Theoretical Studies On The Selective Polymerization Of Conjugated Diene And The Alkylation Of Pyridine With Olefins Catalyzed By Cationic Rare-Earth Metal Complexes

Cationic rare-earth metal complexes showed high activity and high stereoselectivity toward the polymerization of conjugated dienes and functionalization of olefins with alkylpyridines due to their unique chemical properties.However,it is difficult to fully explore the steric and electronic factors governing such activities and selectivities only through the existing experimental methods.In this thesis,density functional theory have been employed to investigate the mechanisms of polymerization of conjugated diene and alkylation of olefins with pyridine derivatives catalyzed by cationic monometallocene,nonmetallocene,and constrained geometry configuration rare-earth metal complexes.Based on the clarified mechanism at the molecular level,the effects of monomer types,coordinative side arms,ligand backbone and similar pyridine substrates on the polymerization activity,regioselectivity,stereoselectivity,and the origin of product selectivity have been elucidated systematically.These results are expected to provide theoretical basis for further design and development of related catalytic systems with higher activity and selectivity.The main results are summarized as follows:(1)The origin of cis-1,4-specific polymerization of butadiene and trans-1,4-isotactic polymerization of trans-1,3-pentadiene(EPD)catalyzed by cationic monometallocene yttrium alkyl species[CpY(o-CH2C6H4NMe2)]+ has been computationally studied.It is found that butadiene,in the form of cis-1,4-prone,can be inserted repeatedly to obtain the cis-type products,which is a kinetic control process.The strong interaction between the catalyst and the cis-isoprene controls the cis-1,4 selectivity.In the case of EPD,cis-4,1-si insertion mode showed the lowest energy barrier to give 4,1-η3-π-anti-pentadienyl unit at the chain initiation step.In the chain propagation stage,the trans-4,1-re insertion of EPD is much easier,which is under thermodynamic controls.The computational results indicate that syndioselective polymerization is kinetically unfavorable owing to steric repulsion between Cp and methyl of EPD in the transition state,thus accounting well for high isoselectivity.(2)DFT studies have been conducted for the difference in the activity and selectivity of isoprene polymerization catalyzed by the cationic rare-earth metal complexes[(C5Me4C6H4OMe-o)Sc(CH2SiMe3)]+(A+),[{C5Me4SiMe2CH2P(O)Ph2}Sc-(CH2SiMe3)]+(B+),and[(C5Me4CH2CH2PPh2)Y(CH2C6H4NMe2-o)]+(C+)with different side arms,respectively.It has been found that trans-1,4 polymerization of isoprene by species A+ prefers insertion-isomerization mechanism.The origin of experimentally observed inertness of species B+ toward isoprene polymerizatio1 is the steric hindrance derived from the crowding of η3-πsyn-allyl species,hampering the continuous coordination and insertion of isoprene monomer.The catalytic activity of isoprene polymerization could be improved via modification of catalyst,such as reducing the size of substituents on phosphorus atoms and the size of Cp ligand,or increasing the radius of central metal.In the C+-catalyzed isoprene polymerization,the active species structure shows the characteristics of the phosphine side arm coordination.Besides,the strong interaction between the Y metal center and isoprene moiety is the main reason for the highly 3,4-isotactic polymerization of isoprene.(3)The process of 3,4-isospecific isoprene polymerization and copolymerization of isoprene with ethylene catalyzed by an yttrium amidinate complex have been carried out theoretically.The results show that at the chain initiation step of isoprene homopolymerization,cis-4,1-re insertion mode of isoprene exhibits the lowest energy barrier and the 1,4-insertion manner is both kinetically and thermodynamically favorable than the 4,3-insertion manner.At the chain propagation stage,the consecutive insertion of isoprene into the Y-C3 bond of Y-allyl species in a cis-1,4-re mode achieves the observed 3,4-isospecific polymerization.The incoming isoprene and the allyl group of polymer chain form endo-exo configuration to ensure the isoselecitive regularity.In the copolymerization process of isoprene and ethylene,ethylene insertion gives thermodynamically unstable η1-Y-alkyl active species,while isoprene insertion gives thermodynamically favorable η3-Y-allyl active species.The homopolymerization of isoprene is both kinetically and thermodynamically favorable than the copolymerization process,which is mainly attributed to a stronger interaction between the η3-anti-Y-allyl species and isoprene.(4)The mechanism of the alkylation of pyridine derivatives with olefins by rare earth metal complexes[C5R5Ln(CH2C6H4NMe2-o)]+(R=H,Me,Ln = Sc,Y)has been computationally studied.It mainly includes three stages:the generation of active species,olefin insertion and subsequent C-H bond activation of another pyridine molecule.On this basis of above mechanism,the reasons for different alkylation products obtained by different pyridine derivatives reacting with olefins were analyzed.(ⅰ)The reaction of 2-tert-butyl-6-methylpyridine with ethylene gives a monobutylated product due to the obvious steric repulsion between the tert-butyl of pyridine moiety and the tert-butyl group of metal-alkyl unit in the resulting species.(ⅱ)The multiethylated products are obtained by the reaction of 2,6-dimethylpyridine with ethylene,which is mainly due to the strong interaction between Y…N(py),thus C-H activation and ethylene insertion occur alternately.(ⅲ)Cis-cyclization products are obtained by the reaction of 2-methylpyridine with 1,5-hexadiene,mainly because the cis-cyclization process is more favorable both kinetically and thermodynamically than the alkyl elimination process after the C=C double bond was inserted into the Sc-C bond.(iv)The reaction of styrene and 2,6-lutidine gives a linear product via 2,1-re insertion reaction mainly because of less geometrical deformation required for this insertion manner compared with 1,2-insertion.(v)The 1,2-si insertion of 1-hexene into C(sp3)-H bond of 2,6-lutidine gives branched products mainly becaue of stronger interaction between olefin moiety and the metal center.Finally,the pyridine alkylation reaction catalyzed by yttrium species was studied.It is found that the C-H bond reactivity under different chemical environment follows the order of a-CH3(1°)>α-CH2(2°)>α-CH(3)>β-CH2(2°)>y-CH3(1°).The calculation results indicate that the Y…N contacts and the NBO charge on Y atom of the corresponding transition state account for the C-H activation energy barrier and thus the C-H reactivity.The shorter the Y…N distance in the transition state,the lower the energy barrier for C-H activation,the higher the activity.The more positive charge on Y atom,the lower energy barrier and thus the higher C-H alkylation reactivity.