Synthese, Struktur und thermische Umlagerungen des 2,6‐Dicyan‐1,5‐dimethylsemibullvalens

Abstract

Zinkiodid-katalysierte Addition von Trimethylsilylcyanid an dasThe zinc iodide-catalyzed addition of trimethylsilyl cyanide to the bicyclo[3.3 .O]octane-2,6-dione 9 yields the sterically defined 0-silylated cyanohydrine 10. Trichlorophosphane oxide in pyridine converts the latter to the unsaturated dinitrile 11. Bromination of 11 using N-bromosuccinimide affords the exo,exo-dibromodinitrile 13. The latter is debrominated by zinc/copper to the dicyanosemibullvalene 4a while the dehydrobromination with potassium tert-butoxide in tert-butyl alcohol yields the bromodicyanosemibullvalene 14. All reactions proceed smoothly giving high yields. Using 14, which exists as a single valence tautomer, as a model of a non-rearranging valence tautomer of 4 a the barrier for the degenerate Cope rearrangement of 4a is estimated from the broadening of the 13C NMR signals to be A G & = 13 kJ mol-'. At 130°C in [D,]benzene, 4a isomerizes to the cyclooctatetraene 16 with a first order rate constant of k = 3.0 s-'.The semibullvalene 4a crystallizes monoclinically in the space group P 2 , / n (No. 14) with four molecules in the unit cell. Selected geometric parameters of 4a are compared to those of cyanosubstituted cyclopropane derivatives and other semibullvalenes.

The Search for Homoaromatic Semibullvalenes. 5.11,5-Dimethyl-2,4,6,8-semibullvalenetetracarboxylic Dianhydride

“…When the pure solid or a solution of either semibullvalene is cooled, the color decreases in intensity and by about −78 °C the color is greatly diminished. Quast et al observed similar results in their most fluxional semibullvalenes and barbaralanes and have correlated this reversible thermochroism with a very flat double minimum potential energy surface and an exceptionally low activation barrier to the Cope rearrangement 27b…”

“…An alternative for studying the solution-phase properties of semibullvalenes is IR spectrometry. The time scale of the IR experiment renders it more amenable than NMR to the investigation of extremely rapid processes. ,27b,c, The observation of two nitrile bands in the IR spectra of 22 ,27c 26 ,37a 27 ,27b and 28 37b supported the conclusion of Quast et al that these molecules were highly fluxional, but not homoaromatic. It would appear that the carbonyl region of the IR spectrum of 10 could be used in a similar diagnostic manner.…”

“…It could be purified by sublimation (200 °C at ambient pressure) and showed no sensitivity to ambient oxygen. This contrasts with the behavior of other semibullvalenes, which frequently undergo thermolysis to the corresponding cyclooctatetraene under relatively mild conditions 26,27 and often tend to react with oxygen. ,27a,b On prolonged deliberate exposure of 10 to the atmosphere it was recovered unchanged along with the product of partial hydrolysis ( 21 , ∼10% after 24 h).

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A High-Precision Carbon-13 Shift Thermometer for the Temperature Range 100–300 K

“…Although several nondegenerate semibullvalenes meet these requirements for large temperature dependence of carbon-13 shifts (13)(14)(15), most of them still possess barriers to exchange which are too high, resulting in line broadening and the emerging of the spectra of the nonrearranging valence tautomers at temperatures well above 100 K. The known accelerating effect of suitably placed cyano groups on the degenerate Cope rearrangement of semibullvalene (20) has led us to conceive the pair of valence tautomers 1 ‫-(‬ 1a and 1b), which is also useful in another context (21). It is a stable, crystalline material (m.p.…”

Lifting of the Degeneracy in Semibullvalenes by Remote and Direct Substituents: A Quantitative Study Using Variable-Temperature Carbon-13 NMR Spectroscopy

“…Many substituted semibullvalenes undergo degenerate Cope rearrangements via transition structures, which may exhibit C 2 , C S , or C 2v symmetry. , There are also semibullvalenes that are completely locked in one of the two possible valence tautomeric structures, e.g., benzosemibullvalenes. , Between these limiting cases, a considerable number of semibullvalenes exist whose Cope equilibria are more or less skewed by virtue of appropriately placed substituents (including monoannelation), ,− deuterium atoms, or heteroatoms 14 . These nondegenerate semibullvalenes merit renewed interest for several reasons.…”