β-Isocupreidinate‒CaAl-layered double hydroxide composites—heterogenized catalysts for asymmetric Michael addition

Abstract

β-isocupreidinate (β-iCu) anions having well-known structure and catalytic activity in various asymmetric reactions, were incorporated into the interlayer gallery of hydrocalumite (CaAl-LDH) by the partial delamination-restacking method. Silylation of the outer surface of the LDH with trimethyl silane was also employed to avoid the adsorption of β-iCu on the outer surface and to block the basic sites there. The obtained materials were characterized by a range of instrumental methods (X-ray diffractometry, scanning electron microscopy, ATR-IR and grazing incidence IR spectroscopies). The catalytic activities of the composites were tested in the asymmetric Michael addition of β-nitrostyrene and ethyl 2-fluoroacetoacetate. The β-iCu-pillared LDHs proved to be active and recyclable catalysts with very good diastereoselectivities and acceptable and in 2-propanol very good enantioselectivities. The silylated composite retained its activity and diastereoselectivity even in the third repeated run, when heptane was the solvent.