03 Fakultät Chemie
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Item Open Access Counterion effects on the mesomorphic and electrochemical properties of guanidinium salts(2024) Ebert, Max; Lange, Alyna; Müller, Michael; Wuckert, Eugen; Gießelmann, Frank; Klamroth, Tillmann; Zens, Anna; Taubert, Andreas; Laschat, SabineIonic liquid crystals (ILCs) combine the ion mobility of ionic liquids with the order and self-assembly of thermotropic mesophases. To understand the role of the anion in ILCs, wedge-shaped arylguanidinium salts with tetradecyloxy side chains were chosen as benchmark systems and their liquid crystalline self-assembly in the bulk phase as well as their electrochemical behavior in solution were studied depending on the anion. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS) experiments revealed that for spherical anions, the phase width of the hexagonal columnar mesophase increased with the anion size, while for non-spherical anions, the trends were less clear cut. Depending on the anion, the ILCs showed different stability towards electrochemical oxidation and reduction with the most stable being the PF6 based compound. Cyclic voltammetry (CV) and density functional theory (DFT) calculations suggest a possible contribution of the guanidinium cation to the oxidation processes.Item Open Access Highly active cooperative Lewis acid : ammonium salt catalyst for the enantioselective hydroboration of ketones(2021) Titze, Marvin; Heitkämper, Juliane; Junge, Thorsten; Kästner, Johannes; Peters, RenéEnantiopure secondary alcohols are fundamental high‐value synthetic building blocks. One of the most attractive ways to get access to this compound class is the catalytic hydroboration. We describe a new concept for this reaction type that allowed for exceptional catalytic turnover numbers (up to 15 400), which were increased by around 1.5-3 orders of magnitude compared to the most active catalysts previously reported. In our concept an aprotic ammonium halide moiety cooperates with an oxophilic Lewis acid within the same catalyst molecule. Control experiments reveal that both catalytic centers are essential for the observed activity. Kinetic, spectroscopic and computational studies show that the hydride transfer is rate limiting and proceeds via a concerted mechanism, in which hydride at Boron is continuously displaced by iodide, reminiscent to an SN2 reaction. The catalyst, which is accessible in high yields in few steps, was found to be stable during catalysis, readily recyclable and could be reused 10 times still efficiently working.Item Open Access Stereoretentive regio‐ und enantioselektive Allylierung von Isoxazolinonen per planar chiralem Palladacyclus‐Katalysator(2022) Yu, Xin; Hu, Lingfei; Frey, Wolfgang; Lu, Gang; Peters, RenéDie katalytische allylische Substitution ist eines der wichtigsten Werkzeuge in der asymmetrischen Synthese zur enantioselektiven Bildung von C-C-Bindungen. Während in vorigen Arbeiten eine hohe Effizienz in Bezug auf Enantio- und Regiokontrolle unter Verwendung verschiedener Katalysatortypen erreicht wurde, besteht eine starke allgemeine Einschränkung in einer sehr ausgeprägten Präferenz für die Bildung von allylischen Substitutionsprodukten mit (E)-konfigurierten C=C-Doppelbindungen. Hier berichten wir, dass mit einem planar-chiralen Palladacyclus-Katalysator unter Verwendung von Isoxazolinonen und Allylimidaten als Substrate ein diastereospezifisches Reaktionsergebnis erzielt wird, wodurch die C=C-Doppelbindungsgeometrie der Allylsubstrate in den hoch enantiomerenangereicherten Produkten beibehalten wird. DFT-Rechnungen zeigen, dass die Reaktionen über einen SN2-Mechanismus und nicht über π-Allyl-Pd-Komplexe ablaufen. Entscheidend für die hohe Kontrolle ist die Stabilisierung des allylischen Fragments im SN2-Übergangszustand durch π-Wechselwirkungen mit den Phenylsubstituenten des Pentaphenylferrocen-Katalysatorkerns.Item Open Access The effect of pooling on the detection of the nucleocapsid protein of SARS-CoV-2 with rapid antigen tests(2021) Berking, Tim; Lorenz, Sabrina; Ulrich, Alexander; Greiner, Joachim; Kervio, Eric; Bremer, Jennifer; Wege, Christina; Kleinow, Tatjana; Richert, ClemensThe COVID-19 pandemic puts significant stress on the viral testing capabilities of many countries. Rapid point-of-care (PoC) antigen tests are valuable tools but implementing frequent large scale testing is costly. We have developed