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The recovery and reuse of the enantioselective catalysts produced by tedious work are important not only from the perspective of green chemistry, but also from the point of view of productivity. Some of the carbohydrate-based crown ethers prepared in our research group were able to generate significant asymmetric induction in certain cases. However, they were not recoverable after the synthesis. Therefore, we modified the most effective structure with a propargyl group so that it can be attached to a polymer with an azide–alkyne reaction. It was investigated whether the position of the bonding affects the activity of the crown ethers, hence, the propargyl group was introduced either to the side chain, to the anomeric center or to the benzylidene protecting group. To anchor the macrocycles, low molecular weight PVC was modified with azide groups in 4% and 10%, respectively. It was found that glucose-based crown ether bearing the propargyl group on the benzylidene unit and grafted to PVC in 4% has the highest activity regarding the enantioselectivity (77% ee). The catalyst was recoverable in the Michael addition of diethyl acetamidomalonate to nitrostyrene and it could be reused five times without the loss of enantioselectivity. [ABSTRACT FROM AUTHOR]

The synthesis of chiral proline-derived squaramide and thiosquaramide organocatalysts, which are capable of the dual activation in asymmetric reactions is reported. The (thio)squaramide moiety can form hydrogen bonds to activate the substrates and to stereocontrol the reaction, while the pyrrolidine unit can form enamines to activate carbonyl compounds via aminocatalysis. Comparing the performance of thiosquaramide to squaramide, the Diels–Alder reaction of (anthracen-9-yl)acetaldehyde and trans -β-nitrostyrene was examined, which has been investigated in the literature using quantum chemical calculations. Both squaramide and thiosquaramide gave excellent yields (up to 99%) and enantiomeric excess values (up to 98%). Moreover, their catalytic performance was compared in conjugate addition of lawsone to β,γ-unsaturated α-keto ester. [ABSTRACT FROM AUTHOR]

The synthesis of α-aryl-α-aminophosphonates was performed by the three-component Kabachnik-Fields reaction of primary amines, benzaldehyde derivatives and dialkyl phosphites in a continuous flow microwave reactor. The target compounds could be obtained in high (~90%) yields without any catalyst in simple alcohols as the solvent. The flow process elaborated required shorter reaction times and lower excess of the reagent, as compared to the "traditional" batch reactions, and allowed the synthesis of the α-aminophosphonates on a somewhat larger scale. [ABSTRACT FROM AUTHOR]

Syntheses of two regioisomeric series of atropisomeric amino alcohols and a comparative study on their application in the enantioselective addition of diethylzinc to benzaldehyde are reported. Systematic modification of the electronic and steric properties of the functional groups resulted in highly efficient catalyst ligands in both series. Quantum-chemical calculations agreed well with the experimental results of this first systematic comparative study on regioisomeric atropisomeric ligands. [ABSTRACT FROM AUTHOR]

In this work, anchoring of cinchona derivatives to trifunctional cores (hub approach) was demonstrated to obtain size-enlarged organocatalysts. By modifying the cinchona skeleton in different positions, we prepared four C3-symmetric size-enlarged cinchona derivatives (hub-cinchonas), which were tested as organocatalysts and their catalytic activities were compared with the parent cinchona (hydroquinine) catalyst. We showed that in the hydroxyalkylation reaction of indole, hydroquinine provides good enantioselectivities (up to 73% ee), while the four new size-enlarged derivatives resulted in significantly lower values (up to 29% ee) in this reaction. Anchoring cinchonas to trifunctional cores was found to facilitate nanofiltration-supported catalyst recovery using the PolarClean alternative solvent. The C3-symmetric size-enlarged organocatalysts were completely rejected by all the applied membranes, whereas the separation of hydroquinine was found to be insufficient when using organic solvent nanofiltration. Furthermore, the asymmetric catalysis was successfully demonstrated in the case of the Michael reaction of 1,3-diketones and trans-β-nitrostyrene using Hub3-cinchona (up to 96% ee) as a result of the positive effect of the C3-symmetric structure using a bulkier substrate. This equates to an increased selectivity of the catalyst in comparison to hydroquinine in the latter Michael reaction. [ABSTRACT FROM AUTHOR]

Numerous cinchona organocatalysts with different substituents at their quinuclidine unit have been described and tested, but the effect of those saturation has not been examined before. This work presents the synthesis of four widely used cinchona-based organocatalyst classes (hydroxy, amino, squaramide, and thiourea) with different saturation on the quinuclidine unit (ethyl, vinyl, ethynyl) started from quinine, the most easily available cinchona derivative. Big differences were found in basicity of the quinuclidine unit by measuring the pKa values of twelve catalysts in six solvents. The effect of differences was examined by testing the catalysts in Michael addition reaction of pentane-2,4-dione to trans-β-nitrostyrene. The 1.6–1.7 pKa deviation in basicity of the quinuclidine unit did not result in significant differences in yields and enantiomeric excesses. Quantum chemical calculations confirmed that the ethyl, ethynyl, and vinyl substituents affect the acid-base properties of the cinchona-thiourea catalysts only slightly, and the most active neutral thione forms are the most stable tautomers in all cases. Due to the fact that cinchonas with differently saturated quinuclidine substituents have similar catalytic activity in asymmetric Michael addition application of quinine-based catalysts is recommended. Its vinyl group allows further modifications, for instance, recycling the catalyst by immobilization. [ABSTRACT FROM AUTHOR]