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 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.