Theoretical Studies On Several Important Chemical Reactions Catalyzed By Imidazolium-Based Ionic Liquids

Ionic liquids(ILs),a new type of compounds,consist of ions and are liquids at room or near room temperature.They have many advantages,such as odorless,non-flammable,low vapor pressure,wide liquid range,adjustable strength,high conductivity,recyclability,flexibility,design ability and so on.So far,ILs has been widely applied in electrochemistry,organic chemistry,materials science,biomass conversion,separation and purification and other research fields.In recent decades,experimental and applied researches using ILs as solvents and catalysts have appeared in an endless stream,and they have made great progress.In contrast,related theoretical research is lagging behind.The understanding of some important organic reaction mechanisms catalyzed by ILs is not clear.Some interesting experimental phenomena and results cannot be understood well using generally chemical knowledges.This hinders the development and utilization of new and efficient ILs catalysts to some extent.Therefore,the theoretical research on catalysis of ILs and the microscopic nature behind macroscopic experimental phenomena have important scientific significance and application prospects.In this dissertation,the thermal stability of imidazolium-based IL and a series of IL-catalyzed organic synthesis and biomass conversion were investigated by quantum chemistry calculations based on the related experimental studies.This paper reveals the influence of the structure of IL on its thermal stability,and illuminates the microscopic nature of some important ILs-catalyzed reactions.Some experimental phenomena have been explained well,and a series of innovative research results have been achieved.These detailed theoretical studies deepened the understanding of the thermal stability of ILs and the ILs-catalyzed chemical reactions.It is expected that these results would provide theoretical guidance for the design of efficient IL catalysts.The main contents and the obtained innovative results of this paper are summarized as follows:1.We investigated the thermal stability of imidazolium-base ILs using density functional theory calculations.The possible decomposition mechanisms of[EMIM]Cl are discussed,and the calculated results show the major decomposition mechanism refers to a bimolecular nucleophilic substitution in which the anion attacks the methyl of the cation to form chloromethane and ethylimidazole.The structures of imidazolium-base ILs have important effect on their thermal stability.The nucleophilicity of anions play a critical role in the decomposition of ILs,[EMIM]Cl shows higher thermal stability than[EMIM]Br and[EMIM]I.Substituents of cation have influences on thermal stability of imidazolium-base ILs.Electron-withdrawing substituents at C4 of cation lead to a decreased thermal stability,while electron-donating substituents have an opposite effect.Increasing the alkyl chain length leads to the decrease of the thermal stability.In general,imidazolium-based ILs with branched alkyl is slightly more stable than their counterparts with linear alkyl.Moreover,it is found that imidazolium-based ILs containing unsaturated side chains possess universally lower thermal stability than their analogues having fully saturated groups,and the thermal stability increase with increased the distance between the unsaturated bond and the nitrogen atom of cation.2.Density functional theory calculations have been carried out on a model system which describes the catalytic reaction by CrCl2 in 1,3-dimethylimidazolium chlorine([MMIM]Cl)IL.The reaction is shown to involve three fundamental processes:ring opening,1,2-H migration,and ring closure.The reaction is calculated to exergonic by 3.8 kcal/mol with an overall barrier of 37.1 kcal/mol.Throughout all elementary steps,both CrCl2 and[MMIM]Cl are found to play substantial roles.The Cr center,as a Lewis acid,coordinates to two hydroxyl group oxygen atoms of glucose to bidentally rivet the substrate,and the imidazolium cation plays a dual role of proton shuttle and H-bond donor due to its intrinsic acidic property,while the Cl-anion is identified as a Bronsted/Lewis base and also a H-bond acceptor.Our present calculations emphasize that in the rate-determining step the 1,2-H migration concertedly occurs with the deprotonation of 02-H hydroxyl group,which is in nature different from the stepwise mechanism proposed in the early literature.The present results provide a molecule-level understanding for the isomerization mechanism of glucose to fructose catalyzed by chromium chlorides in imidazolium chlorine ILs.3.The reaction mechanism of glucose-to-fructose isomerization catalyzed by MnCl2 and 1-methyl-3-(3-sulfobutyl)-imidazolium methylsulfonate([C4SO3HMIM][CH3SO3])ionic liquid(IL)in 1-butyl-3-methylimidazolium chloride([BMIM]Cl)IL was investigated computationally.The