13C PHIP NMR spectra and polarization transfer during the homogeneous hydrogenation of alkynes with parahydrogen

Homogeneously catalyzed hydrogenations of unsaturated substrates with parahydrogen not only lead to strong polarization signals in H-1 NMR spectra, but also can give rise to strong heteronuclear polarization, especially if the hydrogenations are carried out in low magnetic fields. As a typical example, the polarization transfer from protons to carbon nuclei during the hydrogenation of alkynes is outlined for several substrates. In systems containing easily accessible triple bonds, e.g. phenylethyne or 3,3-dimethyl-1-butyne, polarization transfer occurs to all carbon nuclei in the molecule. Accordingly, in NMR spectra recorded in situ all C-13 resonances can be observed with good to excellent signal-to-noise ratios using only a single transient. The qualitative influence of symmetry and electronic aspects of the substrate and its hydrogenation product on the efficiency of the transfer of polarization to the C-13-nuclei are discussed. Copyright (C) 2001 John Wiley Sons, Ltd.