title: Asymmetric Michael Additions of 4-Hydroxycoumarin to β-Nitrostyrenes with Chiral, Bifunctional Hydrogen-Bonding Catalysts.
abstract: Enantioselective Michael additions of 4-hydroxycoumarin to β-nitrostyrenes are catalyzed by different chiral, bifunctional hydrogen-bonding catalysts, based on thiourea- and squaramide motifs. The scope of the catalysis is tested by employing a series of substituted β-nitrostyrenes as well as different solvents. The 3,5-bis(trifluoromethyl)phenyl- and quinine-substituted squaramide catalyst is shown to be the most selective catalyst, resulting in 78% yield and 81% ee. Computational analyses of transition structures with different binding modes show that the most favored transition structure exhibits squaramide (NH)2 binding to an oxygen atom of the enolate nucleophile, while the nitroalkene coordinates via hydrogen bonding to the ammonium function of the quinuclidine unit of the catalyst. Hence, the canted directionality of the squaramide (NH)2 motif, favoring one-atom binding, might be decisive for the selectivity of the reaction. The absolute configuration of the major (-)-(R) enantiomer of the product is assigned computationally according to its optical rotation.
authors.affiliation: Institut für Organische Chemie, Universität zu Köln , Greinstrasse 4, 50939 Köln, Germany.
authors.forename: Florian F
authors.initials: FF
authors.lastname: Wolf
authors.affiliation: Institut für Organische Chemie, Universität zu Köln , Greinstrasse 4, 50939 Köln, Germany.
authors.forename: Helge
authors.initials: H
authors.lastname: Klare
authors.affiliation: Institut für Organische Chemie, Universität zu Köln , Greinstrasse 4, 50939 Köln, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Goldfuss