Parahydrogen-induced polarization in the presence of NMR shift reagents

Abstract

In situ NMR spectroscopy using parahydrogen (p-H2) is a powerful tool to study transition metal-catalyzed hydrogenations because the signals are enhanced by several orders of magnitude. This NMR technique is called parahydrogen-induced polarization (PHIP). Frequently, in the course of such studies, it is desirable either to differentiate overlapping resonances or to discriminate chiral products containing only one asymmetric carbon atom. In regular NMR spectra, this can be achieved using either paramagnetic shift reagents or chiral solvents. Since paramagnetic shift reagents reduce the relaxation times of all nuclei in the system, the compatibility of shift reagents with the PHIP method is not straightforward. Furthermore, shift reagents by themselves could behave as homogeneous catalysts or interfere with their function. Therefore, to explore the potential of either NMR tool, namely using shift reagents and/or chiral solvents, a study was conducted to explore the feasibility of their combinations with the PHIP method. This paper describes the first observation of PHIP-derived polarization signals in the presence of shift reagents. It is demonstrated that a combination of the PHIP method with both lanthanide shift reagents and chiral solvating agents is possible. With the help of lanthanide shift reagents, overlapping polarization signals of several substrates containing different functional groups can be shifted apart, which serves to simplify the analysis of spectra. In addition, it is possible to distinguish between polarization signals of different enantiomers by using either chiral lanthanide shift reagents or chiral solvating agents. The results presented extend the possibilities of the PHIP method in the field of asymmetric hydrogenation. Copyright © 2000 John Wiley & Sons, Ltd.