Unveiling the Ionic Diels-Alder Reactions within the Molecular Electron Density Theory
Luis R Domingo1, Mar Ríos-Gutiérrez1, María José Aurell1
1Department of Organic Chemistry, University of Valencia, Dr. Moliner 50, E-46100 Valencia, Spain.
Molecules (Basel, Switzerland)
|July 2, 2021
Summary
Ionic Diels-Alder reactions are highly reactive due to superelectrophilic iminium cations. Global electron density transfer, not steric repulsion, drives these reactions, offering lower activation energies than neutral imine counterparts.
Area of Science:
- Organic Chemistry
- Theoretical Chemistry
- Computational Chemistry
Background:
- Diels-Alder reactions are fundamental in organic synthesis.
- Ionic Diels-Alder (I-DA) reactions offer unique reactivity pathways.
- Understanding the driving forces behind I-DA reactions is crucial for synthetic applications.
Purpose of the Study:
- To investigate the reactivity and selectivity of ionic Diels-Alder reactions involving iminium cations and cyclopentadiene.
- To elucidate the role of electronic factors, specifically global electron density transfer (GEDT), in I-DA reactions.
- To compare the energetic profiles of I-DA reactions with traditional Diels-Alder reactions.
Main Methods:
- Utilized Molecular Electron Density Theory (MEDT) for theoretical investigation.
- Analyzed activation energies and transition state (TS) characteristics.
- Employed Electron Localization Function (ELF) topological analysis to study bonding changes.
- Studied the influence of solvent effects on reaction energetics and geometries.
Main Results:
- Iminium cations exhibit superelectrophilic character (ω > 8.20 eV), leading to high reactivity in I-DA reactions.
- I-DA reactions show significantly lower activation energies (13–20 kcal·mol-1) compared to neutral imine Diels-Alder reactions.
- Reactions are characterized by low endo selectivity and high regioselectivity, attributed to the cationic nature of the transition states.
- Solvent effects on relative energies and geometries are minimal, with a slight increase in activation energy in acetonitrile due to solvation differences.
Conclusions:
- Global electron density transfer (GEDT) at the transition state is identified as the primary factor governing I-DA reaction features, rather than steric repulsions.
- MEDT provides a robust framework for understanding the mechanistic intricacies of ionic cycloaddition reactions.
- The findings offer valuable insights for designing and controlling stereoselective and regioselective organic transformations.
Keywords:
Molecular Electron Density Theoryasynchronicityglobal electron density transferiminium cationsionic Diels–Alder reactionssuperelectrophilesMore Related Videos
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