Photocatalyzed Coupling-Oxygenation Cascades For Syntheses Of Cyclic Compounds

Solar energy and oxygen are costless,green and valuable reagents for chemical reactions.They have long been explored by chemists to realize challenging organic syntheses.However,due to the high reactivities,especially in the presence of O₂,the active radical intermediates which are formed by the reactions of the substrates and the photo-excited photocatalysts would be very rapidly oxygenated or further oxidized and thereby undergo C-C cleavage pathways.To achieve highly selective and highly effective photocatalytic syntheses reactions,one should take into the account of many factors comprehensively,such as the structures of photocatalysts,the redox potentials of substrates,the polarity of solvent,the interaction mode between catalysts and substrates,and the reaction kinetics.In this PhD thesis,we have focused in the establishment of the photocatalytic reaction cascades,i.e.,the coupling reaction followed by the oxygenation reaction,for the efficient construction of clylic compounds like aryltetralones,endoperoxides,oxazole aldehydes and indole aldehydes from simple substrates.This new strategy has allowed us to overcome the undesired overoxidation of the substrates in the oxidizing conditions of the aerobic solar syntheses.The contents of this dissertation are divided into the following chapters:Chapter I.We briefly introduce the storage and utilization of solar-light,basic concepts about photocatalysis,the classification of the photocatalysts,and the general photocatalytic mechanism.Then we reviewed the recent progresses of the diverse organic reactions using heterogeneous semiconductors and homogeneous molecular photocatalysts,with the aim of discussing the advantages and the shortages of the published strategies.At the end of this chapter,we make a brief perspective on the literature and highlight the scientific significance of this dissertation.Chapter II.We report the TiO₂ photocatalytic cyclization-oxygenation reaction of styrenes for the syntheses of aryltetralones.This efficient photocatalytic system was performed using sunlight as the light source and O₂ as the only oxidant,without any additive.Excellent diastereoselectivities are achieved for intramolecular reactions.Good tolerance to the functional groups is observed.1,2-bis（aryl）cyclobutane,the intermediate of the cascade reaction,is generated in the early stage and can be converted to the corresponding aryltetralone in>90%yields.Details on the effects of solvent polarity,the characteracterization of the active intermediates,the effects of quenching reagents,the source of oxygen,and the reaction mechanism are reported.Chapter III.In order to succeed in the TiO₂ photocatalyzed cyclization of 1,2-bis-substituted ethylene,two approaches are recommended to compensate the steric hinderance of the β-substituent of the substrate.（1）By using the intramolecular alkene cyclization reaction which is greatly beneficial to the nucleophilic addition kinetics.（2）By using the[2+2]cycloaddition products of alkenes as the substrates for TiO₂ photocatalysis.A variety of aryltetralones are then successfully synthesized in good yields.This approach was then successfully applied to synthesize the natural bioactive lignan,（-）-8’-epi-aristoligone,in 22%yield.Chapter IV.I₂/visible light photocatalysis is applied in the syntheses of oxazole aldehydes and indole aldehydes from corresponding propargylic amides.These photocatalytic reactions were carried out with catalytic amounts of I₂ and O₂ as the only oxidant,without any additives or cocatalysts.Due to the π-Lewis acidity,iodine quickly adds to the C≡C bond,resulting in the rapid electrophilic cyclization of the propargylic amide to give a vinyl-iodide intermediate.Regeneration of I₂ involves both the oxidation of iodide ions and the oxidative deiodination of the vinyl-iodide intermediate,in which singlet oxygen plays a key role.The dual functions of I₂,in which iodine acts as both the photocatalyst and the π-Lewis acid,provides a simple and economic approach for the challenging organic synthesis.