A series of highly congested polyphenylbiphenyls and polyarylfluorenes has been prepared and their X-ray structures determined. Decaphenylbiphenyl adopts a very unusual C 1 -symmetric geometry (rather than the more intuitive D 2 geometry) in which one of the central benzene rings is distorted into a boat conformation. AM1 calculations confirm that the C 1 geometry is the ground state but indicate that less highly substituted biphenyls should adopt D 2 geometries. The structure of 2,2′,4,4′,6,6′-hexaphenylbiphenyl supports the latter prediction; this material has crystallographic C 2 symmetry and (except for the orientation of the para phenyl groups) approximate D 2 symmetry in the solid state. Octaphenylfluorenone has been prepared in four steps from tetraphenylcyclopentadienone. Its X-ray structure shows the fluorenone core to be twisted and sterically shielded by the eight peripheral phenyl groups; nevertheless, phenylmagnesium bromide adds easily to the carbonyl group of its equally hindered dimethyl derivative, 2, 3,5,6,7,8-hexaphenyl-1,4-di(p-tolyl)fluorenone. Reduction of the resulting fluorenol with TiCl 3 gives a nonaarylfluorene, 2,3,5,6,7,8,9-heptaphenyl-1,4-di(p-tolyl)fluorene, and its X-ray structure shows distortions similar to those of octaphenylfluorenone.

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structure structure (organic substances) K 9000 -024Polyphenylbiphenyls and Polyphenylfluorenes.-The structures of the title compounds, which are prepared using high-temperature Diels-Alder reactions as the key steps, are investigated by X-ray analysis. -(TONG, L.; LAU, H.; HO, D. M.; PASCAL, R. A. JUN.; J. Am. Chem. Soc. 120 (1998) 24, 6000-6006; Dep. Chem., Princeton Univ., Princeton, NJ 08544, USA; EN)