Cooperative Photocatalysis Of TiO₂ Semiconductor With An Electron Transfer Mediator For Selective Aerobic Oxidation Of Sulfides

In recent years,photocatalytic redox of Ti O₂ under moderate reaction conditions has sparked interest in challenging organic transformations.Nevertheless,Ti O₂ can merely be excited by ultraviolet light(?<400 nm),which limits the progress of this type of reaction.Adsorbing an organic molecule on the surface of Ti O₂ to expand its absorption range for visible light can be a promising strategy to address this problem.Here,a variety of organic molecules containing?-conjugated systems were designed to combine with Ti O₂ to form composite photocatalysts,which were then applied to the photocatalytic selective oxidation of sulfides.Meanwhile,it is detected that the addition of a catalytic amount of electron transfer mediator can ensure the stability of the anchored organic molecules,thereby resulting in the visible light-induced selective aerobic oxidation of sulfide with O₂.The detailed research results are indicated below:(1)With salicylic acid(SA)as the original molecule,1-hydroxy-2-naphthoic acid(1H2NA)and 2-hydroxy-1-naphthoic acid(2H1NA)were afforded when another benzene ring was separately extended at 3,4 or 5,6 positions of the aromatic ring.Then these molecules were assembled with Ti O₂ for the selective oxidation of sulfides.By contrast with SA,2H1NA slightly improved,while the modification effect of 1H2NA on anatase Ti O₂ was significantly enhanced.Meanwhile,(2,2,6,6-tetramethylpiperidin-1-yl)oxyl(TEMPO)was introduced into the system as an electron transfer mediator.The selective oxidation of sulfides driven by blue light with air as the oxidant can be successfully realized.(2)With benzoic acid(BA)as the initial molecule,horizontally extending one or two benzene rings obtained 2-naphthoic acid(2-NA)and 2-anthracene acid(2-AA).Then violet light-induced selective oxidation of sulfides was designed as a probe reaction to verify the photocatalytic activity of Ti O₂ complexes.In addition,triethylamine(TEA)as an electron transfer mediator was introduced to maintain the stability of the anchored aromatic acids.Among the three aromatic acids,2-AA delivered the best result over Ti O₂ due to the most extensive?-conjugated system.Finally,an efficient system by cooperative 2-AA–Ti O₂ photocatalysis with 10 mol%TEA catalysis is established to explore the selective aerobic oxidation of sulfides into sulfoxides with air under violet light irradiation.(3)With 1,2-dihydroxynaphthalene(12DHN)as the original molecule,different ligand molecules were designed in the presence of the?-conjugated system by changing the position and type of the hydroxyl group or expanding the size of the?-conjugation.Then,assembling these organic molecules with Ti O₂ for the photocatalytic oxidation of sulfides.Meanwhile,TEMPO was introduced as an electron transfer mediator in the reaction.Among these?-conjugated molecules,2-AA has the best effect.Finally,an efficient system by cooperative 2-AA–Ti O₂ photocatalysis with 5 mol%TEMPO catalysis is established to explore the selective aerobic oxidation of sulfides into sulfoxides with air under blue light irradiation.In conclusion,a variety of?-conjugated molecules have been designed to assemble with Ti O₂.Then three visible light-induced selective aerobic oxidation systems of sulfides were successfully constructed in the presence of an electron transfer intermediary.This work affords an extending strategy for designing the next-generation ligands for semiconductors to expand visible light-induced selective reactions.