Origin of asynchronicity in Diels-Alder reactions

Pascal Vermeeren*, Trevor A. Hamlin, F. Matthias Bickelhaupt

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

Asynchronicity in Diels-Alder reactions plays a crucial role in determining the height of the reaction barrier. Currently, the origin of asynchronicity is ascribed to the stronger orbital interaction between the diene and the terminal carbon of an asymmetric dienophile, which shortens the corresponding newly formed C-C bond and hence induces asynchronicity in the reaction. Here, we show, using the activation strain model and Kohn-Sham molecular orbital theory at ZORA-BP86/TZ2P, that this rationale behind asynchronicity is incorrect. We, in fact, found that following a more asynchronous reaction mode costs favorable HOMO-LUMO orbital overlap and, therefore, weakens (not strengthens) these orbital interactions. Instead, it is the Pauli repulsion that induces asynchronicity in Diels-Alder reactions. An asynchronous reaction pathway also lowers repulsive occupied-occupied orbital overlap which, therefore, reduces the unfavorable Pauli repulsion. As soon as this mechanism of reducing Pauli repulsion dominates, the reaction begins to deviate from synchronicity and adopts an asynchronous mode. The eventual degree of asynchronicity, as observed in the transition state of a Diels-Alder reaction, is ultimately achieved when the gain in stability, as a response to the reduced Pauli repulsion, balances with the loss of favorable orbital interactions.

Original languageEnglish
Pages (from-to)20095-20106
Number of pages12
JournalPhysical Chemistry Chemical Physics
Volume23
Issue number36
Early online date3 Sep 2021
DOIs
Publication statusPublished - 28 Sep 2021

Bibliographical note

Funding Information:
We thank The Netherlands Organization for Scientific Research (NWO) and the Dutch Astrochemistry Network (DAN) for financial support. This work was carried out on the Dutch national e-infrastructure with the support of SURF cooperative.

Publisher Copyright:
© the Owner Societies 2021.

Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.

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