Investigation On Photocatalytic Reductive Coupling And Hydrogenation Of Aryl Halides With Water As The Reductant

The combination of photocatalysis and organic chemical reactions not only avoids the consumption of fossil energy by traditional heating methods,but also enables organic reactions to be carried out at room temperature and improves the selectivity of the reactions.In conventional organic reduction reactions,sodium borohydride,lithium aluminum hydride,and hydrogen are usually used as the reductant.Compared with these reagents,water is cheap and easy to obtain,the by-product is oxygen,and almost no pollution is produced.Therefore,using water as the reductant for the reductive conversion of organic compounds not only effectively reduces the reaction cost,but also meets the requirements of green chemistry.However,to realize photocatalytic organic reactions with water as the reductant,challenging problems such as the low efficiency of total photolysis of water and the tendency of the generated hydrogen peroxide to poison the catalyst need to be solved.To address these problems,this thesis uses carbon nitride as a photocatalyst and the co-catalyst of additives such as sodium carbonate and iron salts to achieve a reduction coupling reaction with water as the reductant and a hydrogen source to achieve a hydrogenation reaction with water as the reductant and a hydrogen source,as follows and conclusions:The reductive coupling of bromobenzene was achieved by the co-catalysis of sodium carbonate using green water as the reductant and graphite-phase carbon nitride-loaded palladium(Pd/g-C3N4)as the photocatalyst.The effect of various conditions on the reaction was investigated,and the results showed that the optimal experimental conditions was 0.5 mmol substrate,15.00 mg Pd/g-C3N4*(2.8 wt%),0.75 mmol sodium carbonate,5 m L water,3 m L 1,4-dioxane and 420 nm LED light source were reacted for 20 h at room temperature.The effect of functional groups on the reaction was investigated,and the results showed that many substrates(such as cyano,carbonyl,fluoro,chloro,alkoxy,N,N-dimethylamino,hydroxy,aldehyde groups,etc)were able to give the coupling products in high yields.The reaction mechanism was investigated,and the results showed that sodium carbonate is:(1)it can remove hydrogen peroxide and reduce the toxic effect of hydrogen peroxide on the catalyst;(2)it can reduce the H+ concentration in the reaction system,inhibit the generation of H0 and improve the coupling product selectivity.The results showed that the light pretreatment of Pd/g-C3N4 can slightly improve the efficiency of this photocatalyst because the illuminating treatment can effectively increase the content of Pd0 in the catalyst and improve the activity of the photocatalyst.The experimental results of the rotating disc electrode show that the oxygen precipitation half-reaction is a four-electron one-step mechanism process,and the proton reduction half-reaction proceeded by a single electron transfer radical mechanism.The hydrogenation of bromobenzene was achieved by the co-catalysis of Fe species using green water as the reductant and a hydrogen source and graphite-phase carbon nitride-loaded palladium(Pd/g-C3N4)as the photocatalyst.The effect of various conditions on the reaction was investigated,and the results showed that the optimal experimental conditions was 0.5 mmol substrate,15.00 mg Pd/g-C3N4(1.0 wt%),0.15 mmol Ferric chloride,5 m L water,3 m L 1,4-dioxane and 365 nm LED light source was reacted for 20 h at room temperature.The effect of functional groups on the reaction was investigated,and the results showed that many substrates(such as alkoxy,alkyl thiol,cyano,phenolic hydroxyl,ketone carbonyl,carboxyl,formyl,etc)were able to give the hydrogenation products in high yields.When D2 O was used as the reductant instead of H2 O,a series of high-value-added deuterated aromatics were obtained.The reaction mechanism was investigated,and the results showed that Fe species are:(1)it can remove hydrogen peroxide and reduce the toxic effect of hydrogen peroxide on the catalyst;(2)Fe3+ as a lewis acid and H2 O molecule Fe3+ as a lewis can to form complexes with water,this phenomenon can increase the concentration of Hδ+ around metallic palladium,enhancing the production of H0 and improving hydrogenation product selectivity.The experimental results of the rotating disc electrode show that the oxygen precipitation half-reaction is a two-electron two-step reaction mechanism process,and the proton reduction half-reaction proceeded by a single electron transfer radical mechanism.