Uncovering Key Structural Features of an Enantioselective Peptide‐Catalyzed Acylation Utilizing Advanced NMR Techniques

We report on a detailed NMR spectroscopic study of the catalyst‐substrate interaction of a highly enantioselective oligopeptide catalyst that is used for the kinetic resolution of trans‐cycloalkane‐1,2‐diols via monoacylation. The extraordinary selectivity has been rationalized by molecular dynamics as well as density functional theory (DFT) computations. Herein we describe the conformational analysis of the organocatalyst studied by a combination of nuclear Overhauser effect (NOE) and residual dipolar coupling (RDC)‐based methods that resulted in an ensemble of four final conformers. To corroborate the proposed mechanism, we also investigated the catalyst in mixtures with both trans‐cyclohexane‐1,2‐diol enantiomers separately, using advanced NMR methods such as T₁ relaxation time and diffusion‐ordered spectroscopy (DOSY) measurements to probe molecular aggregation. We determined intramolecular distance changes within the catalyst after diol addition from quantitative NOE data. Finally, we developed a pure shift EASY ROESY experiment using PSYCHE homodecoupling to directly observe intermolecular NOE contacts between the trans‐1,2‐diol and the cyclohexyl moiety of the catalyst hidden by spectral overlap in conventional spectra. All experimental NMR data support the results proposed by earlier computations including the proposed key role of dispersion interaction.     

Absolute Quantification of Noncoding RNA by Microscale Thermophoresis

Accurate quantification of the copy numbers of noncoding RNA has recently emerged as an urgent problem, with impact on fields such as RNA modification research, tissue differentiation, and others. Herein, we present a hybridization‐based approach that uses microscale thermophoresis (MST) as a very fast and highly precise readout to quantify, for example, single tRNA species with a turnaround time of about one hour. We developed MST to quantify the effect of tRNA toxins and of heat stress and RNA modification on single tRNA species. A comparative analysis also revealed significant differences to RNA‐Seq‐based quantification approaches, strongly suggesting a bias due to tRNA modifications in the latter. Further applications include the quantification of rRNA as well as of polyA levels in cellular RNA.     

What can reaction databases teach us about Buchwald–Hartwig cross-couplings?

Despite the widespread and increasing usage of Pd-catalyzed C–N cross couplings, finding good conditions for these reactions can be challenging. Practitioners mostly rely on few methodology studies or anecdotal experience. This is surprising, since the advent of data-driven experimentation and the large amount of knowledge in databases allow for data-driven insight. In this work, we address this by analyzing more than 62 000 Buchwald–Hartwig couplings gathered from CAS, Reaxys and the USPTO. Our meta-analysis of the reaction performance generates data-driven cheatsheets for reaction condition recommendation. It also provides an interactive tool to find rarer ligands with optimal performance regarding user-selected substrate properties. With this we give practitioners promising starting points. Furthermore, we study bias and diversity in the literature and summarize the current state of the reaction data, including its pitfalls. Hence, this work will also be useful for future data-driven developments such as the optimization of reaction conditions via machine learning.

An analysis of the entire literature on Pd-catalyzed C–N couplings enables data-driven insight and provides recommendations for reaction conditions.     

Functionality,Effectiveness, and Mechanistic Evaluation of a Multicatalyst‐Promoted Reaction Sequence by Electrospray Ionization Mass Spectrometry

A multicatalytic three‐step reaction consisting of epoxidation, hydrolysis, and enantioselective monoacylation of cyclohexene was studied by using mass spectrometry (MS). The reaction sequence was carried out in a one‐pot reaction using a multicatalyst. All reaction steps were thoroughly analyzed by electrospray ionization (ESI) MS (and MS/MS), as well as high‐resolution MS for structure elucidation. These studies allow us to shed light on the individual mode of action of each catalytic moiety. Thus, we find that under the epoxidation conditions, the catalytically active N‐methyl imidazole for the terminal acylation step is partially deactivated through oxidation. This observation helps to explain the lower efficiency of the catalyst in the last step compared to the monoacylation performed separately. All reactive intermediates and products of the reaction sequence, as well as of the side‐reactions, were monitored, and we present a working mechanism of the reaction.     

