Ortho- and Parahydrogen Induced Nuclear Spin Polarization*

Bargon, Joachim; Kandels, Jörg; Woelk, Klaus

doi:10.1524/zpch.1993.180.part_1_2.065

Cited by 53 publications

(19 citation statements)

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“…Since this complex reacts very slowly with dihydrogen, the following precautions were taken to avoid its formation: Parahydrogen-induced polarization NMR procedure: Parahydrogen was prepared according to the method of Bargon and co-workers. [58] [Rh{(À)-bdpp}(NBD)]ClO 4 (6.7 mg, 9 Â 10 À6 n) and N(nBu) 4 I (6.6 mg, 1.8 Â 10 À5 n, 2 equivalents) were weighed and placed into an NMR tube and brought into the dry box. [D 6 ]benzene (0.6 mL) was then added to make the final [Rh] 15 mm, taking care not to disturb the solid on the bottom.…”

Section: Methodsmentioning

confidence: 99%

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Buriak¹,

Klein

Herrington

et al. 2000

Chem. Eur. J.

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The strong influence of various anions upon the hydrogenation of 2-phenyl-1-butene, catalyzed by chiral rhodium catalysts was investigated. Both sulfonates and halides exert large increases in the enantioselectivity when [Rh[(-)-bdpp](NBD)]ClO4 (bdpp = 2,4-bis(diphenylphosphino)pentane, NBD = 2,5-norbornadiene) is used as the catalyst precursor at high pressures (70 atm) of dihydrogen in nonpolar solvents. A dihydride mechanism similar to that for Wilkinson's catalyst [RhCl(PPh3)3] was shown to be operating at both high- and low-pressure conditions through a combination of catalytic studies, 31P, 1H and parahydrogen-induced polarization (PHIP) NMR experiments. With sulfonate and in neat methanol, however, a mechanistic switch takes place from a dihydride route (dihydrogen addition before olefin binding) at high pressure to an unsaturate route (olefin binding before dihydrogen addition) at low pressures (<30 atm). Olefin isomerization is inhibited by halide addition, but occurs with sulfonate and in neat methanol through what is most likely a pi-allyl mechanism. A detailed understanding of the effects of addition of these anions is crucial for development of new classes of catalysts capable of efficient enantioselective reduction of prochiral olefins lacking a secondary polar binding group.

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Section: Methodsmentioning

confidence: 99%

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Buriak¹,

Klein

Herrington

et al. 2000

Chem. Eur. J.

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“…1); alternatively, 50% para-enriched hydrogen can be prepared in any well-equipped laboratory by cooling hydrogen gas in a bath of liquid nitrogen (vide infra, Section 1.3). It is also possible to prepare ortho-enriched hydrogen, by exploiting the preferential adsorption of orthohydrogen on diamagnetic surfaces at low temperatures [10].…”

Section: The Existence Of Spin-isomers Of Hydrogenmentioning

confidence: 99%

Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry

Duckett

Wood

2008

Coordination Chemistry Reviews

125

127

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“…), 0.7 ml of degassed chloroformd 1 and 5 bar of 50% enriched parahydrogen. This degree of enrichment was achieved via catalytic equilibration over charcoal at 77 K. 12 The tubes were closed, shaken and immediately placed in the NMR probe at room temperature. The FID was recorded using a single 45°( 13 C) r.f.…”

Section: Methodsmentioning

confidence: 99%

¹³C PHIP NMR spectra and polarization transfer during the homogeneous hydrogenation of alkynes with parahydrogen

Schumacher

Kohlmann

Niessen

et al. 2001

Magnetic Reson in Chemistry

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Homogeneously catalyzed hydrogenations of unsaturated substrates with parahydrogen not only lead to strong polarization signals in 1 H NMR spectra, but also can give rise to strong heteronuclear polarization, especially if the hydrogenations are carried out in low magnetic fields. As a typical example, the polarization transfer from protons to carbon nuclei during the hydrogenation of alkynes is outlined for several substrates. In systems containing easily accessible triple bonds, e.g. phenylethyne or 3,3-dimethyl-1-butyne, polarization transfer occurs to all carbon nuclei in the molecule. Accordingly, in NMR spectra recorded in situ all 13 C resonances can be observed with good to excellent signal-to-noise ratios using only a single transient. The qualitative influence of symmetry and electronic aspects of the substrate and its hydrogenation product on the efficiency of the transfer of polarization to the 13 C-nuclei are discussed.

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Ortho- and Parahydrogen Induced Nuclear Spin Polarization*

Cited by 53 publications

References 0 publications

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry

¹³C PHIP NMR spectra and polarization transfer during the homogeneous hydrogenation of alkynes with parahydrogen

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Ortho- and Parahydrogen Induced Nuclear Spin Polarization*

Cited by 53 publications

References 0 publications

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins with Chiral Rhodium Catalysts in the Presence of Anions

Parahydrogen-based NMR methods as a mechanistic probe in inorganic chemistry

13C PHIP NMR spectra and polarization transfer during the homogeneous hydrogenation of alkynes with parahydrogen

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¹³C PHIP NMR spectra and polarization transfer during the homogeneous hydrogenation of alkynes with parahydrogen