Visible Light Photocatalytic Selective Oxidation Reactions Based On Titanium Dioxide And Conjugated Microporous Polymers

The visible light photocatalysis for selective chemical transformations,employing O₂ as oxidant,shows the advantages of sustainable energy,low cost,and environmental protection but is a very challenging undertaking.Herein,we would like to report four kinds of electron donor-acceptor(D-A)type visible light photocatalytic oxidation reactions of heteroatom organic compounds with high efficiency and selectivity over TiO₂ and conjugated microporous polymers(CMPs).First,we choose the S heteroatom of substrate thiol as electron donor and the conduction band of TiO₂ as an electron acceptor to form a surface complex as the photocatalyst,which could drive the visible light photocatalytic selective oxidation of thiols to disulfides in CH₃CN with O₂ as oxidant.The photogenerated electrons are transferred to the conduction band of TiO₂directly from thiols.O₂ takes an essential role in accepting the electrons and protons and finally becomes H₂O₂.AS a result,sulfur radical is photogenerated that reacts with a thiol to give the symmetric disulfides.Moreover,under the treatment with t-butylthiol and triethylamine present in the reaction system,the symmetric products would be transformed to asymmetric disulfides.Excellent reaction selectivity,functional group tolerance,and substrate scope with high isolated yields have been achieved with this protocol.Then,we select organic dye alizarin red S(ARS)as electron donor and the conduction band of TiO₂ as an electron acceptor to form ARS-TiO₂ to carry out the visible light photocatalytic synthesis of imines via a two-step one-pot route.This efficient method involves one step of the highly selective formation of aldehydes from the oxidation of alcohols with O₂ over ARS-TiO₂ photocatalyst,and a subsequent condensation step formed aldehydes with various amines on TiO₂ to afford imines in one pot.Anatase TiO₂ provides the versatile platform for the above course:1)facilitating the electron transfer from dye traversing its conduction band to O₂ in the photocatalytic course;2)the Lewis acid sites of TiO₂ promoting the formation of imines from aldehydes and amines in very high isolated yields.Next,we employ substrate thiol as electron donor and(2,2,6,6-tetramethylpiperidin-1-yl)oxyl(TEMPO)as an electron donor to form a new complex,which could implement a homogeneous visible light photocatalytic course in C₂H₅OH to realize the selective oxidation of thiols into symmetrical disulfides,with O₂ as oxidation at room temperature.The symmetric product is prone to form the asymmetric disulfide when t-butylthiol is introduced in the reaction system without additive.Last,we integrate carbazole electron donor and modified fluorene(gem-difluoromethylenation,dimethylation,and carbonylation on methylene bridge of fluorene),electron acceptor,to form carbazole-fluorene based CMPs as visible light photocatalysts.At first,we study the characteristic of CMPs,including morphology,pore structure,and photoelectricity.Then we implement the selective oxidations of thiols to disulfides,amines to imines,sulfides to sulfoxides by employing CMPs as photocatalysts,O₂ as oxidant under blue light irradiation.The dimethylation functionalized MFC-CMP exhibits outstanding photocatalytic reactivity and cycling utilization.And the above chemical transformations are carried out with excellent functional group tolerance and substrate scope.This work suggests that subtle tweaking in the structural skeleton could give rise to the superior photocatalytic activity for CMPs in selective chemical conversions.In summary,we have accomplished four kinds of electron donor-acceptor type visible light photocatalytic oxidation reactions based on TiO₂,TEMPO,and CMPs.Those successful reactions disclose the diversity of O₂ activation on D-A type of visible-light-driven photocatalysis.Meanwhile,these discoveries declare that selecting or tweaking either the donor or acceptor segment would improve photocatalytic activity in selective chemical conversions.