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

• DOI: 10.1039/p19720002170
• Publication Date: 2004-04-23T15:51:10Z
• Authors: M. G. Barlow; R. N. Haszeldine; W. D. Morton; D. R. Woodward
• Keywords: Diene; Sodium borohydride; Sodium methoxide

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