Issue 31, 2020

Origin of rate enhancement and asynchronicity in iminium catalyzed Diels–Alder reactions

Abstract

The Diels–Alder reactions between cyclopentadiene and various α,β-unsaturated aldehyde, imine, and iminium dienophiles were quantum chemically studied using a combined density functional theory and coupled-cluster theory approach. Simple iminium catalysts accelerate the Diels–Alder reactions by lowering the reaction barrier up to 20 kcal mol−1 compared to the parent aldehyde and imine reactions. Our detailed activation strain and Kohn–Sham molecular orbital analyses reveal that the iminium catalysts enhance the reactivity by reducing the steric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchronous reaction mode and a more significant polarization of the π-system away from the incoming diene compared to aldehyde and imine analogs. Notably, we establish that the driving force behind the asynchronicity of the herein studied Diels–Alder reactions is the relief of destabilizing steric (Pauli) repulsion and not the orbital interaction between the terminal carbon of the dienophile and the diene, which is the widely accepted rationale.

Graphical abstract: Origin of rate enhancement and asynchronicity in iminium catalyzed Diels–Alder reactions

Supplementary files

Article information

Article type
Edge Article
Submitted
22 mai 2020
Accepted
08 jul 2020
First published
09 jul 2020
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2020,11, 8105-8112

Origin of rate enhancement and asynchronicity in iminium catalyzed Diels–Alder reactions

P. Vermeeren, T. A. Hamlin, I. Fernández and F. M. Bickelhaupt, Chem. Sci., 2020, 11, 8105 DOI: 10.1039/D0SC02901G

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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