Theoretical Studies On The C-H Alkylation And C-H Activation Initiated Styrene Polymerization Catalyzed By Rare-earth Metal Complexes

Rare-earth metal complexes have attracted wide attention because of their unique and excellent chemical properties.Especially,the rare-earth cationic catalytic systems have shown excellent catalytic properties in various fields,such as olefin polymerization,C-H bond functionalization,and asymmetric synthesis etc.Although significant progress has been achieved experimentally,the related mechanisms are complicated and have remained unclear.It was experimentally demonstrated that the catalytic performance is influenced by the metal center,ligand type,ligand substituent,and the type of substrate.However,it is hard to clarify the steric and electronic effects of multiple factors through the current experimental means,which has somewhat hampered the further development of these systems.The deep understanding of related mechanisms at the molecular level is of great importance for development of new catalytic systems.In this thesis,the density functional theory?DFT?is used to study the C-H activation reactions,involved in the polymerization of styrene by using pyridine as chain transfer agent,hydroaminoalkylation of tertiary amines with olefins,and the enantioselective C-H addition of pyridine with olefin.The reaction mechanisms,including the orgins of activity,regioselectivity,and stereoselectivity have been elucidated.The main contexts and results are as follows:?1?The styrene polymerization with pyridine as a chain transfer reagent by scandium and gadolinium catalysts has been investigated.The C-H bond activation of pyridine in the chain initiation and chain termination was studied in detail.The syndiospecific styrene polymerization by scandium and gadolinium systems is controlled by steric factors,mainly through steric interaction among the benzene ring of the pre-inserted styrene,the cyclopentadienyl ligand,and the benzene ring of the incoming styrene.In addition,it is shown that the difference in the formation of methylene-or tert-butyl-based functionalized polystyrene by scandium and gadolinium catalysts was observed at the chain termination stage.?2?The reaction mechanism of the hydroaminoalkylation of aliphatic tertiary amines with alkenes has been clarified by DFT calculations.It mainly contains three stages:the generation of active species,the olefin insertion,and the C-H activation of another amine molecule.The factors governing the branched products?1,2-insertion?or the linear products?2,1-insertion?were investigated.The results show that the electron effect is the control factor of regioselectivity during the reaction.With a comparison of theoretical calculation and experimental data,a charge-based prediction model for the prediction of the selectivity of product is proposed.In addition,the reactivity of different types of C-H bonds was also explored.?3?The stereoselectivity of asymmetric alkylation of pyridine with norbornene catalyzed by cation rare-earth metal species is mainly determined by the olefin insertion manner.In Sc-catalyzed system,when the substituent of the chiral ligand is H,the steric effect on stereoselectivity in related transition state is insensitive due to the smaller size of H atom,thus the stereoselectivity is poor.In the case of larger substituent OSi?ⁱPr₃?in size,the steric effect was significant,showing a good stereoselectivity.In addition,the effect of the metal center of the catalysts is also discussed.The calculations show that when yttrium and gadolinium were used,the activation energy barrier for C-H activation of pyridine is too high to be overcomed under the experimental conditions.This could explain the experimental observation that yttrium and gadolinium catalysts are inactive for the asymmetric alkylation of pyridine with norbornene.The above results could be helpful for developing new chiral ligands and new asymmetric synthesis systems.