Improving the Sensitivity of Enantioanalysis with Densely Fluorinated NMR Probes

Abstract

Nuclear magnetic resonance (NMR) spectroscopy has long been utilized as a classic method for chiral discrimination of enantiomers. However, its sensitivity limitations have hindered the detection of analytes at low concentrations. In this study, we present our efforts to overcome this challenge by employing chiral NMR probes that are labeled with a significant number of chemically equivalent 19F atoms. Specifically, we have designed and synthesized three chiral palladium pincer complexes, all of which are labeled with nonafluoro-tert-butoxy groups to enhance detectability. The recognition of enantiomers with the probe induces distinct changes in microenvironments, resulting in differential perturbations on the chemical shift of the 19F atoms in proximity. This method is applicable to the enantiodifferentiation of various amines, amino alcohols, and amino acid esters. The abundance of 19F atoms enables the detection of chiral analytes at low concentrations, which is otherwise challenging to achieve through traditional 1H NMR-based analysis. Two of the probes are constructed with asymmetric pincer ligands with structurally varied sidearms, allowing for facile manipulation of the chiral binding pocket. The C2 symmetrical probe possesses 36 equivalent 19F atoms, enabling the determination of enantiocomposition of samples with concentrations in the low micromolar range.

Chiral Discrimination of Acyclic Secondary Amines by 19F NMR

“…In such cases, we employed a correction coefficient, which was calculated based on the ratio of the 19 F peak area seen in the racemic analyte analysis. Our prior research indicates that using this correction coefficient effectively compensates for the differential affinity of the chiral probe toward the enantiomers, yielding precise enantiomeric excess (ee) evaluations. ,,, As presented in Table , the ee values determined by our methods align closely with the actual enantiocomposition of the samples (for details, see Figures S10–S12 in Supporting Information), showing an average deviation of less than 2%. It is worth noting that our approach bypasses the chemical derivatization often necessitated in chromatographic analyses of amines, paving the way for a quick and direct chiral examination.…”

“…Alternatively, chiral environments can be introduced using chiral aligning solvents, which enable the discrimination of analytes even without polar groups. − Recently, there has been growing attention toward chirality sensing using 19 F-labeled probes. − Given the scarcity of naturally occurring organofluorine compounds, 19 F NMR detection is rarely affected by background signals. − A variety of 19 F NMR-based chiral derivatization agents have been crafted, facilitating the detection and differentiation of various amines, carboxylic acids, and amino acids. , Additionally, 19 F-labeled probes, which can reversibly bind to chiral analytes, have been designed. These probe-analyte adducts typically form instantly, enabling swift enantioanalysis based on distinct 19 F NMR signals for each enantiomer. , This strategy has proven effective for distinguishing chiral alcohols, amides, carboxylic acids, primary amines, nitriles, and more. ,,, However, acyclic secondary amines remain challenging analytes due to their significant steric hindrance around the nitrogen atom and structural flexibility.…”

Bioinspired Fluorine Labeling for 19F NMR-Based Plasma Amine Profiling

“…The study aimed to showcase the profiling of amino acids using easily accessible 400 MHz NMR instruments. However, to improve the detection limit, it is advisable to increase the magnetic field strength and utilize a cryogenic probe. − …”

“…19 F NMR spectroscopy possesses advantageous properties, including 100% natural abundance, a relative receptibility of 0.834, and a wide chemical shift range of over 400 ppm. This technique offers a broader chemical shift range and simpler spectra compared to 1 H NMR, making it effective for analyzing fluorinated compounds. − However, there are several challenges associated with using 19 F NMR for metabolite profiling. The first challenge is the need to introduce fluorine-containing functional groups into the metabolites to enable their detection using 19 F NMR.…”

Chiral Discrimination of Monosaccharides Derivatized with 2-Fluorophenyl Hydrazine Using 19F NMR Spectroscopy

“…In this study, a simple yet effective chiral analysis method was developed for 15 aldose sugars by combining 2-fluorophenyl hydrazone derivatization with the utilization of octahedral chiral gallium ([Ga]­Na) and scandium ([Sc]­Na) complexes. − Unlike other approaches that necessitate optically pure reagents for sugar derivatization, this method utilizes 2-fluorophenyl hydrazine and chiral shift reagents. The high-yielding synthesis of sugar hydrazones without any side products facilitated efficient identification and differentiation of sugar enantiomers using 19 F NMR spectroscopy. ,,− This method offers both quantitative and qualitative analyses, the advantages of a broad analysis window, and the absence of background signals in 19 F NMR spectroscopy.…”