Theoretical Study On The Arylation Via C-F Bond Activation And NHC Catalyzed Intramolecular Cyclization

With the rapid development of quantum chemistry and computer technology,the accuracy of key electron density expressions in quantum chemical calculation methods has been continuously improved.The combination of the main calculation methods and the advantages of avoiding the shortcomings with each other makes calculations of quantum chemistry faster and more accurate.In recent years,the research on the mechanism of organic catalytic reactions using quantum chemical methods is one of the current research hotspots.In this dissertation,two representative organic catalytic reactions?benzene arylation reactions involving carborane anions andnon-polar conversion reactions catalyzed by N-heterocyclic carbene?NHC??were carried out using density functional theory?DFT?method.This thesis consists of five chapters.The first chapter mainly introduces the research progress of arylation reactions and organic reactions catalyzed by NHC.In the second chapter,we briefly introduce the development of quantum chemistry and some commonly used theoretical analysis methods.The third chapter states details of the DFT study to the reaction mechanism and substituent effect of benzene arylation via phenyl cation intermediates.The fourth chapter is about the mechanism and roles of NHC investigations to synthesis of benzoxazolethrough non-polar conversion reaction.Chapter 5 is a brief summary of the work.1.Mechanism and Substituent Effects of Benzene Arylationvia a Phenyl Cation Strategy:A Density Functional Theory StudyIn this study,density functional theory?DFT?calculations were performed to gain insight into the possible reaction mechanism and substituent effects of benzene arylation at the molecular level.The results showed that both[Me₃Si]⁺[WCA]^-and[Et₃Si]⁺[WCA]^-promoted the reaction.The whole catalytic cycle generally involved three processes,namely,carbocation formation,Wheland intermediate formation,and catalyst regeneration.For the second process,the stepwise mechanism?C-C coupling followed by proton transfer?was shown to be preferred over the concerted mechanism,with successive[1,2]/[1,4]-proton transfer as the preferred pathway.C-C coupling,rather than C-H insertion,was shown to be the rate-determining step of the reaction,which was consistent with the kinetic isotope effect experimental results.Bond dissociation energy calculations showed that the ortho-TMS substituent played a major role in activating the C-F bond to facilitate phenyl cation formation,leading to the most weakly C-Cl bonded intermediate,which facilitated the C-C coupling.Ortho-phenyl substituents resulted in more severe steric hindrance of fluoride abstraction and a weaker C-H…?interaction in the C-C coupling transition state compared with meta-phenyl substituents.Better stabilization of the cation center by an electron-rich meta-substituent was concluded to be the main reason for the better yield obtained compared with substrates containing an electron-deficient meta-substituent.2.Reagent Addition Sequence and Equivalent in N-Heterocyclic Carbene-Catalyzed Nonpolar Inversion Enable Conversion from Aldimine to BenzoxazoleMechanism of the oxidative nonpolar inversion reaction catalyzed by N-heterocyclic carbenes?NHCs?to achieve benzoxazoles was firstly investigated in very details.It was revealed that reaction could occur through generally five processes,and for oxidation in the second process,two successive tautomerizations followed by oxidation were demonstrated to be more energetically favorable than the other two pathways.The rate-determining step was disclosed to be the oxidation by3,3'-5,5'-tetra-tert-butyl-4,4'-diphenoquinone?DQ?,and the predicted barrier of 24.2kcal/mol should be a reasonable value for experimental conditions.Afterwards,mechanism calculations to the non-catalyzed reaction indicated that the excessive exothermic property of the initial step should be the main reason for its extremely high barrier.While with participation of NHC,this unfavorable transformation can be deftly prevented according to the specific sequence and amount of reagents addition.In another word,the intrinsic requirement of NHC catalyst formation in situ and the exactly equal amount of the triazolium salt and the base t-Bu OK are the two c ore factors that enable the reaction to occur under mild conditions.