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

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.