Chiral Imidazolium Prolinate Salts as Efficient Synzymatic Organocatalysts for the Asymmetric Aldol Reaction

Abstract

Chiral imidazolium l-prolinate salts, providing a complex network of supramolecular interaction in a chiral environment, have been studied as synzymatic catalytic systems. They are demonstrated to be green and efficient chiral organocatalysts for direct asymmetric aldol reactions at room temperature. The corresponding aldol products were obtained with moderate to good enantioselectivities. The influence of the presence of chirality in both the imidazolium cation and the prolinate anion on the transfer of chirality from the organocatalyst to the aldol product has been studied. Moreover, interesting match/mismatch situations have been observed regarding configuration of chirality of the two components through the analysis of results for organocatalysts derived from both enantiomers of prolinate (R/S) and the trans/cis isomers for the chiral fragment of the cation. This is associated with differences in the corresponding reaction rates but also to the different tendencies for the formation of aggregates, as evidenced by nonlinear effects studies (NLE). Excellent activities, selectivities, and enantioselectivities could be achieved by an appropriate selection of the structural elements at the cation and anion.

Chiral Ionic Liquids Based on l-Cysteine Derivatives for Asymmetric Aldol Reaction

“…The facility with which chirality elements can be introduced into an ionic liquid structure allows for countless potential utilisations of chiral ionic liquids (CILs) as chiral media, additives, or enantioselective catalysts [22][23][24][25]. There exist several works involving CILs as an asymmetric catalyst for the aldol condensation of aldehydes and ketones [26][27][28][29][30][31]. The strategy in these reports relies on the modification of IL ion, imidazolium cation in particular, with an enantiopure fragment ("ion tagging") to provide recoverability and reusability of the chiral catalyst.…”