Comparison between Chlorination and Bromination

The scheme depicts that if the addition of chlorine in 3,4-tetrafluorobenzobi-cyclo[3,2,l]octadienes occurs with the participation of the bond (k ), then that of bromine occurs with the participation of the reagent (k,) to form trans-dibromides with the skeleton retained. This can be accounted for because the bromonium ion is more stable than the chloronium one and possesses higher nucleophility in CHCh. 

Let us summarize the results of the halogenation. The electrophilic attack on the double bond in 3,4-tetrafluorobenzobicyclo[3,2,l]octadienes always occurs from the exo side it is favoured by the Ti-participation of the benzene ring and the jt-participation of the C —C bond. As a rule, such a participation effects a non- 

isomeric a-oxides 337 and 338 interact with HF to form quite different reaction mixtures. The direction of the skeletal 1,2-shift in these reactions is determined by the configuration of the decomposing a-oxide the cr-bond to be shifted is the one that can participate in the transition state of the a-oxide cycle rupture. 

Assuming the a-bond, e.g, that of C —C in the a-oxide 338, being shifted simultaneously with the splitting of the a-oxide cycle, the reaction should result in a classical benzylic cation which would further react with the fluoro-anion non-stereo-specifically  

Indeed, the fluoroalcohol 339 is treated with an acid under more rigid conditions to form a mixture of isomeric alcohols differing in the fluorine configuration at C.  

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