Hexafluorobicyclo hexa-2,5-diene

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

Hexafluorobicyclo[2.2.0]hexa-2,5-diene (2) reverts quantitatively to hexafluorobenzene (1) when heated at 80°C for 4 hours in the vapor phase. 9 2 Dewar benzene 2 is apt to explode violently as a neat liquid,19 but is stable as a solid at — 20 C or in carbon tetrachloride solution at room temperature.18... 

Replacement of fluorine by alkoxy groups also takes place in fluorinated bicyclic alkenes. Hexafluorobicyclo[2.2.0]hexa-2,5-diene (32) when treated with sodium ethoxide in ethanol gives two diethoxy-substituted products.79... 

Pyrrole has also been reported to react with the active dienophile, hexafluorobicyclo[2.2.0]hexa-2,5-diene, to give stable 1 1 and 1 2 adducts (B-77MI30500) and with hexakis(trifluoromethyl)benzvalene to give the 1 1 adduct in low yield (76TL2545). 

Liquid hexafluorobenzene, upon radiolysis with Co y-radiation - produces hexafluorobicyclo[2.2.0]hexa-2,5-diene. In the gas phase, perfluorocyclo-hexadiene is produced as well. 

Hexafluorobicyclo[2,2,0]hexa-2,5-diene has been carefully examined by gas-phase electron diffraction and its molecular structure, which has Cat> symmetry, has the parameters shown in Scheme 29 (bond lengths in pm). TheC-1—C-4 bond is not as long as that in the hexamethyl compound, being similar to the C-3—C-4 bond in perfluorocyclobutene, the angle between the planes of the two cyclobutene portions is 121.8 2.0°, and the olefinic fluorines lie some 7.5 2.0° below these planes. 

Hexafluorobicyclo [2,2,0 ]hexa-2,5-diene (90), the product of photoisomerization of hexafluorobenzene, is a versatile and reactive compound (Vol. 1, p. 263). It is now reported that, at 254 nm, benzvalene and prismane isomers are also produced. Further reactions of the hexadiene (90) have now been described controlled hydrogenation yields a 2,3-dihydro-derivative (91), and, in free-radical additions (see Scheme 26), exo-addition... 

Many polyfluorobenzenes undergo photochemical valence-bond isomerization to give, in most cases, one or more para-bonded isomers. Hexafluoro-benzene gives up to 60% of hexafluorobicyclo[2,2,0]hexa-2,5-diene (111)... 

---