Stereochemistry of intermediates in homogeneous hydrogenation catalysed by tristriphenylphosphinerhodium chloride, employing nuclear magnetic resonance magnetisation transfer

Brown, John M.; Evans, Phillip L.; Lucy, Andrew R.

doi:10.1039/p29870001589

Cited by 51 publications

(54 citation statements)

References 2 publications

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“…[1] The mechanism by which [RhCl(PPh 3 ) 3 ] catalyses the hydrogenation of alkenes has been the subject of extensive research with the core mechanistic pathway proposed to in- 2 ], is never observed in solution under hydrogenation conditions, it has been generated during flash photolysis studies [5] on [RhCl(CO)(PPh 3 ) 2 ] and a solid sample of [RhCl(PtBu 3 ) 2 ], stabilised by the bulky phosphine ligands, has been prepared. [6] Despite the resultant low concentration of [RhCl(PPh 3 ) 2 ], [7,8] it remains kinetically significant since H 2 addition to it is 10 4 times faster than that to [RhCl(PPh 3 ) 3 ]. [9] Indeed, based on results with [RhCl[P(p-tolyl) 3 .…”

Section: Introductionmentioning

confidence: 97%

“…[6] This complex possesses approximately C 2v symmetry, with equivalent and mutually trans-PtBu 3 ligands and matches that proposed by Brown for [RhCl(H) 2 (PPh 3 ) 2 ]. [7] Alkene coordination to the resultant 16 electron dihydride species corresponds to the next step in the cycle, and a number of geometries for [RhCl(H) 2 (alkene)(PPh 3 ) 2 ] are possible, even though this complex has traditionally been depicted with trans phosphines. However, when crystal structure data results were used to enable the energies of potential [RhCl(H) 2 (alkene)(PPh 3 ) 2 ] intermediates to be quantified for many substrates (such as cyclohexene and bicylco[2.2.1]heptene), complexes with trans phosphine arrangements were found to be highly sterically strained due to interactions between the coordinated alkene and the other ligands.…”

Section: Introductionmentioning

confidence: 99%

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Hydrogenation Studies Involving Halobis(phosphine)–Rhodium(i) Dimers: Use of Parahydrogen Induced Polarisation To Detect Species Present at Low Concentration

Colebrooke

Duckett

Lohman

et al. 2004

Chemistry A European J

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Reaction of [RhCl(PPh3)2]2 with parahydrogen revealed that the binuclear dihydride [Rh(H)2(PPh3)2mu-Cl)2Rh(PPh3)2] and the tetrahydride complex [Rh(H)2(PPh3)2(mu-Cl)]2 are readily formed. While magnetisation transfer from free H2 into both the hydride resonances of the tetrahydride and [Rh(H)2Cl(PPh3)3] is observable, neither transfer into [Rh(H)2(PPh3)2(mu-Cl)2Rh(PPh3)2] nor transfer between the two binuclear complexes is seen. Consequently [Rh(H)2(PPh3)2(mu-Cl)]2 and [Rh(H)2(PPh3)2(mu-Cl)2Rh(PPh3)2] are not connected on the NMR timescale by simple elimination or addition of H2. The rapid exchange of free H2 into the tetrahydride proceeds via reversible halide bridge rupture and the formation of [Rh(H)2(PPh3)2(mu-Cl)RhCl(H)2(PPh3)2]. When these reactions are examined in CD2Cl2, the formation of the solvent complex [Rh(H)2(PPh3)2(mu-Cl)2Rh(CD2Cl2)(PPh3)] and the deactivation products [Rh(Cl)(H)PPh3)2(mu-Cl)(mu-H)Rh(Cl)(H)PPh3)2] and [Rh(Cl)(H)(CD2Cl2)(PPh3)(mu-Cl)(mu-H)Rh(Cl)(H)PPh3)2] is indicated. In the presence of an alkene and parahydrogen, signals corresponding to binuclear complexes of the type [Rh(H)2(PPh3)2(mu-Cl)(2)(Rh)(PPh3)(alkene)] are detected. These complexes undergo intramolecular hydride interchange in a process that is independent of the concentration of styrene and catalyst and involves halide bridge rupture, followed by rotation about the remaining Rh-Cl bridge, and bridge re-establishment. This process is facilitated by electron rich alkenes. Magnetisation transfer from the hydride ligands of these complexes into the alkyl group of the hydrogenation product is also observed. Hydrogenation is proposed to proceed via binuclear complex fragmentation and trapping of the resultant intermediate [RhCl(H)2PPh3)2] by the alkene. Studies on a number of other binuclear dihydride complexes including [(H)(Cl)Rh(PMe3)2(mu-H)(mu-Cl)Rh(CO)(PMe3)], [(H)2Rh(PMe3)2(mu-Cl)2Rh(CO)(PMe3)] and [HRh(PMe3)2(mu-H)(mu-Cl)2Rh(CO)(PMe3)] reveal that such species are able to play a similar role in hydrogenation catalysis. When the analogous iodide complexes [RhIPPh3)2]2 and [RhI(PPh3)3] are examined, [Rh(H)2(PPh3)2(mu-I)2Rh(PPh3)2], [Rh(H)2(PPh3)2(mu-I)]2 and [Rh(H)2I(PPh3)3] are observed in addition to the corresponding binuclear alkene-dihydride products. The higher initial activity of these precursors is offset by the formation of the trirhodium phosphide bridged deactivation product, [[(H)(PPh3)Rh(mu-H)(mu-I)(mu-PPh2)Rh(H)(PPh3)](mu-I)2Rh(H)2PPh3)2]

