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.