| 1. |
- Löhmann, Oliver, et al.
(författare)
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Wavelength frame multiplication for reflectometry at long-pulse neutron sources
- 2020
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 91:12
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Tidskriftsartikel (refereegranskat)abstract
- The European Spallation Source (ESS), which is under construction in Lund (Sweden), will be the next leading neutron facility with an unprecedented brilliance and novel long-pulse time structure. A long-pulse source not only provides a high time-average flux but also opens the possibility to tune the resolution by using pulse shaping choppers. Thus, an instrument can readily be operated in either a high flux or a high resolution mode. Several of the shorter instruments at the ESS will employ Wavelength Frame Multiplication (WFM) in order to enable a sufficient resolution while offering a continuous and broad wavelength range. A test beamline was operated until the end of 2019 at the research reactor in Berlin to test components and methods, including WFM, in order to prepare the new facility for the operation of neutron instruments and successful first science. We herein demonstrate the implementation of WFM for reflectometry. By selecting a short pulse mode under the same geometrical configuration, we compare and discuss the results for two reference samples. The reported experiments not only serve to prove the reliability of the WFM approach but also, for the first time, demonstrate the full instrument control, data acquisition and data reduction chain that will be implemented at the ESS.
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| 2. |
- Massaro, Luca, et al.
(författare)
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Stereodivergent Synthesis of Trisubstituted Enamides : Direct Access to Both Pure Geometrical Isomers
- 2019
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 84:21, s. 13540-13548
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Tidskriftsartikel (refereegranskat)abstract
- A stereodivergent strategy has been developed to access either (E)- or (Z)-isomers of trisubstituted enamides. Starting from an extensive range of ketones, it was possible to synthesize and isolate the desired pure isomer by switching the reaction conditions. Lewis acid activation enables the formation of the (E)-isomers in high stereoselectivity (>90:10) and good yields. On the other hand, the use of a Bronsted acid allows the preparation of the (Z)-isomers, again in high selectivity (up to 99:1), with moderate yields.
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| 3. |
- Yang, Jianping, et al.
(författare)
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Combined Theoretical and Experimental Studies Unravel Multiple Pathways to Convergent Asymmetric Hydrogenation of Enamides
- 2021
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:51, s. 21594-21603
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Tidskriftsartikel (refereegranskat)abstract
- We present a highly efficient convergent asymmetric hydrogenation of E/Z mixtures of enamides catalyzed by N,P–iridium complexes supported by mechanistic studies. It was found that reduction of the olefinic isomers (E and Z geometries) produces chiral amides with the same absolute configuration (enantioconvergent hydrogenation). This allowed the hydrogenation of a wide range of E/Z mixtures of trisubstituted enamides with excellent enantioselectivity (up to 99% ee). A detailed mechanistic study using deuterium labeling and kinetic experiments revealed two different pathways for the observed enantioconvergence. For α-aryl enamides, fast isomerization of the double bond takes place, and the overall process results in kinetic resolution of the two isomers. For α-alkyl enamides, no double bond isomerization is detected, and competition experiments suggested that substrate chelation is responsible for the enantioconvergent stereochemical outcome. DFT calculations were performed to predict the correct absolute configuration of the products and strengthen the proposed mechanism of the iridium-catalyzed isomerization pathway.
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