Study On Asymmetric Michael Addition And Aldol Reactions Catalyzed By Immobilized Cinchona Chiral Catalysts

Recently, asymmetric organocatalysis, one of the most active researchfields of asymmetric catalysis, has been received much attention due toits high atom economy. Cinchona alkaloids and their derivatives as onekind of efficient organocatalysts have fascinated researchers. Owing totheir easily modified structure and multiple chiral centres, cinchonaorganocatalysts are widely used in asymmetric Michael addition andasymmetric Aldol reactions. However，the high dosage of the catalyst anddifficulties in separation and recovery restrict their use in industry. Theseproblems resulting from homogeneous process can be overcome by theimmobilization of cinchona catalysts on various supports, which havecalled great attentions from researchers. While heterogeneous cinchonacatalysis in asymmetric dihydroxylation and asymmetric alkylation is stilla major topic at present， the study for heterogeneous catalyst in asymmetric Michael addition and asymmetric Aldol reactions remainsrare.Inorganic materials are considered as ideal supports for chiralcatalysts，because of their high mechanical strength and thermal stability.In this paper, nature cinchona alkaloid and cinchona-derived chiral aminewere immobilized on mesoporous silica and polyoxometalates forasymmetric Michael addition and asymmetric Aldol reactions. Theeffects of different synthetic methods—post-synthesis grafting anddirect co-condensation method, and different supports—SBA-15withplatelet and fiber morphology on the catalytic performance ofheterogeneous catalysts were evaluated in the asymmetric Michaeladdition. The influence of different immobilization strategies, includingcovalent and electrostatic immobilization, on the catalytic performance inthe asymmetric Aldol reactions was also investigated.Quinine was firstly supported on the mesoporous silica SBA-15via post-synthesis grafting. The heterogeneous catalyst was characterized byXRD, FT-IR, DR UV-Vis, TG-DTG, N2sorption, elemental analysis andit was showed that quinine was successfully immobilized on SBA-15andthe prepared catalysts still maintained characteristic mesoporous structureand ordered mesochannels. The effect of different solvents on thecatalytic performance of heterogeneous catalyst was evaluated in theconjugate addition of malononitrile to chalcones. Catalyst recyclingstudies were also carried out. Except4-methoxychalcone, moderate yieldand unsatisfactory ee value were obtained by the heterogeneous catalyst.Compared with post-synthesis grafting, the co-condensation methodoften results in the heterogeneous catalyst with more homogeneouslydistributed active sites on the surface, which might enhance the chiralinduction and offer more suitable reaction microenvironment for thereaction. Herein, mercaptopropyl functionalized mesoporous SBA-15was prepared by co-condensation method and then quinine was incorporated onto the support surface by the free radical addition reaction.The results of XRD, FT-IR, DR UV-Vis, TG-DTG, N2sorption andelemental analysis disclose that quinine was successfully immobilized onthe surface of support. Furthermore, when the dosage of organosilanewas less than20mol%(the molar percentage of silicon atoms in theinitial mixture as organosilane), the heterogeneous catalyst couldmaintain the characteristic mesoporous structure and orderedmesochannels. The effects of different synthesis conditions on catalyticperformance were investigated and the optimal conditions were asfollows: the dosage of organosilane should be20mol%, thecrystallization temperature should be100℃and time should be24h.The heterogeneous catalyst was tested in the conjugate addition ofmalononitrile to chalcones. It was found that the enantioselectivity wassignificantly improved by the catalyst prepared via the co-condensationmethod. However, the yield was basically unchanged due to the long mesochannels in micrometer scale.Mesoporous SBA-15with short mesochannels is proved to be morefavorable for mass transfer than the conventional SBA-15, which alwaysleads to enhanced catalytic activity. In our study, mercaptopropylfunctionalized mesoporous SBA-15with platelet morphology and shortmesochannels was synthesized by co-condensation method and cinchonaalkaloids were then immobilized on such organic-inorganic mesoporoussilica materials. The heterogeneous catalysts were characterized by XRD,FT-IR, DR UV-Vis, TG-DTG, SEM, TEM, N2sorption and elementalanalysis. The characterization results indicated that cinchona alkaloidswere successfully incorporated onto the surfaces and thoese catalystsremain mesoporous structure. SEM and TEM showed that theheterogeneous catalysts have hexagonal platelet morphology and the porelength of the catalysts was about280nm~480nm. The influences of theorganosilane dosage and the structure of cinchona alkaloids on the catalytic performance of heterogeneous cinchona catalyst were alsostudied. It was found that the best dosage of organosilane was10mol%and the heterogeneous quinine catalyst was superior to other cinchonacatalysts. Not only enhanced catalytic activity but also improvedenantioselectivity were obtained by the heterogeneous catalyst with shortmesochannels. Element analysis and FT-IR confirmed that the active sitesbeing covered by the reactant (malononitrile) might be the main reasonfor the loss of activity and enantioselectivity during the recycle process.Electrostatic immobilization is one of the most importantimmobilzation strategies. In our work, cinchona-derived chiral amine wasfirstly supported on polyoxometalate acid by electrostatic interaction. Thecatalyst was characterized by NMR, FT-IR and elemental analysis. Theresults indicated that cinchona-derived chiral amine was successfullyimmobilized on the polyoxometalate acid. The influences of differentimmobilzation strategies, heteropoly acids, solvent and substrates on the catalytic performance of the heterogeneous catalysts were evaluated inthe asymmetric Aldol reaction of aromatic aldehydes. It was found thatcinchona-derived chiral amine covalently immobilized on SBA-15provided racemic product and C-10double bond being destroyed mightbe the main reason. Other influential factors on the catalytic performancewere also studied and the optimal conditions were as follows: theappropriate polyoxometalate acid was phosphotungstic acid whichpossesses the highest acid strength, and the best solvent was acetone. Theexperiments also showed that the space factor from substrate couldeffectively affect the catalytic result. The recycle study indicated that themain reason for the decrease of the catalytic activity was the mass loss ofcatalyst.