The Mechanism Investigation Of Photoreactions Enabled By Uranyl Catalysis

Recently,attention to chemical researchers paid on the development of different photocatalysts to meet the demands of synthesis was significantly increased,owing to the greener and safer property of the photocatalysis strategy.The uranyl cation(UO₂²⁺),as an ideal photocatalyst,was excited with blue light irradiation through ligand-to-metal charge transfer（LMCT）,producing the excited state of uranyl cation(*UO₂²⁺),which is chemical inert toward oxygen molecules,possesses a long half-life up to microseconds,high oxidation potential（E^o=+2.6 V vs SHE,standard hydrogen electrode）and strong ability of hydrogen atom abstraction（BDE C-H>100 kcal/mol,bond dissociation energy）,quenched by substrates via single electron transfer（SET）or hydrogen atom transfer（HAT）.In addition,uranyl peroxide dimer could be obtained from UO₂²⁺with water-splitting under visible-light irradiation.Based on the particular properties of UO₂²⁺,the strategies of Carbon-Nitrogen（C-N）bond,Carbon-Boron（C-B）bond and Carbon-Hydrogen（C-H）bond activation have been developed.（1）From Aniline to Phenol:Carbon-Nitrogen Bond Activation via Uranyl PhotocatalysisC-N bond activation in unprotected anilines,via uranyl photocatalysis of the synergistic effect of SET and oxygen atom transfer（OAT）at ambient environment,has been developed to afford diverse mono-/polysubstitution phenols,including the late-stage modification of natural products and pharmaceuticals.The efficiency of flow reactions were higher than in tube which has been increased up to more than300-fold.The mechanistic studies with radical quenching experiments,UV-visible experiments,fluorescence quenching experiments and ¹H NMR tracking experiments demonstrated that*UO₂²⁺,generated from UO₂²⁺under blue light irradiation,was quenched by protonated anilines through SET mode.¹⁸O labelling experiments and¹⁵N tracking experiments unambiguously depicted that the oxygen source came from water,via water-splitting enabled by UO₂²⁺under visible light irradiation,and the amino group leaved as ammonium salt.Both on-off experiments and quantum yield experiments revealed the existence of a radical chain propagation process during C-N activation.（2）From Boron to hydroxyl:the Oxidation of Carbon-Boron Bond via Uranyl PhotocatalysisC-B bond activation in organoboride,via uranyl photocatalysis of indirect SET mode with triethylamine（Et₃N）as electron donor and oxygen molecules（O₂）as electron acceptor,has been developed to afford multifarious phenols and alcohols,including the late-stage modification of functional molecules.The success of flow operation demonstrated the great potential for industrial application of the strategy.¹⁸O labelling experiments depicted that the oxygen source came from O₂.Combining the experiments of radical trapping and stern-volmer analysis,UO₂²⁺was indicated to be quenched by Et₃N other than organoboride via SET mode,which was further confirmed by mass spectrometry experiment.（3）Stepwise Benzylic Oxygenation via Uranyl PhotocatalysisStepwise oxygenation of primary,secondary,and tertiary benzylic molecules was comprehensively realized under ambient conditions via activation of carbon-hydrogen and carbon-carbon bonds with uranyl photocatalysis,establishing a general strategy for construction of diverse oxidized benzylic compounds even with intense electron-withdrawing and steric hinderance effects.The accuracy of strategy was ensured by the adjustability of UO₂²⁺catalytic activity with additives and solvents,which was disclosed by fluorescence quenching experiment.Kinetic studies indicated that*UO₂²⁺acted on benzylics via HAT processes.Flow apparatus was setup proving the applicability of this oxygenation strategy.