Immobilized Cinchona Alkaloid Chiral Catalysts For Asymmetric Transfer Hydrogenation And Aldol Reactions

Chirality is one of the basic properties of nature. Asymmetric catalysis by chiral catalyst is the most effective method to obtain optically pure compounds. Cinchona alkaloids are a class of natural alkaloids extracted from the bark of the cinchona. Owing to their unique features, such as multiple chiral centers and easily modified structure, cinchona alkaloids and their derivatives have drawn more and more attention in asymmetric synthesis. Nowadays, they have important applications in both asymmetric transfer hydrogenation and asymmetric Aldol reactions. Cinchona alkaloids and their derivatives expressed excellent catalytic activity and enantioselectivity in homogeneous catalysis. However, they often suffer problems in recyclability, cost effectiveness and product contamination under homogeneous catalytic system. To overcome these limitations, the immobilization of such catalysts is considered an effective way. Although heterogeneous cinchona catalysts have received wide concern, their applications in asymmetric transfer hydrogenation and asymmetric Aldol reactions remain rare.Inorganic materials are generally thermal stability and high mechanical intensity, wherein the inorganic mesoporous materials are ideal supports for chiral catalysts due to their large surface areas and uniform pore structures.In this paper, a cheap chiral ligand9-amino epi-cinchonine was anchored for the first time onto mercaptopropyl functionalized short-mesochannel SBA-15which was synthesized by co-condensation method for use in the asymmetric transfer hydrogenation of aromatic ketones. The heterogeneous ligand was characterized by XRD, DR UV-vis, FT-IR, TG, N2sorption, elemental analysis, electron microscopy. The results showed that9-amino epi-cinchonine was successfully immobilized on short-mesochannel SBA-15and that the prepared heterogeneous ligand still maintained characteristic mesoporous structure and ordered mesochannels. After complexing with [Ir(COD)Cl]2, the resulting heterogeneous catalyst was evaluated in the asymmetric transfer hydrogenation of aromatic ketones. The influences of reaction temperature, reaction time, the dosage of mercaptopropyl silane, the amount of catalyst, and the type and amount of base on the catalytic performance over the heterogeneous catalyst were investigated. Compared with the catalyst prepared by post-synthesis grafting and the catalyst supported on conventional SBA-15, the new heterogeneous catalyst exhibited higher catalytic activity and enantioselectivity. Moreover, it showed significantly enhanced enantioselectivity compared to the corresponding homogeneous analogue and exhibited good recyclability.Given the advantages of short-mesochannel SBA-15as support,9-amino epi-cinchonine was also anchored onto short-mesochannel MCM-41. The heterogeneous catalyst was evaluated in the asymmetric transfer hydrogenation of aromatic ketones. It showed comparable activity and enantioselectivity to the catalyst based on short-mesochannel SBA-15, which were much higher than the catalyst based on conventional MCM-41. However, this catalyst presented relatively low stability during the reaction.In our work, cinchona-derived chiral amine was firstly combined with ionic liquids by electrostatic interaction, and used as recyclable catalyst in the asymmetric Aldol reaction of p-nitrobenzaldehyde. The influences of the type and amount of ionic liquid, the type and amount of solvent on the catalytic performance were also studied. Up to95%yield and69%ee were obtained. The recycle study indicated that the activity and enantioselectivity showed no obvious decrease for four runs. The main reasons for the decrease of the catalytic activity might be the covering of active sites by the impurities and the mass loss of catalyst.