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

Abstract

Homogeneously catalyzed hydrogenations of unsaturated substrates with parahydrogen not only lead to strong polarization signals in 1 H 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 13 C 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 13 C-nuclei are discussed.

Parahydrogen‐Induced Polarization: Applications to Detect Intermediates of Catalytic Hydrogenations

“…It could be shown that in systems containing easily accessible triple bonds (Table 12.5) (e.g., ethynylbenzene or 2,2-dimethylbutyne), a polarization transfer takes place to all carbon nuclei in the molecule. Accordingly, all 13 C resonances can be observed in NMR spectra recorded in situ with good to excellent signal-to-noise ratios (SNRs) using only a single transient [42]. This technique has made it possible to identify a Ru-based catalyst system, which yields E-alkenes via homogeneous hydrogenations of alkynes with central triple bonds, as opposed to the more conventional Z-alkenes that result from hydrogenation with Rh-based catalysts [47].…”

“…This frequently simplifies the assignment, thereby facilitating the interpretation of 13 C-NMR spectra of more complex molecules. The polarization phenomena occurring in the 13 C-PHIP-NMR spectra during the in-situ hydrogenation of various alkynes using the catalyst [Rh(cod)(dppb)] + in CDCl 3 at 5 bar of parahydrogen may serve as a characteristic example [42].…”

“…Furthermore, 1-hexyne, 3,3-dimethylbutyne, and ethynylbenzene all have terminal triple bonds, but the electronic structure and accessibility of the p-system of these triple bonds are influenced differently by the catalyst, depending on the group adjacent to the triple bond. Within this series it could be shown how +M and +I effects or steric hindrance influence the hydrogenation and polarization transfer from the protons to the carbon atoms [42]. The six reactions investigated are outlined in Table 12.5 -that is, the starting materials and the corresponding reaction products, together with the numeration of the individual carbon atoms following IUPAC nomenclature.…”

Dihydrogen Transfer and Symmetry: The Role of Symmetry in the Chemistry of Dihydrogen Transfer in the Light of NMR Spectroscopy

“…As a typical example taken from Ref. [74], the polarization transfer from protons to 13 C nuclei during the hydrogenation of 3,3-dimethylbut-1-yne with para-hydrogen and [Rh(cod)(dppb)] + as catalyst is shown in Fig. 21.11.…”

“…Note the extremely high signal/noise ratio in the single shot spectrum, which is a measure for the strong signal enhancement obtained by PHIP (adapted from Ref. [74]). …”