Bromination of benzobarrelene

In the second part of this work we studied the bromination of benzobarrelene at room temperature and at 150°C. Some dipolar addition reactions to the benzobarrelene have been reported by Paquette and al (ref. 7). Surprisingly, there is no report in the literature on bromination of benzobarrelene. However, Barkash and al. (ref. 8) has reported bromination of tetrafluorobenzobarrelene 2 and isolated from complex reaction mixture only compounds 8 and 9 (Scheme 2). 

Scheme 4. Mechanism of formation of products in the bromination of benzobarrelene.

Next we studied high temperature bromination of benzobarrelene at 150 C. NMR analysis indicated that the reaction mixture was very complex and consisted of at least ten products. After repeated column chromatography combined with fractional crystallization we have been able to separate 18 compounds (Scheme 6). Four of them were bromoalcohol compounds 18, 12, 22 and 2fl. After high temperature bromination we expected three isomeric non-rearranged products with benzobarrelene skeleton and isolated 22, 22, and 24 in yields of 34, 9.3, and 6.2 %, respectively. Because of the very close structural similarity we were not able to make a clear-cut differentiation between the stereochemistry of 22 and 24-Therefore, we carried out an X-ray analysis (ref. 9) of the isomer 22-... 

HIGH TEMPERATURE BROMBVATION IVC f D. BROMINATION OF BENZONORBORNADIENE AND BENZOBARRELENE... 

Finally, we would like to conclude that high temperature bromination of bicyclic systems gives more non-rearranged products. If the molecule is more strained, the tendency to rearrange decreases as in the case of benzonorbornadiene. On the other hand, substituents at double bond of benzobarrelene retards also rearrangement (ref. 1). 

In connection with our continuing interest in the temperature bromination reactions (ref. 2) we have been interested in the bromination reactions of benzonorbornadiene 1 and benzobarrelene 2. 

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