2,6‐Dicyano‐1,5‐dimethyl‐4,8‐diphenylsemibullvalene. — Synthesis, Structure and the Reactions with Triplet Oxygen

Abstract

AbstractAddition of trimethylsilyl cyanide to the diphenylbicyclo[3.3.0]octanedione 8c is catalyzed by the potassium cyanide/18‐crown‐6 complex and produces a mixture of the diastereomeric bis[O‐(trimethylsilyl)cyanohydrins] endo‐ and exo‐10c (3:2). The hydrogen fluoride—pyridine complex in phosphorus oxychloride as solvent and, subsequently, an excess of pyridine convert the mixture of diastereomers 10c into the unsaturated y,y′‐diphenyldinitrile 11. This is converted into the red semibullvalene 4 in a single step by treatment with hexachloroethane and concentrated aqueous sodium hydroxide in the presence of tetrabutylammonium hydroxide as phase‐transfer catalyst — Above 30°C, 4 isomerizes in solution to a mixture of the cyclooctatetraenes 12 and 13. While the red crystals of 4 are stable in the atmosphere, in the dark affording the endoperoxide 16 and the yellow 2H‐pyran derivative 17 (7:1) which result from parallel reactions. Only at temperatures as high s 110°C, 16 slowly rearranges to 17 which reacts further to yield well‐defined but still unknown products. The simultaneous formation of 16 and 17 at low temperatures is interpreted in terms of endo and exo attack, respectively, of triplet oxygen at the benzylic carbon atoms of the semibullvalene 4 or the isomeric bicyclic diradical 21. — The structures of the new compounds are based on spectroscopic evidence and X‐ray diffraction analyses of 4, endo‐10c, 11, 16, and 17. The semibullvalene 4 exists as a pair of rapidly rearranging degenerate valence tautomers in solution and in the crystal as well. In the solid state, 4 exhibits apparent C2 symmetry and equal atomic distances C2–C8 and C4–C6 (201.9 pm). Because true degeneracy is highly unlikely in the crystal, the equal distribution of two non‐equivalent valence tautomers at room termperature results from a fortuitous cancelling of the ΔH° and TΔS° terms governing the equilibrium. — The red colour of 4 in the crystal and in solution is due to a maximum at 444 nm which disappears on cooling. Thus, 4 not only belongs to the family of thermochromic semibullvalenes and barbaralanes devoid of a long wavelength chromophor like 1, 6 and 14, but also shows the most intensive maximum at the longest wavelength observed so far.