| 1. |
- Bunrit, Anon, et al.
(författare)
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Brønsted Acid-Catalyzed Intramolecular Nucleophilic Substitution of the Hydroxyl Group in Stereogenic Alcohols with Chirality Transfer
- 2015
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 137:14, s. 4646-4649
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Tidskriftsartikel (refereegranskat)abstract
- The hydroxyl group of enantioenriched benzyl, propargyl, allyl, and alkyl alcohols has been intramolecularly displaced by uncharged O-, N-, and S-centered nucleophiles to yield enantioenriched tetrahydrofuran, pyrrolidine, and tetrahydrothiophene derivatives with phosphinic acid catalysis. The five-membered heterocyclic products are generated in good to excellent yields, with high degree of chirality transfer, and water as the only side-product. Racemization experiments show that phosphinic acid does not promote S(N)1 reactivity. Density functional theory calculations corroborate a reaction pathway where the phosphinic acid operates as a bifunctional catalyst in the intramolecular substitution reaction. In this mechanism, the acidic proton of the phosphinic acid protonates the hydroxyl group, enhancing the leaving group ability. Simultaneously, the oxo group of phosphinic acid operates as a base abstracting the nucleophilic proton and thus enhancing the nucleophilicity. This reaction will open up new atom efficient techniques that enable alcohols to be used as nucleofuges in substitution reactions in the future.
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| 2. |
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| 3. |
- Bunrit, Anon, et al.
(författare)
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H3PO2-Catalyzed Intramolecular Stereospecific Substitution of the Hydroxyl Group in Enantioenriched Secondary Alcohols by N-, O-, and S-Centered Nucleophiles to Generate Heterocycles
- 2020
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 10:2, s. 1344-1352
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Tidskriftsartikel (refereegranskat)abstract
- The direct intramolecular stereospecific substitution of the hydroxyl group in enantiomerically enriched secondary benzylic, allylic, propargylic, and alkyl alcohols was successfully accomplished by phosphinic acid catalysis. The hydroxyl group was displaced by O-, S-, and N-centered nucleophiles to provide enantioenriched five-membered tetrahydrofuran, pyrrolidine, and tetrahydrothiophene as well as six-membered tetrahydroquinolines and chromanes in up to a 99% yield and 100% enantiospecificity with water as the only byproduct. Mechanistic studies using both experiments and calculations have been performed for substrates generating 5-membered heterocycles. Rate studies show dependences in a catalyst, an internal nucleophile, and an electrophile, however, independence in an external nucleophile, an electrophile, or water. Kinetic isotope effect studies show an inverse KIE of k(H)/k(D) = 0.79. Furthermore, phosphinic acid does not promote S(N)1 reactivity. Computational studies support a bifunctional role of the phosphinic acid in which activation of both nucleofuge and nucleophile occurs in a bridging S(N)2-type transition state. In this transition state, the acidic hydrogen of phosphinic acid protonates the leaving hydroxyl group simultaneously as the oxo group partially deprotonates the nucleophile. Thereby, phosphinic acid promotes the substitution of the nonderivatized hydroxyl group in enantioenriched secondary alcohols by uncharged nucleophiles with conservation of the chirality from the alcohol to the heterocycle.
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| 4. |
- Bunrit, Anon, 1986-, et al.
(författare)
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H3PO2-Catalyzed Intramolecular Stereospecific Substitution of the Hydroxyl Group in Stereogenic Secondary Alcohols by N-, O-, and S-centered Nucleophiles to Generate Heterocycles
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The direct intramolecular stereospecific substitution of the hydroxyl group in stereogenic secondary alcohols was successfully accomplished by phosphinic acid catalysis. The hydroxyl group was displaced by O-, S-, and N-centered nucleophiles to provide enantioenriched five- and six-membered heterocycles in good to excellent yields and high enantiospecificity with water as the only by product. Mechanistic studies using both experiments and calculations have been performed. Rate order determination shows first-order dependences in catalyst, internal nucleophile, and electrophile concentrations, however, independence on external nucleophile and electrophile. Furthermore, phosphinic acid does not promote SN1 reactivity. Computational studies support a bifunctional role of the phosphinic acid in which activations of both nucleofuge and nucleophile occur in a bridging SN2-type transition state.
