Origin of the α-Effect in SN2 Reactions

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Abstract

The α-effect is a term used to explain the dramatically enhanced reactivity of α-nucleophiles (R−Y−X:) compared to their parent normal nucleophile (R−X:) by deviating from the classical Brønsted-type reactivity-basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α-effect of a set of anionic nucleophiles, including O-, N- and S-based normal and α-nucleophiles, participating in an SN2 reaction with ethyl chloride using relativistic density functional theory at ZORA-OLYP/QZ4P. Our activation strain and Kohn–Sham molecular orbital analyses identified two criteria an α-nucleophile needs to fulfill in order to show α-effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied–occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO–LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO–LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α-effect or inverse α-effect.

Original languageEnglish
Pages (from-to)20840-20848
Number of pages9
JournalAngewandte Chemie - International Edition
Volume60
Issue number38
Early online date4 Jun 2021
DOIs
Publication statusPublished - 13 Sep 2021

Bibliographical note

Funding Information:
This work was supported by the Netherlands Organization for Scientific Research (NWO) and the Dutch Astrochemistry Network (DAN) for financial support. We thank Pieter C. M. Laan for exploring the viability of this project.

Publisher Copyright:
© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH

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

Keywords

  • activation strain model
  • basicity
  • density functional calculations
  • nucleophilicity
  • α-effect

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