PHIP-Enhanced NMR of Reaction Products

For hydrogenation of styrene and its derivatives over several cationic Rh complexes, in addition to the hyperpolarized multiplets of ethylbenzene the H NMR spectra contained similar polarized multiplets but shifted to a higher field [41,42]. These signals were attributed to the product molecules that have not yet detached from the metal center after the hydrogen-transfer stage was over (e.g., with the aromatic moiety r -coordinated to the Rh(I) center). The results demonstrate that the detachment process can be fairly slow on the NMR timescale. The use of chiral catalysts and/or asymmetrically substituted styrenes led to more complicated spectral patterns. Kinetic studies can be used to measure the rates of formation and decay of such catalyst-product complexes [43]. The fact that the observed product remains coordinated to the catalyst was confirmed [44] in experiments with polarization transfer from the product to the hydrogens of other ligands of the catalyst induced by cross relaxation. 

As PHIP effects in the products are the direct measure of the deviation of the o p ratio of the nuclear spin isomers of H2 from the statistical 3 1 value, the influence of the substrate on the reversibility of oxidative hydrogen addition to the metal center can be studied, as demonstrated for Wilkinson s catalyst and a cationic Rh complex in the presence of phenylacetylene [12,45]. 

PHIP effects were also used to detect reaction intermediates in the mechanistic studies of homogeneously catalyzed hydroformylation reactions [46 - 50]. Interestingly, it was observed that such processes can lead to the so-called one-H PHIP effect when incorporation of only one H atom of pH2 into the product molecule can lead to an observation of hyperpolarization of the aldehyde H atom [46,49], which is most Ukely the result of the nuclear spin evolution in the preceding dihydride intermediates. 

---