Hydrogenation parahydrogen-induced polarization

Duckett381 reported on the use of parahydrogen-induced polarization (PHIP) to delineate the pathways involved in the catalytic hydrogenation of alkenes and alkynes by [Ru3(CO)12 x(PPh3)x] (x = 1 or 2) and showed that the mechanism is highly dependent on the solvent. Bassett and... 

Parahydrogen-Induced Polarization Applications to Detect Intermediates of Catalytic Hydrogenations... 

Lanthanide chelates have been used as aids in studying the products of hydrogenation reactions monitored through parahydrogen-induced polarization (PHIP). Carrying out... 

Parahydrogen induced polarization ( )62 is useful in the detection of short-lived reaction intermediates in hydrogenation processes and is potentially useful in studying reactions of H2 ligands. Apparently because of the rapid equilibrium with the dihydride tautomer in Eq. (5.2) the effect is not seen for pare-H2 addition to W(CO)3(PR3)2 or Mo(CO)(diphosphine)2 systems.10 However, it provides crucial evidence in experiments demonstrating that i/2-H2 can react directly with substrates bound to the same metal center without first undergoing OA (Section 9.5.1). The Mo-norbomadiene (NBD) complex in Eq. (5.10) forms an unstable H2 complex on photolysis, which is proposed to directly transfer H2 to NBD to form norbomene (NBN).63 In situ NMR studies of the photocatalyzed... 

Eisenberg, R. Eisenschmid, T. C. Chinn, M. S. Kirss, R. U. Parahydrogen-induced Polarization and Polarization Transfer in Hydrogenation and Oxidative Addition Reactions. In Homogenatus Transition Metal Catalyzed Reactions-, Moser, W. R, Slocum, D. W., Eds. American Chemical... 

Proved examples, which can be viewed to involve initial intramolecular heterolysis of an H2 ligand, are rare. Unstable but IR-detectable species such as M(CO)4(norbotnadiene)(H2) are proposed intermediates for photocata-lytic hydrogenation of norbornadiene to norbornene and nortricyclene involving direct reaction of H2 ligands. Evidence for direct transfer of to co-bound substrates is limited, but parahydrogen induced polarization (PHIP) provides crucial NMR evidence. The Mo-norbornadiene (NBD) complex in Scheme 4 forms an unstable H2 complex on photolysis that is proposed to directly transfer H2 to NBD to form norbornene (NBN). ... 

Kovtunov KV, Beck IE, Bukhtiyarov VI, Koptyug IV (2008) Observation of parahydrogen-induced polarization in heterogeneous hydrogenation on supported metal catalysts. Angew Chem 120 1514-1517... 

This concept has originally been named PASADENA (Parahydrogen And Synthesis Allow Dramatically Enhanced Nuclear Alignment) [6], but the spectroscopic method based on this phenomenon has subsequently also been called PHIP (Para-Hydrogen Induced Polarization) [7]. In this chapter the abbreviation PH IP will be used throughout. 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

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