Research On Photocatalytic Oxidative Cleavage Of Alkynes And Hydration Of Alkenes By Carbon Nitride Materials

The oxidative cleavage of alkynes and the hydration of alkenes are effective methods for aldehydes,ketones,esters,alcohols and other high value-added compounds.However,most of existing methods use oxidants such as ozone,potassium permanganate,osmium acid and hydrogen peroxide which are difficult to be recovered.Recently,photocatalysis has become a hot topic due to its mild conditions,high yields and atomic economy.Although classical homogeneous photocatalysts such as ruthenium,iridium and other metal complexes and acridinium salts have achieved great success in visible light photocatalysis,their shortcomings are expensive and difficult to be recovered.On this basis,this thesis has efficiently achieved the indirect and direct C≡C oxidative cleavage of alkynes to construct aldehydes and esters by using porous graphitic phase carbon nitride（p-g-C₃N₄）and mixed sodium-potassium poly（heptazine imide）（KNa PHI-II）as recoverable photocatalyst and oxygen as the oxidant;meanwhile,the Meerwein hydration of alkenes in aqueous phase was developed to construct a series of secondary alcohols by mesoporous graphitic carbon nitride（mpg-C₃N₄）.The main contents of the article are divided into three parts as follows:（1）Using p-g-C₃N₄ as the photocatalyst,oxygen as the oxidant,Me CN/H₂O（=9:1 Vol）as the solvent,Cs₂CO₃ as the additive and aryl tetrafluoroborate diazonium salts as the free radical initiator realized C≡C indirect oxidative cleavage of alkynes and prepared 22aldehydes under 250 W Xe lamp irradiation.The reaction can be scaled up to 16-fold gram-scale experiments under sunlight.The p-g-C₃N₄ can be recycled 6 times without significant activity loss after recovery by centrifugation.In addition,12 esters were obtained when HCl O₄ was used as an additive.Mechanistic studies showed the system involved a radical course with oxetane as the main intermediate,while a possible reaction mechanism of photocatalytic radical addition-oxygen oxidation-hydrolysis or alcoholysis was proposed.（2）Using KNa PHI-Ⅱas the photocatalyst,oxygen as the oxidant and 7W UV-LED as the light source,a one-step C≡C oxidative cleavage of alkynes in the aqueous phase was achieved and 16 aldehydes were prepared.Then 16 esters were prepared after the addition of HCl O₄.Also,the system has simple operation and can be scaled up to 16-fold gram-scale experiments.KNa PHI-Ⅱis water-soluble.KNa PHI-II/H₂O can be recycled 5 times after dichloromethane extraction without significant activity loss.Mechanism studies showed the system involved a radical course with superoxide radical and hydroxyl radical were the main oxide species for C≡C oxidative cleavage of alkynes.（3）Photocatalytic Meerwein hydration of alkenes in aqueous phase was achieved by using mpg-C₃N₄ under 250W Xe lamp irradiation to prepare 26 secondary alcohols.The reaction conditions are mild,without any additives or co-solvents and simple operation,and can be scaled up to 20-fold gram-scale experiments under sunlight.In addition,mpg-C₃N₄ can be recycled 5 times without loss of activity after dichloromethane extraction.Mechanistic studies indicated the system underwent a radical addition/hydration tandem process.