Theoretical Studies On Some Synergetic Light/Transition Metal-Catalyzed Reactions And Dual Au/Ag Catalysis

Organometallic chemistry,as an emerging interdiscipline,is at the forefront of modern organic chemistry research.Because of the high reactivity,stability and selectivity,transition metal compounds have become the core of organometallic chemistry,which have attracted great attention of chemists to launch a series of research.It has played an important role in human health,environmental protection,energy development fields and so on.In recent years,the strategy via the visible light-mediated photosensitizer and transition metal-catalyzed reaction has been widely used in synthesis of complex molecules and natural products,which conforms to the development concept of "green chemistry".However,there are some experimental results and phenomena cannot be explained in the experimental research.In addition,the involved reaction mechanism is not clear,and it is difficult to understand the factors that control the reactivity and selectivity,including substituents,ligands,solvents,and additives.These problems limit the development and application of organometallic chemistry.Therefore,theoretical research via performing quantum chemical calculations to explore the reaction mechanism at the molecular level and reveal the microscopic nature of chemical reactions,which is of great significance for designing new synthetic reactions and promotes the development of modern organic chemistry.In this dissertation,on the basis of quantum chemistry calculations,we carried out a series of theoretical studies on some difunctionalizations of alkynes and cross-coupling reactions catalyzed by gold,copper transition metal catalysts.Through performing the systematic theoretical calculations,we revealed the mechanism at the molecular level,established the theoretical model of catalytic cycle,discussed the key factors of controlling the chemoselectivities,stereoselectivities and regioselectivities of the reactions and analyzed the roles of key intermediates.Our DFT calculations rationalized some experimental phenomena.It is expected that these calculated results can provide an important theoretical reference for the subsequent design of more efficient catalytic systems.The main contents and significant innovations of this dissertation are summarized as follows:1.Recently,a photosensitizer-free visible-light-mediated gold-catalyzed 1,2-difunctionalization of alkynes has been developed.However,mechanistic aspects of this unconventional photocatalytic reaction remain largely obscure.With the aid of density functional theory(DFT)and time-dependent(TD-DFT)calculations,we mimicked the photosensitizer-free visible-light-mediated gold-catalyzed 1,2-difunctionalization of 1-phenyl-1-hexyne and focused on two fundamental questions:how does photoredox catalysis occur without assistance of an exogenous photosensitizer under visible light irradiation,and what is the detailed mechanism of the gold-catalyzed 1,2-difunctionalization of alkynes?The results reveal the dual role of the gold(?)complex in lightharvesting and catalysis,where a charge-transfer(CT)complex formed by the association of gold(?)catalyst with PhN2BF4 acts as a photosensitizer,which can undergo an electronic transition between the gold(?)moiety and PhN2BF4 of the CT complex into an excited electronic state and afford a charge-transfer exciplex.The oxidative quenching of the exciplex generates the gold(?)species and diazobenzene radical.The subsequent catalytic cycle proceeds via two parallel pathways,involving the radical addition to gold(?)and gold(?)centers,respectively,and in these two pathways the reductive elimination of gold(?)species is identified as the rate-determining step of the whole reaction.The present study could provide a new understanding for exogenous-photosensitizer-free visible-light-mediated gold-catalyzed processes.This work has been published on Chemistry-A European Journal(2018,24,14119-14126).2.Density functional theory(DFT)calculations were performed to investigate the photosensitizer-free visible-light-mediated goldcatalyzed cis-difunctionalization of alkynes with aryl diazonium salts.The detailed reaction mechanism is established,and the observed regio-and chemoselectivities are rationalized.The results are compared to those of the rhodium-catalyzed cis-difunctionalization of alkynes.It is indicated that the excitation of the aryl diazonium salt initiates the photocatalytic cycle,and the following single electron transfer(SET)between the Au(?)catalyst and the excited aryl diazonium salt affords the key aryl radical.Both gold-and rhodiumcatalyzed reactions involve two major steps:alkyne insertion into the M-N or M-C bond(M=Au,Rh),and C-C or C-N reductive elimination from the M(?)center.The cis-difunctionalized product can be obtained by the trimethylsilyl(TMS)-substituted alkyne through the gold catalysis or by the Ph-substituted alkyne through the rhodium catalysis.The catalyst-dependent reactivity switch of TMS-and Ph-substituted alkynes is attributed to the catalyst-induced shift of the rate-determining step.This work has been published on The Journal of Organic Chemistry(2019,84,16171-16182).3.The C-N coupling of alkyl electrophiles for amine synthesis keeps a relatively less developed region in comparison with that of aryl electrophiles largely due to the difficulty in product-generating C(sp3)-N reductive elimination.The recent work by Hu et al.