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Institute: search.filters.UBSInstitut.Institut für Physikalische ChemieAuthor: search.filters.author.Traa, Yvonne

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    Asymmetric Rh diene catalysis under confinement : isoxazole ring‐contraction in mesoporous solids
    (2024) Marshall, Max; Dilruba, Zarfishan; Beurer, Ann‐Katrin; Bieck, Kira; Emmerling, Sebastian; Markus, Felix; Vogler, Charlotte; Ziegler, Felix; Fuhrer, Marina; Liu, Sherri S. Y.; Kousik, Shravan R.; Frey, Wolfgang; Traa, Yvonne; Bruckner, Johanna R.; Plietker, Bernd; Buchmeiser, Michael R.; Ludwigs, Sabine; Naumann, Stefan; Atanasova, Petia; Lotsch, Bettina V.; Zens, Anna; Laschat, Sabine
    Covalent immobilization of chiral dienes in mesoporous solids for asymmetric heterogeneous catalysis is highly attractive. In order to study confinement effects in bimolecular vs monomolecular reactions, a series of pseudo‐C2‐symmetrical tetrahydropentalenes was synthesized and immobilized via click reaction on different mesoporous solids (silica, carbon, covalent organic frameworks) and compared with homogeneous conditions. Two types of Rh‐catalyzed reactions were studied: (a) bimolecular nucleophilic 1,2‐additions of phenylboroxine to N‐tosylimine and (b) monomolecular isomerization of isoxazole to 2H‐azirne. Polar support materials performed better than non‐polar ones. Under confinement, bimolecular reactions showed decreased yields, whereas yields in monomolecular reactions were only little affected. Regarding enantioselectivity the opposite trend was observed, i. e. effective enantiocontrol for bimolecular reactions but only little control for monomolecular reactions was found.
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    Efficient and spatially controlled functionalization of SBA‐15 and initial results in asymmetric Rh‐catalyzed 1,2‐additions under confinement
    (2021) Beurer, Ann‐Katrin; Kirchhof, Manuel; Bruckner, Johanna R.; Frey, Wolfgang; Baro, Angelika; Dyballa, Michael; Giesselmann, Frank; Laschat, Sabine; Traa, Yvonne
    Selectively functionalized mesoporous silica may considerably advance heterogeneous catalysis through the controlled immobilization of highly selective complex catalysts inside the mesopores. However, spatially controlled functionalization and the precise analytical verification are still a challenge. In this publication, we report a method, which ensures a selective functionalization of the mesopore surface with a clickable linker and thus makes it possible to study confinement effects during catalyzed reactions. First, we passivate the silanol groups on the particle surface and in the pore entrances of the mesoporous silica material SBA‐15 with 1,1,1‐trimethyl‐N‐(trimethylsilyl)silanamine. Then we remove the template by solvent extraction and functionalize the pore walls with 3‐azidopropyltriethoxysilane before we click the catalyst. In initial experiments of asymmetric Rh‐catalyzed 1,2‐addition, we investigate the performance of a catalyst clicked selectively in the mesopores and compare it to the dissolved catalyst as well as to the catalyst immobilized exclusively on the external surface of SBA‐15.
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    Cationic molybdenum imido alkylidene N‐heterocyclic carbene complexes confined in mesoporous silica : tuning transition states towards Z‐selective ring‐opening cross‐metathesis
    (2022) Goldstein, Elizabeth L.; Ziegler, Felix; Beurer, Ann‐Katrin; Traa, Yvonne; Bruckner, Johanna R.; Buchmeiser, Michael R.
    We recently reported a method for selective macro(mono)cyclization of dienes utilizing catalysts confined inside the pores of mesoporous silica, which we believe occurs due to suppression of oligomerization due to pore size. We hypothesized, however, that the system of cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) catalysts immobilized selectively inside the mesopores of silica materials could address much more subtle selectivity differences, such as E/Z selectivity in ring‐opening/cross‐metathesis (ROCM). Upon investigation, we observed that surface‐bound cationic molybdenum imido alkylidene NHC catalysts indeed display an increased Z‐selectivity, especially during the early stages of the reaction. This effect was present when the catalyst was confined inside a pore, as well as when the catalyst was bound to non‐porous silica, which led us to conclude it is an effect caused by the catalyst being bound directly to the surface of a silica material where the proximity of the catalyst to the surface governs the transition state. Kinetic investigations revealed that significant post‐metathesis olefin isomerization occurs, the amount of which seems to be governed by the rate of diffusion of the product away from the active catalyst, with smaller pore sizes resulting in higher Z‐selectivity at higher conversion, attributable to faster diffusion of the product out of the pore than diffusion back into the pore.
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    Investigating the long-term kinetics of Pd nanoparticles prepared from microemulsions and the Lindlar catalyst for selective hydrogenation of 3-hexyn-1-ol
    (2024) Tari, Faeze; Hertle, Sebastian; Wang, Hongguang; Fischer, Julian; Aken, Peter A. van; Sottmann, Thomas; Klemm, Elias; Traa, Yvonne
    The effect of non-saturated corner and edge sites of Pd particles on the long-term selectivity of cis-3-hexen-1-ol in the hydrogenation of 3-hexyn-1-ol was studied in this work. Non-supported Pd agglomerates were synthesized through the microemulsion synthesis route and used at  nalkynol/APdratios between 0.08 and 21 mol/m2for the catalytic conversion of 3-hexyn-1-ol for 20 h. The selectivity of the cis-hexenol product increased by reducing the quantity of Pd catalytic sites (increasing the nalkynol/APdratio) without introducing any modifier or doping agent to poison the nonselective sites. Then, Pd aggregates with fused primary particles and, thus, fewer corner and edge sites were produced through thermal sintering of the agglomerates at 473-723 K. By comparing the catalytic performance of the agglomerates and aggregates, it was observed that at a rather similar kinetic behavior (99.99% conversion and 85-89% selectivity to cis-hexenol), the sintered aggregates could stay selective despite a catalytic surface area about seven times larger. This emphasizes the role of low-coordinated edge and corner sites on the final selectivity of the cis product and demonstrates that thermal sintering allows the number of non-selective sites to be reduced without any need for toxic or organic doping agents or modifiers.