Inversionsbarrieren <i>Ortho,ortho</i>′‐verbrückter Biphenyle

Müllen, Kläus; Heinz, W.; Klärner, Frank‐Gerrit; Roth, Wolfgang R.; Kindermann, Islinde; Adamczak, Oliver; Wette, Michael; Lex, Johann

doi:10.1002/cber.19901231219

Cited by 65 publications

(39 citation statements)

References 55 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…[170][171] For bridged biphenyls the required energy is even higher. [172] Figure 9 Rotational barriers of sterically different ortho substituted biphenyls. All values were obtained by dynamic NMR investigations.…”

Section: Biphenyls In Molecular Electronicsmentioning

confidence: 99%

Shape‐Switchable Azo‐Macrocycles

et al. 2009

View full text Add to dashboard Cite

The synthesis of four shape‐switchable macrocycles comprising different peripheral substituents is described. The macrocycles 1–4 consist of m‐terphenyl semicircles interlinked by two azo joints. These macrocycles were assembled from nitro‐functionalized m‐terphenyl moieties through reductive dimerization. The semicircles were assembled through Suzuki cross‐coupling reactions. The molecular weights of the macrocycles were determined by vapour pressure osmometry, because mass spectrometry failed in the cases of 2 and 3. The E → Z photoisomerization reactions were analysed by UV/Vis spectroscopy complemented by 1H NMR studies. A very slow thermal back‐reaction indicated considerable stabilization of the Z isomer. The reduced efficiency of the thermal back‐reaction probably arises from the reduced degree of freedom due to the mechanical interlinking of the two azo groups. The photostationary state consisted of all‐Z (85 %) and all‐E isomers (15 %). The E → Z transformation induced by irradiation displayed simple exponential kinetics, which indicates pairwise switching of the two azo groups in a macrocycle, at least on the timescale under investigation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

show abstract

Section: Biphenyls In Molecular Electronicsmentioning

confidence: 99%

Shape‐Switchable Azo‐Macrocycles

et al. 2009

View full text Add to dashboard Cite

show abstract

“…Separation or enrichment of single enantiomers enables the determination of inversion barriers by measuring enantiomerization kinetics using chiroptical methods. Alternative techniques like dynamic NMR spectroscopy (DNMR), 13 dynamic capillary gas chromatography (DGC), 14 or high-performance liquid chromatography (DHPLC) 15 are also well established. They need only minute amounts of racemic samples.…”

mentioning

confidence: 99%

Synthesis, resolution, and investigation of the rotational interconversion process of atropisomeric 1,n‐diaza[n]paracyclophanes using cyclodextrin‐mediated capillary zone electrophoresis

et al. 2001

View full text Add to dashboard Cite

A number of variously monosubstituted 1,n-diaza[n]paracyclophanes (n = 10-12), which show planar chirality and atropisomerism due to hindered rotation about single bonds, were synthesized via a classical route to analyze their stereodynamic properties. Racemic analytes with 10- and 11-membered bridges were resolved by capillary zone electrophoresis (CZE) in acidic phosphate buffers (pH 2.5-4.5) employing permethylated beta- and gamma-cyclodextrin as chiral additives. Moreover, cyclodextrin mediated CZE was used in a discontinuously driven mode for investigations of the rotational interconversion process of conformationally labile homologues (n = 11). In stopped-flow experiments, after baseline separation enantiomers were partially enantiomerized in situ inside the capillary by heating. The rate constants (k(enan) = 1/2 k(rac)) and rotational energy barriers (Delta G(++)) were determined from the resulting enantiomeric ratios. Energy barriers between 113-126 kJ/mol were found depending on the substituent of the benzene ring and the degree of ionization of the amino groups in bridgehead positions. The energy barriers increased in order of the substituents: NO(2) >> CF(3) > Br > Cl > CH(3) approximately F. In addition, the rotational energy barriers were decreased by approximately 6-8 kJ/mol in the presence of the chiral selector.

show abstract

“…The fundamental characteristics are already present in bridgedb iphenyls:a na lkyl linker bridges the phenyl rings, which locks them in as pecific conformation (M or P). [16][17][18][19][20][21][22] Adding at hird ring (also conformationally locked by an alkyl bridge)r esultsi nV çgtle's terphenylic Geländer-oligomers. The obtained structures show very similar chiroptical properties and conformational stability to those of shorter helicenes.…”

Section: Introductionmentioning

confidence: 99%

Tuning Helical Chirality in Polycyclic Ladder Systems

Rickhaus

Unke

Mannancherry

et al. 2015

Chemistry A European J

View full text Add to dashboard Cite

Conceptually and experimentally, a new set of helical model compounds is presented herein that allow correlations between structural features and their expression in the secondary structure to be investigated. A cross-linked oligomer with two strands of mismatching lengths connected in a ladder-type fashion serves as a model system. Compensation for the dimensional mismatch leads to the adoption of a helical arrangement. A strategically placed relay ensures the continuity and uniformity of the helix. Upon exchanging the heteroatomic linkage, the helix responds by increasing or decreasing the torsion of the backbone. Inversion of the relay's substitution pattern causes a distortion of the structure, while maintaining the directionality of the helix. Based on a short synthetic protocol with a modular precursor, four closely related "Geländer" oligomers (Geländer is the German word for bannister) were accessed and fully characterized. XRD analysis for one representative of each helical arrangement and complementary computational studies for the remaining derivatives allowed the impact of the alterations on the secondary structures to be studied. Isolation of pure enantiomers of all new Geländer oligomers provided insight into the racemization kinetics and estimation of the racemization barrier. In silico simulation of the electronic circular dichroism spectra of the model compounds enabled the helicity of the isolated samples to be assigned.

show abstract

Inversionsbarrieren Ortho,ortho′‐verbrückter Biphenyle

Cited by 65 publications

References 55 publications

Shape‐Switchable Azo‐Macrocycles

Shape‐Switchable Azo‐Macrocycles

Synthesis, resolution, and investigation of the rotational interconversion process of atropisomeric 1,n‐diaza[n]paracyclophanes using cyclodextrin‐mediated capillary zone electrophoresis

Tuning Helical Chirality in Polycyclic Ladder Systems

Contact Info

Product

Resources

About