Valence-bond isomer chemistry. Part III. Some addition and nucleophilic substitution reactions of hexafluorobicyclo[2,2,0]hexa-2,5-diene

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Abstract
Hexafluorobicyclo[2,2,0]hexa-2,5-diene reacts with chlorine or with bromine in the dark to give a mixture of the corresponding exo,exo- and trans-5,6-dihalogenohexafluorobicyclo[2,2,0]hex-2-enes; further, photochemical reaction with halogen gives a mixture of all-exo- and exo,exo,exo,endo-2,3,5,6-tetrahalogenohexafluoro[2,2,0]hexanes. Catalytic hydrogenation of the diene at –50° gives exo-5H,6H/-hexafluorobicyclo[2,2,0]hex-2-ene. Hydrogen bromide adds photochemically to the diene to give mainly the bicyclohexene arising from cis,exo-addition; cis,exo-addition of bistrifluoromethyl nitroxide to one or both double bonds of the diene is also favoured. The diene is susceptible to nucleophilic displacement of olefinic fluorine atoms; controlled reaction with the nucleophiles sodium methoxide, dimethylamine, methyl-lithium, and sodium borohydride leads to the formation of the appropriate 2-substituted pentafluorobicyclo[2,2,0]hexa-2,5-diene. With potassium benzenethiolate, only the 2,5-disubstituted derivative is formed. Sodium borohydride gives only the 2,6-dihydro-derivative. The diene reacts slowly with lithium halides in dimethylformamide to give halogenopentafluorobenzenes, and with caesium fluoride in formamide to give mainly exo-5H-heptafluorobicyclo[2,2,0]-hex-2-ene. The n.m.r. spectra of the derived bicyclo[2,2,0]-hexa-2,5-dienes, -hex-2-enes, and -hexanes have been partially or completely analysed. The 19F chemical shifts are particularly useful in deciding the configuration of these compounds. The hexadienes show certain characteristic ranges of F,F coupling constants, viz. low values of the four-bond coupling of olefinic fluorines (0–1·6 Hz), substantial five-bond coupling of olefinic fluorines (9·2–14·7 Hz), and substantial three-bond coupling of bridgehead fluorines (7–14·5 Hz). In the bicyclohexanes, the cross-ring coupling of endo-fluorines is very substantial (ca. 40 and 24 Hz); this possibly arises from a ‘through-space contribution’. The C:C stretching frequencies of the hexadienes and hexenes, which more nearly resemble those of polyfluorocyclohexenes than those of polyfluorocyclobutenes, are interpreted in terms of a possible contribution from deformation of a long C–C bond between the 1- and 4-positions.

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