Pep talk: An immobilized peptidic catalyst achieves more than 600 turnovers in a continuous-flow system, allowing the production of chiral γ-nitroaldehydes with excellent stereoselectivities on a scale of >450 mmol (>100 g). Such a high efficiency opens the way for more practical applications of enamine catalysis.
Readily reusable immobilized organocatalysts are important from a practical, economic, and environmental viewpoint. However, their successful development has proven challenging and only limited reaction and recovery cycles have been achieved. We report an extraordinarily robust resinbound tripeptidic organocatalyst that can be readily reused for at least 30 reaction and recovery cycles without loss in catalytic activity or stereoselectivity. The immobilized catalyst can be directly reused for conjugate addition reactions between aldehydes and nitroolefins after a simple filtration from the reaction products. The catalytic efficiency and chemoselectivity of the immobilized peptidic catalysts is so high that a broad range of g-nitroaldehydes were isolated easily after filtration and removal of all volatiles in quantitative yields, excellent diastereoselectivities and enantioselectivities and, most remarkably, perfect analytical purities. The ease of handling allows for a facile scale-up of reactions catalyzed by the immobilized peptidic catalyst. In addition, the results provide a guide for the further development of immobilized organocatalysts.
Hydrophilic polymers having pendant groups of carboxylates or sulfonates have been used as a polymer support for chiral 1,2-diamine monosulfonamides. The polymeric chiral complex prepared from the polymer-supported chiral ligand with ruthenium dichloride·p-cymene was used in the asymmetric transfer hydrogenation of prochiral ketones in water. The balance between hydrophilicity and hydrophobicity of the polymer support influenced both the reactivity and the enantioselectivity of the reaction in water. The chiral polymeric complex having a quaternary ammonium salt structure as the pendant group worked well in water. In most cases the polymer-supported catalyst having a quaternary ammonium sulfonate pendant group showed superior enantioselectivity compared to the corresponding nonsupported model catalyst in the solution system. The polymeric catalysts can be reused without loss of catalytic activity.
Direct support: The title compounds have been used as polymeric organocatalysts (see structure) in the asymmetric alkylation of a glycine derivative to give the corresponding phenylalanine with high enantioselectivity. The polymeric catalyst can be reused without loss of its catalytic activity.
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