Stereo- and regioselective halogenation of norbornenes directed by neighboring group participation.

Directing the outcome of electrophilic addition reactions of norbornenes could be a challenging task. We have found that the ionic addition of bromine to dichloro-norbornene 2 is accompanied with the formation of a bromonium cation followed by anchimeric assistance of juxtaposed chlorine to give five-membered chloronium cation, which upon reacting with bromide gives norbornane 4 in >90% yield. At higher temperatures, however, the radical addition of Br 2 dominates so that dibromo compound 3 appears as the principal reaction product (72%). Stereo- and regioselective rearrangements of norbornenes, reported herein, could be of interest for the syntheses of complex haloalkanes. [ABSTRACT FROM AUTHOR]