Organoboron compounds—XVIII Alkylbromoboranes and relative disproportionation tendencies of alkylhaloboranes

Abstract

The disproportionation tendencies of alkylbromoboranes are significantly different from those of the corresponding alkylchloro and alkylfluoro compounds. Alkyldibromoboranes undergo slow thermal disproportionation to boron bromide and trialkylboranes on fractional distillation while alkyldichloro- and difluoroboranes are completely resistant to disproportionation under the same conditions. Dialkylbromoboranes are completely resistant to thermal disproportionation of dialkylfluoroboranes. That these differences in disproportionation tendencies are the result of thermodynamic rather than kinetic factors has been shown by equilibration experiments. Primary aklyldibromoboranes, dialkylbromoboranes or mixtures thereof are conveniently prepared in high yields by the reaction of appropriate stoichiometric amounts of boron bromide with primary trialkylboranes at 140° or above. Tri isopropylborane, even in the presence of diborane, does not react with boron bromide until extensive isomerization of isopropyl to n-propyl groups has occurred. Tri- s-butylborane reacts with only partial isomerization to give appreciable yields of pure di- s-butylbormoborane.