H. Quast et al., 2,4,6,8-TETRAPHENYLBARBARALANE - AN ORANGE-RED, THERMOCHROMIC HYDROCARBON DEVOID OF A CHROMOPHORE, Chemische Berichte, 126(4), 1993, pp. 1047-1060
The diphenylbicyclo[3.3.1]nonanedione 6 adds phenylcerium(III) dichlor
ide to afford a high yield of the triphenyl-2-oxa-twistanol 8. In cont
rast, phenyllithium yields a mixture of triphenylhydroxyketone exo-7,
tetraphenylbicyclo[3.1.1]nonanediol 9, and 8. The latter is dehydrated
by sulfuric acid in acetic acid to produce the triphenylbicyclo[3.3.1
]nonenone 10 in almost quantitative yield. Addition of phenylcerium(II
I) dichloride to 10 affords a 3:2 mixture of the tetraphenylbicyclo[3.
3.1]nonenols exo- and endo-11 which may be separated by chromatography
. Dehydration of the mixture yields quantitatively the tetraphenyldien
e 12. On treatment with an excess of butylpotassium in pentane, 12 is
converted to the deep violet, crystalline dipotassium salt 17 which ma
y be purified by reprecipitation from its tetrahydrofuran solution wit
h pentane. When the solution of 17 in tetrahydrofuran is slowly added
to an excess of 1,2-dibromoethane at - 60-degrees-C, the dianion is im
mediately oxidized to produce the tetraphenylbarbaralane 4 which is is
olated in 30% yield as orange-red crystals after chromatography. - The
structures of the new compounds are based on spectroscopic evidence a
nd X-ray diffraction analyses of 4, 8, exo-11, and 12. The conformatio
ns in solution are inferred on the basis of vicinal proton coupling co
nstants and a comparison with coupling constants calculated with the a
id of the Karplus equation from torsional angles obtained by X-ray dif
fraction analyses. The conformation of exo-11 in solution closely rese
mbles that present in the crystal. - While the barbaralane 4 exists as
a pair of very rapidly rearranging degenerate valence tautomers in so
lution, the degeneracy is lifted in the crystal lattice. As a result,
the crystal consists of two rapidly rearranging but non-equivalent val
ence tautomers in a ratio of 9:1 as estimated from the apparent atomic
distance C2 - C8 of 4 and the C2 - C8 bond length of the model barbar
alane 18. - The orange-red colour of 4 in the crystal and in solution
results from a pronounced shoulder in the UV/Vis spectrum at 430 nm, t
he intensity of which strongly depends on the temperature. Thus, 4 is
the first barbaralane which exhibits colour though it is lacking a cla
ssical long-wavelength chromophore.