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Going beyond currently reported two electron transformations that formed the core backdrop of asymmetric catalytic site‐selective carbohydrate polyol functionalizations, we herein report a seminal demonstration of an enantioconvergent copper catalyzed site‐selective etherification of minimally protected saccharides through a single‐electron radical pathway. Further, this strategy paves a rare strategy, through which a carboxamide scaffold that is present in some glycomimetics of pharmacological relevance, can be selectively introduced. In light of the burgeoning interest in chiral radical catalysis, and the virtual absence of such stereocontrol broadly in carbohydrate synthesis, our strategy showcased the unknown capability of chiral radical copper catalysis as a contemporary tool to address the formidable site‐selectivity challenge on a remarkable palette of naturally occurring saccharides. When reducing sugars were employed, a further dynamic kinetic resolution type glycosylation can be activated by the catalytic system to selectively generate the challenging β‐O‐glycosides. [ABSTRACT FROM AUTHOR]

In the presence of 1 mol % of a chiral iron porphyrin catalyst, various 3‐arylmethyl‐substituted 2‐quinolones and 2‐pyridones underwent an enantioselective amination reaction (20 examples; 93–99 % ee). The substrates were used as the limiting reagents, and fluorinated aryl azides (1.5 equivalents) served as nitrene precursors. The reaction is triggered by visible light which allows a facile dediazotation at ambient temperature. The selectivity of the reaction is governed by a two‐point hydrogen bond interaction between the ligand of the iron catalyst and the substrate. Hydrogen bonding directs the amination to a specific hydrogen atom within the substrate that is displaced by the nitrogen substituent either in a concerted fashion or by a rebound mechanism. [ABSTRACT FROM AUTHOR]

Vibrational circular dichroism (VCD) spectra of chiral high‐spin organic radicals are expected to show a strong intensity enhancement and are thought to be difficult to predict using state‐of‐the‐art theoretical methods. Herein we show that the chiral triplet nitrene obtained from photochemical cleavage of N2 from enantiopure 2‐azido‐9H‐fluorenol does not feature extraordinarily strong intensities and that the experimental spectra match nicely with calculated ones. Thereby, this study demonstrates the general feasibility of studies on chiral high‐spin organics by matrix‐isolation VCD. [ABSTRACT FROM AUTHOR]

Recording VCD spectra of aqueous solution poses a particular challenge as water is a strong infrared absorber. Likewise, the computational analysis of VCD spectra by means of DFT-based spectral calculations requires the consideration of explicit solvent molecules, thus posing an even greater challenge. Several studies suggested that by modeling the solvent environment with a few water molecules in a micro-solvation approach would be sufficient to describe experimental spectra. For example, using proline at different pH values, we herein show that a change in the relative spatial orientation of a single water molecule in five-fold solvated structures strongly affects the computed VCD spectral signatures and that Boltzmann-weighted spectra do not correctly reproduce the experiment. We thus explored an approach based on molecular dynamics and subsequent DFT-calculations, in which we considered 30 water molecules (about 1.5 solvation shells). Once again, it was found that the Boltzmann-weighted spectra obtained on the basis of several hundred structures did not correctly reproduce experimental signatures, and a simple averaging scheme resulted in well-matching spectra with comparable bandwidths. The rationale behind the procedure was that sampling the configurational space of the solvent molecules is as equally important as the conformational sampling of the solute. For conformationally more flexible molecules, it is assumed that a much larger set of structures will have to be computed in order to properly sample the conformational space of both solute and solvent. [ABSTRACT FROM AUTHOR]

