Study On Oxidative Coupling Reaction Using Carbon Nitride As Photocatalyst Under Visible Light

With the development of the direct functionalization of the C-H bonds,the third generation oxidative coupling with hydrocarbons as substrates has gradually become a new direction for the development of the coupling reaction.The reagents do not need to be further functionalized,which greatly improves the step economy of the reaction,and only two hydrogen atoms are removed after the reaction,which also improves the atom economy.After the step economy and atom economy of the reaction are optimized,the further goal is to make the reaction more efficient,green and mild.Visible-light photocatalysis is a green chemical strategy different from traditional thermal reactions.It utilizes the selective absorption of light by the substance to make visible light pass through the solvent and act on the photocatalyst,and then the substrate conversion is realized through the interaction between the excited photocatalyst and the substrate.Due to the low absorption of visible light by organic solvents,the reaction can maintain room temperature,reducing the energy consumption of the reaction,and improving the selectivity of the reaction.Graphitic carbon nitride（g-C₃N₄）is a relatively ideal heterogeneous visible light photocatalyst.It is a two-dimensional material with rich surface characteristics,which makes it have comparable catalytic properties to homogeneous photocatalysts.In addition,graphite phase carbon nitride has high chemical and light stability,realizing the recycling of the catalyst.Therefore,more and more organic chemists apply it to the organic photocatalysis.This thesis first briefly described the basic concepts and development history of oxidative coupling,and reviewed the classification of oxidative coupling reactions.Afterwards,this thesis give an overview of visible light photocatalytic organic synthesis,and focus on the advantages of graphitic carbon nitride（g-C₃N₄）as photocatalyst.Based on green chemistry concept,graphite phase carbon nitride as photocatalyst under visible light realizes the oxidative coupling/cyclization reaction based on the direct functionalization of C-H bonds.Meanwhile,some mechanism research methods were applied,which revealed the inherent laws of the reaction.The main results of the thesis research are summarized as follows:（1）Using organic polymer semiconductor graphitic carbon nitride（g-C₃N₄）to replace the traditional homogeneous photocatalyst,realize the intramolecular oxidative coupling of thiobenzamide compounds,and synthesize 2-substituted benzothiazole derivatives by internal cyclization.Compared with the previous synthesis method,this method has excellent chemical selectivity and substrate universality,and the reaction conditions are more mild,and the reaction can be realized under the condition of air atmosphere at room temperature and without alkali and organic oxidant.In addition,the cyclic experiment proved that the heterogeneous catalyst is recyclable,which will be more in line with the concept of green chemistry.Mechanism study revealed that absorption of substrate onto g-C₃N₄ took place and facilitated the photoredox transformation,and the reaction kinetics studies have shown that C-H bond cleavage may not be rate-determining step.In addition,O₂ in air act as an electron scavenger to promote the photoredox reaction.（2）Using graphitic carbon nitride（g-C₃N₄）to realize the oxidative coupling/cyclization reaction of alkynes and thiobenzanilide.In this method,alkyne is added to the system of the above strategy（1）to realize the synthesis of isothiochromene derivatives.Compared with the traditional synthesis method,this reaction is based on the direct functionalization of C-H bond oxidative coupling reaction.Only one step reaction can achieve the synthesis of the target product without transition metals,It is worth noting that compared to strategy（1）,the optimal solvent is absolute ethanol,which is more in line with the concept of green chemistry.In addition,the recycling experiment proved that the heterogeneous catalyst is recyclable,which will further reduce the reaction cost.Mechanism studies have shown that oxygen anion radical O₂^·-is a key intermediate in the reaction,which may promote aromatic radicals to form final products via hydrogen atom transfer（HAT）.