Catalysis and Nuclear Magnetic Resonance Signal Enhancement with Parahydrogen

摘要

When catalytic hydrogenations use parahydrogen instead of normal H-2, the NMR signals of reaction products and intermediates can be dramatically enhanced. This phenomenon, called parahydrogen-induced polarization (PHIP), has been used previously to address the mechanisms of H-2 activation by transition metal complexes and clusters in solution. At the same time, it is highly desirable to combine this methodology with heterogeneous catalysis. Potential benefits of such combination include the possibility to develop a hypersensitive NMR-based tool for mechanistic studies of heterogeneous catalytic processes as well as to establish an efficient technology for the production of novel contrast agents for in vivo MRI. To achieve this, one needs to combine the 'homogeneous' hydrogenation mechanism, which incorporates both H atoms of a H-2 molecule into the same product molecule, with the ease of catalyst removal and the possibility to perform the reaction in a continuous mode, the inherent features of heterogeneous catalysis. This field of NMR and MRI research can thus benefit from the current efforts at the interface between homogeneous and heterogeneous catalysis. Here, after a brief introduction of the PHIP phenomenon, the current progress in combining PHIP with heterogeneous hydrogenations is discussed, and future promising directions are outlined.