Synthesis of Hybrid Ferrocene-Proline Amides as Active Catalysts for Asymmetric Aldol Reactions in Water

Ferrocenyl substituents were employed as lipophilic moieties and introduced into a prolinamide scaffold to afford new catalysts that were suitable for use in “on water” conditions. Five proline amides that contain a ferrocene unit were synthesized from easily accessible ferrocenylamines and evaluated as catalysts in the asymmetric direct aldol condensation of cyclohexanone with aldehydes in an aqueous medium. Optimal substrate conversion and stereoselectivity were achieved in brine and in the presence of benzoic acid as an additive. All the examined catalysts displayed good activity with a catalyst loading of 5 mol‐%, and the best results in terms of enantioselectivity were obtained by using a ferrocenyl‐prolineamide with an additional (S) chiral center. In the presence of this catalyst, a variety of aromatic aldehydes underwent a reaction in a reasonable amount time with just 2 equiv. of the cyclic ketone to give the expected aldol products in high yields (65–98 %) and with high stereoselectivities (up to 94 % ee, dr up to 93:7). Hydrophobic proline amides that have a ferrocene fragment incorporated into the molecule proved to be efficient catalysts for asymmetric aldol reactions of aromatic aldehydes and cyclic ketones. These reactions were carried out in brine and in the presence of benzoic acid as an additive.