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| 2. |
- Ahlford, Katrin, et al.
(författare)
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Amino acid-derived amides and hydroxamic acids as ligands for asymmetric transfer hydrogenation in aqueous media
- 2011
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Ingår i: Catalysis communications. - : Elsevier BV. - 1566-7367 .- 1873-3905. ; 12:12, s. 1118-1121
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Tidskriftsartikel (refereegranskat)abstract
- Amides and hydroxamic acids derived from α-amino acids were evaluated as ligands in combination with rhodium and iridium half-sandwich complexes in asymmetric transfer hydrogenation (ATH) of ketones. The reactions were performed in aqueous media using lithium formate as hydride source. The catalyst systems turned out to be highly efficient and ees up to 90% were obtained.
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| 3. |
- Ahlford, Katrin, 1983-
(författare)
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Asymmetric transfer hydrogenation of ketones : Catalyst development and mechanistic investigation
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of ligands derived from natural amino acids for asymmetric transfer hydrogenation (ATH) of prochiral ketones is described herein. In the first part, reductions performed in alcoholic media are examined, where it is found that amino acid-derived hydroxamic acids and thioamides, respectively, are simple and versatile ligands that in combination with [RhCp*Cl2]2 efficiently catalyze this particular transformation. Selectivities up to 97% ee of the corresponding secondary alcohols are obtained, and it is furthermore observed that the two different ligand classes, albeit based on the same amino acid scaffold, give rise to products of opposite configuration. The highly interesting enantioswitchable nature of the two abovementioned catalysts is studied in detail by mechanistic investigations. A structure/activity correlation analysis is performed, which reveals that the diverse behavior of the catalysts arise from different interactions between the ligands and the metal. Kinetic studies furthermore stress the catalyst divergence, since a difference in the rate determining step is established from initial rate measurements. In addition, rate constants are determined for each step of the overall reduction process. In the last part, catalyst development for ATH executed in water is discussed. The applicability of hydroxamic acid ligands is further extended, and catalysts based on these compounds are found to be efficient and compatible with aqueous conditions. The structurally even simpler amino acid amide is also evaluated as a ligand, and selectivities up to 90% ee are obtained in the reduction of a number of aryl alkyl ketones. The very challenging reduction of dialkyl ketones is moreover examined in the Rh-catalyzed aqueous ATH, where a modified surfactant-resembling sulfonylated diamine is used as ligand, and the reaction is carried out in the presence of SDS-micelles. A positive effect is to some extent found on the catalyst performance upon addition of phase-transfer components, especially regarding the catalytic activity in the reduction of more hydrophobic substrates.
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| 4. |
- Ahlford, Katrin, et al.
(författare)
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Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Amino Acid Derived Rhodium Complexes : On the Origin of Enantioselectivity and Enantioswitchability
- 2009
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 15:42, s. 11197-11209
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Tidskriftsartikel (refereegranskat)abstract
- Amino acid based thioamides, hydroxamic acids, and hydrazides have been evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. Catalysts containing thioamide ligands derived from L-valine were found to selectively generate the product with an R configuration (95 % ee), whereas the corresponding L-valine-based hydroxamic acids or hydrazides facilitated the formation of the (S)-alcohols (97 and 91 % ee, respectively). The catalytic reduction was examined by performing a structure–activity correlation investigation with differently functionalized or substituted ligands and the results obtained indicate that the major difference between the thioamide and hydroxamic acid based catalysts is the coordination mode of the ligands. Kinetic experiments were performed and the rate constants for the reduction reactions were determined by using rhodium–arene catalysts derived from amino acid thioamide and hydroxamic acid ligands. The data obtained show that the thioamide-based catalyst systems demonstrate a pseudo-first-order dependence on the substrate, whereas pseudo-zero-order dependence was observed for the hydroxamic acid containing catalysts. Furthermore, the kinetic experiments revealed that the rate-limiting steps of the two catalytic systems differ. From the data obtained in the structure–activity correlation investigation and along with the kinetic investigation it was concluded that the enantioswitchable nature of the catalysts studied originates from different ligand coordination, which affects the rate-limiting step of the catalytic reduction reaction.
