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-ear...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 fromσ-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 duringσ-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 theσ-bond metathesis step.展开更多
基金supported by the National Science Foundation of China(Grants 21372266 and 21772020)the Fundamental Research Funds for the Central Universities(Chongqing University)(No.106112017CDJXY220007)+1 种基金We are also thankful for the project supported by the Science and Technology Research Program of Chongqing Municipal Education Commission(Grant No.KJ1703048)the Graduate Scientific Research and Innovation Foundation of Chongqing,China(No.CYB16034).
文摘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 fromσ-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 duringσ-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 theσ-bond metathesis step.
