Density Functional Theory Studies Of C-H Functionalization Catalyzed By Rare-earth Metal Complexes

The C?H bond is a common chemical bond.Although C?H bond functionalization is relatively inert and difficult to be activated,it plays an important role in synthetic chemistry,biology,and medicinal chemistry.In the past few decades,many metals have been used to catalyze C?H functionalization to obtain corresponding products.Among these metals,rare-earth metal is the strategic resource of our homeland,which has received extensive attention from researchers due to the unique and excellent chemical properties.The rare-earth metal complexes have shown excellent catalytic performance in the process of catalyzing C?H functionalization,but the mechanism,the determinants of selectivity and activity are still unclear.In this thesis,the density functional theory is used to study the direct silylation/borylation reaction of ether substrates and the C?H activation induced[3+2]cyclization reaction catalyzed by rare-earth complexes.The reaction mechanism and the origins of reactivity and selectivity have been explored.The main conclusions are as follows:(1)Based on the reaction of the direct C?H silylation of ethers catalyzed by rare-earth complex,the reaction mechanism and the origins for the regioselectivity were explored.The results indicate that the reactions follow the mononuclear mechanism,which mainly includes two key steps,viz.,C?H activation andσ-bond metathesis.The C?H activation is the selectivity determining step.Since the ortho-C(sp~2)of Ph OMe is more nucleophilic,the activation of the C(sp~2)?H bond is more favorable.As the radius of the heteroatom in the substrates increases,the distortion of the substrate becomes larger in the process of C(sp~2)?H activation,leading to the higher selectivity of C(sp~3)?H activation.(2)The reaction of the[3+2]annulation of aromatic aldimine and styrene catalyzed by rare earth complexes is clarified.The reaction includes four processes:generation of active species,olefin insertion,cyclization,and regeneration of active species.The selectivity determining step is olefin insertion.The results of further study show that the cis-selectivity is preferred when using the catalyst with less steric hindrance,whereas the cis-selectivity will decrease gradually and even shift to trans-selectivity when the steric hindrance of the catalysts increases.Theoretical calculations show that the configuration of the products can be preliminarily predicted by the structure of the catalyst,or the new catalysts can be designed according to the configuration of the desired product.(3)The[3+2]annulation of aliphatic aldimines with olefins in the presence of primary amine additives was explored.The selectivity determining step in the reaction is the olefin insertion step.Besides,the results show that the presence of additives can improve the reactivity because the N?H activation has smaller activation energy than the C?H activation.When there are no additives,the trans-selectivity is mainly due to the stronger interaction between the catalyst and the substrate in the olefin insertion process.The addition of additives makes the structures of the olefin insertion transition states more crowded,resulting in the distortion of the cis-insertion transition state is significantly greater than that of the trans-insertion transition state,thereby obtaining the trans-products.