Conformational analysis of some four-carbon 2,2′-bridged biphenyls
1,2,3,4-Dibenzocyclo-octa-1,3-diene (systematic name: 5,6,7,8-tetrahydrodibenzo[a,c]cyclo-octene) and a number of its 6-monosubstituted and cis- and trans-6,7-disubstituted derivatives have been resolved into enantiomers and diastereoisomers by chiral chromatography. 3JHH-values and empirical force-field calculations indicate that the preferred conformation of the cyclo-octadiene ring is a twist boat-chair (TBC) form with twist boat (TB) forms 4–12 kJ mol–1 higher in energy. In the TBC form, the C-6 (-7) substituents can be oriented in equatorial (e) and axial (a) directions. The monosubstituted compounds appear as e(major) and a forms, the trans-disubstituted as e,e(major) and a,a forms, and the cis-disubstituted as enantiomeric e,a and a,e forms. The e–a, e,e–a,a and e,a–a,e interchanges involve inversion of the biphenyl moiety with barriers in the range 95–102 kJ mol–1, determined by 1H NMR band shape analysis and by following the thermal racemization or epimerization by CD spectroscopy. The free-energy differences between e and a or e,e and a,a forms are reproduced by standard MMP2-85 calculations, except for a few compounds, among them the trans-disubstituted compounds with halogen substituents. For these, replacement of bond dipoles by atomic charges in the calculations led to more realistic results. The e,e–a,a equilibrium of the trans-dibromo compound is remarkably insensitive to solvent polarity.