Supported proline and proline-derivatives as recyclable organocatalysts

Abstract

In the last eight years, L-proline and L-proline derivatives, such as substituted prolinamides or pyrrolidines, have been successfully used as organocatalysts in several reactions. In this critical review we summarize the immobilization procedures of such organocatalysts highlighting their application, recoverability and reusability (86 references).

Cyclic 2:1 and 1:2 Aldehyde‐to‐Acetone Byproduct Adducts in Aldol Reactions Promoted by Supported Proline‐Incorporated Catalysts

“…[2] Despite the questionable economic profitability of developing a successful immobilized proline catalyst, because of the low price of this amino acid and the possibility of its easy recovery from homogeneous reaction mixtures, proline and related molecules provide excellent models for organocatalyst immobilization studies as a result of their simplicity and the ready availability of various derivatives. Moreover, immobilization frequently provides catalysts with an altered reactivity profile that can lead to products different from those obtained in solution and provide valuable mechanistic information.…”

Chiral Ionic Liquids Bearing O‐Silylated α,α‐Diphenyl (S)‐ or (R)‐Prolinol Units: Recoverable Organocatalysts for Asymmetric Michael Addition of Nitroalkanes to α,β‐Enals

“…Furthermore, the majority of or even all known pyrrolidine-based recoverable organocatalysts for asymmetric Michael reactions, including compound 7a, have the (S) configuration of the C2 (C5) carbon atom and allow only one of the two enantiomers of the corresponding Michael adducts to be synthesized. [18][19][20][21][22][23] It should be noted that, with respect to curing CNS disorders, (S) enantiomers of γ-amino acid derivatives 1 being formed in the presence of catalysts (S)-4 are usually less active than the corresponding (R) enantiomers (Scheme 2). [9] In this paper we report for the first time that (S)-or (R)-Michael adducts can be obtained from nitroalkanes 2 and α,β-unsaturated aldehydes 3 in high yields and with high enantioselectivity in the presence of recoverable chiral ionic liquids containing silylated (S)-or (R)-prolinol units that are tagged to the imidazolium cation.…”

“…There are few examples of recoverable organocatalysts for Michael reactions, in particular between carbonyl compounds and nitroalkenes where an alternative enamine-type activation of a nucleophile is put into effect. [18] The chiral inductor in these catalysts is tagged to a polymer, [19] dendritic, [20] perfluoroalkyl, [21] or ionic group. [22] Yet, recoverable organocatalysts of the reactions that proceed via an iminium ion formation step had not been reported until recently.…”

Are Ionic Liquids Suitable Media for Organocatalytic Reactions?

“…[5] Among these functional solvents, acidic or protic ionic liquids have found a wide range of applications. [6] Ionic liquids incorporating a chiral cation or anion can induce interesting levels of stereoselectivity in a range of asymmetric reactions.…”