Theoretical Study On The Reaction Mechanism Of Several Organocatalytic Synthesis

It is of great interest to investigate the chemical reaction anddetermine the reaction mechanism. Now we can theoretically studythe chemical reaction process via quantum chemical programs andthe knowledge of chemistry. In our work,the mechanisms of severalimportant organocatalytic Synthesis reactions, related to carbondioxide with 2,3-epoxypropyl phenyl ether catalyzed by LiBr salt,carbon dioxide with 2-aminobenzonitrile catalyzed by1-butyl-3-methyl imidazolium hydroxide ([Bmim]OH) ionic liquid, thePd-catalyzed coupling of naphthalene allyl chloride withallenyltributylstannane, and Pd-catalyzed coupling ofchloromethylnaphthalene with allenyltributylstannane, have beenextensively investigated at the density functional level of theory.The main contents and results are as follows.1. Mechanism of carbon dioxide with 2,3-epoxypropyl phenyl ethercatalyzed by LiBr salt The mechanism of carbon dioxide with 2,3-epoxypropyl phenylether catalyzed by LiBr salt has been extensively investigated. Ourcalculations reveal that the overall reaction proceeds via thefollowing elementary steps: epoxide ring-opening, carbon dioxideinsertion, and ring-closure of cyclic carbonate. Furthermore,single-point self-consistent reaction field (SCRF) calculations basedon the polarized continuum model (PCM) were performed to evaluatethe solvent effects in N-methylpyrrolidinone (NMP) for the optimizedgas-phase geometries at the B3LYP/6-31G(d,p) level. Since thering-opening of epoxide and the ring-closure of cyclic carbonatehave close activation energies, each of them can be therate-determining step with variation in the reaction conditions(temperature, pressure, and solvent). The overall reaction isexothermic. Although the solvent effects increase the activationenergies, the NMP solvent does not change the general trends forthe reaction potential energy surfaces. In addition, we have alsoconsidered the possibility of NMP as ligand for changing thereaction mechanism. The results show that the NMP as ligand doesnot change the reaction mechanism.2. Mechanism of carbon dioxide with 2-aminobenzonitrile catalyzedby 1-butyl-3-methyl imidazolium hydroxide ([Bmim]OH) ionic liquid.The mechanism of carbon dioxide with 2-aminobenzonitrile catalyzed by [Bmim]OH ionic liquid has been extensively investigated.Three mechanistic pathways are proposed and evaluated. In thefavored pathway, first the OH deprives H1 atom of2-aminobenzonitrile to strengthen its nucleophilic ability; then the[Bmim]+activates the intermediate 3 and nucleophilic cyclization of 3into 4; next the C4-C2 is rotated bond and the C1-N2 bond isformatted; finally, the product is formed. The overall reaction isexothermic. In addition, since the attack of C3 atom of NHC to C1atom from 4 to 12, the C1 O₂ bond cleavage from 12 to 13, and theC1 N2 bond formation from 13 to 14 have close activation freeenergies, each of them can determine the reaction rate with variationin the reaction conditions. Moreover, it is noteworthy that H-bondinginteractions play an important role in stabilizing these structures andtaking the reaction to the energetically favorable pathway.3. Mechanism of Pd-catalyzed coupling of naphthalene allyl chloridewith allenyltributylstannaneThe detailed mechanism of the Pd-catalyzed coupling ofnaphthalene allyl chloride with allenyltributylstannane has beenstudied. The catalyst cycle is suggested to comprise three steps:oxidative addition, transmetalation, and reductive elimination, noneof which contains significantly large barriers. The oxidative additionof naphthalene allyl chloride is confirmed to occur through monophosphine pathway. The step is the rate-determining step due tothe highest energy barrier in the overall cyclic mechanism. In thetransmetalation, start with 3a and 3b, both them take place via thesame four-membered-ring transition state, TS(3/4), to lead to theintermediateη³-allylnaphthalene-η³-propargylPd 4. The result revealsthat the transmetalation is a crucial step for determining thedearomatic product. This is in good agreement with the experimentfindings that propargylic dearomatic product is only the final product.Moreover, 4 is an important intermediate along the reaction pathbecause it facilitates the formation of the intermediateη¹-allylnaphthalene-η³-propargylPdPH38. Then, the reductiveelimination takes place easily along path 2. The path 2 mechanisminvolves isomerization of 8 to the intermediate 9, followed byreductive elimination through coupling of the terminal carbon of theη¹-allenyl ligand with the ortho-carbon of theη³-naphthalene ligand.Direct coupling between the naphthalene allyl chloride and allenylgroups, which gives the Stille coupling product, is found to bekinetically less favorable. It is found that dichloromethane as solventdoes not change the potential energy surface compared to that foundin the gas.4. Mechanism of Pd-catalyzed coupling of chloromethylnaphthalenewith allenyltributylstannane The Pd-catalyzed dearomatization of chloromethylnaphthalene hasbeen investigated. The calculations indicated that monophosphinecomplex is catalytically more active than bisphosphine complex foroxidative addition. The step has the highest energy barrier, thereforebecoming the rate-determining step in the overall cyclic mechanism.The unsaturated speciesη¹-methylnaphthalenePd(PH₃)(Cl) (3c) is acrucial intermediate, which facilitates the coordination ofallenyltributylstannane. The transmetalation step is a crucial step fordetermining the dearomatic product. It is found that reductiveelimination of the dearomatic products from the intermediates occursby coupling of the terminal carbons of theη¹-propargyl ligand andη¹-allenyl ligand with the para-carbon of theη³- methylnaphthaleneligand to form the corresponding allenylated and propargylateddearomatization products. In addition, various C C coupling reactionpathways have been examined.