A DFT Study On The Competing Mechanisms Of PPh₃-catalyzed [3+3] And [3+2] Annulations Between 5-acetoxypenta-2,3-dienoate And 1C,3O-bisnucleophiles

Organocatalysis reaction has become an important field of chemical synthesis,which is an important method to design and develop new chiral drugs,materials and so on.In recent years,many chemists are interested in the advantages of organocatalysts,and organocatalysis has received considerable attention.The interest in this field has increased spectacularly in the last few years as result of both the novelty of the concept and,more importantly,the fact that the efficiency and selectivity of many organocatalytic reactions meet the standards of established organic reactions.Organocatalytic reactions are becoming powerful tools in the construction of complex molecular skeletons.Organocatalysis as a new concept emerged at the 21 st century has been developing within organic chemistry into its own subdiscipline,and its golden age has already dawned.Actually,great progress in organocatalysis has been obtained in both theory and experiment.Theoretical studies of reaction mechanism by computational methods have been greatly facilitated by innovation of computer technology and development of the methods.The theoretical study on the reaction mechanism at the molecular level would be helpful for people to understand the complicated reaction details,obtain the structures of intermediates and transition states,explain the stereoselectivity,and thus provide insights on the ration design of novel and efficient organocatalysts.In this thesis,the competing mechanisms of PPh3-catalyzed [3+3] annulation reaction（including possible pathways a,b and c）and [3 + 2] annulation reaction（including possible pathways a’,b’ and c’）between 5-acetoxypenta-2,3-dienoate and1 C,3O-bisnucleophile and the detailed mechanisms of transamidation reaction in1,4-dioxane,have been theoretically investigated using density functional theory（DFT）.This work discovered the details of the organocatalysis reaction,which should be useful for people to understand the reaction mechanism deeply,at the same time,it provides valuable data on the rational design of organocatalyst and novel reactions.This thesis contains of five chapters as follows:In the 1^st chapter of this thesis,we briefly introduced the background oftheoretical studies of the organocatalysis reaction catalyzed by the four different types of organocatalysts,i.e.Lewis acid,Lewis base,Bronsted acid,and Bronsted base.In the 2^nd chapter of this thesis,we mainly described the generation and the common methods of quantum chemistry,which is involved in this theoretical study on the mechanism of chemical reactions.In the 3^rd chapter of this thesis,the competing mechanisms of PPh3-catalyzed [3+3] annulation reaction（including possible pathways a,b and c）and [3 + 2]annulation reaction（including possible pathways a’,b’ and c’）between5-acetoxypenta-2,3-dienoate and 1C,3O-bisnucleophile have been theoretically investigated using density functional theory（DFT）.Among the six possible pathways,our computational results indicate that pathway c is the most energy favorable pathway at acid condition,whereas pathway b’ is the most energy favorable pathway at base condition.Thus,the different acid/base condition would be the switch for the two competing reactions,which is in agreement with the experiment results.Moreover,the analysis of global reactivity indexes has been carried out to explore the role of the catalyst PPh3,which demonstrates that Lewis base catalyst PPh3 noticeably strengthens the nucleophilicity of the reactant and makes the annulation reactions easier to occur.This work should be helpful for not only understanding the role of PPh3 catalyst in the [3 + 3] and [3 + 2] annulations,but also the role of the real switch（i.e.the acid/base additives）in the competing [3 + 3] and [3 + 2] annulations,and thus provides valuable insights on the rational design of more efficient catalysts for this kind of reactions.In the 4^th chapter of this thesis,we have studied the mechanisms of transamidation reaction in 1,4-dioxane using density functional theory（DFT）.The transamidation reaction can occur easily and obtain good yields in the 1,4-dioxane solvent with catalyst-free and microwave-heating conditions.Nine reaction channels have been investigated in 1,4-dioxane using PCM model at the B3LYP/6-31G（d,p）level.The calculated results indicate the pathway 7 has the lowest energy barrier,and thus is the most favorable reaction pathway,which can give a reasonable explanation of the reaction in the experimental conditions,so this work provide theoretical guide on the development of the novel and efficient transamidation reaction.In addition,wehave also suggested and studied two channels under the Lewis acid BF3-catalyzed condition,and we found the activation energy of the reaction can be lowered significantly,which should be useful on the design of the catalyst for this kind of reactions.The 5^th chapter of thesis gives a summary for all the above works and prospect of the future work.