Issue 1, 2011

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 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.

Graphical abstract: Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes

Article information

Article type
Paper
Submitted
31 Aug 2010
Accepted
22 Oct 2010
First published
18 Nov 2010

Org. Biomol. Chem., 2011,9, 86-91

Racemisation dynamics of torsion angle restricted biphenyl push-pull cyclophanes

J. Rotzler, H. Gsellinger, M. Neuburger, D. Vonlanthen, D. Häussinger and M. Mayor, Org. Biomol. Chem., 2011, 9, 86 DOI: 10.1039/C0OB00650E

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