Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes

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.