Importance of molecular symmetry for enantiomeric excess recognition by NMR

Abstract

Recently prochiral solvating agents (pro-CSA) became a spotlight for the detection of enantiopurity by NMR. Chemical shift non-equivalency in achiral hosts introduced by the presence of chiral guests yields observable...

Critical theoretical analysis of supramolecular chirogenesis in a sterically hindered porphyrin

“…[19d] Position of guests and influence on ECD spectra. Based on the crystallographic data [8] we concluded that one porphyrin cation binds two CSA molecules in each of its cavities. Using this ratio and the P[SO4 2-][HSO4 -]4 crystal data the corresponding host-guest complexes were built with a subsequent conformational search.…”

“…[9b] In particular, we were interested in the α2β2 () atropisomer due to its recent application in the enantiomeric excess discrimination of chiral analytes (CSA) using NMR spectroscopic techniques. [8] As we were unable to obtain the crystal structure of the corresponding adducts we used the α2ß2-P[SO4 2-][HSO4 − ]4 crystal structure [8] as a model. ).…”

“…). [8] The macrocycle in P is highly distorted due to periinteractions between the β-ethyl and meso-aryl residues. [10] To minimize the steric interactions of the peripheral substituents and to a smaller extent to minimize the electrostatic interactions between four core protons the porphyrin adopts overall a saddleshaped structure.…”

“…[10] To minimize the steric interactions of the peripheral substituents and to a smaller extent to minimize the electrostatic interactions between four core protons the porphyrin adopts overall a saddleshaped structure. [11] Normal-structural decomposition (NSD) analysis [11,12] of α2ß2-P[SO4 2-][HSO4 − ]4 [8] and solvated hexacationic porphyrin α2ß2-P showed a major saddle distortion (3.27 -3.92 Å) together with small contributions from ruffling, wave and dom deformations (Figure S1 and Table S1). As a result, two cavities are formed on both sides of the porphyrin (Figure 1).…”

“…For conformational analysis the NMR and X-ray crystallographic data from our previous work were used. [8] We investigated the influence of various geometric factors, such as the position of (R)-CSA, distortion of porphyrin plane, and orientation of both aromatic and non-aromatic peripheral porphyrin substituents on the ECD spectra. Special attention was given to consider the origins of potential mistakes in spectral simulations, such as a lack of significant conformations and accidental agreement of the experimental and theoretical spectra.…”