Atropisomerization of di-para-substituted propyl-bridged biphenyl cyclophanes.
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.
Department of Chemistry, University of Basel, St. Johannsring 19, 4056 Basel, Switzerland.
Jürgen
J
Rotzler
Heiko
H
Gsellinger
Angela
A
Bihlmeier
Markus
M
Gantenbein
David
D
Vonlanthen
Daniel
D
Häussinger
Wim
W
Klopper
Marcel
M
Mayor
D001952:Bridged-Ring Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D009281:Naphthalenes / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism; D013816:Thermodynamics
D001952:Bridged-Ring Compounds; D009281:Naphthalenes; C412330:1,1'-binaphthyl