Story of Hydroboration: Experimentally Generated Mechanistic Insights

Reduction of various unsaturated organic substrates via hydroboration represents one of the most commonly used procedures. Nevertheless, controversial information about the mechanistic insights regarding this particular transformation still exists in the literature. In this work, we show that three-coordinate mono-aminoboranes do not act as effective hydroboration reagents as recently reported unless these boron-containing species are able to form four-coordinate analogues either through dimerization or addition of a base. Furthermore, we also demonstrate that imine-BH3 adduct(s) and mono-amino boranes do not represent key intermediates during Me2S-BH3 catalyzed hydroboration of imines using HBpin as recently reported. In fact, we believe that in both of these reported studies, the Clark mechanism is mainly, if not solely, responsible for the observed/examined hydroboration reactions.