Isomerization

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Jean-françois Carpentier - One of the best experts on this subject based on the ideXlab platform.

  • Insights in the Rhodium-Catalyzed Tandem Isomerization-Hydroformylation of 10-Undecenitrile: Evidence for a Fast Isomerization Regime
    Catalysts, 2018
    Co-Authors: Lucas Le Goanvic, Jean-luc Couturier, Jean-luc Dubois, Jean-françois Carpentier
    Abstract:

    The tandem Isomerization-hydroformylation of 10-undecenitrile (1) into the corresponding linear aldehyde (2) with a Rh-biphephos system was studied and the formation of internal olefin isomers (1-int-x) was monitored over time. The existence of an “Isomerization phenomenon” was evidenced, where fast Isomerization of 1 into up to 70% of 1-int-x followed by fast back-Isomerization of 1-int-x into 1 and, in turn, into 2 occurs. This fast dynamic Isomerization regime is favored at high syngas pressure (40 bar) and low biphephos-to-Rh ratio (5–10), and it is best observed at relatively high catalyst loadings ([1]0/[Rh] ≤ 3000). The latter regime is indeed evanescent, and gives place to a second stage in which Isomerization of internal olefins (and eventual conversion into 2) proceeds much more slowly. The results are tentatively rationalized by the formation of an unstable species that promotes dynamic Isomerization and which slowly vanishes or collapses into a Rh-biphephos species which is the one responsible for hydroformylation.

  • Insights in the rhodium-catalyzed tandem Isomerization-hydroformylation of 10-undecenitrile Evidence for a fast Isomerization regime
    Catalysts, 2018
    Co-Authors: Lucas Le Goanvic, Jean-luc Couturier, Jean-luc Dubois, Jean-françois Carpentier
    Abstract:

    The tandem Isomerization-hydroformylation of 10-undecenitrile (1) into the corresponding linear aldehyde (2) with a Rh-biphephos system was studied and the formation of internal olefin isomers (1-int-x) was monitored over time. The existence of an “Isomerization phenomenon” was evidenced, where fast Isomerization of 1 into up to 70% of 1-int-x followed by fast back-Isomerization of 1-int-x into 1 and, in turn, into 2 occurs. This fast dynamic Isomerization regime is favored at high syngas pressure (40 bar) and low biphephos-to-Rh ratio (5–10), and it is best observed at relatively high catalyst loadings ([1]0/[Rh] ≤ 3000). The latter regime is indeed evanescent, and gives place to a second stage in which Isomerization of internal olefins (and eventual conversion into 2) proceeds much more slowly. The results are tentatively rationalized by the formation of an unstable species that promotes dynamic Isomerization and which slowly vanishes or collapses into a Rh-biphephos species which is the one responsible for hydroformylation. © 2018 by the authors.

Lucas Le Goanvic - One of the best experts on this subject based on the ideXlab platform.

  • Insights in the Rhodium-Catalyzed Tandem Isomerization-Hydroformylation of 10-Undecenitrile: Evidence for a Fast Isomerization Regime
    Catalysts, 2018
    Co-Authors: Lucas Le Goanvic, Jean-luc Couturier, Jean-luc Dubois, Jean-françois Carpentier
    Abstract:

    The tandem Isomerization-hydroformylation of 10-undecenitrile (1) into the corresponding linear aldehyde (2) with a Rh-biphephos system was studied and the formation of internal olefin isomers (1-int-x) was monitored over time. The existence of an “Isomerization phenomenon” was evidenced, where fast Isomerization of 1 into up to 70% of 1-int-x followed by fast back-Isomerization of 1-int-x into 1 and, in turn, into 2 occurs. This fast dynamic Isomerization regime is favored at high syngas pressure (40 bar) and low biphephos-to-Rh ratio (5–10), and it is best observed at relatively high catalyst loadings ([1]0/[Rh] ≤ 3000). The latter regime is indeed evanescent, and gives place to a second stage in which Isomerization of internal olefins (and eventual conversion into 2) proceeds much more slowly. The results are tentatively rationalized by the formation of an unstable species that promotes dynamic Isomerization and which slowly vanishes or collapses into a Rh-biphephos species which is the one responsible for hydroformylation.

  • Insights in the rhodium-catalyzed tandem Isomerization-hydroformylation of 10-undecenitrile Evidence for a fast Isomerization regime
    Catalysts, 2018
    Co-Authors: Lucas Le Goanvic, Jean-luc Couturier, Jean-luc Dubois, Jean-françois Carpentier
    Abstract:

    The tandem Isomerization-hydroformylation of 10-undecenitrile (1) into the corresponding linear aldehyde (2) with a Rh-biphephos system was studied and the formation of internal olefin isomers (1-int-x) was monitored over time. The existence of an “Isomerization phenomenon” was evidenced, where fast Isomerization of 1 into up to 70% of 1-int-x followed by fast back-Isomerization of 1-int-x into 1 and, in turn, into 2 occurs. This fast dynamic Isomerization regime is favored at high syngas pressure (40 bar) and low biphephos-to-Rh ratio (5–10), and it is best observed at relatively high catalyst loadings ([1]0/[Rh] ≤ 3000). The latter regime is indeed evanescent, and gives place to a second stage in which Isomerization of internal olefins (and eventual conversion into 2) proceeds much more slowly. The results are tentatively rationalized by the formation of an unstable species that promotes dynamic Isomerization and which slowly vanishes or collapses into a Rh-biphephos species which is the one responsible for hydroformylation. © 2018 by the authors.