(Nat.Catal.2018,1,120-126)developed an effective strategy for the C-N coupling of alkyl redox-active esters with anilines by merging photoredox catalysis and copper catalysis with an oxoacetic acid ligand(LH2).Here,we present a DFT-based computational study to understand how the special dual catalysis works in a cooperative fashion with the assistance of the ligand.Photoredox catalysis is found to occur most possibly through an oxidative quenching mechanism(Ru?/*Ru?/Ru?/Ru?)with Et3N as the quencher rather than with the experimentally proposed copper complex.Copper catalytic cycle(Cu?/Cu?/Cu?/Cu?)is predicted to proceed via a Cul-oxidation-first pathway instead of the hypothetical aniline-deprotonation-first pathway in the experiment,and the most likely catalytic active species is identified as Cu?LH complex.With the Ru?/Cu?-metallaphotoredox catalysis,the most feasible mechanism for the C(sp3)-N cross-coupling involves six steps:(?)generation of cyclohexyl radical(Cy*)via the single electron transfer(SET)from photoexcited*Ru? to the complex of redox-active ester with Cu?,(?)coordination of aniline to Cu? center,(?)Cy*radical addition to Cu? center,(?)SET between Cu?-cyclohexyl aniline complex and generated Et3N"+,(v)deprotonation of aniline,and(vi)reductive elimination of the Cu?-cyclohexyl amido intermediate to produce the C(sp3)-N coupling product.The Cu? complex is identified to play a dual role in the title reaction,which acts as the promoter in oxidative quenching process and as the catalyst in the copper catalytic cycle.This work has been published on ACS Catalysis(2020,10,5030-5041).4.We have performed a density functional theory(DFT)calculations to study on synergetic Ir/Cu-metallaphotoradox-catalyzed trifluoromethylation of aryl bromides.The calculated results indicated that the photoredox is found to occur through an oxidative quenching mechanism(Ir?/*Ir?/Ir?/Ir?),rather than the reductive quenching mechanism(Ir?/*Ir?/Ir?/Ir?)proposed in the experiment.The single electron transfer(SET)process between the excited photosensitizer*Ir? and the cationic trifluoromethanesulfonic acid can produce the trifluoromethyl radical CF3',and the generation of aryl radical Ar' via SET process from supersilanol to oxidized photosensitizer IrIv.In copper catalytic cycle,the aryl radical Ar*is preferentially oxidative addition to Cu center than the trifluoromethyl radical CF3*.The cleavage of S-CF3 bond is considered as the rate-determining step in the whole catalytic cycle.The present results can better understand the synergetic Ir/Cu-catalyzed C(sp2)-CF3 cross-coupling reaction at the molecular level,which provide a strong theoretical guidance for the further design of new catalytic systems.This work is being in preparation.5.The gold(?)-catalyzed cross-coupling reaction of aryl terminal alkynes with alkynyl hypervalent iodine(?)reagents presents a new strategy for the synthesis of unsymmetrical 1,3-diynes in the presence of AgOTs and 1,10-phenanthroline(Phen).With the aid of DFT calculations,the present study systematically investigated the mechanism of the target transformation.The results show a new Au/Ag co-catalyzed?-activation mechanism for formation of the unsymmetrical 1,3-diyne,which is remarkably different from the previously proposed redox mechanism by experimental authors.The new mechanism emphasizes that a silver(?)complex with a Phen ligand performs the C(sp)-H activation of terminal alkyne to generate the active silver acetylide intermediate,which then nucleophilically attacks the C? atom of the gold(I)-coordinated alkynyl hypervalent iodine reagent in the anti-mode to give the gold vinyl intermediate,and subsequently the unsymmetrical cross-coupling product is formed via a concerted a-elimination and 1,2-phenyl migration step.The calculated barrier is 24.5 kcal/mol for formation of the unsymmetrical 1,3-diyne.In comparison,the formation of the symmetrical 1,3-diyne follows the redox mechanism with a comparatively high barrier of 29.0 kcal/mol.It is confirmed that the alkynyl hypervalent iodine(III)reagent plays dual roles:as an alkyne surrogate and also a chemical oxidant.In addition,the theoretical results reveal the vital roles of AgOTs and Phen,and rationalize the experimental observations of the decisive product originating from unsymmetrical cross-coupling reaction.This work has been published on Catalysis Science&Technology(2019,9,4091-4099).