an inexpensive device for pooling swabs, extracting specimens, and detecting viral antigens with a commercial lateral flow test for the nucleocapsid protein of SARS-CoV-2 as antigen. The holder of the device can be produced locally through 3D printing. The extraction and the elution can be performed with the entire set-up encapsulated in a transparent bag, minimizing the risk of infection for the operator. With 0.35 mL extraction buffer and six swabs, including a positive control swab, 43 ± 6% (n = 8) of the signal for an individual extraction of a positive control standard was obtained. Image analysis still showed a signal-to-noise ratio of approximately 2:1 at 32-fold dilution of the extract from a single positive control swab. The relative signal from the test line versus the control line was found to scale linearly upon dilution (R2 = 0.98), indicating that other pooling regimes are conceivable. A pilot project involving 14 participants and 18 pooled tests in a laboratory course at our university did not give any false positives, and an individual case study confirmed the ability to detect a SARS-CoV-2 infection with five-fold or six-fold pooling, including one swab from a PCR-confirmed COVID patient. These findings suggest that pooling can make frequent testing more affordable for schools, universities, and similar institutions, without decreasing sensitivity to an unacceptable level.Item Open Access Structural characterization of surface immobilized platinum hydrides by sensitivity-enhanced 195Pt solid state NMR spectroscopy and DFT calculations(2024) Atterberry, Benjamin A.; Wimmer, Erik J.; Klostermann, Sina; Frey, Wolfgang; Kästner, Johannes; Estes, Deven P.; Rossini, Aaron J.Supported single-site platinum hydride compounds are promising heterogeneous catalysts for organic transformations. Few methods exist to describe the structures of single-site Pt catalysts with atomic resolution because of their disordered structures and low Pt loadings. Here, we study the compounds formed when bis(tri-tert-butylphosphino)platinum, Pt(PtBu3)2, is supported on dehydroxylated SiO2 or SiO2-Al2O3. First, we obtain magic angle spinning (MAS) 1H, 31P and 195Pt ssNMR spectra of four model Pt phosphine compounds with oxidation states of 0 or +2 and coordination numbers between 2 and 4. These compounds are analogs of potential structures present in the supported compounds. MAS 195Pt ssNMR spectra were obtained using 31P{195Pt} sideband selective J-resolved and J-HMQC experiments. The measured 1H and 31P chemical shifts, 31P-195Pt J-couplings and 195Pt chemical shift (CS) tensors are shown to be diagnostic of oxidation state and coordination number. Room temperature 1H ssNMR spectra of Pt(PtBu3)2 supported on SiO2 or SiO2-Al2O3 show diagnostic hydride NMR signals, suggesting that Pt(PtBu3)2 undergoes oxidative addition, resulting in surface hydrides and Pt–oxygen bonds to the support surface. MAS dynamic nuclear polarization (DNP) enables 31P{195Pt} correlation NMR experiments on the supported compounds. These experiments enable the measurement of the 31P-195Pt J-coupling constants and 195Pt CS tensors. Combined NMR and DFT analyses suggest that the primary surface platinum species are [HPt(PtBu3)2OSi] on SiO2 and [HPt(PtBu3)2]+[Si-O--Al] on SiO2-Al2O3. The Pt-oxygen bond length is dependent on the support and estimated as 2.1-2.3 Å and 2.7-3.0 Å for SiO2 and SiO2-Al2O3, respectively.Item Open Access Synthesis of a peptidoyl RNA hairpin via a combination of solid‐phase and template‐directed chain assembly(2022) Bremer, Jennifer; Richter, Christian; Schwalbe, Harald; Richert, ClemensPeptidoyl RNAs are the products of ribosome‐free, single‐nucleotide translation. They contain a peptide in the backbone of the oligoribonucleotide and are interesting from a synthetic and a bioorganic point of view. A synthesis of a stabilized version of peptidoyl RNA, with an amide bond between the C‐terminus of a peptide and a 3′‐amino‐2′,3′‐dideoxynucleoside in the RNA chain was developed. The preferred synthetic route used an N‐Teoc‐protected aminonucleoside support and involved a solution‐phase coupling of the amino‐terminal oligonucleotide to a dipeptido dinucleotide. Exploratory UV‐melting and NMR analysis of the hairpin 5′‐UUGGCGAAAGCdC‐LeuLeu‐AA‐3′ indicated that the peptide‐linked RNA segments do not fold in a cooperative fashion. The synthetic access to doubly RNA‐linked peptides on a