X-RAY CRYSTALLOGRAPHIC DETERMINATION OF ABSOLUTE-CONFIGURATIONS AND CONFORMATIONS OF 2 CONFORMATIONAL ISOMERS OF 1,2,3,4-DIBENZO-TRANS-6,7-DIBROMO-1,3-CYCLOOCTADIENE, A 2,2'-BRIDGED BIPHENYL

The bridged biphenyl 1,2,3,4-dibenzo-trans-6,7-dibromo-1,3-cyclooctadiene (1b) exists in four stereoisomeric forms as two interconverting enantiomeric pairs, one with axial (a,a) and one with equatorial (e,e) bromine atoms. The stereoisomers can be separated by chromatography on triacetylcellulose. Crystals of the e,e and a,a forms of the pure enantiomers can be separated manually. The structures and absolute configurations of one of the a,a forms and one of the e,e forms were determined by X-ray crystallography, using the anomalous scattering technique. Both stereoisomers were shown to have the eight-membered ring in the twist-boat-chair conformation, confirming predictions by empirical force-field calculation. Two forms were observed by 1H and 13C NMR in solution and were shown by analysis of the coupling constants to be the same as in the crystals. The free energy barrier to e,e-a,a exchange was found by NMR band-shape analysis to be 23.9 ± 0.1 kcal/mol at 460-470 K, and by following the change in the CD spectrum to be 22.85 ± 0.05 kcal/mol at 298-303 K. © 1990, American Chemical Society. All rights reserved.