Theoretical Studies On Transition Metal Catalyzed C-H Activation In Several Important Organic Reactions

Organometallic chemistry is a cross discipline of organic chemistry and inorganic chemistry,as a new subject,the development of organometallic chemistry has aroused widespread concern.As the core of the development of organometallic chemistry,the transition metal organic compounds has become a new hot research field of organic synthesis.Transition metal organic compounds have been widely used in new energy,nano science,life science,the field of medical science.Compared with the experimental study of transition metal organic compounds,the theoretical research lags behind.Some observed phenomena cannot be explained with conventional chemical knowledge.In addition,the molecular mechanism is not clear,and the reaction intermediate can not be tested,these problems will limit the development and application of the new organic transition metal catalyst.Therefore theoretical study has the profound significance to the development of organometallic chemistry.The research on transition metal catalyzed C-H activation generating C-C and C-X bond has become a new hot spot in the field of organic synthesis.Substrate scope is wideand does not needprefunctionalization.Methodof substrate C-H activation can simplifies the synthesis process of chemicals,natural products,agricultural chemicals,polymers,and bulk chemicals.Although C-H activation has made great progress,the catalysts used in these reactions are noble metal catalysts,such as rhodium,ruthenium.palladium,iridium,platinum,nickel.In addition,this kind of reaction needs external oxidant,and external oxidant produces a by-product during the reaction.In order to overcome these disadvantages,people began to look for suitable internal oxidants and new catalystsfor C-H activation reaction.In this dissertation,on the basis of the relevant experimental background,theoretical studies of the mechanism of several organic reactions catalyzed by transition metals,such as Ru,Rh,Co and Fe have been discussed.The microscopic nature of the catalytic reaction is revealed,and the elementary steps involved in these reactions and the thermodynamic and kinetic properties of these reactions are also given.Some achievements have been obtained in the theoretical calculation,and deepened understanding of related chemical reactions and chemical phenomena,which provide an important theoretical guidance for the design of new organic reactions and new organic catalysts.The research contents and innovations of this dissertation are summarized as follows:1.Recent experimental studies on metal complexes of N-heterocyclic carbenes?NHCs?showed a few examples of the unique sequential activation of two C-H bonds of NHC N-methyl group on triruthenium carbonyl clusters,however,the corresponding theoretical calculations showed that the reactions are highly endergonic?>40 kcal/mol?and involve very high barriers?up to-80 kcal/mol?,failing to rationalize the experimental observations that the reactions were carried out under mild conditions?<100 ??.The present work re-examines a representative example of the sequential activation of the two C-H bonds with the aid of density functional theory calculations.From the present results,the reaction is predicted to be endergonic by only 14.7 kcal/mol with an overall barrier of 35.1 kcal/mol,which is in good agreement with the experimental observations that the reaction smoothly proceeds under thermal reaction conditions and that the reversed transformation from the product to reactant occurs when carbon monoxide was gently bubbled into the reaction system at room temperature.2.Theoretical study of the mechanism of twosuccessive N-alkyl C-H bond activations ona phosphine-tethered N-heterocyclic carbene ona triruthenium carbonyl clusterhas been performed by density functional theory?DFT?calculations,and the root causes of selective activation of alkyl groups were analyzed.?1?Themechanism details of two successive N-methylene C-H bond activations on a phosphine-tethered N-heterocyclic carbene on a triruthenium carbonyl cluster been investigated by DFT calculation.Recently the cabeza s group reported a unique successive N-methylene C-H bond activation on N-heterocyclic triruthenium carbene complexes.Here,density functional theory calculations have been performed on this reaction to clarify the molecular-level mechanism of the two C-H bond activations.The calculated results indicated that the reaction occurs sequentially through the following steps:phosphine ligand dissociation from the Ru center followed by rearrangement of ligands on Ru center,the first C-H bond oxidative addition to Ru,the elimination of the first CO ligand with recoordination of the phosphine ligand,the second CO ligand elimination followed by the second C-H bond activation,and hydride migrations between Ru centers.The rate-determining step is the first C-H bond activation,which needs to overcome a barrier of 37.9 kcal/mol.Such a barrier seems to be somewhat higher than expected for the reaction under consideration