title: Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Basel, St. Johannsring 19, 4056 Basel, Switzerland.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Rotzler
authors.forename: Heiko
authors.initials: H
authors.lastname: Gsellinger
authors.forename: Markus
authors.initials: M
authors.lastname: Neuburger
authors.forename: David
authors.initials: D
authors.lastname: Vonlanthen
authors.forename: Daniel
authors.initials: D
authors.lastname: Häussinger
authors.forename: Marcel
authors.initials: M
authors.lastname: Mayor