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| 5. |
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| 6. |
- Ahlford, Katrin, et al.
(författare)
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Fine-tuning catalytic activity and selectivity-[Rh(amino acid thioamide)] complexes for efficient ketone reduction
- 2009
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Ingår i: Tetrahedron Letters. - : Elsevier BV. - 0040-4039 .- 1359-8562. ; 50:46, s. 6321-6324
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Tidskriftsartikel (refereegranskat)abstract
- Amino acid-derived thioamides are prepared and evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. It is found that increasing the steric bulk at the C-terminus of the ligand had a positive impact on both activity and selectivity in the reduction reaction. In order to find the optimum catalyst, a study is performed on a series of thioamide ligands having substituents of varying size.
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| 7. |
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| 8. |
- Ahlford, Katrin, et al.
(författare)
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Rhodium-catalyzed asymmetric transfer hydrogenation of alkyl and aryl ketones in aqueous media
- 2008
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Ingår i: Green Chemistry. - : Royal Society of Chemistry (RSC). - 1463-9262 .- 1463-9270. ; 10:8, s. 832-835
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Tidskriftsartikel (refereegranskat)abstract
- A novel lipophilic rhodium catalyst was evaluated in the enantioselective transfer hydrogenation of ketones in water using sodium formate as the hydride donor, and in the presence of sodium docecylsulfonate. Alkyl alkyl ketones were reduced in good yields and in moderate to good enantioselectivities, and the reduction of aryl alkyl ketones proceeded with excellent enantioselectivity (up to 97% ee).
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| 9. |
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| 10. |
- Coll, Mercedes, et al.
(författare)
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Modular Furanoside Pseudodipeptides and Thioamides, Readily Available Ligand Libraries for Metal-Catalyzed Transfer Hydrogenation Reactions : Scope and Limitations
- 2012
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Ingår i: Advanced Synthesis and Catalysis. - : Wiley. - 1615-4150 .- 1615-4169. ; 354:2-3, s. 415-427
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Tidskriftsartikel (refereegranskat)abstract
- Two new highly modular carbohydrate-based, pseudodipeptide and thioamide ligand libraries have been synthesized for the rhodium- and ruthenium-catalyzed asymmetric transfer hydrogenation (ATH) of prochiral ketones. These series of ligands can be prepared efficiently from easily accessible D-xylose and D-glucose. The ligand libraries contain two main ligand structures (pseudodipeptide and thioamide) that have been designed by making systematic modifications to one of the most successful ligand families developed for the ATH. As well as studying the effect of these two ligand structures on the catalytic performance, we also evaluated the effect of modifying several of the ligand parameters. We found that the effectiveness of the ligands at transferring the chiral information in the product can be tuned by correctly choosing the ligand components (ligand structure and ligand parameters). Excellent enantioselectivities (ees up to 99%) were therefore obtained in both enantiomers of the alcohol products using a wide range of substrates.
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| 11. |
- Nordin, Mikael, et al.
(författare)
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Theoretical study of asymmetric transfer hydrogenation of ketones catalyzed by amino acid derived rhodium complexes
- 2012
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Ingår i: ChemCatChem. - : Wiley. - 1867-3880 .- 1867-3899. ; 4:8, s. 1095-1104
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are employed to study the asymmetric transfer hydrogenation of ketones catalyzed by rhodiumarene complexes containing hydroxamic acid-functionalized amino acid ligands. Firstly, the ligandmetal binding is investigated and it is shown that both the N,N and O,O binding modes Are viable. For each of these, the full free energy profile for the transfer hydrogenation is calculated according to the outer-sphere reaction mechanism. Three factors are demonstrated to influence the stereoselectivity of the process, namely the energy difference between the metalligand binding modes, the energy difference between the intermediate hydrogenated catalyst, and the existence of a stabilizing CHp interaction between the Cp* ligand of the catalyst and the phenyl moiety of the substrate. Theoretical reproduction of the selectivity of a slightly modified ligand that is shown experimentally to yield the opposite enantioselectivity corroborates these results. Finally, a technical observation made is that inclusion of dispersion interactions (using the B3LYP-D2 correction or the M06 functional) proved to be very important for reproducing the enantioselectivity.
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