Proof of a Reversible, Pairwise Hydrogen Transfer during the Homogeneously Rhodium(I)- Catalyzed Hydrogenation of α, β-Unsaturated Carbonic Acid Derivatives with In Situ NMR Spectroscopy and Parahydrogen

摘要

In recent years hydrogenations with para-enriched hydrogen (p-H,) in combination with in situ NMR spectroscopy have been successfully applied to elucidate mechanistic aspects of homogeneously catalyzed hydrogenations. Contrary to the usual Boltzmanndistribution, hydrogenation with p-H, "leads to a selective population of nuclear spin levels, which generates enhanced (up to 10~5) ("polarization") signals in emission and absorption in the ~1H NMR spectra of the hydrogenated products. This leads to ahigh sensitivity for registering these products in low concentration. A prerequisite for detecting polarization signals is a pairwise transfer of both p-H, atoms to the product. This polarization phenomenon is known by the acronyms PASADENA (ParahydrogenAnd Synthesis Allows Dramatically Enhanced Miclear Alignment)/ ALTADENA (Adiabatic Longitudinal Transport After Dissociation Engenders Net Alignment) and PHIP (Parahydrogen Induced Polarization).