Abstract
Prolinamides with double-H potential were prepared and employed as organocatalysts in asymmetric aldol reactions. The catalyst with adamantane showed improved catalytic activity, which was further enhanced by using brine as the solvent. A series of aldol reactions in brine at 0 °C provided good yields (up to 98%) with high diastereoselectivities (>99 : 1) and enantioselectivities (>99%). The prepared catalyst was adsorbed by a nanofibrous film of poly(AN-MA-β-CD) via host–guest interaction in the reaction system. The catalyst was separated from the film by applying ultrasound, with a total recovery of 96.2%. The catalyst was reused up to five times without a significant change in diastereoselectivity and enantioselectivity.
Highlights
Asymmetric aldol reactions play an important role in carbon– carbon bond forming reactions
In 2010, Moorthy demonstrated the use of double-H potential catalysts based on L-proline and ophenylenediamine to catalyze the aldol reaction of p-nitrobenzaldehyde with cyclohexanone in DMF in the presence of TFA with desirable yield (94%), stereoselectivity (>98%), and diastereoselectivity (97 : 3).[4]
We reported the aldol product of p-nitrobenzaldehyde with cyclohexanone in toluene at –20 C was separated by a-CD from the reaction system in Scheme 1.15 a-CD being easy to dissolve in water is to the disadvantage of the overall recycling of the product
Summary
Asymmetric aldol reactions play an important role in carbon– carbon bond forming reactions. Ophenylenediamine catalysts with double hydrogen potential based on L-proline or trans-4-hydroxy-L-proline were synthesized to catalyze aldol reactions of ketone and aldehyde (Fig. 1). The brine was used as medium at 0 C for catalyzing the aldol reaction of aldehyde and ketone, resulting in excellent yields with good stereoselectivity and diastereoselectivity. We employed chiral prolinamide catalysts 1a–1d to screen the direct aldol reaction of p-nitrobenzaldehyde with cyclohexanone at room temperature (Scheme 2).
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