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| 5. |
- Bunrit, Anon, et al.
(författare)
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Nucleophilic Substitution of the Hydroxyl Group in Stereogenic Alcohols with Chirality Transfer
- 2016
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Ingår i: Synlett. - : Georg Thieme Verlag KG. - 0936-5214 .- 1437-2096. ; 27:2, s. 173-176
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Tidskriftsartikel (refereegranskat)abstract
- A brief overview of the development of direct substitution of the hydroxyl (OH) group of alcohols in our research group is presented. By applying a BrOnsted acid, an intramolecular substitution of the OH group in stereogenic alcohols with chirality transfer was achieved. Noteworthy, the intramolecular substitution has a wide scope in respect to both the nucleophile and also the nucleofuge. A mechanistic study by both experiments and DFT calculations revealed a unique reaction pathway in which the BrOnsted acid operates in a bifunctional manner to promote an S(N)2-type reaction mechanism.
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| 6. |
- Kumaniaev, Ivan, et al.
(författare)
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A combination of experimental and computational methods to study the reactions during a Lignin-First approach
- 2020
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Ingår i: Pure and Applied Chemistry. - : Walter de Gruyter GmbH. - 0033-4545 .- 1365-3075. ; 92:4, s. 631-639
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Tidskriftsartikel (refereegranskat)abstract
- Current pulping technologies only valorize the cellulosic fiber giving total yields from biomass below 50 %. Catalytic fractionation enables valorization of both cellulose, lignin, and, optionally, also the hemicellulose. The process consists of two operations occurring in one pot: (1) solvolysis to separate lignin and hemicellulose from cellulose, and (2) transition metal catalyzed reactions to depolymerize lignin and to stabilized monophenolic products. In this article, new insights into the roles of the solvolysis step as well as the operation of the transition metal catalyst are given. By separating the solvolysis and transition metal catalyzed hydrogen transfer reactions in space and time by applying a flow-through set-up, we have been able to study the solvolysis and transition metal catalyzed reactions separately. Interestingly, the solvolysis generates a high amount of monophenolic compounds by pealing off the end groups from the lignin polymer and the main role of the transition metal catalyst is to stabilize these monomers by transfer hydrogenation/hydrogenolysis reactions. The experimental data from the transition metal catalyzed transfer hydrogenation/hydrogenolysis reactions was supported by molecular dynamics simulations using ReaXFF.
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| 7. |
- Meier de Andrade, Ageo, 1990-, et al.
(författare)
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Lignin Intermediates on Palladium : Insights into Keto-Enol Tautomerization from Theoretical Modelling
- 2020
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Ingår i: ChemSusChem. - : John Wiley & Sons. - 1864-5631 .- 1864-564X. ; 13:24, s. 6574-6581
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Tidskriftsartikel (refereegranskat)abstract
- It has been suggested in the literature that keto-to-enol tautomerization plays a vital role for lignin fragmentation under mild conditions. On the other hand, previous modelling has shown that the adsorbed keto form is more stable than enol on the Pd(111) catalyst. The current density functional theory study of lignin model molecules shows that, in the gas-phase, keto is more stable than enol, but on the Pd surface, we find enol conformers that are at least as stable as keto. This supports the experimental result that the favourable reaction pathway for lignin depolymerization involves keto-enol tautomerization. An energy decomposition analysis gives insights concerning the origin of the fine energy balance between the keto and enol forms, where the molecule–surface interaction (−7 eV) and the molecular strain energy (+3 eV) are the main contributors to the adsorption energy.
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| 8. |
- Monti, Susanna, et al.
