Switchable 1,2- and 1,3-Halogen Transposition of Dibromoimidazoles by Activation of Grignard Reagent

A switchable 1,2- and 1,3-halogen transposition reaction of 2,5-dibromoimidazoles is disclosed. Organomagnesium species generated by deprotonation are stable at –20 °C. Heating the organomagnesium at 60 °C to 80 °C leads to 1,3-halogen transposition. In contrast, addition of HMPA promotes 1,2-halogen transposition. These three constitutionally isomeric organomagnesiums from the same substrate undergo electrophilic trapping to provide the corresponding adducts. Several control experiments provide a plausible reaction pathway and a rationale for the contrasting selectivity switch.