Abstract
The ionic Diels–Alder (I-DA) reactions of a series of six iminium cations with cyclopentadiene have been studied within the Molecular Electron Density Theory (MEDT). The superelectrophilic character of iminium cations, ω > 8.20 eV, accounts for the high reactivity of these species participating in I-DA reactions. The activation energies are found to be between 13 and 20 kcal·mol−1 lower in energy than those associated with the corresponding Diels–Alder (DA) reactions of neutral imines. These reactions are low endo selective as a consequence of the cationic character of the TSs, but highly regioselective. Solvents have poor effects on the relative energies, and an unappreciable effect on the geometries. In acetonitrile, the activation energies increase slightly as a consequence of the better solvation of the iminium cations than the cationic TSs. Electron localization function (ELF) topological analysis of the bonding changes along the I-DA reactions shows that they are very similar to those in polar DA reactions. The present MEDT study establishes that the global electron density transfer (GEDT) taking place at the TSs of I-DA reactions, and not steric (Pauli) repulsions such as have been recently proposed, are responsible for the features of these types of DA reactions.
Highlights
The Diels–Alder (DA) reaction between a conjugated diene and an ethylene to yield a cyclohexene, reported for the first time by Diels and Alder in 1928 [1], is one of the most studied organic reactions from a synthetic as well as a theoretical viewpoint [2,3].Accepted: 11 June 2021Published: 14 June 2021Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.Scheme 1
The present Molecular Electron Density Theory (MEDT) study responds to the following questions: (i) What is the origin of the high acceleration and asynchronicity found in ionic Diels–Alder (I-DA) reactions?; (ii) What is the origin of the low endo selectivity in contrast with the high endo selectivity found in P-DA reactions?; and (iii) What is the role of the solvent polarity in these I-PA reactions?
The present MEDT study has been divided into three parts: (i) first, a study of the electronic structure and reactivity at the ground state (GS) of the reagents is performed; (ii) in the second part, a study of the potential energy surface of the I-DA reactions of iminium cations 6, 15–19 with Cp 1 is carried out; and (iii) in the third part, a topological analysis of the bonding changes along the I-DA reactions is presented
Summary
The Diels–Alder (DA) reaction between a conjugated diene and an ethylene to yield a cyclohexene, reported for the first time by Diels and Alder in 1928 [1], is one of the most studied organic reactions from a synthetic as well as a theoretical viewpoint The first step corresponds to the nucleophilic attack of Cp 1 on the carbon of the iminium cation group to produce an acyclic cation intermediate, while the second step is associated with the ring closure of this intermediate along the formation of the second C–N single bond, yielding the final cycloadduct 12 Protonation of both pyridine and imine nitrogen atoms strongly increases the electrophilicity of the imine, and the I-DA reaction takes place along a favorable inverted energy profile, allowing the justification of the large acceleration of the reaction in a strong acid medium [23]. The present MEDT study responds to the following questions: (i) What is the origin of the high acceleration and asynchronicity found in I-DA reactions?; (ii) What is the origin of the low endo selectivity in contrast with the high endo selectivity found in P-DA reactions?; and (iii) What is the role of the solvent polarity in these I-PA reactions?
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