A simple protocol for determination of enantiopurity of amines using BINOL derivatives as chiral solvating agents via1H- and 19F-NMR spectroscopic analysis

Molecular Recognition and Chiral Discrimination from NMR and Multi-Scale Simulations

Chiral molecules are particularly interesting to the pharmaceutical and agrochemical sectors due to their chemical and physical properties. Separation and identification of enantiomers are critical for a broad range of compounds, and discriminating stereoisomers in solution remains a key analytical challenge. Nuclear Magnetic Resonance (NMR) with Matrix-Assisted Diffusion-Ordered Spectroscopy (MAD), in the presence of chiral resolving agents, has emerged as a tool to explore these chiral mixtures. However, insight into the intermolecular interactions that lead to chiral recognition is still limited. Here, we combine experimental MAD studies with computational approaches to investigate the enantioselective discrimination of Mandelic Acid (MA) enantiomers using (R)-BINOL and (S)-BINOL. Molecular dynamics simulations explain the differences in diffusion coefficients for heterochiral complexes. Furthermore, quantum mechanical calculations confirmed enantioselective binding preferences due to differences in Gibbs free energies, highlighting the fundamental interactions and structural criteria that explain the NMR shielding and the diffusion trends. This integrated approach bridges experimental and theoretical studies, offering a comprehensive understanding of chiral recognition mechanisms and elucidating the observed heterochiral preference of BINOL for MA enantiomers. Our findings advance the field of chiral analysis and lay a foundation for future developments for identifying stereoisomers and recognition modes underlying enantioselective binding.

Enantiodifferentiation of Thiohydantoin Derivatives by NMR in the Presence of Chiral Solvating Agent: (1R,2S)-Ephedrine and Chiral Symmetrical and Unsymmetrical Thioureas

2-Thiohydantoin derivatives, including different substitutions at N-1 and C-5 (5-methyl-, 5-isopropyl-, 1-acetyl-5-methyl-, and 1-acetyl-5-isopropyl-) (1-4, respectively), were synthesized by the known literature methods. In these synthetic pathways, it was reported that enantiomerically pure 2-thiohydantions were obtained in the absence of any solvent via the reaction of L-amino acids with thiourea (1&2) and via the reaction of L-amino acids with NH4SCN and acetic anhydride (3&4). However, in this study, in contrary to the previous literature studies, racemic mixtures of 2-thiohydantoins were obtained although the same synthetic methods were used. The racemic nature of 2-thiohydantoin derivatives (1-4) was proved by using 1H NMR analysis in the presence of (1R,2S)-(-)-ephedrine as a chiral auxiliary. In addition, the enantiomers of 3&4 were also resolved on chiral stationary phases by HPLC analyses. Furthermore, newly synthesized unsymmetrical chiral thioureas (S-1&S-2) and previously synthesized symmetrical ones (SS-3&SS-4) were used as chiral solvating agent (CSA) for the enantiodiscrimination of the thiohydantoins (5&6), previously reported. Optimal CSA/substrate ratios were determined for the best enantiodiscrimination.

Luminescent Behavior of Zn(II) and Mn(II) Halide Derivatives of 4-Phenyldinaphtho[2,1-d:1',2'-f][1,3,2]dioxaphosphepine 4-Oxide and Single-Crystal X-ray Structure Determination of the Ligand

The two enantiomers of chiral phosphonate 4-phenyldinaphtho[2,1-d:1',2'-f][1,3,2]dioxaphosphepine 4-oxide, O=PPh(BINOL), were synthesized from the proper 1,1'-bi-2-naphtol (BINOL) enantiomer and characterized. The structure of the (S)-enantiomer was elucidated by means of single-crystal X-ray diffraction. The reaction with anhydrous ZnBr2 afforded complexes having the general formula [ZnBr2{O=PPh(BINOL)}2] that showed intense fluorescence centered in the near-UV region rationalized on the basis of TD-DFT calculations. The corresponding Mn(II) complexes with the general formula [MnX2{O=PPh(BINOL)}2] (X = Cl, Br) exhibited dual emission upon excitation with UV light, with the relative intensity of the bands dependent upon the choice of the halide. The highest energy transition is comparable with that of the Zn(II) complex, while the lowest energy emission falls in the red region of the spectrum and is characterized by lifetimes in the hundreds of microseconds range. Although the emission at lower energy can also be achieved by direct excitation of the metal center, the luminescence decay curves suggest that the band in the red range is possibly derived from BINOL-centered excited states populated by intersystem crossing.