title: On the mechanism of bifunctional squaramide-catalyzed organocatalytic Michael addition: a protonated catalyst as an oxyanion hole.
abstract: A joint experimental-theoretical study of a bifunctional squaramide-amine-catalyzed Michael addition reaction between 1,3-dioxo nucleophiles and nitrostyrene has been undertaken to gain insight into the nature of bifunctional organocatalytic activation. For this highly stereoselective reaction, three previously proposed mechanistic scenarios for the critical CC bond-formation step were examined. Accordingly, the formation of the major stereoisomeric products is most plausible by one of the bifunctional pathways that involve electrophile activation by the protonated amine group of the catalyst. However, some of the minor product isomers are also accessible through alternative reaction routes. Structural analysis of transition states points to the structural invariance of certain fragments of the transition state, such as the protonated catalyst and the anionic fragment of approaching reactants. Our topological analysis provides deeper insight and a more general understanding of bifunctional noncovalent organocatalysis.
authors.affiliation: Institute of Organic Chemistry, Research Centre for Natural Sciences of the Hungarian Academy of Sciences, Pusztaszeri út 59-67, H-1025 Budapest (Hungary), Fax: (+36) 1-438-1145.
authors.forename: Bianka
authors.initials: B
authors.lastname: Kótai
authors.forename: György
authors.initials: G
authors.lastname: Kardos
authors.forename: Andrea
authors.initials: A
authors.lastname: Hamza
authors.forename: Viktor
authors.initials: V
authors.lastname: Farkas
authors.forename: Imre
authors.initials: I
authors.lastname: Pápai
authors.forename: Tibor
authors.initials: T
authors.lastname: Soós
mesh_terms: D000577:Amides / Q000737:chemistry*; D000588:Amines / Q000737:chemistry*; D002384:Catalysis; D015394:Molecular Structure; D055601:Organic Chemistry Phenomena; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D000577:Amides; D000588:Amines
keywords: Michael addition; density functional calculations; organocatalysis; reaction intermediates; reaction mechanisms