Combining vibrational circular dichroism (VCD) spectroscopy with the matrix isolation (MI) technique opens up interesting possibilities to study chiral molecules. MI involves the isolation of guest species in inert solid matrices at cryogenic temperatures. Hence, MI-VCD measures are solid-state VCD measurements, and as such, can suffer from mostly birefringance-related artefacts in the same way as common solid-state VCD measurements. In this contribution, we demonstrate that the sample preparation condition have tremendous impact on the quality and reliability of the recorded MI-VCD spectra. While MI-IR spectra are basically blind to these artefacts, the variation of deposition temperatures and host flow rates seem to control whether high quality MI-VCD spectra are obtained or if depolarization effects lead to completely obscured spectra. For two selected examples, styrene oxide (SO) and 1-phenyl propylene oxide (PPO), we discuss how the various experimental conditions may lead to the aforementioned effects and give a microscopic description of their origin. [ABSTRACT FROM AUTHOR]

Vibrational circular dichroism (VCD) spectroscopy has become an important part of the (stereo-)chemists' toolbox as a reliable method for the determination of absolute configurations. Being the chiroptical version of infrared spectroscopy, it has also been recognized as being very sensitive to conformational changes and intermolecular interactions. This sensitivity originates from the fact that the VCD spectra of individual conformers are often more different than their IR spectra, so that changes in conformational distributions or band positions and intensities become more pronounced. What is an advantage for studies focussing on intermolecular interactions can, however, quickly turn into a major obstacle during AC determinations: solute–solvent interactions can have a strong influence on spectral signatures and they must be accurately treated when simulating VCD and IR spectra. In this perspective, we showcase selected examples which exhibit particularly pronounced solvent effects. It is demonstrated that it is typically sufficient to model solute–solvent interactions by placing single solvent molecules near hydrogen bonding sites of the solute and subsequently use the optimized structures for spectra simulations. This micro-solvation approach works reasonably well for medium-sized, not too conformationally flexible molecules. We thus also discuss its limitations and outline the next steps that method development needs to take in order to further improve the workflows for VCD spectra predictions. [ABSTRACT FROM AUTHOR]

Infrared and vibrational circular dichroism (VCD) spectra are occasionally very sensitive to solute–solvent interactions and show distinct spectral changes when strong solute–solvent hydrogen bonds give rise to conformational changes. In this regard, small peptides are ideal model systems to investigate such solvent effects on IR and VCD spectra as they possess several hydrogen bonding donor sites. In the present study, we investigate serine and serine-phenylalanine, which both are N-protected with Boc and C-capped with n-propylamine. Compared to previously studied model peptides, the serine residue introduces a strong hydrogen bonding site that competes with the amides for intra- and intermolecular interactions. For both compounds, we computationally found that the intramolecular OH·O interactions are preferentially broken by DMSO, but it was not sufficient to model only this particular interaction. Instead, depending on the conformer family, it was necessary to consider different numbers of solvent molecules in the computed structures and the experimental spectra were found to be best described by assuming mixed solvation states. Our analyses show that the IR and VCD spectra of molecules with multiple hydrogen bonding cannot be simulated by simply solvating all donor sites as this would neglect the presence of important conformer families. In turn, these results stresss the need for novel routines to account for solvation in IR and VCD spectra, that help estimating the contributions of different solvations states to the conformational distribution. [ABSTRACT FROM AUTHOR]

Matrix Isolation IR and VCD spectroscopy is used to characterize the self-aggregation of the title compound. It is shown that only the IR spectral region of the OH-/CH-stretching modes is sensitive to hydrogen bonding interactions and that the fingerprint region is not notably affected. In contrast, some characteristic VCD spectral features can be identified in the fingerprint region. [ABSTRACT FROM AUTHOR]

Contrasting cryosolutions and matrix isolation infrared spectroscopy, we investigate weak intermolecular interactions in complexes of iodo trifluoroethene (C2F3I) and N,N-dimethyl ferrocenyl amine as well as the parent ferrocene. In liquid xenon, solely the C–I⋯N halogen bond can be observed, while the confined environment in solid argon allows for the characterization of C–I⋯π and π⋯π bonded complexes. [ABSTRACT FROM AUTHOR]