2,4,6,8-TETRAPHENYLBARBARALANE - AN ORANGE-RED, THERMOCHROMIC HYDROCARBON DEVOID OF A CHROMOPHORE

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.