Bond dissociation energy controlled σ-bond metathesis in alkaline-earth-metal hydride catalyzed dehydrocoupling of amines and boranes: a theoretical study

Dehydrocoupling of amines and boranes is an efficient method for the formation of N-B bonds; however, the strong B-H bond dissociation energy (BDE) always restricts non-catalytic reaction pathways. Therefore, alkaline-earth-metal (Ae) hydrides are used as catalysts for this type of reaction because of their lower Ae-H bond energy. A theoretical study was performed to study the mechanism of Ae-catalyzed dehydrocoupling reactions. The computational results show that such reactions are initiated from sigma-bond metathesis between Ae hydride catalysts and amines to release molecular hydrogen, followed by borane bonding with amino Ae intermediates. Subsequent hydride transfer yields an amino-borane product and, in the process, regenerates the Ae hydride catalyst. Our theoretical calculations revealed that dehydrogenation is the rate-determining step during sigma-bond metathesis in the presence of a magnesium hydride catalyst. We predicted that beryllium hydride could not function as a catalyst because the apparent activation free energy is significantly high. Furthermore, we observed that in calcium or strontium hydride-catalyzed reactions, the rate-limiting step changed to the hydride transfer step. Further density functional theory calculations showed that the BDEs of the Ae-H bond controlled the reactivity of the sigma-bond metathesis step.