Preparation Of Supporting Chiral Catalysts And Catalytic Effects On Asymmetric Aldol Reactions In Water

Asymmetric organic reactions, one of the most important reactions in synthetic organic chemistry, has been intensively studied. Since people reported the direct aldol reaction catalyzed by L-proline under mild reaction condition, organocatalysts such as L-proline has been widely used in asymmetric organic reactions with the advantage of high performance at low cost, easy operation and stable nature. L-Proline has been reported to accelerate the reaction rate and improve enantioselectivity in the presence of organic solvents or a small amount of water, but too much water resulted in low yields with little or no enantioselectivity.Compared with the organic solution, aqueous phase, as a a safe, cheap, and environmentally friendly media, has the great advantages of economic aspect and environmental protection. Therefore, it is significant to develop chiral organocatalysts for the direct asymmetric Aldol reaction in water.In this work, we combined controlled/living radical polymerization with supporting organic chiral catalysts. Herein, we described a general and efficient approach to obtaining compounds and a class of hairy particles supporting organic chiral catalysts, whose surface consisted of a hydrophobic and a hydrophilic part. The compounds and hairy particles supported organocatalysts carried out in asymmetric synthesis reations to investigated the performance of catalyst. The catalytic activity, asymmetric selectivity and recyclability were also further studied. The specific experiments were as follows:(1) Synthesis of the hairy particles supporting organic chiral catalysts via RAFT precipitation polymerization combined with RAFT surface graft copolymerization.The polymer micropheres P(MAA-co-EGDMA) with surface-immobilized dithioester groups were prepared via RAFT precipitation polymerization(RAFTPP), which used CDB as a RAFT chain transfer agent. Then synthesis of chiral monomer N-Boc-O-methacryloyl-L-hydroxyproline used the L-hydroxyproline as material and N-Boc protecting groups for the amine. Finally the hairy particles were prepared via RAFT surface graft copolymerization with the way of P(MAA-co-EGDMA) grafting thermoresponsive polymer-supported organic chiral catalysts. In order to compare, we also prepared respectively two types of the hairy particles, which only contained hydrophobic( polystyrene) or hydrophilic(pnipam) part. After deprotection, the kinds of hairy particles were obtained. The above-obtained particles were characterized with 1HNMR、FTIR、SEM.(2) Synthesis of the compounds supported organic chiral catalystsSynthesis of PECH ionic liquid electrolytes which obtained from reaction of PECH and nitrogen heterocyclic. Organic chiral polymer of L-proline was prepared by RAFT polymerization and deprotected. Then through the role of the ammonia ethanol solution, the compounds supporting organic chiral catalysts were prepared by complexation of polymer ionic liquid electrolytes and chiral polymer of L-proline. The compounds were characterized with FTIR、SEM.(3) Chiral organocatalysts-supported L-proline for direct asymmetric Aldol reaction in waterThe catalytic activity of the synthesized hairy particles and the compounds supported L-proline was tested using a representative aldol reactions between cyclohexanone and 4-nitrobenzaldehyde. For the hairy particles, the reactions were first carried out with 10 mol % catalyst loading at different temperature in pure water for 72 h. We chosed this reaction temperature range 0℃、25℃、35℃、50℃ to investigate the activity and selectivity of catalysts. Catalyst had the advantages of the facile recovery by precipitation or ultrafiltration methods. The recycling potential of catalytic system was studied under the same condition. Finally, We also investigated the reaction between cyclohexanone and several substituted benzaldehydes with the above-obtained catalyst. For the compounds, we chose the reaction temperature of 35℃ to investigate the activity and selectivity of catalysts and observed the reaction results in other reaction conditions. The catalysates were characterized with 1HNMR、HPLC.