Chiral amine and macrocycle fabricated magnetic nanoparticle asymmetrically catalyzed direct aldol reaction

Abstract

Chiral amine and their derivatives have been proved to be powerful organocatalysts for asymmetric transformations. There is a trend to combine organocatalyst with sustainable matrix, such as, magnetic nanoparticles (MNPs) to form a selective and recyclable catalyst. Therefore, silica-coated Fe3O4 MNP was fabricated to establish a large macrocycle with glutamate (MNP-17 and 18), Pro-Glu (MNP-21), or cyclohexyl-1,2-diamine (MNP-28, 29, and 30). Their bifunctional enamine activities were evaluated with direct aldol reaction. Among them, MNP-30 having the largest macrocyclic ring and the longest lipophilic chain achieved better yield and ee value (up to 96%). It seems the affinity toward substrate was enhanced by the macrocyclic effect. The rigidity of cyclohexyl-1,2-diamine contributed the enantioselectivity. Moreover, MNP-30 is recyclable and stayed on high activity and enantioselectivity after 3 recycles.