-guest interactions (Figure 1a).[14] The hollow character of the structure offers a distinct enclosed environment within which entrapped species are exposed to an alternative chemical environment to those found in the bulk media This confined cavity (volume ca 1.5 nm3) acts as a container for regioselective reactions to occur, and has the ability to separate and stabilise entrapped species.[15,16,17] This internal cavity is accessible through twenty surface pores (Figure 1a, yellow), whose rigid nature allows for selective uptake of guest species based on size. We have subsequently demonstrated its noncovalent chiral recognition towards amino alcohol species in aqueous solutions, including the assignment of relative configurations of guest species and the determination of approximate enantiomeric excesses of scalemic mixtures

Results and Discussion

Conclusion

Conflict of Interest