Browsing by Author "Naumann, Stefan"

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Open Access
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.
Open Access
Dual catalysis with an N‐heterocyclic carbene and a Lewis acid : thermally latent precatalyst for the polymerization of ε‐caprolactam
(2020) Altmann, Hagen J.; Steinmann, Mark; Elser, Iris; Benedikter, Mathis J.; Naumann, Stefan; Buchmeiser, Michael R.
So far, the earlier reported strong correlation between basicity of an N‐heterocyclic carbene (NHC) and its reactivity in poly(ε‐caprolactam) (PA6) synthesis resulted in a substantial limitation of applicable carbenes. Here, to overcome this issue, 1,3‐dimethylimidazolium‐2‐carboxylate, an easily accessible, air and moisture‐stable NHC, was applied as thermally latent initiator. In order to compensate for its low basicity, reactivity was enhanced by the addition of both a Lewis acid and an activator to ease the initial polymerization step. The resulting mixtures of ε‐caprolactam, the CO2‐protected NHC, a Lewis acid and N‐acylazepan‐2‐one constitute homogeneous, thermally fully latent “single‐component” blends for the anionic polymerization‐based synthesis of PA6. They can be stored both in the liquid and solid state for days and months, respectively, without any loss in activity. The role of the Lewis acid as well as technical implications of the prolonged pot‐times are discussed.
Open Access
Revealing the monomer gradient of polyether copolymers prepared using N‐heterocyclic olefins : metal‐free anionic versus zwitterionic Lewis pair polymerization
(2023) Kersten, Erik; Linker, Olga; Blankenburg, Jan; Wagner, Manfred; Walther, Patrick; Naumann, Stefan; Frey, Holger
N-Heterocyclic olefin (NHO)-based polymerization pathways for the copolymerization of ethylene oxide (EO) and propylene oxide (PO) are investigated in detail. Employing in situ 1H NMR spectroscopy, both an organocatalytic, anionic polymerization setup (system A) and a zwitterionic, Lewis pair-type approach (system B) are studied comparatively. The obtained kinetics data are fitted to the non-terminal model (Jaacks and Ideal Integrated) and terminal Mayo–Lewis model (Meyer Lowry) to determine the reactivity ratios, revealing striking differences in copolyether microstructure and achievable molar masses. While for the metal-free catalysis (system A) reactivity ratios of rEO = 3.4 and rPO = 0.30 are found, indicating a soft gradient structure, the presence of Mg(HMDS)2 (system B) entails exclusively zwitterionic propagation. This results in enhanced selectivity, displaying corresponding parameters of rEO = 7.9 and rPO = 0.13, in line with the proposed monomer-activated mechanism. The block-like, strongly tapered copolyether microstructure is also reflected in the thermal properties, showing a melting point for the latter sample and much higher molar masses (Mn >50 000 g mol−1). Notably, this study not only identifies capable polymerization systems for EO/PO, but also underlines that via in situ 1H NMR kinetics key questions regarding the polymerization mechanism can be illuminated quickly and reliably, simplifying access to essential structure-property relations.
Open Access
A systematic study of nonionic di‐ and multiborane catalysts for the oligomerization and polymerization of epoxides
(2024) Haug, Iris; Eberhardt, Marc; Krappe, Udo; Naumann, Stefan
Borane catalysis has emerged as a powerful technology in epoxide polymerization. Still, the structure‐activity correlations for these catalysts are not fully understood to date, especially regarding compounds with nonionic backbones. Thus, in this work, 13 different borane catalysts of this respective type are described and investigated for their epoxide oligomerization and polymerization performance, using propylene oxide (PO), 1‐butylene oxide (BO) and allyl glycidyl ether (AGE) as monomers. Structurally, special emphasis is put on catalysts with different linker lengths and linker flexibilities as well as the introduction of more than two borane functionalities. Importantly, this screening is conducted both under typical polymerization conditions as well as under the chain transfer agent (CTA)‐rich conditions relevant for large‐scale production. It is found that suitable preorganization of the borane groups, such as present in biphenyl derivatives, offers a simple route to high‐performing catalysts and quantitative monomer conversion of the investigated epoxides. Furthermore, it is demonstrated that a diborane‐catalyzed oligomerization can be kept active over weeks, whereby repeated addition of monomer batches (14 steps) constantly results in full conversion and well‐defined oligoethers, underlining the practical potential of this method. The absence of co‐initiating counter ions is suggested as an inherent advantage of nonionic catalysts.