The Design And Asymmetric Catalysis Research Of Supported L-proline For The Direct Asymmetric Aldol Reactions

L-proline is excellent catalyst for the direct asymmetric aldol reaction, due to the the mild reaction conditions and the process without prior stoichiometric formation of the nucleophile. However, the insoluble of L-proline in organic solvents often leads to the higer catalyst loading and the lower catalytic efficiency for the L-proline. Furthermore, the homogeneous L-proline is not easily recovered from the reaction system for reuse or recycles. The heterogeneous catalysts often suffer from the decrease of catalytic activities. To address the issue, effects were made to develop novel kinds of L-proline catalyst that can be used as highly effective and reusable catalyst in the direct asymmetric aldol reaction.The direct asymmetric aldol reaction of different aromatic aldehydes with acetone catalyzed by L-proline can be accelerated by the addition of diols or diphenols. The employment of additives can decrease the amount of L-proline and make the reaction perform under free solvent. Especially, the use of catechol with two adjacent hydroxy groups at aromatic ring shows the most efficient. Under the optimized catalytic conditions, only5mol%L-proline with1mol%catechol gives excellent chiral induction (80%ee value) with>90%yield of the corresponding aldol product. Computational study was employed to reveal the positive effects on the reaction. It is found that the additives, especially, the catechol promotes the reaction through intermolecular hydrogen bond between the hydrogen of the hydroxyl group and the oxygen of the carbonyl group, which can activate the carbonyl group of the aldehyde. The catechol bearing two adjacent hydroxyl groups at aromatic ring can form the dual hydrogen bonds with the the oxygen of the carbonyl group in the corresponding aldehyde. Therfore, the combination of L-proline and catechol are very efficient for catalyzing the direct asymmetric aldol reaction to proceeding with high yield in short time.Based on the limited solubility of L-proline in organic solvents and the unique selective solubility of ionic liquids, hydrophilic ionic liquids (ILs) with long chain alkyl substituents were attached onto the L-proline to synthesize a novel surfactant-type IL-functionalized L-proline via covalently bond. The catalytic performances of the catalyst were investigated in the direct asymmetric aldol reaction of aromatic aldehydes with acetone. It was found that the IL-functionalized L-proline was well dipersed in the catalytic reaction and showed high efficiency in the direct asymmetric aldol reaction. Furthermore, the catalyst can be facilely recovered for reuse by centrifugation.Graphene oxide (GO) with larger surface area and the abundant oxygen-containing functional groups, was successfully employed to support the L-proline via a hydrogen-bonding interaction between the -OH and-COOH groups on the graphene sheet and the-COOH and-NH-groups in L-proline. The obtained catalysts were well characterized by FT-IR, and elemental analysis. Compared with the homogeneous L-proline, it showed the comparable reactivity and enantioselectivity in in the direct asymmetric aldol reaction. Moreover, the GO supported L-proline catalyst could be conveniently recovered from the products by centrifugal separation and be reused at least7times without loss of both activity and enantioselectivity.