Open AccessSwitch From Pauli‐Lowering to LUMO‐Lowering Catalysis in Brønsted Acid‐Catalyzed Aza‐Diels‐Alder Reactions
Author(s) -
Yu Song,
Bickelhaupt F. Matthias,
Hamlin Trevor A.
Publication year - 2021
Publication title -
chemistryopen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.644
H-Index - 29
ISSN - 2191-1363
DOI - 10.1002/open.202100172
Subject(s) - homo/lumo , protonation , chemistry , catalysis , brønsted–lowry acid–base theory , reactivity (psychology) , photochemistry , pauli exclusion principle , computational chemistry , organic chemistry , molecule , physics , medicine , ion , alternative medicine , pathology , condensed matter physics
Brønsted acid‐catalyzed inverse‐electron demand (IED) aza‐Diels‐Alder reactions between 2‐aza‐dienes and ethylene were studied using quantum chemical calculations. The computed activation energy systematically decreases as the basic sites of the diene progressively become protonated. Our activation strain and Kohn‐Sham molecular orbital analyses traced the origin of this enhanced reactivity to i) “Pauli‐lowering catalysis” for mono ‐protonated 2‐aza‐dienes due to the induction of an asynchronous, but still concerted, reaction pathway that reduces the Pauli repulsion between the reactants; and ii) “LUMO‐lowering catalysis” for multi ‐protonated 2‐aza‐dienes due to their highly stabilized LUMO(s) and more concerted synchronous reaction path that facilitates more efficient orbital overlaps in IED interactions. In all, we illustrate how the novel concept of “Pauli‐lowering catalysis” can be overruled by the traditional concept of “LUMO‐lowering catalysis” when the degree of LUMO stabilization is extreme as in the case of multi ‐protonated 2‐aza‐dienes.