Theoretical Studies On B–H Bond Activation/Functionalization And The Intermolecular Aminoarylation Of Alkynes

The extensive applications of theoretical and computational chemistry in the reaction mechanism investigations has greatly expanded the application prospects of quantum chemistry,and it has become one of the most effective ways to explore the mechanistic details and fundamental origin of selectivity.In this article,we will present our detailed density functional theory（DFT）study restults towards three reactions related to the activation and functionalization of B–H bond and the intermolecular aminoarylation of alkynes.The thesis consists of five chapters:the first chapter briefly introduces the theories and computational methods,along with the experimental background and research progresses of related organic reactions;the second chapter presents the theoretical investigations to the iridium-catalyzed direct borylation of B（3,6）–H of o-carborane;the third chapter discusses the mechanism and origin of chemoselectivity inverse of the hydroboration of unsaturated compouds when using the N,O-chelated iridium（Ⅰ）complexes;the fourth chapter elucidates how to understand the Z selectivity of the metal-free intermolecular aminoarylation of alkynes;the last chapter summarizes the work and shows the perspective on the future studies.More information about our theoretical study results towards the three reactions is given below:1.Theoretical investigations to the iridium-catalyzed direct borylation of B（3,6）–H of o-carboraneThe recent success of Xie group（Nat.Commun.2017,8,14827）in achieving Ir-catalyzed direct borylation of cage B（3,6）–H of o-carboranes with excellent yield and regioselectivity indicates one of the most exciting developments in regioselective functionalization of carboranes.In the present work,the possible catalytic mechanisms and origin of regioselectivity were studied in very detail by using the DFT method.The computational results show that the Ir（Ⅲ）species should be the actual catalyst,rather than the Ir（Ⅰ）compound as supposed in experiments.Every borylation cycle occurs via two successive oxidative addition-reductive elimination（OxA-ReE）processes,including an unusual Ir（Ⅴ）species in each process.The regioselectivity is determined in the first OxA step of each borylation.Two reaction conformations have been calculated and concluded to be competitive.Different distributions of the C–H…O hydrogen bonds and B–H…O“interactions”are disclosed to be mainly responsible for different energy profiles of the two reaction conformations.The LOL analysis shows a 26-electron-delocalization structure of the o-carborane,which states the main reason of the aromaticity and exceptional chemical stability of this series of carborane（anions）.The highly electron-delocalized structure cooperatively with the inductive effects of the carbon centers make the B（3,6）–H sites most electron deficient,resulting in the most favorable sites to occur the OxA reaction with Ir（Ⅲ）species.This is concluded to be the origin of the regioselectivity of the title reaction.We believe the present work should be very attractive for especially the experimental scientists since it is of great significance in guiding them in the rational design of more efficient catalytic borylation reactions of carboranes with better regioselectivities.2.Theoretical study on mechanism and chemoselectivity of boron hydroboration by the N,O-chelated Ir（Ⅰ）complexIt has been widely reported that the transition metal catalysts can invert the chemoselectivity of hydroboration of multifunctional substrates,but the fundamental reasons for this inversion have rarely been studied.In this work,mechanistic details and chemoselectivity of the hydroboration of various E–C（E=CH,CH₂,O,N,etc.）multiple bonds by using free HBCy₂ or captured HBCy₂ by the N,O-chelated Ir（Ⅰ）complex has been explored by density functional theory calculations.The computational results demonstrate the formation of a 3-centerπcomplex before the4-center transition state when using free HBCy₂.Capture of HBCy₂ by the Ir（Ⅰ）complex results in a hemilabile metallanheterocycle Shimoi-type intermediate in which aδ-[M]...H–B agnostic interaction appears.Afterwards,the E–C functional group goes to intersect thisδ-[M]...H–B interaction rather than the P=O...B coordination bond,and the following hydride transfer is figured out to be rate-determining,the following step is borylation of E–C unsaturated bond and finally dissociation of the product from the catalyst Cat1.According to analysis to the frontier molecular orbitals,the olefins who has typical delocalizedπelectrons tends to give more effective overlap with the quasiπLUMO of HBCy₂,but the aromatic substituents could significantly reduce its reactivity.The polarized carbonyl group facilitates theσcoordination with the metal center,but the vinyl group favors to interact throughπcoordination.The superiority ofσcoordination results in the priority of hydroboration of aldehyde over the olefins.So it is the conversion of the more favored bonding groups by the transition metal that should be fundamentally responsible for the chemoselectivity inversion.3.Understanding the Z selectivity of the metal-free intermolecular aminoarylation of alkynes:a DFT studyThe transition metal-catalyzed hydro-or carboamination of alkynes yields usually cis enamines,denoting the hydrogen or the unsaturated carbon group is located at the same side of the olefine bond with the amine group.Recently,Greaney and coworkers reported a metal-free intermolecular aminoarylation of alkynes,affording tetrafunctionalized enamines with the amine and aryl groups in trans geometry.The Smiles rearrangement was proposed to account for the aminoarylation mechanism,but the Z selectivity（i.e.trans enamine as the main product）is quite difficult to be understood.In the present paper,the aminoarylation mechanism has been thoroughly studied and accordingly,the origin of the Z selectivity has been discussed in detail.The calculated results reveal that the Smiles rearrangement mechanism is able to occur under experimental conditions,but the novel mechanism,in which an alternative pathway is proposed for transformation of the anionic adduct,is more reasonable on account of the lower energy barrier and the more rational explanation to roles of the additive K₂CO₃.The Z selectivity is determined by the inherent requirements of dynamic preference of pathways（the CTI-first pathway in the Smiles rearrangement mechanism and the QZ pathway in the novel mechanism）that promise the Z-geometry product,rather than the conventional presume of isomerization of the E-geometry product in situ under assistance of K₂CO₃.