scale sufficient for structural biology opens the door to the exploration of their structural and biochemical properties.Item Open Access Transitions in solvate crystals of a tetraaryladamantane(2023) Frey, Wolfgang; Schwenger, Alexander; Berking, Tim; Richert, ClemensObtaining high-resolution structures of liquid compounds can be difficult. Encapsulating them in the lattice of a larger organic molecule acting as crystallization chaperone is one option to overcome this difficulty. Tetraaryladamantane ethers can play the role of chaperones, accommodating a range of different guest molecules in their crystals. How well-ordered crystalline arrangements for molecules of different shape are achieved is not clear. Cases in which more than one structure is found may shed light on this phenomenon. Here, we report low-order cubic crystal structures of 1,3,5,7-tetrakis(2,4-dimethoxyphenyl)adamantane (TDA) encapsulating ortho-xylene or cyclohexane, together with better ordered structures obtained after warming the crystals to 60 °C. Evidence for cubic crystal systems was also found for limonene, hexachlorobutadiene and eucalyptol, with a transition to a triclinic system for the former two, but no transition up to 70 °C for the latter. These findings indicate that some solvate structures of TDA can readily undergo structural transitions to less solvated, better ordered systems. Crystals obtained by rapid thermal crystallization may be in kinetically trapped states, and the transition to a solvate-free crystal system appears to have a kinetic barrier that depends strongly on the structure of the liquid guest molecules encapsulated in the lattice.Item 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, SabineCovalent 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.Item Open Access Ribosome‐free translation up to pentapeptides via template walk on RNA sequences(2024) Reußwig, Sabrina G.; Richert, ClemensThe origin of translation is one of the most difficult problems of molecular evolution. Identifying molecular systems that translate an RNA sequence into a peptide sequence in the absence of ribosomes and enzymes is a challenge. Recently, single‐nucleotide translation via coupling of 5′ phosphoramidate‐linked amino acids to 2′/3′‐aminoacyl transfer‐NMPs, as directed by the sequence of an RNA template, was demonstrated for three of the four canonical nucleotides. How single‐nucleotide translation could be expanded to include all four bases and to produce longer peptides without translocation along the template strand remained unclear. Using transfer strands of increasing length containing any of the four bases that interrogate adjacent positions along the template, we now show that pentapeptides can be produced in coupling reactions and subsequent hydrolytic release in situ. With 2′/3′‐aminoacylated mono‐, di‐, tri‐ and tetranucleotides we thus show how efficient translation can be without biomacromolecules.Item Open Access How the way a naphthalimide unit is implemented affects the photophysical and -catalytic properties of Cu(I) photosensitizers(2022) Yang, Yingya; Doettinger, Florian; Kleeberg, Christian; Frey, Wolfgang; Karnahl, Michael; Tschierlei, StefanieDriven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two heteroleptic Cu(I) complexes CuNIphen (NIphen = 5-(1,8-naphthalimide)-1,10-phenanthroline) and Cubiipo (biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthroline-16-one) were prepared and compared with the novel unsubstituted reference compound Cuphen (phen = 1,10-phenanthroline). Beside a comprehensive structural characterization, including two-dimensional nuclear magnetic resonance spectroscopy and X-ray analysis, a combination of electrochemistry, steady-state and time-resolved spectroscopy was used to determine the electrochemical and photophysical properties in detail. The nature of the excited states was further examined by (time-dependent) density functional theory (TD-DFT) calculations. It was found that CuNIphen exhibits a greatly enhanced absorption in the visible and a strong dependency of the excited state lifetimes on the chosen solvent. For example, the lifetime of CuNIphen extends from 0.37 µs in CH2Cl2 to 19.24 µs in MeCN, while it decreases from 128.39 to 2.6 µs in Cubiipo. Furthermore, CuNIphen has an exceptional photostability, allowing for an efficient and repetitive production of singlet oxygen with quantum yields of about 32%.