Insight into the Interactions between Azole-Anion-Based Ionic Liquids and Propargylic Alcohol: Influence on the Carboxylative Cyclization of Propargylic Alcohol with Carbon Dioxide

Abstract

To reveal the interactions between azole-anion-based ionic liquids (AILs) and propargyl alcohol (PA) and their effect on the carboxylative cyclization reaction of PA with CO₂, three AILs, namely, [P₆₆₆₁₄][Im], [P₆₆₆₁₄][4-MeIm], and [P₆₆₆₁₄][4-BrIm], were synthesized and characterized. The interactions between the AILs with 2-methyl-3-butyn-2-ol (MBY) were investigated through IR and ¹H NMR spectra and DFT calculations. The results indicate that azole-based anions can form H-bonds with the hydroxyl (O–H) and the alkynyl hydrogen atoms (C≡C–H) of MBY simultaneously, with the former much stronger than the latter, which is supported by the interaction energy and the distance of the H-bond from DFT calculations. Interestingly, the strength of the former always increases with the mole fraction of AIL (xAIL), while that of the latter first decreases and then increases, reaching a minimum at xAIL ≈ 0.4. Furthermore, the order of the strength of the two H-bonds is as follows: [P₆₆₆₁₄][Im] > [P₆₆₆₁₄][4-MeIm] > [P₆₆₆₁₄][4-BrIm], which is in good agreement with their basicity. According to the interactions between the AILs with MBY and combined with the reaction mechanism of MBY with CO₂ promoted by AILs, it is evident that the increase in the basicity of the AILs enhances the H-bond between the AILs with O–H, thereby enhancing the activation of MBY and the subsequent CO₂ conversion. However, this will also increase the interaction between AILs and C≡C–H, resulting in the deprotonation of C≡C–H and hindering the activation of MBY. Therefore, choosing a suitable basic AIL and controlling its ratio are the keys to the reaction of PA and CO₂.