PARAHYDROGEN INDUCED POLARIZATION - A NEW MAGNIFYING GLASS FOR EXAMINING REACTIONS WITH HYDROGEN

Parahydrogen-included polarization (PHIP), its occurrence and mechanistic implications in homogeneous hydrogenation chemistry, and its appearance in the oxidative addition of H-2 to transition metal centers are described and analyzed. The PHIP phenomenon, which is characterized by unusual NMR absorptions and emissions in product spectra, arises when para-enriched H-2 is employed in hydrogenation of unsaturated organic substrates with a homogeneous metal catalyst or when para-enriched H-2 is added to a metal complex to form a metal dihydride. Examples of PHIP are found in ruthenium phosphine-catalyzed hydrogenations, catalysis by binuclear rhodium complexes, and in H-2 oxidative addition to Ir(I) complexes. The decay of polarization has been shown in the case of asymmetric hydrogenation catalyzed by Rh(chiraphos)(+) to correlate well with the measured rate of reaction. For asymmetric hydrogenation of aprotic substrates using Noyori's Ru(BINAP)(OAc)(2) catalyst (1), PHIP is observed indicating a pairwise hydrogen transfer mechanism. Through the signal enhancement of PHIP, it has been possible to observe Rh hydride species never previously detected including binuclear complexes in the reaction of H-2 with RhCl(CO)(PR(3))(2) (R = Ph, Me) and in hydrogenation catalysis promoted by RhCl(Ph(3))(3). Also observed in the hydrogenation catalysis is the putative olefin dihydride catalytic intermediate.