Mesoporous Organic Polymeric Nano-Bowl-supported Solid Acid/Chiral Primary Amine Organocatalyst With Free Space Around Catalytic Site And Asymmetric Catalytic Tandem Reactions

Solid sulfonic acid is one of the most widely used solid acids in the industry,which has been successfully applied to organic synthesis reactions including esterifi-cation,hydrolysis,alkylation and isomerization.However,organic polymer as a catalyst carrier lacks pore structure,which results in the embedded acid site,the severe mass transfer of reactants to catalytic site and then the poor catalytic efficiency.In order to improve the catalytic performance of organic polymer-supported solid acid,a hollow mesoporous organic polymer nano-bowl-supported solid acid is prepared to achieve the following characteristics:?1?Organic polymer-supported solid acid has unique hydrophobicity and swelling characteristics.?2?The hollow and mesoporous structure can produce a concentration driving effect and provide a transport channel,thereby improve the mass transfer effect of catalytic reaction.?3?Organic polymeric nano-bowl-supported solid acid has free space around acid catalytic sites.The solid acid anchors cinchona alkaloid-derived chiral primary amine?CDNH₂?into its hollow,mesoporous shell via acid-base interaction,which shows good catalytic performance in heterogene-ous acetal hydrolysis/asymmetric aldol tandem reactions.In the aqueous solution containing cobalt salt,using polystyrene nanospheres?PS?as core template,vinyl phenyl isothiouronium hydrochloride?VBIH?as functional mo-nomer,a layer of functionalized polymer is coated on the surface of the PS core by emulsion polymerization.After the PS core is removed by DMF,the as-obtained hollow organic polymer nano-bowl is hydrolyzed in NaOH solution and oxidated by H₂O₂ to afford hollow mesoporous organic polymer nano-bowl-supported solid acid.Poly?St-co-SO₃H?HMOBs-1.Then,hollow mesoporous organic polymer nano-bowl-supported solid acid/chiral primary amine organocatalyst Poly?St-co-SO₃H/CDNH₂?was prepared via the acid-base interaction of cinchona alkaloid-derived primary amine?CDNH₂?with SO₃H in the shell.Meanwhile,two other kinds of organic polymer solid acids Poly?St-co-SO₃H?HMOBs-2 and Poly?St-co-SO₃H?-3 without hollow structure and free space around SO₃H were prepared as contrast catalysts to investigate the function of hollow structure and free space around SO₃H in acetal hydrolysis/asymmetric aldol tandem reactions.From SEM images,Poly?St-co-SO₃H?HMOBs-1 and Poly?St-co-SO₃H/CDNH₂?HMOBs-1 possess a morphology of regular nano-bowl and good monodispersity.TEM images confirm that they have hollow structures.The hydrolysis of thiourea salt is confirmed by the decreased nitrogen content from 3.53%to 0.88%and the disappeara-nce of IR absorption peak at 1659 cm^-1 of-C=N in VBIH monomer.After being oxidiz-ed by H₂O₂,three IR absorption peaks newly emerged at 1216 cm^-1,1162 cm^-1 and 1042cm^-1,which are attributed to the stretching vibration of S=O and-S-O and indicate the formation of sulfonic acid.N₂ adsorption-desorption isotherms shows that the pore sizes increase from 3.39 nm to 8.08 nm and the specific surface areas increases from 16.6m²/g to 97.7 m²/g in the hydrolysis of thiourea salt,which indicates that the hydrolysis of thiourea salt can release free space around thiol site.In this process,metal cobalt salt acts as porogen to construct mesoporous channel in the shell.Owing to the as-fabricated hollow interior,mesoporous shell and free space around site,the specific surface area,acid content and loading capacity of CDNH₂ are sinificantly improved,and a microen-vironment suitable for heterogeneous catalytic reaction is provided.The contrast catalytic performances of three Poly?St-co-SO₃H/CDNH₂?HMOBs-1,Poly?St-co-SO₃H/CDNH₂?HMOBs-2 and Poly?St-co-SO₃H/CDNH₂?-3 are investigated in acetal hydrolysis/asymmetric aldol tandem reactions.According to the yields and stereoselectivities of the product,Poly?St-co-SO₃H/CDNH₂?HMOBs-1 has the best catalytic yield?95%?and stereoselectivies?97%ee anti,anti/syn=89/11?.Under the optimized catalytic conditions?water as solvent,20°C,35 mol%of CDNH₂?,the scope of substrates is extended.It is found that the acetals bearing electron-withdrawing group have good to excellent yields and stereoselectivities?90-95%,74-97%ee anti?.