which was carried out at 66 ?.However,it should benoted that the overall process involves two CO-elimination steps that are irreversible,because the reaction was performed in an open system,where the released CO was continuously purged with an inert gas.Thus,although the barrier of the reaction is relatively higher,the irreversible elimination of CO from the system drive the reaction proceed smoothly.?2?With the aid of DFT,we studied the mechanism of two successive N-methyl C-H bond activations on a phosphine-tethered N-heterocyclic carbene on a triruthenium carbonyl cluster and the thermodynamics and dynamics properties of reaction.Recently the Cabeza's group reported a unique successive N-methyl C-H bond activation on a phosphine-tethered N-heterocyclic triruthenium carbene complexes.With the aid of DFT calculations,we studied the mechanism of two successive N-methyl C-H activation reaction at the molecular level.The calculated results showed that the phosphine migration occurs prior to the C-H activation.CO ligand dissociation is an essential process for the C-H bond activation.The first C-H bond activation is the rate-determining step,which needs to overcome an overall barrier of 39.0 kcal/mol.NBO analysis indicated that the electron density of the methyl group carbon atom attached to the N-heterocyclic carbene is higher than that of the ethylene carbon atom,making the C-H bond of the methyl group more easier to be activated.3.The possible pathways of iron catalyzed dehydrogenation of formic acid in the presence of one molecular lewis acid?LA?and two molecular lewis acid?LA?have beeninvestigated by DFT calculation.The calculated results showed that dehydrogenation of formic acid assisted by two molecular lewis acid?LA?is more kineticfavorable than that assisted by one molecular lewis acid?LA?.The reaction process is as follows:absence of H₂,release of CO₂ accompanied by regeneration of catalyst.The overall maximum on thefree energy surface corresponds to the release of CO₂ with an overall barrier of 23.5 kcal/mol.The lewis acid?LA?can effectively increase the positive charge on the carbonate,and make the transition state of intramolecular rearrangement and the H-bound formateintermediate more stable.which can facilitate P-hydrideelimination and reduce the overall barrier of the release of CO₂.The theoretical results are in good agreement with the experimental observations,and deepen the understanding of the dehydrogenation reaction of formic acid under the conditions of lewis acid.4.The mechanism of cobaltcatalyzed synthesis of indole from hydrazine and alkyne has been studied by performing the density functional theory calculations.According to the experimental prediction mechanism,the pathway,i.e.for C-H activation followed by the alkyne insertion is kinetically unfavorable,because the calculated results showed that the free energy for the insertion of alkyne into the Co-C bond is calculated to be 55.0 kcal/mol.On the basis of the above results,a new mechanism is proposed,and the reaction proceeds as follows:C-H bond activation,deprotonation of the ester-bonded N-Hbond,alkyne insertion,protonmigration,N-C reductive elimination.N-N oxidative addition,protonation.The stable intermediate formed by deprotonation of the ester-bonded N-Hbondplays an important role in the reaction.which can effectively reduce the transition state free energy for alkyne insertion.C-H activation is the rate-determining step,and the overall barrier is calculated to be 31.0 kcal/mol.the calculated results are in good agreement with the isotope-labeled results.The present results not only provide the mechanism details at the molecular level,but also deepen our understanding of the cobaltcatalyzedC-H activation of hydrazide with alkyne under the redox neutral conditions,and provide theoretical guidance for the related experiments.5.This work presents a computational study of the Rh?-catalyzed synthesis of 1.2-benzothiazines from NH-sulfoximines and diazo compounds.Focusing on several representative reactions reported by Bolm et al.?Angew.Chem.Int.Ed.2015.54.12349-12352?.we show the mechanism details of the reaction and the origins of the substituent group effect and regioselectivity.The reaction is indicated to occur through four sequent processes:elimination of dinitrogen,C-H activation,carbene insertion,protonation,and dehydration,and the C-H activation is identified as the rate-determining step with a barrier of 37.2 kcal/mol.The effects of different substituent groups of NH-sulfoximines have been investigated,such as the reactions of phenyl sulfoximine.methoxylbenzene sulfoximine and nitrobenzene sulfoximine with ethyl diazoacetoacetate,and the calculated results showed that differentbarrier of different substituent groups for C-Hactivation results from electronic effect.Theregioselectivity of reaction mainly originate from electronic effect as well as the strong noncovalent interactionrather than steric effect hindrance.The present study rationalizes the experimental observations andprovides valuable guidance for the further research.