Helical molecules are not only esthetically appealing due to their structural beauty, they also display unique physical properties as a result of their chirality. We describe herein a new approach to "Geländer" oligomers by interlinking two oligomer strands of different length. To compensate for the dimensional mismatch, the longer oligo(benzyl ether) oligomer wraps around the oligophenyl backbone. The new "Geländer" oligomer 1 was assembled in a sequence of functional-group transformations and cross-coupling steps followed by final cyclizations based on nucleophilic substitution reactions, and was fully characterized, including X-ray diffraction analysis. The isolation of pure enantiomers enabled the racemization process to be studied by circular dichroism spectroscopy.
The influence of electron donors and electron acceptors of variable strength in the 4 and 4' position of 2 and 2' propyl-bridged axial chiral biphenyl cyclophanes on their atropisomerization process was studied. Estimated free energies ΔG(‡)(T) of the rotation around the central biphenyl bond which were obtained from (1)H-NMR coalescence measurements were correlated to the Hammett parameters σ(p) as a measure for electron donor and acceptor strength. It is demonstrated that the resulting nice linear correlation is mainly based on the influence of the different substituents on the π-system of the biphenyl cyclophanes. By lineshape analysis the rate constants were calculated and by the use of the Eyring equation the enthalpic and entropic contributions were evaluated. Density functional theory calculations show a planar transition state of the isomerization process and the calculated energy barriers based on this reaction mechanism are in good agreement with the experimentally obtained free energies.
The thermodynamics of the atropisomerisation of torsion angle restricted, axial chiral biphenyl-based push-pull cyclophanes were studied. Using (1)H NMR coalescence measurements the rotation barrier around the central C-C bond was determined to be 50 kJ mol(-1) for the propyl-bridged biphenyl derivative 1b, displaying only a negligible solvent dependence. By protonation of the piperidinyl nitrogen as electron donor, the free energy ΔG(‡)(T) of the rotation barrier increased, indicating that the tendency of the push-pull system to planarise may be considered as a driving force for the atropisomerisation. For the more restricted butyl-bridged cyclophane 1c a rotation barrier of ΔG(‡)(T) = 90 kJ mol(-1) was measured using dynamic chromatography. The difference in the free energy of rotation around the central C-C bond probably reflects the crowdedness of the transition states.
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