Conformational analysis of some four-carbon 2,2′-bridged biphenyls

Abstract

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. ³J_HH-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.