Unveiling the Intramolecular Ionic Diels–Alder Reactions within Molecular Electron Density Theory

Luis R Domingo,María José Aurell,Mar Ríos-Gutiérrez

doi:10.3390/chemistry3030061

Abstract

The intramolecular ionic Diels–Alder (IIDA) reactions of two dieniminiums were studied within the Molecular Electron Density Theory (MEDT) at the ωB97XD/6-311G(d,p) computational level. Topological analysis of the electron localization function (ELF) of dieniminiums showed that their electronic structures can been seen as the sum of those of butadiene and ethaniminium. The superelectrophilic character of dieniminiums accounts for the high intramolecular global electron density transfer taking place from the diene framework to the iminium one at the transition state structures (TSs) of these IIDA reactions, which are classified as the forward electro density flux. The activation enthalpy associated with the IIDA reaction of the experimental dieniminium, 8.7 kcal·mol−1, was closer to that of the ionic Diels–Alder (I-DA) reaction between butadiene and ethaniminium, 9.3 kcal·mol−1. However, the activation Gibbs free energy of the IIDA reaction was 12.7 kcal·mol−1 lower than that of the intermolecular I-DA reaction. The strong exergonic character of the IIDA reaction, higher than 20.5 kcal·mol−1, makes the reaction irreversible. These IIDA reactions present a total re/exo and si/endo diastereoselectivity, which is controlled by the most favorable chair conformation of the tetramethylene chain. ELF topological analysis of the single bond formation indicated that these IIDA reactions take place through a non-concerted two-stage one-step mechanism. Finally, ELF and atoms-in-molecules (AIM) topological analyses of the TS associated with the inter and intramolecular processes showed the great similarity between them.

Highlights

The The Diels–Alder Diels–Alder(DA)(DA) reaction reaction[1],[1], which which is is a general class of cycloaddition reactions, is one of the most useful synthetic reactions in is one of the most useful synthetic reactions inOrganic OrganicChemistry Chemistry [2,3]. [2,3]
GaussView program [53] (Version 6.0, Semichem Inc., Shawnee, KS, USA) was ond part, the study of the reaction paths associated with I-DA reaction between butadiene molecular geometries of all the as well as the position12of the EL
The present Molecular Electron Density Theory [12] (MEDT) study was divided into three parts: (i) first, a study of tronic structure and reactivity at the ground state (GS) of the reagents is performe the second part, the study of the reaction paths associated with I-DA reaction butadiene 1 and ethaniminium 18, and those associated with the IIDA reactions

Summary

Introduction

Introduction The The Diels–Alder Diels–Alder(DA)(DA) reaction reaction[1],[1], which which is is a general class of cycloaddition reactions, is one of the most useful synthetic reactions in is one of the most useful synthetic reactions inOrganic OrganicChemistry Chemistry [2,3]. [2,3]. [1], which which is is a general class of cycloaddition reactions, is one of the most useful synthetic reactions in is one of the most useful synthetic reactions inOrganic OrganicChemistry Chemistry [2,3]. By. By varying varying the the nature nature of of the the diene diene and and the theethylene, ethylene, many many different differenttypes typesof ofsix-membered six-memberedcarbocyclic carbocyclic structures structures can can be be built built up. Not not all all possibilities possibilities take take place place

Methods

Results

Conclusion