Tracking the Transition from Pericyclic to Pseudopericyclic Reaction Mechanisms Using Multicenter Electron Delocalization Analysis: The [1,3] Sigmatropic Rearrangement.

Álvaro Pérez-Barcia,Marcos Mandado,Ángeles Peña-Gallego

doi:10.1021/acs.jpca.1c06620

Álvaro Pérez-Barcia, Marcos Mandado + Show 1 more

Open Access

https://doi.org/10.1021/acs.jpca.1c06620

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Abstract

Herein, the power of multicenter electron delocalization analysis to elucidate the intricacies of concerted reaction mechanisms is brought to light by tracking the transition of [1,3] sigmatropic rearrangements from the high-barrier pericyclic mechanism in 1-butene to the barrierless pseudopericyclic mechanism in 1,2-diamino-1-nitrosooxyethane. This transition has been progressively achieved by substituting the migrating group, changing the donor and acceptor atoms, and functionalizing the alkene unit with weak and strong electron-donating and electron-withdrawing groups. Fourteen [1,3] sigmatropic reactions with electronic energy barriers ranging from 1 to 89 kcal/mol have been investigated. A very good correlation has been found between the barrier and the four-center electron delocalization at the transition state, the latter calculated for the atoms involved in the four-centered ring adduct formed along the reaction path. Surprisingly, the barrier has been found to be independent of the bond strength between the migrating group and the donor atom so that only the changes induced in the multicenter bonding control the kinetics of the reaction. Additional insights into the effect of atom substitution and group functionalization have also been extracted from the analysis of the multicenter electron delocalization profiles along the reaction path and qualitatively supported by the topological analysis of the electron density.

Highlights

Multicenter electron delocalization indices (MCIs),1−3 a quantum chemical tool based on the n-electron density function, were proposed 15 years ago as a powerful tool to characterize aromaticity in transition states (TSs) of pericyclic reactions.4 The great sensitivity of MCIs for detecting changes in the electron delocalization patterns within a reacting system was demonstrated
The former is routed on the two-electron probability density,7 whereas the latter is based on the one-electron density so that both are invariant to molecular orbital (MO) transformations
A quantitative tool to characterize concerted reaction mechanisms as pericyclic or pseudopericyclic has been tested. This tool is based on the direct measurement of multicenter electron delocalization through the calculation of MCIs

Summary

Introduction

Multicenter electron delocalization indices (MCIs),− a quantum chemical tool based on the n-electron density function, were proposed 15 years ago as a powerful tool to characterize aromaticity in transition states (TSs) of pericyclic reactions. The great sensitivity of MCIs for detecting changes in the electron delocalization patterns within a reacting system was demonstrated. During the years following the publication of this guiding work, the applications of MCIs in characterizing concerted mechanisms in chemical reactions have been scarce.5,6 This is partly due to the existence of other valuable quantum chemical tools based on the quantum chemical topology, such as the electron localization function (ELF)− or the topological analysis of electron density critical points.. This is partly due to the existence of other valuable quantum chemical tools based on the quantum chemical topology, such as the electron localization function (ELF)− or the topological analysis of electron density critical points.10 The former is routed on the two-electron probability density, whereas the latter is based on the one-electron density so that both are invariant to MO transformations. As was shown very recently for the particular case of Diels−Alder reactions, a key Received: July 26, 2021 Published: September 11, 2021

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