calculation results discovered an updated mechanism involving a concerted 1,2-H migration with an overall free energy barrier of 33.0 kcal/mol.Note that the present 1,2-H migration proceeded simultaneously with two proton transfer processes:the proton transfers from the hydroxyl oxygen of glucose to the deprotonated[C4SO3HMIM]+ cation and another migrates from the protonated[CH3SO3]-anion to another hydroxyl oxygen of glucose.Mn center represents a Lewis acid to stabilize the intermediates and transition states by bounding to glucose throughout the catalytic process.The cation of[C4SO3HMIM][CH3SO3]IL and its counteranion coordinating with Mn center both play substantial roles in a series of proton transfer processes.Additionally,the cation of[BMIM]Cl is critical in providing a polar environment together with the counteranion coordinated with the Mn center to stabilize all structures.The present results could provide help for rationalizing the mechanism of glucose-to-fructose isomerization catalyzed by metal salts and SO3H-functioned ILs in imidazolium-based ILs,and give guidance for the exploitation of more efficient catalysts for biomass conversion.4.DFT calculations have been conducted to gain insight into the mechanism and kinetics of the esterification of a-tocopherol with succinic anhydride catalyzed by a histidine derivative or an imidazoliumbased IL.The two catalytic reactions involve an intrinsically consistent molecular mechanism:a rate-determining,concerted nucleophilic substitution followed by a facile proton-transfer process.The histidine derivative or the IL anion is shown to play a decisive role,acting as a Bronsted base by abstracting the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of a-tocopherol on succinic anhydride.The calculated free energy barriers of two reactions(15.8 kcal/mol for the histamine-catalyzed reaction and 22.9 kcal/mol for the IL-catalyzed reaction)together with their respective characteristic features,the catalytic reaction with a catalytic amount of histamine vs the catalytic reaction with an excessed amount of the IL,rationalize well the experimentally observed kinetics that the former has faster initial rate but longer reaction time while the latter is initiated slowly but completed in a much shorter time.5.A DFT study has been performed on the[C3SO3HMIM][HSO4]IL-catalyzedβ-O-4 linkage cleavage of the lignin model compound,veratrylglycerol-β-guaiacyl ether(VG).The calculated energetically viable pathway consists of five stages:(1)protonation-dehydration,(2)β-H elimination,(3)protonation,(4)hydroxylation,and(5)β-O-4 bond cleavage.The initial deprotonation-dehydration process with a free energy barrier of 29.7 kcal/mol is identified as the bottleneck step.Throughout all elementary steps,both the cation and anion of the IL are fund to play substantial roles.The cation acts as a Br(?)nsted acid,while the counteranion plays a dual role of Bronsted base and proton shuttle.The identified mechanism refers to an unconventional El elimination,which preferably leads to a(Z)-olefin derivative,i.e.(Z)-enol-ether,rather than its(E)-isomer.Distortion/interaction analysis demonstrated that the deformation of the VG moiety,which is induced by the orbital interaction between the developing sp2 hybridized Cβ atom and the β-O atom,controls the stereoselectivity of the El elimination.Our study provides detailed mechanistic insights for the cleavage of β-O-4 linkage in lignin promoted by[C3SO3HMIM][HSO4]IL,and guides the design of efficient functionalized imidazolium IL catalysts for lignin depolymerization.6.The mechanism of the synthesis of pyrano[3,2-c]coumarin catalyzed by H2SO4 and[BSMIM]OTs IL have been investigated theoretically.The calculation results show that the mechanisms of the two catalytic reactions are similar,which involve three stages:1)Michael addition,2)cyclization,and 3)dehydration.H2SO4 and the cation of[BSMIM]OTs play the dual roles of H-shuttle and Bronsted acid in the reaction.The anion of[BSMIM]OTs functions as Br(?)nsted base and H-bond donor.The difference of energy barriers between the two catalyzed reactions(34.9 vs 32.1 kcal/mol)explains the differences of their experimental yields(43%and 84%)qualitatively.The electron delocalization of the reaction site in the rate-determining transition state of the IL-catalyzed reaction is higher than that in the rate-determining transition state of the H2SO4-catalyzed reaction.In addition,the H-π and π-πinteractions existed in the rate-determining transition state lowers the barrier of the IL-catalyzed reaction,but such interactions are absence in the rate-determining transition state of the H2SO4-catalyzed reaction.The current theoretical results clarify the roles of H2SO4 and[BSMIM]OTs IL played in the reaction,essentially explained the experimentally obtained different yields of the two catalytic reaction,and provided guidance for subsequent experimental studies..