Catalytic activation of imine derivatives using novel Lewis acids

The Lewis acid-mediated reactions of imine are one of the most powerful methods for preparation of nitrogen-containing compounds. However, there are few examples of the reactions using catalytic amounts of Lewis acids, because the strong coordination of the products (which are mostly secondly or tertiary amines), deactivates the acids. This article introduces several types of new achiral and chiral Lewis acids which can mediated the reactions of imines catalytically. The essence of the catalytic activation of imines by Lewis acids is the equilibrium between Lewis acids and bases (imines or products), and it has been revealed that rare earth triflates (lanthanide and scandium trifluoromethanesulfonate) are excellent catalysts for this purpose. Imino-aldol reactions, aza Diels-Alder reactions, allylation reactions, cyanation reactions, and three-component reactions of aldehydes, amines, and nucleophiles were successfully carried out in the presence of catalytic amounts of rare earth triflates. Polymer-supported reagents also worked well by using the triflates as catalysts. In addition, it was shown that group IV triflates (Zr and Hi triflates) were effective for catalytic activation of imines. The first truly catalytic asymmetric reactions of imines have been achieved using new chiral Lewis acids. In the presence of a catalytic amount of a chiral rare earth catalyst, imines derived from 2-aminophenol and aldehydes reacted with cyclopentadiene or vinyl ethers to afford 8-hydroxytetrahydroquinoline derivatives in high yields with high diastereo-and enantioselectivities. Moreover, the first catalytic enantioselective Mannich-type reactions of imines with silyl enolates using a novel chiral zirconium catalyst have been developed.High levels of enantioselectivities in the synthesis of chiral beta-amino ester derivatives, beta-amino alcohol derivatives, and tetrahydropyridine derivatives have been achieved using these reactions.