Mark E. Davis - One of the best experts on this subject based on the ideXlab platform.

  • Award Address (Gabor A. Somorjai Award for Creative Research in Catalysis sponsored by the Gabor A. and Judith K. Somorjai Endowment Fund). New heterogeneous catalysts for converting sugars in aqueous media
    2014
    Co-Authors: Mark E. Davis
    Abstract:

    The Isomerization of glucose into fructose is a large-scale reaction for the prodn. of high-fructose corn syrup, and recently, is being considered as an intermediate step in the possible route of biomass to fuels and chems. Here, it is shown that a large pore zeolite that contains tin (Sn-Beta) is able to isomerize glucose to fructose in aq. media with high activity and selectivity. Specifically, a 10 wt% glucose soln. contg. a catalytic amt. of Sn-Beta (1:50 Sn:glucose molar ratio) gives product yields of approx. 46% (wt./wt.) glucose, 31% (wt./wt.) fructose, and 9% (wt./wt.) mannose after 30 and 12 min of reaction at 383 K and 413 K, resp. This reactivity is achieved also when a 45wt% glucose soln. is converted. The Sn-Beta catalyst can be used for multiple cycles, and the reaction stops when the solid is removed, clearly indicating that the catalysis is occurring heterogeneously. With isotopically labeled glucose, it is demonstrated that the Isomerization reaction catalyzed by Sn-Beta in water proceeds by way of an intramol. hydride shift, confirming that framework tin centers in Sn-Beta act as Lewis acids in aq. media. The active site is shown to be Sn that has three bonds to framework oxygen atoms, and reaction rates are strongly dependent on the hydrophobicity of the mol. sieve. The Sn-Beta catalyst is able to perform the Isomerization reaction in highly acidic, aq. environments with equiv. activity and product distribution as in media without added acid. This enables Sn-Beta to couple Isomerizations with other acid-catalyzed reactions, including hydrolysis/Isomerization or Isomerization/dehydration reaction sequences, including starch to fructose and glucose to 5-hydroxymethylfurfural (HMF). Modifications of Sn-Beta (and Ti-Beta) have expanded the types of reactions that can be catalyzed. Some of those reactions include the conversion of glucose to mannose, glucose to sorbose and lactose to lactulose.

  • tin containing zeolites are highly active catalysts for the Isomerization of glucose in water
    Proceedings of the National Academy of Sciences of the United States of America, 2010
    Co-Authors: Manuel Moliner, Yuriy Romanleshkov, Mark E. Davis
    Abstract:

    The Isomerization of glucose into fructose is a large-scale reaction for the production of high-fructose corn syrup (HFCS; reaction performed by enzyme catalysts) and recently is being considered as an intermediate step in the possible route of biomass to fuels and chemicals. Here, it is shown that a large-pore zeolite that contains tin (Sn-Beta) is able to isomerize glucose to fructose in aqueous media with high activity and selectivity. Specifically, a 10% (wt/wt) glucose solution containing a catalytic amount of Sn-Beta (1∶50 Sn:glucose molar ratio) gives product yields of approximately 46% (wt/wt) glucose, 31% (wt/wt) fructose, and 9% (wt/wt) mannose after 30  min and 12 min of reaction at 383 K and 413 K, respectively. This reactivity is achieved also when a 45 wt% glucose solution is used. The properties of the large-pore zeolite greatly influence the reaction behavior because the reaction does not proceed with a medium-pore zeolite, and the Isomerization activity is considerably lower when the metal centers are incorporated in ordered mesoporous silica (MCM-41). The Sn-Beta catalyst can be used for multiple cycles, and the reaction stops when the solid is removed, clearly indicating that the catalysis is occurring heterogeneously. Most importantly, the Sn-Beta catalyst is able to perform the Isomerization reaction in highly acidic, aqueous environments with equivalent activity and product distribution as in media without added acid. This enables Sn-Beta to couple Isomerizations with other acid-catalyzed reactions, including hydrolysis/Isomerization or Isomerization/dehydration reaction sequences [starch to fructose and glucose to 5-hydroxymethylfurfural (HMF) demonstrated here].

František Tureček - One of the best experts on this subject based on the ideXlab platform.

