Parahydrogen-induced polarization in the presence of NMR shift reagents

In situ NMR spectroscopy using parahydrogen (p-H?) is a powerful tool to st udy transition metal-catalyzed hydrogenations because the signals are enhan ced by several orders of magnitude. This NMR technique is called parahydrog en-induced polarization (PHIP). Frequently, in the course of such studies, it is desirable either to differentiate overlapping resonances or to discri minate chiral products containing only one asymmetric carbon atom. In regul ar NMR spectra, this can be achieved using either paramagnetic shift reagen ts or chiral solvents. Since paramagnetic shift reagents reduce the relaxat ion 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 f unction. Therefore, to explore the potential of either NMR tool, namely usi ng shift reagents and/or chiral solvents, a study was conducted to explore the feasibility of their combinations with the PHIP method. This paper desc ribes the first observation of PHIP-derived polarization signals in the pre sence of shift reagents. It is demonstrated that a combination of the PHIP method with both lanthanide shift reagents and chiral solvating agents is p ossible. With the help of lanthanide shift reagents, overlapping polarizati on signals of several substrates containing different functional groups can be shifted apart, which serves to simplify the analysis of spectra. In add ition, it is possible to distinguish between polarization signals of differ ent enantiomers by using either chiral lanthanide shift reagents or chiral solvating agents. The results presented extend the possibilities of the PHI P method in the field of asymmetric hydrogenation. Copyright (C) 2000 John Wiley & Sons, Ltd.