The halogenolysis of organoboranes

Transmercuration of tri-exo-2-norbornylborane followed by chlorinolysis of the resultant organoborane is a low yield (≤5%) method of producing exo-2-norbornyl chloride. The primary source of this low yield is the inefficient transmercuration of the sterically restricted organoborane. Yields of alkyl halide are substantially higher (up to 63%) when the organoborane undergoes direct chlorinolysis promoted by boron trichloride. Even higher yields (approaching 80%) are obtained when brominolysis of the organoborane is carried out. Controls suggest that at least part of the reason for the higher yield with bromine is due to the greater stability of the product under the reaction conditions. Despite the general retention aspects (exo-borane gives largely exo-halide) of these halogenolyses, deuterium labeling studies show that these reactions are accompanied by substantial Wagner-Meerwein rearrangement, very possibly due to boron trihalide induced rearrangement of the norbornyl halide product.