11 research outputs found
Synthetische, C6âfunktionalisierte Aminoflavinkatalysatoren ermöglichen die aerobe Bromierung oxidationsanfĂ€lliger Substrate
Synthetic C6âFunctionalized Aminoflavin Catalysts Enable Aerobic Bromination of OxidationâProne Substrates
Stereodynamic tetrahydrobiisoindole âNU-BIPHEP(O)âs: functionalization, rotational barriers and non-covalent interactions
Stereodynamic ligands offer intriguing possibilities in enantioselective catalysis. âNU-BIPHEPsâ are a class of stereodynamic diphosphine ligands which are easily accessible via rhodium-catalyzed double [2 + 2 + 2] cycloadditions. This study explores the preparation of differently functionalized âNU-BIPHEP(O)â compounds, the characterization of non-covalent adduct formation and the quantification of enantiomerization barriers. In order to explore the possibilities of functionalization, we studied modifications of the ligand backbone, e.g., with 3,5-dichlorobenzoyl chloride. Diastereomeric adducts with Okamoto-type cellulose derivatives and on-column deracemization were realized on the basis of non-covalent interactions. Enantioselective dynamic HPLC (DHPLC) allowed for the determination of rotational barriers of ÎGâĄ298K = 92.2 ± 0.3 kJ molâ1 and 99.5 ± 0.1 kJ molâ1 underlining the stereodynamic properties of âNU-BIPHEPsâ and âNU-BIPHEP(O)sâ, respectively. These results make the preparation of tailor-made functionalized stereodynamic ligands possible and give an outline for possible applications in enantioselective catalysis
Reduced Molecular Flavins as Single-Electron Reductants after Photo-Excitation
Flavoenzymes mediate a multitude of chemical reactions and are catalytically active both in different oxidation states and in covalent adducts with reagents. The transfer of such reactivity to the organic laboratory using simplified molecular flavins is highly desirable and such applications in (photo-)oxidation reactions are already established. However, molecular flavins have not been used for the reduction of organic substrates yet, although this activity is known and well-studied for DNA photolyase enzymes. We report a catalytic method using reduced, molecular flavins as photo-reductants and Îł-terpinene as sacrificial reductant. Additionally, we present our design for air-stable, reduced flavin catalysts, which is based on a conformational bias strategy and circumvents the otherwise rapid reduction of O2 from air. Using our catalytic strategy, we were able to replace super-stoichiometric amounts of the rare-earth reductant SmI2 in a 5-exo-trig cyclization of substituted barbituric acid derivatives. Such flavin-catalyzed reductions are anticipated to be of broad applicability and their straightforward synthesis indicates future use in stereo- as well as site-selective transformations
Enantiodivergent Photochemical Rearrangements Due to Different Coordination Modes at an Oxazaborolidine Lewis Acid Catalyst
A strong enantiodivergence ranging from +92% ee to â45%
ee was observed in the oxadi-Ï-methane rearrangement of 2,4-cyclohexadienones.
Oxazaborolidine-based Lewis acid catalysts of the same absolute configuration
were applied in all cases, and the stereochemical outcome is solely
a function of the oxazaborolidine substituents. Based on the results
of an extended catalyst library screening (27 examples) and by interrogating
plausible catalystâsubstrate complexes in the ground state
with density functional theory (DFT) methods, we could link the switch
in enantioselectivity to a change in substrate binding. If the typical
substrate binding at the convex catalyst side is inhibited by bulky
substituents, our results indicate that substrates instead bind to
the concave side, and enantiomeric products result. Studies by TDDFT
in the S1 excited state further clarified the mechanistic
picture by connecting efficient product formation with trajectories
that reach a conical intersection with more excess energy. Our analysis
was validated by the stereochemical outcome achieved with five structurally
different catalysts
Enantiodivergent Photochemical Rearrangements Due to Different Coordination Modes at an Oxazaborolidine Lewis Acid Catalyst
A strong enantiodivergence ranging from +92% ee to â45%
ee was observed in the oxadi-Ï-methane rearrangement of 2,4-cyclohexadienones.
