2,6-Dicyan-4,8-diphenylbarbaralan

2,6-Dicyano-4,8-diphenylbarbaralane[1] Conjugate addition of the phenylcuprate reagent, obtained from phenyllithium, copper(I) cyanide, and boron trifluoride–diethylether, to the bicyclo[3.3.1]nonadienedione 3 affords the diphenylbicyclo[3.3.1]nonanedione 4 in high yield. Catalyzed by the potassium cyanide/18-crown-6 complex, addition of trimethylsilyl cyanide produces a mixture of the diastereomeric bis[O-(trimethylsilyl)cyanohydrins] exo,exo-, exo,endo- and endo,endo-5. The hydrogen fluoride – pyridine complex in phosphorus oxychloride as solvent and, subsequently, an excess of pyridine convert the diastereomers 5 into the unsaturated γ,γ′1-diphenyldinirile 6. This is brominated by N1-bromosuccinimide to yield the γ,γ′1-dibromodinitriles exo- and endo- 7 (6:1). The predominant diastereomer exo-7 is debrominated by the zinc-copper couple to afford the orange-red title compound 2 in 78% yield. More conveniently, the unsaturated dinitrile 6 is converted to 2 in a single step by treatment with hexachloroethane and concentrated aqueous sodium hydroxide in the presence of tetrabutylammonium hydroxide as phase-transfer catalyst. Surprisingly, low yields of 2 are also obtained when the bis[O-(trimethylsilyl)cyanohydrins] 5 or the unsaturated dinitrile 6 are treated with phosphorus oxychloride in boiling pyridine. – The structures of the new compounds are based on spectroscopic evidence and X-ray diffraction analyses of 2, 4, and endo,endo-5. The conformations of 4 and endo,endo-5 in solution are inferred on the basis of vicinal proton coupling constants and a comparison with coupling constants calculated with the aid of the Karplus equation and torsional angles obtained by X-ray diffraction analyses. – While the barbaralane 2 exists as a pair of very rapidly rearranging degenerate valence tautomers in solution, the degeneracy is lifted in the crystal lattice. As a result, the crystal consists of two rapidly rearranging but non-equivalent valence tautomers in a ratio of 9:1 as estimated from the apparent atomic distance C2–C8 of 2 and the C2–C8 bond length of non-rearranging barbaralanes. – The colour of 2 in the crystal and in solution results from a maximum at 436 nm which increases on heating of the solution to 450 K. Cooling to 77 K results in reversible fading and the disappearance of the maximum. Thus, 2 is a barbaralane like 1 which exhibits colour though it is lacking a classical long-wavelength chromophore.