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Section: Introductionmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Hydrogenation Studies Involving Halobis(phosphine)–Rhodium(i) Dimers: Use of Parahydrogen Induced Polarisation To Detect Species Present at Low Concentration

Colebrooke

Duckett

Lohman

et al. 2004

Chemistry A European J

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“…The supported catalyst was prepared [14,19,20] by the addition of an approximately stoichiometric amount of 2-diphenylphosphinoethyl-functionalized silica gel to a solution of Wilkinsons catalyst in toluene (Scheme 1). The solution was stirred overnight, and the catalyst was filtered, washed with toluene, and dried under vacuum at room temperature.…”

Section: Methodsmentioning

confidence: 99%

Para‐Hydrogen‐Enhanced Hyperpolarized Gas‐Phase Magnetic Resonance Imaging

et al. 2007

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Gas‐phase MRI phantom: Magnetic resonance imaging (MRI) in the gas phase was demonstrated using para‐hydrogen‐induced polarization. H2 enriched in the para (antiparallel) spin state and propylene gas were flowed over solid‐supported Wilkinson's catalyst, and the product, propane gas, was collected in an NMR tube. Enhanced signal intensities were observed in the images of phantoms placed inside the NMR tube (see picture).

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“…The supported catalyst was prepared [39,91,136] by the addition of an approximately stoichiometric amount of 2-diphenylphosphinoethyl-functionalized silica gel to a solution of Wilkinson's catalyst in toluene ( fig. 3.2).…”

Section: Experimental Methodsmentioning

confidence: 99%

“…; and a sup- catalyst is known to undergo rapid ligand exchange [39], and therefore the addition of an approximately stoichiometric amount of 2-diphenylphosphinoethyl-functionalized silica gel to a toluene solution of Wilkinson's catalyst leads to formation of the required catalyst [136].…”

Section: Supported Catalystsmentioning

confidence: 99%

MRI of Heterogeneous Hydrogenation Reactions Using Parahydrogen Polarization

Burt

2008

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The power of magnetic resonance imaging (MRI) is its ability to image the internal structure of optically opaque samples and provide detailed maps of a variety of important parameters, such as density, diffusion, velocity and temperature. However, one of the fundamental limitations of this technique is its inherent low sensitivity. For example, the low signal to noise ratio (SNR) is particularly problematic for imaging gases in porous materials due to the low density of the gas and the large volume occluded by the porous material. This is unfortunate, as many industrially relevant chemical reactions take place at gas-surface interfaces in porous media, such as packed catalyst beds. Because of this severe SNR problem, many techniques have been developed to directly increase the signal strength. These techniques work by manipulating the nuclear spin populations to produce polarized (i.e. non-equilibrium) states with resulting signal strengths that are orders of magnitude larger than those available at thermal equilibrium. 2This dissertation is concerned with an extension of a polarization technique based on the properties of parahydrogen. Specifically, I report on the novel use of heterogeneous catalysis to produce parahydrogen induced polarization and applications of this new technique to gas phase MRI and the characterization of micro-reactors. First, I provide an overview of nuclear magnetic resonance (NMR) and how parahydrogen is used to improve the SNR of the NMR signal. I then present experimental results demonstrating that it is possible to use heterogeneous catalysis to produce parahydrogen-induced polarization.These results are extended to imaging void spaces using a parahydrogen polarized gas. In the second half of this dissertation, I demonstrate the use of parahydrogen-polarized gasphase MRI for characterizing catalytic microreactors. Specifically, I show how the improved SNR allows one to map parameters important for characterizing the heat and mass transport in a heterogeneous catalyst bed. This is followed by appendices containing detailed

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Stereochemistry of intermediates in homogeneous hydrogenation catalysed by tristriphenylphosphinerhodium chloride, employing nuclear magnetic resonance magnetisation transfer

Cited by 51 publications

References 2 publications

Hydrogenation Studies Involving Halobis(phosphine)–Rhodium(i) Dimers: Use of Parahydrogen Induced Polarisation To Detect Species Present at Low Concentration

Hydrogenation Studies Involving Halobis(phosphine)–Rhodium(i) Dimers: Use of Parahydrogen Induced Polarisation To Detect Species Present at Low Concentration

Para‐Hydrogen‐Enhanced Hyperpolarized Gas‐Phase Magnetic Resonance Imaging

MRI of Heterogeneous Hydrogenation Reactions Using Parahydrogen Polarization

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