On the mechanism of bifunctional squaramide-catalyzed organocatalytic michael addition: A protonated catalyst as an oxyanion hole

Bianka Kótai, György Kardos, Andrea Hamza, Viktor Farkas, Imre Pápai, Tibor Soós

Research output: Contribution to journalArticle

77 Citations (Scopus)

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. Finding the path: The mechanism of bifunctional squaramide-promoted Michael addition of prochiral 1,3-dioxo nucleophiles and nitroolefin has been studied on the basis of DFT calculations. Among the investigated mechanistic scenarios, the pathway corresponding to electrophile activation via the protonated amine unit is found to be the most feasible (see figure). For some of the minor stereoisomeric products, alternative pathways are also accessible.

Original languageEnglish
Pages (from-to)5631-5639
Number of pages9
JournalChemistry - A European Journal
Volume20
Issue number19
DOIs
Publication statusPublished - May 5 2014

Keywords

  • Michael addition
  • density functional calculations
  • organocatalysis
  • reaction intermediates
  • reaction mechanisms

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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