Mechanism Of Alkaline-earth Metal Catalyzed B-N Coupling Reaction:A Theoretical Study

Over the past few decades,with the deep understanding of the structure and chemical properties of alkaline-earth metal elements,scientists have synthesized a series of organic alkaline-earth metal compounds with +2 or +1 oxidation state.Alkaline-earth metal complexes have also been widely employed in organic synthesis in recent years,due to its lower price and less toxicity.Among these alkaline-earth metal catalysts,diimine alkaline-earth metal compounds are the most commonly used,which have archived a great success experimentally,while the physicochemical properties and reaction mechanisms have rarely been studied.In view of this,density functional theory were used to explore the aromaticity of the metal heterocycle of alkaline-earth metal hydride and its catalytic mechanism for the B-N ocoupling reaction of boronamine.1.Investigation of the aromaticity of metal heterocycles in alkaline-earth metal hydridecompoundsWe performed aromatic studies on metal heterocycles of ?-diimine alkaline-earth metal hydrides using three aromaticity criteria such as: bond length averaging,NICS and AICD.The calculated results showed that the C-C bond and the C-N bond of the metal heterocyclic ring are respectively between the standard single-double bond lengths.Further analysis indicated that the absolute value of the NICS value is small,and no significant ring current is observed on the ring plane.Therefore,it can conclude that the metal heterocycles in ?-diimine alkaline-earth metal hydrides are non-aromatic.2.Study on the mechanism of alkaline-earth metals catalyzed dehydrogenation couplingof boranes and amines.We have investigated the mechanism of alkaline-earth metals catalyzed dehydrogenation coupling of boranes and amines by theoretical calculations.The alkaline-earth metal assistant ?-bond metathesis reaction pathways of N-H and B-H bond activation and B-N bond formation were proposed and identified.In this pathway,the dehydrogenation process is the rate-determining step over the catalytic cycle,and the activation energy of this process is significantly lower than that of the direct ?-bond metathesis process.These energy discrepancy is mainly attributed to the low bond dissociation energy(BDE)of Mg-H bond.In addition,we also investigated the catalytic processes of beryllium,calcium and strontium.The results show that calcium and strontium exhibit enhanced catalytic activity.While,beryllium fail to catalyse this reaction due to its small atomic radius,large steric hindrance,low metallicity,and strong Be-H bond.3.The mechanism of alkaline earth metal catalyzed hydroboration of carbodiimidesDensity functional theory(DFT)calculations are employed to study the mechanism of alkaline earth metal catalyzed hydroboration of carbodiimides.Our theoretical study revealed that the active catalytic species is a hydridemagnesium complex when magnesium is used as catalyst.A hydride transfer from boron to magnesium regenerates the active catalytic species and yields the hydroboration product,which was considered to be the rate-determining step.Moreover,we also studied the mechanism of calcium or strontium catalyzed corresponding reactions theoretically.The different reaction pathway with calcium or strontium catalyzed corresponding reactions could be attributed to that the radius of calcium or strontium is larger than magnesium significantly.The DFT calculations showed that the activation free energy for the rate-determining step with calcium or strontium catalyst is much lower than that with magnesium catalyst.Therefore,a mild reaction condition might be found with calcium or strontium as catalyst for corresponding reactions.