Enhancing Electrochemical Water‐Splitting Kinetics by Polarization‐Driven Formation of Near‐Surface Iron(0): An In Situ XPS Study on Perovskite‐Type Electrodes

In the search for optimized cathode materials for high‐temperature electrolysis, mixed conducting oxides are highly promising candidates. This study deals with fundamentally novel insights into the relation between surface chemistry and electrocatalytic activity of lanthanum ferrite based electrolysis cathodes. For this means, near‐ambient‐pressure X‐ray photoelectron spectroscopy (NAP‐XPS) and impedance spectroscopy experiments were performed simultaneously on electrochemically polarized La_0.6Sr_0.4FeO_3?δ (LSF) thin film electrodes. Under cathodic polarization the formation of Fe⁰ on the LSF surface could be observed, which was accompanied by a strong improvement of the electrochemical water splitting activity of the electrodes. This correlation suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may open novel paths in the search for electrodes with increased water splitting activity.     

Zinc‐Mediated Formation of Trifluoromethyl Ethers from Alcohols and Hypervalent Iodine Trifluoromethylation Reagents

A (fluor)ry of activity : The transfer of an intact trifluoromethyl group from a hypervalent iodine reagent to an aliphatic alcohol occurs smoothly upon activation by zinc bis(triflimide). This constitutes a straightforward method for the preparation of trifluoromethoxy alkyl derivatives, compounds otherwise difficult to access.

     

Resolving the problem of chromatographic overlap by 3D cross correlation (3DCC) processing of LC, MS and NMR data for characterization of complex glycan mixtures

Chromatographic overlap is a common problem in the analysis of complex mixtures. As a result, it is not possible to identify the components because each resulting NMR or MS spectrum contains multiple components. We introduce three-dimensional cross correlation (3DCC) that dissects NMR spectra of a mixture into spectra of the individual components without actually separating them. Correlation of peaks from MS and NMR profiles along a common LC time domain yields 3DCC NMR spectra of pure components correlated with a mass and a retention time. The method requires an LC run followed by fractionation and recording of MS and NMR spectra. The method is applicable to mixtures of any classes of molecules. Here, we demonstrate its application to a mixture of complex glycans obtained from a glycoprotein. Fourteen glycans eluting within only 3 min showed heavy overlap in the chromatographic run. 3DCC allowed their direct characterization without separation. Some of these structures from the glycoprotein bovine fibrinogen had not previously been described. The 3DCC procedure has been implemented in standard software. Actually, 3DCC can be used for any combination of separation techniques, like LC or GC, combined with two characterization methods like UV, IR, Raman, NMR or MS.     

Enantioselective Synthesis of Tertiary α-Hydroxyketones from Unfunctionalized Ketones: Palladium-Catalyzed Asymmetric Allylic Alkylation of Enolates

Aiming high: The title reaction for the synthesis of tertiary α-hydroxyketones is reported. Protected 1,2-enediol carbonates, the starting materials, were accessed from simple and readily available enol carbonates. Highly functionalized tertiary α-hydroxyketones can be obtained in high yield with excellent enantioselectivity using this method.     

Large scale polarizability calculations using the approximate coupled cluster model CC2 and MP2 combined with the resolution-of-the-identity approximation

We present an implementation of static and frequency-dependent polarizabilities for the approximate coupled cluster singles and doubles model CC2 and static polarizabilities for second-order Mo?ller-Plesset perturbation theory. Both are combined with the resolution-of-the-identity approximation for electron repulsion integrals to achieve unprecedented low operation counts, input-output, and disc space demands. To avoid the storage of double excitation amplitudes during the calculation of derivatives of density matrices, we employ in addition a numerical Laplace transformation for orbital energy denominators. It is shown that the error introduced by this approximation is negligible already with a small number of sampling points. Thereby an implementation of second-order one-particle properties is realized, which avoids completely the storage of quantities scaling with the fourth power of the system size. The implementation is tested on a set of organic molecules including large fused aromatic ring systems and the C(60) fullerene. It is demonstrated that exploiting symmetry and shared memory parallelization, second-order properties for such systems can be evaluated at the CC2 and MP2 level within a few hours of calculation time. As large scale applications, we present results for the 7-, 9-, and 11-ring helicenes.