title: Mechanistic Insights into the Mode of Action of Bifunctional Pyrrolidine-Squaramide-Derived Organocatalysts.
abstract: The catalytic modes of action of three squaramide-derived bifunctional organocatalysts have been investigated using DFT methods. The [5+2] cycloaddition between oxidopyrylium ylides and enals was used as the model reaction. Two primary modes were possible for the different catalysts studied. The preference for one mode over the other was due to the possibility of additional favorable π-π interactions between the hydrogen-bond activated pyrylium ylide and an electron-deficient aromatic ring bonded to the squaramide NH group. The model can be extended to other reactions catalyzed by the same catalysts, such as formal [2+2] cycloadditions between nitroalkenes and α,β-unsaturated aldehydes. The computational results were in excellent concurrence with the available experimental reports on the observed total enantioselectivity and differences in diastereoselectivity depending on the substrate and the reaction.
authors.affiliation: Laboratorio de Síntesis Asimétrica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC, Campus San Francisco, 5009, Zaragoza, Spain.
authors.forename: David
authors.initials: D
authors.lastname: Roca-López
authors.affiliation: Department of Organic Chemistry II, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080, Bilbao, Spain.
authors.forename: Uxue
authors.initials: U
authors.lastname: Uria
authors.affiliation: Department of Organic Chemistry II, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080, Bilbao, Spain.
authors.forename: Efraim
authors.initials: E
authors.lastname: Reyes
authors.affiliation: Department of Organic Chemistry II, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080, Bilbao, Spain.
authors.forename: Luisa
authors.initials: L
authors.lastname: Carrillo
authors.affiliation: Department of Chemistry, Aarhus University, 8000, Aarhus C, Denmark.
authors.forename: Karl Anker
authors.initials: KA
authors.lastname: Jørgensen
authors.affiliation: Department of Organic Chemistry II, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080, Bilbao, Spain. joseluis.vicario@ehu.es.
authors.forename: Jose L
authors.initials: JL
authors.lastname: Vicario
authors.affiliation: Laboratorio de Síntesis Asimétrica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza-CSIC, Campus San Francisco, 5009, Zaragoza, Spain. pmerino@unizar.es.
authors.forename: Pedro
authors.identifier: http://orcid.org/0000-0002-2202-3460
authors.initials: P
authors.lastname: Merino
keywords: asymmetric catalysis; density functional calculations; dienamines; hydrogen bonds; pi interactions; ylides