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Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes

2010; Royal Society of Chemistry; Volume: 9; Issue: 1 Linguagem: Inglês

10.1039/c0ob00650e

1477-0539

Jürgen Rotzler, Heiko Gsellinger, Markus Neuburger, David Vonlanthen, Daniel Häußinger, Marcel Mayor,

Molecular spectroscopy and chirality

The thermodynamics of the atropisomerisation of torsion angle restricted, axial chiral biphenyl-based push-pull cyclophanes were studied. Using 1H 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.