Parahydrogen Induced Polarization: A New Magnifying Glass for Examining Reactions with Hydrogen

Abstract

Parahydrogen-included polarization (PHIP), its occurrence and mechanistic implications in homogeneous hydrogenation chemistry, and its appearance in the oxidative addition of H2 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 H2 is employed in hydrogenation of unsaturated organic substrates with a homogeneous metal catalyst or when para-enriched H2 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 H2 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 H2 with RhCl(CO)(PR3)2 (R = Ph, Me) and in hydrogenation catalysis promoted by RhCl(PPh3)3. Also observed in the hydrogenation catalysis is the putative olefin dihydride catalytic intermediate.