Polyphenylbiphenyls and Polyphenylfluorenes

A series of highly congested polyphenylbiphenyls and polyarylfluorenes has been prepared and their X-ray structures determined. Decaphenylbiphenyl adopts a very unusual C1-symmetric geometry (rather than the more intuitive D2 geometry) in which one of the central benzene rings is distorted into a boat conformation. AM1 calculations confirm that the C1 geometry is the ground state but indicate that less highly substituted biphenyls should adopt D2 geometries. The structure of 2,2‘,4,4‘,6,6‘-hexaphenylbiphenyl supports the latter prediction; this material has crystallographic C2 symmetry and (except for the orientation of the para phenyl groups) approximate D2 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 TiCl3 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.