Hexakis benzvalene

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Partially fluorinated cyclobutene gives at 0 C an ozonide that is stable in dichloromethane solution at room temperature.86 It is decomposed by acid hydrogen peroxide yielding fluorinated butanedioic acid.86 Bubbling ozone into a pentane solution of hexakis(trifluoro-methyl)benzvalene gives a pale-yellow ozonide which is stable at room temperature (Table 5).87... 

Bubbling ozone into a pentane solution ot hexakis(trifluoromethyl)benzvalene gives a pale yellow ozomde, which is stable at room temperatures [27] (equation 18)... 

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). 

Ultraviolet irradiation causes changes in carbon skeleton of many compounds. A classical example is conversion of hexafluorobenzene to hexafluoro Dewar benzene [759], and isomerization of benzene to three isomers. The same is true of hexakis(trifluoromethyl)benzene, which rearranges to the perfluorinated isomers of hexamethyl Dewar benzene (F), prismane (G), and benzvalene (H) [140]. 

Among perfluorinated aromatic compounds, only hexakis(trifluoromethyl)benzene was converted to benzvalene. Using an ultraviolet lamp of longer wavelength (307 nm), benzvalene was selectively obtained in high yield (12)14,1S). This benzvalene is very stable and relatively easy to handle. Its reactions will be discussed in Section 2.5. 

Hexakis(trifluoromethyl)benzvalene is the only perfluorinated benzvalene which has been known so far. It is very stable at room temperature. The thermal and photochemical oc-isomerization of this compound has been reported. 

Next, some reactions of hexakis(trifluoromethyl)benzvalene will be discussed. This compound is very stable as mentioned before and its olefinic double bond reacts as an efficient dienophile. The difference in the reactivity between unsubstituted benzvalene and the hexakistrifluoromethylated derivative is that the former is nucleophilic while the latter is highly electrophilic. Thus, the former does not react with usual 1,3-dienes, in contrast to the latter. Here, mainly describe our results of the several reactions of hexakis(trifluoromethyl)benzvalene which were designed for the synthesis of tetrakis(trifluoromethyl)tetrahedrane. 

Hexakis(trifluoromethyl)benzvalene reacts as an efficient dipolarophile with phenyl azide, diazomethane and ozone to give the corresponding cycloadducts (96). 

The attempted elimination of five-membered heterocycles from the adducts formal from phenyl azide or diazomethane and hexakis(trifluoromethyl)benzvalene was unsuccessful. The photolysis of the ozonide of the benzvalene at room temperature gives the dimer of tetrakis(trifluoromethyl)cyclobutadiene. When the irradiation is carried out in a matrix at —196 °C, the reaction mixture was coloured yellow. The... 

The Diels-Alder reaction of hexakis(trifluoromethyl)benzvalene with furan had been reported 63) but not its stereochemistry. We have examined this reaction and found that it proceeds smoothly with many 1.3-dienes if the steric requirement of the olefin is not large in the transition state where exo-attack occurs as confirmed by X-ray analysis (97)96). Therefore, 2,3-dimethylbutadiene reacts smoothly with hexakis(trifluoromethyl)benzvalene whereas 1,3-pentadiene reacts very slowly and 2,4-hexadiene does not at all. The cycloaddition to cyclobutadiene gives a mixture of endo- and exo-adducts (98)97) ... 

The former assignment of the structures of the cyclobutadiene adducts of hexakis-(trifluoromethyl)benzvalene by Warrener was corrected by the X ray analysis of the bromination product of the exo-adduct98). The reaction of the adduct, formed from hexakis(trifluoromethyl)benzvalene and furan, with bromine yields a ew-adduct, probably because of the high steric requirement of the trifluoromethyl groups on the bridge-head. 

The ozonide of hexakis(trifluoromethyl)benzvalene is reduced with triphenyl-phosphine to a dicarbonyl compound. The attempted conversion of this diketone to the tetrahedrane either directly or via halogeno compounds was unsuccessful, but further treatment of the diketone with triphenylphosphine yields new valence-bond isomers of oxepin (100) 100). The reactions of these isomers will be described in Section 3.4. 

The photoreaction of hexakis(trifluoromethyl)benzvalene with acetylenes gives (2+2)cycloadducts. This reaction does not proceed with electrondeficient acetylenes, such as dialkyl acetylenedicar boxy late. This fact shows that interactions between the electron-rich acetylene and the electron-deficient double-bond of the benzvalene play an important role (101)101>. 

Sensitized photoaddition of hexakis(trifluoromethyl)benzvalene to a series of dialkylacetylenes gave the corresponding [2 + 2] adducts. A yield of 60 % was obtained... 

Diels-Alder addition of maleic anhydride to androsta-14,16-diene is shown from the crystal structure to have occurred by P-attack. Likewise, diffraction data have been employed to support the view that the Diels-Alder addition of substituted 1,3-dienes to hexakis(trifluoromethyl)benzvalene occurs by exo-attack. Structure (23) has been assigned to a naphthalene-diphenylacetylene photo-adduct. ... 

Sensitized [2 + 2] photocycloadditions of alkynes to the olefmic bond in hexakis-(trifluoromethyl)benzvalene have been achieved, and are regarded as a convenient route to the Cg unit. The benzene-sensitized irradiation of cis, trans-cyclodeca-1,5-diene yields a mixture of products containing cis- and trans-l,2-divinylcyclo-hexane, cis,cis-cyclodeca-1,5-diene, and the product of crossed [2 -I- 2] addition... 

Addition of ozone to hexakis(tTifluoromethyI)benzvalene gives the ozonide (183) as a stable yellow liquid. Photolysis of (183) at low temperature in a hydrocarbon matrix led to extrusion of trifluoroacetic anhydride and formation of the syn-dimer (184) of tetrakis(trifluoromethyl)cyclobutadiene (185). Irradiation of (183) in the presence of ethyl azodicarboxylate gives the 1 1 adduct of (185) and ethyl azodi-carboxylate. 

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