The solvent effect on the Diels–Alder reaction in ionic liquids: multiparameter linear solvation energy relationships and theoretical analysis

Ionic Liquids: Applications in Heterocyclic Synthesis

“…The main results of this investigation revealed that the Diels-Alder reaction in the presence of the imidazolium cation proceeds via a concerted mechanism similar to the "uncatalyzed" cycloaddition, although the asynchronicity of the process is increased by the presence of the imidazolium cations. The energetic differences between the reactants and the TSs for the endo and exo approaches, calculated for the three dienophiles in the presence of the above-mentioned cations, were qualitatively in agreement with the experimental data and confirmed the high selectivity in favor of the endo path for the reaction of cyclopentadiene with acrolein or methyl acrylate in a [HBIM]-based IL [Chiappe et al, 2010]. On the basis of these data, it was hypothesized that the interaction between the IL cation and the dienophile may be affected by the whole ionic system, and the expression "clamp-effect" was used to define this interaction.…”

“…Therefore, the consequences of cation-dienophile interaction and of the clamp effect on Diels-Alder reactions is, from a simplistic view, that the interaction with the cation determines the polarization of the double bond of the dienophile, increasing its reactivity, whereas the clamp effect blocks one of the reactants, increasing the probability of efficient stacking in the TS. Another finding verified by Chiappe et al [Chiappe et al, 2010] was that the presence of the IL changes the geometry of the TS for all four pathways considered, deforming the diene-dienophile stacking geometry and enhancing the asynchronicity of the reaction when performed in these solvents. Before examining in detail the solvation aspects, it is necessary to recall that the insertion of a solute in a solvent is characterized by a free energy of solvation that can be approximately divided in two parts: the change in electronic energy of the solute given by electrostatic and dispersion interactions with the solvent, and the change of solvent energy due to the necessary reorganization of the solvent molecules in order to embed the solute.…”

“…However, the situation is much more complicated than that represented by Eqn 1 and Eqn 2, involving a system more complex than an ion pair, and in which kinetic effects also play a role during the solvent reorganization and reorientation. Also, in this review, Chiappe et al [Chiappe et al, 2010] reported the more important findings of theoretical studies on DielsAlder reactions in ILs. The main results of this investigation revealed that the Diels-Alder reaction in the presence of the imidazolium cation proceeds via a concerted mechanism similar to the "uncatalyzed" cycloaddition, although the asynchronicity of the process is increased by the presence of the imidazolium cations.…”

“…Recently, Chiappe et al [Chiappe et al, 2010] published a review about solvent effect on the Diels-Alder reactions in ILs. In this work, the authors discussed the role of ILs in these reactions considering multiparameter linear solvation energy relationships and theoretical analysis.…”

Low melting mixtures in organic synthesis – an alternative to ionic liquids?

“…The Diels-Alder reaction of cyclopentadiene with methyl acrylate was performed in high yields and endo/exo ratios comparable to those using ionic liquids and scCO 2 . 19,69 Metal-catalysed reactions like Suzuki, 20,25 Heck, 25 and Sonogashira reactions, 25 and the Huisgen 1,3-dipolar cycloaddition 25 were also successfully conducted in high yields. By exchanging the sugar melts by a carnitine-urea melt, reduced yields in the Heck reaction, a lower endo/exo ratio in the Diels-Alder reaction, but similar yields in the Huisgen 1,3-dipolar cycloaddition were observed.…”