Reactions of dialkylbromoboranes with alkali metal hydrides in the presence of alkenes. Synthesis of ‘mixed’ trialkylboranes (RA2RBB) in solution

Abstract

Dialkylbromoboranes react steadily with hydroxide-free sodium hydride in diglyme, in the presence of alkenes, to give nearly quantitative yields of partly mixed trialkylboranes. The reaction proceeds too slowly to allow prior clean preparation of dialkylboranes, so reaction in the presence of alkenes possessing hydride-sensitive functional groups is not possible, although terminal alkynes can be hydroborated in situ by a modified experimental procedure. Potassium hydride reacts much more rapidly with dialkylbromoboranes, the reactions being complete within minutes at 20 °C, but the yields are lower. Addition of dibenzo-18-crown-6 leads to an increase in yield but the results are still inferior to those obtained using sodium hydride. Reaction of dialkylbromoboranes with lithium hydride is very sluggish.