(författare)
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ReaxFF Simulations of Lignin Fragmentation on a Palladium-Based Heterogeneous Catalyst in Methanol-Water Solution
- 2018
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:18, s. 5233-5239
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of fragments derived from lignin depolymerization with a heterogeneous palladium catalyst in methanol-water solution is studied by means of experimental and theoretical methodologies. Quantum chemistry calculations and molecular dynamics simulations based on the ReaxFF approach are combined effectively to obtain an atomic level characterization of the crucial steps of the adsorption of the molecules on the catalyst, their fragmentation, reactions, and desorption. The main products are identified, and the most important routes to obtain them are explained through extensive computational procedures. The simulation results are in excellent agreement with the experiments and suggest that the mechanisms comprise a fast chemisorption of identified fragments from lignin on the metal interface accompanied by bond breaking, release of some of their hydrogens and oxygens to the support, and eventual desorption depending on the local environment. The strongest connections are those involving the aromatic rings, as confirmed by the binding energies of selected representative structures, estimated at the quantum chemistry level. The satisfactory agreement with the literature, quantum chemistry data, and experiments confirms the reliability of the multilevel computational procedure to study complex reaction mixtures and its potential application in the design of high-performance catalytic devices.
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| 9. |
- Srifa, Pemikar, et al.
(författare)
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Detecting Important Intermediates in Pd Catalyzed Depolymerization of a Lignin Model Compound by a Combination of DFT Calculations and Constrained Minima Hopping
- 2016
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:41, s. 23469-23479
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory (DFT) calculations, combined with a constrained minima hopping algorithm (global minimum search while preserving the molecular identity), have been performed to investigate important reaction intermediates for the heterogeneously catalyzed beta-O-4' bond cleavage in lignin derivatives. More specifically, we have studied the adsorption properties of a keto tautomer (1-methoxypropan-2-one) and its enol form on a catalytically active Pd(111) surface. In agreement with experiments, we find that for the gas phase molecules the keto tautomer is the most stable. Interestingly, the enol tautomer has a higher affinity to the Pd catalyst than the keto form, and becomes the most stable molecular form when adsorbed on the catalyst surface. The global minimum complex found on the metal surface corresponds to an enolate structure formed when the enol tautomer chemisorbs onto the surface and donates its pi-electrons from the C=C region to two adjacent palladium atoms. The actual formation of a chemical bond to the surface in the case of the enol molecule could be the key to understanding why the enol derivative is needed for an efficient beta-O-4' bond cleavage.
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| 10. |
- Srifa, Pemikar, et al.
(författare)
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Lignin Intermediates on the Palladium Surface : Factors for Structural and Energetic Changes
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this work, dispersion-corrected density functional theory (DFT-D3), has been used to investigate interactions between important intermediates in lignin depolymerization and a palladium catalyst. The keto structure 2-phenoxy-1-phenylethanone and its enol tautomer have been used to model reactive intermediates derived from lignin. To investigate how the adsorption energies are affected by adsorbate coverage, we have used two different Pd(111) super cells; one smaller p(6 × 4) and one larger p(6 × 6). In the gas phase, the staggered conformer of the keto tautomer is more stable than both the eclipsed form of the keto tautomer and as expected much more stable than the E-enol tautomer. However, in interaction with the palladium surface, the E-enol tautomer has a similar binding energy as the keto tautomer. Also, the eclipsed conformer of the keto tautomer is more stable than the staggered conformer of the keto tautomer when adsorbed to the palladium. We found that the coverage, that is concentration of molecules on the surface had a pronounced effect on the adsorption energies. At higher coverage, both the keto and enol models prefer to adsorb on an atop configuration to the surface. Furthermore, we found that both the keto and the enol tautomers bind strongly to the surface through their phenyl rings. Despite the strong binding of the phenyl groups, the enol adsorbs to the surface through a chemisorption by cleavage of the C═C bond, that leads to two types of di-sigma complexes depending on the position of the newly formed Pd–C sigma bonds. The generated complex is a key intermediate in the subsequent depolymerization through cleavage of a C–O bond. Our simulations show that there is an intermolecular repulsion between adsorbates on the surface, and consequently, the molecules were found to bind more strongly to the surface at low coverages (by 8-14 kcal/mol). These results are important for experimental design purposes; as previous experiments have shown that the enol form is key for an efficient β-O-4′ bond cleavage and implies that low concentration reactions are favored.
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