Study On Functionalized Ionic Liquids As Catalysts For [2+2] Photocycloaddition

Ionic liquids as a novel environmentally friendly reaction medium have beenreceived considerable attention, and their applications have been concentrated onorganic synthesis, separation processes, functional material, biological medicine, etc.Ionic liquids have many especial physical properties, such as low volatility, highstability and higher light transmission, and been considered as novel solvents in [2+2]photocycloaddition. Meanwhile, Ionic liquids provided a useful extension to the rangeof reaction mediums that are available for [2+2] photocycloaddition in solid state. Inthis dissertation, ionic liquids were designed for specific photochemical function, suchas reagents and photosensitizers, and focused on the synthesis ofphenacylimidazoliums (PIm) with a photosensitizing chromophore, photochemicalproperties and their applications on [2+2] photocycloaddition. The results will providethe experimental and theoretic basis on the applications of functionalized ionic liquidsand the synthesis of cage compounds.Study on the design, synthesis and photochemical properties ofphenacylimidazoliums (PIm). PIm were synthesized by covalent attachment of1-methylimidazole to aryl ketones, and generated as photosensitizers for [2+2]photocycloaddition. The photostability of PIm was investigated by comparison oftheir structures differences in ground and excited state which were provided byquantum chemistry calculation (density functional theory). The molar absorptioncoefficient of PIm, representing their capacity of light absorbtion, was determined bythe UV-vis absorption spectra. In order to investigate their characteristics asphotosensitizer, the laser flash photolysis experiments were used to study the structurecharacteristics and the triplet lifetimes by the transient absorption spectra andtime-resolved absorbances. The results demonstrated that the photochemicalproperties of PIm were favourable for photoreactions, and they hold promise asphotosensitizers for [2+2] photocycloaddition.Study on phenacylimidazoliums (PIm) as photosensitizers in the [2+2]photocycloaddition of1,4-dihydropyridines. The effects of PIm on thephotocycloadditions were investigated on the optimization of the influence factors aslight source, mode of irradiation and photosensitizer. The [2+2] photocycloadditionsof1,4-dihydropyridines were carried out by irradiation with a250W mediumpressure mercury lamp, in the presence of0.05equiv of PIm. The cage-dimeric3,9-diazatetraasteranes were synthesized by the [2+2] photocycloaddition of1,4-dihydropyridines catalyzed by PIm. The fluorescence quenching and laser flashphotolysis experiments of PIm/1,4-dihydropyridine were performed to determine theformation of intermediate in the proceeds by the transient absorption spectrum and the fluorescence-quenching rate constants. The density functional theory was used toinvestigate the mechanisms of photocycloaddition of1,4-dihydropyridines andelucidate the similarities and differences on the uncatalyzed and PIm-catalyzedreactions. PIm assisted the stabilization of intermediates and reduced the calculatedenergy barrier. All subsequent steps were predicted to be favorable and exothermic,leading to the cage dimers, allowed to explain the efficient PIm-catalyzed effect.Study on phenacylimidazoliums (PIm) as photosensitizers in the [2+2]photocycloaddition of4-aryl-4H-pyrans. The effects of benzophenone and PIm on thephotocycloadditions were investigated on the optimization of the influence factors aslight source, mode of irradiation and photosensitizer (benzophenone and PIm). The[2+2] photocycloadditions of4-aryl-4H-pyrans were carried out by irradiation with a500W high pressure mercury lamp in methanol, in the presence of0.05equiv of PIm.The cage-dimeric3,9-dioxatetraasteranes were synthesized by the [2+2]photocycloaddition of4-aryl-4H-pyran. The laser flash photolysis experiments anddensity functional theory have been performed to rationalize the mechanism ofbenzophenone and PIm photosensitized photocycloadditions. Triplet energy transferoccurred from triplet benzophenone and PIm to4H-pyran, aiming to promote theformation of the lowest excited triplet4H-pyran, while PIm could efficiently inhibitthe Paternò–Büchi reaction.In summary, the phenacylimidazoliums were studied systematically on thesynthesis, photochemical properties and applications on [2+2] photocycloaddition.These results demonstrated that phenacylimidazoliums containing a photosensitizingchromophore can extend the application scopes of ionic liquids and provide theexperimental and theoretic basis on the synthesis of cage compounds.