Theoretical Studies Of C-H Amination Reactions Catalyzed By Transition-Metal Complexes

Transition-metal catalyzed C-H amination has attracted considerable attention,for the sake of high atom economy,and has been developed as an efficient method for constructing C-N module.However,it is still challenging to develope highly acti-ve and selective catalytic reaction that occurs under mild conditions.Given this situation,deep understanding of related reaction mechanism is of great importance.It is hard,however,to conduct mechanistic studies at the molecular and electronic levels though current experimental approaches.In this thesis,a series of C-H aminations by transition-metal complexes have been computationally studied based on density functional theory,and the main results are summarized as follows:(1)The C(sp~2)-H amination of aryl group catalyzed by a Rh(?)complex has been computationally studied.The reaction mechanism involves sequential N-H deprotonation by external base,Rh(?)-mediated C(sp~2)-H activation,SN2-type N-Cl activation,C-N reductive elimination,and final protonation.The SN2-type dominates in the N-Cl/O bond activation and widely works for Ru(?),Rh(?),and Pd(?)catalyzed C(sp~2)-H amination systems.Besides,it is found that the oxidative addition and SN2-type are competitive pathways when the steric effect of transition metal complex is minor and the coordination number is small,however,the reaction kinetically prefers SN2-type when the steric effect of transition metal complex is evident and the coordination number is large.(2)DFT calculations have been carried out for Co(?)-and Rh(?)-catalzyed C(sp~3)-H amination of benzyl group of 8-methylquionline,respectively.These reactions involve sequential C(sp~3)-H activation,decarboxylation,nitrene insertion,and final protonation.It is found that the C(sp~3)-H activation is the rate-determining step in the both Co(?)and Rh(?)systems.The Co(?)complex shows lower activity compared with Rh analogue,this could be explained from two aspects,the smaller atom radius of Co(?)results in the stronger steric repulsion between ligand and substrate,and triplet electronic structure of Co(?)center which hinders the formation of Co-C bond between Co(?)center and electron-rich carbon atom.Theoretically,in contrast with pentamethylcyclopentadienyl Cp*ligand,the Co complex assembling with of less steric cyclopentadienyl Cp ligand and hard base hydridotris(pyrazolyl)borate Tp ligand could have higher activity toward C(sp~3)-H activation.(3)The C(sp~3)-H aminations of allyl group catalyzed by Pd(?)and Rh(?)complexes have been computationally studied,respectively.The Pd(?)-catalyzed reaction mechanism involves sequential solvent-assisted C(sp~3)-H activation,isomerization of allyl group,reductive elimination,and regeneration of active species.The reaction mechanism of Rh(?)system involves sequential C(sp~3)-H activation,N-H deprotonation,cooperative reductive elimination,and regeneration of active species.The regioselectivity in both Pd(?)and Rh(?)systems could be attributed to the electron-delocalized substituents proximal to allyl moiety,resulting in the?-? conjugation with newly formed C=C bond in reductive elimination transition state.(4)The C(sp~3)-H aminations of benzyl group of ethylbenzene catalyzed by Fe(?)and Mn(?)complexes have been computationally studied,respectively.The reaction mechanism involves sequential metal nitrene formation,hydrogen atom transfer,and binding of radicals.Quintet metal nitrene features singly occupied ?*(M-N)orbital(M=Fe,Mn),which could accept one-electron of hydrogen atom transfer.However,it is more difficult for the singly occupied ?*(M-N)orbital to accept two-electron of electrophilic attack process.The hydrogen atom transfer of benzyl C(sp~3)-H is favorable over methyl C(sp~3)-H,the former benefits from the p-? conjugation in corresponding transition state,which could delocalize the unpaired electron from benzylic carbon atom to phenyl moiety,thus stabilizing the transition state.Theoretically,when the Fe and Mn nitrene adopt the square pyramid and trigonal bipyramid structures,the corresponding hydrogen-atom transfer process could proceed more feasibly.