Non-covalent immobilization of asymmetric organocatalysts

Abstract

The immobilization of organocatalysts is one of the most explored approaches to overcome the limitations associated with organocatalytic systems such as high catalyst loading, difficult product separation and catalyst recycling. When compared with the commonly used covalent immobilization methods, which usually involve tedious synthetic manipulations, the non-covalent attachment of organocatalysts to supports offers facile and modular construction of immobilized chiral catalysts with maintained or even improved activity and stereoselectivity. This review summarizes the successful application of non-covalent interactions, such as acid-base interaction, ion-pair interaction, hydrophobic interaction and so on, in assembling recoverable and reusable organocatalysts.

Preparation of Immobilized L‐Prolinamide Via Enzymatic Polymerization of Phenolic L‐Prolinamide and Evaluation of Its Catalytic Performance for Direct Asymmetric Aldol Reaction

“…Among the various catalysts for direct asymmetric aldol reaction, a series of organocatalysts derived from L‐proline are of particular significance. This is why many efforts have been made to modify the structure of L‐proline so as to improve selectivity and reusability, reduce load, and avoid the use of organic solvent . To overcome these drawbacks, researchers also have made attempts to graft organocatalysts with suitable supports .…”

C2‐symmetric sulfonamides as homogeneous and heterogeneous organocatalysts that mimic enzymes in enantioselective Michael additions

“…Having shown the usefulness of C 2 ‐symmetric ligands ( 1a , b , c and 2a , b , c ) as excellent homogeneous organocatalysts in the conjugate 1,4‐Michael addition of carbonylic type of compounds to nitrolefin, we turned our attention to immobilizing these organocatalysts onto silica and exploring their catalytic properties. Heterogenized organocatalysts offer many advantages, such as, easy separation, recyclability, and pores on the silica surface which may mimic natural enzymes; for these reasons, immobilized chiral organocatalysts for asymmetric catalysis have attracted a great deal of attention …”

“…Heterogenized organocatalysts offer many advantages, such as, easy separation, recyclability, and pores on the silica surface which may mimic natural enzymes; for these reasons, immobilized chiral organocatalysts for asymmetric catalysis have attracted a great deal of attention. [40][41][42][43][44] Enantiopure organocatalysts (1a,b,c and 2a,b,c) were immobilized onto silica using 4-(ethylbenzene)sulfonylsilica gel to afford immobilized organocatalyst (5a,b,c and 6a,b,c) (Scheme 3) and tested for their catalytic properties in the conjugate carbonylic addition to βnitrostyrene. Surprisingly, the organocatalysts 1c and 2c, which behaved as excellent homogeneous catalysts (Entries 13 and 16), on immobilization (5c and 6c) led to low yields and racemic mixtures of the final product.…”

2.07 The Aldol Reaction: Organocatalysis Approach

“…Although bond strength in noncovalently supported catalyst is weak, the catalyst itself is directly used for immobilization without need for modification or synthetic steps required for covalent attachment to support. 271 Inclusion of (S)-proline derivatives with β-cyclodextrin is a useful method for noncovalently supported organocatalyst ( Figure 25). The Zhang group demonstrated that the β-cyclodextrin-immobilized catalyst 475 is prepared conveniently by simply heating (4S)-phenoxy-(S)-proline and β-cyclodextrin in ethanol water and by removal of the solvent.…”