Origin of reversal of stereoselectivity for [4+2] cycloaddition reaction between cyclopentadiene and methyl methacrylate in the presence of the chloroloaluminate ionic liquid (1-ethyl-3-methyl-imidazolium chloride): in silico studies

The origin of stereoselectivity for [4+2] cycloaddition reaction of methyl methacrylate with cyclopentadiene was investigated with the B3LYP-D3(BJ)/6-31+G(d)//B3LYP/6-31+G(d) level of theory in the presence of the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMI.sup.+][Cl.sup.-]) and its acidic chloroaluminate melt, 1-ethyl-3-methyl-imidazolium heptachlorodialuminate ([EMI.sup.+][Al.sub.2][Cl.sub.7.sup.-]). The reaction of methyl methacrylate with cyclopentadiene was examined in the gas phase to rationalize the effect of the ionic liquid ion pairs [EMI.sup.+][Cl.sup.-] and [EMI.sup.+][Al.sub.2][Cl.sub.7.sup.-]. The DFT calculated results were found to be in good agreement with the experimentally observed results. The much-discussed hydrogen bonding effect of the imidazolium cation with the dienophile is less important to govern the stereoselectivity for the cycloaddition reaction. The atoms in molecules theory was used to examine the role of hydrogen bonding between the [EMI.sup.+] cation and methyl methacrylate in the transition state geometries. The calculated activation barriers with the M062X/6-31+G(d)//B3LYP/6-31+G(d) and MP2/6-311+G(d,p)//B3LYP/6-31+G(d) levels of theory also predict the similar exo/endo-stereoselectivity trend for the cycloaddition reactions. Key words: DFT, Diels-Alder reaction, cyclopentadiene, methyl methacrylate, ionic liquid. L'origine de la stereoselectivite de la reaction de cycloaddition [4+2] du methacrylate de methyle avec le cyclopentadiene a ete etudiee a l'aide du niveau B3LYP-D3(BJ)/6-31+G(d)//B3LYP/6-31+G(d) de la theorie de la fonctionnelle de la densite en presence du liquide ionique, le chlorure de 1-ethyl-3-methyl-imidazolium ([EMI.sup.+][Cl.sup.-]) et de son melange acide de chloroaluminate, le heptachlorodialuminate de 1-ethyl-3-methyl-imidazolium ([EMI.sup.+][Al.sub.2][Cl.sub.7.sup.-]). La reaction du methacrylate de methyle avec le cyclopentadiene a ete observee en phase gazeuse en vue de rationaliser l'effet des paires d'ions [EMI.sup.+][Cl.sup.-] et [EMI.sup.+][Al.sub.2][Cl.sub.7.sup.-] du liquide ionique. Les resultats de la TFD calcules ont montre une bonne concordance avec ceux obtenus experimentalement. L'effet de liaison hydrogene, souvent etudie, entre le cation imidazolium et dienophile influence peu la stereoselectivite de la reaction de cycloaddition. La theorie quantique des atomes dans les molecules a ete utilisee pour etudier le role de la liaison hydrogene entre le cation [EMI.sup.+] et le methacrylate de methyle dans les geometries d'etat de transition. Les barrieres de transition calculees a l'aide des niveaux M062X/6-31+G(d)//B3LYP/6-31+G(d) et MP2/6-311+G(d,p)//B3LYP/6-31+G(d) de la theorie permettent egalement de predire la meme tendance exo/endo-stereoselective des reactions de cycloaddition. [Traduit par la Redaction] Mots-cles: TFD, reaction de Diels-Alder, cyclopentadiene, methacrylate de methyle, liquide ionique.
Introduction Traditional organic solvents that are generally volatile in nature can be replaced by ionic liquids (ILs) to avoid the emission of volatile organic compounds in our environment. (1) The [...]