Oxazaborolidine-based Lewis acid catalysts of the same absolute configuration
were applied in all cases, and the stereochemical outcome is solely
a function of the oxazaborolidine substituents. Based on the results
of an extended catalyst library screening (27 examples) and by interrogating
plausible catalystâsubstrate complexes in the ground state
with density functional theory (DFT) methods, we could link the switch
in enantioselectivity to a change in substrate binding. If the typical
substrate binding at the convex catalyst side is inhibited by bulky
substituents, our results indicate that substrates instead bind to
the concave side, and enantiomeric products result. Studies by TDDFT
in the S1 excited state further clarified the mechanistic
picture by connecting efficient product formation with trajectories
that reach a conical intersection with more excess energy. Our analysis
was validated by the stereochemical outcome achieved with five structurally
different catalysts
Investigation of Strain-Promoted AzideâAlkyne Cycloadditions in Aqueous Solutions by Capillary Electrophoresis
The Cu-free 1,3-dipolar
cycloaddition of cyclooctynes and azides
is an up-and-coming method in bioorganic chemistry and other disciplines.
However, broad application is still hampered by major drawbacks such
as poor solubility of the reactants in aqueous media and low reaction
rates. It is thus of high demand to devise a fast and user-friendly
strategy for the optimization of reaction conditions and reagent design.
We describe a capillary electrophoresis (CE) study of reaction kinetics
in strain-promoted azideâalkyne cycloadditions (SPAAC) using
substrates with acidic or basic functionalities. This study reveals
that the pH value has a significant effect on reaction rates as a
result of changes in the reactantsâ charge state via protonation
or deprotonation, and the concomitant changes of electronic properties.
This novel experimental setup also enables the study of even more
challenging conditions such as reactions in micelles and we did indeed
observe much faster SPAAC reactions in the presence of surfactants.
Careful combination of the above-mentioned parameters resulted in
the identification of conditions enabling remarkable rate enhancement
by a factor of 80. This electrophoretic method may thus serve as a
versatile, fast and reliable tool for screening purposes in all research
areas applying SPAAC reactions
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Terahertz Spectroscopy of Tetrameric Peptides
Determining the sequence and structure of peptides is crucial for understanding their structure-property relationships. Among many techniques, structures are typically elucidated using nuclear magnetic resonance spectroscopy and single crystal X-ray diffraction measurements. In this study, we present terahertz time-domain spectroscopy (THz-TDS) as a complementary, nondestructive technique that is sensitive to both the primary and secondary structures of tetrapeptides. Using only a few milligrams of peptide, THz-TDS spectra have been measured, some of which have been supported by density functional theory (DFT) calculations, to distinguish six tetrameric peptides with similar primary and secondary structures
CuII-selective bispidineâdye conjugates
The substitution of tetradentate bispidine ligands with rhodamine and cyanine dye molecules, coupled to an amine donor, forming an amide as potential fifth donor, is described. Bispidines are known to lead to very stable CuII complexes, and the coordination to CuII was expected to efficiently quench the fluorescence of dye molecules. However, at physiological pH the amide is not coordinated, as shown by titration experiments and crystallo- graphic structural data of three possible isomers of these complexes. This may be due to the specific cavity shape of bispidines and the JahnâTeller lability of the CuII center. While CuII coordination in aqueous solution leads to efficient fluorescence quenching, experiments show that the complex stabilities are not large enough for CuII sensing in biological media, and possibilities are discussed, how this may be achieved by optimized bispidineâdye conjugates
Stereodynamic QuinoneâHydroquinone Molecules That Enantiomerize at sp<sup>3</sup>âCarbon via Redox-Interconversion
Since
the discovery of molecular chirality, nonsuperimposable mirror-image
organic molecules have been found to be essential across biological
and chemical processes and increasingly in materials science. Generally,
carbon centers containing four different substituents are configurationally
stable, unless bonds to the stereogenic carbon atom are broken and
re-formed. Herein, we describe sp<sup>3</sup>-stereogenic carbon-bearing
molecules that dynamically isomerize, interconverting between enantiomers
without cleavage of a constituent bond, nor through remote functional
group migration. The stereodynamic molecules were designed to contain
a pair of redox-active substituents, quinone and hydroquinone groups,
which allow the enantiomerization to occur via redox-interconversion.
In the presence of an enantiopure host, these molecules undergo a
deracemization process that allows observation of enantiomerically
enriched compounds. This work reveals a fundamentally distinct enantiomerization
pathway available to chiral compounds, coupling redox-interconversion
to chirality