Theoretical Studies On Transition Metal-Catalyzed Selective B-H Functionalization Of O-Carboranes

The ten B-H vertices embedded in the o-carborane possesses extremely similar spatial strucuture and electron environments,making the precise synthesis of functionalized carborane derivatives very challenging.In addition,the continuous development of quantum chemistry and computer science brings about more fast and accurate theoretical predictions.In this context,computational chemistry has been one of most the effective ways to get deep insights into the nature of the reaction,the catalytic mechanism,and the related experimental phenomena.In this thesis,we would present our research results about the mechanism and regioselectivity of the Ir/Pdcatalyzed functionalization of o-carboranes by using the density functional theory（DFT）method.This thesis includes four chapters.The first chapter briefly introduces the general contents of theoretical and computational chemistry,the widely used theoretical analysis methods,and the progress of transition metal-catalyzed regioselective activation functionalization of o-carboranes.The second chapter mainly discusses the reaction mechanism and origin of the B（3）-H functionalization of the Ir-catalyzed amination of o-carborane,when there is no directing group installed.The third chapter documents the theoretical study on the mechanism and origin of the Pd-catalyzed B（4）H arylation of o-carborane,induced by the benzamide directing group on B（9）-H vertex.The fourth chapter is the summary.The following is a brief introduction to the two specific topics:Ir-catalyzed B（3）-H amination:It is of extensive challenges to achieve selective functionalization of o-carboranes without any directing groups.But valuable insights would be obtained for rational design of new catalysts or reactions if the origin of the regioselectivity has been uncovered.In this work,the DFT method was used to study the mechanism and the B（3）-H selectivity of the Ir-catalyzed reaction of o-carboranes with NH₃.The computational results show that the reaction goes through two successive oxidative additions（OxA）and two successive reductive eliminations（ReE）,and the Ir（Ⅴ）complex is an important intermediate.For the initial OxA reaction,the N-H bond of NH₃ to the Ir（Ⅰ）complex is more energetically favorable than the B（3）-H bond of the o-carborane.The subsequent OxA of the B（3）-H bond to the Ir（Ⅰ）complex yields the Ir（Ⅴ）intermediate,which is the regioselective determining step.The special 26 electron delocalized structure of o-carborane was confirmed by the localized orbital positioning function（LOL）analysis,explaining its aromaticity and special chemical stability.Its highly delocalized electronic structure and the synergistic effect of the induced effect of two carbon atoms makes B（3）-H the most electron deficient,so it is easier to undergo an OxA reaction with the Ir（Ⅲ）complex.which is also the fundamental reason for the B（3）-H selectivity.Pd-catalyzed B（4）-H arylation with B（9）-benzamide as the directing group:The mechanism of transition metal-catalyzed B（4,5）-H functionalizations with directing group on the C（1）-H vertex has been extensively studied.An alternative strategy is to assemble a directing group onto the remote B-H verteices.In this case,the charge distribution would be disturbed,and thus to induce the high functionalization selectivity.In this work,the Pd-catalyzed B（4）-H arylation with B（9）-benzamide as the directing group was chosen as the computational model to get deeper insights to this strategy.The computational results indicate that the oxygen atom of the carbonyl in benzamide would coordinated with the metal center,and the activation of the B（4）-H bond by Pd（Ⅱ）is realized by the concerted metallation deprotonation（CMD）process.This step is both the regioselectivity-and rate-determining step of the entire catalytic cycle.The reaction goes through the Pd（Ⅱ）-Pd（Ⅳ）-Pd（Ⅱ）-Pd（0）-Pd（Ⅱ）mechanism to furnish the cross-coupling product.According to the natural population analysis（NPA）,the co-effect of the electron withdrawing of the benzamide at the B（9）position and the induce of the C（1,2）-H vertices makes the B（4）-H bond the most electron deficient（compared with other B-H vertices adjacent to B（9））,and therefore leads to the exclusive B（4）-H selectivity.