Quantifying the Formation of Stereoisomers On-line During an HWE Reaction Employing Benchtop NMR Spectroscopy

The on-line monitoring of chemical reactions employing NMR spectroscopy provides a deeper insight into the kinetics of the reaction in question and its mechanism. Several nuclei like ¹H, ¹⁹F or ³¹P can be detected exclusively or combined in a consecutive experiment loop to follow the course of the reaction and to identify and quantify the chemical moieties of choice as well as possible intermediates or side-products. Within stereoselective synthesis, besides the conversion of the starting material the decisive step for the operator is to define the major and minor stereoisomers of the desired product and to optimize the chemical process towards complete stereoselectivity.

For the quantification of the stereoselective outcome of chemical reactions e.g. the determination of the enantiomeric excess of the reaction product, typically HPLC methods with chiral columns and reference compounds are considered. Nevertheless, when dealing with diastereoisomers as possible reaction products the quantification of the diastereomeric ratio can be determined directly by NMR spectroscopy without the need for reference compounds or a calibration. Within the following application note, we demonstrate for the first time how benchtop NMR spectroscopy can be employed to not only follow the course of a stereoselective reaction on-line but also to determine its stereochemical outcome. For this purpose, the well-known Horner-Wadsworth-Emmons reaction (HWE reaction) was selected using our latest member of the Spinsolve family the Spinsolve 80 MHz ULTRA system.

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