Templating Synthesis Of Pillar[6]arenes And Their Host-Guest Complexes

As the fifth generation of macrocyclic molecules,pillar[n]arenes have attracted much attention due to their perfect rigid columnar structure and unique host-guest chemical properties.Since pillar[n]arenes were reported in 2008,they have been found to have excellent application in supramolecular assembly,ion and molecular recognition,separation and purification of similar compounds,etc.Therefore,they play an increasingly important role in the study of supramolecular chemistry and are now one of the hot spots in supramolecular chemistry.In the synthesis of pillar[n]arenes,pillar[6]arene is the thermodynamic unstable product of the cyclization reaction due to its structural characteristics,which resulted the low yield of the product and the hard control reaction conditions.Therefore,it is still a concern of chemists to find a universal and simple method for the selective synthesis of pillar[6]arene.In this thesis,a simple and efficient method for the synthesis of pillar[6]arene was developed,and at the same time new host-guest complexes with special host-guest binding patterns were observed.The structure and host-guest binding were investigated in detail.In the second chapter,the thesis explored the general applicability of chlorocyclohexane as a template for the synthesis of pillar[6]arene with various substituents.The synthesis yield of linear-alkoxy pillar[6]arene is as high as 50%with more than 99%selectivity.Larger bromocyclohexane and methylcyclohexane cannot be used as templates to synthesize pillar[6]arene.At the same time,unexpected substance was isolated from the product,which shows different but similar behavior to the targeted pillar[6]arene.Multiple techniques were used to characterize this substance,including nuclear magnetic resonance analysis,infrared analysis,thermal analysis,X-ray diffraction analysis and so on.The results showed that the product is a stable host-guest complex formed by pillar[6]arene and chlorocyclohexane.Surprisingly,the complex is only formed during the synthesis of pillar[n]arenes and cannot be made by mechanically mixing the host and guest molecules in solution.And once the complex is formed,it hardly dissociates in solution at room temperature,which shows exceptional stability.This behavior is similar to the "molecular prison",that is,the binding mode of a closed molecular container and a guest molecular.The difference is that the solution containing the host-guest complex can be partially dissociated at high temperature.The X-ray single crystal structure shows that the complex is composed of a pillar[6]arene host containing two chlorocyclohexane guests.The chlorocyclohexane molecules form dimerization through van der Waals interaction.The geometric configuration matching between the guest dimer and the host cavity as well as the complementary surface electrostatic potential are the main reasons for the stable existence of the complex.Density functional theory calculations show that the binding energy between the host and the guest molecule is as high as 193.95 kJ·mol-1,which also reasonably explains the high stability of the composite.In addition to expanding our understanding of supramolecular host-guest binding mode,this result also provides clues to the cyclization process of the template reaction:on the one hand,the guest and chain oligomers combined through intermolecular interactions firstly,and then the chain oligomer is cyclized under the action of the guest template;on the other hand,pillar[n]arene homologues appear in the product first,and then under the action of the template,it transformed into a form that can be combined with it to form a more stable host-guest complex,thereby increasing the yield of specific products.In the third chapter,the thesis further studied the template synthesis of pillar[6]arene,and achieved the efficient and selective synthesis of pillar[6]arene by introducing iodine in the synthesis without the addition of catalyst.After the optimization of temperature,solvent,substrate concentration,iodine concentration and reaction time,the final yield is as high as 92%,and the selectivity of pillar[6]arene is higher than 99%.DFT simulations revealed that in the condensation reaction between 1,4-dialkoxybenzene and formaldehyde,the carbonyl group is activated by I2 through the forming of I···O halogen bond with formaldehyde,thereby initiating the coupling of the C-C bond.In the cyclization process of oligomers,due to high matchment of the diameter of the iodine molecule with the size of the cavity of pillar[6]arene,I2 acts as a template to selectively generate pillar[6]arene through the ?…?-?…? halogen bonds.In view of the hydrophobic nature of the halogen bond,this strategy is not sensitive to water vapor and can be carried out smoothly in a solution containing 5%water,and the yield and selectivity are not significantly affected.This method has the advantages of simplicity,low cost,good selectivity,high yield and universal applicability,and provides a simple method for the synthesis of widely used pillar[6]arene.The last chapter summarizes this thesis and looks forward to the application of halogen bonds in macrocyclic synthetic chemistry.The high degree of directionality,moderate strength and hydrophobicity of halogen bonds can be used as a powerful supplement to hydrogen bonds in organic synthesis,materials chemistry and life sciences.