Theoretical Studies On Several Important Organic Synthesis Reactrous Catalyzed By Transition Metals

Organometallic chemistry has become one of the hottest areas in modern organic chemistry.Recently,transition metal complexes have been widely applied to polymer science,environmental science,biological science and green chemistry due to their high reactivity and high selectivity.Different from the rapid development of Organometallic chemistry in the experimental field,the development of relavent theoretical studies is relatively slow.There are many experimental results and phenomena that can not be well rationalized by general chemical knowledge.In addition,for many improtant chemical reations,the intrinsic mechanisms are still obscure.These greatly impeded the application of transition metal complexes and the development of new catalysts.Therefore,the understanding for the nature of a reaction from a microscopic perspective through theoretical calculations is of great significance to the further development of modern chemistry.In this dissertation,we performed a series of theoretical studies on some important organic synthesis reactions catalyzed by Ru,Rh,Zn transition-metal catalysts.This work aims to elucidate mechanistic details,reaction characteristics and the influences of ligands,solvents as well as additives on the reactions at the molecular level.It is expected that these calculated results can provide an in-depth understanding of these reactions and catalysts,and can provide guidance for further studies.The main contributions and valuable innovations of this dissertation are listed as follows:1.Density functional theory(DFT)calculations have been implemented to clarify the hydrogenation mechanism of a non-active ester:ethyl benzoate(PhCOOC2H5)to ethanol(C2H5OH)and benzyl alcohol(PhCH2OH)catalyzed by RuIIPNN.The calculated catalytic cycle involves two similar hydrogenation processes with the persistent participation of the catalyst:hydrogenation of PhCOOC2H5 to C2H5OH and intermediate PhCHO,and further hydrogenation to PhCH2OH.Three potential mechanisms,named as carbonyl insertion mechanism,stepwise double-hydrogen-transfer mechanism,and direct reduction mechanism,have been investigated in details in different solvents.It is found that the solvent has decisive influence on the hydrogenation mechanism.In a less polar solvent,the reaction prefers a stepwise double-hydrogen-transfer hydrogenation mechanism consisting of dihydrogen activation,stepwise double-hydrogen-transfer,and hydrogen abstraction rather than the carbonyl insertion mechanism proposed in the experimental work.However,in a more polar solvent,the reaction proceeds via the direct reduction mechanism involving the separated ions(PhCH(OC2Hs)O-and monocation)instead of the experimentally proposed hemiacetal intermediate PhCH(OH)OC2H5.The dihydrogen activation as common starting point of the reaction in all three potential mechanisms can be facilitated by the products(C2H5OH and PhCH2OH).The substituent group on an ester has little influence on the reaction mechanism while it greatly affects the reactivity:the electron withdrawing group favors the hydrogenation,while the electron donating group makes the reaction more difficult.These theoretical results are expected to provide valuable guidance for the experimental study of the hydrogenation of non-active esters.This work has been published on Molecular Catalysis(2017,440,120-132).2.Recent development on Rh(III)-catalyzed synthesis of indolines from arylnitrones and alkynes provides an efficient redox-neutral method integrating C-H activation with O-atom transfer.Here density functional theory calculations were performed on the mechanism of a representative model system:[Cp*Rh]2+-catalyzed reaction of phenylnitrone with 1-phenylpropyne.The results suggest that the catalytic cycle involves a Rh(III)-Rh(I)-Rh(III)transformation and consists of C-H activation,alkyne insertion/O-atom transfer,and cyclization-protonation.The C-H activation,the rate-determining step,proceeds via a self-assisted deprotonation mechanism.The alkyne prefers the insertion into Rh-C(sp2)bond rather than Rh-O bond.The calculations locate an intermediate containing an enol unit,which plays a crucial role in facilitating the cyclization process.The theoretical results provide an explanation for the puzzling experimental observations.The regioselectivity of the reaction is stemmed from the electronic effect rather than steric effect and controlled by the alkyne insertion step.The improved yield with the addition of pivalic acid is attributed to the strong deprotonation capability of pivalate,which significantly facilitates the C-H activation via the concerted metalation-deprotonation(CMD)mechanism.This work has been published on J.Organomet.Chem.(2018,854,15-26).3.A DFT study has been carried out to provide insight into the molecular mechanism of the Zinc(?)-catalyzed cross-dehydrogenative coupling of N-propargylanilines with indoles developed by Nakamura et al.(Angew.Chem.Int.Ed.2016,55,6758-6761).The proposed catalytic cycle consists of two subcycles,through which three C-H bonds(two sp2 and one sp3)are activated sequentially.One subcycle carries out the transformation of N-propargylaniline to intermediate product(IMP),N-benzyl-1,4-dihydroquinoline,and another realizes transformation of IMP with indole to the final product.Different from the general knowledge,the activation of the C(sp3)-H is found to be easier than the C(sp2)-H activation.The reaction is demonstrated to be an abnormal catalytic cascade reaction,which features the temporal separation of the multiple catalytic activities of Zinc(?)catalyst.The cascade characteristic illuminated in the present work is instructive for developing controlled and efficient synthesis of various products from catalytic cascade reactions.A cascade reaction can be designed so well that the catalyst can sequentially show its multiple catalytic activities,allowing for the controlled efficient synthesis of desired intermediates and products.The present work also rationalizes the experimental observation for the deuterium labeling study.This work has been published on Catal.Sci.Technol.(2018,8,1997-2007).4.A DFT study has been carried out to provide insight into the molecular mechanism of the base-controlled completely selective linear or branched Rh(I)-catalyzed C-H ortho-alkylation reaction of azines(Angew.Chem.Int.Ed.2017,56,5899-5903).The calculated results provided favorable mechanisms for the reaction in the absence of base or with K3PO4 or KOPiv as catalytic base.The reaction prefers an alkene insertion,protonation mechanism rather than a protonation,C-C reductive elimination mechanism no matter K3PO4 or KOPiv as the catalytic base.In the presence of K3PO4,the reactant Ri is activated via CMD mechanism,wheras the reactant Ri is activated via an oxidative addition and deprotonation process when KOPiv is used as the catalytic base.The theoretical results provide a reasonable explanation for the base controlled selectivity.When K3PO4 is used as the catalytic base,the phosphate anion is strongly bound to the Rh center,which lead to the olefin insertion process being realized via an octahedral transition state.In this case,the steric hindrance becomes a decisive factor for the regioselectivity,resulting in the branched product.When KOPiv is used as the catalytic base,owing to both pivalic acid and chlorine anion are good leaving group,the alkene insertion is accomplished via a quadrilateral transition state.In this cacse,the steric hindrance has little effect on the alkene insertion process,and the electronic effect plays a crucial role for the regioselectivity,leading to the formation of linear product.In the whole,the base-controled regioselectivity is determined in the alkene insertion step,and originated from the different binding ability of the catalytic base to the metal center,which results in different coordination configuration of the ruthenium complex during the olefin insertion process.The corresponding results is being in preparation.