  • Spontaneous Isomerization of Peptide Cation Radicals Following Electron Transfer Dissociation Revealed by UV-Vis Photodissociation Action Spectroscopy
    Journal of The American Society for Mass Spectrometry, 2018
    Co-Authors: Naruaki Imaoka, Camille Houferak, Megan P. Murphy, Huong T. H. Nguyen, Andy Dang, František Tureček
    Abstract:

    Peptide cation radicals of the z-type were produced by electron transfer dissociation (ETD) of peptide dications and studied by UV-Vis photodissociation (UVPD) action spectroscopy. Cation radicals containing the Asp (D), Asn (N), Glu (E), and Gln (Q) residues were found to spontaneously isomerize by hydrogen atom migrations upon ETD. Canonical N -terminal [z_4 + H]^+● fragment ion-radicals of the R-C^●H-CONH- type, initially formed by N−C_α bond cleavage, were found to be minor components of the stable ion fraction. Vibronically broadened UV-Vis absorption spectra were calculated by time-dependent density functional theory for several [^●DAAR + H]^+ isomers and used to assign structures to the action spectra. The potential energy surface of [^●DAAR + H]^+ isomers was mapped by ab initio and density functional theory calculations that revealed multiple Isomerization pathways by hydrogen atom migrations. The transition-state energies for the Isomerizations were found to be lower than the dissociation thresholds, accounting for the Isomerization in non-dissociating ions. The facile Isomerization in [^●XAAR + H]^+ ions (X = D, N, E, and Q) was attributed to low-energy intermediates having the radical defect in the side chain that can promote hydrogen migration along backbone C_α positions. A similar side-chain mediated mechanism is suggested for the facile intermolecular hydrogen migration between the c- and [z + H]^●-ETD fragments containing Asp, Asn, Glu, and Gln residues. Graphical Abstract ᅟ

  • Cascade Dissociations of Peptide Cation-Radicals. Part 2. Infrared Multiphoton Dissociation and Mechanistic Studies of z-Ions from Pentapeptides
    Journal of The American Society for Mass Spectrometry, 2012
    Co-Authors: Aaron R. Ledvina, Thomas W. Chung, Joshua J. Coon, František Tureček
    Abstract:

    Dissociations of z _ 4 ions from pentapeptides AAXAR where X = H, Y, F, W, and V produce dominant z _ 2 ions that account for >50 % of the fragment ion intensity. The dissociation has been studied in detail by experiment and theory and found to involve several Isomerization and bond-breaking steps. Isomerizations in z _ 4 ions proceed by amide trans→cis rotations followed by radical-induced transfer of a β-hydrogen atom from the side chain, forming stable C_β radical intermediates. These undergo rate-determining cleavage of the C_α–CO bond at the X residue followed by loss of the neutral AX fragment, forming x _ 2 intermediates. The latter were detected by energy-resolved resonant excitation collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments. The x _ 2 intermediates undergo facile loss of HNCO to form z _ 2 fragment ions, as also confirmed by energy-resolved CID and IRMPD MS^4 experiments. The loss of HNCO from the x _ 2 ion from AAHWR is kinetically hampered by the Trp residue that traps the OCNH radical group in a cyclic intermediate.

Hiram F Gilbert - One of the best experts on this subject based on the ideXlab platform.

  • reduction reoxidation cycles contribute to catalysis of disulfide Isomerization by protein disulfide isomerase
    Journal of Biological Chemistry, 2003
    Co-Authors: Melissa Schwaller, Bonney Wilkinson, Hiram F Gilbert
    Abstract:

    Abstract Protein-disulfide isomerase (PDI) catalyzes the formation and Isomerization of disulfides during oxidative protein folding. This process can be error-prone in its early stages, and any incorrect disulfides that form must be rearranged to their native configuration. When the second cysteine (CGHC) in the PDI active site is mutated to Ser, the isomerase activity drops by 7–8-fold, and a covalent intermediate with the substrate accumulates. This led to the proposal that the second active site cysteine provides an escape mechanism, preventing PDI from becoming trapped with substrates that isomerize slowly (Walker, K. W., and Gilbert, H. F. (1997) J. Biol. Chem. 272, 8845–8848). Escape also reduces the substrate, and if it is invoked frequently, disulfide Isomerization will involve cycles of reduction and reoxidation in preference to intramolecular Isomerization of the PDI-bound substrate. Using a gel-shift assay that adds a polyethylene glycol-conjugated maleimide of 5 kDa for each sulfhydryl group, we find that PDI reduction and oxidation are kinetically competent and essential for Isomerization. Oxidants inhibit Isomerization and oxidize PDI when a redox buffer is not present to maintain the PDI redox state. Reductants also inhibit Isomerization as they deplete oxidized PDI. These rapid cycles of PDI oxidation and reduction suggest that PDI catalyzes Isomerization by trial and error, reducing disulfides and oxidizing them in a different configuration. Disulfide reduction-reoxidation may set up critical folding intermediates for intramolecular Isomerization, or it may serve as the only Isomerization mechanism. In the absence of a redox buffer, these steady-state reduction-oxidation cycles can balance the redox state of PDI and support effective catalysis of disulfide Isomerization.