Bromination of Norbornene: the Question of Bromonium Ions vs. 6,1-Hydride Shift

Abstract

The reaction of norbornene (1) with bromine has been re-investigated and found to give in addition to bromonortricyclene (2) and 2-exo-bromonorbornane (20) a complex mixture of dibromides of which five (15–19) have been isolated and identified. These dibromides have been found to be kinetically controlled products of ionic addition. Apparently dibromide 15 is also formed to some extent in carbon tetrachloride by a radical mechanism. The product composition from the bromine addition has been compared with that of the analogous chlorination and bromofluorination reactions. Degradation of the 2-exo,3-endo-dibromonorbornane from bromination of 5,6-14C-norbornene has shown that this dibromide is formed by two ionic routes: (a) from a bromonium ion 14 (or its equivalent), and (b) from the cation 12 formed by 6,1-hydride shift. Improvements have been made in the first and final steps of the standard norbornene degradation method.