Enhanced heterogeneous asymmetric catalysis via the acid-base cooperation between achiral silanols of mesoporous supports and immobilized chiral amines

The heterogeneous acid-base cooperation between achiral silanols of mesoporous silica and immobilized chiral amines in asymmetric catalysis has been investigated in this work using direct asymmetric aldol condensation as model reaction. To accomplish the acid-base cooperative asymmetric catalysis as expected, not only the quantity and the acidity of surface hydroxyls have been controlled by the silyation of silica surface and the incorporation of framework aluminum, but also the structure of chiral pyrrolidine amines has been tuned. With appropriate acidity and sufficient quantity of surface silanols, impressive activity acceleration has been achieved for each pyrrolidine amine. For immobilized (2S)-N-(quinine-9-yl)pyrrolidine-2-carboxamid, in which the chiral pyrrolidine amine is modified with neighboring carboxamine moiety, the enantioselectivity has been well preserved, even though the achiral acidic sites compromise the asymmetric induction. The mechanism for the acid-base cooperative asymmetric catalysis between achiral silanols and immobilized chiral amines has been discussed. (C) 2014 Elsevier Inc. All rights reserved.