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On the Mechanism of Bifunctional Squaramide‐Catalyzed Organocatalytic Michael Addition: A Protonated Catalyst as an Oxyanion Hole
Author(s) -
Kótai Bianka,
Kardos György,
Hamza Andrea,
Farkas Viktor,
Pápai Imre,
Soós Tibor
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201304553
Subject(s) - squaramide , bifunctional , protonation , chemistry , organocatalysis , nucleophile , electrophile , michael reaction , oxyanion , catalysis , transition state , enantioselective synthesis , bifunctional catalyst , organic chemistry , ion
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 CC 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.