Employing Iridium Complexes As Photosensitizers To Explore Highly Efficient Visible-light-driven Cycloadditions

Visible light catalysis,having become a powerful tool in the field of organic chemistry by selectively activating specific functional groups or chemical bonds to achieve transformations that are difficult to accomplish with traditional thermal reactions,is favored by chemists.The energy transfer(En T)pathway driven by visible light achieved a series of unique reactions through indirect sensitized substrates,and attracted much attention due to its atomic economy and environmental friendliness.Although photosensitizers play a key role in photocatalytic En T pathway,the design of photosensitizers has not received deserved attention.Aiming at continuing efforts on organic photochemical transformations that fit the demand of sustainable development,we modified the ligands to optimize performance of organometallic complexes,and successfully synthesized a series of transition metal iridium complexes with long-lived excited state,high excited state energy,strong absorption and robust stability.In addition,in order to assess the efficiency of these iridium complexes,we applied them to a series of visible-light-driven cycloaddition reactions that were difficult to be performed with well-established photosensitizers,and efficiently obtained the desired cycloadducts.The corresponding photocatalytic mechanism was thoughtly studied by controlled experiments,electrochemical and spectroscopic studies,which further verified the importance of ligand design to improve the performance of photocatalyst.This dissertation mainly includes the following five aspects:1.The research progress of visible-light-driven En T reaction in recent years is summarized,mainly including the research significance,reaction mechanism and main types.In particular,the application of cycloaddition reactions through visible-light-driven En T pathway and the project design are described concisely.2.Compared with traditional organic molecular photosensitizers,transition metal complexes,such as ruthenium or iridium complexes,have stronger absorption and higher stability,and their ligands can be designed to adjust the redox potentials and triplet state energies.Therefore,we synthesized a series of iridium complexes and characterized their spectroscopic properties.Some iridium complexes have longer lifetime and higher energy on charge-transfer excited states than that of well-established photosensitizers.3.To evaluate the catalytic efficiencies of iridium complexes,high triplet-energy heterocyclic olefin maleimides,which were difficult to be sensitized with common photosensitizers,were selected as energy receptor.And the thiocarbonyl compounds containing unsaturated C=S were used as coupling partners.The thia-Paternò–Büchi reaction between the C=C bond and C=S bond under visible light was realized for the first time,and the corresponding mechanism was discussed by electrochemical and spectroscopic measurements.Quantum yield control experiments show that the iridium complexes designed are more efficient than the current excellent photosensitizers.4.In order to further investigate the general applicability of iridium complexes in photocatalysis,we used the same catalytic system to introduce unsaturated C=N imine as coupling partners,and realized the unprecedented intermolecular cycloaddition reactions via visible-light-driven En T pathway.And we also achieved multicomponent one-pot aza-photocyclization through in situ generated imines formed by corresponding anilines and aldehydes.Finally,the mechanism was verified by the control experiment and spectroscopy analysis.5.The designed iridium complexes were served as photosensitizers for [2+2]cycloadditions between maleimines and unsaturated alkenes/alkynes to achieve3?Azabicyclo[3.2.0]heptane scaffolds.Compared with reported photosensitizers,the designed irdium complexes have better photocatalytic performance,such as low loading,high efficiency,high yield,wide range of substrate scope and good tolerance of functional groups,etc.6.In this chapter,the research of visible-light-driven reactions via En T pathway in this thesis is summarized,and the design of En T photosensitizer and the future development of visible-light-driven En T reactions are prospected.