Hydroboration of imines: intermolecular vs. intramolecular hydride transfer

A crucial step in the formation of mono-aminoboranes (R2N-BH2) from the corresponding imine-BH3 adducts, under mild reaction conditions, is the 1,3-hydride shift. We offer experimental and theoretical insights into this molecular rearrangement according to two mechanistic pathways: intra-molecular (unimolecular) and inter-molecular (bimolecular). Even though a few experimental observations were inconclusive, an isotope-based experiment suggested that the 1,3-hydride shift occurs bimolecularly. In addition, the shape of the potential energy landscape, generated via the computational means, strongly suggests that these reactions are bimolecular. Relative energy gradient analysis enabled to identify the driving forces according to both scenarios and to explain the unusual shape of the minimum energy path in a unimolecular process. Our results imply that elevated temperature (60 degrees C) supplies the reactants with sufficient energy to cross the barrier, whereby each collision with properly oriented reactants results in mono-aminoborane formation.

Automated summary

We investigated the 1,3-hydride shift during the formation of mono-aminoboranes from imine-BH3 adducts. Experimental and theoretical analyses suggest that this molecular rearrangement can occur through either intra-molecular or inter-molecular pathways. Isotope-based experiments and computational simulations support the bimolecular mechanism. Moreover, our results indicate that elevated temperature is necessary for the formation of mono-aminoboranes by supplying sufficient energy to overcome the reaction barrier.